Nanochemistry provides powerful synthetic tools allowingone tocombine different materials on a single nanostructure, thus unfoldingnumerous possibilities to tailor their properties toward diverse functionalities.Herein, we review the progress in the field of semiconductor–metalhybrid nanoparticles (HNPs) focusing on metal–chalcogenides–metalcombined systems. The fundamental principles of their synthesis arediscussed, leading to a myriad of possible hybrid architectures includingJanus zero-dimensional quantum dot-based systems and anisotropic quasi1D nanorods and quasi-2D platelets. The properties of HNPs are describedwith particular focus on emergent synergetic characteristics. Of these,the light-induced charge-separation effect across the semiconductor–metalnanojunction is of particular interest as a basis for the utilizationof HNPs in photocatalytic applications. The extensive studies on thecharge-separation behavior and its dependence on the HNPs structuralcharacteristics, environmental and chemical conditions, and lightexcitation regime are surveyed. Combining the advanced synthetic controlwith the charge-separation effect has led to demonstration of variousapplications of HNPs in different fields. A particular promise liesin their functionality as photocatalysts for a variety of uses, includingsolar-to-fuel conversion, as a new type of photoinitiator for photopolymerizationand 3D printing, and in novel chemical and biomedical uses. This achievement has risen and continuesto strive on the basis of the ongoing significant synthetic advancesin forming, on the one hand, highly controlled colloidal semiconductornanostructures as well as their metal counterparts, on the other hand.Second, such hybrid semiconductor–metal nanoparticles manifesta combination of the original properties of their constituents ona single nanosystem. Moreover, they reveal unique synergistic effectsemerging from the nanoscale semiconductor–metal combination,thus providing an outcome greater than the simple sum of its parts.3 The ongoing developments in synthesis of HNPs along with the in-depthunderstanding of their combined and synergetic physical and chemicalproperties enable their functionality in numerous promising applications.This includes utilization of HNPs in optical, electronic, biomedical,and environmental fields.7 A notable direction is the application of HNPs asefficient photocatalysts for timely challenges including direct solar-to-fuelconversion, waste degradation, and water purification, as novel photopolymerizationand photocuring agents, for antimicrobial functions, and in photodynamictherapies. These different aspects of HNPs are illustrated in Semiconductor–metalhybrid nanoparticles (HNPs) combinetwo disparate materials on a single nanosystem.14Joining semiconductorand metal segments into a single nanoparticleintroduces several fundamental synthetic challenges. The synthesisdesign needs to overcome the competing homogeneous nucleation of eithersemiconductor or metal phase that would form separate nanosystemsinstead of a unified hybrid nanoparticle. Additionally, lattice mismatchingcan hinder the formation of stable semiconductor–metal junctions.Another factor is the competition with cation exchange reactions andmetal diffusion that would generate a nanostructure with altered compositionrather than the desired HNP. Last but not least, a basic challengefor the colloidal nanosystems is the tailoring of the surface coatingneeded both for colloidal chemical stability as well as for chemicaland physical passivation, and in HNPs, both the semiconductor andmetal nanosurfaces must be considered in tandem. This review presents,in The successful synthesis of semiconductor–metalHNPs hasenabled the ongoing study of their emergent properties. The semiconductornanostructure properties that can be tuned by control of the sizeand shape via quantum confinement as well as by altering the compositioncan now be augmented and enhanced by the features of the combinedmetal nanosegment. This opens up novel possibilities, for example,contact metal points directly grown on the semiconductor nanostructurefor facile electrical wiring and as chemical anchor points. Manipulatingthe semiconductor optical properties allows one to achieve absorptionenhancement and emission enhancement or quenching via near-field effectsand interactions induced by coupling to the metal. One of the mostprominent properties arises from the combination of excitonic semiconductorfeatures with the plasmonic metal response and their couplings thatlead to light-induced charge separation from either side of the semiconductor–metalHNPs. This charge separation is affected by several key parameters.Among these parameters are structural effects such as the metal componentsize and type as was demonstrated in various reports on similar CdS-basedHNPs with different metal sites including Au and Pt, all showing size-dependentcharge-transfer dynamics. The light-induced charge-separation characteristicconstitutes the first step that provides excited charge carriers onboth the semiconductor and the metal segments, serving as the basisfor their photocatalytic activity toward different redox reactionpathways. x2–S andAuPt–CuS showed antitumor activity followed by a decrease intumor volume under NIR irradiation in both in vitro and in vivo experiments. The advanced synthetic control of the semiconductor–metalHNPs provides the ability to tailor their characteristics accordingto the requirements of specific photocatalytic applications. A shiningexample is the ability to match the semiconductor nanostructure gapwith a particular illumination source while tailoring the metal segmentfor a specific chemical reaction. This opens the path for using HNPsas a photocatalyst for water splitting to generate hydrogen fuel fromwater while tailoring the absorption to the visible range of the solarspectrum. Fundamental studies of charge separation on model systemssuch as CdS–Au and CdS–Pt nanorods along with advancedinvestigations of favored working conditions such as excitation regimesand alkaline environments for enhanced photocatalytic performancehave been conducted and can set a basis for the future utilizationof such and related systems in addressing “real-world”challenges. Additionally, HNPs have been applied as a new type ofphotoinitiator termed “quantum photoinitiator” for photopolymerizationreactions in general and in 3D printing scenarios in particular aswas demonstrated by CdS–Au nanorods, which were shown to enablephotopolymerization of polyacrylamide. Photodynamic therapies andbioimaging based on HNPs are also of interest in this respect. Remarkably,HNPs such as Au–CuWhiletwo decades of research have led us to an advanced stagein the synthesis of HNPs, understanding their properties, and developingtheir utilization as photocatalysts, there is a vast landscape ofpotential further developments and breakthroughs yet to be discovered.The outlook and perspective of such HNPs is presented in the closingsection of the review, 216 The specificmaterial combination can dictate the physical, chemical, and opticalproperties of the resultant hybrid nanosystem. Often, the HNPs structureand morphology is also a consequence of this material combination,in which one substance promotes the other component’s growthor deposition parameters including orientation and selective sites,for example, due to its own structural properties such as facet reactivity,lattice constant, and surface defects.Over the last three decades, the syntheticpathways to preparesemiconductor–metal hybrid nanoparticles have vastly expandedand developed. These developments allowed the formation of varioushybrid nanosystems composed of different material combinations.17 orTiO2–Au and TiO2–Pt composites.20 However, control over the HNPs structural parameters such as theshape, size, and selective metal deposition became further achievablesince the pioneering demonstration of site-selective Au metal growthon the apexes of CdSe nanorod structures.1 From that point, the diversity of semiconductors and metals usedas building blocks to the formation of HNPs has been enriched extensively.A variety of transition metal chalcogenide have been used for thesemiconductor component including CdS, ZnS, ZnSe, Cu2S,MoS2, MoSe2, WS2, and WSe2, along with a selection of noble and transition metals includingAu, Ag, Pt, Pd, Co, and Ni that have been utilized as the metalliccomponent in the hybrid system.An early example of HNPswas presented through the growth of metalislands on semiconductor nanoparticles (NPs), such as Au, Ag, Cu,and Pt growth on ZnO nanostructures12 As illustratedin In many cases, the desired specificproperty of HNPs, whether itis structural, physical, or chemical, dictates the synthetic strategy.Therefore, several synthetic routes have been developed exploitingthe different characteristics of the two different materials, semiconductorand metal. Although physical deposition methods such as physical vapordeposition (PVD), pulsed laser deposition (PLD), and molecular beamepitaxy (MBE) have been reported and manifested in the literature,one of the most widely used controllable synthetic methods is chemicalreduction deposition.2.1GS, that definesthe heterogeneous deposition reaction and is given by221 and γ2 are the surface energies associated with the respective materials and γ1,2 is thesolid/solid interfacial energy. In a colloidal phase synthesis, theformer two terms can be significantly influenced by adhesion of cappingligands, precursors, and solvent, while the latter depends on thebonding strength and degree of crystallographic compatibility of theconcerned lattices. Hence, in the case of ΔGS > 0, which is a result of a lower energy surfaceofthe secondary material (γ2 < γ1) and/or a good crystallographic matching with the substrate , a layer-by-layer deposition will probably takeplace  growth mode, GS > 0, due to higher energy surfaces of the secondarymaterial (γ2 > γ1) and/or significantlattice mismatch , it will tend todeposit adopting the habit of a discontinuous island-like domain array growth mode, Fundamentally, the underlying mechanism that promotes the formationof heterogeneous growth at the semiconductor–metal interfacecan be derived from the thermodynamic perspective of a similar macrosystemof thin film epitaxial growth over a crystal surface. The growth modeof a second material on a pre-existing substrate is formally dependent on the sign of the total Gibbs free surfaceenergy function, Δf(θ), ranges between 0 (θ = 0°) in thecase of heterogeneous nucleation and 1 (θ = 180°) for homogeneousnucleation. Note that the contact angle is influenced by the structuralcharacteristics of both seed and deposited phases and the surfaceand interface tension at their boundaries.These different deposition routes can be realizedvia specificsynthetic approaches. One of the most common strategies for the synthesisof heterogeneous nanoparticles in general and HNPs in particular is“seeded growth” implemented in solution phase, whichmay be considered as a derivative approach of traditional vapor-phaseheteroepitaxial deposition. According to classical nucleation theory,the energy barrier for further growth on an already formed particlevia heterogeneous nucleation is substantially lower than the activationenergy for the formation of a new particle via homogeneous nucleation,as can be described by the following relationshipIn light of the aboveexplanation, colloidal synthesis involvingdifferent organic stabilizers and other solution species, which affectthe ultimate Gibbs free energy balance, along with kinetic factorssuch as solution supersaturation, various reactive precursor types,and different chemical diffusion conditions that may relieve the misfitstrain provides a multitude of potential pathways to achieve an enormousdiversity of HNPs with different morphologies and architectures.2.2The metal–semiconductornanojunction can be formed by two strategies. As illustrated in 2.1.123 Typically,deposition of Au domains on various semiconductor materials is performedin organic phase solution at room temperature where the metal precursorsare dissolved in this nonpolar environment with the assistance ofsurfactant molecules such as didodecyldimethylammonium bromide (DDAB),while the reduction occurs in the presence of a reducing agent, forexample, dodecylamine (DDA), which can act also as stabilizer forthe nanoparticles. Utilization of this method was expanded to a varietyof HNPs including different semiconductor materials and shapes. Thisincludes 0D structures such as PbS–Au dots24 and Cu2ZnSnS4 (CZTS)–Au cubes.25 Extensive work on synthetic Au decoration hasbeen reported on quasi-1D structures including CdS–Au,29 ZnSe–Au,30 CdSe/CdS–Au,34 CdTe–Au,35 and CZTS–Au36 nanorods and tetrapods. More recently, withthe synthetic development of 2D nanoplatelet-like semiconductor structures(NPLs), their semiconductor–metal hybrid forms were generatedalso. This includes CdS–Au,38 CdSe–Au,41 and CdSe/CdS–Au.43 Various HNPs with differentmorphologies and material compositions which were synthesized viathis approach are shown in Ageneral synthetic strategy for the formation of semiconductor metalHNPs in which metal salt precursors are reduced on the surface ofpresynthesized semiconductor nanoparticles was first demonstratedby Banin et al. for colloidal CdSe nanorods (NRs) with selectivelydeposited Au metal domains on their apexes.44 In addition, binary metal tips were also reportedincluding PtNi and PtCo, where both metal precursors are present duringthe reduction reaction. A similar synthetic route was used to depositPt nanodomains on the apexes of heterostructure semiconductors suchas CdSe/CdS,45 ZnSe/CdS, and ZnTe/CdS nanorods46 and CuInS2 nanoparticles.48 Additionally, other metals were epitaxially deposited under similarconditions including Ni.49 The reactivityof the semiconductor rod apexes was also exploited for the depositionof PdS.50 Dong et al. reported on the formationof CdSe/CdS–Ag NRs by reducing Ag–trioctylphosphine(TOP) complexes with oleylamine,51 therebyavoiding common cation exchange yielding CdSe/CdS–Ag2S heterostructures based on hard and soft (Lewis) acid and base theory.52 In the presence of TOP, which is a soft base,complexes consisting of a soft acid like Ag ions would preferablyform while hampering the driving force of a hard acid, Cd ions, tomigrate outside of the crystal structure. Reduction by oleylaminewas also used for deposition of presynthesized metal nanoparticlessuch as Pt or Pd, which were used as seeds on CZTS nanoparticles toform core/shell and heterodimer architectures.53Modification of this procedure while using oleicacid and oleylamineas stabilizing and reducing agents, respectively, along with higherreaction temperatures allowed for the deposition of additional metalson similar semiconductor components. 4-C8H12)(η3-C8H13)] in toluene withlauric acid and hexadecylamine under H2 atmosphere andelevated temperature.54 Pyun et al. presentedcobalt metal heterogeneous growth on a pre-existing Pt tip servingas a substrate to allow the growth of a second metal phase55 while applying carboxylic acid-terminated polystyreneas a reducing agent used previously for cobalt deposition reactions.56 A unique metal decoration of Ru and Rh metallicframes over Cu2S nanoparticles was achieved by temperature-controlledthermal reduction of Ru or Rh acetylacetonate (acac) with octadecylaminein octyl ether or diphenyl ether/dichlorobenzene solution, respectively.58 Solvothermal formation of semiconductor–metal HNPs was achievedalso in the absence of additional reducing agents and surfactantsas reported by Cozzoli and co-workers. CdS–Co and CdSe/CdS–CoNRs were obtained at high temperature (240 °C) in octadeceneand Co2(CO)8 as the metal precursor.59 Although chemical reduction of metal precursorsis mostly reported in organic solutions, its feasibility was provenalso in aqueous environments. For example, CdSe–Pt nanorodsand nanonets were obtained under different pH values of aqueous solutions.60 Additionally, aqueous condensation of PdII onto CdS NRs was reported by Shemesh et al.50 As Au growth was utilized on 2D nanosystems, implementationof the described oleylamine-assisted synthetic route was also conductedon NPLs. Typically, Pt or Pd metal domains were deposited on the edgesof homogeneous or heterostructure semiconductor NPLs resulting inCdSe–Pt,41 CdSe–Pd,41 CdS–Pt,61 and CdSe/CdS–Pt43 dimethylplatinum(II)as the platinum source and an excess of a tertiary amine (such astriethylamine and diisopropylethylamine). Combination of seeded growthprinciples and photodeposition allowed for size control and bimetallicdeposition. Since the formed metal island serves as an electron sinkfor the photoexcited electrons, the reduction of the nearby metalions takes place on the preexisting metal domain. Li et al. reportedon Pd/Au alloyed metal-tipped CdSe/CdS NRs, as can be seen in 2 andadditional surfactants such as tetraoctylammonium bromide (TOAB) andDDA, an increase in the average diameter is observed due to Pd deposition.66Thefirst demonstrations of photodeposition on metal–chalcogenidesemiconductor NCs were presented for CdS–Pt and CdSe/CdS–PtNRs28 Formation of CdSe/CdS–Pd along with combineddeposition of Au and Pd as the core/shell and alloyed tipped HNPswas presented by Bar-Sadan and co-workers .-workers 4B.67 Au 74 and Au on CdSe/Cd0.5Zn0.5S core/shellNPLs.39 Recently, toward adopting greenersynthesis protocols, photodeposition in ionic liquids in which additionalsurfactants are redundant has been developed for Pt, Au, and Ag depositionon CdSe–CdS NRs,75 achieving comparableresults to the common amine-capped colloidal synthesis in organicmedium. Another recent approach combines flow reaction synthesis withphotodeposition to achieve controllable upscale of the HNPs syntheses.Continuous processes have been already utilized for different semiconductorand metal nanocrystals; nevertheless, Cohen et al. presented thiskind of implementation together with light-induced reaction to achieveCdSe/CdS–Au, ZnSe–Au, and novel ZnSe–Pt NRs.76Other semiconductor structures were shown to allow photoreductionof metal islands including Ni over CdS nanosheets2.2.377 In this procedure, as illustratedin 2 to create a CdO shell over the FePt nanoparticlessurface. A lower reaction temperature (258 °C) resulted in core–shellstructures, while higher temperatures (285–300 °C) promotedheterodimers formation.78 Further controlover this synthetic approach was achieved through a moderate temperatureramp and different reaction times as established by Parak and co-workers.Moreover, this procedure was expanded to form additional classes ofhybrids that comprise a FePt component and a II–VI or IV–VIsemiconductor domain including CdS, ZnS, PbS, and CdSe.79Among the first suggestions for semiconductor–metalHNP formation was partial chemical transformation of the seed material,typically the metal component. During this procedure, the metallicsurface is either oxidized or sulfidized to obtain a thin wettinglayer of metal–oxide or metal–sulfide for a consecutivegrowth of the desired semiconductor phase. This strategy was exploitedby Gu et al. to generate FePt–CdS heterodimer HNPs in a facileone-pot synthesis 5A.77 In 2S HNPs byintroducing sulfur precursors in oleylamine to AuCu-alloyed nanoparticlesunder constant air bubbling.80 Similarapproach of sulfidation in the presence of oleylamine at high temperaturewas demonstrated on PtCu-alloyed NPs, allowing the formation of Pt–CuSHNPs starting from either a Cu-seeded or a Pt-seeded NPs.81 The oxidative environment was necessary to activatethe reaction by forming an oxide surface or intermediate. A generalstrategy for synthesizing binary and ternary HNPs based on sulfidationof the metallic seed was reported by Shi et al., allowing the formationof Au–PbS and Au–PbSe with dumbbell and tripod architectures.82The components of the semiconductor–metalHNPs can alsobe predetermined by the choice of the seed composition. Schaak etal. demonstrated the formation of Au–Cu83 Sulfidation of theAg layer created a Ag2S amorphous matrix that allowed thegrowth of the semiconductor lattice independently of the metalliccore and thus circumventing the lattice mismatch between the two components,as illustrated in x1–Sex, and Au–CdS+PbS.An additional common pathway is the useof Ag deposition on metallicnanoparticles as a preliminary step toward semiconductor–metalHNPs generation. This approach was exploited to produce a crystallinesemiconductor shell over a metallic Au core based on the Lewis acid–basereaction mechanism.2– ions.In this manner, sulfur ions assist to manipulate the reduction kineticsof Ag+ ions, which results in the growth of Ag predominantlyon one side of the preformed metallic nanocubes.84 The metallic heterodimers were used as a platform for theformation of metal–CdS HNPs through successive steps of sulfidationand cation exchange as described above  shell as well as a higher reactiontemperature of the cation exchange reaction led to a larger phaseseparation between Au and CdX to reduce the interfacial and grainboundary energies.85 In addition, Zhaoand co-workers demonstrated the formation of Ag– and Ag2S– (or Ag2Se−) CdS, ZnS, MnS, andCdSe hybrid nanorods via controlled sulfidation (or selenidation)of Ag NPs.86This strategy was generalized for different noble metals , and improved control of Ag growth on metallic seeds was gainedby addition of an appropriate amount of Sed above 5C. ModifDifferent morphologies,architectures, sizes, and shapes are availabledepending on the surface coating, reaction temperature, metal type,precursors, and concentrations, as will be described in detail inthe next section.2.389 several synthesis mechanismshave been developed for the growth of HNPs. Applying one or a sequentialcombination of these strategies allows for good control over the structuralcharacteristics of the HNPs in terms of their morphology and composition,which ultimately leads to different chemical and physical properties.90 This section focuses on controllable synthesisand structural characterization of different semiconductor–metalhybrid nanosystems. This includes site-selective deposition, tunablesize and shape, and a variety of material compositions of both thesemiconductor and the metal segments.As wasdescribed in the former section and previously reviewed extensivelyin the literature,2.3.1Selectivedeposition of either components of the semiconductor–metalHNPs may be dictated by the morphology and crystal structure of thesemiconductor or metal phase. The crystal morphology and surface cappinginduce different chemical reactivity for different facets of the semiconductornanocrystal, which can lead to specific growth of the second-phasematerial on the more reactive facets of the nanocrystals of the first-phasematerial. The architecture of site deposition can be determined throughvarious synthetic approaches. Both thermal deposition and photochemicalreduction methods can be used, resulting in different deposition patternsdependent on surface coating, reaction temperature, metal type, precursors,and concentrations.23 In principle, the preferential growth on the tip sites, in thiscase, is originated from the enhanced reactivity of the (101) or (001)terminal facets of the semiconductor crystal leading to a lower freeenergy barrier for the heterogeneous nucleation on these sites.91 Furthermore, a transition from two tips to asingle tip was observed and ascribed to an electrochemical Ostwaldripening mechanism stabilizing the larger single tip preferably overtwo tips.23 In addition, a controlled site-selectivedeposition where the metal precursor concentration acts as a knobfor different site deposition resulted in the formation of single-tippedCdSe/CdS–Au nanorods along with dumbbell-like structures andbody decoration on the semiconductor surface with increased Au concentrations.92 This control was attributed to the hierarchyof the semiconductor facets reactivities where the sulfur-rich natureof one apex promotes initial deposition of a single Au domain dueto strong Au–S interactions. With the addition of metal precursors,deposition on the less reactive Cd-rich apex occurred. At saturatedconcentration conditions, spontaneous growth over defect sites onthe surface of the semiconductor structure was observed.A first demonstration of site-selectivedeposition dictated by the anisotropically shaped crystal structurewas reported by Mokari et al., presenting Au single-metal domain depositionon the apexes of CdSe and CdS nanorods and tetrapods.24 The depositionof Au occurred favorably on PbS facets with the highest reactivity.In the reported cubic crystal system, the growth rates on common crystalplanes as deduced formerly for various NCs can be ordered as [111]> [110] > [100].94 This suggests that Au would preferentiallygrow on the {111} facets. Because of the different polarities of the(111) and (111) facets, only four of the {111} facets were deposited and the PbS–Au4  nanostructure was formed. Athigh Au precursor concentration, gold would saturate all of the {111}facets and PbS–Au8 was formed. At an even higher Au precursor concentration, a gold-crownedcubic-shaped heterogeneous nanostructure was achieved. This evolutionarymetal deposition as the metal precursor concentration increases isillustrated in Similardependence on the polarity of crystal facets was observedfor PbS–Au HNPs.3 in the presence of smallnanocubes (<25 nm) in aqueous solution resulted in decorated SnS–AuHNPs with sub-5 nm sized metal domains. However, a similar reductionreaction in the presence of large SnS nanocubes (>50 nm) formedlargesingle-metal domain growth on the corner of the nanocubes and a heterodimerstructure  plane of SnS with (111) of Au.An additional example of the semiconductor NCstemplating the Ausite deposition is manifested by Au deposition on SnS nanocubes ofdifferent sizes. Reducing HAuCltructure 6B.95 Thu96 This tendency for tip growth is ascribed to an enhanced intraparticleelectrochemical Ostwald ripening process due to the presence of moresurface atoms in the cone-like structure in comparison to the cylindricalmorphology.Anotherexample of the morphology dependence was demonstrated forcone-like CdSe/CdS tetrapods, which exhibited increased selectivitytoward single Au-tipped deposition as a result of the tapered armstructure, in comparison to regular tetrapods with cylinder-like arms.3 reduction.97 This shell growth is attributed to the lowerligand density on the surface of the pyramidal nanocrystal in comparisonto rod structures. Hence, more surface is accessible to the metalprecursors to be deposited and forms a continuous shell. Further additionof a strong reducing agent or e-beam irradiation was reported to transformthe metal shell to multiple isolated metal domains on the nanocrystalsurface, as shown in Other shapes were found to dictate different hybridarchitectures.Core/shell structures of CdSe–Au dihexagonal pyramidal HNPswere formed following mild AuCl2S3 (ca. 2.81%) to form unique HNPs with Au nanoparticlespositioned at the center of Bi2S3 NRs  on semiconductor nanorod structureswith enhanced selectivity for metal tellurides over the lighter chalcogenides.99 CdSe/CdS NRs with CdTe tips showed specificmetal growth over the CdTe segment with suppressed body decorationover the nanorod surface. This behavior was attributed to a trap-mediatednucleation mechanism, in which metals are rapidly deposited at electron-deficientsurface traps located at the Te–Te bond at the nanocrystalsurface. The reduction potential at these trap states is sufficientto reduce the cations Pt2+, Pd2+, and Au3+.Other methods of controlling the site and the morphologyof themetal deposition are extensively reported in the literature. Pradhanand co-workers exploited the lattice mismatch of Au with Bii2S3 NRs 6D.98 The100 Single-tipped CdS–Au were achievedby lowering the reaction temperature down to 0 °C. At these conditions,phase transition of the alkyl chains of the surface amine ligands(DDA) is taking place and a static phase is formed, preventing thediffusion of Au ions to the semiconductor surfaces. Similar resultswere obtained by replacing the surface ligand with longer alkyl chainamines, octadecylamine, already at 25 °C, which go through phasetransition at ∼32 °C101  on the cornersof the nanoplatelets was observed in the presence of DDA and DDABin toluene. However, using oleylamine in 1-octadecene revealed a quasi-sphericalgrowth at the shorter edges of the NPLs with an Au domain size of∼5.0 nm. present10A. It i43 In allcases, independent of the CdSe core and CdS crown size, Pt domainsare found only at the edges of the NPLs , applying light-assisted metal deposition yielded twolarge metal sites (∼15 nm) per rod in average, yet, surfacedecoration by small metal domains was not entirely arrested.Light-induced reduction is anotherpathway for tuning and selectivelydepositing metals on semiconductor components. A complementary strategyto thermal annealing and Ostwald ripening (described above) in formingone-side Au-tipped CdS or CdSe/CdS NRs was presented by Sönnichsenand co-workers and othersuggested 11.65 Folion time 11e. To n63 While Pt growth on CdS NRs showed heterogeneousgrowth on their surface with up to 6 islands per rod, mostly a single-metaldomain was observed following photodeposition of Pt on CdSe/CdS NRs.Moreover, the location of the Pt island is near the CdSe seed. Thisspecific site deposition is ascribed to the localization of the formedexcitons under illumination. This trend was also observed for CdSe/CdS–AuNRs where a large Au domain is deposited at the CdSe seed region underirradiation conditions  accompanied by trapping of the electrons on surface defects inthat area.Further extension of this selective depositionmethodology wasdemonstrated in photodeposition of Pd nanoparticles on CdSadiation 12B.108 TThe charge transfer of an excited charge carrierto already depositedmetal domains upon light stimulation can be also exploited for selectivedeposition of additional different metal types on top of the formermetal decoration as will be described in detail in 2.3.277 This method was also demonstratedfor the growth of CdS, ZnS, PbS, and CdSe on the FePt nanoparticle’ssurface, achieving improved stability and control by increased surfactantconcentration in the reaction and gradually raising the reaction temperaturein a 5 °C/min rate.79 FePt–CdS/Sesponge-like nanostructures were obtained by first introducing chalcogen(sulfur or selenium) precursors, promoting the formation of nanowires(NWs) of chalcogens which serve to connect FePt nanoparticles.78 Successive addition of Cd(acac)2 allowedformation of the hybrid nanosponges.Another aspect of structural control of the HNPs synthesis is tuningand tailoring the morphology of HNP structures along with their component’ssizes. The size and shape of both the semiconductor and the metalcomponents can affect the physical and chemical properties of suchhybrid nanosystems. Early investigation of semiconductor–metalHNPs showed fair control of the hybrids morphology via temperature-dependencesyntheses. CdS–FePt have been synthesized in core/shell andheterodimer structures depending on the reaction temperature. WhileCd ions reduced on FePt-sulfidized nanoparticles at 100 °C formeda core/shell structure, raising the solution temperature to 280 °Cconverts CdS from its amorphous to a crystalline state, accompaniedby a dewetting process, and results in heterodimers of FePt–CdSHNPs.2X . The degree of Ag2X crystallinitydictated the extent of component separation following cation exchangeto form a family of gradually separated Janus HNPs including Au–CdS,Au–CdSe, and Au–CdTe 2 cation exchange at 2.5,4.5, 7.2, and 8.1 pH values. Janus nanospheres, heterodimers, symmetricdouble-headed nanoparticles, and multiheaded nanoparticles were formed,respectively where metal tip sizes reaching up to 40 nm in fast reactions times(<120 s) were achieved was achieved using laser ablation methods.achieved 15A.115 S aswell 15B.62 Si28An additional degree of control was achieved byconsecutive two-stepsynthesis combining spontaneous chemical reduction and light-inducedreduction deposition. Specifically, following spontaneous metal nucleationin dark conditions to form site-selective small metal islands on theapexes of the CdS NRs, light-induced metal deposition at low temperature(2–4 °C) allowed the metal domain size to be controlledby varying the Au ions/NRs molar ratio 15C.2870 Au metal domains close to 50 nm in size wereobtained by photochemically reducing Au ions on already phase-transferredNRs in ethylene glycol/H2O solvent with polyvinylpyrrolidone(PVP) as the reducing agent. The choice of PVP also allowed furthersurface functionalization of the gold domain since it is only looselycovered by PVP surfactants.Withincreased metal domain sizes, stability and aggregation phenomenahave been reported especially in organic media. A novel synthesisin an aqueous high dielectric environment was reported to achievehigh stability and size control of Au metal tips on CdSe/CdS NRs.119 This strategy resulted in cube-shaped Pt tipswith well-defined (100) facets.In addition, control of the metaldomain shape was achieved byselective deposition of faceted Pt metal tips on CdS nanorods. Bysuccessive metal deposition reactions, nucleation of nonfaceted Pttips on CdS nanorods was followed by facet-selective Pt depositionin the presence of CO molecules that favor the (100) facet growthover the (111) facet due to stronger binding to the former.120 gels,38 and aerogels.122 Recently, Bigall and co-workersshowed the influence of metal decoration architecture on the structuralproperties of HNPs gels and aerogels. While random Au metal decorationon the surface of CdS NRs led to a network which was mainly connectedbetween the NRs (rod to rod connections), Au-tipped CdS NRs resultin gold only in the connection points between the NRs.106 Similar structural effects were reported forCdS NPLs decorated with Pt NPs.121 HybridNPL network gelation reveals a homogeneous distribution of the metalin the cryoaerogel, in contrast to accumulation of Pt–NPs inclose vicinity in the case of mixing and cogelation of NPLs and PtNPs. Such differences in the metal distributions are expected to havea significant impact on the optical and chemical properties of HNP-basedaerogels.Control over the morphologyof HNPs can also dictate their assemblyin more complex formations such as colloidal networks,2.3.3124 Throughout the previous sectionssome examples of bimetallic–semiconductor nanoparticles werereviewed. Such nanosystems with different architectures includingFePt–Cd–chalcogenide core/shell and heterodimers79 along with AuCu–CdS star-like nanostructures125 or spherical CdSe nanoparticles decorated withan alloy of AuPd126 islands were synthesizedvia solvothermal reactions. Either presynthesis of binary/alloyedmetal nanoparticles was used as in the former examples or chemicalreduction in the presence of both metal precursors on the semiconductorsurface or apexes was used as demonstrated for CdS NRs tipped withPtNi and PtCo alloys.44 A different mechanismof exploiting the favorable heterogeneous growth on a pre-existingmetal domain serving as a substrate, over homogeneous nucleation offree-standing metal nanoparticles in solution, was reported by Pyunet al. Activation of small Pt islands at the apexes of such NRs allowedthe formation of a Co shell or a hollow cobalt oxide shell via a Kirkendalleffect in the presence of O2 under high temperature.55Asdescribed above, hybridization of the two different materials, semiconductorand metal, provided new synergistic properties. In addition, combinationof more than one type of semiconductor or metal component can expandthe chemical and physical characteristics of such HNPs. The influenceof such combinations was previously presented in bimetallic nanoparticlesystems, where changes in a variety of properties were observed includingelectric field enhancement effects, modification of surface plasmonproperties, magnetic functionality, and, most notably, enhanced catalyticactivity.66 This strategy was adopted and utilized for theformation of several bimetal tip deposition in different hybrid nanosystems.Au-tipped CdSe/CdS NRs were used as template for photochemical reductionof other noble metals such as Pt68 andPd.67 Controlling the order of metal precursorsaddition and depending on their reduction potential , different morphologies of thebinary metal tip could be achieved, from core/shell through core andalloyed shell and formation of a single-metal core with small secondarymetal islands deposition . In addition,the use of illumination for the photodeposition of the metals facilitatesthe reduction of Ag+ to Ag0. Other hybrid nanosystemssyntheses were also reported to employ photochemical deposition includingNiCo-decorated Zn0.5Cd0.5S nanoparticles128 and PtPd on CdS nanoparticles129 utilizing these structures for photocatalytic applications.This kind of synthetic control, similarlyto the other forms of structural control, can be achieved by a light-inducedroute as well. Selective deposition of Pd on Au-tipped CdSe/CdS NRswas conducted under UV illumination. Exploiting the formed excitonsto provide reductive charges at a specific site, the preexisting Auseed, due to charge transfer from the semiconductor segment to themetal domain, led to the reduction of Pd ions and formation of analloyed AuPd tip which revealed interesting magnetic properties.position 16A. Simiposition 4B.127 Th130 By depositing first the Pt by solvothermalgrowth on one side due to the favored reactive facet, it serves asa blocking material, preventing further growth at this site and promotingsecondary PbS growth on the opposite apex through thermal decompositionof Pb–bis(diethyldithiocarbamate). In a similar manner, asymmetricallytipped PdS–CdSe/CdS–Au NRs were obtained.131 Following Au deposition by common spontaneousmetal growth in the presence of DDA in toluene solution, cation exchangeat 180 °C was performed to achieve PdS on the other rod tip .135 Au loading was formed through reducing HAuCl4 by sodiumcitrate followed by photochemical deposition of Pt, which allowedcontrol of their size and density.Along with the deposition of binary metal domains,disparate metalsand metal chalcogenide types on different sites were also syntheticallyobtained. Mokari et al. exploited the nanorod architecture to synthesizedPbS–CdS–Pt HNPs where the metal and metal chalcogenidedomains are each deposited on the opposite apex of the rod structure16B.130 B rod tip 16C. A dionducted 16D. Sequ43 Au growth, followedby Pt deposition, formed a Pt–Au alloy or core/shell type morphology.Surprisingly, Au deposition on already decorated Pt–CdSe/CdSNPLs resulted in a large Au metal domain mostly on the CdSe core.This observation is in contrast to previous reports on other hybridnanosystems where the Pt metal domains acted as an electron sink,promoting secondary metal deposition over the existing one. In thecase of saturated Pt domains at the edges of the CdS crown, the Augrowth pattern is due to the high tendency of stable Au–Sebond formation at the CdSe–CdS interface and steric hindrancecausing unsaturated S atoms to be less available since it is alreadyoccupied by Pt. On the other hand, on less crowded Pt decoration,small Au island were observed on the edges as well, as the influenceof steric interference became less relevant.Another example of syntheticcontrol via sequential metal depositionwas demonstrated by binary metal deposition of Au and Pt on CdSe/CdScore/crown NPLs.Along with thevariety of deposited metal component combinations,heterostructures of the semiconductor components have also been realizedin the formation of HNPs. As was mentioned throughout the currentand previous sections, different combinations of metal chalcogenideshave been reported. These kinds of heterostructures reveal a differentband alignment and therefore different electronic profile such astype I, type II, and quasi type II. The final energy band alignmentof the HNPs with the specific metal deposition can influence and dictatethe performance of the HNPs in its designated application, where differentapplications require different energy band design. The effect of thematerial combination on the HNPs characteristics and on their utilizationin various applications will be discussed in the following sections.3HNPs hold unique properties whicharise from the combination oftwo disparate materials with distinct different physical and electroniccharacteristics. The hybrid properties are either combined attributesof each of the segments that present both elements properties at thesame single system or often a synergistic combination manifestingnew properties that are a consequence of the semiconductor–metalinterface. Notably, the resulting properties of the combined materialsdepend not only on their type but also on the size and morphologyof the HNPs and the materials interface.3.13.1.1136 This optical alternation is highly pronouncedin HNPs combining semiconductors where the excitonic band-gap transitionand the plasmonic features of the metal segment are spectrally separatedfrom each other.Conjugation of semiconductorand metal segments in a single nanocrystalline system may introducenew hybrid electronic states at the semiconductor–metal interface.Therefore, it is expected that the overall optical and electronicproperties of such hybrid systems will be affected in comparison totheir components. In HNPs, the coupling between excitons and plasmonsbecomes especially strong near resonance in which the exciton energylies in the vicinity of the plasmon peak which results in broadeningand a shift of the first exciton transition in the semiconductor spectra.137 and CdS–Au NRs  modes are well separated and stilldiscernible. Still, some changes are seen , respectively that lead to diminishingof the longitudinal plasmon band of the Au due to the gradually overlappingand ultimately dominating transverse mode of the shell.144Along with the excitonic alternation, the plasmonicresonance inHNPs is red shifted compared to free-standing metal nanoparticles.This shift is ascribed to the change in the dielectric constant ofthe nearby surroundings of the metal component. In general, the plasmafrequency of the bulk metal, ectively 17C.82,13ectively17D.142 F143 This observedshift, as explained above, is related to the larger real part of therefractive index of the semiconductor compared to that of the solventsurrounding the metal tip. Comparison with discrete dipole approximation(DDA) simulations on HNP  on Zn0.67Cd0.33S NPs has shown enhancedformation of electron–hole pairs that led to improved photocatalyticproperties in comparison to isolated Au metal domain decoration (3.5times higher).157 This was attributed tothe strong plasmon coupling of the chain structure inducing highlyintense and localized electromagnetic fields which in turn enhancesthe semiconductor segment absorption. As shown in Additionally, upon excitation of free electronsby incident irradiation,the metal component can induce an intense electric field in its vicinity.S–Au NRs 18B. A gr3.1.2A well-knownand highly applicable optical property of semiconductors is it fluorescencevia spontaneous emission. When conjugating plasmonic metal and semiconductornanomaterials into a single system, this radiative process can beeither enhanced or quenched depending on the hybrid structural characteristics.These effects are strongly dependent on the distance between the metaland the fluorescent semiconductor NC, on the nature of the semiconductor–metalinterface, and on the spectral overlap between the SPR and the emissionspectra.158 The enhancement effect can be attributed to either of thetwo following origins: first, excitation enhancement, which relatesto the increased excitations in the presence of an SPR electric field,also associated with the enhanced absorption discussed in the previoussection and to the surface-enhanced Raman scattering (SERS) phenomena;159 second, emission enhancement, which is affectedby the coupling of the SPR field and the transition dipole momentof the emitting semiconductor NC, leading to an increased radiativerate.160 The fluorescence enhancement isaccompanied by shortened radiative lifetimes due to the inverse relationof lifetime and the radiative rate, which is proportional to the fieldenhancement factor.161Fluorescence enhancement is typically associated withcontrolled separation between the two material components as was exploredin molecule–metal nanoparticle interfaces.168 Demonstration of the distance effect on emission enhancement insemiconductor–metal nanoparticles was reported for CdSe/ZnScore/shell nanoparticles deposited on Au colloids with a defined polyelectrolytespacer allowing controlled distance between the semiconductor nanoparticlesand the gold films.162 Maximum enhancementby a factor of 5 was achieved for a 9-layer spacer (∼11 nm)due to a local enhanced electromagnetic field around the metal nanostructures and 8–10 nm (ZnS) shell thicknesses, withthe yields dropping for thinner and thicker shells 19B.169 T161 as well as for ZnCdSeS–Au HNPs.170 The collective character of an ensemble of interacting Au NPs promotesa strong electromagnetic field and therefore more efficient fluorescenceenhancement compared to single or isolated Au domains.Additional parameters can influence the measure of fluorescenceenhancement as was widely reported previously in the literature. Thenumber of Au domains in the vicinity of emitting semiconductor nanostructureswas found to be a key factor in this optical enhancement as was reportedfor CdTe NWs coaxially surrounded by Au NPs171 Au NPs with diametersin the range of 60–160 nm were investigated. Au NPs largerthan 130 nm in diameter yielded fluorescence enhancement, up to 260%for 160 nm diameter Au NPs compared to the reference sample. Smallersized Au NPs (<130 nm) caused fluorescence quenching  due to the stronglocalization of the near field at the nanorod tips  and single site in the formationof a toothbrush-like structure.174 Althoughquenching processes likely occurred in all nanostructures, the fluorescenceenhancement of the mike-like architecture outcompetes these relaxationroutes due to the overlap energies of the Au SPR and the band edgeof CdSe exciton absorption which promote coupled field enhancementfrom the Au portion of the nanorods.Along with the contributionof the metal component shape and sizeto the fluorescence properties, the effect of the morphology and architectureof the complete hybrid nanosystem has been investigated. Ramamurthyand co-workers reported on the improved up to 9-fold luminescenceenhancement by Au body-decorated CdS NRs, while for the case of CdS–AuNPs, a 5-fold enhancement was observed.3.1.382 Similar results were obtained for CdSe–Au dimers175 and Au–CdS176 core/shell HNPs. Numerous reports on 1D hybrid nanorod structuresobserved similar quenching behavior, including CdSe–Au,177 CdSe–Pt,178 CdSe–Co,54 CdS–Au,177 CdS–Pt,179 CdSe/CdS–Au,33 CdSe/CdS–Ag,51 ZnSe/CdS–Pt,46 and CdTe–Au.180 An example of the CdSe nanodumbbell quenchingtrend with increased metal deposition is provided in As discussedabove, the photophysical properties of HNPs are dictated by the uniqueinteractions of the semiconductor–metal nanojunction. As wasdeduced for the case of fluorescence enhancement, the nanoscale separationbetween the two components of a hybrid nanosystem can also inducequenching effects as the distance between the semiconductor and themetal becomes smaller. In a similar manner to the enhancement effect,this quenching mechanism is attributed to the strong plasmon–excitoncoupling and interactions which increase the nonradiative routes.Moreover, direct growth of the metal component on the semiconductorreduces the distance and the barrier between them, typically causingfluorescence quenching. This reduction in fluorescence intensity isassigned to charge transfer from the excited semiconductor part tothe metal and thus increases the nonradiative rate accompanied bya shorter lifetime. Such effect was observed and reported previouslyin the literature for several different HNPs with different materialcombinations and a variety of morphologies. Prasad and co-workersdemonstrated this quenching effect via PbS and PbSe growth on Au NPswith formation of both a dimer and a core/shell.61 CdSe–X ,41 CdSe/CdS–Au,43 and CdSe/CdS–Pt43 NPLs and CdSe–Pt181 and CdS–Ni74 nanosheets with increasingAu NP size was observed.st decay 20C.183180 Other studieshave presented the influence of the number of either metal or semiconductorsites on the fluorescence efficiency. Mattoussi and co-workers studiedthe effect of increased Au NPs to CdSe/ZnS NPs ratio along with thesemiconductor size and the separation distance between the components.For all semiconductor sizes and separating distances, an increasingratio led to a higher quenching efficiency and faster decay lifetime.184 The authors attributed the quenching mechanismto the strong energy transfer from the emitting NPs to the metal domainsaffecting mostly the nonradiative channel with negligible influenceon the radiative channel. The effect of the inverse ratio of increasednumber of semiconductor NPs to a single plasmonic metal site was demonstratedby an Au nanorod coated with CdSe/ZnS NPs.166 In this work, a shortened lifetime and increased quenching efficiencywere obtained for decreasing semiconductor to metal ratios from 40%to 91% for from 62 to 10 CdSe/ZnS NPs for a single Au NR, respectively that will leadto fluorescence enhancement or deposing one material on the surfaceof the other, allowing charge transfer and therefore fluorescencequenching. The second parameter of spectral overlap can be tuned bythe choice of materials to be conjugated efficiently to promote near-fieldeffects that would enhance the absorption of the adjacent componentand permit effective energy transfer.Therefore, according to the aboveunderstandings regarding bothfluorescence enhancement and quenching as were deduced from the extensiveinvestigations of these optical properties of HNPs, several rulesof thumb can be established for designing either enhanced or quenchedemitting HNPs systems. While the size and shape of both the metaland the semiconductor components can contribute to the extent of desiredoptical property and enhance the overall fluorescence effect of bothenhancement and quenching, the more pronounce parameters are the distancebetween the two segments and the spectral overlap. The first can beset by sorting the suitable synthetic pathway to achieve either sufficientlyseparated HNPs by inserting a separating layer in the form of SiO3.2185 Since the derivative of the tunneling current to voltageis proportional to the local density of states (DOS), the method offersa unique view of the spatial dependence of the electronic levels.Therefore, by scanning along the nanorod structure, the changes inthe local DOS can be visualized. As can be seen in Thesemiconductor–metal interface also manifests unique electricalproperties. Besides the electronic characteristics of each of thecomponents on its own, synergetic features arise due to the conjugationof these two disparate materials. Understanding the synergistic effectsat the semiconductor–metal nanojunction may lead to their utilizationin various electrical applications including their use as electricalcontacts and building blocks for nanoelectronic devices. Early studyof the electronic synergistic effects was conducted on a single CdSe–Aunanodumbbell structure by cryogenic scanning tunneling microscopyand spectroscopy .186 This delocalized conduction band is attributedto the electronic coupling of the Au metallic contact with that ofthe PbS nanorod and the ability of charge-separation processes. Thispartial delocalization forms an n-type behavior that is manifestedby the shift in the (dI/dV)/(I/V) spectrum to the negative bias , allowingan average 6-decade increase in conductivity near zero applied bias creates an excited electron–holepair . Then, excited electrons relax and transfer tothe metal domain, whereas the counter charge carriers are restricted to the semiconductor region. The described separationprocess competes with backward recombination processes, includingelectron–hole recombination and the loss of electrons in themetal, by recombination with the holes in the semiconductor component.The resulting spatial charge separation is the fundamental principlebehind the photocatalytic characteristics of HNPs as will be extensivelydiscussed later. This unique synergistic property of HNPs was extensivelyinvestigated in various HNPs architectures and morphologies. Generally,the efficiency and the dynamics of this process are dependent on thetype, structure, and morphology of both components. Several reviewspreviously discussed and summarized the kinetics and dynamics of thedifferent relaxation routes of excited charge carriers in HNPs, focusingon the effects of different structural and environmental parametersand their implications for different applications.196 A charging effect of the metaldomain through charge transfer was demonstrated for CdSe–Aunanodumbbells in aqueous solutions accumulating up to tens of electronsper metal domain following irradiation.177 Moreover, an in situ single-particle measurement of Au depositionon CdSe/CdS NRs was able to trace the charging effect of the metaldomain with relaxed electrons from the semiconductor conduction bandup to Fermi level equilibration with the latter via plasmonic shiftsand increased optical intensities in their UV–vis spectra.65Early evidenceof charge separation was observed in the form of fluorescence quenchingof emitting semiconductor NPs following metal deposition which wasassigned to the relaxation of the excited electrons in the semiconductorthrough charge transfer to the metal domain. Hence, reduction of thefluorescence lifetime is obtained by forming an additional competitivenonradiative relaxation route . Therefore, ultrafasttransient absorption spectroscopy (TA) is commonly required. Indeed,in the past decade, excited charge carrier relaxation processes inHNPs have been extensively studied by combination of time-resolvedfluorescence and TA measurements  than with the single-phase CdS–PtHNPs (∼4 ps) ,179 their transfer from the semiconductorto the surrounding media or to a sacrificial hole scavenger is slowerthan the analogous electron charge transfer .204HNP intraparticle charge separation as well as interparticleholeremoval were shown to be affected by the structural, surface, andchemical characteristics of the HNPs including their surrounding environment.For example, CdSe-based HNPs such as CdSe–Au NRs revealed hotand band-gap electron-transfer processes during the first 115 and210 fs, respectively 23A.149 S (∼4 ps) 23B.201 N197 Moreover,the former architecture showed almost complete decay, implying aneffective electron transfer to the metal domains showing 10% of theTA signal remaining after 75 ps, while significant TA signal (>20%)was measured along the entire time scale in the single-metal-tippedHNPs  comparedto Au–CdSe–Au and Au–CdSe–Pt compositions .pedHNPs 24A.Thest system 24B.147 Ictively) 24C.134 H28 and CdS–Pt116 with different metalsize domains between 1.5 and 6.2 nm and 0.7 and 3 nm, respectively,showed monotonic increased electron charge-transfer rates with increasedmetal tip sizes (r3), along with weaker contribution through the dependenceof the Fermi energy and work function on the metal domain radius (r).Another key parameter that allows one to controlthe charge-separationefficiency and dynamics is the size of both HNP components, the metallicsegment and the semiconductor light absorber. TA measurements of matchstick-likehybrid structures such as CdS–Auip sizes B. This b205 The charge-transferrates of thermalized electrons from the conduction band to the freeenergy level at the metal domain were measured as 300 and 550 fs forlarge and small Au NPs, respectively, consistent with the previousreports , the transferrates decrease to 1.4 × 108 s–1 (averagelifetime of 4500 ps) , charge transfer to Pt sites has an average time scale of ∼30ps, giving a transfer rate of 2.9 × 104500 ps) 26A.206 Ts gained 26A-e. A s gained 26B.207 Tlanation 26B-c.32 First, thefastest charge-transfer dynamics is seen in the case of the polyethylenimine(PEI)-coated HNPs, slower with l-glutathione (GSH), and slowestwith mercaptoundecanoic acid (MUA)-passivated HNPs with measured half-livesof 100 ps for PEI and 160 and 330 ps for GSH and MUA, respectively.A related second difference is that the decay amplitude over the measurementtime scale in these experiments is also largest in the PEI-coatedsystem. The trend of decay dynamics is attributed to an improved surfacepassivation leading to lower electron–hole trapping which promotedenhanced electron transfer. Moreover, this electron–hole trappingdue to Coulombic interactions in the vicinity of multiple Pt sitesdeposited on CdS NRs promoted faster charge-transfer rates (0.4 ns)in comparison to the same HNPs in the presence of hole scavengers(8 ns).198 Extracting the holes eliminatesthe Coulomb interactions, which leads to the loss of the electronwave function localization near the hole trap sites and thereforethe transfer is delayed. Conversely, fast removal of holes populationsin CdS–Ni HNPs by introducing high alkaline environment conditionsresulted in faster electron transfer to the metal sites due to reducedCoulomb interactions which allowed increased overlap of the electronwave function with increased growth of the metal sites . Onlya small fraction of the excitons (∼13%) can undergo chargeseparation  compared to0D and 1D HNPs  are expected to promotefaster charge transfer and a longer charge-separation distance dueto more uniform quantum confinement and a large surface area, respectively.paration 28A.181 T ∼165ps 28B.74 ThA signal 28C.613.3.2210 and the recently discovered direct plasmon-induced charge transfer(PICT).212 In PHET,hot carriers are initially generated in the metal by plasmon decayand then undergo interfacial transfer to the acceptor component. PICToccurs when there is strong interdomain coupling and mixing of themetal and acceptor energy levels. In PICT, plasmon excitation is directlyaccompanied by a rapid charge-separation process that creates an electronin the acceptor region and a hole in the metal.Indeed, the majority of the charge-separation studies in HNPs havebeen focused on the photoinduced charge transfer from the semiconductorto the metal domain. However, combination of the unique propertiesof an excited plasmonic material, such as enhanced local fields nearthe metal nanostructures, broad spectral tunability, large absorptioncross sections, and superior long-term stability, with excitonic sensitizationthrough the semiconductor has interesting potential for SPR-drivenhot-electron photochemistry applications. This promising route motivatedthe investigations of plasmon-induced hot charge carrier transferacross the interface of the heterostructured system. Within this process,hot carriers generated from plasmon decay can transfer into a nearbysemiconductor through two mechanisms as illustrated in 148 By applyingultrafast spectroscopy with high temporal resolution on similar CdSe–AuNRs, an upper limit of 30 fs for electron transfer could be determined,and further fast back transfer of electrons to the metal domain within200 fs time scale was observed  needs to be extremelyrapid to compete with the fast back-transfer to the metal. In addition,complementary measurements of the charge transfer to the metal followingvisible-light excitation allowed one to present the complete relaxationpathways of excited charge carriers in this nanosystem, signifyingthat the PICT mechanism mostly generates band-gap electrons in CdSesince no hot-electron back-transfer to the gold at the relevant timescale was observed. Nevertheless, in the case of CdS–Au NRs,the Au plasmon band is weakly perturbed and the plasmon-induced hot-electrontransfer occurs through the conventional PHET mechanism. Hence, thehot-electron-transfer rate is noticeably slower (97 fs) due to therapid plasmon decay and the efficiency is reduced (∼2.75%)owing to the competition of hot-electron transfer with ultrafast relaxation.29 However, Liu et al. presented an improved quantumefficiency of hot-electron transfer in CdS–Au NRs via controlover the metal domain sizes. The quantum efficiency of this processincreases from ∼1% to ∼18% as the particle size decreasesfrom 5.5 to 1.6 nm  can outcompete theAuger recombination rate (180 ps), while the relaxation process ofsemiconductor NRs and small Au-tipped NRs is dominate by Auger recombination through opticalor electrical stimulation holds potential in utilizing HNPs in variousapplications that require multielectron reactions such as water splittingand CObination30A.218219 Hence,under theMX state, the efficiency of multiple electron transfer to the metalsite was set to ∼41% compared to 100% of the MX dissociationefficiency calculated for the biextonic state. Note that the differencesbetween these two nanosystems can also originate from the differentstabilizing ligands and solvent environment. Additionally, heterostructuredsemiconductor–metal HNPs such as CdSe/CdS–Pt NRs revealfluence-dependent band-edge exciton dissociation. In this case, thefirst electron-transfer rate was measured with a time constant of192 ps, while the second electron transfer is slowed by the trionCoulombic interaction down to 1700 ps.220 The Coulomb barrier that arises following trion formation was estimatedto increase by ∼60 meV, causing a slower transfer rate, andthe transfer efficiency decreased by 1 order of magnitude.This competitive process can be influenced alsoby the materialcombination of HNPs. CdS–Pt NRs were reported to present slowAuger recombination rates due to trapped holes and spatially separatedexcitons varying between 2 ns and <1 ps for biexcitons and multiexcitons(>20 excitons), respectively 30B.219 H3.4In additionto the former described properties that involve intraparticle characteristics,the photocatalytic ability of such semiconductor–metal HNPsstems from interparticle processes. The advantages of enhanced opticalproperties along with the synergistic light-induced charge separationform the basis for this unique feature. The efficient charge separationacross the semiconductor–metal interface allowed the utilizationof these charge carriers, electrons and holes, at the surface of eachof the hybrid components for additional surface chemistry reactions.Typically, the majority of such chemistry are redox reactions. 177 In additionto in situ degradation measurements, charge retention of electronson the metal tips was observed by using preirradiated experiments.Similar reports of photoinduced dye degradation by numerous differentHNPs were reported using various dye molecules including MB, methylorange (MO), rhodamine B (RhB), and rhodamine 6G (R6G). Cd–chalcogenide-basedHNPs such as CdSe–Au NPs,175 CdS–AuNRs,222 CdSe/CdS–Pt NRs,223 Au–CdS yolk/shell,224 CdS–Aunanorings,225 CdS–Au,226 and CdSe/CdS–Au42 NPLs were all used to explore the mechanism of the photocatalyticreaction and the HNPs structural and chemical effects on this specificproperty. A different mechanism was suggested by some of these reportsdescribing a radical-mediated photocatalysis of dye molecules. Wherefollowing light absorption and charge carriers excitation, superoxideand hydroxyl radicals are generated by electron reduction of molecularoxygen and hole oxidation of H2O molecules, respectively.Consequently, these radicals can further degrade the dye moleculesin their surroundings.One of the most common methodologies to investigateand demonstratethe photocatalytic characteristics of HNPs is via organic dye photoreduction/oxidation.This photocatalytic process, which also bears relevance for variousphotocatalytic purification applications, can be mediated by eitherexcited electrons or holes at both the semiconductor and the metalcomponents. In each of the pathways, degradation of the organic dyesis occurring directly by charge carriers at the hybrid surface orthrough the formation of radicals (mainly in aerobic environments).First demonstration of the photocatalytic performance of HNPs wasreported for CdSe–Au nanodumbbells showing efficient methyleneblue (MB) reduction (up to 64%) following visible-light excitationvia electron reduction of the dye molecules 31A.177 I227 The yolk/shell architecture of Au–CdS was shown to be moreefficient in photocatalytic R6G degradation in comparison to core/shellHNPs where R6G was almost completely degraded in the former form whilethe later achieved 68% bleaching.224 Theauthors attributed this trend to several factors, among them the increasein hole trapping sites that can promote more hydroxyl radicals alongwith the larger surface area of the yolk structure. Other types ofHNPs, such as Au–Bi2S3,98 Au–SnS,95 Au–CZTS,53 Pt–CZTS,53 Pd–CZTS,53 and Cux2–S–CdS shells on Au NRs228 also revealed an improved photocatalytic ability in comparisonto their semiconductor NP component form, exemplified via organicdye degradation.As was described for the other mentioned properties,the photocatalyticproperty is also influenced by the structural effect of the HNPs.In this context of dye degradation, it was reported that the increasedshell thickness of Au–CdS core/shell NPs enhances its photocatalyticactivity due to increased absorption by the larger semiconductor componentthat provides generation of more charge carriers 31B.227 T2O2, the reaction turnover events can be monitored and localized itsposition by applying super-resolution mapping.221 As seen in 222 Similar trends were reported, showing more efficient catalyticperformance under excitonic excitation in comparison to plasmon-inducedphotocatalysis.Moreover, this photocatalytic dye transformationhas been appliedto identify the spatial distribution of the catalytic activity onHNPs. Ha et al. has reported two distinct, incident energy-dependentcharge-separation mechanisms that result in opposite energy flowsand polarities on a single CdS–Au NR. By using Amplex red dye(nonfluorescent) that turns into resorufin (fluorescent) followinglight irradiation of HNPs in the presence of H42 reduction. A complementary photocatalyticapplication route under aerobic conditions generating radicals andreactive oxygen species (ROS) was also realized in the fields of biomedicinealong with environmental and industrial science. The advancement insynthesis control along with the ability to tune the optical and chemicalproperties of HNPs allows a judicious design of a predetermined HNPsthat address the specific physical and chemical requirements of agiven photocatalytic reaction.Utilization of the above-described properties of HNPs is optimallymanifested via photocatalytic applications, perhaps the most notableof which are alternative clean solar-to-fuel conversion in the formof hydrogen generation via photocatalytic water splitting and alsotoward photocatalytic CO4.1232 This reaction pathway opens the way for direct conversion of sustainablesolar energy to chemical energy stored in chemical bonds of hydrogenas an alternative fuel. Utilization of hydrogen in fuel cells to generateelectricity produces back water in a zero emissions cycle.One of the most challenging photocatalytic reactions in the fieldof alternative energy harvesting is the water splitting reaction forhydrogen generation.+/H2 (0 V versus NHE) while the edge ofthe valence band that accumulate the holes should be more positivethan the redox potential of O2/H2O (1.23 V).231 The actual band positions that are needed arein fact larger due to the condition to overcome the overpotential.Honda and Fujishima were the first to show water splitting by a photoelectrochemicalreaction in the early 1970s by using a TiO2 electrode underUV irradiation and a Pt counter electrode with applied bias to forma closed circuit.233 Both wide-gap metal–oxidesemiconductors and narrow-gap metal sulfide semiconductors can inprinciple address the minimal energetic requirements. However, thedifference in the band-gap width determines the absorbance region.Hence, in the pursuit for increasing the solar spectral coverage,the use of narrower gap semiconductors which can absorb in the visibleregion accounting for about one-half of the solar spectrum is morefavorable over the wide-gap common oxide semiconductor absorbing theUV range comprising less than 5% of the solar spectrum.234Toperform complete water splitting, the equilibrated Fermi level ofthe electrons must be at least more negative than the redox potentialof H235 Their advantage over semiconductor NCs related tothe ability to tune their photophysical and chemical properties throughmaterial combination and dimensions has raised great interest towardtheir utilization in such application. Yet, only a handful of reportshave been able to demonstrate full water splitting HNPs as a singlesystem photocatalyst which can effectively promote both reductionand oxidation of water. Therefore, the use of sacrificial agents suchas sulfide/sulfite pair, alcohols, amines, and other electron-donatingmolecules is commonly imposed where the counter charge carrier cannotbe exploited to a complementary redox reaction. HNPs are considered promising photocatalystsfor alternative energyharvesting in this form of hydrogen generation.The significant dependence of the charge carrier’sdynamicson both the HNPs structural properties and the chemical-medium conditions,discussed in 4.1.1The abilityto control the size and morphology of both HNP components along withtheir material combination and their architectures was addressed inprevious sections. This allows a systematic and in-depth investigationof the influence of these structural parameters on the overall photocatalyticefficiency.4.1.1.12 generation quantum efficiency of17.3%, much higher than the 11.4% for their 4.6 nm diameter counterpartand reduced back-recombination from the metal.From the semiconductor component point of view, their size wasshown to affect the hydrogen generation activity in different manners.Optical properties along with charge-separation and e–h recombinationprocesses should be considered in this respect. For smaller nanoparticles,a larger driving force for electron transfer is observed due to thequantum confinement-induced increase in the energetic offset increasingbetween the electronic states of the semiconductor with respect tothe metal, as shown for Pt-decorated CdS HNPs in various sizes 26A in whnterpart32B.206 A207 A similarnonmonotonic photocatalytic hydrogen generation activity was reportedfor Au–CdS core/shell HNPs with optimal 8.2 nm shell thicknesscorrelating with its charge-transfer dynamics as was discussed in 142 Additional size effects of the semiconductorcomponent were demonstrated on CdSe/CdS–Pt core/crown NPLswith different core and crown sizes. Kinetic measurements of hydrogengeneration showed higher photocatalytic water reduction rates forhybrid NPLs with both a large core and crown followed by a large corewith small crown and small core with large crown with calculated apparentquantum efficiencies of 19.3%, 17.8%, and 13.2%, respectively .The advantage of the 2D structure over the 1D morphology is attributedto the elongated lifetime of the charge-separated state in the 2Dplatelet. Applying the hole hopping model to simulate charge recombinationindicates that many more random walk steps are required in the NPLcase before the hole finds the recombination (Pt) site.61Comparing the influence of morphologyon the photocatalytic performanceof different semiconductor components indicates that NPLs are favoredover NRs and spherical HNPs structures. The vicinity of both hybridcomponents in 0D structure in comparison to the other 1D and 2D morphologiesleads to inferior charge separation and faster charge recombination.240 and ZnSe/CdS–Pt201 NRs, have been reported to exhibit enhancedphotocatalytic hydrogen generation in comparison to HNPs with single-phasesemiconductor components due to improved charge separation. A differentstrategy suggested by Li and co-workers includes the alloying of differentelements in the semiconductor component, presented by Cu1.94S–ZnxCdx1–S–Pt HNPs. In this manner, two light-absorbing regionsare synthetically achieved with a tunable energy band gap as a functionof Zn mole fraction that can modify the hybrid band alignment to attaina material combination that manifests more efficient charge transferand photocatalytic activity  of the surface ligand (MUA), and thus, holes were notscavenged effectively in this case, unlike the behavior for the structurewith a ZnSe core.The electronic profile can also determinethe overall energy bandalignment and therefore the actual overpotential for proton reduction.This was manifested by core–shell CdSe/CdS nickel-decoratedHNPs in which the hydrogen generation rate increased with decreasedcore size and increased with the increasing shell thickness.243 A different approach suggested improved chargetransfer to the metal sites by passivating surface traps through postsynthesisaddition of a CdS layer over CdSe–Pt nanodumbbells resultingin an increase of 6.5 times in the photocatalytic hydrogen generationrate compared to bare CdSe–Pt HNPs.244An additional approach for accelerating holescavenging was suggestedvia surface etching of the rod shell in CdSe/CdS–Pt NRs, therebyincreasing the hydrogen production rate by 3–4-fold 32F.243 A30 ZnSe–Pt,76 and CZTS–Au53 NRs were synthesized and shown to achieve photocatalytic hydrogenevolution activity that exceeded their pristine semiconductor complementaryNPs.Importantly, as part of the quest for an alternative nontoxicandearth-abundant material to replace the heavy-metal semiconductor ingeneral and Cd-based HNPs in particular, HNPs such as ZnSe–Au,4.1.1.2The metal component, often named the cocatalyst in the context ofthe photocatalytic activity of HNPs, also plays an important rolein the overall photocatalytic water reduction efficiency. As describedin 245 CZTS–Pt HNPs,25 and CdS NRs decorated with multiple Pt clusters114 (8–68 atoms) showed optimal hydrogen evolution atan intermediate metal size or metal loading  which exhibited an intermediate quantum yield, while thethiolated alkyl ligand showed inferior activity, as presented in 254 However, for HNP-based photocatalysts wherethe photocatalytic reactions take place at the semiconductor segmentvia defect surface states, an inverse trend is expected, where bettersurface passivation caused inferior photocatalytic activity, as wasreported for spherical CdS/CdS core/shell NPs.255The effect of the surface capping ligand on the efficiency of hydrogengeneration was investigated in various hybrid and semiconductor nanosystems.256 Post-treatment HNPs revealednegligible H2 production compared to similar PEI-coatedHNPs. Yet, oxidation processes on the semiconductor surface towardradicals (OH• and O2–•)) were significantly enhanced,and SO32– showed increased hydrogen generationquantum efficiency in the presence of a hole scavenger within thisorder.254 A more negative oxidation potentialleads to a larger driving force for hole removal by sacrificial electron–excitedhole recombination. Moreover, a low hole reduction rate was correlatedwith a loss of stability, attributed to photooxidation of the semiconductorrod itself that also hinders the photocatalytic performance. However,these thermodynamic considerations also should include the natureof the HNPs, structure, and material composition as was demonstratedby testing the photocatalytic water reduction activity of CdS–Ptand CdSe/CdS–Pt NRs with two different hole scavengers, methanoland sulfite. As shown in the former described report, the use of sulfiteindeed allowed relatively higher efficiencies for both HNP systemsin comparison to methanol. Still, comparing these different HNPs inthe presence of each hole scavenger reveals an inverse photocatalyticbehavior where when using methanol a higher quantum efficiency wasobserved for CdSe/CdS–Pt, while a higher quantum efficiencywith sulfite was achieved in CdS–Pt  for reducing strongly bounded holesat the CdS surface compared to holes at the CdSe–CdS core–shellinterface.The effect of the type of thehole scavenger on photoinduced Hn CdS–Pt 36B.240 T2 generation rates by Ni-decoratedCdS NRs at pH 13–14 in comparison with pH 11–12. Ata pH of 14, a steep increase in the reaction rate appeared and animprovement in the photocatalytic function reached a 53% externaland 71% internal quantum yield (71 The underlyingmechanism of this strategy is the formation and utilization of hydroxylions (OH–) as an efficient redox shuttle with highermobility and accessibility to the HNPs surface compared to other commonhole scavenger agents, allowing faster hole reduction and formationof hydroxyl radical coupled with a high-rate catalytic oxidation reactionby the formed radicals of different electron-donating molecules inthe solution such as methanol or TEA. Yet, as illustrated in •/OH–), which follows Nernstian dependence (59 mV per pHunit), crosses and exceeds the energy level of the semiconductor VB,which has a weaker pH dependence (33 mV per pH unit), therefore promotingthe reduction of holes via fast and efficient hydroxyl shuttle agentswhich now dominate the photocatalytic process. Such pH dependencewas observed for CdS–Pt NRs as well, showing a steady increaseof H2 evolution rates upon increasing the pH values frompH 12 (3.6 mmol h–1 g–1) to pH13 (21.8 mmol h–1 g–1).257 Similarly, CdS–Pt NPLs61 and CdSe/CdS–Pt239 NRswere also used to demonstrate the advantage of high alkaline conditions. Simon et al. reported asignificant6-fold increase in Hum yield 36C-a.71 nditions36D. Incr4.1.42 production is clearly manifested throughout theadvanced synthesis control and in-depth investigation of their photocatalyticproperties that derived from their material combination and structureas was discussed in the former sections. Yet, to complete a full watersplitting catalytic cycle that includes both H2 and O2 formation by water reduction and oxidation, respectively,a hybrid system that can exploit in a single excitonic cycle bothcharge carriers, electrons and holes, to promote these two catalyticpathways is desired. Generally, semiconductor–metal HNPs untilnow did not demonstrate efficient activity toward the second half-cellreaction of water oxidation. This is due to the inherent complexityof this multiple-charge carrier reaction that requires four hole chargesto generate molecular oxygen from water along with its slow kineticsand high redox potential. As was addressed above, a long-lived charge-separatedstate ought to be to obtained to enable the four-hole oxidation processrequired to form the O–O bond of molecular oxygen and to effectivelycompete with intraparticle fast relaxation routes that lead to electron–holerecombination or surface oxidation.Thepromising utilization of HNPs as photocatalysts for water reductionand H133 However, no actual photocatalytic measurementsof water splitting were exemplified. Recently, Stolarczyk and colleaguesdemonstrated a full water splitting photocatalytic cycle. The useof well-known CdS–Pt NRs that promote efficient charge separationwas proven to effectively reduce water to generate hydrogen alongwith surface modification by a ruthenium-based complex that was shownto harvest excited holes to catalyze oxygen formation; both waterreduction and oxidation were achieved and measured simultaneously (bpy)Cl2 withdithiocarbamates anchors that attach to the HNP surface allowed afast hole transfer rate within ∼300 fs. This time scale cansuccessfully compete with the above-mentioned relaxation routes thattypically hold a lifetime ranging from 0.7 to 1.0 ps.259 Yet, the efficiency of the oxidation reactionachieved by this hybrid system was up to 0.27% and a molar ratio ofH2 to O2 (∼20:1) that was much lowerthan the stoichiometric value of 2 leave plenty of room for furthersynthetic developments and fundamental investigations of these HNP-basedphotocatalysts for addressing the highly challenging full water splittingreaction.A synthetic effort was madeby Alivisatos and co-workers presenting of Ru–CdSe/CdS–PtHNPs in which the Ru and Pt metal sites served as cocatalysts forthe water oxidation and reduction reactions, respectively.ctively) 37.258 Th4.1.52 and H2 in the same place, raising concerns aboutthe possibility for their reaction and requiring separation processes,altogether challenging the applicability of this approach. Photoelectrochemical(PEC) water splitting has emerged as a path to address these problems.Here, H2 and O2 are formed on a separated photocathodeand photoanode, respectively. As two different semiconductors maybe used for water oxidation and reduction, the band gap of each canbe smaller to maximize light absorption. The required overpotentialfor the chemical reaction is supplied by the electrical propertiesof the semiconductors such as the majority carriers; p-type semiconductorswith high overpotential for water reduction are investigated as photocathodematerials, while n-type semiconductors are used as photoanode materialsdue to the high overpotential for water oxidation. The overpotentialto drive the catalytic reaction is further increased by applying externalelectrical bias, which is yet an additional attribute of the PEC approach.Thus far, HNP PEC cells have been demonstrated as photocathodes photoanodesand tandem cells, but the work in this area utilizing the precontrolledcolloidal HNPs is still quite limited compared to the studies of theirwater splitting performance in suspensions.As discussed in the last section, full water splitting is rare incolloidal HNPs and typically requires a wide band gap due to the highover potential needed to drive the catalytic reactions. On the otherhand, to maximize the absorption of the solar spectrum, small band-gapmaterials are favored. In addition, full water splitting in a colloidalsolution on a single nanostructure leads to the formation of Ox2–Te in comparison to Cux2–Te with photocurrent densities being ∼10 timeshigher.260 Au–CuInS showed enhancementof water splitting of four times in comparison to only CuInS nanostructures.261 Photocurrent response in Au–CuGaS2 over the pure material was also reported to be enhanced.262 This can be attributed to a combination ofthe contributions of the gold catalytic ability and possibly to theplasmonic enhancement of light absorption.Photocathodes madefrom gold-tipped HNPs have been reported to show superior photoelectrocatalyticperformance for the water reduction reaction; for example, Au–Cu2 obtainedfor Ag–CdS NWs is 1.8 times higher than that for the CdS NWphotoanode. As an example of overall water splitting, a tandem cellconsisting of an undoped Au–CdS photoanode and a Cu-doped Au–CdSphotocathode, respectively, modified with cocatalysts was fabricatedand displayed stable H2 and O2 evolution asdemonstrated in 263Photoanodes withHNPs were also demonstrated. Ag–CdS NWshowed a photocurrent density increase by about 4.7 times comparedto that of the pure CdS NW photoanode, whereas the H4.22, while another clean energy source is the reductionof CO2, such that this greenhouse gas is sequestered whileno new carbon is emitted to the atmosphere. The energy barrier ofthe first step in the reduction of CO2 is very high, andtherefore, even two-electron reduction to CO is challenging. If CH4 and CH3OH are the main target products, the generationof CH4 and CH3OH requires the transfer of eightand six electrons, respectively. Due to the high energy barrier andthe multielectron reduction process, this reaction requires tightcontrol on the chemistry of the catalytic metal to favor this reactionover other competitive reactions such as water reduction. A commonstrategy to address this requirement is by coupling the semiconductorto a metal complex to enhance reaction selectivity toward CO2. For example, CdS QDs with Ni(cyclam),264 Ni doping,265 and Co2L complex266 demonstrated selective photocatalytic CO2 to CO conversion. Fe and Co complexes with CuInS2 were also used to selectively reduce CO2 to CO in water.269 The metal atoms can also change the selectivity of the CO2 reduction products. Under full solar spectrum irradiation, CO asthe main product with 97.2% selectivity was observed on bare CdS,whereas the intentional introduction of an optimized amount of Rumetal enables CO2 reduction to CH4 with 97.6%selectivity.270Water splitting is one way to form carbon-free fuel in the formof H2 reduction by metal tip deposition is sofar less common for chalcogenide-based HNPs. For tip-decorated Cu2S–Pt, the CO formation rate was 3.02 μmol h–1 g–1, 2 orders of magnitude higherthan that for random decoration of Pt. Methane evolution was alsoobserved for the tip-decorated sample, albeit with a much smallerformation rate, 0.13 μmol h–1 g–1 , which is >5 times the activity of only Cu/TiO2.274 Another example for tunablephotocatalytic CO2 reduction activity by CdS was shownfor Au/MoS2-tipped CdS NWs. This study reported syngasgeneration with a H2/CO ratio ranging from 0.35 to 3.6under visible-light irradiation by simply altering the Au particlesize.275 In this work, CO2 reductionto methanol over noble metal-free hybrid semiconductor photocatalyst(Ni2P/CdS) under visible light was also reported.276Visible-light photocatalysis using Cd–chalcogenidewas alsodemonstrated by combining CdSe with Pt/TiO4.3The abilityto utilize the exciton in HNP for photocatalytic reactions can befurther extended to various additional applications. In this section,we show several examples of how these excited charge carriers canbe exploited for industrially relevant processes. This includes theuse of HNPs for photoinduced catalysis of selective organic reactions,and we also discuss their use as photoinitiators for polymerization.Last, we describe some examples of sensors, including chemical detectorsand photodetectors made by HNPs.4.3.12O, CO2, and O2, the metal is crucial for the selectivitytoward specific reactions. The use of HNPs for selective photocatalysisis also exemplified by using different metal tips to direct the conversionof molecules toward the desired chemical product utilizing a photocatalyticreaction route. Photochemical dehydrogenation and hydrogenolysis ofbenzyl alcohol was studied on CdS–Pt and CdS–Pd nanorods.CdS–Pt favors dehydrogenation (H2) over hydrogenolysis(toluene) with a ratio of 8:1, whereas CdS0.4Se0.6–Pd favors hydrogenolysis over dehydrogenation by 3:1.277As shownin the above sections for the reduction of H7S4–Pd nanostructure demonstrated photocatalyticactivity for Suzuki coupling reactions of iodobenzene with differentreagents, selective oxidation of benzyl alcohol, and hydrogenationof nitrobenzene under 808, 980, and 1500 nm irradiation.278 Selective reduction of nitroaromatic compoundswas also enhanced by Au on CdS NWs.279 Theimportance of the HNPs for selective hydrogenation reactions was alsodemonstrated in the selectivity of catalytic reaction products. Catalysisof the hydrogenation of cinnamaldehyde shows that while 3-phenyl-1-propanolwas the only product over Pt–Cu2S HNPs, CuPt–Cu2S HNPs exhibited a higher conversion rate and selectivitytoward hydrocinnamaldehyde which was also enhanced compared to CuPtonly.280The use of HNPs to catalyze organic chemicalreactions was extended also to the near-infrared region, where theCu4.3.25 The mechanism is either by direct reduction/oxidation ofthe monomer itself or by some mediator such as amine or hydroxyl radicals.283 The enhanced charge separation in HNPs and the formation of ROScan also be harnessed for this application. The effect of metal tipin the photopolymerization of polyacrylamide was studied in CdS–AuHNPs. The kinetics of the polymerization degree was followed by theFTIR signature of the acrylamide monomer double bond. CdS–Audemonstrated three times faster polymerization compared to CdS NRswithout a metal tip  and in the absence of water. The suggested initiationmechanism in this case was by surface-mediated hole transfer fromthe semiconductor to the monomers, either directly to the double bondof the monomers or via the surface ligands, polyethylenimine is another imaging technique studied in the context of HNP activity.High image contrast at the tumor sites could be observed after Au–Cux2–S or AuPt–CuS HNP injection,significantly larger than that of iopromide or iohexol  imaging  imaging was demonstrated on hybrid Au–Cuhell NPs 43A.294 I iohexol 43B.291,2 imaging 43C.291,22. Bimodal labels with colorimetric andfluorescent readout were fabricated via a layered sequential assemblystrategy. This resulted in amplified signals from the assemblies ofindividual single nanoparticles and allowed colorimetric and fluorescentdetection of cystatin C (Cys C) after surface conjugation with antibodies.301In the above examples of imaging techniques, thereis no band-gapemission of the semiconductor in the HNPs and their contribution isthe plasmon absorption or structural characteristic. This is due tothe quenching of the emission when the metal is in direct contactwith the semiconductor. Growth of an insulating layer between thesemiconductor and the metal can prevent this quenching. In this way,the presence of the metal plasmon can contribute to the enhancementof the semiconductor emission. Such structures were demonstrated aslabels for immunoassay by the structures of Au and CdSe/CdS/ZnS NPsembedded in SiO5303 Moreover, addressing the stability of HNPs in diverse demandingphotocatalytic scenarios is also important. In this context, expandingthe functionality while controlling the HNP surface coating whilealso utilizing inorganic ligands may be further addressed.This reviewsurveyed the remarkable accumulated research on semiconductor–metalHNPs, from their synthesis to properties to emergent photocatalyticapplications. Yet, there is a prominent future for such HNPs thathinges on continued further research and innovation to meet the requirementsfor efficient photocatalytic reactions. Among the challenges, it isimportant to consider the introduction of novel synthetic approachestoward the formation of more complex HNP architectures. There is alsoa need to expand the HNP family to green and environmentally friendlysemiconductor–metal systems. For example, the earth-abundantcopper– and zinc–chalcogenide metal combination hasnot yet reached the full potential in their development owing to limitedspectral coverage of the visible spectrum, the electronic properties,the charge extraction efficiency, or the chemical stability in aqueoussolutions. III–V semiconductors as components in HNPs alsohold promise due to their compliance with regulatory restrictionsand their potential also as photocatalytic systems but have yet tobe fully studied in these contexts. Recent progress in the synthesisof anisotropic shapes of InP nanocrystals and their surface controlopens the way for development of new HNPs based on these materials.Open questions concerning the fundamental nature of the nanoscalesemiconductor–metal interplay in HNPs remain and call for furtherexperimental and theoretical investigations. With the developmentof plasmonic photocatalysis, HNPs could offer a unique platform fordual combined excitonic and plasmonic photocatalysis in a single nanosystems.Yet, following this avenue motivates the study of electronic structureand charge carrier dynamics and relaxation routes across the excitonic–plasmonicinterface. Harnessing the plasmonic excitation route requires oneto extract the charge carriers on an ultrafast time scale from eitherside of the HNPs, which can only be achieved via further multidisciplinaryefforts combining synthesis, theory, and advanced experiments. Anadditional interesting avenue is to integrate HNPs as building blocksin new photocatalytic applications such as photoelectrochemical solarto fuel conversion schemes, which also constitutes a highly promisingdirection of further research.