Recent Advances in the Design and Photocatalytic Enhanced Performance of Gold Plasmonic Nanostructures Decorated with Non-Titania Based Semiconductor Hetero-Nanoarchitectures

: Plasmonic photocatalysts combining metallic nanoparticles and semiconductors have been aimed as versatile alternatives to drive light-assisted catalytic chemical reactions beyond the ultraviolet (UV) regions, and overcome one of the major drawbacks of the most exploited photocatalysts (TiO 2 or ZnO). The strong size and morphology dependence of metallic nanostructures to tune their visible to near-infrared (vis-NIR) light harvesting capabilities has been combined with the design of a wide variety of architectures for the semiconductor supports to promote the selective activity of speciﬁc crystallographic facets. The search for e ﬃ cient heterojunctions has been subjected to numerous studies, especially those involving gold nanostructures and titania semiconductors. In the present review, we paid special attention to the most recent advances in the design of gold-semiconductor hetero-nanostructures including emerging metal oxides such as cerium oxide or copper oxide (CeO 2 or Cu 2 O) or metal chalcogenides such as copper sulﬁde or cadmium sulﬁdes (CuS or CdS). These alternative hybrid materials were thoroughly built in past years to target research ﬁelds of strong impact, such as solar energy conversion, water splitting, environmental chemistry, or nanomedicine. Herein, we evaluate the inﬂuence of tuning the morphologies of the plasmonic gold nanostructures or the semiconductor interacting structures, and how these variations in geometry, either individual or combined, have a signiﬁcant inﬂuence on the ﬁnal photocatalytic performance. species; (ii) O 2 adsorbed on previously generated Ce(III) sites forming Ce(IV)-O-O · (iii) that radical can remove α -H of benzyl alcohol yielding Ce(IV)-O-OH; (iv) the radical dehydrogenated benzylic alcohol combines with Ce(IV)-O-OH to produce the ﬁnal benzaldehyde product and H 2 O 2 , and simultaneously, an electron returning to the Au bands to ensure electron-hole pair recombination. charge separation under visible light irradiation and its further electron migration through AuNRs to enhance tipped Pt reservoir; Induced both the resonance energy transfer and hot electron transfer processes and synergistically boosted the photocatalytic reaction


Introduction
Since the first reported example of heterogeneous photocatalysis in 1911 applied to the degradation of Prussian Blue by ZnO powder and illumination [1], the degree of sophistication and complexity in photocatalyst design has experienced a huge development [2]. In heterogeneous photocatalysis, the process is initiated by the interaction between incident photons and the catalyst. The photon absorption by the catalyst (typically a semiconductor) leads to the promotion of valence band electrons into the conduction band, thereby creating electron-hole pairs. Those carriers can induce the subsequent

Metal-Semiconductor Hetero-Nanostructures: Different Configurations and Light-Driven Activation Mechanisms
A key feature of plasmonic photocatalysts is related with the photo-induced generation of highly energetic electrons (hot electrons) generated via LSPR [40]. The distribution of these hot electrons can be described by the Fermi equation using an elevated effective temperature [41]. After light absorption, LSPR decay may occur either radiatively, through re-emitted photons, or non-radiatively, for instance through transfer of hot electrons [42], generally through intraband excitations within the conduction band [40,43] thereby causing electrons from occupied energy levels to be excited above the Fermi energy. Hence, after coupling with metals, typical semiconductors used in photocatalysis can capture these hot electrons and generate reactive species by using visible-NIR light. The formation of metal-semiconductor heterojunctions allows hot electrons to be accepted into semiconductor conduction band and carry on the photocatalytic process ( Figure 2a). The energetic barrier formed Catalysts 2020, 10,1459 4 of 27 at the metal-semiconductor interface is called Schottky barrier [44]. Hot electrons are injected into semiconductor conduction band when their energy is superior to Schottky barrier energy (E SB ) which is lower than the bandgap of semiconductor (E g ) [45]. After the hot electron generation, holes are also generated in the plasmonic structure as illustrated in Figure 2b. For this mechanism to occur there must be a good interaction between the metal and the support, which make critical the synthesis step of these hybrid materials.
Catalysts 2020, 10, x FOR PEER REVIEW 4 of 27 formation of metal-semiconductor heterojunctions allows hot electrons to be accepted into semiconductor conduction band and carry on the photocatalytic process ( Figure 2a). The energetic barrier formed at the metal-semiconductor interface is called Schottky barrier [44]. Hot electrons are injected into semiconductor conduction band when their energy is superior to Schottky barrier energy (ESB) which is lower than the bandgap of semiconductor (Eg) [45]. After the hot electron generation, holes are also generated in the plasmonic structure as illustrated in Figure 2b. For this mechanism to occur there must be a good interaction between the metal and the support, which make critical the synthesis step of these hybrid materials. (b) hot electron injection process. When a plasmonic structure as, in example, AuNR is irradiated with light with an energy equal to its LSPR, electrons mainly coming from conduction band are excited. If their energy overcomes the Schottky barrier energy (ESB), they can be injected into the semiconductor conduction band to further perform the photocatalytic process; (c) mechanisms involved in plasmonic-semiconductor heterostructures (adapted from [46]). Diverse physical processes may take (b) hot electron injection process. When a plasmonic structure as, in example, AuNR is irradiated with light with an energy equal to its LSPR, electrons mainly coming from conduction band are excited. If their energy overcomes the Schottky barrier energy (E SB ), they can be injected into the semiconductor conduction band to further perform the photocatalytic process; (c) mechanisms involved in plasmonic-semiconductor heterostructures (adapted from [46]). Diverse physical processes may take place between plasmonic nanostructure and semiconductor to promote valence band electrons to conduction band energy levels.
As shown in Figure 3b, it is possible to differentiate between core-shell, dumbbell, Janus, and yolk-shell configurations attending to the semiconductor distribution around the metallic cores [38] with several implications in their photocatalytic activity. Structures with exposed surfaces of both the metal and the semiconductors (i.e., dumbbell and Janus) exhibit a superior photocatalytic performance due to the continuous exposure of the reactants with the generated charge carriers (e − /h + ) [27,40,[60][61][62][63][64]. Different anisotropic heterostructures have been prepared with SiO2 [65], Pt [66,67], Fe2O3 [68], Cu2O [60], CeO2 [63], and TiO2 [27]. Yolk-shell structures (Figure 3b) are characterized by a hollow shell and an inner plasmonic core with several benefits for the photocatalytic process: (i) the presence of a hollow shell ensures higher specific surface area as it possesses an inner and external part. (ii) The small thickness of shells shortens charge diffusion distance, reducing possible bulk recombination processes, and (iii) void space present allows reflection of light inside the hollow shell, causing light scattering, and boosting the number of available photons [69].  The morphology of Au nanostructure determines LSPR shift. Au nanospheres, prisms, cages, urchin-like or rods exhibit different electron confinement, entailing different UV-vis. spectra; (b) Plasmonic-semiconductor heterostructures with different architectures, including core-shell (plasmonic nanoparticle as core and semiconductor as a surrounding shell), dumbbell (plasmonic AuNR where the semiconductor is selectively deposited on tips), Janus (plasmonic nanoparticle and semiconductor attached with both surfaces available for the substrates), and yolk-shell (plasmonic nanoparticle inside a voided semiconductor structure).
As shown in Figure 3b, it is possible to differentiate between core-shell, dumbbell, Janus, and yolk-shell configurations attending to the semiconductor distribution around the metallic cores [38] with several implications in their photocatalytic activity. Structures with exposed surfaces of both the metal and the semiconductors (i.e., dumbbell and Janus) exhibit a superior photocatalytic performance due to the continuous exposure of the reactants with the generated charge carriers (e − /h + ) [27,40,[60][61][62][63][64]. Different anisotropic heterostructures have been prepared with SiO 2 [65], Pt [66,67] [63], and TiO 2 [27]. Yolk-shell structures (Figure 3b) are characterized by a hollow shell and an inner plasmonic core with several benefits for the photocatalytic process: (i) the presence of a hollow shell ensures higher specific surface area as it possesses an inner and external part. (ii) The small Catalysts 2020, 10, 1459 6 of 27 thickness of shells shortens charge diffusion distance, reducing possible bulk recombination processes, and (iii) void space present allows reflection of light inside the hollow shell, causing light scattering, and boosting the number of available photons [69].

Evaluation of Au-CeO 2 Anisotropic Hetero-Nanoarchitectures
The combination of plasmonic Au nanostructures with cerium oxide as complex hybrid architectures represents an interesting example of synergistic photocatalyst that maximizes the positive properties of each individual counterpart beyond the most explored TiO 2 or ZnO hybrids [70][71][72]. The redox properties of Ce endow this oxide with large capability to transport oxygen via Ce(IV)-Ce(III) pair redox cycles within its framework [21]. As a result, an extensive number of oxygen vacancies are present in ceria-based materials surface [73], favoring a continuous cycle of catalytic reaction-regeneration in oxidation reactions [21,[74][75][76][77]. Nevertheless, CeO 2 exhibits two major drawbacks for photocatalytic purposes: (i) CeO 2 possesses a band gap of 3.2 eV burdening its photocatalytic response to the ultraviolet region; (ii) CeO 2 possesses low carrier mobility, hindering its transport to surface after the electron-hole pair photogeneration to react with the targeted substrates [78,79] .
Regarding the generation of the plasmonic hybrid photocatalyst, (i) CeO 2 as an n-type semiconductor possesses high density of states in its conduction band, which confers a good electron-accepting capability [40] and forms a heterojunction and Schottky barrier with Au to allow a proper hot-electron injection [21,22] (Figure 4). Therefore, the synthesis and development of Au-CeO 2 photocatalysts to maximize the redox properties of ceria has boosted the research on optimizing the interaction of these materials through an extended variety of imaginative configurations. Thus far, CeO 2 has been successfully deposited onto nanorods (both forming dumbbell [22] and core-shell [21,22,80] structures), spheres [21,81], or hollow cages [82].
Catalysts 2020, 10, x FOR PEER REVIEW 6 of 27 AuNR where the semiconductor is selectively deposited on tips), Janus (plasmonic nanoparticle and semiconductor attached with both surfaces available for the substrates), and yolk-shell (plasmonic nanoparticle inside a voided semiconductor structure).

Evaluation of Au-CeO2 Anisotropic Hetero-Nanoarchitectures
The combination of plasmonic Au nanostructures with cerium oxide as complex hybrid architectures represents an interesting example of synergistic photocatalyst that maximizes the positive properties of each individual counterpart beyond the most explored TiO2 or ZnO hybrids [70][71][72]. The redox properties of Ce endow this oxide with large capability to transport oxygen via Ce(IV)-Ce(III) pair redox cycles within its framework [21]. As a result, an extensive number of oxygen vacancies are present in ceria-based materials surface [73], favoring a continuous cycle of catalytic reaction-regeneration in oxidation reactions [21,[74][75][76][77]. Nevertheless, CeO2 exhibits two major drawbacks for photocatalytic purposes: (i) CeO2 possesses a band gap of 3.2 eV burdening its photocatalytic response to the ultraviolet region; (ii) CeO2 possesses low carrier mobility, hindering its transport to surface after the electron-hole pair photogeneration to react with the targeted substrates [78,79] .
Regarding the generation of the plasmonic hybrid photocatalyst, (i) CeO2 as an n-type semiconductor possesses high density of states in its conduction band, which confers a good electronaccepting capability [40] and forms a heterojunction and Schottky barrier with Au to allow a proper hot-electron injection [21,22] (Figure 4). Therefore, the synthesis and development of Au-CeO2 photocatalysts to maximize the redox properties of ceria has boosted the research on optimizing the interaction of these materials through an extended variety of imaginative configurations. Thus far, CeO2 has been successfully deposited onto nanorods (both forming dumbbell [22] and core-shell [21,22,80] structures), spheres [21,81], or hollow cages [82]. In general, the synthesis of core-shell Au-CeO2 nanostructures has been typically reported via hydrothermal treatment of a chelated Ce precursor [21,22,[80][81][82]. Li et al. [21] reported the first In general, the synthesis of core-shell Au-CeO 2 nanostructures has been typically reported via hydrothermal treatment of a chelated Ce precursor [21,22,[80][81][82]. Li et al. [21] reported the first synthesis of Au-CeO 2 nanostructures with core-shell configuration. One of the problems facing CeO 2 selective deposition on Au nanostructures was the rapid condensation of Ce precursors to form CeO 2 . By using ethylenediaminetetraacetic acid (EDTA) as chelating agent, fast hydrolysis of Ce (III) ions could be prevented [83]. Thus, tuning Ce (III)/EDTA ratios allowed a fine control of CeO 2 deposition rates. Regarding the synthesis of dumbbell and Janus Au-CeO 2 systems, two outstanding contributions have been demonstrated in the recent literature by Pan et al. [63] and Jian et al. [22], respectively. It was possible to control the selective growth of CeO 2 on AuNR tips by controlling the amount of cetyltrimethylammonium bromide (CTABr) adsorbed onto the Au surface, which acted as a blockade of CeO 2 nucleation.
The first reported examples of Au-CeO 2 hybrids in photocatalysis were structurally analogous to the conventional counterparts traditionally used in heterogeneous catalysis for CO oxidation reactions. Kominaim et al. [84,85] reported the use of Au nanoparticles dispersed onto nanostructured CeO 2 supports to oxidize formic acid under visible light irradiation [84,85]. Nevertheless, Au-CeO 2 photocatalysts consisted in the random deposition of Au nanoparticles onto a relatively large CeO 2 support that prevented a deep understanding of the photocatalytic mechanism. As mentioned before, Li et al. [21] reported the first core-shell configuration for Au-CeO 2 nanoparticles. They tested the use of both spheres and rod-shaped Au plasmonic cores. The Au@CeO 2 catalysts exhibited different LSPR absorption maxima at different wavelengths within the visible-NIR region (530, 591, 715 nm, respectively) [21]. These hybrid configurations were employed towards the photooxidation of benzylic alcohol. The catalysts were subjected to calcination post-treatments to remove the excess of the directing surfactant employed to grow the ceria shell (CTABr), which did not possess any influence in the morphology/crystallinity of the sample. The influence of this calcination treatment was thoroughly evaluated upon irradiation with two different excitation sources (Xe lamp (λ > 420 nm) and laser (λ = 671 nm)). For Xe lamp, a systematic study of Au@CeO 2, LSPR 530nm was performed. It was found that the Au@CeO 2 calcined sample exhibited the highest photocatalytic activity, suggesting that the removal of CTABr enhanced the formation of Schottky barrier between Au and CeO 2 and facilitated a better charge carrier transfer in the catalyst. Upon laser irradiation, a similar trend was confirmed for the uncalcined and calcined samples, respectively. Calcined Au@CeO 2, LSPR 591nm showed the highest photocatalytic activity. The authors attributed the enhanced response to the better match between the LSPR absorption maximum of the calcined hybrid (redshifted after calcination to 680 nm) and the incident laser wavelength at 671 nm.
As previously pointed out in Figure 2c (vide supra), not only the hot electron injection pathway can supply electrons in the semiconductor conduction band. Interestingly, the generation of heat caused by one of the decay mechanisms caused by LSPR [86] can also promote the excitation of electrons from the valence band to the conduction band. As control experiments, Li et al. [21] studied the influence of the temperature in the catalytic activity in the absence of light. They found a considerable catalytic activity at the temperature that reached the system under illumination in darkness conditions, suggesting that part of the photocatalytic activity was thermally induced. As a potential mechanism (Figure 5a), the authors suggested a series of redox steps (vide infra): (i) electron injection into Ce(IV) conduction band, which generated Ce(III) species; (ii) O 2 adsorbed on previously generated Ce(III) sites forming Ce(IV)-O-O· (iii) that radical can remove α-H of benzyl alcohol yielding Ce(IV)-O-OH; (iv) the radical dehydrogenated benzylic alcohol combines with Ce(IV)-O-OH to produce the final benzaldehyde product and H 2 O 2 , and simultaneously, an electron returning to the Au bands to ensure electron-hole pair recombination. generating H species that further convert 4-NP into 4-aminophenol. On the other hand, CeO2 is able to withdraw e − from AuNR that absorbs H + from H2O yielding the same H active species [88]. This phenomenon could not occur in the case of core-shell AuNR@CeO2 since Au facets remained unexposed to the liquid reaction media and yielded a lower catalytic activity. Figure 5. (a) Hot electron injection mechanism for a core-shell Au@CeO2 configuration (adapted from [21]). Hot electrons generate Ce (III) species that can capture and activate O2 which furtherly reacts with the substrate (benzylic alcohol). The generated Ce hydroperoxide (Ce-O-OH) reacts with the radical intermediate, yielding the final aldehyde; (b) photocatalytic mechanism for a dumbbell Au-CeO2 structure (adapted from [22]). The generation and consumption (and thus, photocatalysis) of both hot electrons/holes is enhanced by promoting the accessibility of hole-acceptor molecules to the metallic surface and thanks to the accommodating role of oxygen vacancies (OVs) present in the ceria nanostructures. Core-shell architectures hinder this accessibility, accumulating hot holes and slowing down the catalytic cycle.
Following a similar methodology, Jia et al. [22] combined CeO2 with AuNRs forming a dumbbell heterostructure ( Figure 5b) and evaluated their photocatalytic performance in the N2 photofixation with NIR irradiation. CeO2 rich surface in oxygen vacancies (OVs) (Ce(III) sites) chemisorbed N2 that could be reduced to NH3 by injected plasmonic hot electrons, breaking triple bond N-N. Due to the exposure of Au facets to the reaction media, as-generated hot holes could be consumed by a hole scavenger (in this particular case, methanol) to close the photocatalytic cycle. Core-shell AuNR@CeO2 nanostructures prevented the availability of hot holes and the accessibility of CH3OH to the active (a) Hot electron injection mechanism for a core-shell Au@CeO 2 configuration (adapted from [21]). Hot electrons generate Ce (III) species that can capture and activate O 2 which furtherly reacts with the substrate (benzylic alcohol). The generated Ce hydroperoxide (Ce-O-OH) reacts with the radical intermediate, yielding the final aldehyde; (b) photocatalytic mechanism for a dumbbell Au-CeO 2 structure (adapted from [22]). The generation and consumption (and thus, photocatalysis) of both hot electrons/holes is enhanced by promoting the accessibility of hole-acceptor molecules to the metallic surface and thanks to the accommodating role of oxygen vacancies (OVs) present in the ceria nanostructures. Core-shell architectures hinder this accessibility, accumulating hot holes and slowing down the catalytic cycle.
Alternatively, Wang et al. [80] demonstrated that the plasmon-induced hot-electron injection under NIR illumination in CeO 2 coated AuNRs with Janus configuration accelerated photo Fenton-like reactions using H 2 O 2 as substrate. Again, the hot electron injection in Ce(IV) induced the formation of Ce(III) that acted as active species in Fenton-like reactions [87] with the generation of highly reactive ·OH that subsequently facilitated the degradation of an organic model pollutant. As depicted in Figure 5b, a heterostructure with plasmonic exposed faces possess important photocatalytic advantages. Pan et al. [63] demonstrated that half-encapsulated AuNRs with CeO 2 possessed better activity in the catalytic reduction of 4-nitrophenol (4-NP) with NaBH 4 , in comparison with their core-shell AuNR@CeO 2 counterparts. Hot electrons were transferred from the plasmonic AuNR into CeO 2 , generating holes on Au surface. BH 4 can donate electrons to AuNR generating H species that further convert 4-NP into 4-aminophenol. On the other hand, CeO 2 is able to withdraw e − from AuNR that absorbs H + from H 2 O yielding the same H active species [88]. This phenomenon could not occur in the case of core-shell AuNR@CeO 2 since Au facets remained unexposed to the liquid reaction media and yielded a lower catalytic activity.
Following a similar methodology, Jia et al. [22] combined CeO 2 with AuNRs forming a dumbbell heterostructure ( Figure 5b) and evaluated their photocatalytic performance in the N 2 photofixation with NIR irradiation. CeO 2 rich surface in oxygen vacancies (OVs) (Ce(III) sites) chemisorbed N 2 that could be reduced to NH 3 by injected plasmonic hot electrons, breaking triple bond N-N. Due to the exposure of Au facets to the reaction media, as-generated hot holes could be consumed by a hole scavenger (in this particular case, methanol) to close the photocatalytic cycle. Core-shell AuNR@CeO 2 nanostructures prevented the availability of hot holes and the accessibility of CH 3 OH to the active sites, which reflected in a huge difference in the NH 3 generation rate (114.3 vs. 18.44 µmol·h −1 ·g −1 ) [22]. Thus, CeO 2 emerges as a promising photocatalytic material in combination with Au plasmonic nanostructures, being the main highlights the regeneration of highly desired OVs due to hot electron transference from Au and the photothermo-induced oxygen mobility [21]. The high oxygen mobility has shown to have influence during the photothermo-catalytic process, as demonstrated by Li's experiments with temperature and in the absence of light [21,89]. CeO 2 as n-type semiconductor [90] and as oxygen ion conductor possess a unique charge separation effect: when a photon is absorbed, an electron is excited from the valence band (orbital O 2p ) to an empty conduction band (orbital Ce 4f ), forming a Ce 4+ (e − )/O 2− (h + ) pair. The as-generated electrons remain localized in Ce (IV) centers favoring the charge separation and consequently, reducing the recombination phenomena ( Figure 6) [90,91].
Catalysts 2020, 10, x FOR PEER REVIEW 9 of 27 sites, which reflected in a huge difference in the NH3 generation rate (114.3 vs. 18.44 µmol·h −1 ·g −1 ) [22]. Thus, CeO2 emerges as a promising photocatalytic material in combination with Au plasmonic nanostructures, being the main highlights the regeneration of highly desired OVs due to hot electron transference from Au and the photothermo-induced oxygen mobility [21]. The high oxygen mobility has shown to have influence during the photothermo-catalytic process, as demonstrated by Li's experiments with temperature and in the absence of light [21,89]. CeO2 as n-type semiconductor [90] and as oxygen ion conductor possess a unique charge separation effect: when a photon is absorbed, an electron is excited from the valence band (orbital O2p) to an empty conduction band (orbital Ce4f), forming a Ce 4+ (e − )/O 2− (h + ) pair. The as-generated electrons remain localized in Ce (IV) centers favoring the charge separation and consequently, reducing the recombination phenomena ( Figure 6) [90,91]. Cited reports confirm an effective synergistic effect in the photocatalysis between LSPR and CeO2 and an enhancement of the photocatalysis with Janus-type heterostructures [22,80]. Hence, synthetized Au-CeO2 nanostructures interact with visible and NIR light, remarking the promising interest of these materials in environmental, biomedical applications and solar energy harvesting, among others [69,[91][92][93][94][95][96][97][98].

Evaluation of Au-Cu2O/Au-CuS Anisotropic Heteronanostructures
Cuprous oxide has attracted much attention as a photocatalyst due to its remarkable features as a semiconductor, and its extended variety of active roles in solar and energy applications of paramount interests for the Sustainable Development Goals targeted by most of the worldwide governments' agendas. Copper (I) oxide materials have been widely used in the degradation of organic compounds in solution and their photocatalytic activity has been studied against a large number of potential pollutants, such as organic material [99,100], drugs [101] or nutrients [102]. In addition, studies are also emerging where copper (I) oxide is being used in the generation of molecular hydrogen [32,103]. It is a highly abundant p-type semiconductor on earth with an energy band gap (Eg) of ≈ 2.17 eV that has a low cost and good absorption capabilities in the visible range Cited reports confirm an effective synergistic effect in the photocatalysis between LSPR and CeO 2 and an enhancement of the photocatalysis with Janus-type heterostructures [22,80]. Hence, synthetized Au-CeO 2 nanostructures interact with visible and NIR light, remarking the promising interest of these materials in environmental, biomedical applications and solar energy harvesting, among others [69,[91][92][93][94][95][96][97][98].

Evaluation of Au-Cu 2 O/Au-CuS Anisotropic Heteronanostructures
Cuprous oxide has attracted much attention as a photocatalyst due to its remarkable features as a semiconductor, and its extended variety of active roles in solar and energy applications of paramount interests for the Sustainable Development Goals targeted by most of the worldwide governments' agendas. Copper (I) oxide materials have been widely used in the degradation of organic compounds in solution and their photocatalytic activity has been studied against a large number of potential pollutants, such as organic material [99,100], drugs [101] or nutrients [102]. In addition, studies are also emerging where copper (I) oxide is being used in the generation of molecular hydrogen [32,103]. It is a highly abundant p-type semiconductor on earth with an energy band gap (E g ) of ≈ 2.17 eV that has a low cost and good absorption capabilities in the visible range when compared with other semiconductors [50,104]. However, some drawbacks have been also consistently reported for this oxide such as: (i) rapid recombination of electron-hole pairs; (ii) limited absorption of the semiconductor in the visible range; or (iii) natural tendency to form larger structures or disproportionation (especially in liquid media). The combination with anisotropic plasmonic cores to generate metal-semiconductor heterostructures represents an appealing alternative to overcome these problems [56,105,106]. These hybrid nanostructures offer two remarkable features, the Schottky barrier and the LSPR that improves the photocatalytic response by decreasing the carrier recombination rates and expanding the absorption range to the visible.
The selective growth of copper oxide onto plasmonic Au cores has been a trendy topic widely studied by multiple researchers in recent years [32,56,[105][106][107][108]. However, the final morphology and selective exposure of specific crystalline facets has also attracted great interest in the field of photocatalysis in terms of both selectivity and reactivity [106,107,109]. The generation of core shell Au-Cu 2 O hybrid structures with high precision has been reported in the recent literature [107,109,110]. Several parameters such as the nature of the reducing agents, the concentrations of reducing agent or pH values have been systematically evaluated to modulate the morphology, shape, and length of the oxide shells and favor the preferential exposure of selected crystal facets [32,109] (see Figure 7). For instance, Kuo et al. [109] reported the controlled synthesis of different morphologies including cubic structures, truncated cubes, cuboctahedra, or truncated octahedra to octahedra upon tuning the variation of NH 2 OH. On the other hand, Kuo et al. [106] explored the influence of the different morphologies of AuNPs coated Cu 2 O shell on the final photocatalytic performance towards the methyl orange (MO) degradation. They showed how core-shell architectures improved MO degradation from 18 to 50 µmol·cm −2 in comparison with pristine Cu 2 O nanostructures and how the influence was higher in the case of the cube-shaped counterparts. This was an example where the plasmonic core helped in the transfer of carriers and promoted a better conductivity in a photo-electronic device. Yuan et al. [107] also developed hybrid AuNRs-copper oxide nanoparticles where they explored variations on the type of reducing agent to tune the preferential octahedral or cubo-octahedral morphologies (see Figure 7a). Both structures were generated by Ostwald ripening process. Smaller crystals were dissolved and redeposited onto larger crystals when different reductants were added. Hydrazine as stronger reductant was reported to promote the copper species to their more stable morphology. They also tested their photocatalytic activity towards the degradation of MO. It was observed how hybrid structures enhanced more activity in comparison with analogous bare semiconductor counterparts. The Au internal core performed in a dual way, LSPR promoted electron injection into the semiconductor and also acted as a charge sinker to increase charge migration and separation. This was a representative example on how the presence of the Au core considerably affects the activity of the system and how the morphology of the external shell (corner-truncated octahedral vs. regular octahedral Cu 2 O) is also highly relevant in the absorption and degradation of MO. The truncated octahedral core-shell led to 94% MO conversion from an original 10 mg·L −1 MO concentration in 80 min [107]. This influence was also reported in Cu 2 O nanoparticles with different morphologies that also exhibited a much higher photocatalytic activity in rhombohedral shaped structures due to the more selective MO absorption over <110> faces for subsequent further oxidation [111] (see Figure 7a). Recent studies have also reported the use of hollow Au cores to build Au-Cu 2 O hybrids have also reported a successful tunability of their optical properties depending on their final architecture (Janus vs. core-shell). They reported an effective plasmon-induced energy transfer favored by the hollow Au nature that improved the MO degradation rates [56]. Other recent report by Xu et al. [112] has established a strong influence of the final heterostructure configuration on the capability to generate photocurrent. In this case, the used solid gold seeds incubated with a ligand that shifted the core-shell configuration towards a Janus-like structure. Additional studies have also exploited the synergy of Au-Cu2O hybrid materials. For instance, Yu et al. developed [32] a hot-dog-like hybrid configuration where they selectively deposited copper oxide on the transverse part of large aspect ratio AuNRs, leaving the edges completely exposed [32]. It was possible to change the kind of Cu2O-AuNR architecture from core-shell to tips-exposed hotdog type structure by reducing the amount of copper precursor added to the synthesis. The preferential epitaxial growth of Cu2O over the transversal part of AuNR promoted a first nucleation over these sides, leaving the edges completely uncoated under low copper concentration conditions. In a sequential manner, they even incorporated very thin TiO2 layers on top of the copper oxide and established a critical evaluation of the photocatalytic activity for each hybrid configuration towards hydrogen production. The charge separation efficiency was significantly higher for the hot-dog configuration than for the bare Cu2O or the fully-covered core-shell counterparts. The hot-dog configuration also exhibited an enhanced photocatalytic response almost 24-fold higher than the Cu2O nanoparticles (80 µmol H2 g −1 h −1 vs. 3.4 µmol H2 g −1 h −1 ) and four times higher than the coreshell configuration (19.3 µmol H2 g −1 h −1 ). The authors partially attributed the outperformance of the hot-dog configuration to the generation of four hot-spots at the interface of the non-coated AuNR and the semiconductor shells (see Figure 7b) of the hot-dog configuration. They concluded that this interface was more efficient in the transport of carriers due to the direct interaction of holes with glucose (used as sacrifice agent) and to the more efficient generation of hot electrons to induce the reduction step toward H2 generation (see Figure 7b) [32]. The results were even better in the case of the ternary shell due to the more efficient p-n junction formed with the addition of a titania layer. Additional studies have also exploited the synergy of Au-Cu 2 O hybrid materials. For instance, Yu et al. developed [32] a hot-dog-like hybrid configuration where they selectively deposited copper oxide on the transverse part of large aspect ratio AuNRs, leaving the edges completely exposed [32]. It was possible to change the kind of Cu 2 O-AuNR architecture from core-shell to tips-exposed hot-dog type structure by reducing the amount of copper precursor added to the synthesis. The preferential epitaxial growth of Cu 2 O over the transversal part of AuNR promoted a first nucleation over these sides, leaving the edges completely uncoated under low copper concentration conditions. In a sequential manner, they even incorporated very thin TiO 2 layers on top of the copper oxide and established a critical evaluation of the photocatalytic activity for each hybrid configuration towards hydrogen production. The charge separation efficiency was significantly higher for the hot-dog configuration than for the bare Cu 2 O or the fully-covered core-shell counterparts. The hot-dog configuration also exhibited an enhanced photocatalytic response almost 24-fold higher than the Cu 2 O nanoparticles (80 µmol H 2 g −1 h −1 vs. 3.4 µmol H 2 g −1 h −1 ) and four times higher than the core-shell configuration (19.3 µmol H 2 g −1 h −1 ). The authors partially attributed the outperformance of the hot-dog configuration to the generation of four hot-spots at the interface of the non-coated AuNR and the semiconductor shells (see Figure 7b) of the hot-dog configuration. They concluded that this interface was more efficient in the transport of carriers due to the direct interaction of holes with glucose (used as sacrifice agent) and to the more efficient generation of hot electrons to induce the reduction step toward H 2 generation (see Figure 7b) [32]. The results were even better in the case of the ternary shell due to the more efficient p-n junction formed with the addition of a titania layer.
Even though ternary heterostructures are not the main focus of this review, it is worth mentioning recent efforts to build Z-schemed TiO 2 -Au-Cu 2 O photoelectrodes [30] or the addition of a third component of small Au NPs to the Au-Cu 2 O hybrids reported by Yu et al. [113]. They designed a more complex core-shell AuNR@Cu 2 O (octahedral) architecture additionally decorated with AuNPs ( Figure 8). This ternary composite was able to increase the photocatalytic activity of the binary material by incorporating AuNPs, improving the transfer and separation of displaced charges through the semiconductor. Using higher wavelengths where Cu 2 O is not able to absorb, light would penetrate through the structure reaching AuNR. Hot electrons could be generated via LSPR effect and injected into the conduction band of the semiconductor. Hot electrons and holes generated in the conduction band of Cu 2 O could be promoted from bulk Cu 2 O to the surface of the catalyst where the electrons are mainly trapped by Au nanoparticles to form singlet oxygen radical from the O 2 present on the surface. Meanwhile, holes places mainly in the surface of Cu 2 O p-type semiconductor are used for the direct MO degradation. An electron transfer "push-pull" synergetic effect was enhanced with the help of both AuNR core and AuNPs on the surface of Cu 2 O.
Catalysts 2020, 10, x FOR PEER REVIEW 12 of 27 AuNPs (Figure 8). This ternary composite was able to increase the photocatalytic activity of the binary material by incorporating AuNPs, improving the transfer and separation of displaced charges through the semiconductor. Using higher wavelengths where Cu2O is not able to absorb, light would penetrate through the structure reaching AuNR. Hot electrons could be generated via LSPR effect and injected into the conduction band of the semiconductor. Hot electrons and holes generated in the conduction band of Cu2O could be promoted from bulk Cu2O to the surface of the catalyst where the electrons are mainly trapped by Au nanoparticles to form singlet oxygen radical from the O2 present on the surface. Meanwhile, holes places mainly in the surface of Cu2O p-type semiconductor are used for the direct MO degradation. An electron transfer "push-pull" synergetic effect was enhanced with the help of both AuNR core and AuNPs on the surface of Cu2O. Sang Woo Han and co-workers conducted a comprehensive and systematic study where they developed hybrid plasmonic structures and explored how the variation of configuration affected the photocatalytic performance for hydrogen production using methanol as hole scavenger [60]. They synthesized plasmonic gold nanostars (Au NSs) and partially deposited copper monoxide (Cu2O) tuning the stabilizing agent (sodium dodecyl sulfate (SDS), (CTABr), or Polyvinylpyrrolidone (PVP) (see Figure 8b). The Janus-like structures with <111> vertices completely covered by the semiconductor were achieved by using PVP as a stabilizing agent. A complete core-shell type configuration was achieved with the aid of sodium dodecyl sulfate (SDS) as a stabilizing agent and the structure with the exposed vertices was achieved by combining two surfactants, CTABr and SDS. The nature of each stabilizing agent was crucial for the selective deposition of the semiconductor (Cu2O) onto the plasmonic Au NSs. Another important parameter to consider is the curvature of the surface of the plasmonic Au cores. In this systematic study, other Au seeds were used to prepare Janus-like architectures. It was observed how more shaped seeds could satisfactorily enhance the Sang Woo Han and co-workers conducted a comprehensive and systematic study where they developed hybrid plasmonic structures and explored how the variation of configuration affected the photocatalytic performance for hydrogen production using methanol as hole scavenger [60]. They synthesized plasmonic gold nanostars (Au NSs) and partially deposited copper monoxide (Cu 2 O) tuning the stabilizing agent (sodium dodecyl sulfate (SDS), (CTABr), or Polyvinylpyrrolidone (PVP) (see Figure 8b). The Janus-like structures with <111> vertices completely covered by the semiconductor were achieved by using PVP as a stabilizing agent. A complete core-shell type configuration was achieved with the aid of sodium dodecyl sulfate (SDS) as a stabilizing agent and the structure with the exposed vertices was achieved by combining two surfactants, CTABr and SDS. The nature of each stabilizing agent was crucial for the selective deposition of the semiconductor (Cu 2 O) onto the plasmonic Au NSs. Another important parameter to consider is the curvature of the surface of the plasmonic Au cores. In this systematic study, other Au seeds were used to prepare Janus-like architectures. It was observed how more shaped seeds could satisfactorily enhance the proposed architecture. However, it was not possible for less shaped seed structures using PVP as a stabilizing agent. In contrast, samples with less sharp vertex led to the preferential formation of core-shell type structures.
Yu et al. [60] also found that the use of PVP as a capping agent allowed a stronger interaction with low curvature sites from the Au NSs apexes, thereby leading to a preferential nucleation and growth of Cu 2 O tips at those positions (see Figure 8b). Homogeneous core-shell heterostructures required the use of SDS instead of PVP. The random assembly and weaker protecting capability of SDS enabled a complete overgrowth of Cu 2 O. In vertex-exposed Cu 2 O-Au NSs a binary mixture of CTABr/SDS was used. The electrostatic interaction between both surfactants induced the generation of more stable micelles in aqueous solution that preferentially absorbed in the high-curvature sites, leaving the rest of Au nanostructure more accessible for Cu 2 O nucleation. To further analyze the influence of the angle and the surfactants in the synthesis of anisotropic structures, AuNRs with different aspect ratios and binary mixtures of two surfactants (CTAB and SDS) were also evaluated by Yu and coworkers. They observed that the formation of Janus-type structures was preferentially induced as the aspect ratio of the AuNRs increased [60]. The authors studied the photocatalytic response of the different AuNSs-copper oxide configurations. A clear correlation between the efficiency of conversion of solar energy and the topology could be established during the hydrogen generation using methanol as sacrificing agent. Better conversions were achieved in partially covered structures compared to fully covered structures. Furthermore, no catalytic response was observed in the absence of the catalyst or in the presence of non-coated plasmonic cores. Furthermore, copper oxide of physical mixtures of both Cu 2 O and AuNSs did not render any significant conversion, thereby reinforcing the synergistic role of the plasmonic photocatalyst with a well-defined architecture (especially for the Janus-type configurations).
The comparison of the photocatalytic activities among core shell-type structures with different plasmonic structures (spheres and stars) in their core can be explained considering the excitation capacity of both materials at different wavelengths. Using excitation wavelengths of 700/750 or 800 nm, the sharp surfaces of AuNSs caused a much higher plasmonic excitation than their spherical counterpart. Janus-like architectures present higher catalytic activity than Core-Shell structures. Not covered Au highly increased the final photocatalytic activity of hybrid structure letting the oxidation reaction of hole scavengers to occur for methanol, whereas core-shell architecture dramatically deprives the accessibility of Au surface to hole scavengers.
It is shown how using identical AuNSs cores with a Cu 2 O overgrowth forming anisotropic heterostructures, induced completely different photocatalytic outcomes. It was observed a strong influence of Au-Cu 2 O plasmon energy transfer. Coated vertex Au nanostar enhanced higher plasmon excitation in comparison to vertex exposed Au NSs configurations and leading the photocatalytic activity of hybrid structures under those experimental conditions [60]. To better understand the interaction mechanism between the plasmonic core and the outer semiconductor layer, the AuNSs were completely coated with a thin insulating SiO 2 layer prior to the subsequent deposition of copper oxide. The hybrids with the SiO 2 interlayer exhibited a considerable decrease in activity, pointing out that the mechanism of action was carried out by plasmon-induced photocatalysis [60].
Au-Cu 2 O hybrid structures are being also currently explored for biomedical applications [114][115][116][117] such as photodynamic therapy treatment of cancer cells. Xu et al. [118] performed in vitro studies with AuCu-based materials, taking advantage of their ability to generate toxic reactive oxygen species (ROS) and its highly efficient phototherapy. AuNRs were used to selectively deposit copper on one of the sides of the Au structure using hexadecylamine as a protective ligand and sequentially making a coating with Au, which modified the structure inducing a galvanic replacement reaction and decreasing the amount of copper in the sample.
Interestingly, there are additional copper-based heterostructures where the semiconductor deposited over the plasmonic structure of Au is a Cu-based chalcogenide. These semiconductors are not expensive and more stable p-type semiconductors with a well-defined LSPR in the visible region. Furthermore, the amount of free charges in the material can be tuned upon modification of experimental parameters. Generating non-stoichiometric copper deficient structures of CuS induces the formation of transporters in the p-type semiconductor and exhibits subsequent properties such as photothermal catalysis or photocatalysis through the generation of ROS [119]. This type of structure can be made from previously formed copper oxides, giving rise to yolk shell-type structures where the plasmonic core was enclosed in an external copper chalcogenide shell and this can move freely within the voided structure [120]. To carry out the synthesis, it is typically necessary to obtain a copper (I) oxide structure through the conventional protocol, growing copper oxide following an epitaxial growth process. Later, the sample is sulfurized by introducing a sulfide precursor, such as Na 2 S in an acidic medium, and it is allowed to evolve until the yolk-shell type structure is formed, maintaining the crystalline faces of the original Cu 2 O structure. The catalytic ability of Au-CuS yolk shell structures to form ROS has been used in cancer chemotherapeutic therapies. Zhang et al. used the photothermal and photodynamic properties of this particular hybrid to perform a co-therapy using the resonance energy transfer (RET) mechanism [116].
Wang et al. [121] synthetized hybrid core-shell Au-CuS nanostructures directly without the need to use Cu 2 O as a sacrificial agent in the formation of the nanostructure and modified the final properties of the photocatalyst through the non-stoichiometric growth of CuS at different amounts of copper. They carried out a systematic study in which they evaluated the photocatalytic properties at different wavelengths, ranging from UV to Infrared through the visible spectrum and determined the influence of the different parts of the hybrid nanoparticle on the activity of the catalysts. The plasmonic characteristics of the Au nucleus could be measured by changing its dimensions and modifying the thickness of the CuS coating deposited around it. On the other hand, the plasmonic characteristics of the copper sulfide shell could be modified by changing the concentration of free holes, which is closely linked with the crystalline phase of copper sulfide and the Cu:S ratios found. Nanostructured Au can act as a photosensitizer (see Figure 2), being able to generate more electron-hole pairs in the semiconductor by means of RET mechanisms and thus improve the photocatalytic activity in the visible spectrum. Moreover, charge recombination in the surface of the semiconductor can be partially suppressed with the help of metals and charge migration through the bulk of Cu 2 O could be enhance more easily in the presence of Au. The controlling deposition of copper around Au nanostructure enables an overall downsize of the hybrid nanoparticles compared with conventional Cu 2 O nanostructures. It also allows a more selective deposition of Cu 2 O. All these encountered possibilities appearing with hybrid structures have a positive influence to enhance the photocatalytic response of the hybrid.

Evaluation of Au-CdS Anisotropic Heteronanostructures
Other semiconductors commonly used in photocatalysis reactions are metal chalcogenides. However, some of their most remarkable features, such as the absorption of light at certain wavelengths or the separation of charges are far from optimal. For this reason, alternatives have been proposed to improve the photocatalytic properties of these systems when we use a range of wavelengths that encompasses the visible or infrared spectrum while achieving a better and more effective charge separation. An interesting alternative that arises from the need to solve this problem is to create hybrid metal-semiconductor nanostructures that allow the use of the visible-NIR spectrum thanks to the metal's plasmonic properties. Moreover, a wisely metal incorporation in semiconductor to form hybrid structure also improve the charge separation and its photocatalytic activity. Metal chalcogenides have been in the spotlight for the past years due to their versatility and potential number of applications in the energy field (i.e., photovoltaics, imaging). In addition, chalcogenides semiconductors have been also studied for photocatalytic emerging applications in solar energy conversion and renewable energy development [34,[122][123][124][125][126][127][128][129].
Great interest has been devoted in recent years to the development of photocatalysts able to exploit the sun's energy to generate combustible, thus trying to reduce the amount of fossil fuels used [130,131]. One of the most interesting alternatives is H 2 generation as fuel using solar energy and semiconductor-based photocatalysts are postulated as one of the most promising alternatives to achieve this objective [132]. Some interesting features for solar-assisted photocatalysis in H 2 generation could be: [133] (i) It is necessary to find a semiconductor that has a narrow band gap and is highly efficient in absorption light; (ii) the band edge has to be more negative than standard hydrogen reduction potential; (iii) it should have good enough charge separation and migration to provide enough active sites.
There is a wide variety of photocatalysts but CdS is posited as a promising candidate for the generation of hydrogen. It has an optimal band gap of 2.4 eV that allows it to have photocatalytic activity under wavelengths of up to 500-600 nm in addition to a conduction band edge suitable for the generation of H 2 [134]. On the other hand, this semiconductor suffers from a rapid recombination of charges that make it inefficient. As an alternative to improve the migration and separation of charges, the generation of metal-semiconductor hybrids appears again as an alternative recently explored in the field [61,[133][134][135][136][137]. In this particular hybrid, the rich existing chemistry of quantum dots has boosted the search for a wide variety of architectures searching the optimal coupling of energy levels necessary to improve the photocatalytic outcome towards H 2 production [34,136]. Developing anisotropic structures such as dumbbell type bimetallic particles [135,136], Janus-type particles [62], or yolk-shell structures [34,138] have reported considerable increases in the activity of the systems in comparison with more conventional core-shell type structures. No considerable activity enhancements have been shown in this latter configuration in comparison with the bare CdS semiconductor.
Zhao et al. [137], developed core-shell type hybrid structures and verified their efficiency in photocatalytic reactions for the production of hydrogen. They were able to synthesize concentric nanostructures where the plasmonic Au-core was completely covered by the semiconductor. Alternatively, they were also able to generate eccentric structures where the core of Au was shifted, having part of internal Au core exposed (see Figure 9a). For the synthesis of these nanostructures, a two-step multistage was necessary in which, first, silver-coated Au structures were generated in a core-shell configuration. Subsequently, the silver shell was sulfurized and a mixture of these nanoparticles were allowed to evolve with the cadmium precursor and certain conditions to enhance Au-CdS Janus-like/core shell-like nanoparticles. Zhao et al. reported [62] the need of eccentric nanoparticles of silver sulfide with partial crystallinity for further evolve into the generation of Janus-type structures. The catalytic studies showed that anisotropic materials with partially exposed Au core possessed much higher photocatalytic activity than completely coated core-shell configurations. A remarkable 730-fold enhancement in the H 2 evolution rates was detected for the anisotropic dimer (7.3 mmol H 2 g −1 h −1 vs. 0.9 µmol H 2 g −1 h −1 , respectively). The authors claimed that increasing the temperature was necessary to ensure the proper presence of a well-defined crystalline phase. This phase was the niche that enabled a rational control of the anisotropy degree (see Figure 9a). In addition the partial exposure of the plasmonic Au was also determinant to maximize the SPR effects [62].
The photocatalytic activity of these hybrid structures was proved to have a highly symmetry dependence [137]. Janus-type particles were the most active, followed by analogous semiconductor particles and ending with core-shell Au-CdS nanoparticles. Furthermore, the Janus hybrids with the more pronounced anisotropy also exhibited the highest photoactivity using 300 W Xe lamp with application of a 400 nm cut-off long pass filter for water splitting reaction. This was attributed to the supporting role of Au acting as electron sink that could delay e/h + recombination rates and perform water reduction reaction. Upon excitation at wavelengths higher than 400 nm, a rapid electron transfer occurs between the conduction layers of the semiconductor (CdS) towards the Au surface thereby enabling a more efficient water reduction to generate hydrogen. However, the nanostructured bare CdS or core-shell Au-CdS configurations prevent the separation of charges in such an efficient way and in the case of core-shell particles, the electrons remain trapped in the core of the particle and precluding the photocatalyst from an efficient hydrogen evolution reaction [137].
In an attempt to expand the photocatalytic response towards the less energetic NIR range and enhance core anisotropy, Li et al. [139] explored the possibilities of plasmonic AuNRs as inner plasmonic core. In this case, AuNRs were completely covered with silver to generate silver sulfide in a second step that would end up giving rise to CdS under certain conditions. Wang et al. [140] also developed more complex core shell-like nanostructures generating multilayered chalcogenide shells containing different combinations of Bi 2 S 3 and CdS (Figure 9b,c). These samples were tested for dye photodegradation under visible region being the ternary combinations the most active in comparison with the binary counterparts (Au-Bi 2 S 3 or Au-CdS). The enhancement observed in the ternary component was attributed to the transfer of electrons from the exposed semiconductor (CdS) with a 2.4 eV band gap passing through the Bi 2 S 3 semiconductor intermediate layer with a 1.32 eV band gap until reaching the internal Au core. However, holes as positive charges, take longer to move from the valence layer from one semiconductor to another, enabling charges separation and, therefore, increasing photocatalytic activity. Moreover, hot electron injection from visible activated dumbbell-like AuNR to semiconductor can considerably increase the photoactivity compared with bare semiconductor or semiconductor coated conventional GNRs (see Figure 9b,c).
Catalysts 2020, 10, x FOR PEER REVIEW 16 of 27 component was attributed to the transfer of electrons from the exposed semiconductor (CdS) with a 2.4 eV band gap passing through the Bi2S3 semiconductor intermediate layer with a 1.32 eV band gap until reaching the internal Au core. However, holes as positive charges, take longer to move from the valence layer from one semiconductor to another, enabling charges separation and, therefore, increasing photocatalytic activity. Moreover, hot electron injection from visible activated dumbbelllike AuNR to semiconductor can considerably increase the photoactivity compared with bare semiconductor or semiconductor coated conventional GNRs (see Figure 9b,c). Figure 9. (a) Concentric core-shell (I), eccentric core-shell (II), and Janus (III, IV) hybrid structures (adapted from [137]); (b) AuNR@Bi2S3 hybrid structure and its excitation mechanism under visible light irradiation; (c) AuNR@Bi2S3@CdS core-(double) shell hybrid structure. Excitation mechanism for a more efficient carrier migration and its considerable improvement when both semiconductors were coupled in a double shell structure (adapted from [140]).
Xu et al. [136] developed different metal-CdS hybrid bimetallic structures and evaluated their photocatalytic activity to form benzaldehyde and H2. They synthesized three types of plasmonic heteronanostructures: CdS coated AuNRs, CdS coated-Pt coated AuNRs, and CdS coated-Pt tipped AuNRs and compared their photocatalytic activity. It was observed an increasing order of activity: Au@CdS < Au@Pt@CdS < Au-Pt@CdS (50 vs. 100 vs. 150 µmol H2 g −1 h −1 , respectively). They also carried out controls with the CdS semiconductor and metallic nanoparticles, but the activity decreased considerably in both cases. In this study, the use of AuNRs improved the harvesting of visible-NIR light and induces a local electric field (Figure 2) that is capable of promoting the generation and separation of charges from the CdS semiconductor ( Figure 10). The anisotropically deposited platinum at the edge of the nanorod acted as an electron sinker or electron reservoir enabling the electrons to be directed to the places where the reduction reaction of the protons takes place to obtain H2. In addition, the asymmetric separation of both metals served as a highway for an efficient transport of electrons that partially prevent electron-hole recombination (see Figure 10). This Figure 9. (a) Concentric core-shell (I), eccentric core-shell (II), and Janus (III, IV) hybrid structures (adapted from [137]); (b) AuNR@Bi 2 S 3 hybrid structure and its excitation mechanism under visible light irradiation; (c) AuNR@Bi 2 S 3 @CdS core-(double) shell hybrid structure. Excitation mechanism for a more efficient carrier migration and its considerable improvement when both semiconductors were coupled in a double shell structure (adapted from [140]).
Xu et al. [136] developed different metal-CdS hybrid bimetallic structures and evaluated their photocatalytic activity to form benzaldehyde and H 2 . They synthesized three types of plasmonic heteronanostructures: CdS coated AuNRs, CdS coated-Pt coated AuNRs, and CdS coated-Pt tipped AuNRs and compared their photocatalytic activity. It was observed an increasing order of activity: Au@CdS < Au@Pt@CdS < Au-Pt@CdS (50 vs. 100 vs. 150 µmol H 2 g −1 h −1 , respectively). They also carried out controls with the CdS semiconductor and metallic nanoparticles, but the activity decreased considerably in both cases. In this study, the use of AuNRs improved the harvesting of visible-NIR light and induces a local electric field (Figure 2) that is capable of promoting the generation and separation of charges from the CdS semiconductor ( Figure 10). The anisotropically deposited platinum at the edge of the nanorod acted as an electron sinker or electron reservoir enabling the electrons to be directed to the places where the reduction reaction of the protons takes place to obtain H 2 . In addition, the asymmetric separation of both metals served as a highway for an efficient transport of electrons that partially prevent electron-hole recombination (see Figure 10). This represents another good example of how smart designs and proper understanding of the carrier transport mechanisms facilitates an improved photo-response outperforming the simpler configurations.
Catalysts 2020, 10, x FOR PEER REVIEW 17 of 27 Figure 10. Schematic representation of the energy band multichannel in bimetallic@CdS heterostructures (adapted from [136]); semiconductor charge separation under visible light irradiation and its further electron migration through AuNRs to enhance tipped Pt reservoir; Induced both the resonance energy transfer (RET) and hot electron transfer processes and synergistically boosted the photocatalytic reaction (adapted from [136]).
Wu et al. [135] explored an analogous combination of Au-Pt-CdS components structured in coreshell (Au@CdS) or AuPt-CdS dumbbell like configurations with different coverage degree (see Figure  11a). These hybrids were systematically tested in the photocatalytic hydrogen generation reaction under visible or NIR light irradiation [135]. The photocatalytic activities were compared when systems were faced with wavelengths belonging to the visible or the near infrared light and it was seen that the order in the activity depended not only on the type of structure used, but also on the wavelength used because of the mechanisms enhancement. When the catalysts were excited with wavelengths within the visible range, the order in the catalytic activity was as follows: Au-Pt-CdS >> Au@CdS > CdS >> Au-Pt. In contrast, when NIR illumination was used, the activity dropped significantly, and the order of the catalytic activity was: Au-Pt > Au-Pt@CdS > Au > Au@CdS. Still, the H2 evolution rates (in the micromolar range) were less remarkable than those reported for the heterodimers (vide supra).  [136]); semiconductor charge separation under visible light irradiation and its further electron migration through AuNRs to enhance tipped Pt reservoir; Induced both the resonance energy transfer (RET) and hot electron transfer processes and synergistically boosted the photocatalytic reaction (adapted from [136]).
Wu et al. [135] explored an analogous combination of Au-Pt-CdS components structured in core-shell (Au@CdS) or AuPt-CdS dumbbell like configurations with different coverage degree (see Figure 11a). These hybrids were systematically tested in the photocatalytic hydrogen generation reaction under visible or NIR light irradiation [135]. The photocatalytic activities were compared when systems were faced with wavelengths belonging to the visible or the near infrared light and it was seen that the order in the activity depended not only on the type of structure used, but also on the wavelength used because of the mechanisms enhancement. When the catalysts were excited with wavelengths within the visible range, the order in the catalytic activity was as follows: Au-Pt-CdS >> Au@CdS > CdS >> Au-Pt. In contrast, when NIR illumination was used, the activity dropped significantly, and the order of the catalytic activity was: Au-Pt > Au-Pt@CdS > Au > Au@CdS. Still, the H 2 evolution rates (in the micromolar range) were less remarkable than those reported for the heterodimers (vide supra).  [135]) and comparative excitation pathways: Au-assisted CdS charge separation and Au-Pt hot electron transfer is enhanced in the anisotropic structure. Hot electron transfer or RET is not easily achieved when Pt shell is deposited over AuNRs (adapted from [135]); (c) schematic representation of photoactivation mechanisms in yolk-shell Au-CdS nanostructures (adapted from [138]).
Under sunlight illumination, CdS semiconductor deposited on the sides of Pt-Au bimetallic nanoparticle induced both the RET and hot electron transfer processes and synergistically boosted the photocatalytic H2 generation. However, when catalysts were activated by infrared light, Au-Pt bimetallic structure became the most active architecture indicating that excited hot electrons can further transfer to tipped Pt, but it is more complicated for electrons to enhance further migration from AuNR to CdS semiconductor (see mechanisms in Figure 11b). It was also observed how the catalytic activity of CdS coated metal tipped AuNRs (Au-Pt-CdS) was more active than its concentric counterpart Au@Pt@Cds when nanocatalyst were activated with visible light. The transversal LSPR induced electric field cannot reach the outside CdS semiconductor due to the presence of Pt NPs between the inner Au core and the external CdS layer (see Figure 11b). Pt nanoparticles around AuNR prevented the charge transfer between CdS and Au internal core. It has been observed how strategically modulating the metallic core of the structure by incorporating anisotropy or integrating several metals in a controlled way, the activity of the system was highly increased.
Likewise, yolk-shell structures have been recently reported as promising alternative hybrids with enhanced photocatalytic outcome provided by the selective control of the shell properties [138,141]. Han et al. [138] developed yolk-shell structures containing a plasmonic AuNR. The photocatalytic activity towards the generation of H2 was compared to analogous core-shell type architectures and hollow CdS nanoparticles [138]. Using hole scavengers such as Na2S or Na2SO3 to avoid photocorrosion of the CdS and exciting upon visible wavelengths, the yolk-shell structure generated a hydrogen conversion over 27 (1.7 mmol H2 g −1 h −1 ) or 12 (0.5 mmol H2 g −1 h −1 ) times greater  [135]) and comparative excitation pathways: Au-assisted CdS charge separation and Au-Pt hot electron transfer is enhanced in the anisotropic structure. Hot electron transfer or RET is not easily achieved when Pt shell is deposited over AuNRs (adapted from [135]); (c) schematic representation of photoactivation mechanisms in yolk-shell Au-CdS nanostructures (adapted from [138]).
Under sunlight illumination, CdS semiconductor deposited on the sides of Pt-Au bimetallic nanoparticle induced both the RET and hot electron transfer processes and synergistically boosted the photocatalytic H 2 generation. However, when catalysts were activated by infrared light, Au-Pt bimetallic structure became the most active architecture indicating that excited hot electrons can further transfer to tipped Pt, but it is more complicated for electrons to enhance further migration from AuNR to CdS semiconductor (see mechanisms in Figure 11b). It was also observed how the catalytic activity of CdS coated metal tipped AuNRs (Au-Pt-CdS) was more active than its concentric counterpart Au@Pt@Cds when nanocatalyst were activated with visible light. The transversal LSPR induced electric field cannot reach the outside CdS semiconductor due to the presence of Pt NPs between the inner Au core and the external CdS layer (see Figure 11b). Pt nanoparticles around AuNR prevented the charge transfer between CdS and Au internal core. It has been observed how strategically modulating the metallic core of the structure by incorporating anisotropy or integrating several metals in a controlled way, the activity of the system was highly increased.
Likewise, yolk-shell structures have been recently reported as promising alternative hybrids with enhanced photocatalytic outcome provided by the selective control of the shell properties [138,141]. Han et al. [138] developed yolk-shell structures containing a plasmonic AuNR. The photocatalytic activity towards the generation of H 2 was compared to analogous core-shell type architectures and hollow CdS nanoparticles [138]. Using hole scavengers such as Na 2 S or Na 2 SO 3 to avoid photocorrosion of the CdS and exciting upon visible wavelengths, the yolk-shell structure generated a hydrogen conversion over 27 (1.7 mmol H 2 g −1 h −1 ) or 12 (0.5 mmol H 2 g −1 h −1 ) times greater than the core-shell type and hollow semiconductor structures (0.02 mmol H 2 g −1 h −1 ). The authors also compared the catalytic activity of the yolk-shell Au-CdS hybrid system with an analogous system containing an insulating intermediate layer of silicon oxide Au@SiO 2 -CdS [138]. No significant differences were observed when compared both photocatalysts under the same experimental conditions. For further explanation, several mechanisms were proposed.
The thermal activity of the rod was tentatively considered as one of the possible causes of the increase in activity, but the thermal evolution of the system for all photocatalysts was very similar. Since higher activity was achieved by using Au-coated plasmonic structures covered with SiO 2 , it was concluded that plasmon energy transfer or plasmon-induced RET (see Figure 10) was not the mechanism by which these systems increased their activity. The researchers determined that the main cause for this process was radiative relaxation of the SPR of the AuNRs, such as near-field enhancement and resonant photon scattering (see Figures 2 and 11c). More recent studies have concurred in finding higher activity for the yolk-shell type [34]. Yolk-shell type structures have the ability to generate a very large surface-to-volume ratio, increasing the amount of surface exposed for photocatalysis and decreasing the amount of bulk material to reduce charge recombination probabilities. In addition, plasmonic Au is capable of generating better charge separation as well as potential light scattering of the light inside the nanoparticle (see Figure 11c).
We have seen several examples of hybrid architectures, where not only the type of metal incorporated (Au, Pt) was important, but its arrangement with respect to the CdS semiconductor played a crucial role in enhancing the catalytic activity. Yolk-shell type structures provided a better optical response thanks to the light scattering generated in the internal part of this void structure. Dumbbell-like Au-Pt type bimetallic structures allow better migration and separation of charges, solving one of the most important problems of this type of semiconductor. It is important to highlight the development of CdS-based complex hybrid structures where a good design and configuration of the constructed architecture allows us to considerably improve the activity of these systems when it is compared with other less complex hybrid structures, such as bare CdS or core-shell architectures.

Conclusions
TiO 2 and ZnO assembled with plasmonic metals are still the most explored and developed heteronanostructures obtained using semiconductors. The search for novel alternatives that can expand and take advantage of the full-solar irradiation spectra has stimulated the search for novel alternatives that can provide additional features in terms of more efficient harvesting properties, more effective carrier transport and enhanced selectivities towards specific end-products. In this regard, the need to find newer and cleaner routes for energy production, the need to improve the efficiency of decontamination of harmful pollutants and the willingness to promote the revalorization of wastes or unwanted byproducts has boosted the research in the area of photocatalysis. Nowadays, the innovation in the design of smart combination of metal-semiconductors continue to be a very promising alternative. In this review, we presented some of the most promising and versatile alternative gold-semiconductor heterostructures. The main results highlighted throughout this revision of the most recent literature suggests an important morphology dependence in the final outcome and how less-conventional designs beyond core-shell configurations may pave the way for important breakthroughs in forthcoming years. Furthermore, the selection of anisotropic metals also represents an appealing strategy to maximize the virtues of plasmon excitation. Finally, we would like to make emphasis on the need to establish exquisite control and definition of these hetero-nanoarchitectures at the nanoscale level in order to establish a clear progress in biomedical applications, where size control and reproducibility are key for clinical translation.