**2. Metal/semiconductor hybrid nanocrystal synthesis**

According to the classical hetero epitaxial growth theory [8], when a secondary material (referred to as ''2'') has to be deposited over a preexisting seed substrate of a different material (denoted as ''1''), total Gibbs free surface energy change function ΔGs can be described as follows: Δ*G*s=*γ*<sup>1</sup> −*γ*<sup>2</sup> + *γ*1,2, where *γ*1 and *γ*2 are the surface energies of seed substrate and the deposited materials, respectively, the solid/solution interfacial energies of colloidal nanostruc‐ ture in the liquid medium, and *γ*1,2 is strain-related solid/solid interfacial energy that depends on the bonding strength and degree of crystallographic compatibility between materials 1 and 2. As shown in Figure 1, if material 2 exposes lower energy surfaces (*γ*2 < *γ*1) and/or attains good lattice matching with the material 1 (*γ*1,2 is small), then it is easy to get continuous and uniform core/shell nanostructure (Δ*G*s>0, Frank—van der Merwe (FM) growth mode). On the contrary, if material 2 is featured by higher energy surfaces (*γ*2 > *γ*1) and/or *γ*1,2 is high owing to the large lattice mismatch, then it will tend to deposit as a discontinuous island-like domain to be heterodimer structure (ΔGs <0, Volmer—Weber (VW)growth mode) or even separate from each other. Therefore, the metal/semiconductor hybrid nanocrystal synthesis should follow the mechanism of Figure 1. However, in colloidal phase, *γ*1 and *γ*2 would be strongly influenced by adhesion of solvent, capping ligands and precursors, thus changing *γ*1,2 too [9,10]. Considering of the significant impact of binding of organic stabilizers or other solution species to the surface energy terms and therefore altering the ultimate Δ*G*s, the colloidal phase synthesis will be potential for the control of large lattice mismatch-directed shape evolution and morphology control when choosing different solvent, capping ligands, and reactant precursors.

energy to drive chemical reactions since increasing energy demand and environmental pollution create a pressing need for clean and sustainable energy solution. The high efficient separation and collection of photoexcited electrons (e-) and holes (h+) are the key points to get high efficient photocatalysis applications. Hybrid nanocrystals composed of semiconductor and metal components are receiving extensive attention in recent years due to their high efficient separation of photoexcited electrons and holes and potentional photocatalysis applications [1,4–5]. Furthermore, hybrid nanocrystals composed of semiconductor and plasmonic metal components are receiving extensive attention. The simultaneous existence and coupling of localized surface plasmon resonance induced plasmon and excitons in semiconductors, as well as the synergistic interactions between the two components [1, 6–7].

This review focuses on recent research efforts to synthesize metal/semiconductor hybrid nanocrystals to understand and control the photocatalytic applications. First, we summarize the synthesis methods and recent presented metal/semiconductor morphologies, including heterodimer, core/shell, and yolk/shell. The metal clusters and nanocrystals deposition on semiconductor micro/nanosubstrates with well-defined crystal face exposure will be clarified into heterodimer part. The outline of this synthesis part will be the large lattice mismatchdirected interface, contact, and morphology evolution. For detailed instructions on each

Second, the recent upcoming photocatalysis applications and research progress of these hybrid nanocrystals will be reviewed, including the photocatalytic hydrogen evolution (water splitting), photoreduction of CO2, and other newly emerging potential photosynthesis applications of metal/semiconductor hybrid nanocrystals. Finally, we provide a summary and outlook on the future of this topic. From this review, we try to facilitate the understanding and further improvement of current and practical metal/semiconductor hybrid nanocrystals and

According to the classical hetero epitaxial growth theory [8], when a secondary material (referred to as ''2'') has to be deposited over a preexisting seed substrate of a different material (denoted as ''1''), total Gibbs free surface energy change function ΔGs can be described as follows: Δ*G*s=*γ*<sup>1</sup> −*γ*<sup>2</sup> + *γ*1,2, where *γ*1 and *γ*2 are the surface energies of seed substrate and the deposited materials, respectively, the solid/solution interfacial energies of colloidal nanostruc‐ ture in the liquid medium, and *γ*1,2 is strain-related solid/solid interfacial energy that depends on the bonding strength and degree of crystallographic compatibility between materials 1 and 2. As shown in Figure 1, if material 2 exposes lower energy surfaces (*γ*2 < *γ*1) and/or attains good lattice matching with the material 1 (*γ*1,2 is small), then it is easy to get continuous and uniform core/shell nanostructure (Δ*G*s>0, Frank—van der Merwe (FM) growth mode). On the contrary, if material 2 is featured by higher energy surfaces (*γ*2 > *γ*1) and/or *γ*1,2 is high owing to the large lattice mismatch, then it will tend to deposit as a discontinuous island-like

synthesis, the readers are referred to the corresponding literature.

296 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

**2. Metal/semiconductor hybrid nanocrystal synthesis**

photocatalysis applications.

**Figure 1.** General metal/semiconductor hybrid nanocrystal synthesis sketches by traditional heterogeneous epitaxy growth mechanism: (A) Franck–van der Merwe (FM) mode; (B) Volmer–Weber (VM) mode.

## **2.1. Metal/semiconductor heterodimers by direct heterogeneous deposition**

A broad family of metal/semiconductor hybrid nanocrystals has been obtained by accom‐ plishing direct heterogeneous nucleation and growth of one or more secondary material layers onto preformed metal NC seeds serving as starting "cores" [9,10]. Synthetic strategies aim, on one side, at inhibiting homogeneous self-nucleation of isolated NCs made of the shell material and, on the other side, at achieving size- and shape-mediated deposition of the shell beyond limitations imposed by misfit strain at the metal/semiconductor interface. Practical techniques to realize these objectives rely on the colloidal synthesis, the "active" surface of "core" and the slow-nucleation of another material to deposit on "core" homogeneously, depending on the inherent chemical accessibility of the "core" or "seeds" as well on the reactivity of the shell molecular precursors (generally nonaqueous) solution media. The regulation of the thermo‐ dynamics and kinetics of reactions, namely, the temperature and selection of suitable surfaceadhering organic ligands or surfactants and the optimal reactant injection rate all critically influence on the temporal evolution of the supersaturation degree.

To date, disparate combinations of metals, semiconductors, and oxides have been addressed by manipulating direct heterogeneous deposition pathways [11–25]. Here, we typically review most recent achievement in the field, the metal/semiconductor heterodimers (as scheme in Figure 1). Representative transmission electron microscopy (TEM) and high-resolution TEM examples of core–shell NCs can be found in Figure 2, respectively.

The one-pot synthesis of metal/semiconductor heterodimers mostly focus on the bifunctional heterodimers of nanoparticles, such as the conjugate of quantum dots (QDs) and magnetic nanoparticles reported by Gu et al. [20], the noble metal/metal oxide heterodimers by Wang et al. [21] and Wu et al. [22], and the bimagnetic FePt–iron oxide heterodimer nanocrystals by Figuerola et al. [23] and Cozzoli et al. [24] (see also Figure 2).

**Figure 2.** (A) FePt/CdS heterodimers by one-pot synthesis: (a) the scheme of synthesis process, (b) a TEM image, and (c) a high-resolution TEM image of FePt/CdS heterodimers. Adapted from Gu et al. [20] with permission; copyright American Chemical Society. (B) Seed-mediated high temperature growth of M/metal oxide heterodimers: (a) the scheme of synthesis, (b) 5–12 nm Au–Fe3O4, (c) 5–12 nm Ag-Fe3O4, and (d) 5–12 nm Pt–Fe3O4. Adapted from Wang et al. [21] with permission; copyright American Chemical Society. FePt/In2O3 heterodimers were adapted from Wu et al. [22] with permission; copyright American Chemical Society. (C) One-pot, two-step colloidal strategy to prepare FePt/ iron oxide heterodimer nanocrystals: (a) the scheme of synthesis method; (b) the TEM image of FePt/ Iron Oxide heter‐ odimers; and (c) the HRTEM image of FePt/ Iron Oxide heterodimer. Adapted from Figuerola et al. [23] and Cozzoli et al. [24] with permission; copyright American Chemical Society.

The Au/ CdSe or Au/CdS heterodimer nanocrystals, because of the noble metal and II-VI QDs, are potential for photocatalysis applications. Banin et al. used surface nucleation and growth of a second phase to get Au/CdSe nanorod (NR) heterodimers, the first time to realize siteselective growth of Au NPs on CdSe NRs tips (see also Figure 3A) [13, 25, 26]. Du et al. used similar way to directly deposit the Au NPs on CdS NRs to get heterodimers (see also Figure 3B) [27]. The as-obtained Au/CdS and Au/CdSe NR heterodimers promote the electron/hole separation and exhibit promising photocatalytic activity for the water-splitting reaction in photoelectrochemical cells and photodegradation applications.

To date, disparate combinations of metals, semiconductors, and oxides have been addressed by manipulating direct heterogeneous deposition pathways [11–25]. Here, we typically review most recent achievement in the field, the metal/semiconductor heterodimers (as scheme in Figure 1). Representative transmission electron microscopy (TEM) and high-resolution TEM

The one-pot synthesis of metal/semiconductor heterodimers mostly focus on the bifunctional heterodimers of nanoparticles, such as the conjugate of quantum dots (QDs) and magnetic nanoparticles reported by Gu et al. [20], the noble metal/metal oxide heterodimers by Wang et al. [21] and Wu et al. [22], and the bimagnetic FePt–iron oxide heterodimer nanocrystals by

**Figure 2.** (A) FePt/CdS heterodimers by one-pot synthesis: (a) the scheme of synthesis process, (b) a TEM image, and (c) a high-resolution TEM image of FePt/CdS heterodimers. Adapted from Gu et al. [20] with permission; copyright American Chemical Society. (B) Seed-mediated high temperature growth of M/metal oxide heterodimers: (a) the scheme of synthesis, (b) 5–12 nm Au–Fe3O4, (c) 5–12 nm Ag-Fe3O4, and (d) 5–12 nm Pt–Fe3O4. Adapted from Wang et al. [21] with permission; copyright American Chemical Society. FePt/In2O3 heterodimers were adapted from Wu et al. [22] with permission; copyright American Chemical Society. (C) One-pot, two-step colloidal strategy to prepare FePt/ iron oxide heterodimer nanocrystals: (a) the scheme of synthesis method; (b) the TEM image of FePt/ Iron Oxide heter‐ odimers; and (c) the HRTEM image of FePt/ Iron Oxide heterodimer. Adapted from Figuerola et al. [23] and Cozzoli et

The Au/ CdSe or Au/CdS heterodimer nanocrystals, because of the noble metal and II-VI QDs, are potential for photocatalysis applications. Banin et al. used surface nucleation and growth of a second phase to get Au/CdSe nanorod (NR) heterodimers, the first time to realize siteselective growth of Au NPs on CdSe NRs tips (see also Figure 3A) [13, 25, 26]. Du et al. used

examples of core–shell NCs can be found in Figure 2, respectively.

298 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

Figuerola et al. [23] and Cozzoli et al. [24] (see also Figure 2).

al. [24] with permission; copyright American Chemical Society.

**Figure 3.** (A) TEM images showing controlled growth of gold onto the tips of CdSe nanorods. (a, b) The size of the gold tips can be controlled by varying the amount of gold precursor added during growth. (c, d) HRTEM images of a single nanodumbbell (c) and a nanodumbbell tip (d); the CdSe lattice for the rod in the center and gold tips at the rod edges can be identified. (From [25, 26], reprinted with permission from the AAAS.) (B) (a) TEM image, (b) HRTEM image, and (c) HAADF-STEM image of Au/CdS NRs hybrid nanocrystals. (d) XRD patterns of hybrid nanocrystals of Au/CdS NRs. (e) HAADF-STEM images in which the size of Au NPs is ∼8 nm. (f) TEM images of Au/CdS hybrid nanocrystals in which the size of Au NPs is ∼3 nm. Adapted from Du et al. [27] with permission; copyright Wiley-VCH Verlag GmbH & Co. KGaA.

Colloidal metal/semiconductor hybrid nanoparticles contain multiple nanoscale domains fused together by solid-state interfaces. They represent an emerging class of multifunctional lab-on-a-particle architectures that underpin future advances in solar energy conversion, photocatalysis. Buck et al. reported that the known direct heterogeneous deposition could be applied in a predictable and stepwise manner to build complex hybrid nanoparticle architec‐ tures that include M–Pt–Fe3O4 (M=Au, Ag, Ni, Pd) heterotrimers, MxS–Au–Pt–Fe3O4 (M=Pb, Cu) heterotetramers, and higher-order oligomers based on the heterotrimeric Au–Pt–Fe3O4 building block [18]. This synthetic framework conceptually mimics the total synthesis ap‐ proach used by chemists to construct complex organic molecules (see Figure 4).

In other words, the advantage of direct heterogeneous deposition colloidal method is it could get heterodimers with high monodispersity. The disadvantages are as follows: (1) The metal part or semiconductor part could not be mediated in more larger size range, such as tens of nanometers to hundreds of nanometers due to the Lar Mer growth mechanism of high

**Figure 4.** (A–D) TEM images showing the stepwise direct heterogeneous deposition of linear nanoparticle heterote‐ tramers: (A) Pt nanoparticles, (B) Pt–Fe3O4 heterodimers, (C) Au–Pt–Fe3O4 heterotrimers, and (D) Cu9S5–Au–Pt–Fe3O4 heterotetramers. (E–H) TEM images and schematic representations showing a series of metal/semiconductor hetero‐ dimers (same synthetic conditions for all steps) aimed at understanding the site-selective deposition of Ag onto Pt– Fe3O4 heterodimers: (E) Ag grown off Fe3O4 nanoparticles, (F) Ag grown off Pt nanoparticles, (G) Ag grown indiscrim‐ inately off both Fe3O4 and Pt nanoparticles when both are present as a physical mixture, and (H) Ag grown exclusively off the Pt domain when Pt and Fe3O4 are directly attached as heterodimers. Reprinted from Buck et al. [18] with per‐ mission (Copyright of Nature group 2011, Macmillan Publishers Limited).

temperature organic phase synthesis. (2) The interface between metal and semiconductor is still not controlled well to be clear enough because of the size of two parts mostly still limited at <10 nm range. The lattice mismatch-induced stain energy here is usually too large to get more defects on the interface, which is not helpful for the highly efficient photoinduced electron/hole separation and the following photocatalysis applications.

## **2.2. Large lattice mismatch-directed shape evolution and morphology control**

The growth of monocrystalline semiconductor-based metal/semiconductor hybrid nanostruc‐ tures with modulated composition, morphology, and interface strain are the prerequisite for exploring their plasmon–exciton coupling, efficient electron/hole separation, and enhanced photocatalysis properties. As the schematic process in Figure 1, different from generally used one-pot epitaxial growth on performed metal nanoparticle seeds, Ouyang et al. and Zhang et al. unprecedentedly took two steps of facile chemical thermodynamics processes to maximally adjust the surface energies *γ*2, *γ*1,2, and then ΔGs to change their overgrowth modes from FM mode to VW mode gradually. Furthermore, Ouyang et al. and Zhang et al. unprecedentedly used the cation exchange initiated the topotactic in situ conversion from amorphous to be single-crystalline structure [28–33]. In this case, the more flexible size range (from smaller than 10 nm to tens of nanometers, to hundreds of nanometers, and to micrometer scale), morphol‐ ogy (isotropic and anisotropic), interface, and single-crystallinity of metal and semiconductor part could be tailored synergistically.

By controlling soft acid–base coordination reactions between molecular complexes and colloidal nanostructures, Zhang et al. and Ouyang et al. showed that chemical thermodynam‐ ics could drive nanoscale monocrystalline growth of the semiconductor shell on metal nanosubstrates and then enhanced light–matter-spin interactions in these judiciously engi‐ neered nanostructures could be achieved [28, 3].

temperature organic phase synthesis. (2) The interface between metal and semiconductor is still not controlled well to be clear enough because of the size of two parts mostly still limited at <10 nm range. The lattice mismatch-induced stain energy here is usually too large to get more defects on the interface, which is not helpful for the highly efficient photoinduced

**Figure 4.** (A–D) TEM images showing the stepwise direct heterogeneous deposition of linear nanoparticle heterote‐ tramers: (A) Pt nanoparticles, (B) Pt–Fe3O4 heterodimers, (C) Au–Pt–Fe3O4 heterotrimers, and (D) Cu9S5–Au–Pt–Fe3O4 heterotetramers. (E–H) TEM images and schematic representations showing a series of metal/semiconductor hetero‐ dimers (same synthetic conditions for all steps) aimed at understanding the site-selective deposition of Ag onto Pt– Fe3O4 heterodimers: (E) Ag grown off Fe3O4 nanoparticles, (F) Ag grown off Pt nanoparticles, (G) Ag grown indiscrim‐ inately off both Fe3O4 and Pt nanoparticles when both are present as a physical mixture, and (H) Ag grown exclusively off the Pt domain when Pt and Fe3O4 are directly attached as heterodimers. Reprinted from Buck et al. [18] with per‐

The growth of monocrystalline semiconductor-based metal/semiconductor hybrid nanostruc‐ tures with modulated composition, morphology, and interface strain are the prerequisite for exploring their plasmon–exciton coupling, efficient electron/hole separation, and enhanced photocatalysis properties. As the schematic process in Figure 1, different from generally used one-pot epitaxial growth on performed metal nanoparticle seeds, Ouyang et al. and Zhang et al. unprecedentedly took two steps of facile chemical thermodynamics processes to maximally adjust the surface energies *γ*2, *γ*1,2, and then ΔGs to change their overgrowth modes from FM mode to VW mode gradually. Furthermore, Ouyang et al. and Zhang et al. unprecedentedly used the cation exchange initiated the topotactic in situ conversion from amorphous to be single-crystalline structure [28–33]. In this case, the more flexible size range (from smaller than

electron/hole separation and the following photocatalysis applications.

mission (Copyright of Nature group 2011, Macmillan Publishers Limited).

300 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

**2.2. Large lattice mismatch-directed shape evolution and morphology control**

**Figure 5.** Schemes of large lattice mismatch-directed shape evolution and morphology control by Ouyang et al. and Zhang et al.: (A) Nonepitaxial growth process and mechanism of hybrid core–shell nanostructures with substantial lat‐ tice mismatches. From Zhang et al. [28], reprinted with permission from the AAAS. (B) The cation exchange reaction initiated by different phosphines or phosphites (R3P) and phosphine-initiated cation exchange for precisely tailoring composition and properties of metal/semiconductor nanostructures. (C) The schematic process of controllable structur‐ al symmetries in Au/CdX (X = S, Se, and Te) hybrid structures with large lattice mismatch by two steps of in situ chem‐ ical conversion. The a-Ag2X means amorphous Ag2X, c-Ag2X means crystalline Ag2X. Adapted from Gui et al. [31] and Zhao et al. [32] with permission; copyright Wiley-VCH Verlag GmbH & Co. KGaA.

First, as shown in Figure 5A, based on the Lewis acid–base reaction mechanism, where the entire nanostructure is spatially confined by an amorphous matrix, the monocrystalline growth of the semiconductor shell is fully directed by chemical thermodynamic properties of reactions within the matrix; the shell's lattice structure can be independent of that of the core NPs, thus circumventing the limitations imposed by epitaxial strategies. Starting from the core NPs, an overlayer of metal with soft Lewis acidity is grown onto the core. For all hybrid core– shell structures, we choose an Ag metal overlayer. The silver shells can be modified to form silver compound shells (Ag2X) with an amorphous structure, providing a crucial platform for the next chemical transformation stage, ultimately leading to monocrystalline growth. It has been demonstrated that nanoscale chemical transformations, such as cation exchange, represent a versatile route for converting one crystalline solid to another [34, 35]. They show that this process can be harnessed to drive the single-crystal growth by carefully controlling the thermodynamic properties of reaction (1).

Tributylphosphine (TBP) was selected because it is a soft base and can behave as a phasetransfer agent to transport metal ions (Mn+ ) to the surface of the core NPs by binding to free cations in solution. The high acid softness of Ag+ favors the exchange process between Ag+ in the amorphous matrix and Mn+ in solution as long as the softness of Mn+ is small enough to result in a positive ΔG.

Second, based on above research achievements, as shown in Figure 5B, Zhang et al. further studied the cation exchange reaction here. It has been explored that different phosphines could modulate the thermodynamic and kinetic parameters of the cation exchange reaction to synthesize complex semiconductor nanostructures [31]. Here, we take the examples of cation exchange between N+ in amorphous N2E (E means chalcogen) nanoparticles (NPs) and M2+ (such as Cd2+) ions in solution, as shown in reaction (2) and Figure 1. Besides TBP, many other phosphine choices have been studied to mediate the thermodynamics and kinetics of reaction (2). Initiated by trace phosphine (R3P), the crystallization, the morphology, and the composi‐ tion of metal/ME core/shell NCs have been tailored well.

The thermodynamics and kinetics of different phosphine and phosphite agents to synthesize semiconductor shell have been studied. The prerequisite to forward the cation exchange is the lone-pair electrons of P atom in phosphines or phosphites. Based on this, different π-accepting and σ-donating capabilities of them to M and N ions make reaction (2) to be exothermic (ΔG reaction < 0). Different phosphine coordinating to Ag+ and Cd2+ and their coordination abilities study further prove reaction (2) is exothermic in principle. Therefore, thermodynamics and kinetics of reaction (2) could be mediated. The stronger coordination ability of R3P to N+ than to M2+ ions enables in situ conversion of amorphous N2E nanoparticles to be ME NCs. Although many kinds of phosphine or phosphite are applicable in reaction (2) thermodynamically, the steric effect derived from carbonyl ligands, such as alkyl and aryl ligands, would influence the kinetics of reaction (2), and the crystallization and composition of produced ME NCs distinctly. They took the example of cation exchange from amorphous Ag2S nanostructure to singlecrystalline CdS nanostructure. The kinetic activity order here is PEt3 > P(MeO)3 > P(EtO)3 > P(n-Bu)3 > PPh3 > P(n-Oct)3 > P(PhMe)3 > P(PhOMe)3. This is almost consistent with their σ donation ability order and contrary to their π back-bonding order. In particular, the steric hindrance from carbonyl ligands would influence σ donation ability and then chemical kinetics distinctly. That is the reason why P(PhOMe)3 phosphines have low activities to reaction (2). These findings are concordant with Chad Tolman's classic phosphine ligands ordering in terms of their electron-donating ability and steric bulk. Besides preserving the original shape and size, phosphine-initiated cation exchange reactions show potential to precisely tune the crystallinity and composition of metal/semiconductor hybrid nanocrystals.

First, as shown in Figure 5A, based on the Lewis acid–base reaction mechanism, where the entire nanostructure is spatially confined by an amorphous matrix, the monocrystalline growth of the semiconductor shell is fully directed by chemical thermodynamic properties of reactions within the matrix; the shell's lattice structure can be independent of that of the core NPs, thus circumventing the limitations imposed by epitaxial strategies. Starting from the core NPs, an overlayer of metal with soft Lewis acidity is grown onto the core. For all hybrid core– shell structures, we choose an Ag metal overlayer. The silver shells can be modified to form silver compound shells (Ag2X) with an amorphous structure, providing a crucial platform for the next chemical transformation stage, ultimately leading to monocrystalline growth. It has been demonstrated that nanoscale chemical transformations, such as cation exchange, represent a versatile route for converting one crystalline solid to another [34, 35]. They show that this process can be harnessed to drive the single-crystal growth by carefully controlling

Tributylphosphine (TBP) was selected because it is a soft base and can behave as a phase-

the amorphous matrix and Mn+ in solution as long as the softness of Mn+ is small enough to

Second, based on above research achievements, as shown in Figure 5B, Zhang et al. further studied the cation exchange reaction here. It has been explored that different phosphines could modulate the thermodynamic and kinetic parameters of the cation exchange reaction to synthesize complex semiconductor nanostructures [31]. Here, we take the examples of cation

(such as Cd2+) ions in solution, as shown in reaction (2) and Figure 1. Besides TBP, many other phosphine choices have been studied to mediate the thermodynamics and kinetics of reaction (2). Initiated by trace phosphine (R3P), the crystallization, the morphology, and the composi‐

The thermodynamics and kinetics of different phosphine and phosphite agents to synthesize semiconductor shell have been studied. The prerequisite to forward the cation exchange is the lone-pair electrons of P atom in phosphines or phosphites. Based on this, different π-accepting and σ-donating capabilities of them to M and N ions make reaction (2) to be exothermic (ΔG

study further prove reaction (2) is exothermic in principle. Therefore, thermodynamics and kinetics of reaction (2) could be mediated. The stronger coordination ability of R3P to N+

to M2+ ions enables in situ conversion of amorphous N2E nanoparticles to be ME NCs. Although many kinds of phosphine or phosphite are applicable in reaction (2) thermodynamically, the steric effect derived from carbonyl ligands, such as alkyl and aryl ligands, would influence the kinetics of reaction (2), and the crystallization and composition of produced ME NCs distinctly. They took the example of cation exchange from amorphous Ag2S nanostructure to singlecrystalline CdS nanostructure. The kinetic activity order here is PEt3 > P(MeO)3 > P(EtO)3 > P(n-Bu)3 > PPh3 > P(n-Oct)3 > P(PhMe)3 > P(PhOMe)3. This is almost consistent with their σ donation ability order and contrary to their π back-bonding order. In particular, the steric hindrance

in amorphous N2E (E means chalcogen) nanoparticles (NPs) and M2+

) to the surface of the core NPs by binding to free

favors the exchange process between Ag+

and Cd2+ and their coordination abilities

in

than

the thermodynamic properties of reaction (1).

302 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

transfer agent to transport metal ions (Mn+

result in a positive ΔG.

exchange between N+

cations in solution. The high acid softness of Ag+

tion of metal/ME core/shell NCs have been tailored well.

reaction < 0). Different phosphine coordinating to Ag+

Third, as illustrated in Figure 5C, Zhang et al. further showed that the structural symmetry of such metal/semiconductor hybrid heterostructures can be finely tuned with controllable separation between metal and semiconductor components by taking advantage of chemical thermodynamics-directed colloidal strain tuning, from symmetric core–shell to asymmetric heterodimer gradually by in situ conversion of amorphous/crystalline Ag2X shell to be single crystalline CdX. Typically, nanoparticle/solution interfacial energies and heterointerfacial energy (due to the lattice mismatch) can be strongly influenced by adhesion of solvent, capping ligands and precursors in colloidal phase [9]. Here, Zhang et al. took a facile two-step approach to maximize these effects to precisely control gradual shape evolution from symmetric core/ shell to asymmetric heterodimer nanostructures, as shown in Figure 5C [32]. This approach is fundamentally different from generally used one-pot epitaxial overgrowth with metal nanoparticle seeds [11-25]. First, they started with concentric Au–Ag core/shell. The fact of close lattice feature between Au and Ag makes it possible to achieve precise control of core size, shell thickness, and monodispersity based on Frank-van der Merwe growth mode [9]. Moreover, the higher reactivity of silver metal nanostructures than gold enables the in situ conversion of Ag shell to silver chalcogenide (Ag2X) without modification of the Au metal core. Depending on reaction condition (such as temperature and reactants), the crystallinity of Ag2X can be controlled from amorphous to partial crystalline, thus leading to different lattice mismatch between Au core and Ag2X shell. As a result, different interfacial lattice strain between Au and Ag2X can be induced to initiate phase separation to a certain extent. Different from the long time aging-induced elemental Au diffusion in Au–Ag2X core/shell nanocrystals via Ostwald ripening, the Au–Ag2X here was used as intermediate precursor to carry out next step quickly to get Au/CdX heteronanocrystals. Second, they utilized cation exchange process based on the theory of hard-soft acid and base to realize the in situ conversion of Ag2X to monocrystalline CdX [28]. The cation exchange-induced cation rearrangement and crystallog‐ raphy texture transformation provide further impetus to shift CdX shell gradually because of the larger strain energy between Au and monocrystalline CdX. In general, the higher crystal‐ lization of Ag2X shell as well as the higher reaction temperature of cation exchange reaction leads to larger phase separation between Au and CdX to reduce interfacial and grain boundary energies. These tunable relocations of gold to CdX in quantum size region would enable maximum degree of tunability of their optoelectronic properties coupling.

### **2.3. Metal/semiconductor core/shell nanocrystals**

The growth of single-crystal semiconductor-based heterostructures with modulated composition is a prerequisite for exploring fundamental nanoscale semiconductor physics [36] and can offer technological devices with optimum characteristics, including enhanced optical properties with high quantum yields [37], engineered electronic band gaps [38–40], and various solid-state optoelectronic properties [41–43]. Unintentional crystalline imperfec‐ tions (such as polycrystallinity, dislocations, and other structural defects) lead to perform‐ ance degradation or even premature failure of devices. For example, although the optical quality of semiconductor CdSe nanoparticles (NPs) could be improved by an overlayer of epitaxially grown CdS or ZnS, problems appear once the shell thickness becomes larger than the critical layer thickness (about two monolayers) due to the existence of straininduced defects [38, 44, 45].

Current methods that achieve high-quality monocrystalline heterostructures are all based on epitaxial growth, as shown in Figure 1, which requires moderate lattice mismatches (<2%) between the two different materials. This lattice-matching constraint is a severe obstacle, particularly for growth of core–shell nanostructures with (quasi)spherical core NPs with highly curved surfaces that present many different crystallographic facets [46]. In addition to such lattice-matching requirements, the issues related to differences in crystal structure, bonding, and other properties have been found to inhibit epitaxial growth of dissimilar hybrid materials such as monocrystalline semiconductors on metals [47]. Attempts to use epitaxy to achieve hybrid metal core–semiconductor shell nanostructures have been unsuccessful, resulting in either polycrystalline semiconductor shells or anisotropic structures with segre‐ gation of the core and shell. This is only because as Figure 1 schemed, under large lattice mismatch (>40%), the semiconductor nanostructures would grow as small island-shaped NPs on metal core surface to decrease the surface strain energy. In this case, the Volmer—Weber (VW) mode growth would happen. Thus, based on these direct heterogeneous deposition methods, such as Figures 2–4 demonstrated, the polycrystalline semiconductor shell formed finally. As Figure 6 demonstrated, the Klimov, Talapin, and Wang groups have tried these kinds of method to prepare Au–PbS, Co–CdSe, Au–CdS, and Au–ZnS core/shell hybrid nanocrystals. Although they are highly monodispersed and could self-assemble into super‐ lattice, the polycrystalline shell and too many defects at the interface limited their usefulness, especially in photocatalysis applications [48–53].

The photocatalysis application in the integration of semiconductor nanocrystals with noble metals uses the localized surface plasmon resonance (LSPR) effect of the metal component to enhance the light absorption, charge separation, and facilitate the absorbed light energy transfer from the metal to the semiconductor component for technologically important lightinvolved applications. Abundant studies have been devoted to the controllable productions of various hybrid nanocrystals. Heterostructure preparation is the basis for any application of semiconductor/noble metal hybrid nanocrystals. The crystalline imperfections of each component and the defects on the interface lead to the performance degradation or lost. Currently, high-quality monocrystalline heterostructures are usually produced by thermal decomposition epitaxial growth or through cation-exchange processes. The reported studies of monocrystalline hybrid nanocrystal synthesis are mainly based on epitaxial growth, which requires a moderate lattice mismatches between the different components. The latticemismatching constraint seriously limits the application of this method, particularly for growth of core/shell hybrid nanocrystals with highly curved surfaces. Therefore, it is a great challenge to synthesize the large lattice-mismatching semiconductor/metal hybrid nanocrystals with monocrystalline compounds and clear interface. Cation exchange reaction is a successful Metal/Semiconductor Hybrid Nanocrystals and Synergistic Photocatalysis Applications http://dx.doi.org/10.5772/61888 305

optical properties with high quantum yields [37], engineered electronic band gaps [38–40], and various solid-state optoelectronic properties [41–43]. Unintentional crystalline imperfec‐ tions (such as polycrystallinity, dislocations, and other structural defects) lead to perform‐ ance degradation or even premature failure of devices. For example, although the optical quality of semiconductor CdSe nanoparticles (NPs) could be improved by an overlayer of epitaxially grown CdS or ZnS, problems appear once the shell thickness becomes larger than the critical layer thickness (about two monolayers) due to the existence of strain-

Current methods that achieve high-quality monocrystalline heterostructures are all based on epitaxial growth, as shown in Figure 1, which requires moderate lattice mismatches (<2%) between the two different materials. This lattice-matching constraint is a severe obstacle, particularly for growth of core–shell nanostructures with (quasi)spherical core NPs with highly curved surfaces that present many different crystallographic facets [46]. In addition to such lattice-matching requirements, the issues related to differences in crystal structure, bonding, and other properties have been found to inhibit epitaxial growth of dissimilar hybrid materials such as monocrystalline semiconductors on metals [47]. Attempts to use epitaxy to achieve hybrid metal core–semiconductor shell nanostructures have been unsuccessful, resulting in either polycrystalline semiconductor shells or anisotropic structures with segre‐ gation of the core and shell. This is only because as Figure 1 schemed, under large lattice mismatch (>40%), the semiconductor nanostructures would grow as small island-shaped NPs on metal core surface to decrease the surface strain energy. In this case, the Volmer—Weber (VW) mode growth would happen. Thus, based on these direct heterogeneous deposition methods, such as Figures 2–4 demonstrated, the polycrystalline semiconductor shell formed finally. As Figure 6 demonstrated, the Klimov, Talapin, and Wang groups have tried these kinds of method to prepare Au–PbS, Co–CdSe, Au–CdS, and Au–ZnS core/shell hybrid nanocrystals. Although they are highly monodispersed and could self-assemble into super‐ lattice, the polycrystalline shell and too many defects at the interface limited their usefulness,

The photocatalysis application in the integration of semiconductor nanocrystals with noble metals uses the localized surface plasmon resonance (LSPR) effect of the metal component to enhance the light absorption, charge separation, and facilitate the absorbed light energy transfer from the metal to the semiconductor component for technologically important lightinvolved applications. Abundant studies have been devoted to the controllable productions of various hybrid nanocrystals. Heterostructure preparation is the basis for any application of semiconductor/noble metal hybrid nanocrystals. The crystalline imperfections of each component and the defects on the interface lead to the performance degradation or lost. Currently, high-quality monocrystalline heterostructures are usually produced by thermal decomposition epitaxial growth or through cation-exchange processes. The reported studies of monocrystalline hybrid nanocrystal synthesis are mainly based on epitaxial growth, which requires a moderate lattice mismatches between the different components. The latticemismatching constraint seriously limits the application of this method, particularly for growth of core/shell hybrid nanocrystals with highly curved surfaces. Therefore, it is a great challenge to synthesize the large lattice-mismatching semiconductor/metal hybrid nanocrystals with monocrystalline compounds and clear interface. Cation exchange reaction is a successful

induced defects [38, 44, 45].

304 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

especially in photocatalysis applications [48–53].

**Figure 6.** The metal/semiconductor core/shell NCs with polycrystalline semiconductor shell by direct heterogeneous deposition method. (A) Au–PbS core/shell NCs. (B) Co–CdSe core/shell NCs. Adapted from Lee et al. [48] and Kim et al. [49], respectively, with permission; copyright American Chemical Society. (C) Au–TiO2 core/shell NCs. Adapted from Fang et al. [53] with permission; copyright Royal Society of Chemistry. (D) Au–CdS and Au–ZnS core/shell NCs. Adapted from Sun et al. [54] with permission; copyright Wiley-VCH Verlag GmbH & Co. KGaA.

synthetic method to preparation hybrid large lattice-mismatching heterodimer and core/shell nanocrystals with monocrystalline compounds and clear interface, which cannot be obtained by conventional epitaxial techniques [48–54]. Based on the Lewis acid–base reaction between molecular complexes and colloidal nanocrystals, the amorphous nanostructures can be transferred into monocrystalline compounds. Hence, by combining the sulfidation, seleniza‐ tion, or hyperoxidation of silver with cation exchange, other monocrystalline metal chalcoge‐ nide, selenide, or oxide nanostructures can been nonepitaxially grown on the large latticemismatching metal surface [28–33]. For the catalytic applications of semiconductor/noble metal hybrid nanocrystals, the key challenges are to obtain a clear semiconductor–metal interface and precise control over the size and shape of the heterostructures. Via cation exchange reaction, a series of semiconductor/metal hybrid nanocrystals can be obtained from concentric core–shell to nonconcentric heterodimer with precisely controlled separation and clear interface between the semiconductor and noble metal compounds. The symmetry evolution of semiconductor/metal hybrid nanocrystal has led to novel control of light absorp‐ tion and photocatalytic activity, which indicates the advantage of cation exchange nonepitaxial growth and the importance of nanoscale interface control. For the phosphine-initiated cation exchange, different phosphines have been used to modulate the thermodynamic and kinetic process of the cation exchange reaction in semiconductor nanocrystal synthesis. By using different phosphines, the crystallinity, composition, morphology, and related properties of semiconductors can be precisely controlled.

Different from the reports in Figure 6, Ouyang et al. and Zhang et al. used the nonepitaxial growth scheme (Figure 5A) to get metal–semiconductor core/shell NCs with single crystalline semiconductor shell. The size of metal, semiconductor shell, and the composition of semicon‐ ductor shell could be precisely controlled (Figures 7–9) [28].

**Figure 7.** Au–CdS core–shell nanostructures with monocrystalline shell. (A) Typical TEM image showing uniform core–shell nanostructures. Scale bar, 20 nm. (B–E) High-resolution TEM images of core–shell nanostructures from (A). Whereas Au core NPs can manifest monocrystalline (B), single-fold twin (C), fivefold twin (D), and multiple-twin (E) lattice structures, all CdS shells are monocrystalline. The red lines highlight the lattice orientations within the Au core NPs. Scale bar, 5 nm. (F) XRD pattern of Au–CdS sample shown in panel A. Bulk Au [red solid lines, Joint Committee on Powder Diffraction Standards (JCPDS) #04-0784] and wurtzite CdS (blue solid lines, JCPDS #41-1049) are also pro‐ vided for reference and comparison. (Inset) A ball-and-stick molecular model of Au–CdS, illustrating a cubic core and wurtzite shell. (G–J) Angle-dependent high-resolution TEM characterization. The sample depicted has a larger shell thickness than the one in panel A to emphasize the extremely high-quality crystallinity of the shell. The CdS shell shows perfect monocrystalline features without detectable structural defects under a different viewing angle. Scale bar, 5 nm. From Zhang et al. [28], reprinted with permission from the AAAS.

clear interface between the semiconductor and noble metal compounds. The symmetry evolution of semiconductor/metal hybrid nanocrystal has led to novel control of light absorp‐ tion and photocatalytic activity, which indicates the advantage of cation exchange nonepitaxial growth and the importance of nanoscale interface control. For the phosphine-initiated cation exchange, different phosphines have been used to modulate the thermodynamic and kinetic process of the cation exchange reaction in semiconductor nanocrystal synthesis. By using different phosphines, the crystallinity, composition, morphology, and related properties of

Different from the reports in Figure 6, Ouyang et al. and Zhang et al. used the nonepitaxial growth scheme (Figure 5A) to get metal–semiconductor core/shell NCs with single crystalline semiconductor shell. The size of metal, semiconductor shell, and the composition of semicon‐

**Figure 7.** Au–CdS core–shell nanostructures with monocrystalline shell. (A) Typical TEM image showing uniform core–shell nanostructures. Scale bar, 20 nm. (B–E) High-resolution TEM images of core–shell nanostructures from (A). Whereas Au core NPs can manifest monocrystalline (B), single-fold twin (C), fivefold twin (D), and multiple-twin (E) lattice structures, all CdS shells are monocrystalline. The red lines highlight the lattice orientations within the Au core NPs. Scale bar, 5 nm. (F) XRD pattern of Au–CdS sample shown in panel A. Bulk Au [red solid lines, Joint Committee on Powder Diffraction Standards (JCPDS) #04-0784] and wurtzite CdS (blue solid lines, JCPDS #41-1049) are also pro‐ vided for reference and comparison. (Inset) A ball-and-stick molecular model of Au–CdS, illustrating a cubic core and wurtzite shell. (G–J) Angle-dependent high-resolution TEM characterization. The sample depicted has a larger shell thickness than the one in panel A to emphasize the extremely high-quality crystallinity of the shell. The CdS shell shows perfect monocrystalline features without detectable structural defects under a different viewing angle. Scale bar,

semiconductors can be precisely controlled.

306 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

ductor shell could be precisely controlled (Figures 7–9) [28].

5 nm. From Zhang et al. [28], reprinted with permission from the AAAS.

**Figure 8.** Large-scale (left) and high-resolution (right) TEM images of different hybrid core–shell nanostructures with various combinations of the core and shell components. All semiconductor shells show monocrystalline features. Scale bars for large-scale and high-resolution TEM images are 20 and 5 nm, respectively. (A) Au–CdSe; (B) Au–CdTe; (C) FePt–CdS; (D) Au–PbS; (E) Au–ZnS; and (F) Pt–CdS. From Zhang et al. [28], reprinted with permission from the AAAS.

**Figure 9.** Growth of complex hybrid core–shell nanostructures with tailored structures and compositions of the mono‐ crystalline shells. (A–C) Control of the monocrystalline cation species within the shell: the case of Au–(CdS+PbS). (A) Schematic of the growth procedure. (B) Large-scale TEM image. Scale bar, 20 nm. (C) (Top) High-resolution TEM im‐ age. Blue and green dashed arc curves highlight the monocrystalline CdS and PbS regimes, respectively. CdS and PbS manifest distinct lattice planes that can be assigned to (100) and (220), respectively. Scale bar, 5 nm. (Bottom) Singleparticle EDS measurements in the CdS and PbS regimes. Peaks from Cd, Pb, and S elements are highlighted. (D–F) Control of the monocrystalline anion species within the shell: the case of Au–CdS1–aSea. (D) Schematic growth proce‐ dure. (E) Large-scale TEM image. Scale bar, 20 nm. (Inset) High-resolution TEM image showing the monocrystalline alloy shell. Scale bar, 5 nm. (F) XRD patterns highlighting lattice evolution from CdSe to CdS with different ratio a. From Zhang et al. [28], reprinted with permission from the AAAS.

Based on the strategy in Figure 5B, Zhang et al. use air-stable PPh3 for the first time to initiate cation exchange reaction (2). Figure 10 demonstrates the Au–CdS, Pt–CdS core/shell NCs preparation by PPh3 initiated reaction (2). Following the nonepitaxial growth process we published before [28] after cation exchange from amorphous Ag2S shell, Au–CdS and Pt–CdS NCs could preserve thick monocrystalline CdS shell. LRTEM and HRTEM images in Figure 10A–D confirmed their good crystallization. Especially, as shown in Figure 10D, despite of anisotropic shape of Pt nanocube, the thick CdS shell has good single-crystallinity. PPh3 further facilitates the monocrystalline engineering to break through critical layer thickness limit of heteroepitaxy. PPh3 is "green" choice than TBP because it is air-stable enough to enable reaction (2) under more flexibly conditions, such as higher temperature and longer time to facilitate versatile ME NCs crystallization. Moreover, besides crystallization tailoring, PPh3 could initiate cation exchange to get more complex heterostructures with precise compositional tailoring. Figure 11A and B showed the LRTEM and HRTEM images of as-prepared Au– CdS1-xSex core/shell nanocrystals with ternary single-crystal alloys shell. The S-to-Se ratio could be tailored (CdS0.58Se0.42 and CdS0.45Se0.55, obtained by EDS elemental analysis) to engineer their band gaps. The consistent shift of their powder XRD peaks from CdS1-xSex shell (Figure 11C) and the distinct colloid color changing (insert in Figure 3D) confirmed the homogeneous composition modulation. The strong visible light absorption (550–700 nm) (Figure 11D) due to the surface plasmon resonance (SPR) and exciton coupling in as-prepared Au–CdS1-xSex NCs indicated their potential photocatalysis and photovoltaic applications [55].

**Figure 10.** Core/shell metal–semiconductor NCs with thick monocrystalline CdS shell synthesized by PPh3 initializa‐ tion under large lattice mismatches: LRTEM and HRTEM images of Au–CdS NCs (A) and Pt–CdS NCs with Pt nano‐ cube core (B-D). Adapted from Gui et al. [31] with permission; copyright Wiley-VCH Verlag GmbH & Co. KGaA)

Recently, through the strategy in Figure 5C, Zhang et al. have demonstrated evolution of relative position of Au and CdX in Au–CdX (X means S, Se, and Te) from symmetric to asymmetric configuration (Figures 12 and 13) [32].

Metal/Semiconductor Hybrid Nanocrystals and Synergistic Photocatalysis Applications http://dx.doi.org/10.5772/61888 309

Based on the strategy in Figure 5B, Zhang et al. use air-stable PPh3 for the first time to initiate cation exchange reaction (2). Figure 10 demonstrates the Au–CdS, Pt–CdS core/shell NCs preparation by PPh3 initiated reaction (2). Following the nonepitaxial growth process we published before [28] after cation exchange from amorphous Ag2S shell, Au–CdS and Pt–CdS NCs could preserve thick monocrystalline CdS shell. LRTEM and HRTEM images in Figure 10A–D confirmed their good crystallization. Especially, as shown in Figure 10D, despite of anisotropic shape of Pt nanocube, the thick CdS shell has good single-crystallinity. PPh3 further facilitates the monocrystalline engineering to break through critical layer thickness limit of heteroepitaxy. PPh3 is "green" choice than TBP because it is air-stable enough to enable reaction (2) under more flexibly conditions, such as higher temperature and longer time to facilitate versatile ME NCs crystallization. Moreover, besides crystallization tailoring, PPh3 could initiate cation exchange to get more complex heterostructures with precise compositional tailoring. Figure 11A and B showed the LRTEM and HRTEM images of as-prepared Au– CdS1-xSex core/shell nanocrystals with ternary single-crystal alloys shell. The S-to-Se ratio could be tailored (CdS0.58Se0.42 and CdS0.45Se0.55, obtained by EDS elemental analysis) to engineer their band gaps. The consistent shift of their powder XRD peaks from CdS1-xSex shell (Figure 11C) and the distinct colloid color changing (insert in Figure 3D) confirmed the homogeneous composition modulation. The strong visible light absorption (550–700 nm) (Figure 11D) due to the surface plasmon resonance (SPR) and exciton coupling in as-prepared Au–CdS1-xSex

308 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

NCs indicated their potential photocatalysis and photovoltaic applications [55].

**Figure 10.** Core/shell metal–semiconductor NCs with thick monocrystalline CdS shell synthesized by PPh3 initializa‐ tion under large lattice mismatches: LRTEM and HRTEM images of Au–CdS NCs (A) and Pt–CdS NCs with Pt nano‐ cube core (B-D). Adapted from Gui et al. [31] with permission; copyright Wiley-VCH Verlag GmbH & Co. KGaA)

Recently, through the strategy in Figure 5C, Zhang et al. have demonstrated evolution of relative position of Au and CdX in Au–CdX (X means S, Se, and Te) from symmetric to

asymmetric configuration (Figures 12 and 13) [32].

**Figure 11.** Au–CdS1–xSex NCs synthesized by PPh3 initialization. (A, B) LRTEM and HRTEM images of prepared Au– CdS0.58Se0.42 NCs and Au–CdS0.45Se0.55 NCs. (C) XRD patterns comparison of them. (D) UV-Vis extinction spectra com‐ parison of them. Inserted pictures showed their colloid color. Adapted from Gui et al. [31] with permission; copyright Wiley-VCH Verlag GmbH & Co. KGaA)

**Figure 12.** TEM characterizations on controllable structural symmetry from core/shell to heterodimers. (A, B) Au–Ag2S with amorphous (A) and crystalline Ag2S (B) shells. The inserts are HRTEM images of a-Ag2S (A) and c-Ag2S shell (B); scale bar, 5 nm. Red solid lines are guides for the eye, distinguishing the Au core and c-Ag2S shell boundaries, respec‐ tively. (C–F) Au/CdS hybrid nanostructure with controllable structural symmetry. (C) Concentric core/shell. (D) Non‐ concentric core/shell. (E, F) Heterodimers. The inset diagrams highlight the phase separation-induced Au/CdS morphologies. Scale bar: 20 nm. (G–J) HRTEM images highlight the shape separations of panels C–F, respectively. Scale bar: 2.5 nm. Adapted from Zhao et al. [32] with permission; copyright Wiley-VCH Verlag GmbH & Co. KGaA.

**Figure 13.** Large-scale TEM images of shape evolutions in Au/CdSe and Au/CdTe hybrid nanostructures with ∼5 nm sized Au. (A) Concentric core/shell of Au/CdSe; (B, C) heterodimer of Au/ CdSe; (D) heterodimer of Au/CdTe. The in‐ set diagrams highlight the phase separation-induced Au/CdSe and Au/CdTe morphologies. Scale bar: 20 nm. Adapted from Zhao et al. [32] with permission; copyright Wiley-VCH Verlag GmbH & Co. KGaA.

Based on Figure 5, the metal/semiconductor core/shell NCs can further lead to fine tuning of plasmon–exciton coupling, different hydrogen photocatalytic performance, and enhanced photovoltaic, electrical properties.
