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Comprehensive Study of All-Solid-State Z-Scheme Photocatalytic Systems of ZnO/Pt/CdZnS

We have investigated a Z-scheme based on a ZnO/Pt/CdZnS photocatalyst, active in the presence of a complex medium composed of acetic acid and benzyl alcohol, the effects of which on the catalyst stability and performance are studied. Transmission electron microscopy images showed uniformly dispersed sub-nanometer Pt particles. Inductively coupled plasma and X-ray photoelectron spectroscopy analyses suggested that Pt is sandwiched between ZnO and CdZnS. An apparent quantum yield (AQY) of 34% was obtained over the [ZnO]4/1 wt %Pt/CdZnS system at 360 nm, 2.5-fold higher than that of 1%Pt/CdZnS (14%). Furthermore, an AQY of 16% was observed using [ZnO]4/1 wt %Pt/CdZnS, which was comparable to that of 1 wt %Pt/CdZnS (10%) at 460 nm. On the basis of these results, we proposed a charge transfer mechanism, which was confirmed through femtosecond transient absorption spectroscopy. Finally, we identified the two main factors that affected the stability of the catalyst, which were the sacrificial reagent and the acidic pH.


INTRODUCTION
Photocatalytic water splitting for hydrogen production employing semiconductor-based photocatalysts is considered to be the "Holy Grail" of solar energy conversion and storage. 1 However, this photocatalytic process is largely limited by four factors: (1) stability with respect to photocorrosion, (2) a narrow band gap of the photocatalysts to absorb more light and suitable band edges/maximum−minimum, which match with the redox potential/level of water, (3) environmental friendliness, and (4) easy access and cost-effectiveness. These four conditions are often mutually exclusive for practical applications. Furthermore, fast charge recombination of the photocatalysts seriously reduces the solar-to-fuel conversion efficiency. 2 Therefore, it is almost impossible to find a single semiconductor photocatalyst that satisfies all of these requirements. For instance, ZnO is the preferred material for the photoelectrode in hydrogen production. However, it has an energy band gap of about ∼3.2 eV; therefore, it mostly absorbs the ultraviolet portion of the solar spectrum, which is less than 5% of total solar energy. As an alternative, cadmium sulfide-based narrow band gap semiconductors, such as Cd x Zn 1−x S, also have been extensively investigated due to their band gap tunability, and better charge mobility compare to those of CdS. 3 The photocatalytic activity of various morphologies of Cd x Zn 1−x S, including nanoparticles, 4 nanotwins, 5 nanoflowers, 6 and volvox-like structures, 7 was investigated. Despite the suitable band gap and high quantum efficiency, these types of CdS-based photocatalysts suffer from catalytic decay even with sacrificial reagents because the sulfide is easily oxidized to elemental sulfur by photogenerated holes. Therefore, effective removal of holes from CdS-based light absorbers is the key for long-term stability of this type of photocatalysts. Nevertheless, these limitations of CdS-based single semiconductors can be mitigated by designing a heterogeneous p−n junction system. A p−n junction is a boundary or interface between p-type and n-type semiconductors, which improves charge separation through band bending. However, in this process, the redox potentials of electrons and holes are reduced; 8 as a result, it is impossible to obtain a system that has higher charge separation as well as higher or suitable redox potentials simultaneously. Therefore, it is necessary to explore new types of multijunction photocatalysts that can overcome the drawbacks of a p−n junction system. The actual solution for this problem can be found in nature; for instance, plants use a Z-scheme photocatalytic process to convert H 2 O and CO 2 into carbohydrate and O 2 , which is a two-step photoexcitation process. 1,9 In step 1, the highest occupied molecular orbital (HOMO) electrons of an oxygen-evolving complex (photosystem II, PS II) are excited to the lowest unoccupied molecular orbital (LUMO) of PS II under solar light and then the electrons in LUMO of PS II are transferred to the HOMO of ferredoxin-NAPD reductase (PS I), through the electron mediator. In step 2, the HOMO (photosystem I, PS I) electrons of PS I are excited to its LUMO (PS I). As a result, the photogenerated electrons and holes are left in the LUMO (PS I) and the HOMO (PS II), respectively. Finally, the photogenerated electrons in the LUMO (PS I) are used to reduce CO 2 , whereas the photogenerated holes in the HOMO of PS II are used to oxidize H 2 O. In light of that, artificial Zscheme-based photocatalytic systems have been widely investigated. 10 The nature-mimicked Z-scheme photocatalytic system not only shows high redox potentials but also has a slow charge recombination rate in addition to enhanced light absorption. 9b Despite their limitations as mentioned above as single semiconductors, ZnO and Cd x Zn 1−x S together can form an efficient Z-scheme system due to their compatible lattice structures and suitable band gap alignment, which lead to excellent charge migration through the interface and consequently facilitate the effective interband charge transfer from Cd x Zn 1−x S to ZnO. 11 12 However, to the best of our knowledge, there are no systematic studies on this particular photocatalyst in terms of the charge transfer mechanism and stability due to the complexity of the system. Therefore, we used ZnO/Pt/CdZnS as a model system to perform a detailed analysis on the Z-scheme.
In this work, we first synthesized and characterized ZnO/Pt/ Cd 0.8 Zn 0.2 S and its two main components, Pt/ZnO and Pt/ Cd 0.8 Zn 0.2 S, and performed detailed analysis of their contributions to the photocatalytic activity of ZnO/Pt/Cd 0.8 Zn 0.2 S. Then, we studied the effect of Pt content on the overall photoactivities of ZnO/Pt/Cd 0.8 Zn 0.2 S and evaluated the factors that affect the stability of this type of photocatalysts. In addition, we also investigated charge carrier dynamics using femtosecond transient absorption (TA) spectroscopy. Finally, we proposed the possible charge transfer mechanism and verified experimentally the proposed Z-scheme.

RESULTS AND DISCUSSION
2.1. Characterization. The X-ray diffraction (XRD) patterns of various catalysts are depicted in Figure 1. The ZnO/Pt/CdZnS hybrid system shows a clear mixture of hexagonal ZnO and cubic Cd 0.8 Zn 0.2 S phases. The three diffraction peaks of all of the Cd 0.8 Zn 0.2 S-based materials with 2θ; values of 26.9, 44.6, and 52.8°represent the (111), (220), and (311) 13 planes of the zinc-blend phase, respectively. All peaks are broad due to the high content of Cd in the solid solution, which causes a significant increase of unit cell parameters because of large differences of their ion radius (Cd 2+ (0.78 Å) and Zn 2+ (0.68 Å)) 14 and small particle sizes (<10 nm) of each component, as shown by transmission electron microscopy (TEM) images ( Figure 3). Because the solid solution of Cd 0.8 Zn 0.2 S was synthesized at low temperature (60°C), the main crystal composition is a zinc-blend cubic phase (Figure 1). Similar results were reported by Chen et al., 14 where a minor wurtzite phase starts to appear once the Cd content is higher than 48% in Cd x Zn 1−x S solid solution when the system was synthesized at low temperature. The two small shoulder peaks at 25.3 and 28.5°in Figure 1, corresponding to the (100) and (101) crystal planes of wurtzite, indicate a mixed phase in the solid solutions (Cd 0.8 Zn 0.2 S and 1%Pt/Cd 0.8 Zn 0.2 S), but it is not obvious in the case of the hybrid system ([ZnO] 4 /1%Pt/Cd 0.8 Zn 0.2 S). However, high-resolution TEM images of [ZnO] 4 /0.1%Pt/ Cd 0.8 Zn 0.2 S ( Figure 3) show some wurtzite hexagonal phases of Cd 0.8 Zn 0.2 S. Therefore, one can conclude that both the solid solution and the hybrid system are formed by mixed phases but the dominant phase is the zinc-blended phase. Because lattice parameters of cubic Cd x Zn 1−x S solid solutions follow Vergard's law, 15 which describes a linear relationship between x and lattice parameters, the Cd contents (x) were calculated according to eq 1.
Here, a(Cd x Zn 1−x S) is the lattice parameter of the Cd x Zn 1−x S solid solution, a(CdS) and a(ZnS) are the lattice parameters of CdS and ZnS, respectively, and x is the molar fraction of Cd in the solution. The Cd contents in the solid solutions were calculated to be 0.78 ± 0.02. Because the error is less than 3%, the Cd content is assumed to be the same in all of the cases as 0.8. All of the characteristic peaks of Cd 0.8 Zn 0.2 S, whether alone or with ZnO, show a clear shift toward a higher angle, indicating the formation of a solid solution. The crystal sizes of ZnO plus Cd 0.8 Zn 0.2 S both in separate and mixed phases in the Z-scheme are calculated according to Scherrer's equation. The results show that the crystal sizes of ZnO and Cd 0.8 Zn 0.2 S are the same in both conditions, which are around 10 and 6 nm, respectively.
The optical band gaps of the Cd 0.8 Zn 0.2 S-based systems were determined by the Tauc plot; 16 the optical band gap (E g ) can be obtained in the wavelength range 450−750 nm from the straight line plot between (αhυ) 1/2 (α is the absorption coefficient) and photon energy (hυ). There are slight differences in the E g , as shown in Figure 2, which could be associated with slight differences of Cd concentration in the solid solution during sample preparation. The E g of the Cd 0.8 Zn 0.2 S-based hybrid systems were found to be between 2.2 and 2.3 eV (Figure 2, inset). Figure 3 shows the TEM images of an overall view of asprepared [ZnO] 4 /Pt/Cd 0.8 Zn 0.2 S, which is constituted by small

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Article particles, quite homogeneous in size. An enlargement of the area inside the square in Figure 3a shows a very homogeneous distribution of particles with an average particle size of 5 nm (Cd 0.8 Zn 0.2 S), which is slightly smaller than that estimated by XRD ( Figure 1). It is interesting to note, however, that particles with different crystallinities are seen, ranging from almost amorphous to very well faceted ones. This is particularly visible in the insets of the Fourier transform (FT) image of the particle in Figure 3b. The spots at 3.36 and 3.16 Å correspond well to the (002) and (101) crystallographic planes of hexagonal Cd 0.8 Zn 0.2 S, whereas the spots at 2.72 and 3.14 Å represent the (200) and (111) planes of the cubic crystal phase, respectively. Furthermore, the (101) plane of hexagonal ZnO is also observed. In Figure 3c, the several small particles with high electron contrast (marked by white arrows) can be ascribed to a Pt-containing phase. They are subnanometric and very well dispersed. HAADF-STEM was performed to study the microstructure of the catalyst. The EDX analysis shows strong signals of Zn, Cd, and S; they are likely the mixed phases of ZnO and Cd 0.8 Zn 0.2 S. Both phases are extremely well mixed. Figure 1S shows the X-ray photoelectron spectroscopy (XPS) survey spectra of 0.1%Pt/Cd 0.8 Zn 0.2 S, 1%Pt/Cd 0.8 Zn 0.2 S,
The range of binding energies of Zn 2p 3/2 of all of the samples are between 1022.2 ± 0.2 eV and they are close to each other which are similar to that reported for ZnO 17 Figure S2). Interestingly, the surface-adsorbed water is only detected on the surface of a nonhybrid system such as 0.1%Pt/ Cd 0.8 Zn 0.2 S and 1%Pt/Cd 0.8 Zn 0.2 S. Furthermore, the signal intensity of Zn−O became much weaker in the case of ZnO/ 1%Pt/Cd 0.8 Zn 0.2 S compared to that of ZnO/0.1%Pt/ Cd 0.8 Zn 0.2 S, which can be interpreted in terms of encapsulation of ZnO by Cd 0.8 Zn 0.2 S in the hybrid systems. The S 2p 3/2 peaks of the samples are close to 161.0 eV, characteristic of the presence of S 2− species on the surface 20 ( Figure 4a). Furthermore, no other sulfur species are detected, which indicates that S 2− species were not oxidized during the synthesis.
The peaks at around 404.7 eV in Figure 4c correspond to Cd 3d 5/2 and are ascribed to Cd−S bonds. 7 The binding energies of Pt 4f 7/2 of the samples are almost identical (Figure 4b). In general, the Pt 4f 7/2 binding energy of bulk metal platinum is around 71.1 eV, whereas the binding energy of Pt 4f 7/2 increases with decreasing the particle size of Pt. 21,22 Then, the binding energy of Pt 4f 7/2 recorded at 72.0 eV indicates the presence of very small Pt particles on the surface, in accordance with high-resolution TEM (Figure 3). Inductively coupled plasma (ICP) and XPS analyses suggest that Pt may be sandwiched between ZnO and Cd(Zn)S. This is because 1 wt % Pt, detected by ICP and TEM, is not detectable by XPS in the same hybrid system, even though XPS could detect concentrations down to 0.1 wt % Pt in the absence of ZnO (the nonhybrid system, Figure 4b).

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Article 2.2. Photocatalytic Tests. Photocatalytic hydrogen generation was evaluated at 42.5 mW/cm 2 using an aqueous solution of benzyl alcohol and acetic acid (2.5−2.5% v/v) as a sacrificial reagent, which was reported to yield high quantum yields. 12 The pH of the solution was 2.5. Figure 5 shows a comparison of the photocatalytic hydrogen generation activity of the as-prepared samples. The highest hydrogen production rate was achieved for [ZnO] 4 /1%Pt/Cd 0.8 Zn 0.2 S, which is likely due to the effect of the Z-scheme between ZnO and Cd 0.8 Zn 0.2 S through Pt. No H 2 was detected when Pt/ZnO alone was used even after 5 h of irradiation, suggesting that ZnO was not stable at pH = 2.5. However, ZnO/Pt/Cd 0.8 Zn 0.2 S showed good stability and better activity under such a condition due to the encapsulation of ZnO by Cd 0.8 Zn 0.2 S. To study the effect of each component of ZnO/Pt/Cd 0.8 Zn 0.2 S on the Zscheme, different combinations of ZnO, Pt, and Cd 0.8 Zn 0.2 S were prepared (Pt/ZnO and Pt/Cd 0.8 Zn 0.2 S). The hydrogen generation activities in the broad light spectrum (360−700 nm) and the corresponding quantum yields at 365 and 460 nm were compared to those of ZnO/Pt/Cd 0.8 Zn 0.2 S. Both ZnO/Pt/ Cd 0.8 Zn 0.2 S and Pt/Cd 0.8 Zn 0.2 S showed much higher hydrogen generation rates than those of Cd 0.8 Zn 0.2 S regardless of the quantity of Pt. This could arise from the increase in the chargeseparation efficiency because of the electron transfer from the semiconductor to Pt. In addition, the photocatalytic activity of [ZnO] 4 /1%Pt/Cd 0.8 Zn 0.2 S was around 50% higher than that of 1%Pt/Cd 0.8 Zn 0.2 S, which highlights the positive contribution of ZnO to the photoactivity. To quantify the actual contribution of ZnO, the apparent quantum yields (AQYs) of both photocatalysts were measured using the same quantities present in the hybrid system (Table 2). For instance, the maximum hydrogen generation rate of [ZnO] 4 /1%Pt/Cd 0.8 Zn 0.2 S was obtained with 33 mg of catalyst in 30 mL of sacrificial reagent (33 mg cat/30 mL sac.), which contain 10 mg of Cd 0.8 Zn 0.2 S and 23 mg of ZnO, respectively. The result reveals that when the catalysts were illuminated at 365 nm, where both ZnO and Cd 0.8 Zn 0.2 S were activated at the same time, the AQY of [ZnO] 4 /1%Pt/Cd 0.8 Zn 0.2 S (33 mg/30 mL) was over 2 times higher than that of 1%Pt/Cd 0.8 Zn 0.2 S (10 mg/30 mL), whereas 1%Pt/ZnO was not producing any hydrogen. Therefore, it can be concluded that the [ZnO] 4 /1%Pt/Cd 0.8 Zn 0.2 S hybrid system follows a Z-scheme mechanism at 365 nm, as shown in Figure  6a. In this case, the Z-scheme simultaneously generates valence band (VB)-holes on ZnO with a higher oxidation potential and conduction band (CB)-electrons on Cd 0.8 Zn 0.2 S with a higher reduction potential compared to those of ZnO and Cd 0.8 Zn 0.2 S. Consequently, the maximum redox potentials of the hybrid system were utilized, thus resulting in higher photocatalytic activities. Interestingly, a significantly higher AQY was also obtained by ZnO 4 /1%Pt/Cd 0.8 Zn 0.2 S compared to that of 1% Pt/Cd 0.8 Zn 0.2 S when both catalysts were illuminated at 460 nm (Table 2). Although only the Cd 0.8 Zn 0.2 S portion of [ZnO] 4 / 1%Pt/Cd 0.8 Zn 0.2 S was exited at 460 nm, a higher AQY was achieved likely because the photogenerated electrons from CB of Cd 0.8 Zn 0.2 S were relocated to the CB of ZnO and then transferred to Pt, resulting in a better charge separation, which in turn leads to a higher activity. A similar phenomenon was reported on various hybrid materials. 23 In contrast, deactivations were observed at both 365 and 460 nm when the [ZnO] 4 /0.1%Pt/Cd 0.8 Zn 0.2 S versus the 0.1%Pt/Cd 0.8 Zn 0.2 S (Table 2) which implies the importance of Pt content in the hybrid system. Apparently, the amount of Pt needs to be optimized for constructing an efficient Z-scheme. The effect of Pt on the Z-scheme is currently subjecting to a further study.
Accordingly, we proposed the charge transfer mechanisms for both cases separately, as shown in Figure 6. The overall solar-driven Z-scheme involves four steps, as described in Figure 6a. The steps are as follows: (1)  To further validate the proposed mechanism, femtosecond transient absorption (TA) measurements were performed on ZnO, ZnO/Pt, Cd 0.8 Zn 0.2 S, Cd 0.8 Zn 0.2 S/Pt, and ZnO/Pt/ Cd 0.8 Zn 0.2 S following 350 nm excitation, as shown in Figure  7A. The details of TA spectra are described in Figure 7A, where the spectral signatures were identified for both electrons and holes for ZnO and Cd 0.8 Zn 0.2 S. The spectral features of   Figure 7A). Therefore, the TB feature at 390 nm and PA at 538 nm of ZnO were chosen to monitor the effect of Z-scheme ( Figure  7B). All of the transient features of ZnO/Pt, ZnO/Pt/ Cd 0.8 Zn 0.2 S at various wavelengths were fitted with biexponential and the relevant fitted parameters are summarized in Table

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Article 3. The electron feature of ZnO/Pt/Cd 0.8 Zn 0.2 S at 392 nm shows faster decay compared to that of ZnO/Pt, whereas the hole feature at 538 nm displays a negligible change in both ZnO/Pt/Cd 0.8 Zn 0.2 S and ZnO/Pt ( Figure 7B). This indicates faster depopulation of ST electron of ZnO through Z-scheme. According to the proposed mechanism, as shown in Figure 6, the electron generated on ZnO was quenched with the hole generated on Cd 0.8 Zn 0.2 S through Pt, which resulted in faster electron decay compared to that of ZnO/Pt. On the other hand, the lifetime of the hole is not affected that much. Consequently, this process made it possible for the electron from Cd 0.8 Zn 0.2 S and the hole from ZnO to participate in redox reactions. This kind of carrier dynamics features agrees well with the description of Z-scheme, as shown in Figure 6.
The stability of the photocatalyst is considered to be the most important factor for practical applications. To investigate the stability of the ZnO/Pt/Cd 0.8 Zn 0.2 S system, the time dependency of the photocatalytic H 2 production over [ZnO] 4 / 0.1%Pt/Cd 0.8 Zn 0.2 S from an aqueous solution containing 2.5− 2.5% BnOH−CH 3 CO 2 H at 23% light intensity (42.5 mW/ cm 2 ) of a xenon lamp was performed. The initial pH of the sacrificial reagent was around 2.5. The results (Figure 8) show that the H 2 generation rate declines continuously and then stops completely after 2500 min.
Although the catalysts aggregated physically after a prolonged time, there was no color change of the catalyst. Furthermore, a similar H 2 generation rate was obtained after washing the used catalyst with ethanol. This phenomenon suggests that benzyl aldehyde (BzCHO) which was quantified by GC-MS, as one of the reaction intermediates, is strongly adsorbed on the surface of the catalyst, subsequently inhibiting the photocatalytic reaction. Moreover, the XPS analysis of catalysts before and after the reaction (Table 4) reveals a change of Cd/Zn ratio from 1:2.7 to 1:1.6. This could be due to Zn leaching during H 2 generation, owing to the acidic nature of the sacrificial reagent.
According to the experimental results and visual observation, one can conclude that the two main reasons for the deactivation of the photocatalyst are strong adsorption of BzCHO and dissolution of ZnO.

CONCLUSIONS
In summary, a uniformly dispersed ZnO/Pt/Cd 0.8 Zn 0.2 S photocatalyst exhibits high photocatalytic activity for hydrogen production when 2.5−2.5% v/v BnOH/CH 3 CO 2 H was used as the sacrificial reagent under xenon lamp irradiation (total light flux: 42.5 mW/cm 2 ). In particular, the highest photocatalytic rate (10.2 mmol/h g) and AQY (34%) were achieved with [ZnO] 4 /1%Pt/Cd 0.8 Zn 0.2 S, which are significantly higher than those of 1%Pt/ZnO and 1%Pt/Cd 0.8 Zn 0.2 S. These results are attributed to the effective Z-scheme formation between ZnO and Cd 0.8 Zn 0.2 S through Pt. A Z-scheme-based charge transfer mechanism was proposed by performing a time-resolved transient absorption study. Finally, a systematic study was conducted on the photocatalytic stability of the system. The results indicate that there are two main reasons for catalytic deactivation of ZnO/Pt/Cd 0.8 Zn 0.2 S, namely, (1) blockage of active sites caused by strong adsorption of BzCHO and (2) ZnO dissolution under acidic conditions, which will eventually destroy the Z-scheme. This work not only represents a systematic study on the ZnO/Pt/Cd 0.8 Zn 0.2 S system but also provides new insights into the understanding of photocatalytic activities of metal oxide/metal/Cd(Zn)S hybrid systems.

EXPERIMENTAL SECTION
4.1. Synthesis of ZnO/Pt/CdZnS. All reagents were of analytical grade, purchased from Sigma-Aldrich, and used without further purification. The detailed synthetic procedure used for preparing Cd 0.8 Zn 0.2 S-based hybrid materials is described below. 12 4.1.1. Pt/ZnO. 12 Zn(CH 3 COO) 2 ·2H 2 O (2.64 g, 12 mmol) was added to methanol (210 mL) in a 500 mL three-neck RBF, and the temperature was raised to 60°C. After 10 min, 30 mL of a methanolic solution of KOH (1.50 g, 26.7 mmol) was added dropwise to the reaction solution. The clear solution was stirred for another 2 h at 60°C and then 50 mL of water was added. The resulting white precipitate was filtered, washed with water−methanol (1:1), and dried at 80°C for overnight to give ZnO in quantitative yield. H 4 PtCl 6 (1 mg Pt/mL, 12 mL) was   6 (1 mg/mL (Pt)) in BnOH/AcOH (2.5−2.5% v/v). The resulting mixture was illuminated under UV (λ = 360 nm) light with the intensity of 5 mW/cm 2 for 4 h. The resulting solution was filtered, washed with water, and dried at 80°C overnight to obtain the desired product 1%Pt/Cd 0.8 Zn 0.2 S in quantitative yield.
4.2. Characterization. UV−vis absorption spectra of the powdered catalysts were collected over the wavelength range of 250−700 nm on a Thermo Fisher Scientific spectrophotometer equipped with praying mantis diffuse reflectance accessory. Absorbance (A) and reflectance (% R) of the samples were measured. The reflectance (% R) data were used to calculate the band gap of the samples using the Tauc plot (Kubelka− Munk function).
XRD spectra were recorded using a Bruker D8 Advance Xray diffractometer. Cu Kα (λ = 1.5406 Å) radiation over the range of 2θ; interval between 20 and 90°with a step size of 0.010°and a step time of 0.2 s/step was used.
The XP spectra of the samples were collected by a Thermo Scientific Escalab 250 XI XP spectrometer with an Al Kα X-ray source. The X-ray spot size was 650 × 650 μm 2 . Charge compensation was carried out using a standard flood gun. Before collecting XPS data, samples were etched using Ar ions for 5 min at an ion energy of 1000 eV. Data were acquired using the settings given in Table 5.
All peaks were corrected with respect to the binding energy of the adventitious C 1s peak at 284.5 eV. All peaks were fitted using the SMART background option and Lorentzian/ Gaussian.
The concentrations of Zn, Cd, S, and Pt were measured by inductively coupled plasma optical emission spectroscopy on a Varian 720-ES instrument. TEM samples were dispersed in alcohol, and a drop of the suspension was placed over a grid with holey-carbon film. The TEM images were collected using a FEI Tecnai F20 microscope operating at 200 kV.
4.3. Photocatalytic Test. Photocatalytic reactions were performed in a 137 mL volume Pyrex glass reactor using 6−30 mg of catalyst. Various volume ratios (30 mL) of benzyl alcohol/acidic acid (BnOH/AcOH) aqueous solutions were used. The slurry was purged with N 2 gas to remove any O 2 and subjected to constant stirring before the reaction. The reactor was then exposed to UV light (100 W ultraviolet lamp H-144GC-100 Sylvania par 38 with a flux of around 5 mW/cm 2 at a distance of 5 cm). Similarly, to evaluate the UV + visible light activity, a xenon lamp (Asahi spectra MAX-303) with a total flux of 42.5 mW/cm 2 (UV ∼ 3.0 mW/cm 2 and visible (up to 650 nm)) at a distance of 2 cm was used. Product analyses were performed by a gas chromatograph equipped with a thermal conductivity detector connected to a Porapak Q packed column (2 m) at 45°C, and N 2 was used as a carrier gas. The BzCHO content was monitored by GC-MS from Thermo Scientific (Waltham, MA USA) equipped with trance 1300 (GC) and ISQ single quadrupole (MS). Apparent quantum yields (AQYs) at various wavelengths defined by eq 1 27 were calculated by data obtained using monochromatic light-emitting diode (365− 750 nm) at a distance of 2 cm. The corresponding light intensities were measured with a GL Spectis 5.0 Touch spectrometer. 4.4. Femtosecond Transient Absorption (TA) Measurements. Time-resolved absorption decays of the catalyst suspension in water were measured with a pump−probe setup, in which a white light continuum probe pulse was generated in a 2 mm thick sapphire plate contained in an Ultrafast System LLC spectrometer using few microjoules energy of an 800 nm pulse. The fundamental output delivered by a Ti/sapphire femtosecond regenerative amplifier operating at 800 nm with 35 fs pulses and a repetition rate of 1 kHz. Spectrally tunable (240−2600 nm) femtosecond pulses generated by an optical parametric amplifier (Light Conversion Ltd) and a white light continuum were used, respectively, as the pump (excitation) and probe beams in a pump−probe experimental setup (Helios). The details of the experimental setup can be found elsewhere. 28 ■ ASSOCIATED CONTENT

* S Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b00767.
XP survey spectra of ZnO/CdZnS-based systems (PDF)

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