Photoelectrocatalytic Hydrogen Generation Enabled by CdS Passivated ZnCuInSe Quantum Dot-Sensitized TiO2 Decorated with Ag Nanoparticles

Here we present the photoelectrocatalytic hydrogen generation properties of CdS passivated ZnCuInSe (ZCISe) quantum dots (QDs) supported by TiO2 nanowires decorated with Ag nanoparticles. In this configuration, Ag nanoparticles were sandwiched between the photo-electrons collector (TiO2) and photo-sensitizers (ZCISe), and acted as an electron relay speeding up the charge carrier transport. ZCISe and CdS enabled the optical absorption of the photoelectrode ranging from ultraviolet to near infrared region, which significantly enhanced the solar-to-chemical energy conversion efficiency. A photocurrent of 10.5 mA/cm2 and a hydrogen production rate of about 52.9 μmol/h were achieved under simulated sunlight (1.5 AG).


Introduction
Hydrogen generation by solar energy water splitting with a semiconductor photocatalyst is an interesting way to use renewable resources [1][2][3]. The solar-to-chemical energy conversion efficiency for a given photocatalyst is heavily dependent on light absorption and the rate between charge carriers separation (or migration to the surface) and electron-hole recombination [4][5][6][7], with an ideal semiconductor photocatalyst that presents high absorption and charge carriers separation. Hence, many semiconductor photocatalytic materials have been exploited for sustainable development. TiO 2 is still the most widely used semiconductor due to its chemical stability, low cost, and good catalytic activity. For instance, Minero studied the relationship between light absorption and degradation rate in four TiO 2 specimens, which was helpful for the degradation of formic acid at a low concentration [8]. TiO 2 was also employed by Emeline to research the effects of light intensity and phenol concentration on the degradation [9]. Lasa researched plenty of work on photocatalysis and concluded that TiO 2 photocatalysis also depended on the crystal structure of TiO 2 , cocatalyst, substrates, and sacrificial agents [10]. Using the sol-gel method, Rusinque and co-worker prepared mesoporous TiO 2 for photocatalysis and hydrogen production reaching up to 210 cm 3 STP (standard temperature and pressure) when using the 1.00 wt%-Pd on TiO 2 [11]. However, TiO 2 can only absorb the UV light due to wide bandgap, therefore limiting the overall efficiency [12,13]. In order to extend its light absorption into the visible light region, a narrow bandgap semiconductor combined with TiO 2 has been used to form a heterostructure [14]. I-III-VI 2 group QDs, such as CuInS 2 , CuInSe 2 , AgInS 2 , AgInSe 2 , etc., displayed narrow band gap, small self-absorption, large coefficient, and luminescence of the near-infrared wavelength region, and they have been successfully utilized in photocatalysis [15].

Preparation of ZCISe QDs
The ZCISe QDs were prepared by a hot-injection technique [18]. A mixture containing 3 mL DDT, 1 mL ODE, and 5 mL OAm was added to a 20 mL three-neck flask containing ZnCl 2 (0.05 mmol), CuCl (0.03 mmol), and InCl 3 .4H 2 O (0.1 mmol) under a nitrogen atmosphere. The mixture was heated to 160 • C at a rate of 15 • C/min. Then, the Se (Se powder, 0.3 mmol) precursor solution with 0.1 mL OAm and 1 mL DDT, was injected into the flask. After remaining at 160 • C for a definite amount of time (10 min, 20 min, 30 min), the mixture was cooled in an ice bath. After 20 min, the nanocrystals were precipitated with excess ethanol and centrifuged at 6000 rpm for 5 min. Then, the supernatant was discarded, and the precipitate was redispersed with 3 mL dichloromethane. After centrifugation at 10,000 rpm for 10 min, the supernatant was collected and stored at 4 • C.

Decoration of Ag NPs on TiO 2 Nanowires
TiO 2 nanowires were synthesized based on our previous method [22]. For the decoration of Ag NPs, TiO 2 nanowires were immersed in 0.002 M AgNO 3 aqueous solution and exposed to ultraviolet light at different times (5 min, 10 min, 15 min, 20 min, 25 min) [23]. Then, the Ag NPs loaded on the TiO 2 nanowires were washed with ultrapure water three times.

Loading ZCISe QDs on TiO 2 /Ag
ZCISe QDs were decorated on the TiO 2 /Ag nanowires by dropping 20 µL ZCISe QD solution in CCl 3 onto the nanowires, and then dried in an oven at 50 • C. This process can be repeated several times to get the optimal loading of ZCISe QDs. Finally, the electrode was annealed at 450 • C in an N 2 flow for 30 min to improve the crystalline and stability.

Fabrication of TiO 2 /Ag/ZCISe/CdS Nanocomposite
CdS NPs were deposited on the TiO 2 /Ag/ZCISe composite with a successive ionic layer adsorption and reaction (SILAR) method to decrease the surface defects of the ZCISe QDs. In brief, 0.05 M cadmium nitrate in ethanol was used as cation source, and 0.05 M sodium sulfide in methanol/water (1/1) was used as anion source. The TiO 2 /Ag/ZCISe nanocomposite was immersed Nanomaterials 2019, 9, 393 3 of 11 in Cd precursor for one minute followed by flushing with ethanol, and then immersed in sodium sulfide precursor for another minute followed by rinsing with methanol. This counted as one SILAR cycle. For the present work, this process was repeated for 5 cycles.

Characterization
UV-vis absorption measurements were obtained on a Shimadzu-NIR-2550 spectrophotometer (Guangzhou, China). Photoluminescence (PL) spectra were recorded on a Hitachi F-7000 fluorescence spectrophotometer (Hitachi High-Tech, Tokyo, Japan). The structure of the ZCISe QDs was identified via an X-ray diffractometer (XRD) on a D8 advance (Bruker, AXS, Madison, WI, USA) with Cu-Ka radiation. The morphology of the sample was characterized by scanning electron microscopy (SU8010, Hitachi, Hong Kong, China) and transmission electron microscopy (Tecnai G 2 F20 S-Twin, FEI, Hillsboro, OR, USA). Elemental analysis was carried out on an energy disperse X-ray spectrometer (EDX) fitted with TEM. The performances were measured with a CHI electrochemical workstation (CHI660E, Shanghai Chenhua Instrument Co. Ltd., Shanghai, China). The light source was a 300 W Xe lamp equipped with a band pass filter at 500 nm with a bandwidth (full width at half maximum) of 15 nm to remove IR and UV radiations, and a cutoff filter for eliminating radiation below 800 nm, respectively.

Photoelectrochemical Performance and Hydrogen Generation Test
The photoelectrochemical test was carried out in a standard three-electrode system composed of an Ag/AgCl reference, a Pt sheet counter electrode, and samples of the working electrode in an electrolyte containing 0.35 M Na 2 SO 3 and 0.24 M Na 2 S, which was purged with N 2 for 30 min prior to measurements. The incident light from a 300 W Xe lamp was filtered to match the AM 1.5 G spectrum. The hydrogen production test was essentially the same as the material's performance test, except the electrolyte was replaced with pure water. The amount of hydrogen gas generated during 6 h of irradiation was collected in a home-made device [24].

Characterization of ZCISe QDs
An injection route was used to synthesize the Zn-Cu-In-Se QDs (Scheme 1). Firstly, zinc, copper, and indium chlorides were solubilized in ODE using OAm, which prevented slow nucleation and rapid aggregation of the resulting materials, and DDT reduced the colloidal stability of the QDs particularly at high temperatures as ligands. Additionally, the presence of DDT was critical to the synthesis. According to the theory of Hard-Soft-Acids-Base, the reactive Cu + ion, a soft acid, is preferentially complexed by dodecanethiol, a soft base [25,26]. After degassing, the solution was heated under nitrogen. Then we observed the change of solution from turbid to clear and finally to light yellow, which indicated the metal chloride was dissolved. Meanwhile, Se powder was solubilized in OAm and DDT ligands. After injecting the Se precursor into the flask, the color of the solution changed to black red immediately. As the reaction time increased, it first turned bright red and then turned black, which indicated nanocrystal nucleation. Figure 1 shows the PL emission and UV-Vis absorption spectra of ZCISe QDs with different reaction times at 160 • C, with the corresponding PL emission photographs shown in the inset of  Figure 2b), show that the ZCISe QDs are identical with an average particle size of 4.1 ± 0.3 nm. The homogeneous size distribution in the TEM observation is consistent with the distinct excitonic peak observed in the absorption spectra ( Figure 1b) and calculated size from XRD.

Characterization of TiO 2 /Ag/ZCISe/CdS Nanocomposite
The TiO 2 nanowires are a smooth porous network with the top loosely arranged with an average diameter and length corresponding to 40 nm and 3 µm, respectively, as shown in Figure 3a. High magnification of the TiO 2 nanowires is shown in the inset of Figure 3a. After subsequently assembling with Ag NPs (Figure 3b) and ZCISe QDs (Figure 3c), lots of tiny particles anchor onto the TiO 2 nanowires which consequently turn rough. Figure 3d represents the SEM image of TiO 2 /Ag/ZCISe/CdS. One can see that TiO 2 /Ag/ZCISe are almost completely coated by a thin CdS layer without the naked TiO 2 surface, which can greatly decrease the surface defects that are related to the charge carriers' recombination. The TiO2 nanowires are a smooth porous network with the top loosely arranged with an average diameter and length corresponding to 40 nm and 3 μm, respectively, as shown in Figure 3a. High magnification of the TiO2 nanowires is shown in the inset of Figure 3a. After subsequently assembling with Ag NPs (Figure 3b) and ZCISe QDs (Figure 3c), lots of tiny particles anchor onto the TiO2 nanowires which consequently turn rough. Figure 3d represents the SEM image of TiO2/Ag/ZCISe/CdS. One can see that TiO2/Ag/ZCISe are almost completely coated by a thin CdS layer without the naked TiO2 surface, which can greatly decrease the surface defects that are related to the charge carriers' recombination.   The TiO2 nanowires are a smooth porous network with the top loosely arranged with an average diameter and length corresponding to 40 nm and 3 μm, respectively, as shown in Figure 3a. High magnification of the TiO2 nanowires is shown in the inset of Figure 3a. After subsequently assembling with Ag NPs (Figure 3b) and ZCISe QDs (Figure 3c), lots of tiny particles anchor onto the TiO2 nanowires which consequently turn rough. Figure 3d represents the SEM image of TiO2/Ag/ZCISe/CdS. One can see that TiO2/Ag/ZCISe are almost completely coated by a thin CdS layer without the naked TiO2 surface, which can greatly decrease the surface defects that are related to the charge carriers' recombination.   Figure 5a shows the UV-vis spectra of the TiO2 nanowires, TiO2/Ag, TiO2/Ag/ZCISe, and TiO2/Ag/ZCISe/CdS. After the sensitization of ZCISe QDs, the absorption spectrum of the TiO2 nanowires was significantly enhanced. The TiO2/Ag/ZCISe/CdS electrode exhibited red-shift absorption at a wavelength range between 680 and 1000 nm. The surface passivation can enhance the absorption spectra towards long wavelength region compared with those of pure TiO2 and TiO2/Ag/ZCISe. This red-shift may indicate that the QDs charge carriers wave functions tunnel into the surrounding of the CdS shell. UV-Vis spectra were also measured to reveal the band gaps of the as-prepared samples, using Tauc relation [27,28],

Optical Absorption
where α is the light absorption coefficient, A is a constant, h is the Planck's constant, ν is light frequency, and Eg is the band gap energy. The optical band gaps of TiO2/Ag/ZCISe/CdS, TiO2/Ag/ZCISe, and TiO2/Ag correspond to 1.75 eV, 2.7 eV, and 2.8 eV in Figure 5b.  Figure 5a shows the UV-vis spectra of the TiO 2 nanowires, TiO 2 /Ag, TiO 2 /Ag/ZCISe, and TiO 2 /Ag/ZCISe/CdS. After the sensitization of ZCISe QDs, the absorption spectrum of the TiO 2 nanowires was significantly enhanced. The TiO 2 /Ag/ZCISe/CdS electrode exhibited red-shift absorption at a wavelength range between 680 and 1000 nm. The surface passivation can enhance the absorption spectra towards long wavelength region compared with those of pure TiO 2 and TiO 2 /Ag/ZCISe. This red-shift may indicate that the QDs charge carriers wave functions tunnel into the surrounding of the CdS shell. UV-Vis spectra were also measured to reveal the band gaps of the as-prepared samples, using Tauc relation [27,28],

Optical Absorption
where α is the light absorption coefficient, A is a constant, h is the Planck's constant, ν is light frequency, and Eg is the band gap energy. The optical band gaps of TiO 2 /Ag/ZCISe/CdS, TiO 2 /Ag/ZCISe, and TiO 2 /Ag correspond to 1.75 eV, 2.7 eV, and 2.8 eV in Figure 5b.

Electrochemical Characterization
A three-electrode system was used to determine the photocurrent density of different electrodes. It is well known that the optimized photoreduction deposition time of Ag nanoparticles benefit to enhance the photocurrent density. The photocurrent densities of TiO2/Ag monotonically increased with the increasing of Ag loading content corresponding to the reaction time, whereas the photocurrent density sharply decreased as the reaction time reached 20 min. Fifteen minutes was chosen for the single-step photoreduction deposition time to improve the photoelectrochemical properties as shown in Figure S1A. Notably, by characterizing the SEM images of TiO2/Ag with different deposition times ( Figure S2, Detailed Analysis), the obtained results were consistent with the change trend of the photocurrent density. In addition, the amount of ZCISe QDs and CdS for the photocurrent density also played a key role in the proposed photoelectrode. The photocurrent density of TiO2/Ag/ZCISe and TiO2/Ag/ZCISe/CdS increased and then decreased with the increase in the quantity of ZCISe and the number of layers covered by CdS (Figure S1 B,C). Due to the excessive load which interfered with the absorption of light by the photoelectrodes, the optimal quantity and the number of cycles were 1 mL and 5 times, respectively. Figure 6a, b shows the I-t curves of TiO2, TiO2/Ag, TiO2/Ag/ZCISe, and TiO2/Ag/ZCISe/CdS ptotoelectrodes. It can be seen that the photocurrent density of TiO2/Ag displayed a small increase compared with the pristine TiO2 nanowires . After loading the ZCISe QDs, the photocurrent density increased about 10 times, reaching 3.66 mA/cm 2 . Further loading the CdS shell, the photocurrent density increased about 2.8 times more than TiO2/Ag/ZCISe, reaching 10.5 mA/cm 2 . In order to determine the function of each part in the proposed photoelectrode, the photocurrent densities of different modified photoelectrodes were measured under chopped illumination of AM 1.5 G light with a cutoff filter and a band pass filter for obtaining the different wavelength as shown in Table S1. Under the visible light (500 nm ± 15 nm), ZCISe, Ag/ZCISe, and CdS modified TiO2 exhibited the largest photocurrent densities. TiO2/ZCISe and TiO2/Ag/ZCISe also had a non-negligible response in the IR region, indicating the role of ZCISe QDs in absorbing IR. However, due to the lower intensity of IR light, the obtained photocurrent density were less than the above results. After connecting the parts together, the proposed photoelectrode showed an excellent photoelectrochemical performance in accordance with the UV-vis spectra ( Figure 5).

Electrochemical Characterization
A three-electrode system was used to determine the photocurrent density of different electrodes. It is well known that the optimized photoreduction deposition time of Ag nanoparticles benefit to enhance the photocurrent density. The photocurrent densities of TiO 2 /Ag monotonically increased with the increasing of Ag loading content corresponding to the reaction time, whereas the photocurrent density sharply decreased as the reaction time reached 20 min. Fifteen minutes was chosen for the single-step photoreduction deposition time to improve the photoelectrochemical properties as shown in Figure S1A. Notably, by characterizing the SEM images of TiO 2 /Ag with different deposition times ( Figure S2, Detailed Analysis), the obtained results were consistent with the change trend of the photocurrent density. In addition, the amount of ZCISe QDs and CdS for the photocurrent density also played a key role in the proposed photoelectrode. The photocurrent density of TiO 2 /Ag/ZCISe and TiO 2 /Ag/ZCISe/CdS increased and then decreased with the increase in the quantity of ZCISe and the number of layers covered by CdS ( Figure S1B,C). Due to the excessive load which interfered with the absorption of light by the photoelectrodes, the optimal quantity and the number of cycles were 1 mL and 5 times, respectively. Figure 6a,b shows the I-t curves of TiO 2 , TiO 2 /Ag, TiO 2 /Ag/ZCISe, and TiO 2 /Ag/ZCISe/CdS ptotoelectrodes. It can be seen that the photocurrent density of TiO 2 /Ag displayed a small increase compared with the pristine TiO 2 nanowires. After loading the ZCISe QDs, the photocurrent density increased about 10 times, reaching 3.66 mA/cm 2 . Further loading the CdS shell, the photocurrent density increased about 2.8 times more than TiO 2 /Ag/ZCISe, reaching 10.5 mA/cm 2 . In order to determine the function of each part in the proposed photoelectrode, the photocurrent densities of different modified photoelectrodes were measured under chopped illumination of AM 1.5 G light with a cutoff filter and a band pass filter for obtaining the different wavelength as shown in Table S1. Under the visible light (500 nm ± 15 nm), ZCISe, Ag/ZCISe, and CdS modified TiO 2 exhibited the largest photocurrent densities. TiO 2 /ZCISe and TiO 2 /Ag/ZCISe also had a non-negligible response in the IR region, indicating the role of ZCISe QDs in absorbing IR. However, due to the lower intensity of IR light, the obtained photocurrent density were less than the above results. After connecting the parts together, the proposed photoelectrode showed an excellent photoelectrochemical performance in accordance with the UV-vis spectra ( Figure 5).

Photocatalytic Performance of Hydrogen Generation
The photocatalytic activities of the as-prepared TiO2, TiO2/Ag, TiO2/Ag/ZCISe, and TiO2/Ag/ZCISe/CdS photoelectrodes were evaluated by hydrogen production under high-power xenon lamp irradiation. As shown in Figure 7, after 6 h irradiation by the xenon lamps, the H2 yields by the different photoelectrodes were about 1.8, 2, 4, and 7 mL, respectively. Notably, the related photocatalytic rate constant of hydrogen evolution by TiO2/Ag/ZCISe/CdS reached 52.9 μmol/h, which was approximately 3.88 times of that by the photoelectrode of pure TiO2 nanowires. This is due to the sensitization of ZCISe QDs which broadens the light absorption range of the TiO2/Ag/ZCISe/CdS electrode, increases the separation efficiency of electrons-holes, and generates more electrons. The CdS passivates the surface of the QDs and inhibits the recombination of electron holes. In addition, the decoration of Ag NPs promoted the transmission of electrons and enhanced the photocatalytic activity.

Mechanism of Charge Transfer in TiO2/Ag/ZCISe/CdS
Based on the photoelectric characterization of composite electrode and hydrogen production performance, the mechanism of charge transfer in TiO2/Ag/ZCISe/CdS was discussed using PL emission spectra as shown in Figure 8. When ZCISe QDs were loaded onto TiO2 nanowires, a distinct quenching of the emission was observed when compared with the composite structure of FTO/ZCISe(FTO, The SnO2 transparent conductive glass of doped F.), which demonstrated deactivation of the excited ZCISe via electron transfer to the TiO2. In the same way, electrons can be transferred from CdS NPs to ZCISe QDs and then to TiO2 nanowires by comparing FTO/ZCISe/CdS and TiO2/ZCISe/CdS. Moreover, the emission further significantly quenched when Ag NPs were decorated between TiO2 nanowires and ZCISe to form the TiO2/Ag/ZCISe sandwich, suggesting that

Photocatalytic Performance of Hydrogen Generation
The photocatalytic activities of the as-prepared TiO 2 , TiO 2 /Ag, TiO 2 /Ag/ZCISe, and TiO 2 /Ag/ ZCISe/CdS photoelectrodes were evaluated by hydrogen production under high-power xenon lamp irradiation. As shown in Figure 7, after 6 h irradiation by the xenon lamps, the H 2 yields by the different photoelectrodes were about 1.8, 2, 4, and 7 mL, respectively. Notably, the related photocatalytic rate constant of hydrogen evolution by TiO 2 /Ag/ZCISe/CdS reached 52.9 µmol/h, which was approximately 3.88 times of that by the photoelectrode of pure TiO 2 nanowires. This is due to the sensitization of ZCISe QDs which broadens the light absorption range of the TiO 2 /Ag/ZCISe/CdS electrode, increases the separation efficiency of electrons-holes, and generates more electrons. The CdS passivates the surface of the QDs and inhibits the recombination of electron holes. In addition, the decoration of Ag NPs promoted the transmission of electrons and enhanced the photocatalytic activity.

Photocatalytic Performance of Hydrogen Generation
The photocatalytic activities of the as-prepared TiO2, TiO2/Ag, TiO2/Ag/ZCISe, and TiO2/Ag/ZCISe/CdS photoelectrodes were evaluated by hydrogen production under high-power xenon lamp irradiation. As shown in Figure 7, after 6 h irradiation by the xenon lamps, the H2 yields by the different photoelectrodes were about 1.8, 2, 4, and 7 mL, respectively. Notably, the related photocatalytic rate constant of hydrogen evolution by TiO2/Ag/ZCISe/CdS reached 52.9 μmol/h, which was approximately 3.88 times of that by the photoelectrode of pure TiO2 nanowires. This is due to the sensitization of ZCISe QDs which broadens the light absorption range of the TiO2/Ag/ZCISe/CdS electrode, increases the separation efficiency of electrons-holes, and generates more electrons. The CdS passivates the surface of the QDs and inhibits the recombination of electron holes. In addition, the decoration of Ag NPs promoted the transmission of electrons and enhanced the photocatalytic activity.

Mechanism of Charge Transfer in TiO2/Ag/ZCISe/CdS
Based on the photoelectric characterization of composite electrode and hydrogen production performance, the mechanism of charge transfer in TiO2/Ag/ZCISe/CdS was discussed using PL emission spectra as shown in Figure 8. When ZCISe QDs were loaded onto TiO2 nanowires, a distinct quenching of the emission was observed when compared with the composite structure of FTO/ZCISe(FTO, The SnO2 transparent conductive glass of doped F.), which demonstrated deactivation of the excited ZCISe via electron transfer to the TiO2. In the same way, electrons can be transferred from CdS NPs to ZCISe QDs and then to TiO2 nanowires by comparing FTO/ZCISe/CdS and TiO2/ZCISe/CdS. Moreover, the emission further significantly quenched when Ag NPs were decorated between TiO2 nanowires and ZCISe to form the TiO2/Ag/ZCISe sandwich, suggesting that

Mechanism of Charge Transfer in TiO 2 /Ag/ZCISe/CdS
Based on the photoelectric characterization of composite electrode and hydrogen production performance, the mechanism of charge transfer in TiO 2 /Ag/ZCISe/CdS was discussed using PL emission spectra as shown in Figure 8. When ZCISe QDs were loaded onto TiO 2 nanowires, a distinct quenching of the emission was observed when compared with the composite structure of FTO/ZCISe(FTO, The SnO 2 transparent conductive glass of doped F.), which demonstrated deactivation of the excited ZCISe via electron transfer to the TiO 2 . In the same way, electrons can be transferred from CdS NPs to ZCISe QDs and then to TiO 2 nanowires by comparing FTO/ZCISe/CdS and TiO 2 /ZCISe/CdS. Moreover, the emission further significantly quenched when Ag NPs were decorated between TiO 2 nanowires and ZCISe to form the TiO 2 /Ag/ZCISe sandwich, suggesting that pronounced charge separation occurred within the TiO 2 /Ag/ZCISe. The PL quenching of the composite agrees with an initial expectation of high efficient charge carriers transport pathway.
On the basis of the results and discussion above, the schematic diagram and photocatalytic mechanism for the TiO 2 /Ag/ZCIS/CdS photoelectrode is proposed and schematically exhibited in Scheme 2. Under solar irradiation, CdS, ZCISe QDs, and TiO 2 absorbed the light and excited. Electron-hole pairs are generated on CdS/ZCISe QDs, and the electrons are quickly transferred into the conduction band of TiO 2 nanowires through Ag NPs. Then, electrons interact with H 2 O to produce hydrogen. pronounced charge separation occurred within the TiO2/Ag/ZCISe. The PL quenching of the composite agrees with an initial expectation of high efficient charge carriers transport pathway. On the basis of the results and discussion above, the schematic diagram and photocatalytic mechanism for the TiO2/Ag/ZCIS/CdS photoelectrode is proposed and schematically exhibited in Scheme 2. Under solar irradiation, CdS, ZCISe QDs, and TiO2 absorbed the light and excited. Electron-hole pairs are generated on CdS/ZCISe QDs, and the electrons are quickly transferred into the conduction band of TiO2 nanowires through Ag NPs. Then, electrons interact with H2O to produce hydrogen.

Conclusions
In summary, oil-soluble ZCISe QDs were prepared by a simple hot-injection method, and a promising strategy with enhanced photocatalytic efficiency of CdS NPs, ZCISe QDs, and Ag NPsbased sandwich electrode configuration is present. The excellent architecture of TiO2/Ag/ZCISe/CdS showed good light-harvesting behavior and notably promoted photocatalytic hydrogen production rate of about 52.9 μmol/h. It is believed that the positive effect between Ag, ZCISe, and CdS as the components of co-catalyst on the photocatalytic hydrogen production activity, efficiently suppress charges recombination, improve interfacial charge transfer, and eventually provide good photocatalytic reaction centers.
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1: The dependence of photocurrent on the loaded Ag nanoparticals, ZCISe and CdS amount which is represented by pronounced charge separation occurred within the TiO2/Ag/ZCISe. The PL quenching of the composite agrees with an initial expectation of high efficient charge carriers transport pathway. On the basis of the results and discussion above, the schematic diagram and photocatalytic mechanism for the TiO2/Ag/ZCIS/CdS photoelectrode is proposed and schematically exhibited in Scheme 2. Under solar irradiation, CdS, ZCISe QDs, and TiO2 absorbed the light and excited. Electron-hole pairs are generated on CdS/ZCISe QDs, and the electrons are quickly transferred into the conduction band of TiO2 nanowires through Ag NPs. Then, electrons interact with H2O to produce hydrogen.

Conclusions
In summary, oil-soluble ZCISe QDs were prepared by a simple hot-injection method, and a promising strategy with enhanced photocatalytic efficiency of CdS NPs, ZCISe QDs, and Ag NPsbased sandwich electrode configuration is present. The excellent architecture of TiO2/Ag/ZCISe/CdS showed good light-harvesting behavior and notably promoted photocatalytic hydrogen production rate of about 52.9 μmol/h. It is believed that the positive effect between Ag, ZCISe, and CdS as the components of co-catalyst on the photocatalytic hydrogen production activity, efficiently suppress charges recombination, improve interfacial charge transfer, and eventually provide good photocatalytic reaction centers.
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1: The dependence of photocurrent on the loaded Ag nanoparticals, ZCISe and CdS amount which is represented by Scheme 2. Schematic diagram of TiO 2 /Ag/ZCISe/CdS photoelectrode and its mechanism of photocatalytic hydrogen production.

Conclusions
In summary, oil-soluble ZCISe QDs were prepared by a simple hot-injection method, and a promising strategy with enhanced photocatalytic efficiency of CdS NPs, ZCISe QDs, and Ag NPs-based sandwich electrode configuration is present. The excellent architecture of TiO 2 /Ag/ZCISe/CdS showed good light-harvesting behavior and notably promoted photocatalytic hydrogen production rate of about 52.9 µmol/h. It is believed that the positive effect between Ag, ZCISe, and CdS as the components of co-catalyst on the photocatalytic hydrogen production activity, efficiently suppress charges recombination, improve interfacial charge transfer, and eventually provide good photocatalytic reaction centers.

Supplementary Materials:
The following are available online at http://www.mdpi.com/2079-4991/9/3/393/s1, Figure S1: The dependence of photocurrent on the loaded Ag nanoparticals, ZCISe and CdS amount which is represented by the ultraviolet light exposure time (A), the amount of dropwise (B) and the number of soak (C) of the corresponding electrode, Figure S2: SEM images of TiO 2 /Ag with photoreduction deposition time of 5 min (A,A 1 ), 10 min (B,B 1 ), 15min (C,C 1 ), 20 min (D,D 1 ) and 25 min (E,E 1 ) with different magnification, Table S1: Photocurrent density of different modified photoelectrode under three condition.