Quantum-Dot-Sensitized Nitrogen-Doped ZnO for Efficient Photoelectrochemical Water Splitting

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1 DOI: /ejic Quantum-Dot-Sensitized Nitrogen-Doped ZnO for Efficient Photoelectrochemical Water Splitting Chih Kai Chen, [a] Yen-Ping Shen, [a] Hao Ming Chen, [a] Chih-Jung Chen, [a] Ting-Shan Chan, [b] Jyh-Fu Lee, [b] and Ru-Shi Liu* [a] CLUSTER ISSUE Keywords: Water splitting / Doping / Zinc / Quantum dots Fossil fuels have been used for several decades and have resulted in increased greenhouse gases and pollutants. Currently, clean and renewable energy is in demand. Hydrogen appears to be a good candidate for clean energy because the only product of its reaction with oxygen is water. Water splitting by solar energy is a potential method for the generation of hydrogen in future applications. This study investigates the use of a CdTe quantum-dot-sensitized ZnO:N nanowire arrays for water splitting. The proposed method resulted in considerably enhanced photocurrent and stability. The electronic structures of the ZnO:N materials are also determined by O K-edge X-ray absorption spectroscopy. The incorporation of nitrogen into the ZnO nanostructure is determined by X-ray photoelectron spectroscopy and Zn K-edge X-ray absorption spectroscopy; the nitrogen incorporation changes the electronic state and, thus, increases the water-splitting performance. Introduction The use of solar energy and water to synthesize fuels is vital to sustainable development beyond fossil fuels. Photocatalytic and photoelectrochemical (PEC) water splitting by using semiconductor materials to generate hydrogen and oxygen have attracted attention worldwide because of their renewable fuel generation, which benefits the environment. PEC water splitting combines electrical generation and electrolysis into a single system. Semiconductor materials have been the focus of investigation as the key component in solar energy conversion systems. [1 4] To achieve efficient PEC water splitting, the band gap of the semiconductor must exceed 1.6 to 1.7 ev to exhibit sufficient overpotential for water splitting, because the theoretical difference in the equilibrium potentials of the water-splitting reactions is 1.23 V at 25 C. [5] For PEC cells for water splitting, metal oxide photoelectrodes with wide band gaps, such as ZnO and TiO 2, [6] are severely limited by their low absorption of visible light. Various approaches have been adopted to increase their absorption in the visible region, including doping with heteroatoms and sensitization with dyes or quantum dots (QDs). Semiconductor QDs, such as CdS, [7] CdSe, [8] CdTe, [3] and InP QDs, [9] have been coated on metal oxide semiconductor nanostructures as photoelectrode sensitizers. This approach improves the absorption in the vis- [a] Department of Chemistry, National Taiwan University, Taipei, Taiwan rsliu@ntu.edu.ew [b] National Synchrotron Radiation Research Center, Hsinchu, Taiwan ible region and, thus, enhances the water-splitting performance. Zinc oxide is a multifunctional semiconductor material with a direct band gap of ca. 3.2 ev, high electron mobility, low electrical resistance and high electron-transfer efficiency. To utilize solar energy, impurities that frequently cause dramatic changes in the electrical and optical properties can be introduced. [10 12] The introduced impurities generate a new energy level in the band gap of pristine semiconductor materials. The energy level of ZnO indicates that the conduction band consists mainly of Zn 4s and Zn 4p states and that the valence band is composed of the O 2p state. [13] The lowest conduction-band energy level of ZnO approaches the reduced hydrogen energy level. Moreover, the energy level difference between the highest valence-band energy level of ZnO and the water oxidation potential is large. Therefore, doping with lower-electronegativity impurities results in a more negative energy level compared with the highest valence-band energy level of ZnO and, thus, narrows the band gap. For this purpose, N has been widely used as a dopant to modify the electronic structures of metal oxide semiconductors because of its similar size to oxygen and the low formation energy required to substitute O. [14] However, the majority of studies on ZnO:N have focused on its PEC performance. [10,14,15] Reports on sensitized ZnO:N, which has more potential for efficient PEC water splitting, have been few. In the current study, we investigate the use of a CdTe QD-sensitized ZnO:N nanowire arrays in water splitting. The one-dimensional ZnO:N nanowires allow a unidirectional transport of electrons and, thus, reduce the prob- 773

2 ability of electron hole recombination. CdTe sensitization could further improve absorption in the visible region. These effects result in considerable enhancement of the photocurrent and stability of the photoelectrode. Results and Discussion ZnO nanowire arrays were synthesized on the entire surface of a fluorine-doped tin oxide (SnO 2 :F, FTO) glass substrate by using a modified hydrothermal method. The synthesized ZnO:N nanowire arrays were annealed in ammonia at 500 to 700 C for 30 min and then in nitrogen for another 30 min. CdTe QDs were deposited on the ZnO:N nanowire array by chemical-bath deposition, followed by thermal treatment to remove the linker between the QDs and the ZnO:N nanowires. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the specific nanostructures of the ZnO:N and ZnO:N@CdTe samples. The SEM images (Figure 1, a and b) show the compact and vertically aligned ZnO nanowires on the FTO substrate. The average nanowire diameter is ca. 150 nm, and the average length is around 4.5 μm. The nanostructures of the ZnO nanowire array were unchanged after they had been annealed in ammonia at 500 to 700 C (Figure 1, c e). onset at ca. 720 nm; this indicates that the use of the solution as a sensitizer would enable a wider range of visible wavelengths in sunlight to be harvested. Figure 2 (a) shows a typical HRTEM micrograph of the CdTe QDs. The size of the nanoparticles is ca. 4 6 nm, and they are well separated and roughly spherical with clear lattice fringes. The average particle size obtained from the images was nm. The particles are oriented along the (311) axis in the plane of the images with a lattice spacing of 0.20 nm. This value is consistent with that of CdTe bulk crystals (JCPDS file no ). The CdTe quantum dots were then deposited on the surface of the ZnO nanowire array for further structural and photoelectrochemical measurement. Figure 2. (a) HRTEM image of the CdTe QDs. (b) Absorption spectrum of a suspension of the CdTe QDs in water. The inset displays photographs of a CdTe QD solution under ambient and UV light (365 nm). Figure 1. Scanning electron microscopy (SEM) images of (a) crosssectional and (b) top views of ZnO. SEM images of ZnO:N at annealing temperatures of (c) 500, (d) 600, and (e) 700 C. Figure 2 displays the high-resolution TEM (HRTEM) image and the absorption spectrum of the prepared CdTe QDs in solution. Figure 2 (b) shows the absorption spectrum of CdTe QDs prepared by wet-chemical processes. The prepared CdTe QDs were capped by mercaptopropionic acid (MPA) ligands, which could help the CdTe QDs disperse in water and self-assemble on the surfaces of the ZnO nanowires. The inset displays photographs of solutions of CdTe QDs under ambient and UV light with a wavelength of 365 nm; the CdTe QDs are highly dispersed in water. The solution absorbs in the visible region with an The HRTEM image of ZnO annealed in ammonia at 700 C shows that ZnO:N has a single-crystal structure with a 0002 growth direction (Figure 3, a). The HRTEM image of the ZnO:N nanowire decorated with CdTe QDs and the corresponding energy-dispersive X-ray spectroscopy (EDS) spectrum are shown in Figure 3 (c and d). Part c of Figure 3 shows that the QDs are directly attached to the ZnO nanowire surface. An abrupt transition was observed between the (0002) lattice planes of the ZnO nanowires and the (311) lattice planes of the CdTe QDs. This result is consistent with that of the bulk CdTe cubic crystal and proves that individual ZnO nanowires are covered with CdTe nanoparticles with diameters of ca. 5 to 6 nm, which corresponds to the diameter from the previous HRTEM image of the CdTe QDs. The EDS spectrum of the ZnO:N (700)@CdTe (Figure 3, d) shows that the CdTe QDs are attached to the surface of the ZnO nanowires. Quantitative analysis reveals that the surface-modified ZnO nanowire array contains ca. 1 % CdTe QDs. X-ray diffraction (XRD) studies were conducted to examine the structural properties of the ZnO:N nanowires (Figure 4). The samples prepared at different temperatures yielded similar diffraction patterns. Pattern indexing showed that all diffraction peaks are consistent with the wurtzite ZnO structure with lattice constants of a = Å and c = Å. The high-intensity (002) diffraction peaks in the patterns indicate that the reflections from the (002) plane of the ZnO nanowires are stronger than those from other planes because of the [00l]-oriented nanowire growth. 774

3 Figure 4. XRD patterns of ZnO, ZnO:N (500), ZnO:N (600), and ZnO:N (700). Figure 3. (a) HRTEM image of the ZnO:N nanowires. (b) TEM image of ZnO:N (c) HRTEM image of ZnO:N (d) EDS spectrum of ZnO:N The lattice constants and phase changes of the ZnO and ZnO:N nanowires show no significant differences; therefore, N doping has no significant effect on the lattice. To investigate the N incorporation in the ZnO:N nanowires, X-ray photoelectron spectroscopy (XPS) was performed to determine the quantitative concentration and the chemical state of N. The results are shown in Figure 5. A plot of the N concentration in the ZnO:N nanowires versus the ammonia annealing temperature is shown in Figure 5 (a). To prevent errors in the oxygen concentration measurements of the ZnO nanowires on the FTO substrate, all XPS samples were prepared on a silicon substrate. As the annealing temperature increased from 500 to 700 C (Figure 5, b d), the N concentration slightly increased from 1.0 to 4.0 wt.-%. The diffusion distance of nitrogen increases with increasing annealing temperature and, subsequently, the doped N concentration increases. High-resolution XPS studies were then performed to identify the chemical state of the N dopant in the ZnO:N nanowires (Figure 5, b). The core level spectrum of the N 1s region shows an asymmetric broad peak centered at ev. All experimental line profiles show two peaks centered at and ev. The peak centered at ev is attributed to typical Zn N bonds and, thus, confirms the successful doping of N in the ZnO crystal structure. [10,14] The peak centered at ev is characteristic of the N 1s binding energy in amines. Therefore, N is successfully doped at the O sites of ZnO during the nitridation reaction. Figure 5. (a) X-ray photoelectron (XPS) plot of the ZnO:N nanowire nitrogen concentration vs. the annealing temperature. High-resolution XPS spectra of (b) ZnO:N (500), (c) ZnO:N (600), and (d) ZnO:N (700). UV/Vis absorption spectroscopy was conducted to investigate the optical changes in the ZnO:N samples. The results are shown in Figure 6. The absorption edge of the pristine ZnO nanowires is at 380 nm. Ammonia annealing at 500 to 700 C produced a pale yellow ZnO:N nanowire electrode and caused a redshift of the absorption edge to 550 nm. The broader absorption in the nm range is attributed to changes in the band structure of the ZnO nanowires as a result of nitridation. The first excitonic peak of the CdTe QDs at 690 nm was not exhibited by the ZnO:N nanowires and this could possibly improve their absorption in the visible light region. The UV/Vis absorption spectra of the ZnO:N@CdTe samples exhibit almost the same curve from 550 to 800 nm and are shown in Figure 6 (b). The increased absorption in the visible light region of ZnO:N@CdTe compared with that of the ZnO:N samples is attributed to the decorated CdTe QDs on the ZnO nanowires surface. Nitridation of the ZnO nanowires in combination with CdTe QD sensitization induces the efficient harvest of solar light over a wider range of wavelengths. 775

4 Figure 6. UV/Vis absorption spectra. (a) ZnO, ZnO:N (500), ZnO:N (600), and ZnO:N (700) nanowires. (b) ZnO:N (700), ZnO:N ZnO:N and ZnO:N nanowires. As a proof of concept for the photoactivity under light illumination, PEC studies were performed by using 0.5 m Na 2 SO 4 (ph 6.8) as supporting electrolyte. Figure 7 (a) displays a set of linear-sweep voltammograms that were recorded with pristine ZnO nanowires and ZnO:N nanowire arrays. The ZnO:N (700) nanowire array showed a pronounced photocurrent starting at ca. 0.2 V, which increased to 0.31 ma/cm 2 at 0.5 V under illumination. The photocurrent density of the ZnO:N nanowire array was higher (ca ma/cm 2 ) than that of the pristine ZnO nanowires (ca ma/cm 2 ) at 0.5 V, which suggests that the synthesized ZnO:N is more efficient at harvesting solar light and converting it to electricity than the pristine nanowires. The two most important metrics associated with photocurrent are the plateau current and the onset potential. [5,16] The plateau current depends mainly on the photogenerated holes and electrons that reach the semiconductor liquid junction and, instead of recombining, subsequently react with water. The overpotential must be considered when the onset potential is determined. A large overpotential is caused mainly by the slow kinetics of water oxidation and results in the accumulation of holes and electrons on the surface. Subsequent surface recombination occurs until sufficiently positive potentials are applied. Modification of the electrode surface lowers the kinetic barrier to interfacial charge transfer and, thus, reduces the required overpotential and shifts the curve to the left. The onset potentials of the ZnO:N nanowires prepared at different annealing temperatures are more negative than that of pristine ZnO. These samples exhibited identical behavior, which suggests that their surfaces are similar. Notably, the plateau Figure 7. Photoelectrochemical (PEC) performance measurement in 0.5 m Na 2 SO 4 under 100 mw/cm 2. Linear-sweep voltammograms of (a) ZnO:N and (b) ZnO:N@CdTe. (c) Photoconversion efficiency of bare ZnO, ZnO:N, and ZnO:N@CdTe. (d) Measured IPCE spectra of bare ZnO, ZnO:N, and ZnO:N@CdTe. (e) Chronoamperometric measurement of ZnO:N@CdTe. (f) Gas evolution of the ZnO and ZnO:N(700)@CdTe nanowire photoelectrodes. 776

5 current of the ZnO:N annealed at 500 C is nearly equal to that of the pristine ZnO nanowires; therefore, a low dopant concentration does not significantly improve the plateau photocurrent. The improvement in the plateau current is caused by the formation of the ZnO:N nanowires; the ZnO:N nanowires can generate more photoelectrons by harvesting a higher amount of sunlight. To improve the PEC performance, we sensitized the ZnO:N nanowires with CdTe QDs, which could extend the absorption region of the photoelectrode to allow the harvesting of more visible light to generate photoelectrons. The linear-sweep voltammograms of the ZnO:N@CdTe photoelectrodes are shown in Figure 7 (b). After sensitization, the onset potential slightly shifted to a more negative potential. The ZnO:N@CdTe heterojunction increased the charge-transfer efficiency and, thus, reduced the overpotential. The plateau current of the ZnO:N@CdTe is higher (ca ma/cm 2 ) than that of the pristine ZnO (ca ma/cm 2 ) and ZnO:N nanowire arrays. This increase is attributed to the photoelectrons generated by the CdTe QDs. We performed amperometric I t studies to investigate the photoresponses of ZnO:N and ZnO:N@CdTe over time. The energy conversion efficiency (η) of a photoelectrochemical cell is calculated as follows: j p is the measured photocurrent density in ma/cm 2 0, E rev denotes the standard reversible potential, which is 1.23 V vs. the normal hydrogen electrode (NHE), I 0 is the intensity of incident light in mw/cm 2, and E app is the electrode potential between the working electrode and the counterelectrode at which the photocurrent was measured under illumination. The plot of efficiency against applied potential (Figure 7, c) reveals a maximum efficiency of ca. 0.75%, which is obtained at an applied potential of +0.5 V. Importantly, the ZnO nanowires that were sensitized with CdTe QD solution were over two times as efficient as bare ZnO nanowires and showed a typical photoconversion efficiency of 0.28 %. The photoconversion efficiency of ZnO:N- (700)@CdTe is greater than that of the ZnO:N (700) photoanode; this could be attributed to the contribution of the quantum dot sensitization. To quantify the photoresponse of ZnO:N@CdTe photoanodes, incident-photon-to-current-conversion efficiency (IPCE) measurements were performed to examine their photoresponse as a function of incident light wavelength (Figure 7, d). IPCE can be expressed as: IPCE = (1240 I)/(λ J light ) I is the photocurrent density, J light is the measured illumination, and λ is the wavelength of the incident light. The ZnO:N nanowires exhibited substantially greater IPCE than bare ZnO nanowires in both the visible and UV regions, primarily because of the increase in light absorption caused by the nitridation. The ZnO:N nanowires that were sensitized with CdTe QDs exhibited photoactivity over a broader wavelength range ( nm with an IPCE value of ca. 4%) because of the nitridation and the sensitization. At the same incident wavelength ( nm), the higher IPCE of the ZnO:N(700)@CdTe composite revealed that it was more efficient than bare ZnO for the separation and/or collection of photoexcited electrons in the visible region; this finding is consistent with the larger potential difference between the conduction bands of CdTe and ZnO. In the absorption spectrum of ZnO:N(700)@CdTe, it is worth noting that the photoanode does not have any photoactivity at wavelengths longer than 660 nm. This may be because the photoenergy in the longer wavelength is not enough to overcome the overpotential to drive the water-splitting reaction. The I t curves of the ZnO:N@CdTe samples with cutoff light cycles at 100 mw/cm 2 and 0.5 V are shown in Figure 7 (e). Extremely low dark currents at 10 7 ma/cm 2 were observed during the measurement. The photocurrent intensities of ZnO:N and ZnO:N@CdTe show a pile in the photoresponse, which is caused by the transient effect of power excitation. The photocurrent then rapidly returned to the steady state, which indicates an efficient photoelectron transfer. The I t curves of all samples show that the photocurrent does not decrease with increasing measurement time and, thus, confirm rapid electron transport and photoelectrode stability. To demonstrate the occurrence of the water-splitting reaction under simulated solar light illumination, the gas evolution of PEC was measured by using a two-electrode system with 0.5 V bias under a solar simulator with power density of 100 mw/cm 2 (Figure 7, f). Approximately 20.8 and 6.8 μmolh 1 of H 2 and 10.2 and 3.3 μmolh 1 of O 2 were produced when the ZnO:N(700 and pristine ZnO photoelectrodes were used, respectively, which indicates that water splitting was more efficient on the ZnO:N@CdTe photoelectrode. The slightly decreased oxygen evolution after 2 h of measurement may have contributed to the low Faraday efficiency. These findings show that the combination of N-doped ZnO nanowires and CdTe QD sensitization can improve the efficiency of PEC water splitting. To investigate the electronic state of the ZnO:N nanowires, the O K-edges of all ZnO:N nanowires were determined by X-ray absorption spectroscopy (XAS). The results are shown in Figure 8. The white-line peaks correspond to the O 1s to O 2p transition, which indicates that N was incorporated into the ZnO nanostructure and increased the valence state of oxygen. The increased oxygen Figure 8. X-ray absorption near-edge structure measurement of the ZnO:N nanowire O K-edge. 777

6 valence state results in increased adsorption of water on the ZnO surface for the water-splitting reaction; [17,18] therefore, the water-splitting performance improves. Although XPS has demonstrated the incorporation of N in the ZnO nanostructures, little structural information concerning the zinc atoms was revealed. Extended X-ray absorption fine structure (EXAFS) is caused by local correlations around the absorbing atom; it provides a shortrange structural probe and yields results concerning specifically the nearest-neighbor interatomic distances and coordination numbers. [19,20] EXAFS can provide more convincing evidence of the structural parameters of the ZnO:N nanowire array. The 01C1 beam line of the National Synchrotron Radiation Research Center (NSRRC), Taiwan was designed for such experiments. Structural parameters from each spectrum were obtained by EXAFS refinement (Table 1). The EXAFS spectra of the Zn K-edge (Figure 9) were used to determine the structural parameters based on a two-shell model that involves Zn O, Zn N, and Zn Zn shells to characterize the short-range structure around the Zn atoms. Figure 9 shows the Zn K-edge spectra of the ZnO and ZnO:N nanowires arrays. In pristine ZnO nanowires, these results suggest that the interatomic distance scattered from the first nearest-neighboring O atoms and the second nearest-neighboring Zn atoms are ca. 1.9 and 3.2 Å, respectively. A strong peak at ca Å in the Fourier transform (FT) of the Zn K-edge EXAFS spectrum of the ZnO nanowires with phase correction suggests that central Zn atoms are surrounded by O atoms in first shell scattering. Another strong peak (ca. 3.2 Å with phase correction) indicates that the second shell around the Zn atoms includes neighboring Zn atoms. The coordination numbers (CNs) of the Zn O (2.5) and Zn Zn (11.8) paths were similar to the theoretical values (Zn O: 4 and Zn Zn: 12). This is consistent with the value for the bulk wurtzite phase of ZnO. For the ZnO:N nanowire array, another peak (ca. 1.5 Å with phase correction) scattered from first shell atoms indicated that the first shell around the Zn atoms may be affected by the incorporation of N atoms and may include neighboring atoms of two elements (O and N). It was worth noting that the second-shell scattering (ca. 3.2 Å) around the central Zn Table 1. Zn K-edge EXAFS structural parameters of ZnO, ZnO:N (500), ZnO:N (600), and ZnO:N (700) nanowires. Sample Path R [Å] CN σ 2 [Å 2 ] ΔE [ev] ZnO Zn O 1.97(3) 2.5(2) (5) 5.7(3) Zn Zn 3.23(2) 11.8(5) (2) 1.0(5) Zn O 1.97(6) 2.2(4) (4) 6.6(7) ZnO:N (500) Zn N 1.49(5) 0.3(3) (3) 7.5(6) Zn Zn 3.21(5) 12.1(7) (5) 0.4(3) Zn O 1.98(5) 2.9(2) (3) 7.9(4) ZnO:N (600) Zn N 1.53(3) 0.8(3) (6) 0.5(8) Zn Zn 3.21(3) 12.5(6) (2) 1.1(7) Zn O 1.99(2) 2.8(1) (3) 11.1(2) ZnO:N (700) Zn N 1.52(4) 1.2(3) (3) 18.4(2) Zn Zn 3.20(2) 12.3.(9) (4) 2.9(4) atom was identical in the ZnO and ZnO:N nanowire arrays; this could be attributed to the existence of only Zn atoms in the second scattering shell. The CN of the Zn N nanowires increases with increasing annealing temperature and, subsequently, the doped N concentration increases. This demonstrated that the ZnO:N nanowire array exhibited the wurtzite crystal structure, which is able to provide the additional potential advantage of improved charge transport over traditional doping approaches and results in a dramatic increase in the plateau current. Figure 9. EXAFS spectra of the Zn K-edge for ZnO, ZnO:N (500), ZnO:N (600), and ZnO:N (700) nanowires. Conclusions We have demonstrated significant improvements in the CdTe-sensitized ZnO:N photoelectrode that result in a dramatic increase in the plateau photocurrent as well as a substantial shift in the onset potential. A photocurrent of 0.46 ma/cm 2 at 0.5 V was observed. This value is over 300 % higher than that of pristine ZnO nanowires. The proposed method resulted in higher photoactivity than traditional QD sensitization and also caused hydrogen generation. X-ray absorption spectroscopy demonstrated the incorporation of N atoms in ZnO crystal and its effect on the electronic structure. Experimental Section Chemicals and Substrates: Zinc nitrate, absolute ethanol, zinc acetate, and Te powder were purchased from Sigma Aldrich. Sodium borohydride and hexamethylenetetramine (HMT) were obtained from Acros Organics. Cadmium chloride and mercaptopropionic acid (MPA) were purchased from Fluka. FTO substrates (F:SnO 2, Tec 15, 10 Ω/sq) were purchased from the Hartford Glass Company. All chemicals were used as received. Synthesis of ZnO-Seeded Substrates: A solution of zinc acetate in absolute ethanol (0.01 m, 100 ml) was mixed by ultrasonic agitation. The FTO substrates were dipped in the zinc acetate solution for 10 s and then blow-dried with an argon stream. The procedure was repeated several times. The FTO substrates were annealed at 350 C for 30 min to yield a layer of ZnO seeds. Synthesis of ZnO:N Nanorods: The seeded substrates were horizontally suspended in a reagent solution containing zinc nitrate (0.06 m) and HMT (0.06 m) in a Teflon vessel. The vessel was 778

7 placed in an autoclave and then heated to 110 C for nanowire growth. After 24 h of growth, the nanowire substrates were removed from the autoclave, washed with distilled water, and then dried in air. The nanowire substrates were baked at 450 C for 30 min. Synthesis of CdTe QDs: NaBH 4 (0.08 g) was treated with Te powder (0.127 g) in distilled water (3.0 ml) for 4 h to produce 1 m of a sodium hydrogen telluride (NaHTe) solution. The NaHTe solution (1.5 ml) was added to a N 2 -saturated mixture (74.8 ml; ph 10.8) of 38 mm MPA and 16 mm CdCl 2 to give a final Cd 2+ :MPA:HTe molar ratio of 1:2.4:0.5. The mixture was heated under reflux at 90 C for 3 h. The solution changed from dark red to orangeyellow. The CdTe QDs were purified by centrifugation in absolute ethanol to remove free ligands such as MPA and unreacted precursor ions. Synthesis of ZnO:N@CdTe Photoelectrode: The prepared ZnO nanorods were placed in an Al 2 O 3 crucible for nitridation at 500, 600, and 700 C for various durations under a NH 3 atmosphere with a flow rate of 100 cm 3 /min. The ZnO:N photoelectrodes were immersed in a CdTe solution for 24 h to prepare the ZnO:N@CdTe photoelectrode. PEC Characterization: Electrochemical characterization was conducted by using three-electrode-based methods. The ZnO:N@CdTe photoelectrode was used as the working electrode. A Ag/AgCl electrode was used as the reference electrode, and a platinum plate was the counterelectrode. PEC was determined in a 0.5 m Na 2 SO 4 (ph 6.8) solution, which served as the supporting electrolyte. The photoelectrode was illuminated by an AM 1.5 solar simulator at 100 mw/cm 2. The I V characteristics of the PEC were recorded at 25 C by using a potentiostat (Eco Chemie AUTOLAB, Netherlands) and the General Purpose Electrochemical System software. Hydrogen and oxygen generation were determined by using a twoelectrode system with 0.5 V bias under solar light simulation at 100 mw/cm 2. Chronoamperometric measurements were conducted by using a three-electrode system with 0.5 V bias under solar light simulation at 100 mw/cm 2. The ZnO:N@CdTe photoelectrode was used as the working electrode, and a Pt wire was used as the counterelectrode. The accumulated H 2 and O 2 in the glass system were measured with a China Chromatography 2000 gas chromatograph equipped with a flame ionization detector. UV/Vis Absorption Spectra Measurement: The UV/Vis absorption spectra of CdTe QD suspensions, ZnO, and ZnO:N@CdTe were obtained at room temp. with a SHIMADZU UV-700 spectrophotometer by using 1 cm wide quartz cells. Characterization: High-resolution transmission electron microscope (HRTEM) images, electron diffraction patterns, and elemental maps were captured with a JEOL JEM-2100F electron microscope. The morphology of the nanowires was investigated with a JEOL JSM-6700F field-emission scanning electron microscope (FE-SEM). Elemental analysis was conducted by using an inductively coupled plasma atomic emission spectrometer (Shimadzu ICPS-1000III) and an elemental analyzer (Flash EA 1112 series/ CE Instruments). X-ray photoelectron spectroscopy (XPS, Al-K α radiation, λ = 8.34 Å) was performed with a PHI Quantera instrument. A series of XAS measurements of the synthesized samples were made by using synchrotron radiation at room temp. Measurements were made at the Zn K-edge (9659 ev) with the sample held at room temp. The 01C1 beam line of the National Synchrotron Radiation Research Center (NSRRC), Taiwan was designed for such experiments. Acknowledgments The authors thank the National Science Council of Taiwan (contract number NSC M MY3) for financial support. Mrs. C.-Y. 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