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DOI:10.1002/ejic.201301310

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Quantum-Dot-Sensitized Nitrogen-Doped ZnO forEfficient 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] andRu-Shi Liu*[a]

Keywords: Water splitting / Doping / Zinc / Quantum dots

Fossil fuels have been used for several decades and haveresulted in increased greenhouse gases and pollutants. Cur-rently, clean and renewable energy is in demand. Hydrogenappears to be a good candidate for clean energy because theonly product of its reaction with oxygen is water. Water split-ting by solar energy is a potential method for the generationof hydrogen in future applications. This study investigatesthe use of a CdTe quantum-dot-sensitized ZnO:N nanowirearrays for water splitting. The proposed method resulted in

IntroductionThe use of solar energy and water to synthesize fuels is

vital to sustainable development beyond fossil fuels. Photo-catalytic and photoelectrochemical (PEC) water splitting byusing semiconductor materials to generate hydrogen andoxygen have attracted attention worldwide because of theirrenewable fuel generation, which benefits the environment.PEC water splitting combines electrical generation and elec-trolysis into a single system. Semiconductor materials havebeen the focus of investigation as the key component insolar energy conversion systems.[1–4] To achieve efficientPEC water splitting, the band gap of the semiconductormust exceed 1.6 to 1.7 eV to exhibit sufficient overpotentialfor water splitting, because the theoretical difference in theequilibrium potentials of the water-splitting reactions is1.23 V at 25 °C.[5] For PEC cells for water splitting, metaloxide photoelectrodes with wide band gaps, such as ZnOand TiO2,[6] are severely limited by their low absorption ofvisible light. Various approaches have been adopted to in-crease their absorption in the visible region, including dop-ing with heteroatoms and sensitization with dyes or quan-tum dots (QDs). Semiconductor QDs, such as CdS,[7]

CdSe,[8] CdTe,[3] and InP QDs,[9] have been coated on metaloxide semiconductor nanostructures as photoelectrode sen-sitizers. This approach improves the absorption in the vis-

[a] Department of Chemistry, National Taiwan University,Taipei, TaiwanE-mail: [email protected]://www.ch.ntu.edu.tw/~rsliu/

[b] National Synchrotron Radiation Research Center,Hsinchu, Taiwan

Eur. J. Inorg. Chem. 2014, 773–779 © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim773

considerably enhanced photocurrent and stability. The elec-tronic structures of the ZnO:N materials are also determinedby O K-edge X-ray absorption spectroscopy. The incorpora-tion of nitrogen into the ZnO nanostructure is determinedby X-ray photoelectron spectroscopy and Zn K-edge X-rayabsorption spectroscopy; the nitrogen incorporation changesthe electronic state and, thus, increases the water-splittingperformance.

ible region and, thus, enhances the water-splitting perform-ance. Zinc oxide is a multifunctional semiconductor mate-rial with a direct band gap of ca. 3.2 eV, high electron mo-bility, low electrical resistance and high electron-transferefficiency.

To utilize solar energy, impurities that frequently causedramatic changes in the electrical and optical properties canbe introduced.[10–12] The introduced impurities generate anew energy level in the band gap of pristine semiconductormaterials. The energy level of ZnO indicates that the con-duction band consists mainly of Zn 4s and Zn 4p states andthat the valence band is composed of the O 2p state.[13] Thelowest conduction-band energy level of ZnO approachesthe reduced hydrogen energy level. Moreover, the energylevel difference between the highest valence-band energylevel of ZnO and the water oxidation potential is large.Therefore, doping with lower-electronegativity impuritiesresults in a more negative energy level compared with thehighest valence-band energy level of ZnO and, thus, nar-rows the band gap. For this purpose, N has been widelyused as a dopant to modify the electronic structures ofmetal oxide semiconductors because of its similar size tooxygen and the low formation energy required to substituteO.[14] However, the majority of studies on ZnO:N have fo-cused on its PEC performance.[10,14,15] Reports on sensitizedZnO:N, which has more potential for efficient PEC watersplitting, have been few.

In the current study, we investigate the use of a CdTeQD-sensitized ZnO:N nanowire arrays in water splitting.The one-dimensional ZnO:N nanowires allow a unidirec-tional transport of electrons and, thus, reduce the prob-

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ability of electron–hole recombination. CdTe sensitizationcould further improve absorption in the visible region.These effects result in considerable enhancement of thephotocurrent and stability of the ZnO:N@CdTe photoelec-trode.

Results and Discussion

ZnO nanowire arrays were synthesized on the entiresurface of a fluorine-doped tin oxide (SnO2:F, FTO) glasssubstrate by using a modified hydrothermal method. Thesynthesized ZnO:N nanowire arrays were annealed in am-monia at 500 to 700 °C for 30 min and then in nitrogen foranother 30 min. CdTe QDs were deposited on the ZnO:Nnanowire array by chemical-bath deposition, followed bythermal treatment to remove the linker between the QDsand the ZnO:N nanowires. Scanning electron microscopy(SEM) and transmission electron microscopy (TEM) wereused to characterize the specific nanostructures of theZnO:N and ZnO:N@CdTe samples. The SEM images (Fig-ure 1, a and b) show the compact and vertically alignedZnO nanowires on the FTO substrate. The averagenanowire diameter is ca. 150 nm, and the average length isaround 4.5 μm. The nanostructures of the ZnO nanowirearray were unchanged after they had been annealed in am-monia at 500 to 700 °C (Figure 1, c–e).

Figure 1. Scanning electron microscopy (SEM) images of (a) cross-sectional and (b) top views of ZnO. SEM images of ZnO:N at an-nealing 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 CdTeQDs in solution. Figure 2 (b) shows the absorption spec-trum of CdTe QDs prepared by wet-chemical processes.The prepared CdTe QDs were capped by mercaptopropi-onic acid (MPA) ligands, which could help the CdTe QDsdisperse in water and self-assemble on the surfaces of theZnO nanowires. The inset displays photographs of solutionsof CdTe QDs under ambient and UV light with a wave-length of 365 nm; the CdTe QDs are highly dispersed inwater. The solution absorbs in the visible region with an


onset at ca. 720 nm; this indicates that the use of the solu-tion as a sensitizer would enable a wider range of visiblewavelengths in sunlight to be harvested. Figure 2 (a) showsa typical HRTEM micrograph of the CdTe QDs. The sizeof the nanoparticles is ca. 4–6 nm, and they are well sepa-rated and roughly spherical with clear lattice fringes. Theaverage particle size obtained from the images was5.3�0.8 nm. The particles are oriented along the (311) axisin 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. 89-3053). The CdTe quantum dots werethen deposited on the surface of the ZnO nanowire arrayfor further structural and photoelectrochemical measure-ment.

Figure 2. (a) HRTEM image of the CdTe QDs. (b) Absorptionspectrum of a suspension of the CdTe QDs in water. The insetdisplays photographs of a CdTe QD solution under ambient andUV light (365 nm).

The HRTEM image of ZnO annealed in ammonia at700 °C shows that ZnO:N has a single-crystal structure witha �0002� growth direction (Figure 3, a). The HRTEM im-age of the ZnO:N nanowire decorated with CdTe QDs andthe corresponding energy-dispersive X-ray spectroscopy(EDS) spectrum are shown in Figure 3 (c and d). Part c ofFigure 3 shows that the QDs are directly attached to theZnO nanowire surface. An abrupt transition was observedbetween the (0002) lattice planes of the ZnO nanowires andthe (311) lattice planes of the CdTe QDs. This result is con-sistent with that of the bulk CdTe cubic crystal and provesthat individual ZnO nanowires are covered with CdTenanoparticles with diameters of ca. 5 to 6 nm, which corre-sponds to the diameter from the previous HRTEM imageof the CdTe QDs. The EDS spectrum of the ZnO:N(700)@CdTe (Figure 3, d) shows that the CdTe QDs areattached to the surface of the ZnO nanowires. Quantitativeanalysis reveals that the surface-modified ZnO nanowirearray contains ca. 1 % CdTe QDs.

X-ray diffraction (XRD) studies were conducted to ex-amine the structural properties of the ZnO:N nanowires(Figure 4). The samples prepared at different temperaturesyielded similar diffraction patterns. Pattern indexingshowed that all diffraction peaks are consistent with thewurtzite ZnO structure with lattice constants of a = 3.250 Åand c = 5.207 Å. The high-intensity (002) diffraction peaksin the patterns indicate that the reflections from the (002)plane of the ZnO nanowires are stronger than those fromother planes because of the [00l]-oriented nanowire growth.


Figure 3. (a) HRTEM image of the ZnO:N nanowires. (b) TEMimage of ZnO:N (700)@CdTe. (c) HRTEM image of ZnO:N(700)@CdTe. (d) EDS spectrum of ZnO:N (700)@CdTe.

The lattice constants and phase changes of the ZnO andZnO:N nanowires show no significant differences; there-fore, N doping has no significant effect on the lattice. Toinvestigate the N incorporation in the ZnO:N nanowires,X-ray photoelectron spectroscopy (XPS) was performed todetermine the quantitative concentration and the chemicalstate of N. The results are shown in Figure 5. A plot of theN concentration in the ZnO:N nanowires versus the ammo-nia annealing temperature is shown in Figure 5 (a). To pre-vent errors in the oxygen concentration measurements ofthe ZnO nanowires on the FTO substrate, all XPS sampleswere prepared on a silicon substrate. As the annealing tem-perature increased from 500 to 700 °C (Figure 5, b–d), theN concentration slightly increased from 1.0 to 4.0 wt.-%.The diffusion distance of nitrogen increases with increasingannealing temperature and, subsequently, the doped N con-centration increases. High-resolution XPS studies were thenperformed to identify the chemical state of the N dopant inthe ZnO:N nanowires (Figure 5, b). The core level spectrumof the N 1s region shows an asymmetric broad peak cen-tered at 396.3 eV. All experimental line profiles show twopeaks centered at 396.3 and 399.6 eV. The peak centeredat 396.3 eV is attributed to typical Zn–N bonds and, thus,confirms the successful doping of N in the ZnO crystalstructure.[10,14] The peak centered at 399.6 eV is characteris-tic of the N 1s binding energy in amines. Therefore, N issuccessfully doped at the O sites of ZnO during the nitrid-ation reaction.


Figure 4. XRD patterns of ZnO, ZnO:N (500), ZnO:N (600), andZnO:N (700).

Figure 5. (a) X-ray photoelectron (XPS) plot of the ZnO:Nnanowire 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 inves-tigate the optical changes in the ZnO:N samples. The resultsare shown in Figure 6. The absorption edge of the pristineZnO nanowires is at 380 nm. Ammonia annealing at 500 to700 °C produced a pale yellow ZnO:N nanowire electrodeand caused a redshift of the absorption edge to 550 nm. Thebroader absorption in the 400–550 nm range is attributed tochanges in the band structure of the ZnO nanowires as aresult of nitridation. The first excitonic peak of the CdTeQDs at 690 nm was not exhibited by the ZnO:N nanowiresand this could possibly improve their absorption in the vis-ible light region. The UV/Vis absorption spectra of theZnO:N@CdTe samples exhibit almost the same curve from550 to 800 nm and are shown in Figure 6 (b). The increasedabsorption in the visible light region of ZnO:N@CdTe com-pared with that of the ZnO:N samples is attributed to thedecorated CdTe QDs on the ZnO nanowires surface. Nitrid-ation of the ZnO nanowires in combination with CdTe QDsensitization induces the efficient harvest of solar light overa wider range of wavelengths.


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 (500)@CdTe, ZnO:N (600)@CdTe, and ZnO:N(700)@CdTe nanowires.

As a proof of concept for the photoactivity under lightillumination, PEC studies were performed by using 0.5 m

Na2SO4 (pH 6.8) as supporting electrolyte. Figure 7 (a) dis-plays a set of linear-sweep voltammograms that were re-corded with pristine ZnO nanowires and ZnO:N nanowirearrays. The ZnO:N (700) nanowire array showed a pro-nounced photocurrent starting at ca. –0.2 V, which in-creased to 0.31 mA/cm2 at 0.5 V under illumination. Thephotocurrent density of the ZnO:N nanowire array was

Figure 7. Photoelectrochemical (PEC) performance measurement in 0.5 m Na2SO4 under 100 mW/cm2. 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 spectraof bare ZnO, ZnO:N, and ZnO:N@CdTe. (e) Chronoamperometric measurement of ZnO:N@CdTe. (f) Gas evolution of the ZnO andZnO:N(700)@CdTe nanowire photoelectrodes.


higher (ca. 0.31 mA/cm2) than that of the pristine ZnOnanowires (ca. 0.15 mA/cm2) at 0.5 V, which suggests thatthe synthesized ZnO:N is more efficient at harvesting solarlight and converting it to electricity than the pristinenanowires. The two most important metrics associated withphotocurrent are the plateau current and the onset poten-tial.[5,16] The plateau current depends mainly on the photo-generated holes and electrons that reach the semiconduc-tor–liquid junction and, instead of recombining, sub-sequently react with water. The overpotential must be con-sidered when the onset potential is determined. A large ov-erpotential is caused mainly by the slow kinetics of wateroxidation and results in the accumulation of holes and elec-trons on the surface. Subsequent surface recombination oc-curs until sufficiently positive potentials are applied. Modi-fication of the electrode surface lowers the kinetic barrierto interfacial charge transfer and, thus, reduces the requiredoverpotential and shifts the curve to the left. The onset po-tentials of the ZnO:N nanowires prepared at different an-nealing temperatures are more negative than that of pristineZnO. These samples exhibited identical behavior, whichsuggests that their surfaces are similar. Notably, the plateau


current of the ZnO:N annealed at 500 °C is nearly equal tothat of the pristine ZnO nanowires; therefore, a low dopantconcentration does not significantly improve the plateauphotocurrent. The improvement in the plateau current iscaused by the formation of the ZnO:N nanowires; theZnO:N nanowires can generate more photoelectrons byharvesting a higher amount of sunlight. To improve thePEC performance, we sensitized the ZnO:N nanowires withCdTe QDs, which could extend the absorption region of thephotoelectrode to allow the harvesting of more visible lightto generate photoelectrons. The linear-sweep voltammog-rams of the ZnO:N@CdTe photoelectrodes are shown inFigure 7 (b). After sensitization, the onset potential slightlyshifted to a more negative potential. The ZnO:N@CdTeheterojunction increased the charge-transfer efficiency and,thus, reduced the overpotential. The plateau current of theZnO:N@CdTe is higher (ca. 0.46 mA/cm2) than that of thepristine ZnO (ca. 0.15 mA/cm2) and ZnO:N nanowire ar-rays. This increase is attributed to the photoelectrons gener-ated by the CdTe QDs. We performed amperometric I–tstudies to investigate the photoresponses of ZnO:N andZnO:N@CdTe over time.

The energy conversion efficiency (η) of a photoelectro-chemical cell is calculated as follows:

jp is the measured photocurrent density in mA/cm2, Erev0

denotes the standard reversible potential, which is 1.23 Vvs. the normal hydrogen electrode (NHE), I0 is the intensityof incident light in mW/cm2, and Eapp is the electrode po-tential between the working electrode and the counterelec-trode at which the photocurrent was measured under illumi-nation. The plot of efficiency against applied potential (Fig-ure 7, c) reveals a maximum efficiency of ca. 0.75%, whichis obtained at an applied potential of +0.5 V. Importantly,the ZnO nanowires that were sensitized with CdTe QDsolution were over two times as efficient as bare ZnOnanowires and showed a typical photoconversion efficiencyof 0.28 %. The photoconversion efficiency of ZnO:N-(700)@CdTe is greater than that of the ZnO:N (700) pho-toanode; this could be attributed to the contribution of thequantum dot sensitization. To quantify the photoresponseof ZnO:N@CdTe photoanodes, incident-photon-to-cur-rent-conversion efficiency (IPCE) measurements were per-formed to examine their photoresponse as a function ofincident light wavelength (Figure 7, d). IPCE can be ex-pressed as:

IPCE = (1240 � I)/(λ � Jlight)

I is the photocurrent density, Jlight is the measured illumina-tion, and λ is the wavelength of the incident light. TheZnO:N nanowires exhibited substantially greater IPCEthan bare ZnO nanowires in both the visible and UV re-gions, primarily because of the increase in light absorptioncaused by the nitridation. The ZnO:N nanowires that weresensitized with CdTe QDs exhibited photoactivity over abroader wavelength range (430–660 nm with an IPCE value


of ca. 4%) because of the nitridation and the sensitization.At the same incident wavelength (430–660 nm), the higherIPCE of the ZnO:N(700)@CdTe composite revealed that itwas more efficient than bare ZnO for the separation and/orcollection of photoexcited electrons in the visible region;this finding is consistent with the larger potential differencebetween the conduction bands of CdTe and ZnO. In theabsorption spectrum of ZnO:N(700)@CdTe, it is worth not-ing that the photoanode does not have any photoactivity atwavelengths longer than 660 nm. This may be because thephotoenergy in the longer wavelength is not enough toovercome the overpotential to drive the water-splitting reac-tion. The I–t curves of the ZnO:N@CdTe samples with cut-off light cycles at 100 mW/cm2 and 0.5 V are shown in Fig-ure 7 (e). Extremely low dark currents at 10–7 mA/cm2 wereobserved during the measurement. The photocurrent inten-sities of ZnO:N and ZnO:N@CdTe show a pile in thephotoresponse, which is caused by the transient effect ofpower excitation. The photocurrent then rapidly returnedto the steady state, which indicates an efficient photoelec-tron transfer. The I–t curves of all samples show that thephotocurrent does not decrease with increasing measure-ment time and, thus, confirm rapid electron transport andphotoelectrode stability. To demonstrate the occurrence ofthe water-splitting reaction under simulated solar light illu-mination, the gas evolution of PEC was measured by usinga two-electrode system with 0.5 V bias under a solar simula-tor with power density of 100 mW/cm2 (Figure 7, f).Approximately 20.8 and 6.8 μmolh–1 of H2 and 10.2 and3.3 μmolh–1 of O2 were produced when the ZnO:N(700 °C)@CdTe and pristine ZnO photoelectrodes were used,respectively, which indicates that water splitting was moreefficient on the ZnO:N@CdTe photoelectrode. The slightlydecreased oxygen evolution after 2 h of measurement mayhave contributed to the low Faraday efficiency. These find-ings show that the combination of N-doped ZnO nanowiresand CdTe QD sensitization can improve the efficiency ofPEC water splitting.

To investigate the electronic state of the ZnO:Nnanowires, the O K-edges of all ZnO:N nanowires were de-termined by X-ray absorption spectroscopy (XAS). The re-sults are shown in Figure 8. The white-line peaks corre-spond to the O 1s to O 2p transition, which indicates thatN was incorporated into the ZnO nanostructure and in-creased the valence state of oxygen. The increased oxygen

Figure 8. X-ray absorption near-edge structure measurement of theZnO:N nanowire O K-edge.


valence state results in increased adsorption of water on theZnO surface for the water-splitting reaction;[17,18] therefore,the water-splitting performance improves.

Although XPS has demonstrated the incorporation of Nin the ZnO nanostructures, little structural informationconcerning the zinc atoms was revealed. Extended X-rayabsorption fine structure (EXAFS) is caused by local corre-lations around the absorbing atom; it provides a short-range structural probe and yields results concerning specifi-cally the nearest-neighbor interatomic distances and coordi-nation numbers.[19,20] EXAFS can provide more convincingevidence of the structural parameters of the ZnO:Nnanowire array. The 01C1 beam line of the National Syn-chrotron Radiation Research Center (NSRRC), Taiwan wasdesigned for such experiments. Structural parameters fromeach 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 ona two-shell model that involves Zn–O, Zn–N, and Zn–Znshells to characterize the short-range structure around theZn atoms. Figure 9 shows the Zn K-edge spectra of the ZnOand ZnO:N nanowires arrays. In pristine ZnO nanowires,these results suggest that the interatomic distance scatteredfrom the first nearest-neighboring O atoms and the secondnearest-neighboring Zn atoms are ca. 1.9 and 3.2 Å, respec-tively. A strong peak at ca. 1.97 Å in the Fourier transform(FT) of the Zn K-edge EXAFS spectrum of the ZnOnanowires with phase correction suggests that central Znatoms are surrounded by O atoms in first shell scattering.Another strong peak (ca. 3.2 Å with phase correction) indi-cates that the second shell around the Zn atoms includesneighboring Zn atoms. The coordination numbers (CNs) ofthe Zn–O (2.5) and Zn–Zn (11.8) paths were similar to thetheoretical values (Zn–O: 4 and Zn–Zn: 12). This is consis-tent with the value for the bulk wurtzite phase of ZnO. Forthe ZnO:N nanowire array, another peak (ca. 1.5 Å withphase correction) scattered from first shell atoms indicatedthat the first shell around the Zn atoms may be affected bythe incorporation of N atoms and may include neighboringatoms of two elements (O and N). It was worth noting thatthe 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 σ2 [Å2] ΔE [eV]

ZnO Zn–O 1.97(3) 2.5(2) 0.0035(5) 5.7(3)Zn–Zn 3.23(2) 11.8(5) 0.0104(2) 1.0(5)Zn–O 1.97(6) 2.2(4) 0.0036(4) 6.6(7)

ZnO:N Zn–N 1.49(5) 0.3(3) 0.0020(3) –7.5(6)(500)Zn–Zn 3.21(5) 12.1(7) 0.0106(5) 0.4(3)Zn–O 1.98(5) 2.9(2) 0.0027(3) 7.9(4)

ZnO:N Zn–N 1.53(3) 0.8(3) 0.0057(6) 0.5(8)(600)Zn–Zn 3.21(3) 12.5(6) 0.0105(2) –1.1(7)Zn–O 1.99(2) 2.8(1) 0.0024(3) 11.1(2)

ZnO:N Zn–N 1.52(4) 1.2(3) 0.0101(3) 18.4(2)(700)Zn–Zn 3.20(2) 12.3.(9) 0.0099(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 inthe second scattering shell. The CN of the Zn–N nanowiresincreases with increasing annealing temperature and, sub-sequently, the doped N concentration increases. This dem-onstrated that the ZnO:N nanowire array exhibited thewurtzite crystal structure, which is able to provide the ad-ditional potential advantage of improved charge transportover traditional doping approaches and results in a dra-matic 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 theCdTe-sensitized ZnO:N photoelectrode that result in a dra-matic increase in the plateau photocurrent as well as a sub-stantial shift in the onset potential. A photocurrent of0.46 mA/cm2 at 0.5 V was observed. This value is over300 % higher than that of pristine ZnO nanowires. The pro-posed method resulted in higher photoactivity than tradi-tional QD sensitization and also caused hydrogen genera-tion. X-ray absorption spectroscopy demonstrated the in-corporation of N atoms in ZnO crystal and its effect on theelectronic structure.

Experimental SectionChemicals and Substrates: Zinc nitrate, absolute ethanol, zinc acet-ate, and Te powder were purchased from Sigma–Aldrich. Sodiumborohydride and hexamethylenetetramine (HMT) were obtainedfrom Acros Organics. Cadmium chloride and mercaptopropionicacid (MPA) were purchased from Fluka. FTO substrates (F:SnO2,Tec 15, 10 Ω/sq) were purchased from the Hartford Glass Com-pany. All chemicals were used as received.

Synthesis of ZnO-Seeded Substrates: A solution of zinc acetate inabsolute ethanol (0.01 m, 100 mL) was mixed by ultrasonic agita-tion. The FTO substrates were dipped in the zinc acetate solutionfor 10 s and then blow-dried with an argon stream. The procedurewas repeated several times. The FTO substrates were annealed at350 °C for 30 min to yield a layer of ZnO seeds.

Synthesis of ZnO:N Nanorods: The seeded substrates were horizon-tally suspended in a reagent solution containing zinc nitrate(0.06 m) and HMT (0.06 m) in a Teflon vessel. The vessel was


placed in an autoclave and then heated to 110 °C for nanowiregrowth. After 24 h of growth, the nanowire substrates were re-moved from the autoclave, washed with distilled water, and thendried in air. The nanowire substrates were baked at 450 °C for30 min.

Synthesis of CdTe QDs: NaBH4 (0.08 g) was treated with Te pow-der (0.127 g) in distilled water (3.0 mL) for 4 h to produce 1 m of asodium hydrogen telluride (NaHTe) solution. The NaHTe solution(1.5 mL) was added to a N2-saturated mixture (74.8 mL; pH 10.8)of 38 mm MPA and 16 mm CdCl2 to give a final Cd2+:MPA:HTe–

molar ratio of 1:2.4:0.5. The mixture was heated under reflux at90 °C for 3 h. The solution changed from dark red to orange-yellow. The CdTe QDs were purified by centrifugation in absoluteethanol to remove free ligands such as MPA and unreacted precur-sor ions.

Synthesis of ZnO:N@CdTe Photoelectrode: The prepared ZnOnanorods were placed in an Al2O3 crucible for nitridation at 500,600, and 700 °C for various durations under a NH3 atmospherewith a flow rate of 100 cm3/min. The ZnO:N photoelectrodes wereimmersed in a CdTe solution for 24 h to prepare the ZnO:N@CdTephotoelectrode.

PEC Characterization: Electrochemical characterization was con-ducted by using three-electrode-based methods. The ZnO:N@CdTephotoelectrode was used as the working electrode. A Ag/AgCl elec-trode was used as the reference electrode, and a platinum plate wasthe counterelectrode. PEC was determined in a 0.5 m Na2SO4 (pH6.8) solution, which served as the supporting electrolyte. The pho-toelectrode was illuminated by an AM 1.5 solar simulator at100 mW/cm2. The I–V characteristics of the PEC were recorded at25 °C by using a potentiostat (Eco Chemie AUTOLAB, Nether-lands) and the General Purpose Electrochemical System software.Hydrogen and oxygen generation were determined by using a two-electrode system with 0.5 V bias under solar light simulation at100 mW/cm2. Chronoamperometric measurements were conductedby using a three-electrode system with 0.5 V bias under solar lightsimulation at 100 mW/cm2. The ZnO:N@CdTe photoelectrode wasused as the working electrode, and a Pt wire was used as the coun-terelectrode. The accumulated H2 and O2 in the glass system weremeasured with a China Chromatography 2000 gas chromatographequipped with a flame ionization detector.

UV/Vis Absorption Spectra Measurement: The UV/Vis absorptionspectra of CdTe QD suspensions, ZnO, and ZnO:N@CdTe wereobtained at room temp. with a SHIMADZU UV-700 spectro-photometer by using 1 cm wide quartz cells.

Characterization: High-resolution transmission electron micro-scope (HRTEM) images, electron diffraction patterns, and elemen-tal maps were captured with a JEOL JEM-2100F electron micro-scope. The morphology of the nanowires was investigated with aJEOL JSM-6700F field-emission scanning electron microscope(FE-SEM). Elemental analysis was conducted by using an induc-tively coupled plasma atomic emission spectrometer (ShimadzuICPS-1000III) and an elemental analyzer (Flash EA 1112 series/CE Instruments). X-ray photoelectron spectroscopy (XPS, Al-Kα

radiation, λ = 8.34 Å) was performed with a PHI Quantera instru-ment. A series of XAS measurements of the synthesized samples


were made by using synchrotron radiation at room temp. Measure-ments were made at the Zn K-edge (9659 eV) with the sample heldat room temp. The 01C1 beam line of the National SynchrotronRadiation Research Center (NSRRC), Taiwan was designed forsuch experiments.

Acknowledgments

The authors thank the National Science Council of Taiwan (con-tract number NSC 101-2113-M-002-014-MY3) for financial sup-port. Mrs. C.-Y. Chien of Precious Instrument Center (NationalTaiwan University) is thanked for assistance with the TEM experi-ments.

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Received: October 7, 2013Published Online: November 14, 2013

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