Composition-Graded ZnxCd1–xSe@ZnO Core–Shell Nanowire Array

Dec 29, 2011 - photoelectrochemical (PEC) solar hydrogen generation ... By using such core−shell nanowire arrays as photoanodes for solar hydrogen ...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/JPCC

Composition-Graded ZnxCd1−xSe@ZnO Core−Shell Nanowire Array Electrodes for Photoelectrochemical Hydrogen Generation Hongxing Li,† Chuanwei Cheng,† Xianglin Li,† Jinping Liu,† Cao Guan,† Yee Yan Tay,‡ and Hong Jin Fan*,† †

Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371, Singapore ‡ School of Materials Science and Engineering, Nanyang Technological University, 639798, Singapore S Supporting Information *

ABSTRACT: One-dimensional oxide nanostructure arrays are widely investigated as photoelectrodes in solar cells or photoelectrochemical (PEC) solar hydrogen generation applications, for which it is highly desirable for the electrode to have a broad light absorption and an efficient charge separation. In this work, a composition-graded ZnxCd1−xSe@ ZnO core−shell nanowire array is prepared through temperature-gradient chemical vapor deposition (CVD) of ZnxCd1−xSe layer onto the pregrown ZnO nanowires. The core−shell nanowire array photoelectrodes yield a continuous absorption edge from 2.7 (460 nm) to 1.77 eV (700 nm) across the sample surface. The core−shell heterostructure facilitates the photogenerated electron−hole pair separation and the electron transfer from ZnCdSe to ZnO. By using such core−shell nanowire arrays as photoanodes for solar hydrogen generation via a PEC cell, a photocurrent density of ∼5.6 mA/cm2 is achieved under 1 sun solar light illumination at zero bias versus Ag/ AgCl. This method may be useful in the design of multijunction nanostructured semiconductor photoelectrodes toward more efficient solar fuel devices.

■

INTRODUCTION Much attention has been focused on the high-efficiency PEC splitting of water using oxide semiconductors (e.g., ZnO, TiO2) as a form of solar energy harvesting and storage.1−7 However, most of these metal oxides have large band gaps, which prevents efficient absorption of the sunlight in the visible region and overall efficiency. To extend the activity of a photoelectrode into the visible light region, 1D nanowire arrays (such as ZnO or TiO2) sheathed with narrow bandgap semiconductor nanocrystal (e.g., CdS, CdSe, PbS) are emerging as a new photoelectrodes for these application.8−14 In this case, light is absorbed by either phase, but the photogenerated electrons and holes can quickly separate into different phases because the topology dictates that the interfacial boundary is always nearby. The morphology of the nanowires then provided the photoinjected electrons with a direct electrical pathway to the photoanode. As has been reported by David et al,15 the highly ordered transparent TiO2 nanotube array exhibits much better photoenergy conversion performance than the same weight of TiO2 nanoparticles film sensitized by CdS QDs. For solar energy harvesting device, the highest conversion efficiency would be achieved by having a wide range of band gaps that matches the entire solar spectrum.16,17 However, all existing work including the QDs-sensitized oxide photoelectrodes is mainly based on the binary semiconductors with © 2011 American Chemical Society

fixed composition and absorption edge. As compared with the binary semiconductors, the ternary alloy semiconductors have shown the capability of growing alloy with continuously tunable composition and band gap and even achieving spatial composition-graded nanostructures on a single substrate.18,19 Their useful properties make them promising candidates for important applications such as solar energy conversion. Recently, Lee et al. reported a CdSSe layer-sensitized TiO2 nanowire arrays under different Se doping and showed an improved PEC performance from that of the pure CdS and CdSe-sheathed TiO2 photoelectrodes.20 In this Article, we report a new photoelectrodes with composition-graded ZnxCd1−xSe@ZnO core−shell nanowire array on the transparent FTO substrate. As depicted in Scheme 1 (left), the ZnO nanowire arrays were grown directly on the 2 cm × 2 cm FTO substrate, and a continuous ZnxCd1−xSe layer (with x from 0 to 1) was deposited on the ZnO nanowires by a temperature-gradient CVD method. Because the shell composition in the ZnxCd1−xSe@ZnO nanowire array varies along the length direction of the substrate, the light absorption edge of the core−shell nanostructures spans a wide range of the solar Received: May 22, 2011 Revised: November 27, 2011 Published: December 29, 2011 3802

dx.doi.org/10.1021/jp204747w | J. Phys. Chem. C 2012, 116, 3802−3807

The Journal of Physical Chemistry C

Article

Scheme 1. Composition-Graded ZnxCd1−xSe@ZnO CoreShell Nanowire Array Photoelectrode Structure (Left) and the Charge Transfer Process from ZnxCd1−xSe to ZnO (Right)

Figure 1. Schematic diagram of the reactor setup. The FTO substrate grown with ZnO nanowire array was placed downstream of the gas flow, where the temperature lowers from A to B (540 at T1 and 450 °C at T3). For synthesis of the ZnxCd1−xSe@ZnO (x ≈ 0.5) sample, the substrate was placed vertically at T2, where the local temperature is ∼500 °C.

10 nm was successfully coated on the ZnO nanowires. The composition gradient of the ZnCdSe alloy nanolayer was modulated by the temperature-gradient along the length of the substrate with the sample closer to the end of the tube having higher Cd content. Sample Characterizations. The morphology and microstructures of the as-prepared ZnxCd1−xSe@ZnO core−shell nanowire array were examined using JEOL JSM-6700F fieldemission scanning electron microscopy (FE-SEM) and JEM 2010F transmission electron microscopy (TEM). The positiondependent element composition analysis was performed by in situ energy-dispersive X-ray spectroscopy (EDS, taken in the SEM). To determine the elemental composition along the length direction of the substrate, we accurately positioned the sample using an encoded x-y translation stage, and the locations were recorded. The diffuse reflection spectra of the ZnxCd1−xSe/ ZnO core−shell nanowire arrays with different Cd content along the length direction of the substrate were measured by moving the excitation spot from one end of the substrate to the other. The diameter of excitation spot during the UV−vis measurements was focused to ∼1 mm. PEC Measurements. The PEC measurement of the ZnxCd1−xSe@ZnO nanowire array photoelectrodes was carried out in electrolytes containing 0.35 M Na2SO3 and 0.24 M Na2S (pH 11.5) with Zaner IM6 electrochemical workstation. The as-prepared ZnxCd1−xSe@ZnO core−shell nanowire array on FTO electrode was used as the working electrode, a Pt foil as the counter electrode, and a saturated Ag/AgCl electrode as the reference electrode. The photocurrent densites versus measured potential (I−V curve) measurements were performed in a standard three-electrode configuration under AM 1.5G simulated sunlight from a 150 W Xe lamp (Sciencetech SS150). The incident light intensity was calibrated to 100 mW/ cm2.

spectrum. Furthermore, the type-II band alignment of the ZnxCd1−xSe-ZnO heterojunction allows an efficient transfer of photoexcited electrons from ZnxCd1−xSe to ZnO (Scheme 1 (right)). Whereas the electrons contribute to the hydrogen generation reaction at the Pt cathode, holes are consumed for an oxidation reaction at the anode/electrolyte interface.14,21 As a result of this, together with the good electron transport property of hydrothermal-grown ZnO nanowires, a good PEC performance is achieved.

■

EXPERIMENTAL SECTION Synthesis of ZnO Nanowire Arrays. The ZnO nanowire arrays were prepared by using a hydrothermal growth route. In a typical synthesis process, the transparent conductive F/SnO2 (FTO) substrates were first seeded with an oriented ZnO thin film (∼20 nm) by using the sol−gel dip-coating method. The substrates were then annealed at 450 °C for 30 min to obtain oriented ZnO seed layers. The growth solution consists of 0.025 M zinc nitrate, 0.025 M hexamethylenetetramine (HMTA), and 0.01 M PEI (end-capped, molecular weight 800 g/mol LS, Aldrich). The seeded substrates were then placed in 50 mL of the above growth solution. The container was sealed and heated to 95 °C for 12 h without stirring. After growth, the resultant samples were removed and rinsed thoroughly with ethanol to get rid of any residual reactants and dried in air at 80 °C. Finally, the as-grown ZnO nanowire arrays were annealed at 450 °C for 1 h. Coating of ZnxCd1−xSe Shells. To deposit compositiongradient of ZnxCd1−xSe outlayer on the ZnO nanowire arrays, a short tube furnace with sharp temperature gradient was used. Considering the lower melting point of CdSe than ZnSe, CdSe powder was positioned at the upstream inside the quartz-tube furnace to make sure that the CdSe can vaporize with the ZnSe at a similar speed. The FTO substrate grown with ZnO nanowire arrays was placed downstream of the gas flow where the temperature was between 450 and 540 °C (shown in Figure 1). An appropriate selection of the temperature range that covers the full temperature range required to grow the full composition of ZnxCd1−xSe in a single run of growth was found to be crucial for the successful deposition of the gradient ZnxCd1−xSe nanolayer on the ZnO nanowire arrays. The furnace was heated to the temperature around 820 °C and maintained at this temperature for ∼20 min. The pressure in the quartz tube was controlled at 300 mbar using a 2% H2 in Ar gas mixture that acted as a carrier gas. After 20 min of deposition, the ZnxCd1−xSe alloy outlayer with typical thickness around 5−

■

RESULT AND DISCUSSION Figure 2a shows a photograph of the pristine ZnO nanowire array (left) and the spatially composition-graded ZnxCd1−xSe@ ZnO core−shell nanowire array (right) on the conductive FTO substrates. Compared with the pure ZnO nanowire arrays, the as-grown ZnxCd1−xSe@ZnO shows an obvious color variation gradually from yellow to red and dark along the length direction of the FTO substrate. Figure 2b is a low-magnification SEM image of the pristine ZnO nanowire arrays, which covers uniformly the whole surface of the FTO substrate. The length of the nanowires is ∼ μm, and the diameters are in the range of 50−100 nm. The neighboring ZnO nanowires are slightly bent 3803

dx.doi.org/10.1021/jp204747w | J. Phys. Chem. C 2012, 116, 3802−3807

The Journal of Physical Chemistry C

Article

accordance with the gradual composition change as determined optically. The microstructure of the Zn xCd1−xSe@ZnO coaxial heterogeneous nanowires was investigated based on TEM. Figure 3a is a typical TEM image of the ZnxCd1−xSe@ZnO

Figure 3. (a) Typical TEM image of the ZnxCd1−xSe@ZnO coaxial nanowire. (b) Low-magnification high-resolution TEM image of the ZnxCd1−xSe@ZnO structure. (c) High-resolution TEM image taken from the edge of the out ZnCdSe nanolayer. (d) TEM image of the bare ZnO nanowire. Inset is the corresponding high-resolution TEM image.

Figure 2. (a) Optical image of the pristine ZnO (left) and as-grown spatially composition-graded ZnxCd1−xSe@ZnO core−shell nanowire arrays (right) on conductive FTO substrate. (b,c) SEM image of the pristine ZnO nanowire arrays. (e,f) SEM image of the ZnxCd1−xSe@ ZnO core−shell nanowire arrays. (f) Spatially EDS spectra of the ZnxCd1−xSe@ZnO core−shell nanowire arrays taken from different positions along the length direction of the substrate.

coaxial nanowire. It can be seen that the alloy shell is deposited over the entire surface of the ZnO nanowire. It is an important point that a heterojunction is formed on the whole nanowire surface, which is beneficial to the charge separation during their photoelectrochemical applications (to be discussed below). A closer view of the surface reveals that the ZnCdSe shell, ∼5 nm in thickness, is granular-like with a clear interface with the ZnO core. (See Figure 3b.) A partial interface diffusion and alloying between the ZnO core and ZnCdSe shell might have occurred, as indicated by the rough interface compared with the smooth surface of the pristine ZnO nanowire (Figure 3d). The highresolution TEM image (Figure 3c) taken from the edge of the shell verified that the shell is composed of ZnCdSe nanocrystals with diameters of 3−5 nm, which aggregated into a continuous film covering the entire surface of the ZnO nanowires. As for comparison, Figure 3d and its inset show a TEM image and the corresponding high-resolution TEM image of a bare ZnO nanowire, respectively, confirming that the pristine ZnO nanowires are single-crystalline with smooth surfaces. To demonstrate further that the band gap of the ZnxCd1−xSe shell can be well-tuned by their composition variation, we collected the spatial UV−vis diffuse reflection absorption spectra of the as-obtained ZnxCd1−xSe@ZnO nanowire sample at different spots along the length of the substrate. (See Figure 4a.) One can clearly see that each examined point along the sample length exhibits a steep absorption edge. Relative to the absorption edge of the bare ZnO nanowire array (a control sample) near 380 nm, the spectral position of the absorption edge red shifts continuously from 460 to 700 nm with the decrease in the composition x (Zn ratio) in the core−shell nanowires. Such a continuous shift of the absorption edge is in good agreement with the results of ZnxCd1−xSe films and singlecrystal nanowires.22,23 For direct bandgap semiconductors, the optical absorption near the band edge follows24

and joined into bundles. (See Figure 2c.) The bending and bundling of nanowires is a typical result of the balance between the tensile strain and the capillary interaction derived from their large aspect ratios and the surface tension of water during the hydrothermal synthesis process. In addition, the individual ZnO nanowires have a smooth lateral surface and uniform diameter along their entire length. These structural properties may qualitatively favor the formation of heterojunction with an abrupt interface. After the deposition with a ZnxCd1−xSe outlayer for 20 min, the overall alignment of the nanowire array is still retained, as shown in Figure 2d. Figure 1e shows a higher magnification SEM image of ZnxCd1−xSe@ZnO core− shell nanowire array. As compared with the pristine ZnO nanowires, the ZnxCd1−xSe@ZnO nanowires exhibit a larger diameter and rougher surface, which indicates the formation of a core−shell structure. Moreover, the ZnxCd1−xSe shells have similar thicknesses along the length of the nanowires, and nearly all the nanowires are coated. The spatial chemical compositions of ZnxCd1−xSe@ZnO nanowire array along the length direction of the substrate were further identified by EDS spectra at different positions. The EDS data (Figure 2f) collected from varied points of ZnxCd1−xSe@ZnO nanowire arrays demonstrate the existence of Zn, Cd, Se, and O elements. It is seen that when scanned from the yellow end of the sample surface to the black end, the Cd concentration evolves with a trend opposite to that of Zn. This is in 3804

dx.doi.org/10.1021/jp204747w | J. Phys. Chem. C 2012, 116, 3802−3807

The Journal of Physical Chemistry C

Article

nanowire arrays. Figure 5a shows the I−V curves of hybrid nanowire array photoelectrodes measured both in dark and

Figure 4. (a) In situ spatial UV−vis absorption spectra of the ZnxCd1−xSe@ZnO nanowire array along the length direction of the substrate. (b) Bandgap energy (Eg) of the core−shell nanowire array plotted versus the Zn concentration x.

(αhν)2 = A(hν − Eg )

Figure 5. (a) Photocurrent versus voltage in a solution containing 0.35 M Na2SO3 and 0.24 M Na2S (pH 11.5) under AM 1.5 G at 100 mW/ cm2 illumination for the composition-graded ZnxCd1−xSe@ZnO core− shell nanowire arrays electrode. (b) Amperometric I−t curves of the composition-graded ZnxCd1−xSe@ZnO core−shell nanowire arrays electrode at a zero bias versus Ag/AgCl voltage at 100 mW/cm2 with light on/off cycles.

(1)

where α, ν, and Eg are the absorption coefficient, optical frequency, and band gap energy, respectively. A is a constant. Therefore, the bandgap (Eg) can be estimated from a plot of (αhν)2 versus the photon energy (hν). From the spectra for various positions along the length direction of the substrate (Figure 4a), the corresponding band gaps were determined to be Eg = 2.70, 2.53, 2.41, 2.24, 2.06, 1.96, and 1.77 eV. The bandgap energy Eg = 2.7 eV corresponds to ZnSe (x = 1) and Eg = 1.77 eV to Zn0.05Cd0.95Se. These values are in good agreement with the previously reported values for ZnSe and CdSe nanowires.25 The dependence of the bandgap energy on composition (x) was found to follow a quadratic relationship as follows26

Eg (x) = 0.41x 2 + 0.54x + 1.74

under illumination. A dark scan shows a very small current density in the range of 10−1 mA/cm2, whereas under light illumination, a pronounced photocurrent density is observed, implying efficient charge separation and transfer. Typically, the photocurrent density increases from the onset potential around −1.2 V versus Ag/AgCl and approaches ∼5.6 mA/cm2 at 0 V. As for the pristine ZnO photoelectrode, the photocurrent onset potential (Von) is 0.25 V versus Ag/AgCl, which is much smaller than that of the core−shell nanowire arrays electrode. The measured potential versus Ag/AgCl was also converted to the reversible hydrogen electrode (RHE) scale according to the Nernst equation27

(2)

which is also plotted in Figure 4b. Therefore, when used as photoelectrodes, such ZnxCd1−xSe@ZnO core−shell nanowire array can absorb light that spans continuously from blue to red edge of the solar spectrum by controlling the shell composition variation. Note that whereas Pan and coworker have reported a spatial grading of bandgap engineering in ternary CdSSe nanowires on a single chip,18 the novelty of our present work is to extend this method to the controllable synthesis of core− shell structured hybrid nanowires with spatially gradient absorption edge on a single FTO electrode. Electrochemical measurements were carried out to evaluate the PEC properties of photoanodes composed of both pristine ZnO and composition-graded ZnxCd1−xSe@ZnO core−shell

0 ERHE = EAg/AgCl + 0.059pH + EAg/AgCl

(3)

where ERHE is the converted potential versus RHE, EoAg/AgCl = 0.1976 V at 25 °C, and EAg/AgCl is the experimentally measured potential versus the Ag/AgCl reference. With this scaling (Figure 5), the photocurrent density is ∼7.4 mA/cm2 at 1.23 V versus RHE, 30 times higher than that (∼0.25 mA/cm2) of the pristine ZnO cell (Figure S1 in Supporting Information). The significantly higher photocurrent onset potential value in the core−shell nanowire arrays demonstrates a shift in the Fermi level to a more negative potential as a result of a coupling 3805

dx.doi.org/10.1021/jp204747w | J. Phys. Chem. C 2012, 116, 3802−3807

The Journal of Physical Chemistry C

Article

ZnO photoelectrocatalytic cells measured under open circuit condition under 1 sun light intensity. The radius of each arc is correlated with the charge-transfer ability of the corresponding photoelectrode. One can see that the charge-transfer impedance at the ZnO-ZnCdSe-electrolyte interface is much smaller than that of the bare ZnO-electrolyte interface under the same condition (as shown in the Nyquist plots in Figure 6a). Correspondingly, the higher characteristic frequency (1− 100 Hz) in the ZnCdSe-ZnO composite (as shown in the Bode phase plots in Figure 6b) suggests that the electron lifetime in the ZnO-ZnCdSe composite is shorter than that in pristine ZnO electrode. The equivalent circuit of the ZnO-ZnCdSe electrode is also depicted in the inset of Figure 6a. Here Rs is the series resistance, and R and C represent the total charge transfer resistance and capacitance of the space charge region, respectively, including charge transfer across the FTO/ZnOZnCdSe interface and the ZnCdSe/electrolyte interface. W is the Warburg impedance. RPt and CPt are the resistance and capacitance of the Pt-coated cathode, respectively, which is identified by the high-frequency semicircle in Figure 6b (103 to 105 Hz). Although a ZnCdSe alloy layer deposition results in a higher electron injection, it also increases the recombination possibility of the electrons and causes a smaller lifetime, but in the ZnCdSe composite electrode, the reduction of lifetime could be compensated by the highly improved light-harvesting efficiency and greatly reduced the charge transfer resistance, hence resulting in improved overall PEC performance. To understand the intrinsic electronic properties of the ZnxCd1−xSe@ZnO nanowire arrays in contact with the electrolytes, we also performed the electrochemical impedance measurements at the fixed frequency in the dark to determine the capacitance of the nanowire arrays and the flat band potential at the semiconductor/electrolyte interface. The capacitance can be described by the Mott−Schottky (M−S) equation35,36

between the ZnO core and ZnxCd1−xSe shell in the composite system.28,29 Moreover, no saturation of photocurrent was observed in the ZnxCd1−xSe@ZnO photoelectrode at a more positive potential, which indicates efficient charge separation in core−shell structures upon illumination.30 Figure 5b shows the amperometric current−time (I−t) curves of the compositiongraded ZnxCd1−xSe@ZnO nanowire array electrode with light on/off cycles at 0 V versus Ag/AgCl under AM1.5G 1 Sun illumination. Upon illumination, a spike in the photoresponse was observed, which might be due to the transient effect in the power excitation,31 and the photocurrent then returned to a steady state after a few seconds. The steady values are 80−90% of initial currents. The photocurrent value dropped to nearly zero instantaneously when the incident light was turned off and returned to the original value only when the light was turned on again. The changes of both “on” and “off” currents are nearly vertical, which indicates that charge transport in the composite material proceeds very quickly. To show an idea of the composition dependent PEC property, we prepared ZnSe@ZnO, ZnxCd1−xSe@ZnO (x ≈ 0.5), and CdSe@ZnO nanowires samples under similar growth condition as control samples for comparison. Figure S2 of the Supporting Information shows a set of I−V curves recorded from these nanowire array photoanodes at AM1.5 1 sun illumination. All photoanodes show pronounced photoresponse under light illumination. The ZnSe@ZnO photoanode gives a photocurrent density of 2.4 mA/cm2 at 0 V (vs Ag/AgCl). With increasing Cd content in the shells, the photocurrent density increases to 4.2 and 6.5 mA/cm2 for the ZnxCd1−xSe@ZnO (x ≈ 0.5) and CdSe@ZnO photoanodes, respectively. The increase in photocurrent density can be ascribed mainly to the expanded absorption spectrum range of the ZnxCd1−xSe@ZnO core−shell nanowire arrays with more Cd,32 consistent with the results of UV−vis measurements. Electrochemical impedance spectroscopy has been proved to be a powerful tool to study charge transfer and recombination processes in solar cells. From applying appropriate equivalent circuits and physical models, the transport rate and lifetime of the electron can be derived on the nanoscale.33,34 Figure 6 shows the impedance spectra of the ZnCdSe-ZnO and pristine

1/C 2 = (2/εε0e0NdA2)[(V − VFB) − kT /e0]

(4)

where C is the charge space capacity, ε is the dielectric constant of ZnCdSe, ε0 is the electric permittivity of vacuum, e0 is the electron charge, Nd is the donor density, A is the area, k is the Boltzmann constant, T is the absolute temperature, V is the electrode applied potential, and VFB is the flat band potential. The measured capacitance decreases monotonically with increasing potential as described by the M−S relation. VFB of the ZnxCd1−xSe@ZnO nanowire arrays can be determined from the extrapolation of X intercepts in the M−S plot (1/C2 vs V) between −0.25 and 0.5 V versus Ag/AgCl, which is found to be −0.76 V (as shown in Figure 7). The positive slope indicates that the ZnxCd1−xSe@ZnO nanowire array is n-type material with electron conduction. Furthermore, the VFB of the ZnxCd1−xSe@ZnO nanowire arrays is much more negative than that of the VFB′ (−0.05 V) of pristine ZnO nanowire array (shown as inset of Figure 7). Such a big negative variation of the flat band potential suggests a higher carrier concentration in the composite system and more efficient charge transfer from ZnxCd1−xSe shell to the ZnO core and then to the transparent conducting oxide current collector.37

■

Figure 6. Impedance spectra measured at open circuit condition under 1 sun illumination. (a) Nyquist plots. (b) Bode phase plots. Frequency range: 0.1−105 Hz. Inset is the equivalent circuit of the photoelectrochemical cell.

CONCLUSIONS Nanowire arrays containing ZnO core and spatial compositiongraded ZnxCd1−xSe shell have been fabricated by a temperature3806

dx.doi.org/10.1021/jp204747w | J. Phys. Chem. C 2012, 116, 3802−3807

The Journal of Physical Chemistry C

Article

Sunkara, M.; McFarland, E. W.; Domen, L.; Miller, E. L.; Turner, J. A.; Dinh, H. N. J. Mater. Res. 2010, 25, 1−16. (8) Lin, C. J.; Lu, Y. T.; Hsieh, C. H.; Chien, S. H. Appl. Phys. Lett. 2009, 94, 113102. (9) Cesar, I.; Kay, A.; Martinez, J. A. G.; Grätzel, M. J. Am. Chem. Soc. 2006, 128, 4582−4583. (10) Zhu, W.; Liu, X.; Liu, H. Q.; Tong, D. L.; Yang, J. Y.; Peng, J. Y. J. Am. Chem. Soc. 2010, 132, 12619−12626. (11) Sun, W. T.; Yu, Y.; Pan, H. Y.; Gao, X. F.; Chen, Q.; Peng, L. M. J. Am. Chem. Soc. 2008, 130, 1124−1125. (12) Wang, X. N.; Zhu, H. J.; Xu, Y. M.; Wang, H.; Tao, Y.; Hark, S. K.; Xiao, X. D.; Li, Q. ACS Nano 2010, 6, 3302−3308. (13) Kang, Q.; Liu, S. H.; Yang, L. X.; Cai, Q. Y.; Grimes, G. A. ACS Appl. Mater. Interfaces 2011, 3, 746−749. (14) Wang, G. M.; Yang, X. Y.; Qian, F.; Zhang, J. Z.; Li, Y. Nano Lett. 2010, 10, 1088−1092. (15) David, R. B.; Prashant, V. K. Adv. Funct. Mater. 2009, 19, 805− 811. (16) Ning, C. Z.; Pan, A. L.; Liu, R. B. IEEE Photovoltaic Spec. Conf., 34th 2009, No. 1−3, 2289−2292. (17) Caselli, D. A.; Ning, C. Z. Opt. Express 2011, 19, A686−A694. (18) Pan, A. L.; Zhou, W. C.; Leong, E. S. P.; Liu, R. B.; Chin, A. H.; Zou, B. S.; Ning, C. Z. Nano Lett. 2009, 9, 784−788. (19) Pan, A. L.; Liu, R. B.; Sun, M. H.; Ning, C. Z. ACS Nano 2010, 4, 671−680. (20) Sung, T. K.; Kang, J. H.; Jang, D. M.; Myung, Y.; Jung, G. B.; Kim, H. S.; Jung, C. S.; Cho, Y. J.; Park, J.; Lee, C. L. J. Mater. Chem. 2011, 21, 4553−4561. (21) Liu, D.; Kamat., P. V. J. Phys. Chem. 1993, 97, 10769−10773. (22) Ammar, A. H. Physica B 2001, 296, 312−318. (23) Xu, H. Y.; Liang, Y.; Liu, Z.; Zhang, X. T.; Hark, S. K. Adv. Mater. 2008, 20, 3294−3297. (24) Smith, R. A. Semiconductors, 2nd ed.; Cambridge University Press: Cambridge, U.K., 1978; p P313. (25) Venugopal, R.; Lin, P. I.; Chen, Y. T. J. Phys. Chem. B 2006, 110, 11691−11696. (26) Yoon, Y. J.; Park, K. S.; Heo, J. H.; Park, J. G.; Nahm, S.; Choi, K. J. Mater. Chem. 2010, 20, 2386−2390. (27) Grätzel, M Nature 2001, 414, 338−344. (28) Wang, H.; Bai, Y. S.; Zhang, H.; Zhang, Z. H.; Li, J. H.; Guo, L. J. Phys. Chem. C 2010, 114, 16451−16455. (29) Lee, Y. L.; Chi, C. F.; Liau, S. Y. Chem. Mater. 2010, 22, 922− 927. (30) Yang, X. Y.; Wolcott, A.; Wang, G. M.; Sobo, A.; Fitzmorris, R. C.; Qian, F.; Zhang, J. Z.; Li, Y. Nano Lett. 2009, 9, 2331−2336. (31) Chen, H. M.; Chen, C. K.; Chang, Y. C.; Tsai, C. W.; Liu, R. S.; Hu, S. F.; Chang, W. S.; Chen, K. H. Angew. Chem., Int. Ed. 2010, 49, 5966−5969. (32) Xu, J.; Yang, X.; Wang, H. K.; Chen, X.; Luan, C. Y.; Xu, Z. X.; Lu, Z. Z.; Roy, V. A. L.; Zhang, W. J.; Lee, C. S. Nano Lett. 2011, 11, 4138−4143. (33) Wang, Q.; Moser, J. E.; Grätzel, M. J. Phys. Chem. B 2005, 109, 14945−14953. (34) Pan, X.; Chen, C. H.; Zhu, K.; Fan, Z. Y. Nanotechnology 2011, 22, 235402. (35) Zhang, W. D.; Jiang, L. C.; Ye, J. S. J. Phys. Chem. C 2009, 113, 16247−16253. (36) Morrison, S. R. Electrochemistry at Semiconductor and Oxidized Metal Electrodes; Plenum Press: New York, 1980; Chapter 5. (37) Lin, Y. J.; Zhou, S.; Sheehan, S. W.; Wang, D. W. J. Am. Chem. Soc. 2011, 133, 2398−2401.

Figure 7. Mott−Schottky plot of 1/C2 versus applied potential (voltage vs Ag/AgCl) in complete darkness at an AC frequency of 103 Hz and an AC amplitude of 7 mV. Inset shows the Mott−Schottky plot of the pristine ZnO nanowire array under the same condition.

gradient CVD of ZnCdSe layer on preprepared ZnO nanowire array and tested as photoelectrode for PEC hydrogen generation. The element composition and subsequently the bandgaps of the ZnxCd1−xSe shell can be tuned across the photoelectrode surface, allowing a continuous light absorption within the solar spectrum. A photocurrent intensity of ∼5.6 mA/cm2 at a potential of zero (vs Ag/AgCl) and a photocurrent onset potential of about −1.2 V is achieved (vs Ag/AgCl). The present study opens a new route to the design of composition-graded alloy nanolayer (e.g., CdSSe)-coated nanowire array photoelectrodes for full-visible spectrum solar energy applications such as semiconductor-sensitized solar cells and PEC hydrogen generation devices.

■

ASSOCIATED CONTENT

S Supporting Information *

I−V curve of the pristine ZnO nanowire photoanode, I−V curves of the photoanodes based on the ZnSe@ZnO, ZnxCd1−xSe@ZnO (x ≈ 0.5), and CdSe@ZnO nanowire array. This material is available free of charge via the Internet at http://pubs.acs.org.

■ ■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

ACKNOWLEDGMENTS This work is supported by the start-up funding from Nanyang Technological University to H.J.F. (M58110048). We thank Mr. Sun Gengzhi and Prof. Lianxi Zheng for allowing us to use their CHI electrochemical workstation.

■

REFERENCES

(1) Fujishima, A.; Honda, K. Nature 1972, 238, 37−38. (2) Chen, X. B.; Liu, L.; Yu, P. Y.; Mao, S. S. Science 2011, 331, 746− 750. (3) Wolcott, A.; Smith, W. A.; Kuykendall, T. R.; Zhao, Y. P.; Zhang, J. Z. Small 2009, 5, 104−111. (4) Bingham, S.; Daoud, W. A. J. Mater. Chem. 2011, 21, 2041−2050. (5) Tilley, S. D.; Cornuz, M.; Sivula, K.; Grätzel, M. Angew. Chem., Int. Ed. 2010, 49, 6405−6408. (6) Ahn, K. S.; Yan, Y.; Shet, S.; Jones, K.; Deutsch, T.; Turner, J.; Jassim, M. A. Appl. Phys. Lett. 2008, 93, 163117. (7) Chen, Z. B.; Jaramillo, T. F.; Deutsch, T. G.; Shwarsctein, A. K.; Forman, A. J.; Gaillard, N.; Garland, R.; Takanabe, K.; Heske, C.; 3807

dx.doi.org/10.1021/jp204747w | J. Phys. Chem. C 2012, 116, 3802−3807