- Nano Express
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CdS nanoparticles sensitization of Al-doped ZnO nanorod array thin film with hydrogen treatment as an ITO/FTO-free photoanode for solar water splitting
© Hsu and Chen; licensee Springer. 2012
Received: 27 August 2012
Accepted: 19 October 2012
Published: 25 October 2012
Aluminum-doped zinc oxide (AZO) nanorod array thin film with hydrogen treatment possesses the functions of transparent conducting oxide thin film and 1-D nanostructured semiconductor simultaneously. To enhance the absorption in the visible light region, it is sensitized by cadmium sulfide (CdS) nanoparticles which efficiently increase the absorption around 460 nm. The CdS nanoparticles-sensitized AZO nanorod array thin film with hydrogen treatment exhibits significantly improved photoelectrochemical property. After further heat treatment, a maximum short current density of 5.03 mA cm−2 is obtained under illumination. They not only are much higher than those without CdS nanoparticles sensitization and those without Al-doping and/or hydrogen treatment, but also comparable and even slightly superior to some earlier works for the CdS-sensitized zinc oxide nanorod array thin films with indium tin oxide (ITO) or fluorine-doped tin oxide (FTO) as substrates. This demonstrated successfully that the AZO nanorod array thin film with hydrogen treatment is quite suitable as an ITO/FTO-free photoanode and has great potentials in solar water splitting after sensitization by quantum dots capable of visible light absorption.
In recent years, hydrogen energy has found increased attention as a renewable and clean energy source in scientific community and government organizations[1–3]. Also, it has enormous potential to be developed as a new substitutive energy resource for solving energy crisis in the future. Among many methods for the generation of hydrogen, solar water splitting is a particularly attractive one because of the environmental friendliness and the abundance of water source[4–7]. Fujishima and Honda were the first to demonstrate the concept of water splitting in a series of experiments using titanium dioxide as a photoanode. Up to date, many efforts have been done on the development of photoelectrodes to improve the efficiency of hydrogen generation[6, 7, 9–12].
TiO2 is the typical photoelectrode material most extensively examined. ZnO is a cheap and safe semiconductor. Its energy-band structure and physical properties are similar to those of TiO2, but it has higher electronic mobility which is favorable for electron transport. So, it has the potential as an alternate of TiO2 in photovoltaic or photoelectrochemical devices. However, both TiO2 and ZnO are not photocatalytic in the visible light region. Since most of the solar frequency spectrum intensity is located in the wavelength range of 400–800 nm, the drawback of nonabsorbing ability in the visible light region significantly limited their hydrogen generation efficiency in the photoelectrochemical cells. So an important strategy was focused on their doping with carbon or nitrogen[15–17], the adsorption of dyes[18, 19], the deposition of quantum dots[20–26], and the use of other semiconductor metal oxides capable of visible light absorption such as WO3[27, 28] and Fe2O3[29, 30] to enhance the absorption of visible light. Furthermore, to enhance the charge-transport property by increasing the direct electron conduction, the other important strategy was the development of their 1-D nanostructures such as nanorods[24, 31, 32], nanowires[16, 21, 22, 26, 33], and nanotubes[25, 34, 35].
On the other hand, it is mentionable that photoactive materials of photoelectrodes are usually coated or grown on the transparent conducting oxide (TCO) thin film for collecting electron efficiently. The suitable TCO materials are the SnO2, In2O3, and ZnO-based binary semiconductor compounds and the multicomponent oxides composed of combinations of these binary compounds. Since the undoped oxide thin films have lower conductivity and are unstable at a high temperature, impurity doping is usually necessary in practical use. The typical examples include the F-doped SnO2 (FTO), Sb-doped SnO2 (ATO), Sn-doped In2O3 (ITO), and Al-doped ZnO (AZO). For solar energy conversion to date, the most common TCOs in dye-sensitized solar cell (DSSC), solar water splitting, and quantum dot-sensitized solar cell (QDSSC) are ITO and FTO. However, as has been known, the rare metal indium is expensive. So, for DSSC, QDSSC, and the solar-driven water splitting in a photoelectrochemical cell, FTO has become the better choice. Moreover, some SnO2- and In2O3-based double layers or triple layers such as TiO2/ITO, Nb-doped TiO2/ITO, FTO/ITO, SnO2/ITO, and TiO2/ATO/ITO have also been developed to improve the energy conversion efficiency[38, 39].
Besides the SnO2- and In2O3-based TCOs, AZO is another attractive TCO material because of its nontoxicity, relative abundance, low cost, thermal stability, and durability in hydrogen plasma[36, 40]. Furthermore, Lee et al. has also reported that the ZnO nanowire-based DSSC using AZO as the TCO substrate showed superior cell performance than that using FTO as the TCO substrate. This revealed that AZO could be used to replace ITO or FTO as the TCO substrate in DSSCs, QDSSCs, and the photoelectrochemical cell for water splitting. In addition, it is also mentionable that some efforts have been made on the development of other ITO-, FTO-, and even TCO-free electrodes in solar cells, such as the grapheme- and poly(3,4-alkylenedioxythiophene) (PEDOT)-based electrodes[42, 43].
As stated above, ZnO not only is the photoactive material of photoelectrode, but also can be used as a TCO substrate after Al-doping and hydrogen treatment. Recently, we synthesized the ZnO nanorod array thin film and demonstrated that appropriate Al-doping and hydrogen treatment could lead to the significant transparency improvement and 1,000-fold conductivity enhancement. This revealed that the AZO nanorod array thin film with hydrogen treatment possessed the functions of TCO thin film and photoelectrode simultaneously. Thus, the AZO nanorod array thin film with hydrogen treatment might be used directly as an ITO- and FTO-free photoelectrode. This made the fabrication of photoelectrode simple and low-cost because the use of expensive rare metal was avoided and the pre-fabrication of an extra TCO substrate is not necessary. Although the ZnO nanowire- or nanorod-based DSSC has been reported, TCO substrate was still used, and the phoelectrode using AZO has showed superior cell performance than using FTO. So, such an AZO nanorod-based photoelectrode without an extra TCO substrate has great potentials in DSSCs, QDSSCs, and the photoelectrochemical cell for water splitting. Based on this reason, in our more recent work, the AZO nanorod array thin film with hydrogen treatment has been demonstrated to possess good photoresponse and stability. Also, a preliminary test showed that its sensitization by cadmium sulfide (CdS) nanoparticles via the chemical bath deposition method could enhance the hydrogen generation efficiency efficiently because of the significant absorption of CdS nanoparticles over a wide wavelength range in the visible light region which made them useful in the development of nanocomposite photoanodes for photoelectrochemical water splitting[20–26].
Accordingly in this work, a comprehensive study has been done to develop the CdS nanoparticles-sensitized AZO nanorod array thin film as a nanocomposite photoanode for solar water splitting without an extra TCO substrate. The effect of cycle number for the chemical bath deposition of CdS nanoparticles on the photoelectrochemical properties was studied. For comparison, the photoelectrochemical properties of CdS nanoparticles-sensitized thin films without Al-doping and/or hydrogen treatment were also examined. In addition, the effect of post-heat treatment was also examined to enhance the hydrogen generation efficiency.
AZO nanorod array thin film was synthesized in a chemical bath according to our previous work. Firstly, for the deposition of ZnO seed layer, 0.4 M zinc acetate solution was prepared by dissolving zinc acetate in 11 mL of 2-methoxyethanol via sonication for 0.5 h and mixing with 0.5 mL of monoethanolamine. Then this solution was kept in a water bath at 60°C for 1 h and aged at room temperature for another 2 days. The resulting solution was deposited on the glass substrate (0.1 mL on a square of 2.5 × 2.5 cm2) by a spin coater at a rate of 3,000 rpm for 30 s, and then the as-deposited thin film was dried in a furnace at 350°C for 10 min to evaporate the solvent and remove organic residuals. After repeating the spin coating and drying procedures for ten times to obtain the required thickness, the obtained thin film was put into a furnace and calcined in air at 550°C for 2 h to yield the ZnO seed layer.
For the growth of AZO nanorod array thin film on the ZnO seed layer, 15 mL of aqueous solution containing zinc nitrate (0.004 M) and aluminum nitrate (Al/Zn molar ratio/20%) was mixed with the mixture of 0.46 ml diethylenetriamine and 15 mL water. After sonication for 10 min to dissolve the precursor, the solution pH was adjusted to 11.5 with 10 M NaOH to yield the deposition solution. Then the ZnO seed layer-coated glass substrate was immersed into the deposition solution and kept in an oven at 95°C for 6 h. After cooling to room temperature naturally, the glass substrate grown with AZO nanorods was washed with water and ethanol several times to remove the organic residue and dried at 70°C in an oven for 2 h. In performing hydrogen treatment to increase the crystallinity, remove organic residual, and enhance the conductivity, the as-grown AZO nanorod array thin film was annealed in Ar/H2 (97/3) atmosphere at 400°C with a gauge pressure of 0.4 kg/cm2 for 2 h. For comparison, the thin films without Al-doping and/or hydrogen treatment were also prepared according to the above method. According to our previous work, the resulting AZO nanorods had an average diameter of 64.7 ± 16.8 nm and an average length of about 1.0 μm.
CdS nanoparticles were decorated on the surface of AZO nanorods via a chemical bath deposition. At first aqueous solution of 30 mL containing 0.001 M cadmium nitrate, 0.005 M thiourea, and 1 M ammonium hydroxide was prepared as the deposition solution. Secondly, the AZO nanorod array thin film with hydrogen treatment was put in the deposition solution at room temperature for 1 h and then at 60°C for another 1 h. The chemical bath deposition was repeated for the desired cycle number (1–5), and the deposition solution was refreshed for each cycle. Finally, the product was washed with ethanol for several times and then dried in the oven. For the decoration of CdS nanoparticles on the thin films without Al-doping and/or hydrogen treatment, the cycle number was fixed at 3. To make sure the formation of CdS nanoparticles, the above chemical bath deposition process was also conducted in the absence of AZO nanorod array thin film for comparison.
The surface morphology was observed by a high-resolution field emission scanning electron microscopy (JEOL SEM 6700F, JEOL Ltd., Tokyo, Japan). The transmission electron micrograph (TEM), energy dispersive X-ray (EDX) spectroscopy, and high-resolution lattice image were analyzed by a high-resolution field emission transmission electron microscopy (HRTEM, JEOL Model JEM-2100F). The crystalline structures were characterized by X-ray diffraction (XRD) analysis on a Rigaku D/max-ga X-ray diffractometer (Rigaku Corporation, Tokyo, Japan) at 40 kV with Cu Kα radiation (λ = 0.1542 nm). The optical absorption spectra were analyzed using a Jasco V-570 UV–VIS spectrophotometer (Jasco Inc., Easton, MD, USA). The photoluminescence spectra were measured on a Hitachi F-4500 fluorescence spectrophotometer (Hitachi Company, Hong Kong, China) with a xenon lamp as the excitation source.
The CdS nanoparticles-sensitized AZO nanorod array thin film with hydrogen treatment was fabricated as the photoelectrode by sticking copper wire on the ZnO seed layer and secured with conducting copper tap. The photoelectrode was sealed on all edges with epoxy resin to reduce leakage current. Photoelectrochemical measurement was conducted on a Zahner IM6ex electrochemical workstation (Zahner- Elektrik GmbH & Co. KG, Kronach, Germany) in a standard three-electrode configuration with the above photoelectrode as the working electrode, Pt wire as the counter electrode, and a BAS (West Lafayette, IN, USA), Model MF-2502 Ag/AgCl electrode as the reference electrode. The electrolyte solution contained 0.25 M Na2S and 0.35 M Na2SO3 (pH 13). A 350 W Xe lamp (FL-88) was used as the solar simulated source with AM 1.5 filter (Oriel Instruments Corporation, Stratford, CT, USA, model 81094). The irradiance measurement was detected with a power meter (Newport Opto-Electronics Technologies (Wuxi) Co., Ltd., Jiangsu, China, model 842-PE), and full power irradiation was fixed at 100 mW/cm2 throughout this work. A lens (Newport, model LFM-1A) focused light on the working electrode with a surface area of 1 cm2. Linear sweep voltammograms ranges were from −0.5 to 0.4 V and the scanning rate was 10 mV/s. By the same method, the photoelectrochemical properties of the CdS nanoparticles-sensitized ZnO thin films without Al-doping and/or hydrogen treatment were measured for comparison. The light switch was tested at a bias of 0 V (vs. Ag/AgCl potential) and 180 s in a cycle with light on and off. The stability of AZO nanorod array thin film with hydrogen treatment was examined by current–voltage (C-V) scanning from −0.5 to 0.4 V for 1 or 50 cycles at a scanning rate of 10 mV/s under illumination and by measuring the current variation with time under illumination at 0.5 V for 2 h.
Results and discussion
The photoluminescence spectra at an excitation wavelength of 460 nm, at which CdS nanoparticles exhibited significant absorption, were also examined. As shown in Figure 5b, a broad emission occurred in the wavelength of 480–600 nm was observed for AZO nanorod array thin film with hydrogen treatment. Because the excitation at 460 nm cannot result in the band-to-band emission of AZO, the emission observed should be resulted from the surface or defect states[49–51]. After sensitization by CdS nanoparticles for 3 and 5 cycles, significant quenching in photoluminescence intensity was observed. As stated above, this was because the sensitization of CdS nanoparticles improved the quality of AZO surface and reduced the interfacial charge-recombination.
Furthermore, as shown in Figure 6, the maximum photocurrent densities were about 1.1, 3.6, and 1.8 mA/cm2 for AZO(H)-CdS (1), AZO(H)-CdS (3), and AZO(H)-CdS (5), respectively. Also, the short current densities (i.e., the current at a zero bias potential) could be determined at 0.03, 0.215, 3.21, and 1.73 mA/cm2 for AZO(H), AZO(H)-CdS (1), AZO(H)-CdS (3), and AZO(H)-CdS (5), respectively. It was noteworthy that the enhancement increased and then decreased with the increasing cycle number. For AZO(H)-CdS (1) and AZO(H)-CdS (3), it was reasonable that more cycle numbers could result in a higher photocurrent density owing to the increased surface coverage of AZO nanorods by CdS nanoparticles as observed in their SEM images (Figure 1). As for the AZO(H)-CdS (5), its lower enhancement than AZO(H)-CdS (3) might be due to the deposition of excess CdS nanoparticles. Excessive CdS sensitization might cause the increase in the distance for the electron transportation of outer CdS nanoparticles to AZO surface, and also increase the interaction probability of CdS-CdS nanoparticles which might act as the trap or recombination centers of electron–hole pairs. Thus, it was suggested that the monolayer deposition of CdS nanoparticles on the surface of AZO nanorods could produce a maximum photocurrent. This result was in good agreement with the earlier works on the dye or quantum dots-sensitized[53–55] solar cells. Moreover, it was mentionable that the photocurrent density for AZO(H)-CdS (3) was comparable and even slightly superior to some earlier works for the CdS-sensitized ZnO nanorod array thin films with ITO, FTO, or metallic Ti foil as substrates[21, 22, 26, 56]. Thus, the AZO nanorod array thin film with hydrogen treatment indeed could be used as an ITO/FTO-free photoanode, and its performance for solar water splitting could be significantly improved by the sensitization with the quantum dots capable of visible light absorption.
The AZO nanorod array thin film with hydrogen treatment has been sensitized by CdS nanoparticles successfully via chemical bath deposition as a novel ITO/FTO-free composite photoelectrode for solar water splitting. The sensitization not only did not destroy the 1-D morphology of AZO nanorod array thin film, but also could efficiently increase the absorption around 460 nm and reduce the electron–hole recombination of AZO nanorods via the FRET mechanism. The CdS nanoparticles decorated on AZO nanorods had a hexagonal structure and the diameters of 5.1-6.5 nm. By increasing the cycle number, the loading of CdS nanoparticles was raised and could significantly enhance the photoelectrochemical property of AZO nanorod array thin film with hydrogen treatment. When a monolayer of CdS nanoparticles was formed on AZO nanorods, the maximum short current density under illumination could be obtained as 3.21 mA/cm2 which was much higher than those without CdS nanoparticles sensitization and those with CdS nanoparticles sensitization but without Al-doping and/or hydrogen treatment. Such a good performance was comparable and even slightly superior to some earlier works for the CdS-sensitized ZnO nanorod array thin films with ITO, FTO, or metallic Ti foil as substrates. In addition, the CdS nanoparticles-decorated AZO nanorod array thin film with hydrogen treatment also exhibited good photosensitivity, reproducibility, and stability. After further annealing at 300°C for 0.5 h, the maximum short current density under illumination could be raised to 5.03 mA/cm2. Accordingly, we successfully demonstrated that the AZO nanorod array thin film with hydrogen treatment could be used as a novel ITO/FTO-free photoanode, and its performance for solar water splitting could be significantly improved by CdS nanoparticles sensitization.
This work was performed under the auspices of the National Science Council of the Republic of China to which the authors wish to express their thanks, under contract number NSC 97-2221-E-006-119-MY3.
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