Enhanced Photoelectrochemical Performance of Nanocomposite Film

Mar 1, 2010 - Enhanced Visible Photovoltaic Response of TiO2 Thin Film with an All-Inorganic ... ACS Applied Materials & Interfaces 2010 2 (7), 1912-1...
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J. Phys. Chem. C 2010, 114, 5211–5216

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Enhanced Photoelectrochemical Performance of Nanocomposite Film Fabricated by Self-Assembly of Titanium Dioxide and Polyoxometalates Zhixia Sun, Lin Xu,* Weihua Guo, Bingbing Xu, Shuping Liu, and Fengyan Li Key Laboratory of Polyoxometalates Science of Ministry of Education, College of Chemistry, Northeast Normal UniVersity, Changchun 130024, P. R. China ReceiVed: NoVember 09, 2009; ReVised Manuscript ReceiVed: January 21, 2010

In order to fabricate photoelectrochemical devices, multilayer films composed of positively charged titanium dioxide (TiO2) colloids and anionic tungsto(molybdo)phosphate (PW12 and P2Mo18) or poly(styrenesulfonate) (PSS) have been prepared by the layer-by-layer (LbL) self-assembly method. These films were characterized by UV-vis spectroscopy, IR spectra, and atomic force microscopy. Photocurrent transient measurement suggested that the photocurrent response of the PW12/TiO2 composite film was highly dependent on film thickness (viz., the deposited number of layers). Both current-voltage curves and photocurrent transient measurement demonstrate that the photocurrent and power conversion efficiency of the PW12/TiO2 composite film were markedly enhanced in comparison with that of the P2Mo18/TiO2 film and the PSS/TiO2 film. Furthermore, the PW12/TiO2 composite film also exhibited the most significant photoelectrooxidation activity for methanol. These results provide valuable information for photovoltaic and photoelectrochemical applications. 1. Introduction In recent years, the photoelectrochemical properties of nanostructured titania (TiO2) films have attracted considerable research in many areas such as photoelectrocatalysts, solar photovoltaic cells, and water splitting devices.1-5 However, most of these applications suffer from a dissatisfactory quantum efficiency. Generally, the efficiency is greatly limited by a high electron-hole recombination rate or a low photogenerated charge transfer rate. To solve these problems, various strategies have been adopted, including morphological control, metal modification, and combination with other materials, etc.6-10 Among various efforts for improvement of the efficiency, a powerful approach was the use of electron acceptors to transfer electrons from the conduction band (CB) of TiO2 to an inert electrode. Such a donor-acceptor structure could retard effectively the fast charge pair recombination. Polyoxometalates (POMs), a well-known class of inorganic nanoclusters with a metal-oxygen framework, have an intrinsic ability to accept electrons readily.11,12 Simultaneously, POMs can undergo reversible multielectron redox reactions in retaining intact structure;13,14 this made POMs an efficient electron scavenger to enhance the photoelectrochemical response. Choi and Park reported that the use of POMs facilitated photogenerated electron transfer from TiO2 to a collector electrode to improve the overall photoefficiency.15 Recently, phosphotungstic acid played a vital role to enhance energy conversion efficiency in a photovoltaic device of TiO2 nanodisc films.16 Moreover, a POMs-modified TiO2 nanotube electrode showed great photocatalytic and photoelectrocatalytic ability for degradation of recalcitrant organic pollutants.17,18 Besides the above-mentioned main points, the surface morphology of nanostructured TiO2 film is also an important factor to influence the photoelectrochemical performance. Some preparation methods of TiO2 films were explored, such as casting,19 sputtering,20 and chemical vapor deposition,21 but it * To whom correspondence should be addressed. Phone: +86-43185099668. Fax: +86-431-85099668. E-mail: [email protected].

still remains a challenge to precisely control morphology, composition, and film thickness at the nanoscale. To satisfy this demand, the layer-by-layer (LbL) self-assembly technique relying on electrostatic absorption of oppositely charged species has been developed as a simple and efficient approach for the construction of nanocomposite films. This method could hold an advantage to control morphology, structure, and film thickness at the molecular level.22,23 Recently, POMs/TiO2 films prepared by the LbL method have displayed high photocatalytic activity against both 2-propanol and acetone.24 To our knowledge, however, research on the photoelectrochemical performance of the POMs/TiO2 nanocomposite films constructed by the LbL self-assembly method has rarely been performed so far. In this work, we fabricated the nanocomposite films of positively charged TiO2 colloids and anionic tungstophosphate (H3PW12O40, denoted as PW12) by the LbL self-assembly method, conducting an investigation on the photoelectrochemical performance of such multilayer nanocomposite films. For comparison, the negatively charged component of LbL films is replaced by inactive polyanion poly(styrenesulfonate) (PSS) or molybdophosphate (H6P2Mo18O62, denoted as P2Mo18); the latter possesses much stronger oxidation capability than PW12. Also, the influence of film thickness (viz., the deposited number of layers) and film structure on the photoelectrochemical performance was investigated. The research demonstrated that the LbL method is attractive for constructing photoelectrochemical systems by controlling film thickness and film structure, and the POMs/TiO2 composite film is a promising route for developing new photovoltaic and photoelectrochemical materials. 2. Experimental Section 2.1. Materials. PW12 and P2Mo18 were prepared according to literature method and identified by UV-vis adsorption spectra and cyclic voltammetry.25 The colloidal dispersion of TiO2 was synthesized by hydrolysis of tetra-n-butyl titanate according to the reported procedure.26 Transmission electron microscopy (TEM) revealed that the mean size of the colloidal particles

10.1021/jp910665b  2010 American Chemical Society Published on Web 03/01/2010

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Figure 1. Schematics of the self-assembly of the (PW12/TiO2)n film.

was ca. 8 nm (Supporting Information, Figure S1). The crystal structure of TiO2 was anatase, as confirmed by X-ray diffraction (XRD) analysis (Supporting Information, Figure S2). 3-Aminopropyltrimethoxysilane (APS) and PSS (MW 70 000) were purchased from Aldrich and used without further treatment. Other reagents were of AR grade. The water used in all experiments was deionized to a resistivity of 18 MΩ cm. 2.2. Preparation of Composite Film. Quartz substrates, silicon wafers (Si〈100〉, polished on one side), and ITO-coated glass were cleaned according to a literature procedure.27 First, APS-modified substrates were dipped into HCl (pH ) 2.0) for 20 min, followed by washing with deionized water and drying in nitrogen. Then the substrate-supported precursor films were alternately dipped into the PW12 (1 × 10-3 M in pH ) 1.5 HNO3) solution and the TiO2 colloidal dispersion for 10 and 1 min, respectively, and rinsed with deionized water after each dipping. Subsequently, (PW12/TiO2)n thin films were obtained by repeating these procedures. Finally, the samples were heated at 523 K for 60 min to establish electrical contact among the nanoparticles. The schematics of the self-assembly of the (PW12/ TiO2)n films are illustrated in Figure 1. For comparison, (P2Mo18/ TiO2)n and (PSS/TiO2)n thin films were prepared in a similar way. 2.3. Thin Film Characterization. UV-vis absorption spectra of quartz-supported films were recorded on a 756CRT UV-visible spectrophotometer after each layer deposition. Atomic force microscopy (AFM) measurements were performed in air with a SPI3800N Probe Station. IR spectra were recorded with Nicolet Magna 560 FT-IR Spectrometer. All photoelectrochemical experiments were performed on a CHI660C Electrochemical Workstation (Shanghai Chenhua Instrument Corp., China) at room temperature. Linear sweep voltammetry was used to obtain current-voltage (I-V) curves. Photoelectrochemical oxidation for methanol was measured by a photocurrent transient experiment. A three-electrode system was employed in a quartz cell with an Ag/AgCl (saturated KCl) electrode as the reference electrode, a platinum foil as the counter electrode, and the composite films-assembled ITO glass as the working electrode. All photocurrent transient experiments were carried out at a constant bias of 0 V. A 6 W UV lamp with a monochromatic wavelength (365 nm) was used as a light source, and the average intensity of UV irradiance reaching the composite films was measured to be ca. 105 µW · cm-2. The illumination area of working electrode was set constant at 2.5 × 0.9 cm2. All photoelectrochemical measurements were done in a 0.1 M Na2SO4 electrolyte which was exposed to air. 3. Results and Discussion 3.1. UV-Vis Absorption Spectra and IR Spectra. TiO2 colloidal particles are amphoteric; whether the particle surface is positively or negatively charged depends on the pH of the

Figure 2. UV-vis absorption spectra of multilayer films (PW12/TiO2)n on quartz substrates with n ) 1-5 (from lower to upper curves). The solid line represents spectra after TiO2 deposition. The dashed line represents spectra after PW12 deposition. (Inset) Relationship of absorbance at 248 nm after PW12/TiO2 deposition vs the number of layers.

surrounding media.28 Herein, as TiO2 nanoparticles are positively charged in HNO3 aqueous solution, the spontaneous LbL selfassembly of the anionic POMs and the cationic TiO2 onto the substrate depends basically on the Coulomb force between the oppositely charged species. UV-vis spectroscopy was used to monitor the LbL adsorption process. Figure 2 shows the UV-vis absorption spectra of the (PW12/TiO2)n multilayers (with n ) 1-5) assembled on a precursor-coated quartz substrate (on both sides). Both PW12 and TiO2 possess absorption bands in the UV region. Accordingly, the absorbance was increased with both depositions at 248 nm. In addition, the absorbance at 248 nm was observed to increase linearly with the number of layers as shown in the inset of Figure 2. This indicates that the PW12/TiO2 multilayer films have been constructed uniformly and homogeneously. A similar behavior is observed for the PSS/TiO2 and P2Mo18/TiO2 multilayer films. In addition, UV-vis data can provide information on the amount of TiO2 deposited on quartz substrate. The surface coverage (Γ) of TiO2 on the LbL films can be calculated according to the equation Γ ) NAAnλTiO2/2nλ,29 where NA is Avogadro’s constant, AnλTiO2 (AnλTiO2 ) Anλtotal - AnλPOMs, n g 1) is the absorbance of TiO2 at a given wavelength λ, Anλtotal is the total absorbance, AnλPOMs is the absorbance of POMs, λ is the molar extinction coefficient of TiO2 (M-1 cm-1), and n is the number of bilayers. Therefore, we can easily get the following results: the surface coverage of TiO2 in the (PW12/ TiO2)3 film is approximately two times larger than that in the (P2Mo18/TiO2)3 film but is close to that in the (PSS/TiO2)3 film. Moreover, the surface coverage of TiO2 in the (P2Mo18/TiO2)3 film is ca. 1.6 times larger than that in the (PW12/TiO2)1 film. IR spectra have been carried out to examine the structure stability of POMs in the LbL films. Figure 3 portrays the IR spectra of the (PW12/TiO2)12 film (a), the (P2Mo18/TiO2)12 film (b), and the (PSS/TiO2)12 film (c) on the Si substrate. In comparison with curve c, curves a and b exhibit characteristic bands of POMs at 700-1100 cm-1. The peaks (curve a) observed at 1079, 943, 840, and 778 cm-1 are assigned to υas(P-Oa), υas(WdOd), υas(W-Ob-W), and υas(W-Oc-W), respectively. The peaks (curve b) at 1070, 933, 835, and 764 cm-1 correspond to υas(P-Oa), υas(ModOd), υas(Mo-Ob-Mo),

Self-Assembly of Titanium Dioxide and Polyoxometalates

Figure 3. IR spectra of the (PW12/TiO2)12 film (a), the (P2Mo18/TiO2)12 film (b), and the (PSS/TiO2)12 film (c) on the Si substrate. Asterisks denote characteristic peaks of POMs.

Figure 4. AFM images of (A) the (PSS/TiO2)3 film and (B) the (PW12/ TiO2)3 film on the silicon wafer.

and υas(Mo-Oc-Mo), respectively (Od, terminal oxygen; Oa, central tetrahedral oxygen; Ob, bridging oxygen of two octahedra sharing a corner; Oc, bridging oxygen sharing an edge). These results suggest that the structure of POMs is retained in the films. 3.2. Investigation of Film Morphology. AFM can provide detailed information about the surface morphology and homogeneity of the deposited film. We have taken the (PW12/TiO2)3 film as representative of the POMs/TiO2 films. Figure 4A and 4B shows AFM images of the (PSS/TiO2)3 film and the (PW12/ TiO2)3 film, respectively. Both samples show typical spherical or granular patterns similar to other classes of TiO2-deposited LbL films.30,31 Such a morphologic feature is due to the aggregation behavior of TiO2 colloidal nanoparticles.32,33 Figure 4A shows that TiO2 agglomerates have a size of ca. 25 nm. Correspondingly, Figure 4B shows nanoparticles coagulated into larger scale agglomerates which are approximately 60 nm in diameter. On the other hand, the average thickness of the (PW12/ TiO2)3 film is approximately 22 nm, which is larger than that of the (PSS/TiO2)3 film (∼18 nm). In addition, vertical grain structure of the multilayer surface can be observed from threedimensional AFM images, which show that the distribution of aggregated particles is almost uniform.

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Figure 5. Photocurrent versus time curves of the (PW12/TiO2)n films of different layers (n ) 1-5) under 0 V versus Ag/AgCl bias. (Insert) Relationship of the photocurrent of the multilayered films and the number of layers.

3.3. Photocurrent Transient Measurement. To investigate the photoelectrochemical performance of the LbL films, the photocurrent response experiments have been carried out under pulsed UV irradiation at a constant bias of 0 V. Figure 5 displays the anodic photocurrent responses obtained for films with different numbers of bilayers (PW12/TiO2)n (n ) 1-5). The photocurrent transients are prompt, steady, and reproducible during several on/off cycles of the UV light irradiation. Interestingly, as shown in the inset of Figure 5, the steady-state photocurrent increases gradually as the number of bilayers increases and then reaches a plateau with n ) 3. This increase is attributed to more TiO2 nanoparticles effectively absorbing the UV light and injecting electrons. However, the photocurrent of the (PW12/TiO2)5 is slightly decreased compared to that of the (PW12/TiO2)4, which is assigned to slower electrontransfer rates from the film to the ITO electrode as the number of bilayers increases, namely, the electron conductivity becomes lower with the increase of film thickness.34 On the other hand, in the case of illumination, when the film thickness is thicker than the light penetration depth, the film far from the light source absorbs few incident photons,35 because the particles adsorbed in the outer part of the ITO hinder the light penetrating deeper. The above results indicate that the photocurrent responses of the (PW12/TiO2)n films are highly dependent on film thickness, that is, we could get the ideal photocurrent intensity of various films through precisely controlling film thickness via the LbL technique. Figure 6 shows the photocurrent changes of the (PW12/TiO2)3 film (a), the (PSS/TiO2)3 film (b), and the (P2Mo18/TiO2)3 film (c) under the chopped UV light irradiation in 0.1 M Na2SO4 at a constant bias of 0 V. The (PW12/TiO2)3 film and (PSS/TiO2)3 film represent fast photocurrent responses, whereas the (P2Mo18/ TiO2)3 film shows a slow one when the UV light is regularly switched on and off. A plausible explanation is deep electron traps which act as trap filling sites36 present in the (P2Mo18/ TiO2)3 film and then can be filled by injected electrons,37 thus displaying the slow photocurrent response. Additionally, we observed a nearly 2-fold increase in the photocurrent of the (PW12/TiO2)3 film as compared to that of the (PSS/TiO2)3 film. Under the case that the surface coverage of TiO2 in the (PW12/ TiO2)3 film is close to that in the (PSS/TiO2)3 film, PW12 can be considered as an efficient electron scavenger to improve the inherent photoelectrochemical response of TiO2. On the other hand, the photocurrent response of the (PW12/TiO2)3 film is

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Figure 8. Redox potentials and electron-transfer processes.

Figure 6. Photocurrent responses of (a) the (PW12/TiO2)3 film, (b) the (PSS/TiO2)3 film, and (c) the (P2Mo18/TiO2)3 film in a 0.1 M Na2SO4 electrolyte.

Figure 7. Current-voltage curves of (a) the (PW12/TiO2)3 film, (b) the (PSS/TiO2)3 film, and (c) the (P2Mo18/TiO2)3 film obtained upon 365 nm irradiation.

much higher than that of the (P2Mo18/TiO2)3 film (Figure 6), suggesting that PW12 is the more efficient component for enhancing the photoelectrochemical performance of the POMs/ TiO2 LbL films. These results demonstrate that photocurrent generation significantly depends on the layered structure of the LbL films. In these TiO2-deposited LbL films systems, the (PW12/TiO2)3 film exhibited superior performance with the greatest photocurrent generation efficiency. 3.4. Current-Voltage (I-V) Curves. To further explore the photoelectron-conversion efficiency of the LbL films, the I-V curves (Figure 7) for the (PW12/TiO2)3 film (a), the (PSS/ TiO2)3 film (b), and the (P2Mo18/TiO2)3 film (c) have been measured upon illumination. The (PSS/TiO2)3 film shows a power conversion efficiency (η) of 0.22% with short-circuit current (Jsc) ) 1.16 µA · cm-2, open-circuit voltage (Voc) ) 0.42 V, and fill factor (ff) ) 0.48. For the (P2Mo18/TiO2)3 film, the I-V curve exhibits a power conversion efficiency of η ) 0.12% with Jsc ) 0.74 µA · cm-2, Voc ) 0.36 V, and ff ) 0.49. In contrast, the (PW12/TiO2)3 film has the highest power conversion efficiency of 0.33% with Jsc ) 1.64 µA · cm-2, Voc ) 0.41 V, and ff ) 0.52. Obviously, the (PW12/TiO2)3 film displays a much higher power conversion efficiency than the others, which is consistent with the results of the photocurrent transient measurements.

It is clear that the (PW12/TiO2)3 film exhibits a higher photocurrent response and η than the (PSS/TiO2)3 film, indicating that PW12 should play the role of an efficient electron shuttle between the TiO2 CB and the ITO electrode. The redox potential of PW12 and the CB level of TiO2 are +0.22 and -0.5 V versus NHE, respectively,24 thus suggesting that electron transfer from the TiO2 CB to PW12 is a favorable exothermic process. When the LbL film is fabricated with PW12 and TiO2, the fast electron-hole recombination on the surface of TiO2 can be retarded effectively, thus improving the photoelectrochemical performance. However, for the (PSS/TiO2)3 film, the photocurrent response and η only depend on the inherent properties of TiO2. Hence, PW12, as a negatively charged component in the LbL films, is crucial in enhancing the overall photoelectronconversion efficiency. In addition, the (P2Mo18/TiO2)3 film reveals a smaller photocurrent response and η than the (PW12/TiO2)3 film. This may be owing to the smaller surface coverage of TiO2 in the (P2Mo18/ TiO2)3 film from the above analysis of UV-vis data, possibly implying that the bigger sized P2Mo18 particle is not favorable for the LbL assembly. Nevertheless, it is difficult to explain the photocurrent response solely from the surface coverage of TiO2, because the photocurrent response of the (P2Mo18/TiO2)3 film is close to that of the (PW12/TiO2)1 film (see Figure 5) despite the smaller surface coverage of TiO2 in the (PW12/TiO2)1 film. Otherwise, POMs is reduced by electrons from the TiO2 CB and then reduced POMs transfer electrons to the ITO electrode. Since the redox potential of PW12 and P2Mo18 is +0.22 and +0.66 V,38 reoxidation of reduced P2Mo18 is more difficult than that of reduced PW12.39 The redox potentials and electron-transfer processes are schematically illustrated in Figure 8. This may be an additional reason which results in the smaller photocurrent response for the (P2Mo18/TiO2)3 film. Therefore, the redox ability of POMs is crucial for enhancing the photoelectrochemical performance in TiO2-deposited LbL films. In this study, the photoelectric conversion mechanism of the (PW12/TiO2)3 film could be explained by the following processes: (1) the band-gap excitation from the valence band to the CB of TiO2 under illumination, (2) electron transfer from the CB of TiO2 to PW12, and (3) electron transfer from PW12 to the ITO electrode. In other words, the photogenerated electrons of TiO2 can be extracted and transferred to PW12, which effectively retards the fast charge pair recombination. Thus, the (PW12/TiO2)3 film exhibited superior photoelectrochemical performance. However, the (PSS/TiO2)3 film has high resistance due to the insulating anionic polymer layers (PSS) used in the assembly process. The increase of the internal resistance will lead to slow photogenerated electron transfer from the CB of TiO2 to the ITO electrode, thus reducing the photocurrent of the films. For the (P2Mo18/TiO2)3 film, reoxidation of reduced P2Mo18 is extremely difficult, which probably results in the recombination of reduced P2Mo18 and photogenerated holes of TiO2. Therefore, film structure is important toward improvement

Self-Assembly of Titanium Dioxide and Polyoxometalates

J. Phys. Chem. C, Vol. 114, No. 11, 2010 5215 0.1-0.5 M range, which obeys the Langmuir-Hinselwood kinetics equation.42 4. Conclusion POMs/TiO2 films were successfully fabricated by the LbL self-assembly method. Photocurrent transient measurement indicated that the photocurrent response of the PW12/TiO2 composite film was strongly dependent on film thickness. In addition, compared to the P2Mo18/TiO2 film and the PSS/TiO2 film, the PW12/TiO2 composite film demonstrated the highest photocurrent, power conversion efficiency, and photoelectrooxidation activity for methanol; this is because such a composite film could promote the efficiency of charge separation and transport of electrons. The results obtained from this study show that the appropriate combination with POMs could modulate the light response, which is potentially useful in many applications including photocatalysis, solar photovoltaic cells, and water splitting devices. Acknowledgment. The authors are thankful for the financial support from the Natural Science Foundation of China (Grant No. 20731002 and 20971019). This work was also supported financially by the Program for Changjiang Scholars and Innovative Research Team in University and by the Science Foundation for Young Teachers of Northeast Normal University (Grant No. 20090403). Supporting Information Available: TEM image and XRD pattern of TiO2 colloids. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 9. (A) Dependence of photocurrent on methanol concentration of (a) the (PW12/TiO2)3 film, (b) the (PSS/TiO2)3 film, and (c) the (P2Mo18/TiO2)3 film in 0.1 M Na2SO4. (B) Plot of methanol concentration to photocurrent, C/Iph, vs C for (a) the (PW12/TiO2)3 film, (b) the (PSS/TiO2)3 film, and (c) the (P2Mo18/TiO2)3 film.

of the photoelectrochemical performance of the TiO2-deposited LbL films system. 3.5. Photoelectrochemical Oxidation of Methanol. To probe the photoelectrocatalytic activity of the LbL films, the photoelectrooxidation process of methanol was investigated at a constant bias of 0.5 V vs Ag/AgCl. Figure 9A shows the photocurrents obtained for the (PW12/TiO2)3 film (a), the (PSS/ TiO2)3 film (b), and the (P2Mo18/TiO2)3 film (c) in different concentrations of methanol under UV light irradiation. The presence of methanol increases the photocurrent responses of these films, and, in particular, the (PW12/TiO2)3 film has the highest photocurrent in each concentration of methanol. This indicates that the (PW12/TiO2)3 film possesses a more significant photoelectrooxidation activity for methanol. It is well known that the photogenerated hole can oxidate methanol either by formation of reactive hydroxyl radicals or by direct charge transfer. From the comparison of the concentration dependence on the photocurrent (Figure 9A) with model predictions,40 we propose that the increase in the photocurrent measured in the presence of methanol can be attributed to direct hole transfer, which is similar to the results reported by Maruga´n and coworkers.41 Furthermore, Figure 9B displays the concentration/ photocurrent (C/Iph) versus concentration plot for methanol oxidation at the (PW12/TiO2)3 film (a), the (PSS/TiO2)3 film (b), and the (P2Mo18/TiO2)3 film (c). The kinetics plot of the photoelectrooxidation of methanol appears linearly in the

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