Investigation on the photoconductivity of polyoxometalates

Abstract

Polyoxometalates (POMs), a class of molecular metal-oxo cluster compounds based mainly on Mo, W, and V elements, have shown superior physicochemical properties and various applications. In this study, we investigate the photoconductivity of Keggin-type tungsten-series POMs, H₃PW₁₂O₄₀, Na₃SiW₁₂O₄₀, Na₃GeW₁₂O₄₀ and Na₆H₂W₁₂O₄₀, considering that photoconductivity can be closely related to the potential applications in sensors, photodetectors, photovoltaics, and photocatalysis. The results from the photoconductivity measurements demonstrate that the photoconductive performance of the POM is clearly dependent on the type of POM. Moreover, H₃PW₁₂O₄₀ exhibited a much higher photocurrent than the others; this may be due to the different photogenerated electron–hole recombination rate and photogeneration efficiency of the POMs. Furthermore, H₃PW₁₂O₄₀ exhibited a good sensing performance for ethanol gas.

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1. Introduction

In recent years, solar cells have been widely studied with the growing requirement for the generation of electricity from sunlight, including dye sensitized cells, organic solar cells, perovskite solar cells, etc.^1,2 As a core component in the solar cell, the development of semiconductor materials has attracted considerable attention because of the urgent demand for low cost production and the solar energy conversion of high efficiency.³ However, most semiconductor materials applied in solar cells 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.^4,5 To solve these problems, various strategies have been adopted, including morphological control, metal modification, and combination with other materials, etc.^6–8

POMs, one kind of molecular metal oxides, have structural variety and special physicochemical properties. They have showed broad applications in catalysis, molecule-based functional materials and molecular magnetism.^9–11 In particular, one of the fundamental properties of POM clusters is the capability for accepting electrons. In addition, POMs can undergo reversible multielectron redox reactions in retaining intact structure;^12–14 this made POMs an efficient electron scavenger to capture and transport photogenerated electrons from the CB of semiconductor and then improve the photovoltaic response. Choi and Park investigated that the use of POMs facilitated photogenerated electron transfer from titanium dioxide (TiO₂) to a collector electrode to improve the overall photoefficiency.¹⁵ POMs could also improve the energy conversion efficiency of a photovoltaic device containing TiO₂ nanodisc films.¹⁶ Furthermore, our group previously reported the preparation of POMs/inorganic semiconductors (TiO_2, CdS, Cu₂O and SnO₂, etc.) nanocomposite films to enhance the photovoltaic performance.¹⁷ Meanwhile, Vasilopoulou group investigated the use of reduced POMs as cathode interlayers in organic optoelectronic devices to achieve significant enhancement of device performance.¹⁸ In these photovoltaic devices, POMs played a vital role in improvement of their energy conversion efficiency. On one hand, POM can be used as electron acceptor to capture the photogenerated electrons from semiconductors,^15,16,19 which facilitates charge separation and restrains electron–hole recombination. On the other hand, POMs can be excited to generate the photocurrents under UV irradiation.²⁰

Therefore, the intrinsic nature of the electron–hole recombination and photogenerated charge transfer in POM would have a great effect on photoelectric property of POM-based composite materials. Photoconductivity corresponds to an increase of conductivity due to the excess carriers generated by the absorption of photons. Furthermore, photoconductivity can reflect both the free carrier density and the competitive result from exciton photogeneration, electron–hole recombination and trapping. More importantly, the photoconductive materials can find potential applications in sensors, photodetectors, photovoltaics, photocatalysis.^21–23 In earlier researches, since POMs were traditionally considered as the insulators due to their wide band gap, the investigation on the photoconductivity of POMs has been overlooked in previous works. In this work, we investigate the photoconductive properties of Keggin-type tungsten-series POMs, H₃PW₁₂O₄₀ (PW₁₂), Na₃SiW₁₂O₄₀ (SiW₁₂), Na₃GeW₁₂O₄₀ (GeW₁₂) and Na₆H₂W₁₂O₄₀ (H₂W₁₂) and their optical band gap values are obtained from the UV-vis diffuse reflectance spectra. We found that these POM samples could display steady and reproducible photocurrent responses, while PW₁₂ exhibited much higher photocurrent than others. These results of photoconductivity measurement provide an insight to explain why POMs could quickly transfer electron into external circuit and retard electron–hole recombination. In addition, the PW₁₂ device exhibited a fine sensing performance for ethanol gas, which is the first example of using only POM as the gas sensing material (Fig. 1).

Fig. 1 The UV-vis diffuse reflectance spectra of PW₁₂ (a), SiW₁₂ (b), GeW₁₂ (c) and H₂W₁₂ (d).

2. Experimental

2.1. Materials

PW₁₂, SiW₁, GeW₁₂ and H₂W₁₂ were prepared according to literature method and identified by UV-vis adsorption spectra, infrared spectra (IR), energy dispersive X-ray (EDX), X-ray diffraction (XRD) and cyclic voltammetry of four POMs (Fig. S1–S5†).^24,25 The water used in all experiments was deionized to a resistivity of 18 MΩ cm.

2.2. Characterization

Diffuse reflectance UV-vis spectra were measured from 200 to 800 nm on a Varian Cary 500 UV-vis NIR spectrometer equipped with a 110 mm diameter integrating sphere at room temperature. A barium sulfate (BaSO₄) pellet was used as the standard with 100% reflectance. IR spectra were recorded with a Nicolet Magna 560 FT-IR Spectrometer. UV electronic spectra were obtained on a Varian Cary 50 Conc UV-vis spectrophotometer. XRD analyses were performed with a Rigaku D/max-3c X-ray diffractometer, using CuKα1 radiation (λ = 1.5405 Å). Field-emission scanning electron microscopy (Hitachi SU-8010 FE-SEM) was used to investigate the surface morphology.

All photoelectrochemical experiments were performed on a CHI660C Electrochemical Workstation (Shanghai Chenhua Instrument Corp., China) at room temperature. A 500 W xenon lamp (320 nm ≤ λ ≤ 780 nm) was used as an irradiation source. The average irradiation intensity reaching the films was about 50 mW cm⁻². A two-electrode system was employed with each piece of the ITO as an electrode. The photoconductivity of the samples was measured by monitoring their photocurrent response under light irradiation. All photocurrent experiments were carried out at a constant bias of 0.6 V. In addition, all the tests were operated at a relative humidity of 40 ± 5%.

2.3. Fabrication of photoconductive device

The fabrication of photoconductive device includes two steps. First, the electrodes for photoconductive device were made as the following process: first, to protect the unetaching parts of the ITO glass and ensure the same etching length of all ITO glass, we covered the ITO glass with hook tapes. Secondly, the tape was cut by a sharp knife to get a gap and the length of gap is ca. 2 cm and the gap was coated by enough Zn powder. At last, the conducting layer was detached from the ITO glass by moving the cotton (containing enough 2 M hydrochloride solution) on the gap covered enough Zn powder. Then, the etched substrate was cleaned with surfactant and rinsed with acetone and ethanol and deionized water and finally dried by air flow. Each piece of the ITO was adopted as an electrode.²⁶

Finally, the PW₁₂ powder (0.3 g) was dispersed in ethanol (1 mL) by ultrasonication. And then the obtained mixture was coated over the surface of the gap between the two electrodes by doctor blade technique, getting a composite film. After air drying, the photoconductive device formed. Fig. S6† shows the device structure. For comparison, SiW₁₂, GeW₁₂ and H₂W₁₂ photoconductive devices were prepared in a similar way.

To investigate the gas sensitivity of the samples, the device was placed in an airproof test box with a quartz window. A special volume of volatile solution was injected into the box with a microsyringe. Following injection, ethanol solution, which is highly volatile at room temperature, will produce volatile vapors. In this system, air was used both as a reference gas and a diluting gas to get desired concentrations of ethanol.

3. Results and discussion

UV-vis diffuse reflectance spectroscopy is an important analytical tool for characterizing the optical properties of POMs. Fig. 1 shows the UV-vis spectra of PW₁₂ (a), SiW₁₂ (b), GeW₁₂ (c) and H₂W₁₂ (d). All the samples possess broad absorption bands in the UV region, which indicated that these POMs exhibited light-harvesting capabilities at the UV region. In addition, the measurement of diffuse reflectance UV-vis spectra for solid state sample is a convenient and reliable spectroscopy means of obtaining E_g. In the means, the band gap value was determined by the intersection point between the energy axis and the line extrapolated from the linear portion of the absorption edge in a plot of Kubelka–Munk function F against energy E (Fig. 2).

E = 1240/λ (1)

F = (1 − R)²/2R. (2)

λ is wavelength. R is the reflectance of an infinitely thick layer at a given wavelength. As shown in Table S1,† the E_g values of Keggin-type tungsten-series POMs located at the range of wide band gap, which is similar to TiO₂ and ZnO.

Fig. 2 K–M function versus energy (eV) curve of PW₁₂ (A), SiW₁₂ (B), GeW₁₂ (C) and H₂W₁₂ (D).

SEM can provide the detailed information on the surface morphology and the homogeneity of films. Fig. 3 showed the SEM images of four POMs films. All films surface is rough and consists of block-shaped patterns. Fig. S7† displayed the SEM of the bridging-gap and its width is roughly 106 ± 20 μm.

Fig. 3 SEM images of PW₁₂ film (A), SiW₁₂ (B), GeW₁₂ (C) and H₂W₁₂ (D).

To investigate the photoconductive performance of the samples, photocurrent response experiments (I–t) of these photoconductive devices have been carried out at a constant bias of 0.6 V under Xe lamp irradiation. Fig. 4 displays the photocurrent responses of the photoconductive device with different types of POMs (PW₁₂, SiW₁₂, GeW₁₂ and H₂W₁₂) when the irradiation is switched on and off. All the samples show steady and reproducible photocurrent responses during several on/off cycles of the Xe lamp irradiation. The photocurrent response intensity follows the order H₂W₁₂ < GeW₁₂ < SiW₁₂ < PW₁₂, and PW₁₂ exhibits much higher photocurrent than others. On one hand, this may be attribute to the different absorption efficiency of different type of POMs. As shown in Fig. S1,† the PW₁₂ and SiW₁₂ displayed similar and higher absorbance, GeW₁₂ and H₂W₁₂ displayed similar and lower absorbance. On the other hand, although the PW₁₂ and SiW₁₂ displayed similar absorbance, the PW₁₂ exhibited much higher photocurrent than SiW₁₂. GeW₁₂ and H₂W₁₂ also showed the same results. This may be due to the different photogenerated electron–hole recombination rate of POMs, PW₁₂ have lower photogenerated electron–hole recombination rate than others. This is in accordance with previous reports.^17,27

Fig. 4 Photocurrent responses of PW₁₂ (a), SiW₁₂ (b), GeW₁₂ (c) and H₂W₁₂ (d) devices under Xe lamp irradiation.

To further explore the photoconductive performance of the POMs, the current–voltage (I–V) curves (Fig. 5) for the POMs device have been measured. Compared the dark current, the POMs device show a remarkable enhancement in the photocurrent in each voltage. Taken the bias of 0.6 V as representative, the photocurrent is about 5.8-fold (PW₁₂), 2.3-fold (SiW₁₂), 1.7-fold (GeW₁₂), 1.5-fold (H₂W₁₂) respectively, which is in accordance with the photocurrent response experiments.

Fig. 5 Current–voltage (I–V) curves of PW₁₂ (A), SiW₁₂ (B), GeW₁₂ (C) and H₂W₁₂ (D) devices under Xe lamp irradiation.

It is well known that the photoconductivity results from the competition between the photogenerated electron (charge carrier) and the electron–hole recombination. Since different anionic compositions of POMs have different energy band gaps (E_g value) between HOMO and LUMO (Table S1†), which are directly related with the capabilities to generate photoexcited electron. For example, PW₁₂ and SiW₁₂ have almost the same light absorbance efficiency, but the photocurrent of PW₁₂ is much higher than that of SiW₁₂. Therefore, according to the I–t and I–V curves, we can conclude that the photoconductivity depends on the type of POMs, namely the compositions and structures of the anions.

Previous reports have demonstrated that POMs could share similar photochemical characteristics of TiO₂ photocatalysts;^28–30 that is, POMs represented the analogues of TiO₂. UV excitation of POMs induces an electron transfer from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), which corresponds to a ligand to metal charge transfer (from O 2p to W 4d). This can be considered as a similar process to the bandgap excitation from the valence band (VB) to the conduction band (CB) in the TiO₂. Then the photoexcited electron as charge carrier can transfer from the LUMO to the photoanode under the bias voltage, and then the charge carrier (photoexcited electron) migrates into the photocathode through the electric circuit, so that the photoexcited electron is returned to POMs. Thus, the mechanism for the POM photoconductivity generation is proposed as follows. With illumination and bias voltage, POMs can be excited and produce the photogenerated electron transfer from HOMO to LUMO. Then the bias voltage brings about the electron directional migration, and thus generates the photocurrent.

At present, as the most common photoconductive, the metal oxide semiconductor TiO₂ has drawn considerable attention. In this study, the photocurrent responses experiment of the photoconductive device with pure TiO₂ (P25) has been carried out in the same way with POMs. According to the related literatures and the results (Fig. S8†), we can estimate that PW₁₂ has better photoconductive properties than TiO₂.^31–33

In order to probe gas sensing performance of these devices, we investigated their responsive behaviors for ethanol under Xe lamp irradiation. Fig. 6A shows the photocurrent responses of the PW₁₂ device in different concentrations of ethanol. The photocurrent responses gradually increase with the sequentially increasing ethanol concentration. When ethanol is injected, gas–solid photocatalytic oxidation–reduction reactions can occur on the surface of POMs under light irradiation. Ethanol as a hole scavenger can trap the photogenerated hole, and thus suppressing the photogenerated electron–hole recombination and increasing the photocurrent responses of the devices.^34,35 The sensitivity (S) of device is defined as S = (I_M2 − I_M1)/I_M1, where I_M1 is the maximum photocurrent before ethanol gas injection and I_M2 is the maximum photocurrent after addition of ethanol gas. As shown in Fig. 6B, PW₁₂ device exhibits significant gas sensing activity for ethanol. And Fig. S9† showed the photocurrent responses of PW₁₂ device to ethanol concentration (0–100 ppm) and the detection limit³⁶ was <25 ppm.

Fig. 6 Photocurrent responses (A) and the sensitivities (B) of the PW₁₂ device in different concentrations of ethanol.

4. Conclusions

The photoconductive performance of Keggin-type tungsten-series POMs has been demonstrated by photocurrent response measurement. The PW₁₂ exhibited much higher photocurrent than others; this may be due to different photogenerated electron–hole recombination rate and different photogeneration efficiency of POMs. Moreover, the PW₁₂ photoconductive device exhibited significant gas sensing activity for ethanol. This study demonstrates that Keggin tungsten-series POMs can display photoconductivity, and the photoconductive performance of the POMs is clearly dependent on the type of POMs. These results of photoconductivity measurement clearly reveal why POMs could quickly transfer electron into external circuit and retard electron–hole recombination. Meanwhile, the PW₁₂ device showed a good sensing performance for ethanol gas, suggesting its potential applications in developing new-type photoelectric sensing device.

Acknowledgements

The authors are thankful for the financial support from the Natural Science Foundation of China (Grant No. 21273031 and 21401020). This work is also supported by the Fundamental Research Funds for the Central Universities (14QNJJ012).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra15544h

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