Mesoporous TiO2-Based Photoanode Sensitized by BiOI and

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Mesoporous TiO2‑Based Photoanode Sensitized by BiOI and Investigation of Its Photovoltaic Behavior Yu Zhang,† Qi Pei,† Jiachen Liang,† Ting Feng,† Xin Zhou,† Hui Mao,† Wei Zhang,† Yoshio Hisaeda,‡ and Xi-Ming Song*,†,‡ †

Liaoning Key Laboratory for Green Synthesis and Preparative Chemistry of Advanced Materials, College of Chemistry, Liaoning University, Shenyang 110036, China ‡ Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, 744 Motooka, Nishiku, Fukuoka 819-0395, Japan Downloaded by CENTRAL MICHIGAN UNIV on September 9, 2015 | http://pubs.acs.org Publication Date (Web): September 9, 2015 | doi: 10.1021/acs.langmuir.5b02248

S Supporting Information *

ABSTRACT: We reported a novel BiOI/mesoporous TiO2 photoanode for solar cells, which was fabricated with BiOI attached onto a threedimensional mesoporous TiO2 film by a chemical bath deposition (CBD) method. BiOI was revealed as an efficient and environmental friendly semiconductor sensitizer to make TiO2 respond to visible light. Based on this photoanode, mesoporous TiO2-based solar cell sensitized by BiOI exhibited promising photovoltaic performance. Meanwhile, the optimization of photovoltaic performance was also achieved by varying cycles of deposition immersions. The highest open circuit voltage and short circuit current of the solar cell can reach 0.5 V and 1.5 mA/cm2, respectively.

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BiOI nanosheets-decorated electrospun TiO2 nanofibers.21 Wang and co-workers developed a series of BiOI/TiO2nanorod array photoanodes grown on fluorine-doped tin oxide (FTO) glass by using a solvothermal/hydrothermal method.22 It is easily expected that BiOI is a promising material for sensitizers and is apt to form heterostructures with TiO2. However, previous studies have mainly focused on the photocatalytic activities of BiOI/TiO2 composites for the degradation of phenol23 and methyl orange.20,24 To the best of our knowledge, there are few reports on BiOI as an effective sensitizer to be applied to solar cells to promote good photovoltaic performance. In the present study, we designed a novel BiOI/mesoporous TiO2 photoanode for solar cells, which was fabricated with BiOI attached onto a three-dimensional mesoporous TiO2 film by the chemical bath deposition (CBD) method. The photoanode displays some excellent characteristics, such as low processing cost, easy fabrication, good light scatter effect, and environmental friendliness. The growth of BiOI particles can be controlled by varying cycles of deposition immersions. Mesoporous TiO2 with appropriate pore size resulted in more efficient and uniform loading of the BiOI, and also served as a light scattering layer.25 The mesoporous TiO2-based solar cell sensitized by BiOI displayed good photovoltaic performance.

INTRODUCTION Recently, dye-sensitized solar cells (DSSCs) have attracted increasing attention because of their high power conversion efficiency and low processing cost.1−5 The conventional DSSC consists of a wide band gap semiconductor photoanode, a photosensitizer, a redox electrolyte, and a counter electrode. As photosensitizer, the major issue for organic dyes is not only the difficult aspects in fabrication but also stability.6 Hence, in recent years inorganic semiconductors have been usually used to replace organic dyes. Conceiving this, a tremendous amount of metal chalcogenide compounds, such as CdS,7 Ag2S,8 PbS,9,10 SnS211 and CuInS2,12 were combined with wide band gap semiconductors, and their applications in photovoltaic devices were reported. However, these sensitizers are prone to decompose and they are environmentally hazardous, which limits their practical applications. Bismuth oxyiodide (BiOI), an important II−VI semiconductor with a narrow band gap of 1.7−1.9 eV, has demonstrated excellent photocatalysis in the degradation of organic pollutants because of its unique optical properties and promising industrial applications in prior studies.13−19 Zhang et al. demonstrated the powder BiOI/TiO2 heterostructure as an efficient photocatalyst, exhibiting higher photocatalytic activity than the single-phase BiOI or TiO2 in the presence of visible light,20 which indicated that BiOI and TiO2 have matching band potentials, leading to the efficient generation and separation of photoinduced carriers. Liao et al. reported a successful attempt at the fabrication and characterization of © XXXX American Chemical Society

Received: June 19, 2015 Revised: August 12, 2015

A

DOI: 10.1021/acs.langmuir.5b02248 Langmuir XXXX, XXX, XXX−XXX

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Langmuir Meanwhile, the photovoltaic performance of the solar cell was also optimized by adjusting the CBD cycles.

Downloaded by CENTRAL MICHIGAN UNIV on September 9, 2015 | http://pubs.acs.org Publication Date (Web): September 9, 2015 | doi: 10.1021/acs.langmuir.5b02248

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MATERIALS AND METHODS

The mesoporous TiO2 microspheres with suitable pore size (average 8 nm) and high surface area (∼197 m2/g) were synthesized through the process described in our earlier study.26 The obtained mesoporous TiO2 (0.2 g) microspheres were mixed with absolute alcohol (1 mL), followed by ultrasonic and magnetic stirring until the particles were dispersed homogeneously. The resulting suspension was applied onto a conducting glass substrate (FTO) by doctor blading. Subsequently the TiO2 film was annealed in air at 450 °C for 30 min. BiOI sensitized mesoporous TiO2 films were prepared through the CBD process. A 0.0020 mol sample of Bi(NO3)3·5H2O (Sinopharm Chemical Reagent Co., ≥ 98.0%) was dispersed in 20 mL of ethylene glycol, and an equivalent mole of KI (Sinopharm Chemical Reagent Co., ≥ 99.5%) was slowly added into the system, leading to the formation of Bi(OCH2CH2OH)I2.27 The homogeneous solution was denoted as solution A. The TiO2 films were first immersed into solution A for 2 min, rinsed with ethanol, dried by an air blower, and then immersed into ultrapure water for 2 min, rinsing and drying likewise. The twostep procedure forms one cycle, and the incorporated amount of BiOI would be increased by repeating the cycle. These BiOI/mesoporous TiO2 working electrodes were cohered together with platinized conducting glasses by epoxy resin.28 The electrolyte was composed of I2, BMII, NBII, CuSCN, and 3-methoxypropionitrile (liquid electrolyte DHS-E36, Dalian HeptaChroma SolarTech Co. Ltd.). For comparison, pure BiOI coated on FTO film was also prepared by using the same procedures with nine CBD cycles. X-ray diffraction (XRD) patterns were characterized on a Bruker (Germany) D8 Advance diffractometer with Cu Kα radiation (1.54056 Å; 60 kV, 80 mA) in the range of 20−80° (2θ). Scanning electron microscopy (SEM) images were determined by using a FESEM JSM6700F microscope. UV−vis diffuse reflectance spectroscopy (DRS) measurements were obtained on a UV−vis spectrometer (Shimadzu UV-2550) using BaSO4 as a reference standard. Analysis of the X-ray photoelectron spectra (XPS) was performed on an ESCLAB 250 using Al as the exciting source. In photovoltaic measurements, the BiOI/ mesoporous TiO2 working electrodes together with platinized conducting glasses serve as a prototype solar cell device. The current density−voltage (I−V) curves were recorded by an electrochemistry workgroup (CHI 660b, Shanghai). A 500 W xenon lamp (CHFXQ500W, Beijing Trusttech Co. Ltd.) was used as the light source and a filter plate (simulated AM 1.5 sunlight, Beijing Trusttech Co. Ltd.) was used to control the wavelength of light. The output light intensity was about 100 mW/cm2, which was measured with a radiometer (Photoelectronic Instrument Co., attached to Beijing Normal University, China). The effective area of the solar cell is 0.25 cm2. The photocurrent density−time (I−t) curves were also carried out using this photovoltaic measurement. The solar cells were directly tested in chopping mode, light on−off repeatedly. The FB potentials of mesoporous TiO2 and BiOI were determined from Mott−Schottky plots recorded by electrochemistry workgroup (CHI 660b, Shanghai). A three electrode single compartment immersed in 0.5 M Na2SO4 solution was used for capacitance analysis. The as-prepared film coated on FTO was used as a working electrode while Ag/AgCl and platinum were used as reference and counter electrodes, respectively.

Figure 1. X-ray diffraction patterns of (a) mesoporous TiO2 film and (b) BiOI/mesoporous TiO2 film prepared after 9 CBD cycles.

BiOI with a tetragonal structure (JCPDS file No. 73-2062). These results provide evidence of the successful coating of BiOI on the surface of mesoporous TiO2. XPS data have also been used to characterize the chemical structure of BiOI/mesoporous TiO2, which is shown in Supporting Information, Figure S1. Two peaks with binding energies around 158.9 and 164.3 eV shown in Figure S1a can be ascribed to the signals of Bi 4f7/2 and Bi 4f5/2, which indicates that Bi is in the form of Bi3+ assigned to BiOI.29 The peaks with binding energy of 630.1 and 618.4 eV (Figure S1b) are associated with I 3d5/2 and I 3d3/2, respectively, which can be assigned to I in BiOI.23 In Figure S1c, the binding energy peaks corresponding to Ti 2p are around 458.5 eV (Ti 2p3/2) and 464.3 eV (Ti 2p1/2), in good agreement with that in TiO2.20,30 The O 1s core level spectrum from Figure S1d fits the strong peak at 529.7 eV and the shoulder at 531.8 eV that is attributed to bulk Ti−O bonds and the O−H bonds of the surface-adsorbed water.21,31 Figure 2 gives SEM images of (a) mesoporous TiO2 and BiOI/mesoporous TiO2 prepared after (b) 3, (c) 7, and (d) 11 CBD cycles, which were synthesized by the procedures as Scheme 1. Figure 2a shows that the mesoporous TiO2 has good spherical morphology with an average size of about 300 nm in diameter. That is to say, the photoanode based on mesoporous TiO2 can significantly enhance optical absorption due to the light scattering effect derived from the submicrometer size.32,33 Figure 2 panels b−d depict the assembly and formation process of BiOI via simple hydrolysis. As illustrated in the figure, mesoporous TiO2 microspheres were coated by BiOI uniformly after 3 CBD cycles (Figure 2b). With the increasing number of deposition cycles, the aggregative particles became flakes for the further growth. Owing to the adsorption of EG molecules on the BiOI surface, these flakes tended to cross-link,27,34 then the crossed networks were formed as shown in Figure 2d. Figure 2c exhibits a coexistence of both flakes and networks of BiOI structure. The consecutive deposition of BiOI on the surface of mesoporous TiO2 was accomplished by a sequence of color changes from transparent pale yellow to translucent deep orange (digital photographs depicted in Scheme 1d). Figure 3 shows the UV−vis diffuse reflectance spectra of (a) mesoporous TiO2, BiOI/mesoporous TiO2 prepared after (b) 3, (c) 5, (d) 7, (e) 9, and (f) 11 CBD cycles and (g) pure BiOI.

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RESULTS AND DISCUSSION Structural Studies. The X-ray diffraction (XRD) patterns of mesoporous TiO2 film and BiOI/mesoporous TiO2 film prepared after nine CBD cycles are depicted in Figure 1. Four peaks appearing at 25.2, 37.8, 47.8, and 55.1 degrees are assigned to (101), (004), (200) and (211) orientations of anatase TiO2 (JCPDS file No. 21-1272), respectively. After decoration by BiOI, additional diffraction peaks around 2θ of 29.8, 31.7, and 45.5 degrees appeared in the pattern and can be attributed to the characteristic peaks (012), (110), and (020) of B


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Figure 2. SEM images of (a) mesoporous TiO2 and BiOI/mesoporous TiO2 prepared after (b) 3, (c) 7, and (d) 11 CBD cycles.

Scheme 1. Proposed Growth Routes of BiOI Sensitized Mesoporous TiO2 Film

Figure 3. UV−vis diffuse reflectance spectra of (a) mesoporous TiO2, BiOI/mesoporous TiO2 prepared after (b) 3, (c) 5, (d) 7, (e) 9, and (f) 11 CBD cycles and (g) pure BiOI. Inset: Plots of the (ahν)1/2 vs photon energy (hν).

The absorption edge of mesoporous TiO2 and pure BiOI is located at about 380 and 670 nm respectively, which is consistent with anatase TiO2 and tetragonal BiOI according to ref 16 and ref 35. After the CBD procedures, all the BiOI/ mesoporous TiO2 samples display apparent response in the visible region (>400 nm). Furthermore, the increased absorbance of the spectra and obvious redshift of the absorption edge position are observed with an incremental CBD cycle (Figure 3b−f), indicating an increased amount and the growth of attached BiOI. As we know, small particles are prone to agglomerate to form bigger ones, so the increasing number of deposition cycles would lead to a slow assembly process and the formation of uniform structures of BiOI. For

semiconductors, the adsorption near the band edge follows the formula:36 A(hν − Eg)n/2, where α, ν, Eg and A represent absorption coefficient, light frequency, energy band gap, and a constant, respectively; n depends on the characteristics of optical transition of the semiconductor, n = 1 for indirect transition and n = 4 for direct transition. As for TiO2 and BiOI, the value of n is 4.37 Thus, the band gap energy of mesoporous TiO2 (inset a) is evaluated to be 3.02 eV, and the band gap energy of BiOI (inset g) is evaluated to be 1.72 eV. The band gap energy differences between various BiOI/mesoporous TiO2 C


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(inset b−f) and pure BiOI (inset g) are mainly induced by the size quantization effect. The Mott−Schottky equation was conducted to identify the flat band potential of the as-prepared mesoporous TiO2 and BiOI. According to the Mott−Schottky equation, a linear relationship of 1/C2 versus applied potential can be obtained, and the negative and positive slopes correspond to p- and ntype conductivities, respectively. The results in Figure 4 panels

Figure 5. (a) Schematic energy-band diagram for isolated mesoporous TiO2 and BiOI; (b) energy-band diagrams for mesoporous TiO2/BiOI heterogeneous structure, and the separation of charge carriers under irradiation.

photogenerated holes in the VB of BiOI. Thus, this p−n structure will allow the separation of photoinduced electrons and holes. Photoelectrochemical Performance. Current−voltage curves of the BiOI/mesoporous TiO2 solar cells, which were measured under illumination (simulated AM 1.5 sunlight) with a powder density of 100 mW/cm2, are shown in Figure 6, and

Figure 4. Variation of capacitance (C) with the applied potential in 0.5 M Na2SO4 presented in the Mott−Schottky relationship for (a) mesoporous TiO2 and (b) pure BiOI. The capacitance was determined by electrochemical impedance spectroscopy.

Figure 6. Current density−voltage curves of (a) mesoporous TiO2 photoanodes, (b) BiOI photoanodes, and BiOI/mesoporous TiO2 photoanodes with various (c) 3, (d) 5, (e) 7, (f) 9, and (g) 11 CBD cycles under simulated AM 1.5 sunlight with a light intensity of 100 mW/cm2.

a and b show that the mesoporous TiO2 is an n-type semiconductor and BiOI is a p-type semiconductor. Thus, the BiOI/mesoporous TiO2 can be seen as an n−p heterogeneous structure, which would be very conductive to the photogenerated charges separation. As reported earlier, the flat band potential represents the apparent Fermi level of a semiconductor in equilibrium with a redox couple.38,39 Therefore, we can propose the change of energy band structure of the two semiconductors before and after the contact as shown in Figure 5 panels a and b. The Fermi level of TiO2 and BiOI are 0.05 and 1.74 V (vs AgCl), respectively. When TiO2 and BiOI were brought in contact, the Fermi level should be at equilibrium conditions, a p-n heterogeneous structure can be formed after the contacting of the n-type TiO2 and p-type BiOI. The electrons in the VB of BiOI would be excited into the CB when exposed to visible light. Then, the photogenerated electrons were transferred from BiOI to the CB of TiO2, leaving the

the main photovoltaic parameters are listed in Table 1. According to the results, the photovoltaic performance of a single BiOI working electrode is relatively poor, which may be due to its weak electron transporting ability and P-type characteristic. Obviously, those solar cells based on BiOI/ mesoporous TiO2 films display good ability in converting the light to electric current compared to single mesoporous TiO2based or BiOI-based solar cells. This may be ascribed to the effective light absorbance and electrons injection of the heterogeneous structure. As it is revealed in Figure 6, with an increase of CBD cycles, the overall efficiency values reach a maximum (nine CBD cycles) and then decrease. On one hand, there should be relatively little BiOI attached on the D


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Table 1. Photovoltaic Parameters Obtained from the Current Density-Voltage Curves for the Cells (This Data Is the Average of Tests Recorded on Three Different Devices) Based on Mesoporous TiO2 Photoanode, BiOI Photoanode and BiOI/ Mesoporous TiO2 Photoanodes with Various CBD Cycles under a Light Intensity of 100 mW/cm2 Jsc (mA/cm2)

sample


mesoporous TiO2 BiOI BiOI/mesoporous BiOI/mesoporous BiOI/mesoporous BiOI/mesoporous BiOI/mesoporous

TiO2, TiO2, TiO2, TiO2, TiO2,

3 CBD cycles 5 CBD cycles 7 CBD cycles 9 CBD cycles 11 CBD cycles

0.28 0.12 0.77 0.92 1.09 1.52 1.05

± ± ± ± ± ± ±

Voc (V)

0.02 0.01 0.03 0.02 0.02 0.02 0.01

0.34 0.41 0.50 0.50 0.53 0.49 0.51

mesoporous TiO2 film when the number of CBD cycles was under 9. Thus, relatively few photogenerated charges would be excited, resulting in relatively weak photoelectric conversion efficiencies. On the other hand, excessive accumulation of BiOI will enhance layer thickness with the increasing CBD cycles, which may increase the recombination probability of photoinduced electrons and holes in the film.40 The mesoporous TiO2 based-solar cell sensitized by BiOI can attain the highest short-circuit currents of 1.52 mA/cm2 and open-circuit voltages of 0.49 V when illuminated with light intensity of 100 mW/ cm2. Compared to our previous report,40 the difference between using BiOI and CdS quantum dots (QDs) as light harvesting media in photoelectrochemical performance of DSSCs is very close. However, BiOI has its peculiar advantage in environmental friendliness, which makes it a promising material for solar cells. To further confirm the separation of photogenerated charge carriers in the samples, photocurrent experiment was carried out under simulated AM 1.5 sunlight. The experimental results of mesoporous TiO2 and BiOI sensitized mesoporous TiO2 are shown in Figure 7. The photocurrent density of a single TiO2

± ± ± ± ± ± ±

0.02 0.03 0.02 0.01 0.02 0.01 0.02

η (%)

FF 0.42 0.44 0.51 0.50 0.51 0.51 0.52

± ± ± ± ± ± ±

0.01 0.02 0.03 0.03 0.03 0.01 0.02

0.04 0.02 0.20 0.23 0.29 0.38 0.28

± ± ± ± ± ± ±

0.006 0.003 0.02 0.003 0.02 0.02 0.02

mesoporous TiO2 after nine CBD cycles is the highest when we compared the transient photocurrent response.

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CONCLUSIONS In this study, we reported novel solar cells fabricated by adopting a simple chemical bath deposition method with BiOI sensitized mesoporous TiO2 as the photoelectrodes. On the basis of this photoanode, the highest efficiency of mesoporous TiO2-based solar cell sensitized by BiOI reached 0.38% with the short-circuit current of 1.52 mA/cm2 and open-circuit voltage of 0.49 V, exhibiting good photoelectrochemical property. For our photoelectrochemical system, the photogenerated holes can be consumed by the electrolyte, which ensure the stability of BiOI. These results show that this novel BiOI sensitized mesoporous TiO2 photoanode is worth wide study for its prospective application in areas such as DSSCs and photoelectrochemical hydrogen production.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b02248. XPS spectra of BiOI/mesoporous TiO2 prepared after nine CBD cycles (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-24-62207794. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the National Natural Science Foundation of China (Nos. 21203082, 51273087, 51203072), Natural Science Foundation of Liaoning Province (Educational Department) (No. LT2011001), the Research Fund for the Doctoral Program of Liaoning Province (No. 20131042).

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Figure 7. Photocurrent of (a) mesoporous TiO2 photoanodes, (b) BiOI photoanodes, and BiOI/mesoporous TiO2 photoanodes with various (c) 3, (d) 5, (e) 7, (f) 9, and (g) 11 CBD cycles under simulated AM 1.5 sunlight on/off cycles.

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Mesoporous TiO2-Based Photoanode Sensitized by BiOI and

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