BiOI-Based Photoanode with n-p Heterogeneous Structure for

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FeO/BiOI -based photoanode with n-p heterogeneous structure for photoelectric conversion Yu Zhang, Ying Li, Weining Sun, Chunxue Yuan, Baoxin Wang, Wei Zhang, and Xi-Ming Song Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02969 • Publication Date (Web): 29 Sep 2017 Downloaded from http://pubs.acs.org on September 30, 2017

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Fe2O3/BiOI -based photoanode with n-p heterogeneous structure for photoelectric conversion

Yu Zhang, Ying Li, Weining Sun, Chunxue Yuan, Baoxin Wang, Wei Zhang, Xi-Ming Song*

Liaoning Key Laboratory for Green Synthesis and Preparative Chemistry of Advanced Materials, College of Chemistry, Liaoning University, Shenyang 110036, China

* Corresponding authors. E-mail: [email protected]; Tel: +86-24-62207794

Abstract We report a promising photoanode material of Fe2O3/BiOI for efficient photoelectric conversion in solar cells, which was fabricated with BiOI attached onto a one-dimensional Fe2O3 nanorods array. The two semiconductors of p-type BiOI and n-type Fe2O3 formed a heterogeneous structure for efficient charge separation. The highest open circuit voltage and short circuit current of the solar cell can reach 0.41 V and 4.89 mA/cm2, respectively. This study opens an available field to develop low cost and environment-friendly photoelectric materials for solar cells.

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1. Introduction The third generation solar cells, such as dye-sensitized solar cells, perovskite cells, quantum dot solar cells and hybrid solar cells, have attracted worldwide attention due to their cost-effective fabrication and relatively high device performance in the past two decades.1-8 TiO2 as a core electrode material has been widely used because of its excellent electron transporting properties in most of the third generation solar cells,9-11 where varied photosensitizers (organic dye, perovskite, quantum dots) were used to absorb solar light. Under illumination, the photogenerated electrons transfer from the photosensitizers to the conduction band (CB) of TiO2, and then diffuse to the electrode. However, the photosensitizers with complex structures could enhance the manufacturer's cost and introduce highly toxic cadmium and lead chalcogenide elements.12-16 In addition, the photogenerated electrons are almost exclusively originated from the sensitizers in these solar cells, because TiO2 with a wide band gap cannot be excited by visible light.9 So, instead of TiO2, an electron-tansporting material that could absorb visible light to produce photogenerated electrons may be a good choice in developing next generation solar cells. α-Fe2O3, a common semiconductor with a narrow-band gap of 2.1 eV, possesses the advantages of easy fabrication, low cost and nontoxic nature.17-20 But the short lifetime of photogenerated charge carriers, low electron mobility and short hole diffusion length greatly limit its further application in photoelectric system.21,22 Recently, the photoelectrochemical (PEC) performance of Fe2O3 has been improved by ion doping and compounding semiconductors, and some progress is achieved in 2


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the area of its PEC water splitting. Fu and co-workers prepared a Ti-doped α-Fe2O3 with Al3+ treatment for photoanode in PEC water splitting.23 Negative shift of the onset potential and strong increase in photocurrent density were achieved in the PEC water oxidation cell. Xu and co-workers introduced a cocatalyst of Ni(OH)2 to the Ti-doped α-Fe2O3 photoanode.24 They found the Ni(OH)2 layer can store holes produced in Ti-Fe2O3, leaving the photogenerated electrons in the Ti-Fe2O3 layer, resulting in the enhancement of urea oxidation. Xia and co-workers prepared a high-performance BiVO4/α-Fe2O3 photoanode for PEC water splitting.25 They confirmed the Fe2O3 reduced the accumulation of holes on the surface and thus reduced charge recombination. These results reveal that enhanced photogenerated charges separation exists in the Fe2O3-based heterostructures. As far as we know, there have been no reports on Fe2O3 as an effective electrode material of solar cells for photoelectric conversion. In this work, BiOI, an easily obtained and small toxic semiconductor with narrow band gap,26-30 was used to construct a heterogenerous structure with Fe2O3 nanorods array. Since BiOI is a p-type semiconductor,31 the two semiconductors can form the p-n heterogenerous structure, which can greatly promote the separation of electron-hole pairs. Then, a novel solar cell was designed based on this one-dimensional Fe2O3/BiOI photoanode, and the short circuit current of the photovoltaic device was up to 4.89 mA/cm2. The charge separation mechanism was revealed by time-resolved photovoltaic results from the view of dynamics in the system. 3


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2. Experimental section 2.1. Materials Sodium nitrate (NaNO3), Bismuth nitrate (Bi(NO3)3·5H2O), ethylene glycol ((CH2OH)2), Potassium iodide (KI) were

purchased

from Sinopharm Chemical

Reagent Company Limited. Ferric chloride (FeCl3) was purchased from Tianjin Yonda Chemical Reagent Company Limited. Fluorine-doped tin oxide (FTO) glass was purchased from OPV Tech New Energy Company Limited. All the chemicals in the experiment were analytical grade and used without further purification. 2.2. Preparation of α−Fe α− 2O3 nanorods film Fe2O3 photoanode was prepared through a hydrothermal method.23A 50 mL aqueoussolution of 0.15 M FeCl3 and 1.0 M NaNO3 was removed to a reaction kettle. Subsequently, an FTO glass was put into it with the conducting plane facing down, then the reaction kettle was put in a vacuum oven and heated to 100oC for 12 h. After cooling down to the room temperature, the FTO with Fe2O3 film grown on was fully washed in deionized water and annealed in air at 550oC for 2 h at a ramp rate of 2 o

C·min-1.

2.3. Preparation of Fe2O3/BiOI nanocomposites The Fe2O3/BiOI nanocomposites were prepared via a chemical bath deposition (CBD) process.32 0.0020 mol Bi(NO3)3·5H2O (Sinopharm Chemical was dissolved in 20 mL of ethylene glycol, the equimolar amounts of KI was following slowly added into the mixed solution to form Bi(OCH2CH2OH)I2. Firstly, the as-prepared Fe2O3 films were immersed into the above solution for 2min, and then washed with ethanol, 4


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dried by an air blower. Secondly, the films were immersed into ultrapure water for 2min, washed and dried following the same steps above. One CBD cycle was combined with two steps mentioned above. With the cycles increased, the amount of BiOI would be increased too. For comparison, pure BiOI film was also prepared by using the same procedures with eight CBD cycles. 2.4. Characterizations X-ray diffraction (XRD) patterns were characterized on a Bruker (Germany) D8 Advance diffractometer with Cu Kα radiation in the range of 20o-80o (2θ). Scanning electron microscopy (SEM) images were recorded by a FESEM JSM-6700F microscope equipped with EDAX attachment to probe elemental analysis. The high resolution transmission electron microscopic (HRTEM) imaging was performed on a HITACHI H-7650 microscope. XPS analysis was performed by using an ESCALAB 250 instrument with Al Kα radiation (15 kV, 150 W) under a pressure of 4×10-8 Pa. UV–vis diffuse reflectance spectroscopy (DRS) measurements were obtained on an UV–vis spectrometer (Shimadzu UV-2550).In photovoltaic measurements, a 500 W xenon lamp (CHFXQ500W, Beijing Trusttech Co. Ltd.) with a filter plate (simulated AM 1.5 sunlight, Beijing Trusttech Co. Ltd.) was used as the light source. The output light intensity was 100 mW/cm2, which was measured with a radiometer (Photoelectronic Instrument Co., attached to Beijing Normal University, China). The Fe2O3/BiOI working electrodes together with platinized FTO counter electrode serve as a prototype solar cell device. The current density-voltage (I–V) curves were recorded by an electrochemistry workgroup (CHI660E, Shanghai). The effective area 5


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of the solar cell is 0.25 cm2. The photocurrent density-time (I–t) curves with light on-off cycles were also carried out using this photovoltaic measurement, and the solar cells were directly tested in chopping mode with no applied bias. The incident photon-to-electron conversion efficiency (IPCE) was calculated from the photocurrent under monochromatic light and the corresponding light intensity. The monochromatic light was obtained by monochromatic filters (Beijing Trusttech Co. Ltd.).The flat band (FB) potentials of BiOI and Fe2O3were determined from Mott-Schottky plots recorded by electrochemistry workgroup (CHI660E, Shanghai). A three-electrode single compartment immersed in 0.5M Na2SO4 solution was used for capacitance analysis. The Fe2O3/BiOI film was used as a working electrode while Ag/AgCl electrodes and platinum electrodes were used as reference electrode and counter electrodes, respectively. Time-resolved photovoltage (TPV) measurement system consisted of a laser pulse radiation (wavelength 532 nm, pulse width 5 ns, excitation intensity 50 µJ cm-2) from a Nd: YAG laser (Polaris II, New Wave Research, Inc.), a 500 MHz digital phosphor oscilloscope (TDS 5054, Tektronix) with a preamplifier and a samplechamber.33 The samplechamber like a parallel-plate capacitor consisted of the Fe2O3/BiOI film on the FTO substrate, a piece of 10 µm thick mica and a platinum wire gauze electrode. The TPV measurements were performed in air atmosphere and at room temperature. The surface photovoltage spectrum was recorded by a system including a source of monochromatic light, a lock-in amplifier (SR830-DSP) with a light chopper (SR540), a sample cell and a computer. In the photovoltaic cell the FTO with the Fe2O3/BiOI acted as a bottom electrode. A glass 6


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substrate covered with ITO was used for the top electrode and a spacer of mica was inserted between the two electrodes.

Figure 1. SEM images of top view and cross-sectional view (a) a single Fe2O3 film, (b) a single BiOI film after 8 CBD cycles, (c) Fe2O3/BiOI nanocomposite after 12 CBD cycles. The scale bar of the insets is 500nm; (d) proposed growth routes of Fe2O3/BiOI nanocomposite step by step.

3. Results and discussion The size and morphology of the Fe2O3, BiOI and Fe2O3/BiOI nanocomposites were investigated via field emission scanning electron microscope (FESEM), as shown in Figure 1. According to the top view of pure Fe2O3 in Figure 1(a), the nanorods were dense and vertically aligned with FTO substrate. The thickness of Fe2O3 nanorods film is about 600 nm from the cross-sectional view. Figure 1(b) shows the single BiOI 7


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film based on FTO substrate obtained via simple hydrolysis, which exhibits a flake-like networks structure. Figure 1(c) presents the FESEM image of the obtained Fe2O3/BiOI heterogeneous structures. As illustrated in the figure, the Fe2O3nanorods were coated by BiOI networks structure uniformly after 12 CBD cycles, and the thickness of Fe2O3/BiOI nanocomposites reaches 1 µm, indicating BiOI had well coated on the Fe2O3 nanorods. The fabrication process of Fe2O3/BiOI nanocomposites film can be schematically illustrated in Figure 1(d). Firstly, large-scale ordered Fe2O3 nanorods were prepared on FTO substrates. Then, BiOI was deposited on the surface of Fe2O3 film via a hydrolysis process using CBD method. The consecutive deposition of BiOI on the surface of Fe2O3 was accomplished by a sequence of color changes from red to dark brown red after 2, 4, 6, 8, 10, 12, 14 CBD cycles. Figure S1 give the EDS results of the samples, it can be found the contents of BiOI (12 CBD) cycles increased when comparing with BiOI (8 CBD). XPS data have also been used to characterize the chemical structure of Fe2O3/BiOI, which is shown in Figure S2 in the Supporting Information. According to the XPS data in Figure S2(a), two peaks centered at 158.6 and 163.9 eV can be attributed to Bi 4f

7/2

and Bi 4f

5/2

region for

BiOI (Bi3+).34 The peaks with binding energy of 630.1 and 618.6 eV are associated with I 3d5/2 and I 3d3/2, respectively, which can be assigned to I in BiOI ( Figure S2(b)).35 In Figure S2(c), the peaks corresponding to Fe 2p are around 710.7 eV (Fe 2p3/2 ) and 724.4 eV (Fe 2p1/2 ), which is consistent with those in Fe2O3.36 The peaks of O 1s spectra at 529.7 eV and the shoulder at 531.8 eV were assigned to lattice oxygen and hydroxyl group (Figure S2(d)).35 8


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Figure 2. (a) X-ray diffraction patterns of Fe2O3, BiOI, and Fe2O3/BiOI nanocomposites based on FTO glass; (b) UV-visible light absorption spectra of Fe2O3, BiOI, and Fe2O3/BiOI nanocomposites prepared after 2, 4, 6, 8, 10, 12, 14CBD cycles, the inset are the photos of samples.

Figure 2(a) depicts the X-ray diffraction (XRD) patterns of Fe2O3, BiOI and Fe2O3/BiOI nanocomposites prepared after 12 CBD cycles. After subtracting the diffraction peaks of FTO substrate, the two peaks appearing at 2θ=35.6o and 64.0o can be vested to (110) and (300) orientations of α-Fe2O3 (JCPDS 86-0550). The three diffraction peaks appearing at 2θ=29.7o, 31.7o and 45.5o can be attributed to characteristic peaks (012), (110) and (020) of BiOI with a tetragonal structure (JCPDS 73-2062). The HRTEM image of Fe2O3/BiOI nanocomposite is shown in Figure S3. Two sets of lattice fringes with the interplanar spacing of 0.300 nm and 0.145 nm were found, corresponding to BiOI (JCPDS 73-2062) and α-Fe2O3 (JCPDS 86-0550), respectively. These results provide that the BiOI was successful coating on the surface 9


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of Fe2O3. UV-vis diffuse reflectance spectra of the Fe2O3/BiOI samples prepared after 2, 4, 6, 8, 10, 12, 14 CBD cycles was described in Figure 2(b). It is noticed that the Fe2O3/BiOI nanocomposites exhibit a broad and apparent absorption edge in the visible region. The absorption edge of Fe2O3 and pure BiOI is located at about 590 and 630 nm respectively, corresponding to the band gaps of 2.1 eV and 1.97 eV using the formula E = 1240/λonset.37 Furthermore, with the increasing of CBD cycles, a red shift of the absorption edge position of Fe2O3/BiOI nanocomposites occurs, indicating the growth of attached BiOI from quantum size.38 As we know, small particles are prone to agglomerate to bigger ones, so the increasing number of deposition cycles would lead to a slow assembly process and the formation of uniform structures of BiOI.

10


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Figure 3. Variation of capacitance (C) with the applied potential in 0.5M Na2SO4 presented in the Mott Schottky relationship for pure BiOI after 8 CBD cycles (a) and Fe2O3 (b), the capacitance was determined by electrochemical impedance spectroscopy; Schematic energy-band diagram for isolated BiOI, Fe2O3 and Fe2O3/BiOI heterogeneous structure (c).

The Mott Schottky equation was used to identify the conductivity type and flat band potential (Ef) of the as-prepared Fe2O3 and BiOI. A linear relationship of 1/C2 versus applied potential was obtained based on the Mott Schottky equation, and negative and positive slopes correspond to p- and n-type conductivities, respectively.39As shown in Figure 3(a) and 3(b), BiOI is a p-type semiconductor and Fe2O3 is an n-type semiconductor, and thus the 11


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Fe2O3/BiOI nanocomposites can be thereby considered as an n-p heterogeneous structure. According to the previous reports,40,41 the positions of the conduction band level (EC) of Fe2O3 and the valance band level (EV) of BiOI are 0.08 eV and 2.22 eV (vs Ag/AgCl), respectively, and the corresponding EV and EC can be determined by the equation EC= EV - Eg. Eg in the equation has been calculated from the UV–vis spectra (Figure 2(b)), which are 2.10 eV and 1.97 eV. Hence, the EV of Fe2O3 and EC of BiOI are 2.18 eV and 0.25 eV. In addition, the previous reports presented that the flat band potential represents the apparent Fermi level of a semiconductor in equilibrium with a redox couple.39,42 Therefore, the Fermi level of Fe2O3 and BiOI are 0.96 eV and 1.70eV (vs Ag/AgCl) obtained from the flat band potential.The change of energy band structure of the two semiconductors before and after the contact can be depicted as follows. As is shown in Figure3(c), the Fermi level of p-type semiconductor BiOI was close to valence band (VB) and the Fermi level of n-type semiconductor of Fe2O3 was on the contrary close to conduction band (CB). When BiOI and Fe2O3werecontacted to each other (see Figure 3(c) right), the Fermi level should be at equilibrium conditions, and an n-p heterogeneous structure can be constructed.43,44 Then, the photogenerated electrons were transferred from BiOI to the CB of Fe2O3, and photogenerated holes were transferred from Fe2O3 to VB of BiOI, forming effective separation of interfacial charge carriers.

12


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Figure 4. Current–voltage (I–V) characteristic curves (a) and photocurrent experiments with light on-off cycles (b) for Fe2O3, BiOI after 8 CBD cycles, and Fe2O3/BiOI nanocomposites prepared after 2, 4, 6, 8, 10, 12, 14 CBD cycles under AM 1.5 sunlight illumination with a power density of 100 mW/cm2; (c) The transients photovoltage of the Fe2O3/BiOI photoelectrode with 12 CBD cycles. The wavelength and intensity of the laser pulse are 532 nm and 50 µJ cm-2 ; (d) Schematic illustration of photogenerated charge transfer in the Fe2O3/BiOI-based solar cell.

Table 1.Photovoltaic parameters obtained from the current density-voltage curves for the cells based on BiOI (8 CBD cycles) photoanode, Fe2O3photoanode, and Fe2O3/BiOI photoanodes with various CBD cycles under a light intensity of 100 mW/cm2. 13


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Jsc(mA/cm2)

Voc (V)

FF(%)

η (%)

Fe2O3

0.58

0.10

25.9

0.015

BiOI

1.10

0.41

35.5

0.16

Fe2O3(BiOI2)

2.96

0.37

25.6

0.28

Fe2O3 (BiOI4)

3.96

0.39

25.3

0.39

Fe2O3 (BiOI6)

3.97

0.37

33.4

0.49

Fe2O3 (BiOI8)

4.39

0.38

31.8

0.53

Fe2O3 (BiOI10)

4.02

0.41

29.1

0.49

Fe2O3 (BiOI12)

4.89

0.36

31.2

0.55

Fe2O3 (BiOI14)

4.03

0.37

26.2

0.39

Figure 4(a) shows current–voltage (I–V) characteristic curves of the solar cells under illumination (simulated AM 1.5 sunlight) with a power density of 100 mW/cm2, and the main photovoltaic parameters are listed in Table 1. On the basis of results, single Fe2O3 working electrode is relatively poor in the photovoltaic performance, which is in accordance with its inferior electron transporting ability. However, those solar cells based on Fe2O3/BiOI photoanodes showed a marked rise in converting the light to electric current when comparing to the single Fe2O3-based or BiOI-based solar cells. This was considered to be attributed to the effective light absorbance and sufficient charge separation in the heterogeneous structure. By increasing the CBD cycles, more BiOI will be deposited on the surface of Fe2O3, and more charge carriers could be excited. The Fe2O3/BiOI-based solar cell with 12 CBD cycles can acquire the highest short-circuit current of 4.89 mA/cm2, PCE of 0.55%, open-circuit voltage 14


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of 0.36 V, and fill factor (FF) of 0.31. Meanwhile undue CBD cycles would bring about overmuch deposition of BiOI, which may enhance the recombination probability of photogenerated charges.12,31 The photocurrent experiments with light on-off cycles (see Figure 4(b))further confirmed the separation of photogenerated charge carriers in the solar cells. Compared with the single Fe2O3-based or BiOI-based electrodes, the photocurrents of Fe2O3/BiOI electrodes got obvious improvement and good cyclic stability. In addition, the same solar cells have been retested after two months. According to the Table S1, the photoelectric conversion efficiencies of the cells fell only slightly, which is because the BiOI and Fe2O3 are stable inorganic crystals. The XRD peaks of BiOI and Fe2O3 before and after photoelectric test (see Figure S4) remain the same, which further prove the stability of the two semiconductors. The TPV measurements were carried out to reveal the kinetics of the photogeneration of excess carriers in the Fe2O3/BiOI nanostructure. From the TPV responses shown in Figure 4(c), the peak at the level of 10−6 s reflects the nature of separation mode of drift undera built-in electric field for photogenerated charges in the composite film,45-47 which is in accordance with the above discussion on the n-p heterogenerous structure. It is known that there will be an interfacial charge transfer when n-type semiconductor contacts with p-type semiconductor, that is to say, a built-in electric field (or energy band bending) will be formed at their interface. Then the photogenerated electrons and holes will be separated by drift in a short timescale (10-6 s).45-47 The photoelectric activity based on different wavelength light irradiation was 15


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investigated by surface photovoltage spectrum and IPCE spectrum, which is shown in Figure S5 and S6 in the supporting information. It can be seen both photovoltaic response and IPCE is consistent with its absorption spectrum, further proving the separation of photogenerated electrons and holes. Figure 4(d) gives a charge transfer mechanism in the solar cell, unlike traditional TiO2-based photoanode, both Fe2O3 and BiOI can be excited due to their narrow band gaps, and Fe2O3 can also serve as the electron transporting materials. Therefore it is easily demonstrated that Fe2O3/BiOI heterogeneous structure is capable of facilitating the separation of photogenerated charge carriers and improving the photoelectric conversion performance. 4. Conclusions In summary, a novel Fe2O3/BiOI photoanode for solar cells has been successfully fabricated via a simple chemical bath deposition method. The highest efficiency of Fe2O3/BiOI-based solar cell reached 0.55 % with the short-circuit current of 4.89 mA/cm2 and open-circuit voltage of 0.36 V, displaying superior photoelectrochemical property. The n-p heterogeneous structure greatly promoted the separation of photogenerated electrons and holes via an interfacial built-in field. These results show that this novel low cost and environment-friendly Fe2O3/BiOI-based photoanode is worth studying for its prospective application. Acknowledgements The authors are grateful to the National Natural Science Foundation of China (Grant Nos. 21203082 and 51273087). References 16


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SEM images of top view and cross-sectional view (a) a single Fe2O3 film, (b) a single BiOI film after 8 CBD cycles, (c) Fe2O3/BiOI nanocomposite after 12 CBD cycles. The scale bar of the insets is 500nm; (d) proposed growth routes of Fe2O3/BiOI nanocomposite step by step. 516x397mm (150 x 150 DPI)


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Figure 2. (a) X-ray diffraction patterns of Fe2O3, BiOI, and Fe2O3/BiOI nanocomposites based on FTO glass; (b) UV-visible light absorption spectra of Fe2O3, BiOI, and Fe2O3/BiOI nanocomposites prepared after 2, 4, 6, 8, 10, 12, 14 CBD cycles, the inset are the photos of samples. 496x193mm (150 x 150 DPI)


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Variation of capacitance (C) with the applied potential in 0.5 M Na2SO4 presented in the Mott Schottky relationship for pure BiOI after 8 CBD cycles (a) and Fe2O3 (b), the capacitance was determined by electrochemical impedance spectroscopy; Schematic energy-band diagram for isolated BiOI, Fe2O3 and Fe2O3/BiOI heterogeneous structure (c). 467x435mm (150 x 150 DPI)


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Figure 4. Current–voltage (I–V) characteristic curves (a) and photocurrent experiments with light on-off cycles (b) for Fe2O3, BiOI after 8 CBD cycles, and Fe2O3/BiOI nanocomposites prepared after 2, 4, 6, 8, 10, 12, 14 CBD cycles under AM 1.5 sunlight illumination with a power density of 100 mW/cm2; (c) The transients photovoltage of the Fe2O3/BiOI photoelectrode with 12 CBD cycles. The wavelength and intensity of the laser pulse are 532 nm and 50 µJ cm-2 ; (d) Schematic illustration of photogenerated charge transfer in the Fe2O3/BiOI-based solar cell. 477x369mm (150 x 150 DPI)


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