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Abstract
The photogenerated charge separation mechanism in α-Fe2O3-ZnFe2O4 is investigated with surface photovoltaic response. The enhanced photovoltaic response of α-Fe2O3-ZnFe2O4 clearly presents a special position near 373 nm (3.32 eV) for molar ratio 1:1 and a larger wavelength range for the molar ratio 6:1. The special position near 373 nm (3.32 eV) is also observed in absorption spectra for a pristine α-Fe2O3 and α-Fe2O3-ZnFe2O4 composite with molar ratios of 2:1 and 1:1. The same position indicates that a possible higher level in the conduction band of α-Fe2O3 corresponding to the position near 3.32 eV could exist. The result suggests that, besides the reported photogenerated electrons transfer in α-Fe2O3-ZnFe2O4 composite from ZnFe2O4 with higher conduction band to α-Fe2O3 with lower conduction band, the first transition could occur from the higher level in the conduction band of α-Fe2O3 to ZnFe2O4 due to energy level matching.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
α-Fe2O3 is one of photoelectric materials with visible light sensitivity (band gap 2.2 eV [1–5]). However, the high recombination of photogenerated charges induces the low photoelectric response intensity [6]. To enhance the photoelectric response intensity, many means have been investigated [7–10]. For example, Zn2SnO4 was adopted to enhance the photogenerated charge separation of α-Fe2O3 [10]. The other composites such as ZnFe2O4-based composites were reported with enhanced photoelectric response [11–15]. The enhanced photoelectric responses are always ascribed to electric transfer from the material with higher conduction band bottom to the other with lower conduction band bottom. However, further explanations for the different enhancement positions of photoelectron response have not been found.
The photoexcitation electrons could not always transfer to the conduction band bottom, i.e. the photoexcitation electrons absorbed higher photon energy than band gap could transfer to the higher levels in the conduction band. For the two materials situation with higher and lower conduction bands, the photoexcitation electrons with higher energy than higher conduction band bottom, which conduction band bottom could be chosen to transfer? The reports of photoexcitation electric transfer to the material with lower conduction band bottom rather the other with higher conduction band bottom should be against the energy level matching principle [10–15]. It is reasonable, when obtaining higher energy than band gap, depending on the energy level matching principle, the photoexcitation electrons could transfer to the close conduction band bottom (higher energy). It is meaningful to investigate its possibility: the photoexcitation electrons transfer could direct to the higher conduction bottom rather than the lower conduction band bottom due to energy level matching.
In order to explore the possibility, different composite molar ratios of α-Fe2O3-ZnFe2O4 have been prepared with different photovoltage enhancement. The photovoltaic property of the composite α-Fe2O3-ZnFe2O4 is enhanced notably at a special position near 373 nm (especially for molar ratio 1:1) and within a larger wavelength scope (especially for the molar ratio 6:1). The absorption spectra show obviously increasing absorption band near 373 nm as long as the content of α-Fe2O3 is higher than that of ZnFe2O4. A possible higher energy level corresponding to the absorption wavelength near 373 nm could exist in the conduction band in α-Fe2O3. Therefore, the enhancement mechanism of photoelectric response of the composite α-Fe2O3-ZnFe2O4 could relate to the photogenerated electrons transfer from the possible higher level in the conduction band of α-Fe2O3 to ZnFe2O4.
2. Experimental
Analytical reagents of Fe(NO3)3·9H2O and Zn(NO3)2·6H2O were used as raw materials. Deionized water about 40 mL dissolves the raw materials in a magnetically stirred flask. The molar ratios of Fe(NO3)3·9H2O and Zn(NO3)2·6H2O were set corresponding to that of α-Fe2O3:ZnFe2O4 with 1:0, 6:1, 4:1, 2:1, 1:1, 1:2, 1:4 and 0:1. The obtained transparent aqueous solutions were dried in an air dry oven at about 120 °C for overnight. The dried precursors were sintered at 500 °C for 5 h in a muffle furnace to obtain the α-Fe2O3-ZnFe2O4 composites. For the molar ratio 0:1, the obtained sample with a few α-Fe2O3 and ZnO was purified with excessive 30% HCl solution and then rinsed with deionized water and heated at 120 °C for overnight.
X-ray diffraction (XRD) patterns were recorded on a DX-2700 diffractometer with a step 0.04° to analyze the crystal structure. Raman spectra were obtained on a red laser (633 nm) Raman spectrometer (Renishaw RM1000). The surface photovoltage spectroscopy (SPS) measurements were carried out with a home-built apparatus including a 500W xenon lamp (CHF XM500W, Beijing Trusttech Co. Ltd), a double-grating monochromator (Zolix SP500), a lock-in amplifier (SR830-DSP) with an optical chopper (SR540) and a sample cell. The diffuse reflectance spectra were collected on a UV–vis spectrophotometer (Varian Cary 5000) with BaSO4 as the reference and were converted to the absorbance data through the Kubelka–Munk method. The microstructures of the samples were investigated using field-emission scanning electron microscopy (FESEM).
3. Results and discussion
3.1 Crystal structure
Figure 1 shows the XRD patterns of α-Fe2O3-ZnFe2O4 with molar ratios of 4:1, 2:1, 1:1, 1:2 and 1:4. It is found that, the diffraction peaks of the composite are in good agreement with the standard diffraction patterns of ZnFe2O4 (JCPDS 22-1012) and α-Fe2O3 (JCPDS 33-0664) [13,16–19]. The peaks located at 18.20°, 29.93°, 35.30°, 36.82°, 42.90°, 46.95°, 53.17°, 56.69, 62.25°, and 73.50° can be readily ascribed to the characteristic peaks of the cubic phase of ZnFe2O4. The diffraction peaks of the sample located at 24.16°, 33.18°, 35.26°, 42.90°, 49.50°, 53.20°, 62.28° and 64.08° correspond to α-Fe2O3 (JCPDS no.33-0664). No diffraction peak from impurities suggests the high phase purity of the composite. The relative intensities of ZnFe2O4 peaks decrease with the molar ratio of Fe2O3:ZnFe2O4.
Raman spectra of α-Fe2O3-ZnFe2O4 composites in the region 100–1200 cm−1 at room temperature are carried out to further characterize the variation of the structure with molar ratios of 1:0, 6:1, 4:1, 2:1, 1:1, 1:4 and 0:1 (see Fig. 2).
Fig. 2. Raman spectra of α-Fe2O3-ZnFe2O4 composites with molar ratios of 1:0, 6:1, 4:1, 2:1, 1:1, 1:4 and 0:1.
The pristine α-Fe2O3 has trigonal structure that belongs to the space group with $D_{3d}^6$ (D3d space group). The bands at about 240, 287, 402, 604, 652 cm−1 are assigned to Eg vibrations of α-Fe2O3. The band at about 220 cm−1 belongs to A1g vibrations of α-Fe2O3 [20,21]. Spinel ZnFe2O4 has cubic structure that belongs to the space group with $O_h^7$ (Fd3 m space group). The observed bands at about 343 and 470 cm−1 are attributed to transverse F2g modes, 245 cm−1 to Eg vibration, and 638 cm−1 to longitudinal A1g of ZnFe2O4 [17,22,23].
With decreasing the molar ratio of α-Fe2O3:ZnFe2O4, the relative intensities of Raman bands at about 220 and 287 cm−1 corresponding to α-Fe2O3 decrease gradually. For the molar ratio of 1:1, the Raman bands corresponding to that of α-Fe2O3 are more intense than that of ZnFe2O4. Only after the molar ratio of Fe2O3:ZnFe2O4 decreases to 1:4, the intensities of Raman bands corresponding to α-Fe2O3 decrease notably and more bands are corresponding to ZnFe2O4. It suggests that the Raman activity of α-Fe2O3 is greater than that of ZnFe2O4.
3.2 Enhanced surface photovoltaic response and explanation
Figure 3(a) shows the SPS spectra of α-Fe2O3-ZnFe2O4 composites with molar ratios of 1:0, 6:1, 4:1, 2:1, 1:1, 1:2, 1:4 and 0:1. Both α-Fe2O3 and ZnFe2O4 present weak photoelectric response and their primary photoelectric responses concentrate in the wavelength range at 350-475 nm (see the inset in Fig. 3(a)). The result suggests that their photogenerated charge separation presents low efficient within 300-600 nm and relatively higher photogenerated charge separation is in the range 350-470 nm. However, even for the molar ratio at 6:1 with much low content of ZnFe2O4, the surface photovoltage of α-Fe2O3 is enhanced notably within larger wavelength range. For the molar ratio at 4:1, even though the enhancement of photovoltage becomes weak but the relative intensity of photovoltage corresponding to 373 nm (3.32 eV) increases, especially, for the molar ratios at 2:1 and 1:1, the photovoltage (vs 3.32 eV) is equal (2:1) and much larger (1:1) than that of the sample with ratio at 6:1. For more content of ZnFe2O4 with molar ratios at 1:2 and 1:4, the photovoltage (vs 3.32 eV) remains the same intensity as that of the molar ratio at 4:1, even though there is obvious decrease for longer wavelength light. The photovoltage enhancement presents not stable trend but only at special molar ratios of 6:1 and 1:1, which could suggest deep mechanism to explore.
Fig. 3. (a) SPS spectra of α-Fe2O3-ZnFe2O4 composites; (b)-(c) Photovoltages of α-Fe2O3-ZnFe2O4 composites with molar ratios of 6:1, 4:1, 2:1, 1:1 at 400 nm, 373 nm.
In order to analyze the deep mechanism of photovoltage enhancement of α-Fe2O3-ZnFe2O4 composites, the absorption spectra of them are performed (Fig. 4(a)). It is found that near 373 nm, except for the samples with molar ratios at 1:2, 1:4 and 0:1 with slowly decreasing absorption, the other samples have obviously increasing absorption, especially for the sample with molar ratio at 1:1. The result shows that obviously increasing absorption could present near 373 nm as long as the content of α-Fe2O3 is higher than that of ZnFe2O4. It suggests that a possible higher energy level corresponding to the absorption wavelength near 373 nm could exist in the conduction band in α-Fe2O3. Through the absorption spectra of α-Fe2O3-ZnFe2O4 composites, the band gaps are estimated to be 2.13, 2.11, 2.14, 2.10, 2.14, 2.26, 2.23, 2.36 eV for molar ratios α-Fe2O3:ZnFe2O4 at 1:0, 6:1, 4:1, 2:1, 1:1, 1:2, 1:4, 0:1, respectively. The inconsistent increase of band gaps with the molar ratio shows indirect relationship between absorption or photovoltage and band gap.
Fig. 4. (a) Absorption spectra of α-Fe2O3-ZnFe2O4 composites; (b)-(i) Band gap estimation of α-Fe2O3-ZnFe2O4 composites and observation of absorption increasing near 3.32 eV depending on the absorption spectra (wavelength nm transfer to energy eV).
Depending on the results of surface photoelectric response of α-Fe2O3-ZnFe2O4 composites and the analysis of band structures of α-Fe2O3/ZnFe2O4 composite [24], it is reasonable that there is possible higher level in the conduction band of α-Fe2O3 (Fig. 5). The possible level near 3.32 eV is responsible for the stronger photoelectric response and stronger absorption band near wavelength 373 nm. It is considered that, the electrons in the valence band of α-Fe2O3 absorb enough photons and transfer to the higher level and then to conduction band of ZnFe2O4 enhancing photogenerated charges separation due to energy level matching. The next transition of photogenerated charges in the conduction band of ZnFe2O4 could from ZnFe2O4 to α-Fe2O3. The photogenerated charges transitions from the higher level in the conduction band of α-Fe2O3 to conduction band bottom of ZnFe2O4 and then to the conduction band bottom of α-Fe2O3 could increase the life of photogenerated charges. The explanation enriches the analysis of J. P. Dhal [25] and X. Y. Liu [26]: the photoexcitation electrons could transfer from the semiconductor with higher conduction band bottom to the other with lower conduction band bottom.
Fig. 5. Proposed mechanism of photogenerated electron transfer from a possible higher level in α-Fe2O3 to conduction band bottom of ZnFe2O4 and then to conduction band bottom of α-Fe2O3 in α-Fe2O3/ZnFe2O4 composite.
In order to investigate the further mechanism for the enhancement of photovoltage within larger wavelength range for the sample with molar ratio of 6:1, the microstructure analysis of the composites is performed (see Fig. 6). α-Fe2O3 shows obvious particle aggregation. For the sample with molar ratio 6:1, the particles present less aggregation and obvious holes like honeycombs. With increasing the content of ZnFe2O4 from the molar ratio 6:1 to 1:1, the particle aggregation reduces obviously. Especially for ZnFe2O4, the average particle size decrease and much less aggregation is observed. The microstructure change with molar ratio of the α-Fe2O3-ZnFe2O4 composites could suggest that heavy aggregation (α-Fe2O3) and complete dispersion (near ZnFe2O4) for the particles are not helpful for the photogenerated charges separation. The sample with molar ratio 6:1, less aggregation and obvious holes could be helpful for the photogenerated charges separation within larger wavelength scope. The sample with molar ratio 6:1, less aggregation and obvious holes supply the channels for photogenerated electrons to enhance the photogenerated charges separation. However, the aggregation grains obstruct the photogenerated electronic movement, and the photogenerated electrons move in decentralized directions in the complete dispersion grains with too lager space.
4. Conclusions
α-Fe2O3-ZnFe2O4 composite with different molar ratios have been synthesized and identified by XRD and Raman. For the molar ratio at 6:1 with much low content of ZnFe2O4, the surface photovoltage of α-Fe2O3 is enhanced notably within larger wavelength range. For the molar ratios at 2:1 and 1:1, the photovoltage (vs position 3.32 eV) is equal (2:1) and much larger (1:1) than that of the sample with ratio at 6:1. An obviously increasing absorption could present near 373 nm as long as the content of α-Fe2O3 is higher than that of ZnFe2O4. A possible higher energy level corresponding to the absorption wavelength near 373 nm could exist in the conduction band in α-Fe2O3. The results suggest that the electrons in the valence band of α-Fe2O3 absorb enough energy and transfer to higher levels of α-Fe2O3 and then to conduction band bottom of ZnFe2O4 contributing to the photogenerated charge separation. On the other hand, the multi-channel microstructures for the sample with molar ratio 6:1 are helpful for the photogenerated charge transfer. The investigation enriches the explanation that photoexcitation electrons transfer from the semiconductor with higher conduction band bottom to the other with lower conduction band bottom. The longer time in the conduction band for photogenerated charges is critical for enhancing the photogenerated charge separation.
Funding
National Natural Science Foundation of China (NSFC) (11404093, 61350012); Key Scientific Research Project of Colleges and Universities in Henan Province (18A140014); Science and Technology Department, Henan Province (182102210241); Industrial Science and Technology Research Projects of Kaifeng, Henan Province (1501049).
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Nos. 11404093, 61350012), the Key Scientific Research Projects of Henan Province, China (No. 18A140014), Henan Science and Technology Development Project (182102210241), and Industrial Science and Technology Research Projects of Kaifeng, Henan Province, China (No. 1501049).
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