Novel Fe2O3/{101}TiO2 nanosheet array films with stable hydrophobicity and enhanced photoelectrochemical performance

https://doi.org/10.1016/j.matchemphys.2021.125226Get rights and content

Highlights

  • Thorn-like Fe2O3 were deposited on the {101} surface of anatase nanosheet array film.

  • α-Fe2O3/{101} facets have larger band offset value.

  • Fe2O3/TiO2 films demonstrate excellent hydrophobicity without further modification by low energy chemicals.

Abstract

Single crystal TiO2 nanosheet (TNS) arrays with coexposed {101} and {001} facets were synthesized on FTO conductive glass directly, thorn-like Fe2O3 were deposited on the {101} surface of TNS by a chemical bath deposition method. Taking advantage of different energy band positions of {101} and {001} facets, the higher band offset value between Fe2O3 and {101} facets was achieved, which would accelerate the photoexcited electrons and holes separation between Fe2O3 and {101} facets of TNS and enhance photoelectrochemical properties. Furthermore, without modification by low energy chemicals, such novel Fe2O3/TNS heterostructure electrodes exhibited stability hydrophobicity. After UV irradiation for 240 min, the contact angle of the Fe2O3/TNS (80min) films retained hydrophobicity (134.8°), which is approximately 7.1 times higher than that of bare TNS films. The optimal composite photoelectrode exhibited the highest transient photocurrent intensity (48 μA), which is about 4.4 times superior over the bare TNS. The superior photoelectrochemical properties and stability hydrophobicity were attributed to the suitable band edge positions and hydrophobic thorn-like α-Fe2O3.

Introduction

Recently, solar driven water splitting through photocatalysis has attracted wide attention because it could directly convert the solar energy to valuable chemical energy [[1], [2], [3], [4]]. For the purpose of improving the conversion efficiency of solar energy, many materials (such as Fe2O3 [5], TiO2 [6], NaNbO3 [7], and MoS2 [8]) have been demonstrated to be suitable for PEC water splitting. Design suitable semiconductor electrode with high efficiency light absorption, efficient charge separation has been the core of the research. Due to its physicochemical inertness, nontoxicity, abundant reserves and low cost, anatase titanium dioxide (TiO2) has been widely used in smart surface coatings, photonic crystals, and photovoltaic cells [[9], [10], [11], [12]]. It is known that the properties of nanomaterials are not only related to the crystal phase, morphology, crystal defect but also exposed crystal facets. The atomic configuration and electronic structure variations between different exposed facets, which result in differences in magnetic, dielectric, Optical, and chemical properties [13,14]. Based on theoretical calculations and experimental studies, compared to the {101} facet, {001} facet of anatase TiO2 has higher reactivity [[15], [16], [17], [18]]. Li et al. synthesized ultra-thin anatase nanosheets, showing that the hydrogen evolution rates increases from 4335 to 7381 μmol h−1 g−1 with the increase {001} facets percentage area from 69% to 82% [19]. Sun et al. prepared TiO2 films with dominant {001} facets and observed the TiO2 film with 84% of exposed {001} facets shows the highest photocurrent [20]. Pan et al. reported facet dependent photocatalytic activities show complex trends, crystals with 24% {001} facet have higher activities than that with 40% {001} facet areas, and crystals with 53% {010}, 14% {001} and 33% {101} facet areas exhibit the highest activities [17]. Ohno [21] and Tachikawa [22] demonstrated that {001} & {101} facets are oxidative & reductive sites by photochemical deposition technology and single-molecule imaging, respectively. On account of the theoretical calculations, Yu and co-workers proposed the “surface heterojunction” concept and showed that the {001} and {101} facets of anatase exhibit different band edge positions [23].

Nonetheless, pure TiO2 nanomaterial is restricted by its broad band gap and rapid h+/e recombination rate. Various TiO2-based heterojunctions have been explored, and they exhibited superior separation efficiency of electron-hole pairs [[24], [25], [26], [27]]. Inspired by the different oxidative and reductive sites of the {001} and {101} facets and their different energy band positions of anatase, the deposition of noble metal and semiconductor nanostructures on designated crystal facets was reported. In previous studies, PbO2 and Pt were deposited on specific crystal facets of rutile and anatase particles by Ohno's group, the results revealing the spatial separation of photoinduced h+/e pair [21,[28], [29], [30], [31]]. More recently, Zhang et al. reported high-energy {110} and {001} facets were easier to the stabilization of Au atoms, which would enhance the photocatalytic activity [32]. Toupance et al. prepared NiO−TiO2(101) nanocrystals by means of photodeposition technique, and 0.1 wt% NiO−TiO2(101) exhibited highest activities [33]. Due to the chemical stability, appropriate band gap (≈2.2 eV), earth abundance, Fe2O3 is one of the most promising photoanode materials, however, its application is hindered by the low carrier mobility and short photogenerated charge carriers lifetime [[34], [35], [36]]. Hence, coupling of Fe2O3 with a suitable energy-band semiconductor is frequently employed [[37], [38], [39]]. Recently, Fe2O3/TiO2 heterostructures have been fabricated, Deng et al. reported the preparation of a Ti3C2 MXene derived TiO2 nanosheets/hematite photoanode, which exhibited an increased photocurrent density of 1.72 mA/cm2 at 1.23 V vs. RHE [40]. Singh et al. investigated Fe2O3–TiO2 heterostructure via first-principles calculations, demonstrated that the Fe2O3–H:TiO2 heterostructure shows a type II band alignment [41]. Li et al. designed a TiO2/α-Fe2O3/Cu:NiOx/Co-Pi photoanode, which exhibited higher photocurrent and durability for solar water splitting [42].

Herein, single crystal TiO2 nanosheet (TNS) arrays were synthesized on the SnO2:F (FTO) conductive glass directly, which facilitates photoinduced electron transfer between TiO2 nanosheet and FTO substrate interface. The TNS arrays are almost vertical on the FTO substrate, rectangular sidewalls are the {001} facets and the upper surface of arrays film are {101} facets of anatase crystal [43], which resulted in the inclined deposition of Fe2O3 nanocrystals on the {101} facets.

On the other hand, for outside optoelectronic devices, such as solar cells, the long-term durability of superhydrophobicity to rainwater, heat, as well as UV radiation is a key factor. TiO2 films have been discovered to exhibit photoinduced hydrophilicity when subjected to UV irradiation [[44], [45], [46]]. From a practical point of view, it is desirable to construct film surfaces possessing persistent superhydrophilicity. Recently, researchers have designed and fabricated specific superhydrophobic TiO2 surfaces through creating hierarchical micro-nanostructures modifying with low energy organic chemicals [[47], [48], [49], [50]]. However, modified organic materials are environmentally unfriendly and poor durable, especially when they are exposed to UV irradiation or high temperature.

In this work, TiO2 films with coexposed {101} & {001} facets were fabricated, then, the thorn-like Fe2O3 were deposited on the {101} surface of TNS by chemical bath deposition (CBD) method. The higher band offset value between Fe2O3 and {101} facets is expected to be valuable for the transportation of electrons. Interestingly, such Fe2O3/TiO2 composite films demonstrate excellent hydrophobicity without further modification by low energy chemicals. Furthermore, this hydrophobicity was stable, even when they exposed to UV light for 240 min. These might lead to its application in outdoor photovoltaic devices and self-cleaning coatings.

Section snippets

Synthesis of TiO2 nanosheet array films

In the present work, TiO2 nanosheet array films were synthesized via hydrothermal process. Briefly, 13 mL hydrochloric acid (36.5–38 wt%), 0.5 mL of tetrabutyl titanate, and 0.25 g ammonium fluotitanate were dissolved in 17 mL of deionized water, under magnetic stirring for 30 min. Then, the obtained transparent mixture was poured into a 50 mL Teflon-lined stainless-steel autoclave with a piece of FTO glass. After hydrothermal treatment at 170 °C for 16 h, the samples were rinsed with deionized

X-ray diffraction analysis

Fig. 1 shows the XRD patterns of TNS and Fe2O3/TNS composite films. The diffraction peaks at 2θ = 25.25°, 37.67°, 47.98°,53.83°, 54.92° and 62.68° can be labeled as the (101), (004), (200), (105), (211), (204) plane of pure anatase phase (JCPDS No. 21-1272). Notably, compared with TiO2 deposited by other method, the intensity of the (004) peak is increased, which is due to the large area exposed of {001} facets. After decorated with Fe2O3, two weak diffraction peaks appeared at 2θ = 33.13°,

Conclusion

In this paper, TiO2 films with coexposed {001} and {101} facets were synthesized on FTO substrates, then, the thorn-like Fe2O3 were deposited on the {101} surface of TNS by CBD method. Such novel Fe2O3/TNS heterostructural electrodes exhibit enhanced photoelectrochemical properties and stability hydrophobicity under UV light irradiation for 240 min. The enhanced photoelectrochemical performance of the Fe2O3/TNS can be ascribed to the thorn/flake-like electrode structure combine with high

CRediT authorship contribution statement

Lei Yang: Conceptualization, Methodology, Writing – review & editing. Chengjie Yao: Software. Ruyi Wang: Software. Liang Jiang: Software. Wenjing Zhu: Software. Mengyao Wang: Software. Lingli Liu: Writing – review & editing. Dewei Liang: Writing – review & editing. Lei Hu: Writing – review & editing. Chonghai Den: Writing – review & editing. Qiyi Yin: Writing – review & editing. Miao Zhang: Writing – review & editing. Gang He: Writing – review & editing. Jianguo Lv: Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (Nos. 61804039 and 11774001), and the Natural Science Foundation of Anhui Province (1808085QE126), the Program of Anhui Province for Outstanding Talents in University (gxyq2019115, gxbjZD31), Hefei University Talent Research Fund Project (20RC35), and the University Synergy Innovation Program of Anhui Province (GXXT-2021-013).

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