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Anatase TiO2 sheet-assisted synthesis of Ti3+ self-doped mixed phase TiO2 sheet with superior visible-light photocatalytic performance: Roles of anatase TiO2 sheet

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Abstract

On the basis of measurements, such as field emission scanning electron microscope, UV–Vis diffuse reflectance spectra, X-ray diffraction, electron paramagnetic resonance, photoluminescence spectra, and photocurrent measurements, the roles of anatase TiO2 sheet on synthesizing Ti3+ self-doped mixed phase TiO2 nanosheets (doped TiO2 (A/R, TiO2 (A))) and on improving the performance for photocatalytic CO2 reduction were explored systematically. High surface area anatase TiO2 nanosheets (TiO2 (A)) as a substrate, structure directing agent, and inhibitor, mediated the synthesis of Ti3+ self-doped mixed phase TiO2 nanosheets. Addition of TiO2 (A) significantly improved not only visible light absorption of doped TiO2 (A/R, TiO2 (A)), but also the efficiency of photo-excited charges separations due to the existence of interfacial regions of anatase-rutile TiO2 junctions. Finally, a possible mechanism for interfacial charge transfer at the anatase-rutile TiO2 interface and for photocatalytic CO2 reduction over Pt loaded doped TiO2 (A/R, TiO2 (A)) were proposed.

Graphical abstract

The roles of anatase TiO2 sheet on synthesizing Ti3+ self-doped mixed phase TiO2 nanosheets (doped TiO2 (A/R, TiO2 (A))) and on improving the performance for photocatalytic CO2 reduction were explored systematically.

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Introduction

Titanium dioxide (TiO2) has been widely investigated as a photocatalyst for various energy and environmental applications, such as producing clean fuel (e.g., hydrogen, methane) and eliminating environmental pollutants (e.g., organic dyes, nitrogen oxide, carbon monoxide), because of its excellent chemical/photochemical stability, good biocompatibility, non-toxicity and low cost [1], [2], [3], [4]. However, the photocatalytic performance of TiO2 is still rather low owing to its wide band gap (⩾3 eV) and the rapid recombination of photo-excited charge carriers, etc. Various strategies have been exploited to improve the light absorption of TiO2, such as doping, hydrogenation, sensitization, surface plasmon resonance (e.g., Ag, Au), and coupling TiO2 with a narrow band gap semiconductor (e.g., CdS, C3N4, AgX (X = Cl, Br, or I)) [5], [6], [7], [8], [9], [10], [11]. Among these strategies, element doping has been demonstrated as an effective tactic to extend the absorption edge of TiO2 to longer wavelengths. However, introducing foreign atoms usually results in newly generated recombination centers of photo-excited carriers [5]. Recently, self-structural modifications through rearranging atoms have been proven as an efficient strategy in not only extending the absorption spectra of TiO2, more importantly, but also simultaneously eliminating the side effect of introducing extra carriers recombination centers [12], [13], [14], [15].

Zuo et al. synthesized a highly stable Ti3+ self-doped rutile TiO2 visible-light-active photocatalyst by a facile hydrothermal method using titanium powder and hydrochloric acid as precursors [16]. However, the size of the obtained Ti3+-doped rutile TiO2 was larger, and the corresponding surface area was very low (only ca. 5 m2/g), which were unfavorable for surface photocatalytic reactions [17]. Loading active components onto high surface area supports has been demonstrated as an effective tactic to overcome the drawback of low specific surface area [18]. Additionally, compared with single-phase anatase or rutile TiO2, mixed phase TiO2 usually exhibits exceptional photocatalytic performance because suitable band alignments promotes the spatial separation of photo-excited charge carriers [19], [20], [21]. Generally, a large interfacial contact area and intimate contact between two parts or phases are essential for highly efficient hetero-geneous or hetero-phase junctions. Inspired by highly efficient electron transfer in chloroplasts, 2D-2D junctions would help to accomplish efficient separations of photo-excited carriers [22]. Noteworthily, constructing a 2D-2D junction structure may enhance evidently interfacial contact areas because of its unique face-to-face contact mode [23]. Since seeds could dramatically inhibit homogeneous nucleation and predictably steer the growth of the resultant crystals, the seed-mediated solution-phase growth method is reliable in synthesizing various types of nanostructures [24]. Besides, surface hydroxyl groups could act as heterogeneous nucleation sites for the hydrolysis of guest metal ions, which facilitates chemical bonding of the resulting metal oxides to the surface of host materials [25].

On the basis of the aforementioned analysis, we synthesized Ti3+ self-doped mixed phase TiO2 nanosheets by a two-step hydrothermal method, and investigated the influence of anatase TiO2 sheets substrate on the performance of visible light photocatalytic CO2 reduction [26]. In our previous paper, however, the critical roles of anatase TiO2 sheet and the transfer mechanism of photo-excited carriers (i.e., electron and hole) were not reported or elaborated in detail. Therefore, we elucidated these aspects more explicitly in the current paper. Additionally, the introduction of moderate metallic Pt on photocatalysts surface as a co-catalyst could not only boost the separation of photo-generated electron-hole pairs, but also enhance the selectivity for CH4 [27], [28], [29]. Consequently, photocatalytic CO2 reduction to CH4 under visible light irradiation (405 nm  λ  723 nm) was chosen as a probe reaction to evaluate Pt-loaded Ti3+ self-doped mixed phase TiO2 nanosheets photocatalyst. The roles of anatase TiO2 sheet, the effects of Pt on the performance of photocatalytic CO2 reduction, and the electron transfer mechanism during photocatalysis were investigated systematically.

Section snippets

Materials

Titanium powder (99.99 wt.%), titanium (IV) butoxide (⩾99 wt.%), chloroplatinic (IV) acid hexahydrate (Pt wt.%  37.5%), and concentrated hydrofluoric acid solution (⩾40 wt.%) were purchased from xxx and used without further purification. Be careful! Hydrofluoric acid (HF) is strongly corrosive and consequently should be handled very meticulously. The reactant gases (i.e., H2 and CO2) and argon (as a balance and carrier gas) were of ultra high purity (Airgas) and used without further purification.

Preparation of photocatalysts

Serving as A structure directing agent

Fig. 1 showed the FESEM images of TiO2 (A), P25 TiO2, doped TiO2 (R) and doped TiO2 (A/R). Obviously, (i) both TiO2 (A) (Fig. 1a) and doped TiO2 (A/R, TiO2 (A)) (Fig. 1c) were composed of nanosheets; (ii) compared to TiO2 (A), the thickness of doped TiO2 (A/R, TiO2 (A)) was larger, whereas its length hardly changed. As demonstrated previously, surface hydroxyl groups could act as heterogeneous nucleation sites for the hydrolysis of metal ions [25]; a flat wall or larger seeds may accelerate

Conclusions

On either synthesizing Ti3+ self-doped mixed phase TiO2 nanosheets (doped TiO2 (A/R, TiO2 (A))) or improving the performance for photocatalytic CO2 reduction over Pt loaded doped TiO2 (A/R, TiO2 (A)), TiO2 (A) played vital roles. In the former, TiO2 (A) sheets (i) as a structure directing agent, mediated the synthesis of doped TiO2 (A/R, TiO2 (A)) through boosting vigorously the heterogeneous nucleation under the assistance of surface hydroxyl groups and refraining the newly formed Ti3+

Acknowledgements

We acknowledge the financial support from University of South Carolina start-up funding. This work was also supported financially by the foundation of Henan Educational Committee (17A150036), the College Young Backbone Teacher Foundation of Henan Province (2014GGJS-177), and Jiyuan Vocational and Technical College.

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