Ti³⁺-TiO₂/Ce³⁺-CeO₂ Nanosheet heterojunctions as efficient visible-light-driven photocatalysts

Highlights

• A novel Ti³⁺-TiO₂/Ce³⁺-CeO₂ nanosheet heterojunction is fabricated.

• It owns the bandgap of 2.6 eV and extends the photoresponse to visible light region.

• It exhibits excellent visible-light-driven photocatalytic performance.

• It is due to synergy of Ti³⁺, Ce³⁺ co-doping and nanosheet heterojunction.

Abstract

Novel Ti³⁺-TiO₂/Ce³⁺-CeO₂ nanosheet heterojunctions are successfully prepared via a facile hydrothermal approach combined with a wet-chemical deposition precipitation and an in situ solid-state chemical reduction strategy. The structures of the as-prepared samples are characterized in detail by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy and UV–vis diffuse reflectance spectroscopy. The results show that Ti³⁺ and Ce³⁺ are both doped into the lattice of TiO₂ and CeO₂, respectively, which narrow the band gap to 2.7 eV and extend the photoresponse to visible light region. The photocatalytic degradation rate of methyl orange and methylene blue are as high as 93.3% and 97.1% under visible-light illumination, respectively. The high degradation rate is attributed to the efficient doping of Ti³⁺ and Ce³⁺, formation of surface oxygen vacancy and heterojunction, which favors the absorption of visible light and separation of photogenerated charge carriers.

Introduction

With the development of science and the improvement of human living standard, industrial waste and atmospheric waste have become series environmental problems which restrict further development of economy [[1], [2]]. In recent years, people are aware of the importance of sustainable development and seek a number of ways to degrade pollutants [[3], [4]]. Among the various approaches, heterogeneous photocatalysis is one of the main methods for the degradation of pollutants [[5], [6]]. TiO₂, with the excellent properties including low-cost, low toxicity, good chemical stability, energy-saving, environmentally friendly nature [[7], [8], [9]], has attracted infinite interest as a potential semiconductor photocatalyst and is widely researched for pollutants in both gas and liquid phases [[10], [11]]. Unfortunately, two major shortcomings restrict the photocatalytic activity of TiO₂ [[12], [13], [14]]. One is the wide band gap of anatase, which is ∼3.2 eV, resulting in barely absorption in visible light region, which restricts its visible-light-driven photocatalytic activity [[15], [16]]. The other one is the rapid recombination of photoinduced electron-hole pairs, which results in the low efficiency of photoinduced chemical reactions [17]. For the purpose of overcoming the unavoidable disadvantages of TiO₂ photocatalyst, a variety of methods are developed, including non-metal and metal elements doping, semiconductor heterojunction fabrication, surface sensitization, and so on [[13], [18]]. Among them, semiconductor coupling is an efficient method to solve the disadvantage of single component TiO₂ [19].

Apart from the most commonly used TiO₂ photocatalyst, cubic fluorite cerium dioxide (CeO₂) has band gap energy similar to that of TiO₂ [20]. Owing to the special redox property, high thermal stability, high oxygen storage capacity, and easy conversion between Ce(III) and Ce(IV) oxidation states [[21], [22]], CeO₂ is of great importance in many oxidation reactions, which can act as an oxygen buffer by releasing/recovering oxygen. So it is widely used in combination with TiO₂ to improve the light absorption property and photo-degradation performance [23]. Coupling TiO₂ and CeO₂ could reduce the recombination of photo-generated electron hole pairs. In addition, various morphologies of TiO₂ micro/nanostructures, such as nanotubes, nanoparticles, nanocubes, nanosheets, films, and so on [[24], [25], [26], [27], [28]], have been successfully synthesized. Among them, nanosheets have attracted lots of attention due to the unique 2D structure [29]. Loading CeO₂ on the surface of 2D TiO₂ nanosheet to fabricate heterojunction will be good candidate for efficiently accelerating the separation and transfer of photon-induced electron hole pairs as well as increasing the efficiency of photocatalysis process [30].

Recently, Mao et al. designed an effective approach for enhancing the visible light photocatalytic performance of TiO₂ through Ti³⁺ self-doping using the high pressure hydrogenation strategy [31]. The color of as-prepared hydrogenated TiO₂ changed from white to black, which resulted in the absorption of light not only confined to visible light but even extended to the infrared light region and then displayed higher photocatalytic performance than that of the pristine white TiO₂ [32]. From then on, different synthesis methods are devoted to prepare black TiO₂ materials, including plasma assisted hydrogenation, high pressure hydrogenation, high-temperature Al vapor reduction, and chemical reduction [[33], [34], [35], [36]]. In the above methods, chemical reduction is the simplest and low-cost. An effortless method to reduce TiO₂ using sodium borohydride (NaBH₄) was reported [[37], [38], [39]]. This extremely facile hydrogenation strategy is based on the discovered fact that after NaBH₄ treatment, durable defects (Ce³⁺, Ti³⁺, and oxygen vacancies) and surface disorder can generate on the surface of TiO₂ and CeO₂ [[40], [41], [42], [43]]. This is a double-doping process, which can result in the large enhanced solar energy utilization of TiO₂/CeO₂.

In this paper, we present a mild hydrothermal synthetic method, combined with wet-chemical deposition precipitation and an in situ solid-state chemical reduction approach for fabricating the Ti³⁺ and Ce³⁺ double-doping nanosheet heterojunctions. The photocatalytic mechanism of Ti³⁺-TiO₂/Ce³⁺-CeO₂ was described. And the direction of electron transfer and mechanism of photocatalysis were explained. The prepared Ti³⁺-TiO₂/Ce³⁺-CeO₂ heterojunction photocatalysts with narrow band gap of ∼2.70 eV extend the photoresponse to visible light region. The photocatalytic activity and electrochemical property of the prepared photocatalyst have been tested under visible-light irradiation by removing methyl orange (MO) and methylene blue (MB). In addition, the possible photocatalytic mechanism is also proposed.