Visible light responsive La and Fe co-doped NaTaO3 photocatalysts: Local structure around dopants
Introduction
Hydrogen fuel produced via photocatalytic water splitting has great potential as a replacement for fossil fuel to fulfill our steadily increasing energy demand [1], [2], [3], [4]. Water is a naturally abundant reactant [5], and solar energy will never be exhausted [6]. Being equally important, photocatalytic water splitting produces no harmful byproducts [2]. The separation of H2 and O2 mixtures is also doable with the development of advanced membrane technology [7].
After more than three decades of exploring water splitting reactions, over one hundred photocatalyst materials have been reported to split water into H2 and O2 [4]. Among the most promising is NaTaO3 [8], [9], [10], [11], [12], [13], along with SrTiO3 [14], KTaO3 [15] and Ga2O3 [16]. In particular, when NaTaO3 is doped with La cations, its optimized apparent quantum yield reaches 56% under UV light [8]. As a wide band gap (4 eV) material, however, NaTaO3 is inactive under visible light. This bottleneck limits its application in a real setting upon exposure to sunlight. As is known, visible light occupies up to 45% of the sunlight that reaches the earth's surface, and therefore, the visible light absorption capability of the material is determinant of the energy conversion efficiency [17]. To sensitize La-doped NaTaO3 to visible light, transition metals, including Cr [18], [19], [20], [21], Mn [22], Fe [23] and Co [24], have been introduced as co-dopants. A co-dopant is intended to narrow the band gap, while La cation balances the cationic charge. Nevertheless, water splitting under visible light with those dopant pair systems proceeded at a relatively low rate. The highest H2 evolution rates over NaTaO3 doped with La and Cr (3 + 3 mol%) [18], La and Fe (2 + 2 mol%) [23] and La and Co (10 + 10 mol%) [24] in the presence of a Pt cocatalyst and sacrificial reagents were 4.0, 0.8, and 4.2 µmol h−1, respectively. Given the low H2 evolution rates, a deeper understanding of structure is obviously required to rationally design future visible light responsive materials based on such a promising scheme of charge-compensated dopants.
The purpose of this study is to reveal the local structures of La and Fe cations doped in NaTaO3 photocatalyst using X-ray absorption spectroscopy. No attention has been paid thus far to addressing such an issue. NaTaO3 doubly doped with La and Fe cations was able to split water under visible light with methanol as the sacrificial reagent [23]. The La and Fe guests were found to control the water splitting activity of the NaTaO3 host. Here, upon the double doping of NaTaO3, we hypothesize that La3+ substitutes for Na+, while Fe3+ substitutes for Ta5+, forming a LaFeO3-NaTaO3 solid solution. We expect the prepared LaFeO3-NaTaO3 solid solution here to be a good platform for visible light photocatalysis. Indeed, solid solution composed of two isostructural semiconductors has been an attractive approach to realize single photoexcitation-based water splitting under visible light. The band gap and band edge level can be conveniently tuned by varying the fraction of the semiconductors constructing the solid solution, as was in the case of the ZnS-CuInS2 solid solution [25], for instance.
Section snippets
Experimental
We prepared the doubly doped NaTaO3 (hereinafter referred to as NTO) with the solid-state method. First, appropriate amounts of reactants: Na2CO3 (99.8%, Kanto), Ta2O5 (99.99%, Rare Metallic), La2O3 (99.99%, Wako), and Fe2O3 (99.9%, Wako) were mixed and precalcined in an air atmosphere at 1173 K for 1 h, and subsequently calcined at 1423 K for 10 h. An excess of Na2CO3 that was 5 mol% out of stoichiometry, was added to compensate for sodium volatilization during calcination. Here, the La/Ta and
Visible light sensitization of NTO
For sensitizing NTO to visible light, we employed La and Fe cations as dopants. The particle shape became more cubic and the particle size decreased after double doping, as shown in Fig. S1 (Supplementary Materials). Either the shape or the size of the particles was seen to be identical when the dopant concentration was increased ten times. The particle shape and size seemed to be insensitive to the dopant concentration.
Let us inspect the light absorption capability of NTO upon the double
Conclusion
We revealed the local structures of dopants accommodated in a visible light responsive photocatalyst, La and Fe co-doped NaTaO3. Upon double doping, La cations occupied the Na-sites, and Fe cations occupied the Ta-sites, forming a LaFeO3-NaTaO3 solid solution, as confirmed by EXAFS. The local environment around La and Fe cations in the doubly doped samples was not sensitive to the dopant concentration and was identical to LaFeO3. We thus suggest that a LaFeO3-NaTaO3 solid solution formed in all
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.
Acknowledgments
We thank JSPS KAKENHI (Grant: JP18F18029, JP16H02250, JP18KK0161 and JP19H00915) for financially supporting this research. X-ray absorption measurement was performed under the approval of the Photon Factory Advisory Committee (Proposal: 2016G057 and 2018G078).
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