Influence of structural features on the photocatalytic activity of NaTaO3 powders from different synthesis methods

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Abstract

Perovskite-like NaTaO3 photocatalyst powders have generally been synthesized with a solid-state method, which formed the orthorhombic phase that has a direct band gap and a Ta–O–Ta bond angle of ca. 163°. The present work reports a sol–gel synthesis, in which NaTaO3 nanoparticles were obtained at a temperature as low as 500 °C by using sodium acetate and tantalum chloride as the starting materials and citric acid as the complexing agent. Because of the low-temperature condition used in the synthesis, the sol–gel NaTaO3 was of the monoclinic phase that has an indirect band gap, high densities of states near the band edges, and a Ta–O–Ta bond angle close to 180°. Thanks to the surface area as well as the electronic and crystalline structures, the sol–gel NaTaO3 was considered more favorable to photocatalytic reactions than the solid-state material. This interpretation was supported by the finding that the sol–gel NaTaO3 exhibited a remarkably higher photocatalytic activity in water splitting than the solid-state material.

Graphical abstract

The sol–gel synthesized NaTaO3 showed an indirect bandgap, i.e. phonons involved in transition, and a higher water-splitting rate under illumination than the solid state one.

Introduction

Powders with semiconductor characteristics have been widely employed in photocatalytic or photoelectrochemical systems because they are capable of generating charge carriers by absorbing photon energies. The separation effectiveness of the photo-induced charge carriers is an important factor in determining the photocatalytic activity of the powders. Among the photo-catalyzed reactions, photocatalytic water splitting for H2 evolution has attracted much research attention in recent years because H2 is considered a prospective fuel and the process presents a clean and renewable source for hydrogen energy [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53]. Oxide powders containing tantalum have shown promising stability and catalytic activity for water splitting into H2 and O2 under irradiation [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]. Among these catalysts, perovskite-like NaTaO3 loaded with NiO cocatalyst exhibited high activity in water splitting under UV-light irradiation [5], [6], [7], [8], [9], [10], [11], [12], [13]. The NiO cocatalyst promotes transfer of photo-induced electrons in the conduction band of NaTaO3 and enhances charge separation [8]. This results in the high activity of NiO/NaTaO3. Apart from cocatalyst loading, changing the crystalline structure of the principal catalyst may eventually affect the effectiveness of the charge separation as well. The present work intends to demonstrate the influence of structural features on the photocatalytic activity for NaTaO3 powders derived from different synthesis methods.

The high photocatalytic activity of tantalates was attributed to factors such as a suitable conduction band level consisting of Ta5d and the efficient carrier-delocalization caused by the proper distortion of TaO6 connections [8], [15], [20]. Both factors are associated with the crystalline structure, which can be synthesis route-dependent. The solid-state synthesis method was widely used to prepare the vast majority of the tantalates acting as photocatalysts [5], [6], [7], [8], [9], [10], [11], [12], [13]. A few studies developed other synthetic routes for the tantalates [13], [16]. The solid-state method requires high-temperature and long-duration calcination, leading to grain growth (thus small surface area) and loss of stoichiometry of the products [16]. The sol–gel method has been used in syntheses of nanosized materials with considerably larger surface area [54], [55]. In addition, the low-temperature feature in sol–gel synthesis may affect the final crystalline structure of NaTaO3.

Both the solid-state and sol–gel methods were executed in the present work, in an attempt to synthesize NaTaO3 with a crystalline structure that tends to facilitate separation of the photo-induced charges without the assistance of a cocatalyst. The differences in the structure or property resulting from the change in the synthesis method were extensively explored. We demonstrated that the sol–gel route synthesized a NaTaO3 powder with structural features, including surface morphology and crystalline and electronic structures, that are favorable to photocatalytic reactions.

Section snippets

Experimental

In the sol–gel synthesis of NaTaO3, reagent grade chemicals: sodium acetate (CH3COONa; Nihon Shiyaku, Japan), tantalum chloride (TaCl5; Aldrich, USA), and hydrated citric acid (C6H8O7·H2O; Riedel-de Haën, Germany) were used as the starting materials. A schematic summarizing the procedure for the sol–gel synthesis is shown in Fig. 1. As indicated, the solutions of sodium acetate, tantalum chloride and citric acid (as the complexing agent) were mixed to prepare a sol solution with a Na/Ta/citric

Results and discussion

The SEM images of the NaTaO3 powders are shown in Fig. 2, in which the solid-state and sol–gel synthesized specimens are designated as SS and SG, respectively. The particles of SS were cube-like with a size of 2–3 μm, as shown in Fig. 2a. There were ordered nanosteps observed on the particle surface. The inset of Fig. 2a shows a width of ca. 10–20 nm for the nanosteps. This ordered nanostructure was reported to constitute the external surface of NaTaO3 powders doped with lanthanum [7], [8], [10].

Conclusion

The present work compared the structures of NaTaO3 powders synthesized from a developed sol–gel method and the conventional solid-state method. The sol–gel method produced NaTaO3 nanoparticles with high crystallinity at a temperature as low as 500 °C, while the solid-state method required a temperature of 1200 °C. Because of the temperature difference, the NaTaO3 powders synthesized from the sol–gel and solid-state methods were of the monoclinic and orthorhombic phases, respectively. The

Acknowledgements

This research was supported by the National Science Council of Taiwan (NSC 95-2120-M-006-001 and NSC 95-2221-E-006-408-MY3).

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