Ethanol assisted synthesis of anatase nanobelts with improved crystallinity and photocatalytic activity

Highlights

• Ethanol assisted synthesis of TiO₂ nanobelts.

• Improved anatase crystallinity.

• Higher photocatalytic activity.

Abstract

A modified alkaline hydrothermal method via adding the ethanol treatment to the intermediates was developed to synthesize TiO₂ nanobelts, in which the main phase is anatase. Compared with the previous reported TiO₂ nanobelts obtained without the ethanol treatment, the new TiO₂ nanobelts obtained through the ethanol assisted route are with much improved anatase crystallinity and a sharply reduced amount of TiO₂-B phase, as well as a significantly higher photocatalytic activity that is even better than P25 for degrading Rhodamine-B under the ultraviolet light irradiation, which apparently correlates to the increased contents and crystallinity of anatase. The mechanism of ethanol treatment is also discussed based on the FTIR results.

Introduction

Deterioration of environmental condition and shortage of natural resources have been attracting considerable public attention during recent years. Titanium dioxide (TiO₂), as a kind of effortlessly available and stable nontoxic semiconductor, has been widely investigated in photocatalysis [1], [2], [3], solar cell [4], and Li-ion battery applications [5], [6], with a goal to struggle with these intractable problems. Since the pioneer work that Fujishima and Honda found the phenomenon of water splitting by TiO₂ electrodes [7], physicochemical performances of TiO₂ have been found to be highly related to the geometrical morphology, size, degree of crystallinity and the surface structure, and reduction in size to the nanoscale has been the trend of investigation in recent years [8]. However, for the widely studied nanoparticles, benefits from the quantum size effect and the increased reactive sites could be canceled by severe agglomeration and high recombination rates of the photoinduced carriers. Moreover, small sizes of nanoparticles also mean an increase difficulty to recycling them from commonly used aqueous solutions [9], poorer repeatability and then increased costs.

One-dimensional (1D) TiO₂ nanostructures, especially the nanobelts, have been extensively studied due to their relatively stable morphology at high calcination temperature, unique surface atomic arrangement and oriented migration path for photogenerated charge carriers [9], [10], [11]. Among various demonstrated approaches to synthesize TiO₂ nanobelts, alkaline hydrothermal method possesses relatively concise preparation steps, low cost and good repeatability [12], [13]. The general process in this method contains the phase transformation among Na₂Ti₃O₇, H₂Ti₃O₇ and TiO₂. Moreover, phase of the resulted TiO₂ nanobelts may include TiO₂-B, anatase and rutile which depend on the subsequent calcination temperature [9], [14]. TiO₂-B is an exotic monoclinic polymorph with mesoporous structures which is thought to be an intermediate phase between H₂Ti₃O₇ and anatase during the calcinating process [9], [15], [16]. In general, among all kinds of titania polymorphs, anatase exhibits a better activity than rutile and TiO₂-B [9], [17]. However, low calcination temperature may result in a low degree of crystallinity and then a high percentage of TiO₂-B, which will seriously slash the activity of TiO₂ nanobelts. On the other hand, high calcination temperature will induce distortion and fusion of the nanobelts, leading to a low specific area which then decreases the surface active sites [9]. Majority of previous attentions were focused on fabricating bicrystal TiO₂ nanobelts consisting both TiO₂-B and anatase parts [18]. However, photocatalytic activity of the bicrystal TiO₂ nanobelts was still inferior to the commercial TiO₂ nanoparticles (P25) [19], [20].

With the goal of finding new ways able to enhance the nanobelts’ photocatalytic activity, in this work we developed an ethanol-assisted alkaline hydrothermal approach. In contrast to the acid assisted approach in some previous researches [18], [19], [20], this new process is more convenient and facile, and the resulting products own increased contents and crystallinity of anatase if using the same calcination temperature. The resulting nanobelts show much improved photocatalytic activity that is better than P25 and previous reported TiO₂ nanobelts, when decomposing Rhodamine-B (RB) under the ultraviolet light illumination.