Elsevier

Optical Materials

Volume 114, April 2021, 110938
Optical Materials

Research Article
Physicochemical properties and photocatalytic de-NOx performance of TiO2 nanostructures via microwave hydrothermal strategy

https://doi.org/10.1016/j.optmat.2021.110938Get rights and content

Highlights

  • One dimensional TiO2 nanostructures were successfully synthesized via microwave hydrothermal method.

  • Effects of NaOH concentration on the physicochemical properties of as-prepared TiO2 were analyzed.

  • Heterojunctions among TiO2 nanoparticles, nanosheets or nanotubes reduced the recombination rate of charge carriers.

  • As-prepared TiO2 products showed a significant improvement in de-NOx conversion in comparison with TiO2-P25.

Abstract

TiO2 nanostructures have been prepared via microwave hydrothermal process using different NaOH concentrations. The physicochemical properties of the products were systematically investigated by using UV–vis Diffuse Reflectance Spectra, X-ray Photoelectron Spectra, Photoluminescence Spectra, Raman, N2 adsorption-desorption, and Transmission Electron Microscope analysis. The results indicated a close correlation between the NaOH concentration and different types of nanostructured titania products obtained from the microwave-assisted hydrothermal process. At low NaOH concentrations (4 M and 6 M), the precursor TiO2 nanoparticles partially converted into a mixture of one dimensional (1D) nanostructures (e.g., nanorods or nanowires) and 2D nanostructures such as nanosheets. As the NaOH concentration is increased to 8 M and 10 M, the obtained TiO2 products contain nanosheet and nanotube-like structures, respectively, with a significantly larger specific surface area. The photocatalytic performance of the synthesized TiO2 products, prepared under different NaOH concentrations, were evaluated through the toxic NOx gas removal efficiencies. The nanostructured TiO2 samples prepared at a higher NaOH concentration have shown improved de-NOx efficiencies than the TiO2–P25 precursor.

Introduction

The photocatalytic TiO2 nanomaterial has gained the most attention compared to other photocatalytic semiconductors in the field of photocatalysis. The reason may relate to its fascinating properties, including chemical stability, non-toxicity, low cost, and safe to the environment and living beings [1,2]. Moreover, TiO2 has widely been applied to various areas, such as photocatalytic degradation of waste materials, water splitting to produce H2, and solar cells [[3], [4], [5], [6]].

Currently, TiO2 nanomaterials are being synthesized via different strategies, including sol-gel, hydrothermal, deposition, and oxidation methods. In the hydrothermal process, the products are precipitated and recrystallized at a high temperature and high vapor pressure in an aqueous medium [7,8]. The reaction system is carried out in a closed autoclave for about one day under controlled pressure and temperature. On the other hand, microwave irradiation has often been introduced in the hydrothermal method as a heat source instead of a traditional oven to shorten the reaction time. The polar molecules or conducting ions interact with microwaves (f = 0.3–300 GHz and λ = 1 mm to 1 m) and get heated [9]. During the interaction with microwave, polar molecules or ions try to orient with the rapidly changing alternating electric field, resulting in raising the local temperature due to rotation, friction, and collision of molecules.

With the application of microwave heating, the reaction time of the hydrothermal process is shortened from several days down to a few hours. That is because the microwave energy goes directly into the reacting materials, crossing the container walls without any losses. Therefore, the reactants can get enough energy in a short time. Moreover, this technology provides a uniform distribution of heat energy leading to better reproducibility via excellent control of experimental conditions [10,11]. In contrast, a typical hydrothermal method always requires extra energy to heat the container walls, and often overheating takes place, which in turn reduces the uniformity of obtained products.

In the hydrothermal and microwave hydrothermal methods, the process parameters such as precursors, hydrothermal duration, reaction temperature, and pre-and post-treatments play a vital role in the morphology, structure, and properties of the products. In these experimental conditions, the effects of different alkaline media (such as NaOH, KOH, etc.) on the structural changes have been reported in some literature. For example, Cui et al. [12] demonstrated that the tubular structure did not form under low NaOH content (3 M). However, increasing the NaOH concentration to 5 and 8 M, the obtained products consisted of nanosheets and nanotubes. Pure nanotubes were synthesized under the action of 10 M NaOH. According to Wu et al. [13], multiwall structured titanate nanotubes were prepared at high NaOH concentrations (8–12 M) for 90 min. Yuan et al. [14] indicated that a small amount of TiO2 nanotubes was observed as the content of NaOH was lower than 5 M or as strong as 20 M. Moreover, the formed morphologies of TiO2 were significantly influenced by the different alkaline media. Accordingly, TiO2 nanoribbons were formed in the 10 M NaOH solution at 180 °C; whereas, TiO2 nanowire structures were observed in the KOH treated samples. However, TiO2 nanoparticles were obtained in the LiOH solution [14]. Similarly, the tube-like and rod-like morphologies were also reported under 10 M NaOH [15].

The physicochemical properties of the photocatalysts have a major impact on the efficient degradation of pollutants in the aqueous and gaseous phases. According to Verbruggen et al. [3], an ideal gas phase photocatalyst should possess a high specific surface area, adsorption ability, and high photon utilization efficiency. Their study also indicated that the zero-dimensional TiO2 (nanoparticles) could benefit from the high surface to gas volume ratios and short charge carrier diffusion distances. In contrast, one-dimensional TiO2 materials (nanotubes, nanobelts, nanowires, and nanorods) are found advantageous for both higher surface area and fast interfacial charge transfer. In addition, Adan et al. [16] reported that the efficiency of charge separation and the increase of electron transport velocity through TiO2 nanotubes, compared to the TiO2 nanoparticles, had made 1D TiO2 nanostructure materials ideal for solar light applications.

In this study, the effects of NaOH concentration during the microwave-assisted hydrothermal process on the structure and physicochemical properties of the TiO2 products were investigated, along with their NOx removal efficiencies.

Section snippets

Materials

TiO2–P25 (Degussa) (P25) was used as a precursor source. NaOH (98%, Samchun Chemical), HCl (35–37%, Sigma) were used to synthesize the TiO2 nanomaterials. During the synthesis process, the chemicals and precursor materials were utilized directly without any further treatments.

Preparation of TiO2 nanostructures

TiO2 nanostructures were prepared using a hydrothermal strategy with the assistance of the microwave treatment based on the literature [14] with a slight modification.

In the first step, a calculated amount of NaOH was

Physicochemical characterizations of samples

The physicochemical properties of the nanostructured photocatalytic materials greatly influence their photocatalytic performance. In addition, the properties depend on synthesis route, synthesis condition, reaction medium, and their concentration, etc. Therefore, the changes in physicochemical properties of nanostructured TiO2 photocatalyst with respect to different concentrations of NaOH during the synthesis process have been investigated.

Conclusion

In this study, TiO2 nanostructures were prepared successfully via the microwave-assisted hydrothermal strategy using different concentrations of NaOH and P25 as the precursor materials. The physicochemical properties (structure, phase, surface area, band gap energy, light absorption, PL, etc.) of the products revealed a dependence of the NaOH concentration to produce the different grades of the nanostructured titania products from the microwave-assisted hydrothermal process. Titania nanosheets

CRediT authorship contribution statement

Hao Huy Nguyen: Conceptualization, Methodology, Writing – original draft. Adriana Martinez-Oviedo: Data curation. Tae-Ho Kim: Resources, Supervision. Bhupendra Joshi: Validation, Methodology. Lung Nhat Dang Quang: Visualization. Gobinda Gyawali: Conceptualization, Formal analysis, Writing – review & editing, Supervision.

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

The authors would like to acknowledge Prof. Soo Wohn Lee from Global Research Laboratory at Sun Moon University, Korea, for providing the necessary facilities to carry out the research.

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