Research ArticlePhysicochemical properties and photocatalytic de-NOx performance of TiO2 nanostructures via microwave hydrothermal strategy
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.
References (30)
TiO2 photocatalysis for the degradation of pollutants in gas phase: from morphological design to plasmonic enhancement
J. Photochem. Photobiol. C Photochem. Rev.
(2015)- et al.
Degradation of organic dyes using spray deposited nanocrystalline stratified WO3/TiO2 photoelectrodes under sunlight illumination
Opt. Mater.
(2018) - et al.
Construction of TiO2@Bi2WO6 hollow microspheres by template method for enhanced degradation of ethylene under visible light
Opt. Mater.
(2021) - et al.
Polymer nanocomposites comprising PMMA matrix and ZnO, SnO2, and TiO2 nanofillers: a comparative study of structural, optical, and dielectric properties for multifunctional technological applications
Opt. Mater.
(2021) - et al.
Effect of microwave-assisted hydrothermal process parameters on formation of different TiO2 nanostructures
Catal. Today
(2016) - et al.
Facile microwave-assisted hydrothermal synthesis of TiO2 nanotubes
Mater. Lett.
(2012) - et al.
Synthesis of titania nanotubes by microwave irradiation
Solid State Commun.
(2005) - et al.
Titanium oxide nanotubes, nanofibers and nanowires
Colloids Surfaces A Physicochem. Eng. Asp.
(2004) - et al.
Hydrothermal synthesis and characterization of TiO2 nanostructures prepared using different solvents
Mater. Lett.
(2018) - et al.
Understanding the effect of morphology on the photocatalytic activity of TiO2 nanotube array electrodes
Electrochim. Acta
(2016)