Gallium Oxide Nanostructures for Wastewater Treatment: Photocatalytic Degradation of Methyl Orange

Abstract

In recent years, various fabrication methods of photocatalytic materials have been developed for photo degradation of organic and inorganic pollutants from consumable resources. Particularly, inorganic semiconductor nanomaterials have been considered as most promising agents for photocatalytic applications due to their remarkable physical and chemical properties with large effective surface area, and a variety of morphologies, such as nanorods, cubes, spheres and flowers, synthesized by cost effective chemical route. This work is directed towards the development of group III metal oxide nanostructures of Ga2O3 which have been recognized as an important material for several applications including catalysts, gas sensors, solar cells, and photodetectors. Typically, the Ga2O3 nanostructures were obtained by calcination of gallium oxide hydroxide (GaOOH) synthesized via a chemical bath method. Then, as-prepared GaOOH nanostructures were calcined at different temperatures of 500–1000°C for obtaining Ga2O3 nanostructures. This system was characterized by traditional tools like XRD, FESEM, EDX, UV-Vis to investigate the phase information, morphological features, composition and the information about band gap of the same. Also, the photocatalytic performance of Ga2O3 nanostructures was studied by time evolved UV-vis absorption spectrum of degradation of Methyl Orange (MO) solution. Under UV irradiation for 120 min, the Ga2O3 nanorods exhibited a high photodegradation efficiency of (98%). This work proposes a simple cost-effective eco-friendly route for synthesis of Ga based oxide photocatalysts for wastewater treatment.

Introduction

I. INTRODUCTION

With faster technological development, industrial wastewater is threatening us with their increasing pollution due to rapid development of polymer, photographic, textile, dyeing industries. Different types of organic pollutants such as dyes, phenol and its derivatives are the most potential contaminations liberated from various productions due to printing and dyeing in paper, textile, paints, leathers; oil refining; polymeric resin production; coal gasification; coking plants etc. [1] Nowadays removal of wastes from water is really a big challenge as some conventional methods like activated carbon adsorption, solvent extraction and common chemical oxidation frequently suffer from significant drawbacks including high cost or generation of hazardous by products. For example, more toxic chlorinated compounds may form during water purification by chlorination method [2].

In the past decades various metal oxide nano particles such as TiO2, ZnO,WO3have drawn considerable attention as a promising candidate for photocatalysis due to exhibiting their high efficiency in decomposition of a wide range of stubborn organic pollutants into carbon dioxide and water under UV irradiation [3-6]. However, it is necessary to investigate new strategies to enhance the photocatalytic activity that is by reducing the size of a metal oxide and improving its surface-to-volume ratio, porosity, structural uniformity, stability etc. Five different polymorphisms exist in Ga2O3 like α, β, γ, δ, ?-gallia. Among these crystalline phases, β-Ga2O3 having monoclinic crystal structure possess excellent thermal and chemical stability. Being a wide band gap semiconductor β- Ga2O3 (Eg = 4.9 eV) [7] is UV transparent and displays a promising prospect in the fields of photocatalysis, including the degradation of organic pollutants [8], Hydrogen evolution [9], CO2 reduction [10] etc. Ga2O3has been considered as a cost-effective material for water decontamination because of its superior charge separation, favourable mobility of the photo-generated electrons and its capability for converting light energy into chemical energy. Here we have attempted to prepare the porous gallium oxide nanostructures with large surface to volume ratio via a relatively simple, cost effective, environmental friendly wet chemical bath method followed by calcination. In our work, β-phase Ga2O3 was prepared via a facile chemical bath method and proper calcination.

Then to get the information about crystallinity, phase, morphology, composition etc, we have characterized our sample by conventional tools like X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray analysis (EDX) etc. The as-synthesized materials displayed exceptional photocatalytic performance under UV irradiation for the decomposition of organic pollutant Methyl orange (MO) to get the pure water under ambient conditions. The efficiency of degradation was correlated with the porous feature of the nanostructures.

II. EXPERIMENTAL

All the reagents used in the synthesis procedure were analytical pure grade chemicals. Bar-like Ga2O3 were prepared by a simple chemical bath method followed by calcination. To prepare the growth solution initially the mixture of hydrated Gallium nitrate (Ga(NO3)3.nH2O) (0.1 M) and ammonium hydroxide (NH4OH) was stirred and heated at 105?C maintaining pH 9 to produce white precipitate of Gallium Oxide hydroxide bars. Further the as-synthesized GaOOH samples were calcined in an oven at a heating rate of 10?C per min for 2 hours, then that temperature was maintained for 3 hours to obtain gallium oxide structures of β phase.

Traditional characterizations with X-ray diffractometer (Bruker D8 Advanced), field emission scanning electronmicroscopy (FESEM, Hitachi S-4800), EDX spectrophotometer attached with FESEM (EDS, Thermo Scientific attached with Hitachi S-4800) were carried out to analyze crystal structure, morphology and chemical composition of the synthesized Ga2O3 nanostructure. For photocatalytic experiment, 40 ml of 10-5 M methyl orange dye and 0.03 g of the as-prepared Ga2O3 powder sample were taken as pollutant and catalyst. The dye-catalyst solution was stirred in dark condition for 30 mins before UV exposure. Afterwards the solutions containing the powder sample was subjected to UV irradiation using two 40W UV tube (Phillips) emitting wavelength of 254.6 nm (UVC). The time evolved absorption spectra were recorded with the solutions collected in different time intervals using UV-Vis spectrophotometer.

III. RESULTS AND DISCUSSION

The XRD pattern of the as prepared samples can be found in fig. 1. It can be clearly seen that the GaOOH microbars and Ga2O3 microbars exhibit good crystallinity with the later following β phase. The lattice planes for the GaOOH and β-Ga2O3 samples were correlated with JCPDS card no: 06-0180 and 76-0573 respectively. The intense lattice peaks indicate the proper crystallinity of Ga2O3 samples.

The detail analysis of morphology has been depicted in fig. 2. It can be clearly seen from fig. 2(a) that the GaOOH samples are adequately uniform in size and shape with compact surface structure. Whereas fig. 2(b) shows that the Ga2O3 samples are almost same in shape, with only difference of surface porosity. The pore distribution was found to be uniform. The occurrence of pores is directly correlated to the temperature effect. As the GaOOH microbars are subjected to high temperature annealing, the internal water molecules vaporized leaving appreciable vacant space within the structure.

Conclusion

Porous ?-Ga2O3 microbars were synthesized via cost effective chemical route. Traditional characterizations were performed to investigate structural morphological and compositional properties. The synthesized sample was subjected to photocatalysis experiment to check its ability in degrading methyl orange. The sample showed efficient dye degradation performance with degradation rate constant of 0.02264. This work therefore establishes porous ?-Ga2O3 samples as efficient material for wastewater treatment remediation.

References

[1] Z. Carmen and S. Daniela, “Textile Organic Dyes – Characteristics, Polluting Effects and Separation/Elimination Procedures from Industrial Effluents – A Critical Overview”, Book Chapter 3, pp. 55-86. [2] M. Deborde and U. von Gunten, Water Research, vol. 42, pp. 13-51, January 2008. [3] S. G. Kumar and L. G. Devi, dx.doi.org/10.1021/jp204364a |J. Phys. Chem. A, vol. 115, pp. 13211–13241, 2011. [4] K. Intarasuwan, P. Amornpitoksuk, S. Suwanboon, P. Graidist, Separation and Purification Technology, vol. 177, pp. 304–312, 2017. [5] V. Iliev, D. Tomova, L. Bilyarska, Journal of Photochemistry and Photobiology A: Chemistry, vol. 351, pp. 69–77, 2018. [6] C. A. Aggelopoulosa, M. Dimitropoulosa, A Govatsia,L. Sygelloua, C. D. Tsakirogloua, S. N. Yannopoulos, Applied Catalysis B: Environmental, vol. 205, pp. 292–301, 2017. [7] S.I. Stepanov, V.I. Nikolaev, V.E. Bougrov and A.E. Romanov, Rev.Adv. Mater. Sci. vol. 44, pp. 63-86, 2016. [8] M. Bagheri, A. R. Mahjoub, A. A. Khodadadi and Y. Mortazavi, RSC Adv., vol. 4, pp. 33262–33268, 2014. [9] L. Li, B. Ma, H. Xie, M. Yue, R. Cong, W. Gao and T. Yang, RSC Adv., vol. 6, pp. 59450–59456, 2016. [10] Y. Pan, Z. Sun, H. Cong, Y. Men, S. Xin, J. Song, and S. Yu, Nano Research, vol. 9(6), pp. 1689–1700, 2016. [11] M. Nazim, A. A. P. Khan, A. M. Asiri and J. H. Kim, ACS Omega, vol. 6, pp. 2601–2612, 2021.

Copyright

Copyright © 2023 Uday Das. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.