CuO nanoparticles prepared by alcohol-assisted hydrothermal synthesis for gas-sensing application: effect of pH value

CuO nanoparticles were synthesized by alcohol-assisted hydrothermal method at various pH values, using sodium hydroxide as a precursor for pH control. The chemical composition, morphological and structural properties of the obtained CuO nanoparticles were investigated by the Raman and energy-dispersive X-ray spectroscopies, the field-emission scanning electron microscopy, and the X-ray diffraction.
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CuO nanoparticles prepared by alcohol-assisted hydrothermal synthesis for gas-sensing application: effect of pH valueCommunications in Physics, Vol. 33, No. 4 (2023), pp. 435-446DOI: https://doi.org/10.15625/0868-3166/18283CuO nanoparticles prepared by alcohol-assistedhydrothermal synthesis for gas-sensing application:effect of pH valueNguyen Manh Hung1,† , Phung Dinh Hoat2 , Pham Tien Hung2 , Nguyen Van Hoang11 Department of Materials Science and Engineering, Le Quy Don Technical University, Hanoi 100000, Vietnam2 Department of Physics, Le Quy Don Technical University, Hanoi 100000, VietnamE-mail: † hungnm@lqdtu.edu.vnReceived 22 April 2023Accepted for publication 21 November 2023Published 6 December 2023Abstract. CuO nanoparticles were synthesized by alcohol-assisted hydrothermal method at vari-ous pH values, using sodium hydroxide as a precursor for pH control. The chemical composition,morphological and structural properties of the obtained CuO nanoparticles were investigated bythe Raman and energy-dispersive X-ray spectroscopies, the field-emission scanning electron mi-croscopy, and the X-ray diffraction. The results reveal that although the pH value does not affectthe formation of the CuO phase, it strongly influences the crystalline size, the morphology, and theparticle-agglomeration level of CuO. The differences in the morphology and the crystalline sizeof CuO nanoparticles are ascribed to various H+ /OH- ratios of the growth solution. Meanwhile,with the optimized particle size, the CuO nanoparticles-based sensor can be used as a potentialcandidate for CO and/or H2 detection.Keywords: alcohol-assisted hydrothermal; agglomeration; CuO; pH control; H2 detection.Classification numbers: 65.80.-g; 82.60.Qr.1. Introduction Cupric oxide (CuO) nanoparticles have attracted a lot of interest from scientists around theworld due to their special properties. It is well-known that CuO nanoparticles could be widelyutilized for different applications, including magnetic recording [1], photoelectrochemical watersplitting [2], energy conversion [3], catalyst [4], and gas sensors [5]. In these applications, CuO©2023 Vietnam Academy of Science and Technology436 CuO nanoparticles prepared by alcohol-assisted hydrothermal synthesis for gas-sensing application: effect of pH valuenanoparticles exhibit outstanding characteristics in a comparison to their bulk counterparts. Themain factor that leads to such differences is that CuO nanoparticles possess extremely small grainsizes allowing them to make unique chemical and physical properties. Among them, large specificsurface area, high porosity, strong catalytic activities, and advanced magnetic characteristics areoutstanding properties that are usually used for a number of various applications [6–8]. How-ever, these properties strongly depend on the particle size, crystalline size, and morphology ofCuO nanoparticles. Therefore, it is expected that these parameters of CuO nanoparticles could beeffectively controlled for practical applications. Until now, many solutions have been proposed for an effective synthesis procedure of CuOnanoparticles, for example, hydrothermal, solvothermal, sol-gel methods, co-precipitation, ther-mal decomposition, etc. Among them, the thermal decomposition method of copper salts or theirhydroxides was used as a simple method to prepare CuO nanoparticles in large amounts [9]. How-ever, one of the key limitations of this method is that it is difficult to adjust the desired morphol-ogy and size of CuO nanoparticles. In the meantime, sol-gel and co-precipitation methods requirehigh-skilled workers. As a result, the repeatability of products prepared by these methods has beenproven relatively low. On the contrary, the hydrothermal method possesses high reproducibilityin comparison to others mentioned above while allowing the synthesis of CuO nanoparticles inlarge quantities. However, another aspect of hydrothermal CuO synthesis that needs mentioning isthat finding the optimal synthesizing conditions of CuO nanoparticles is a complicated and time-consuming process. Many parameters need to be controlled during a hydrothermal process, suchas pH, the concentration of the chemical precursors, solvent, and growing temperature. Because ofthese reasons, a lot of work has been conducted to intend an optimal process for the hydrothermalsynthesis of CuO nanoparticles so far and various results have been reported [10–13]. However,the obtained results show that the morphology and size of CuO are widely distributed when theinput parameters fluctuate. This thing leads to significant limitations of CuO nanoparticles in prac-tical applications. Therefore, the researches on the influence of the hydrothermal parameters on themorphological and structural properties of CuO nanoparticles are necessary and should be donefurther. In the meaning of such intent, in this study, we have investigated the effect of pH on themorphological and structural properties of CuO nanoparticles synthesized by the alcohol-assistedhydrothermal method. The CO and H2 gas-sensing properties of the CuO nanoparticles-basedsensor were also considered in detail.2. Experiment2.1. Synthesis of CuO nanoparticles and sensor fabrication The chemical reagents of copper acetate (Cu(CH3 COO)2 , ≥ 98%) and diethylene glycol(DEG, C4 H10 O3 , ≥ 99%) were purchased from Sigma-Aldrich Co., Ltd (USA). The absoluteethanol solution (C2 H5 OH, 99.5%) and sodium hydroxide (NaOH, ≥ 97%) were supplied bySamchun pure chemical Co., Ltd (Korea). All the chemical reagents were used in the suppliedstate without further purification. Firstly, 1 g copper acetate was dissolved into 20 ml deionized(DI) water until obtaining a blue and transparent solution. Secondly, diethylene glycol was addedto the solution to reach a solution volume of 100 ml. The pH of the solution then wa ...