Structural and thermal properties of pure and chromium doped zinc oxide nanoparticles

Pure ZnO and Cr-doped ZnO nanoparticles have been synthesized via a facile chemical co-precipitation route and their structural, thermal characteristics were discussed systematically. In the experimental producer, the doping concentration has varied the range, 0.05–0.1 M, while calcined at 600 °C. The influence of Cr-doping on the physical characteristics of ZnO nanoparticles was investigated and addressed. As-prepared samples were analyzed via XRD, FTIR, TGA/DTA, BET, and ICP-MS. XRD analysis shows that ZnO and Cr doped ZnO nanoparticles with average particle sizes between 23 and 39 nm were successfully developed with hexagonal wurtzite structure. The FTIR spectroscopy analysis confirms the existence of chromium in the doped ZnO nanoparticles and the formation of ZnO. The TGA/DTA analysis shows that Cr–ZnO nanoparticles are more thermally stable than ZnO nanoparticles. Moreover, the dopant concentration has been analyzed via ICP-MS and showed a good agreement with the expected chromium concentration. The BET surface area measurement shows that 176.25 m2/g and 287.17 m2/g for un-doped ZnO, and 0.1 M Cr-doped ZnO nanoparticles, respectively. Hence, doping of Cr enhances the surface area and thermal stability. Thus, Cr–ZnO nanoparticles show good thermal stability, and high surface area, which is an excellent characteristices of nanomaterials. Pure ZnO and Cr–ZnO nanoparticles have been synthesized using a facile and cost effective technique. The as prepared samples of pure ZnO and Cr–ZnO nanoparticles show wurtzite structure. Cr doped ZnO nanoparticles showed a higher surface area than that of un-doped ZnO nanoparticles. The synergetic effect of Cr and ZnO nanoparticles attributed to good thermal stability and high surface area. Pure ZnO and Cr–ZnO nanoparticles have been synthesized using a facile and cost effective technique. The as prepared samples of pure ZnO and Cr–ZnO nanoparticles show wurtzite structure. Cr doped ZnO nanoparticles showed a higher surface area than that of un-doped ZnO nanoparticles. The synergetic effect of Cr and ZnO nanoparticles attributed to good thermal stability and high surface area.


Introduction
Recently, nanoscale materials have revealed advanced occasions for several technological utilization [1][2][3]. Because of their unique, optical, catalytic, electrical, and magnetic characteristics and enhanced physical characteristics such as thermal, or chemical and mechanical, metal oxide nanoparticles are widely employed for many applications like magnetic materials, cosmetics, batteries, pharmaceuticals, catalysts, optical devices, protective coatings structured materials, and biomaterials [4][5][6][7]. Thus, the preparation of metal oxide semiconductors with different sizes and morphology has gained significant attention due to their excellent chemical stability and thermal property [8][9][10][11]. Among different nanomaterials, ZnO nanoparticles (NPs) have been extensively employed in many industrial sectors such as electro-optical devices, gas sensors, photocatalysts, antimicrobial agents, antibiotics, and electrode materials [12][13][14][15]. ZnO can be obtained in different crystallized forms such as rock salt, zincblende, and wurtzite with the doping of ZnO nanoparticles with transition metals, like Ag, Sb, Cr, Mn, Fe, P, In, Co, Li, Al, Ga, As, Sn, Mg, etc., have been observed to perform fine control of morphology and size [29][30][31]. From the various transition metals, Ag ions have been taken as dopants, due to their simple link matrices, and high-corrosion resistance as well as their hardness [32][33][34][35]. Furthermore, among the various transition metals, Cr ions have been gained great attention as a dopant due to their Cr 3+ radius, which is closer to that of Zn 2+ . Hence, Cr 3+ can simply dope with the ZnO crystal lattice or replace the site of Zn 2+ in ZnO [36]. The purpose of this study is to examine the impact of Cr doping in structural and thermal characteristics of ZnO NPs [37][38][39]. There are various methods for the development of ZnO and Cr doped ZnO like sol-gel method, chemical coprecipitation, microemulsion, thermal decomposition, spray pyrolysis, electrochemical deposition, and hydrothermal, solid-state reaction techniques have been utilized mostly [40][41][42]. Hence, cost-effective, low-temperature, fast, simple, and scalable synthesis methods are preferable for the preparation of NPs [43,44]. Among these, a simple and low-cost chemical co-precipitation route has been selected for the synthesis of ZnO and Cr doped ZnO nanoparticles from zinc nitrate hexahydrate and chromium acetate tetrahydrate using sodium hydroxide as a precipitating agent [45,46]. The advantages of the chemical co-precipitation technique from others for preparing nanoparticles are to obtain smaller particle size, pure particles, and better homogeneity [47][48][49]. In this work, the development of ZnO and Cr doped ZnO nanoparticles have been reported via a simple and low-cost route. Thus, the prepared products have been analyzed via XRD, FTIR, TGA/DTA, BET, and ICP-MS. In the analysis Cr, doped ZnO nanoparticles show better thermal stability and high surface area.

Materials
In this work the chemicals utilized were analytical grade and used without further refinement. Zinc nitrate hexahydrate (Zn(NO 3 ) 2 ·6H 2 O), sodium hydroxide (NaOH), ethanol, and chromium nitrate tetrahydrate (Cr(NO 3 ) 2 ·4H 2 O) were obtained from Merck Limited, India. Deionized was used as a solvent.

Preparation of ZnO and Cr-doped ZnO
In this work, nanoparticles of pure and Cr doped ZnO with 0.05, 0.075, and 0.1 M of Cr, have been prepared via co-precipitation approach using zinc nitrate hexahydrate, sodium hydroxide (NaOH) and chromium acetate tetrahydrate as precursor materials. For synthesis of Cr-doped ZnO NPs, 2.5 g of zinc nitrate hexahydrate was dissolved in 80 ml deionized water and (0.05, 0.075, and 0.1 M) amount chromium acetate tetrahydrate dissolved in 100 ml was added to the above solution. 20 mL of 2 M NaOH solution was slowly added into the starting materials under vigorous stirring until pH of the solution become 12. The solution acquired was stirred for 2 h at 80 °C. The obtained product were filtered and washed for several times with deionized water and ethanol to remove the impurities. The as-prepared nanoparticles were dried in air at 100 °C for 6 h. Finally, Annealing of the product was provided out at 600 °C for 4 h. The samples were calcined at 600 °C for 4 h, due to energy from the heat would improve the vibration and diffusion of the atoms in the lattice, thereby crystallizing the atoms. The experimental setup and additional explanations of the systems are shown in Fig. 1.

Characterization
The thermal characteristics of as-prepared materials are analyzed by TGA and DTA. Structural properties of ZnO and Cr-ZnO nanoparticles was examined via XRD (SHI-MADZU, MAXima_X XRD-7000) with Cu-K α radiation in the theta-2theta angle range of 20° and 80°. The functional groups and other impurities exist in of ZnO and Cr-ZnO nanoparticles have been analyzed via Fourier Transform Infrared spectroscopy (FTIR) (JASCO MODEL FT-IR 6660) in the wavelength range of 400-4000 cm −1 . The Brunauer-Emmett-Teller (BET) technique was conducted to estimate the total surface area of as-prepared nanomaterial using Quanta chrome Instruments version 11.0. Inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7500A) was used to analyse the concentration of residual doping elements in the mixture at the end of the preparation. Before testing, a strong acid solution has been used to dissolve all samples.

Structural analysis
The XRD patterns of pure ZnO and Cr doped ZnO nanoparticles are illustrated in Fig. 2. As shown in Fig. 2 where "K" is the Scherer constant, "λ" is the wavelength of the radiation sources, "β" is the full width at the half maximum intensity (FWHM) and "θ" is the peak position. where "ε" is strain and "θ" is the Bragg's angle in degrees. Moreover, the dislocation density (δ) values of as-prepared samples were determined using the following equation, which is tabulated in Table 1 [53].
where "δ" is the dislocation density and "D" is the crystallites size. The value of dislocation density was seen to be increased as the dopant increases (Table 1). Therefore, the XRD spectra of pure and doped ZnO NPs shows that Cr 3+ ions are exist within the lattice structure of ZnO via substituting certain Zn 2+ ions from its lattice position without forming chromium-related oxide phases. Moreover, the radius of Zn 2+ ion (0.74 A°) is different from the radius of Cr 3+ ion (0.63 A°), and this variation in ion radius size will cause internal lattice strain within the lattice structure of Cr 3+ ion doped ZnO NPs. The internal lattice micro-strain of the Cr 3+ ion-doped ZnO nanoparticles is larger than that of the pure ZnO NPs. As the percentage of Cr 3+ ion doping in the ZnO structure increases, they gradually increase.

BET surface area analysis
The BET technique was conducted to estimate the total surface area of the as-prepared nanoparticles. The surface area of ZnO and Cr doped ZnO nanoparticles at different dopant concentration was estimated. The results concerning BET surface area shows a dramatically change increment in surface area due to Cr-doping. The BET surface area measurement shows of un-doped ZnO and 0.1 M Crdoping ZnO nanoparticles are shown in Table 1. The BET surface area result shows that the prepared nanomaterials with high surface area. Thus, as approved by XRD analysis, crystallite size diminishes; consequently, the surface area grows. Hence, improving the porosity of the surface increases the surface area of nanoparticles. Therefore, higher surface area formation in the doped nanoparticles may be accredited to the thermal decomposition of Cr species during calcination process [6,54].

Dopant concentration analysis
The amount of chromium incorporated into ZnO NPs ware analysed using inductively coupled plasma mass spectroscopy (ICP-MS). We have prepared various Cr doped ZnO NPs samples to accurately measure the concentration of chromium. Table 2 shows the ICP-MS results of pure ZnO and Cr doped ZnO NPs. As shown in Table 2, there is a very good agreement among the expected chromium percentage using stoichiometry and the actual measured chromium concentration incorporated in the ZnO nanostructures [55].

Thermal property analysis
The TGA/DTA analysis was conducted to study the thermal stability of ZnO and Cr-ZnO nanoparticles. Figure 4 illustrates the TGA/DTA curve of ZnO and Cr-ZnO nanoparticles, which was scanned at the rate of 15 °C/min in the range of 30-800 °C under air atmosphere. For pure ZnO nanoparticles, the first weight loss of 2.8% from 25 to 174 °C is because of the evaporation of water molecules on the surface of ZnO nanoparticles. Then, the sample illustrates the second weight loss of 1.97% and a in the range of 174-311 °C, which attributed to the removal of organic components. The final weight loss of 3.63% in the range of 311-540 °C is the crystallization of as-prepared material during the heating process. The crossponding DTA peaks are observed at 345 °C, 402 °C and 464 °C (Fig. 4a). For pure Cr-ZnO nanoparticles, the first weight loss of 1.2% from 25 to 287 °C is because of the evaporation of water molecules on the surface of ZnO nanoparticles. Then, the sample displays the second weight loss of 5.23% and a in the range of 287-480 °C, which attributed to the removal of organic components. The final weight loss of 0.2% in the range of 480-656 °C is the crystallization of as-prepared material during the heating process. The crossponding DTA peaks are observed at 315 °C, 326 °C and 340 °C (Fig. 4b). Hence, TGA analysis showed that total weight loss of 8.13% and 6.63% for ZnO and Cr-ZnO nanoparticles, respectively. Thus, Cr-ZnO nanoparticles are more thermally stable than ZnO [59].

Conclusions
In this paper, ZnO and chromium doped zinc oxide (Cr-ZnO) nanoparticles were developed via a simple chemical co-precipitation route by changing the doping concentration in the range 0.05-0.    Author contributions AKW was involved in the experiments, data analysis and discussion. GAM, NYM, TAY and MAT drafted the manuscript. DWA and NGH supervised the experiments and edit the final manuscript.
Funding The authors declare that no financial interests.

Conflict of 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.
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