Zirconium-Doped Chromium IV Oxide Nanocomposites: Synthesis, Characterization, and Photocatalysis towards the Degradation of Organic Dyes

: Degradation of organic dyes and their byproducts by heterogeneous photocatalysts is an essential process, as these dyes can be potentially discharged in wastewater and threaten aquatic and xerophyte life. Therefore, their complete mineralization into nontoxic components (water and salt) is necessary through the process of heterogeneous photocatalysis. In this study, Zr/CrO 2 (Zirconium-doped chromium IV oxide) nanocomposite-based photocatalysts with different compositions (1, 3, 5, 7 & 9 wt.%) were prepared by an environmentally friendly, solid-state reaction at room temperature. The as-prepared samples were calcined under air at 450 ◦ C in a furnace for a speciﬁc period of time. The synthesis of Zr/CrO 2 photocatalysts was conﬁrmed by various techniques, including XRD, SEM, EDX, FT-IR, UV-Vis, and BET. The photocatalytic properties of all samples were tested towards the degradation of methylene blue and methyl orange organic dyes under UV light. The results revealed a concentration-dependent photocatalytic activity of photocatalysts, which increased the amount of dopant (up to 5 wt.%). However, the degradation efﬁciency of the catalysts decreased upon further increasing the amount of dopant due to the recombination of holes and photoexcited electrons.


Introduction
Water pollution is one of the major environmental issues that arises from the discharge of organic dyes in wastewater, which is a threat to both aquatic and human life [1]. Various methods used for the removal of these organic dyes from wastewater are reported in the literature [2], such as advanced oxidation, biological methods, and adsorption processes. [3] Among these methods, heterogeneous photocatalysis under UV light or artificial light is a popular choice [4]. In this regard, different types of metal and metal oxide nanoparticles Zr/CrO 2 materials were determined through diffused reflectance spectroscopy (DRS). Finally, photocatalytic experiments were carried out using the different samples under visible light.

Structural and Morphological Characterization
The prepared Zr/CrO 2 photocatalysts were characterized by their crystal structure, crystallite size, and phase composition by X-Ray diffraction technique. Figure 1a shows the XRD patterns of the synthesized Zr1CrO 2 , Zr3CrO 2 , Zr5CrO 2 , Zr7CrO 2 , and Zr9CrO 2 materials, which were tetragonal in structure and single phase due to the presence of (110), (101), (200), and (111) diffraction planes. The lattice parameters of CrO 2 a = b = 4.421 Å, c = 2.916 Å were almost similar to those of TiO 2 (a = b = 4.593 Å, c = 2.959 Å), which is considered an efficient photocatalyst [30]. When doping a metal, the size of the dopant is an important parameter. First, the dopant size should approximately match that of the host metal (Cr or O). Second, if it is necessary to occupy the distance between the host atoms, an appropriately sized dopant is required; in the case of Cr-O, Zr, has an atomic radius of 0.160 nm. The (4 + ) ionic radius is 0.080 nm smaller than the distance between the Cr-O, which is 0.18 nm to 0.21 nm. It is clear from the XRD pattern in Figure 1b that the two most intensive peaks at (110) and (101) were slightly shifted towards the high angle (2θ) with low d-spacing after the incorporation of Zr in CrO 2 . This indicated that the Cr atoms had been replaced by Zr in the tetragonal structure of CrO 2 due to substitution, which distorted the crystal structure of the host material on the basis of differences in the ionic radii of chromium and zirconium atoms [31,32]. The XRD pattern of Zr/CrO 2 showed few peaks of Cr 2 O 3 , which is considered to be one of the most stable oxides of chromium [33]; at high temperatures, CrO 2 is converted to Cr 2 O 3 . The crystallite size obtained using the Scherrer equation [34] revealed an average size of the NPs of between 33.9 nm and 45 nm. Notably, with varying zirconium doping from 1 to 9 wt.%, the peak intensities also increased, along with the zirconium concentration. towards the high angle (2θ) with low d-spacing after the incorporation of Zr in CrO2. This indicated that the Cr atoms had been replaced by Zr in the tetragonal structure of CrO2 due to substitution, which distorted the crystal structure of the host material on the basis of differences in the ionic radii of chromium and zirconium atoms [31,32]. The XRD pattern of Zr/CrO2 showed few peaks of Cr2O3, which is considered to be one of the most stable oxides of chromium [33]; at high temperatures, CrO2 is converted to Cr2O3. The crystallite size obtained using the Scherrer equation [34] revealed an average size of the NPs of between 33.9 nm and 45 nm. Notably, with varying zirconium doping from 1 to 9 wt.%, the peak intensities also increased, along with the zirconium concentration. SEM was used for crystal morphology and shape observations of all the synthesized samples. The SEM images are shown in Figure 2. These images indicate that the shape and morphology of Zr/CrO2 photocatalysts change with increasing Zr concentration. The particles seemed to agglomerate with an increase of Zr content and exhibit a flake shaped morphology, with average crystalline sizes of 33.9 nm to 45.3 nm. The particles were dispersed properly in all samples, which is evident in Figure 2. To determine the elemental composition of all the samples, EDX was used, as shown in Figure 3. The EDX analysis showed that the material was in pure form with no elemental impurities present in the samples, while the concentration of Zr increased gradually. (b) XRD spectra demonstrating peak shifts of (110) and (101) diffraction planes. SEM was used for crystal morphology and shape observations of all the synthesized samples. The SEM images are shown in Figure 2. These images indicate that the shape and morphology of Zr/CrO 2 photocatalysts change with increasing Zr concentration. The particles seemed to agglomerate with an increase of Zr content and exhibit a flake shaped morphology, with average crystalline sizes of 33.9 nm to 45.3 nm. The particles were dispersed properly in all samples, which is evident in Figure 2. To determine the elemental composition of all the samples, EDX was used, as shown in Figure 3. The EDX analysis showed that the material was in pure form with no elemental impurities present in the samples, while the concentration of Zr increased gradually.   In order to further confirm the synthesis of all as-prepared Zr/CrO 2 -based photo catalysts, FTIR spectra were measured, as shown in Figure 4 and Table 1. After calcinations, the synthesized photocatalysts were free from impurities and moisture. The IR spectra exhibited peaks at 3100, 3010, and 1986 cm −1 belonging to C-H, C-C, and C=O functional groups. The peaks at 3400 and 1239 cm −1 represented O-H and C-O functional groups [35]. Different modes of vibration of CrO 2 and Zr-O were also visible in the IR spectra.  In order to further confirm the synthesis of all as-prepared Zr/CrO2-based photo catalysts, FTIR spectra were measured, as shown in Figure 4 and Table 1. After calcinations, the synthesized photocatalysts were free from impurities and moisture. The IR spectra exhibited peaks at 3100, 3010, and 1986 cm −1 belonging to C-H, C-C, and C=O functional groups. The peaks at 3400 and 1239 cm −1 represented O-H and C-O functional groups [35]. Different modes of vibration of CrO2 and Zr-O were also visible in the IR spectra.      The as-prepared Zr/CrO 2 photocatalysts (Zr1CrO 2 , Zr3CrO 2 , Zr5CrO 2 , Zr7CrO 2 , and Zr9CrO 2 ) were also investigated for absorption in the 200-800 nm wavelength region, as shown in Figure 5a. Pure CrO 2 absorbance peaks appeared at 257 nm and 346 nm [33]; however, after doping, the absorbance peaks shifted to a higher wavelength [38]. For band gap studies, the Diffuse Reflectance Spectroscopy (DRS) technique was used. From the DRS spectra, the band gap of the as-prepared photocatalysts was calculated using the Tauc, Davis and Mott relation, given by (αhν)1/2 = A(hν-Eg), where α, ν, A, and Eg are the absorption coefficient, light frequency, proportionality constant, and band gap, respectively [39][40][41]. From the plot for (αhν)1/2 vs. energy (hν) shown in Figure 5b, the band gap could be evaluated by extrapolating the straight line to the axis intercept. By using the DRS technique, followed by the numerical relation given above, the band gaps of all the samples were calculated. The DRS studies of the prepared material showed band gaps of 4.3, 3.2, 2.9, 2.87, 2.7, and 2.4 eV for pristine CrO 2 , Zr1CrO 2 , Zr3CrO 2 , Zr5CrO 2 , Zr7CrO 2 , and Zr9CrO 2 , respectively, as shown in Table 2. The DRS study revealed that after doping, the band gap decreased from 4.3 eV to 2.4 eV, which made the samples capable of absorbing light in the UV region, ultimately enhancing their photocatalytic degradation capabilities.   BET was employed to determine the surface area and pore size of the prepared material. The results, shown in Table 3, indicated that by increasing the doping concentration of zirconium in chromium IV oxide, the surface area of the host metal oxide (CrO2) also increased, which indicated that the maximum loading of the dopant and the degradation  BET was employed to determine the surface area and pore size of the prepared material. The results, shown in Table 3, indicated that by increasing the doping concentration of zirconium in chromium IV oxide, the surface area of the host metal oxide (CrO 2 ) also increased, which indicated that the maximum loading of the dopant and the degradation capacity of the material had been enhanced due to the large surface area, as shown [42]. Both MB and MO are heterocyclic aromatic compounds, which are also known as azo dyes bearing the functional group R − N = N −Ŕ . Both R andŔ are aryl groups that can be degraded using OH radicals produced by metal-doped metal oxide nanoparticles that are activated by UV light in the presence of aqueous media, producing carbon dioxide, water, and minerals as byproducts. These dyes are known to alter the physicochemical properties of soil and poison water bodies, and can cause serious damage to the flora and fauna in the environment. The UV-visible spectroscopic technique was used to study the effect of dopant concentration and time during the degradation of methylene blue (MB) and methyl orange (MO) in aqueous media in the presence of catalysts. During the experiments, the concentrations both of dyes (MB and MO) and photocatalysts were kept constant. The mixtures were prepared by dissolving 0.2 g of photocatalysts in 50 mL aqueous solution of each dye; before placing them under UV-light irradiation, these mixtures were kept in a dark place for 20 min to achieve complete adsorption and desorption equilibrium. The zero-time readings were taken, and the mixtures were studied under UV-light from 0-140 min.
The photocatalytic experiments showed that as the dopant concentration increased, the efficiency of the photocatalyst also increased. However, this trend was followed only up to 5 wt.%. After further increasing the amount of dopant, the efficiency of the samples dropped; this was due to the recombination of photoexcited electrons and holes and the energy absorbed by electrons during excitation that were released in the form of radiation [43]. The photocatalysts showed maximum efficiency from 1 wt.% to 5 wt.%, beyond which the efficiency decreased for the Zr7CrO 2 and Zr9CrO 2 samples. This fact can be clearly seen from the UV/Visible degradation study of MB and MO dyes. Initially, the degradation rates of MB and MO dyes were fast due to high concentration of dye molecules; however, the rate decreased gradually due to the deficiency of dye molecules [44,45]. The effect of contact time and dopant concentration on catalysts with MB and MO dyes are shown in Figures 6 and 7, respectively, which indicate that by increasing the time, the catalytic efficiency also increases up to 140 min. degradation rates of MB and MO dyes were fast due to high concentration of dye m cules; however, the rate decreased gradually due to the deficiency of dye mole [44,45]. The effect of contact time and dopant concentration on catalysts with MB and dyes are shown in Figures 6 and 7, respectively, which indicate that by increasing the the catalytic efficiency also increases up to 140 min.

The MB and MO Kinetic Study
The reaction (degradation) kinetics of both MB and MO were studied by placing the prepared mixtures under UV-light for different intervals of time, i.e., 0-140 min. The initial rate (r 0 ) of degradation of methylene blue and methyl orange increased for Zr1CrO 2 , Zr3CrO 2 , and Zr5CrO 2 as the concentration of zirconium increased, while the initial rate (r 0 ) decreased in the case of Zr7CrO 2 and Zr9CrO 2 due to the recombination of photoexcited electrons and holes. The Langmuir-Hinshelwood model can be used to understand the kinetics of photocatalytic degradation of MB and MO dyes as [46,47]: where r 0 denotes the initial reaction rate mg L −1 min −1 , C represents the dye concentration mg L −1 , and t represents the reaction time (min). In addition, k and Kc represent the Langmuire-Hinshelwood reaction rate constant mg L −1 min −1 and langmuire adsorption equilibrium constant L mg −1 . For small amounts of pollutants (KC << 1), pseudo-firstorder kinetics can be applied, as given by the following equations: where apparent constant k app is the kinetics parameter, and C 0 and C are initial and residual concentrations of MB and MO dyes in the aqueous phase before and after light irradiation, respectively. The k app can be found from (ln C 0 C ) vs. irradiation time. For all photocatalysts, the initial degradation rates (r 0 = k app × C 0 ) of 10 mg L −1 MB and MO were calculated, and the results are shown in Figure 8a,b, respectively.
To find the rate of degradation of MB and MO dyes, the pseudo first order kinetics can be applied, as given by the following equation as [48]: where C 0 represents the initial concentration before irradiation, C t represents the concentration after irradiation, t represents the time of irradiation, and K is a constant which can be found as a slope after plotting, as shown in Figure 9a,b. The photocatalytic efficiency of the prepared catalysts is dependent on the concentration of dopant (Zr 4+ ) in (CrO 2 ) nanoparticles. As the concentration of the dopant increases, the photocatalytic efficiency also increases up to 5 wt.%, whereas for 7 wt.% and 9 wt.%, the catalytic efficiency decreases due the recombination of holes and photoexcited electrons. The dopant enhanced the photocatalytic efficiency of the host materials due to the presence of Zr 4+ ions, which gain electrons during photo-excitation by converting to Zr 3+ , which is transferred to O 2 generating superoxide radical O 2 •− , preventing photo-corrosion.
Catalysts 2021, 11, x FOR PEER REVIEW 11 of 16 adsorption equilibrium constant L mg . For small amounts of pollutants 1 , pseudo-first-order kinetics can be applied, as given by the following equations: ln where apparent constant is the kinetics parameter, and and are initial and residual concentrations of MB and MO dyes in the aqueous phase before and after light irradiation, respectively. The can be found from (ln vs. irradiation time. For all photocatalysts, the initial degradation rates ( ) of 10 mg L MB and MO were calculated, and the results are shown in Figure 8a,b, respectively.
To find the rate of degradation of MB and MO dyes, the pseudo first order kinetics can be applied, as given by the following equation as [48]: where represents the initial concentration before irradiation, represents the concentration after irradiation, represents the time of irradiation, and K is a constant which can be found as a slope after plotting, as shown in Figure 9a,b.
The photocatalytic efficiency of the prepared catalysts is dependent on the concentration of dopant (Zr 4+ ) in (CrO2) nanoparticles. As the concentration of the dopant increases, the photocatalytic efficiency also increases up to 5 wt.%, whereas for 7 wt.% and 9 wt.%, the catalytic efficiency decreases due the recombination of holes and photoexcited electrons. The dopant enhanced the photocatalytic efficiency of the host materials due to the presence of Zr ions, which gain electrons during photo-excitation by converting to Zr , which is transferred to O2 generating superoxide radical O2 •− , preventing photocorrosion.

MB and MO Percent Photo Catalytic Degradation Curve of Zr-doped CrO2 as Function of time
To study the effects of contact time and dopant concentration on the percent degradation of MB and MO, 10 ppm dye solutions were prepared in deionized water.
The amount of prepared catalysts and dyes (MB and MO) were kept constant. The efficiencies of the prepared photocatalysts varied for different dopant concentrations. The MB dye was degraded up to 50.5%, 57.4%, and 64.4% by Zr1CrO , Zr3CrO , and Zr5CrO photocatalysts after 140 min, respectively, whereas 41.8% and 37.6% of MB dye was degraded by Zr7CrO and Zr9CrO , respectively (see Figure 10a,b). In the case of MO degradation, photocatalysts followed the same pattern as above. The MO dye was degraded up to 48.4%, 52.2%, and 55.4% by Zr1CrO , Zr3CrO , and Zr5CrO photocatalysts, respectively, after 140 min, whereas 38.3% and 26.5% of MO dye degraded by Zr7CrO and Zr9CrO respectively (see Figure 11a,b). On the basis of percentage of degradation of MB and MO dyes, it is clear that after 5 wt.% doping, the photocatalytic efficiency did not increase, because the doping crossed the optimum level; as a result, the electrons and holes were recombined, also known as photo-corrosion [49].  dye solutions were prepared in deionized water. The amount of prepared catalysts and dyes (MB and MO) were kept constant. The efficiencies of the prepared photocatalysts varied for different dopant concentrations. The MB dye was degraded up to 50.5%, 57.4%, and 64.4% by Zr1CrO 2 , Zr3CrO 2 , and Zr5CrO 2 photocatalysts after 140 min, respectively, whereas 41.8% and 37.6% of MB dye was degraded by Zr7CrO 2 and Zr9CrO 2 , respectively (see Figure 10a,b). In the case of MO degradation, photocatalysts followed the same pattern as above. The MO dye was degraded up to 48.4%, 52.2%, and 55.4% by Zr1CrO 2 , Zr3CrO 2 , and Zr5CrO 2 photocatalysts, respectively, after 140 min, whereas 38.3% and 26.5% of MO dye degraded by Zr7CrO 2 and Zr9CrO 2 respectively (see Figure 11a,b). On the basis of percentage of degradation of MB and MO dyes, it is clear that after 5 wt.% doping, the photocatalytic efficiency did not increase, because the doping crossed the optimum level; as a result, the electrons and holes were recombined, also known as photo-corrosion [49].   Figure 10a,b). In the case of MO degradation, photocatalysts followed the same pattern as above. The MO dye was degraded up to 48.4%, 52.2%, and 55.4% by Zr1CrO , Zr3CrO , and Zr5CrO photocatalysts, respectively, after 140 min, whereas 38.3% and 26.5% of MO dye degraded by Zr7CrO and Zr9CrO respectively (see Figure 11a,b). On the basis of percentage of degradation of MB and MO dyes, it is clear that after 5 wt.% doping, the photocatalytic efficiency did not increase, because the doping crossed the optimum level; as a result, the electrons and holes were recombined, also known as photo-corrosion [49].

Tentative Mechanism
The proposed mechanisms of Zr-doped CrO2 nanoparticles for the degradation of MB and MO dyes are as follows: i.
After illumination with UV light, the electrons move from the conduction band to the valence band, leaving a positive hole in the valence band. CrO2 + hv → CrO2 (h + vb) + CrO2 (e − cb) ii.
The electrons in the conduction band of Zr-doped CrO2 can be trapped by dopant Zr 4+ , thereby holding up the recombination process of electrons.
CrO2 ( Finally, the holes act as oxidizing agents and electrons act as reducing agents for the degradation of MB and MO dyes in aqueous solution. • OH (hv) + Pollutant → Degradation product

Conclusions
Zr-doped CrO2 nanoparticles were prepared on the basis of different concentrations of dopant (Zr) through a one-step, solid-state reaction method. XRD analysis confirmed the doping of Zr in CrO2 by replacing the Cr with Zr. This optical study verifies that as the dopant concentration increases, the absorbance peaks shifts to higher wavelengths and lower energy as a result of the decreasing band gap of the host material. Moreover, other techniques such as FT-IR and EDX confirmed the Zr doping in CrO2, which shows that the photocatalysts are suitable for UV light absorption due to the reduction in the band gap. It was also observed that as the concentration of dopant increases, the photocatalytic efficiency of the catalysts also increases up to 5 wt.%. Therefore, Zr-doped CrO2 photocatalysts could potentially be used for wastewater treatment.

Tentative Mechanism
The proposed mechanisms of Zr-doped CrO 2 nanoparticles for the degradation of MB and MO dyes are as follows: i.
After illumination with UV light, the electrons move from the conduction band to the valence band, leaving a positive hole in the valence band. • OH (hv) + Pollutant → Degradation product

Materials
The chemicals used for the preparation of zirconium-doped, chromium IV oxide photocatalysts were of analytical grade, and used as such without further purification. Zirconium metal powder, CrO 2 powder, DI water, methylene blue, and methyl orange were purchased from Merck Ltd. (Darmstadt, Germany). Ethanol was purchased from Sigma-Aldrich (St. Louis, MO, USA).

Synthesis of Zr/CrO 2 Photocatalyst by Solid-State Reaction Method
Zirconium-doped, chromium IV oxide photocatalysts (Zr/CrO 2 ) were prepared by a solid-state, environmentally friendly reaction at room temperature. Zirconium metal powder and chromium IV oxide powder were mixed in definite molar ratios (1:1, 3:1, 5:1, 7:1, and 9:1) using a mortar and pestle with a small amount of ethanol, and ground for several minutes until the powder was sufficiently mixed. After mixing, the prepared photocatalysts were calcined at 450 • C in a muffle furnace for 1 h to remove any impurities. The prepared photocatalysts were then characterized by various techniques, i.e., XRD, SEM, EDX, FT-IR, UV-Vis, and BET, and were used for the degradation of methylene blue and methyl orange as such. Degradation studies were carried out by UV/Visible spectrophotometer (Perkin Elmer lambda 35, Waltham, MA, USA).

Photocatalytic Experiments
The photocatalytic activity was examined by dissolving 0.2 g of each catalyst (solid), i.e., Zr1CrO 2 , Zr3CrO 2 , Zr5CrO 2 , Zr7CrO 2 , and Zr9CrO 2 , in 50 mL aqueous solution of MB and MO dyes (10 ppm). The reaction vessel was placed in the dark for complete desorption and adsorption equilibrium for 20 min with continuous stirring. Zero-time readings were taken before the samples were placed under UV light (10 watt) at 15 cm distance for maximum utilization of the light at different intervals of time (10-140 min) for their complete mineralization. Afterwards, all samples were filtered through a 0.45 µm (PVDF) filter and then analyzed using a UV/Visible spectrophotometer (Carry 50) to determine the degradation, reaction kinetics, and concentration.

Conclusions
Zr-doped CrO 2 nanoparticles were prepared on the basis of different concentrations of dopant (Zr) through a one-step, solid-state reaction method. XRD analysis confirmed the doping of Zr in CrO 2 by replacing the Cr with Zr. This optical study verifies that as the dopant concentration increases, the absorbance peaks shifts to higher wavelengths and lower energy as a result of the decreasing band gap of the host material. Moreover, other techniques such as FT-IR and EDX confirmed the Zr doping in CrO 2 , which shows that the photocatalysts are suitable for UV light absorption due to the reduction in the band gap. It was also observed that as the concentration of dopant increases, the photocatalytic efficiency of the catalysts also increases up to 5 wt.%. Therefore, Zr-doped CrO 2 photocatalysts could potentially be used for wastewater treatment.