Morphological, microstructural and photocatalytic characterization of undoped and Ni, Co doped Fe2O3 particles synthesized by sonochemical method

In this study, an abundant and eco-friendly photocatalytic material, Fe2O3 particles were synthesized by sonochemical method. Morphological and microstructural investigations of synthesized undoped and Ni, Co-doped Fe2O3 particles were performed. The effect of particle morphology and microstructure on its photocatalytic performance was further investigated. Comparative studies for evaluating particle crystallite sizes were conducted by Williamson-Hall (W-H) method and modified Debye-Scherrer (MDS). Crystallite sizes and lattice strains of Fe2O3 induced by process parameters were calculated by W-H method based on uniform deformation model (UDM). The crystallite sizes of the synthesized powders were calculated in the range of 200 nm and 76 nm by Williamson-Hall analysis. In addition to structural investigation, dislocation density of the synthesized particles was calculated by Williamson-Smallman relation. Afterwards, photocatalytic performance of Fe2O3 particles was investigated in detail. The photodegradation of methylene blue solutions in the presence of light in 20 min with samples 3,4, and 5 in 20 min were 0.937, 0.896, and 0.855, respectively. Moreover, the photodegradation of methylene blue solution with sample 5 for 15, 30, and 45 min were 0.9, 0.828, and 0.757, respectively. A photocatalytic activity of 24.25% has been observed under optimum conditions for the time interval of 45 min.

magnetic hysteresis curves with doping of nickel at different rates were investigated [28]. Transition element (Cu, Ni, Co) doped Fe 2 O 3 nanoparticles for photocatalytic applications were synthesized by Satheesh et al. Magnetic and photocatalytic properties of synthesized powders were investigated. It was reported that acid red 27 (AR27) degradation rate of Cu doped Fe 2 O 3 particles was the highest. As solution pH is among important factors that affect photodegradation, photodegradation activity of particles synthesized at pH levels between 3 and 9 was investigated. Highest photodegradation degree of 98.05% was reported at pH 6. Effect of catalyst and organic die concentrations was also investigated [29]. Sn doped Fe 2 O 3 particles were synthesized by Mansour et al. through coprecipitation method. Sn was doped at molar concentrations of 0.01%, 0.03%, and 0.06%. Reduction in crystallite size and bandgap was reported. Highest Rhodamine B (RhB) degradation rate was reported for 0.06% Sn doped Fe 2 O 3 particles [30].
In many studies in the literature, the effect of Fe 2 O 3 morphology on photocatalytic activity was investigated. In the study of Yan et al., Rhodamine (RhB) degradation of Fe 2 O 3 particles synthesized in the form of rings was investigated under UV light in the 400-800 wavelength range [31]. In another study, photocatalytic activity of nanoplate shaped hematite particles synthesized through microwave assisted solvothermal method was investigated against salicylic acid by Sun et al. [32]. By using different precipitating agents (NaCl, NaBr), Wang et al. synthesized Fe 2 O 3 particles and investigated the difference in photocatalytic activity. Particles synthesized by NaCl showed higher surface area, therefore had a higher light absorbance and photocatalytic activity was increased [33]. Photocatalytic activity of hematite produced by sol-gel method at different calcination temperatures was investigated by Boumaza et al. Calcination temperature affected the surface area and therefore altered the photochemical activity [34].
In recent years, ultrasonic waves have been preferred in many studies to synthesize different materials. When liquids are exposed to strong sound waves, high-and low-pressure waves are formed, which causes small air bubbles to form, and when the bubbles reach a certain size, they split immediately. Current and turbulence emerged this way provides on one hand a strong agitation, on the other hand a homogeneous mixture is obtained. Moreover, turbulence splits the agglomerated particles and therefore is very effective in shrinking soft and hard particles [35]. It was reported that usage of ultrasonic waves in the synthesis process instead of conventional mixing and heating methods shortened the synthesis duration.
In this study, undoped, Ni and Co-doped Fe 2 O 3 particles were synthesized through various process parameters via sonochemical method. The effect of process parameters including calcination time, the amount of ethylene glycol, the addition of PVP, and Ni, Co doping on photocatalytic activity of Fe 2 O 3 was revealed. The particle morphologies of the synthesized hematite particles were manipulated by process parameters. The synthesized particles phase analysis was conducted by XRD. On the basis of XRD peaks, crystallite sizes were calculated by mathematical models including W-H and MDS methods. Moreover, dislocation densities were calculated by Williamson-Smallman model. In order to investigate the short-term photocatalytic activity of the synthesized particles, photocatalytic tests were conducted by methylene blue (MB) solution under a halogen light for different durations. The photodegradation of organic dyes was characterized by a UV-Vis spectrophotometer.

Materials and methods
Firstly, ethylene glycol was prepared. Afterwards, 1% wt polyvinylpyrrolidone (PVP, mn: 10000) was dissolved in ethylene glycol in an ultrasonic bath. The base solution was prepared using a 200 mL ethylene glycol -PVP mixture and 100 mL deionized water. Iron nitrate, nickel nitrate and cobalt nitrate (Fe(NO 3 ) 3 .6H 2 O, Ni(NO 3 ) 3 .6H 2 O and Co(NO 3 ) 3 .6H 2 O Carlo Erba) solutions were prepared. Then, iron nitrate (0.1mol/L) solution was prepared by mixing of nickel nitrate (vol. 1.5%) and cobalt nitrate (vol. 1.5%) for producing of Ni, Co doped Fe 2 O 3 . Afterwards, gelation was initiated by adding ammonia (NH 4 OH, Merck) to iron nitrate solution. After the beginning of gelation, solution was mixed with an ultrasonic homogenizer for 6 min. After 48 h, precipitates were filtrated and dried in the oven for 24 h. Different samples were calcined at 700 °C and 900 °C for 3 h. After calcination, Fe 2 O 3 formation was observed. Experimental parameters are given in Table 1. Crystal structures of synthesized particles were analyzed by X-ray Diffractometer (Bruker AXS/Discovery D8) with monochromatic CuKα tube in the range of 10-90° within 0.02° steps and phase analyzes were conducted by X'Pert HighScore program. The effect of process parameters on crystallite sizes was conducted by Williamson-Hall analysis based on uniform deformation model. In order to compare the results obtained by Williamson-Hall analysis, the modified Debye-Scherrer method was also used to calculate crystallite sizes. Dislocation density was calculated by Williamson-Smallman analysis from the calculated crystallite sizes. In order to observe the effect of process parameters on powder morphologies, synthesized particles were analyzed by field emission gun scanning electron microscopy (FEG-SEM, Philips XL30). Methylene blue solutions (10 mg/L) were prepared to investigate the photocatalytic performances of synthesized particles. These solutions were mixed for 30 min in a dark box to reach absorption-desorption balance. Afterwards, hematite particles were added to methylene blue solutions with a ratio of 5 g/L. Fifty milliliters of suspension was mixed under a 100-W halogen lamp with a wavelength range of 350-1050 nm at a 15-cm distance. Samples were repeatedly taken from suspension during mixing to analyze the effect of exposure time. The catalyst material was separated from the suspension by centrifuge (Hettich, Rotina 420R). Spectral analysis was conducted by UV-Vis spectrometer (Analytikjena, Specord 200 Plus).

Microstructural investigation of undoped and Ni, Co doped Fe 2 O 3 particles
Phase analyzes of undoped and Ni, Co-doped Fe 2 O 3 particles synthesized under different conditions were conducted and XRD patterns are given in Figure 1. Reference patterns of synthesized particles were determined as 01-084-0307 by X'Pert HighScore Plus software. The crystal structure of synthesized particles belongs to the rhombohedral crystal system and the space group is R-3c. Despite the changes in the production parameters, it was observed that the crystal system did not change. The determined calcination temperatures were suitable for Fe 2 O 3 crystallization. Effect of process parameters on crystal sizes was conducted by Williamson-Hall analysis based on uniform deformation model. Initially, 7 peaks with high diffraction intensities were selected and peak widths were determined by X'Pert High Score Plus software. Straininduced peak broadenings are considered as crystal defects and shown as ε = βs/tanθ. The equations below are derived from Scherrer and ε = βs/tanθ equations. Peak broadening was calculated by subtracting instrumental peak broadening from measured peak broadening as shown in Equation 1.
(1) Peak broadening consists of crystallite size and crystallite strain as shown in Equation 2.
(4) (5) β hkl denotes peak broadening, θ denotes diffraction angle, whereas k denotes shape factor (in this case: 0.84), λ is the wavelength of CuKα radiation (λ= 0,154184 nm), D denotes average crystallite size and ε is average lattice strain. β hkl cosθ -4εsinθ graph is plotted for calculating crystallite size and strain. Values were subjected to linear fitting. The slope of the line equals strain. The point where the line intercepts the y-axis equals crystallite size.
Graphs plotted by Williamson-Hall analysis are given in Figure 2.
Williamson-Hall analysis based on uniform deformation model revealed that crystallite sizes were 204, 178, 150, 107, and 76 nm, indicating that nanostructured Fe 2 O 3 particles were obtained. After Ni and Co doping, a reduction in crystallite size was observed. Moreover, PVP usage reduced agglomeration during calcination and crystallite sizes were decreased with increased PVP amount. The decrement in crystallite size was observed when calcination temperature was lowered from 900 °C to 700 °C. Similar observations were reported in previous studies [36,37]. Modified Debye-Scherrer analysis was conducted to compare the results of Williamson-Hall analysis. Graphs of modified Debye-Scherrer analysis are given in Figure 3.   According to the modified Debye-Scherrer analysis, crystallite sizes of samples were calculated as 113, 107, 92, 79, and 67 nm. Retrieved results were relatively smaller and in accordance with Williamson-Hall's analysis. As expected, crystallite size values evaluated by the MDS method differ from the ones calculated by the W-H method owing to the negligence of the lattice strain. Due to the presence of tensile stress in the lattice, MDS method calculates smaller crystallite sizes than the W-H method. In case of compressive stresses, MDS method calculates higher crystallite sizes than the W-H method. Similar observations were reported in previous studies [38][39][40][41][42].
Dislocation densities were calculated by Williamson-Smallman analysis from crystallite sizes obtained by Williamson-Hall analysis. Williamson-Smallman equation is given in Equation 6.
(6) Calculated dislocation densities and crystallite sizes are given in Table 2.
An increment in dislocation density with reduced crystallite size was observed.

Investigation of powder morphologies of undoped, Ni and Co-doped Fe 2 O 3 particles
In order to investigate the effects of process parameters on powder morphology, scanning electron microscopy imaging was conducted. Figure 4 represents SEM images with secondary electrons images of undoped, Ni and Co-doped Fe 2 O 3 particle. From scanning electron microscopy images, it was observed that synthesized Fe 2 O 3 particles were agglomerated during the calcination step. In Ni and Co-doped samples, linear voids on grains were caused by doping elements. PVP agent was added and calcination temperature was decreased for reducing agglomeration. In order to compare the effect of PVP addition and calcination temperature, samples 3, 4, and 5 were imaged by scanning electron microscopy at different magnifications. Figure 5 illustrates SEM images with secondary electrons images of samples 3, 4, and 5.
Ethylene glycol and PVP were used to prevent to agglomeration of particles. In particular, PVP was used to encapsulate particles. Samples synthesized by the sonochemical route and contained PVP were not agglomerated and had a porous structure. Agglomeration behavior was not observed when the calcination temperature was lowered. Moreover, at lower calcination temperatures, samples obtained a spherical morphology.

Investigation of photocatalytic performance of undoped, Ni and Co doped Fe 2 O 3 particles
In order to investigate the effect of powder morphology and microstructure on photocatalytic performance, photocatalysis experiments were conducted comparatively at different durations.  Initially, photocatalysis experiments of agglomerated particles were conducted and their results are given in Figure 6. According to UV-Vis spectrums of samples 1 and 2, particles did not exhibit photocatalytic performance. These particles were proven to be agglomerated from SEM imaging and agglomeration was considered to affect photocatalytic performance dramatically. In order to investigate the photocatalyst performances of not agglomerated particles, comparative UV-Vis spectrums of samples 3, 4, and 5 were plotted and results are given in Figure 7.
Samples 3, 4, and 5 were subjected to photocatalysis experiments for 20 min. The best performance was obtained from sample 5, which was not agglomerated during the calcination step. Investigating the SEM images and XRD patterns, sample 5 had a lower particle size than other samples. According to Williamson-Hall analysis, sample 5 had the lowest crystallite size. As sample 5 showed the highest photocatalytic activity, in order to investigate the effect of light exposure duration, it was exposed to halogen light for 45 min and changes in the absorbance with time is measured by UV-Vis spectrometer. UV-Vis spectrum is given in Figure 8.     An increment in methylene blue degradation was observed with a prolonged photocatalytic activity experiment. The effect of particle morphology and microstructural properties on photocatalytic performance was investigated and although the crystallite sizes were able to be decreased by doping elements, agglomerated particles did not show photocatalytic activity. Particles with porous shapes that were not agglomerated exhibited highest photocatalytic activity.
The photodegradation of MB in the presence of light by samples 3, 4, and 5 for 20 min was calculated. Figure 9a, represents the photodegradation of MB in the presence of light by samples 3, 4, and 5 for 20 min. Figure 9b illustrates the photodegradation rate for time interval of 45 min.
From the photodegradation results of sample 3,4, and 5, it is clear that sample 5 shows higher photocatalytic rate than samples 3 and 4. Therefore, sample 5 was chosen for the prolonged photodegradation experiment. Moreover, the photodegradation of methylene blue solution with sample 5 for 15, 30 and 45 min were 0.9, 0.828, and 0.757, respectively. Figure 10 shows linearly fitted curve of ln(C 0 /C) as a function of irradiation time for sample 5.
A linear behavior of Figure 10 shows the pseudo first order degradation kinetics. The slope of linear fit of this graph is 0.006, this value indicates the reaction constant.
The efficiency of the photocatalytic activity is calculated from the formula: Table 3 illustrates the efficiency of the degradation of MB by samples 3, 4, and 5 in 20 min. The efficiency of photodegradation of MB by samples 3, 4, and 5 were 6.20%, 10.30%, 14.50%, respectively, in 20 min.  16.6%, and 30% in 40 min, respectively. Increasing the amount of cobalt favored the increase of the photocatalytic activity of the samples [43]. In another study, Ilkme et [44]. Gu et al. studied the effect of the addition of PVP on the photocatalytic activity of TiO 2 , the presence of PVP in the sol-gel process has been proved to be efficient in the photodegradation of methylene red under UV light [45]. Similarly, Phuruangrat et al. evaluated the effect of weight contents of PVP in the solutions on the synthesized photocatalytic BiOCl powders. The synthesized BiOCl nanoplate powders with PVP addition exhibit photodegradation of Rhodamine B (RhB) under visible light irradiation compared to BiOCl nanoplates without PVP addition. The weight content of PVP was ascribed to play a significant role in the morphology and size of BiOCl powders. BiOCl nanoplate powders with PVP addition exhibited the best photocatalytic efficiency for RhB photodegradation of 97.61% in 240 min [46]. Similarly, in our work, the addition of PVP plays a significant role in the photodegradation of MB and Fe 2 O 3 powders with PVP addition showed the best degradation efficiency of MB.

Conclusion
In this study, undoped Fe 2 O 3 and Ni and Co-doped Fe 2 O 3 particles were synthesized from Fe(NO 3 ) 3 solution via sonochemical method. SEM image of the synthesized particles revealed that samples 1 and 2 were agglomerated during calcination. Samples 3, 4, and 5 had a porous structure due to PVP addition since PVP encapsulated the synthesized particles. The change in the calcination temperature from 900°C to 700°C prevented the agglomeration. The crystal structure of particles was identified as rhombohedral by XRD analysis. In order to investigate the effect of process parameters on microstructural properties, Williamson-Hall, modified Debye-Scherrer analyzes were conducted based on the XRD peak broadenings. The crystallite size of agglomerated sample 1 was calculated at 204 nm, in Co and Ni-doped samples, crystallite size is reduced as cobalt and nickel atomic radii were smaller than iron. PVP addition during synthesis prevented agglomeration and reduced the crystallite sizes. Moreover, lowering the calcination temperature prevented the formation of tensile stresses in lattice and crystallite sizes were reduced. The crystallite size of sample 5 was calculated at 76 nm. The crystallite sizes were also calculated by modified Debye-Scherrer analysis and as this calculation did not involve lattice strains, results differed from Williamson-Hall analysis, however, the results were parallel for both methods. The dislocation densities were calculated by Williamson-Smallman analysis. An increment in dislocation density was observed with reduced crystallite size. The agglomerated particles with reduced crystallite size by doping elements still did not show photocatalytic activity. Particles with a porous shape exhibited the highest photocatalytic activity. The photodegradation of MB solutions in the presence of light in 20 min with samples 3, 4, and 5 in 20 min were 0.937, 0.896, and 0.855, respectively. Moreover, the photodegradation of MB solution with sample 5 for 15, 30, and 45 min were 0.9, 0.828, and 0.757, respectively. A photocatalytic activity of 24.25% has been observed under optimum conditions for the time interval of 45 min.