A simple co-precipitation technique is proposed for synthesis of tin oxide (SnO2) microrods. Stannous chloride and urea were used during synthesis. X-ray powder diffraction (XRD) analysis revealed that the annealed product consists of SnO2 microrods having tetragonal unit cells, while scanning electron microscopy (SEM) analysis revealed the rod-like morphology of a synthesized product. These synthesized microrods are used as photocatalyst for the degradation of reactive black 5 (RB5). Degradation kinetics of RB5 are monitored under daylight in different concentrations of hydrogen peroxide (H2O2) and catalyst. The percentage of RB5 conversion is also calculated at various concentrations of hydrogen peroxide and catalyst which demonstrate that RB5 shows high catalytic degradation at high concentrations of hydrogen peroxide and catalyst.
Industries contribute greatly to the contamination of water resources. A wide range of hazardous toxic substances are continually released into the water body of the environment due to a lack of effective source treatment techniques. Synthetic dyes play a major role in the contamination of water through industrial waste. These dyes show stable behavior because of their aromatic structures, so synthetic dyes cannot be easily degraded. Their stable structure prevents the absorbance and reflection of sunlight. These dyes are also mutagenic and toxic in nature. Their direct release into the ecosystem disturbs its balance, therefore removal of these synthetic dyes from water resources is necessary to avoid these effects (Chowdhury & Balasubramanian 2014). Photocatalysis plays an important role in the purification of water by removing the biological and chemical contaminants from water resources. Photocatalysis also causes the breakdown of organic contaminants into mineral salts (Wang et al. 2006). In a photocatalytic reaction, an electron-hole pair is created with the help of a catalyst which further generates free radicals. These free radicals break down the aromatic structure of dye molecules (Julkapli & Bagheri 2014). Semiconductor materials are used as photocatalyst to remove the dyes from water resources because they can efficiently oxidize organic substances. Semiconductor materials are used as photocatalysts in various fields (hydrogen gas production (Yoong et al. 2009), water splitting (Zou et al. 2001), dismantling of micro-organisms such as bacteria (Robertson et al. 2012), degradation of organic dyes (Chakrabarti & Dutta 2004)). Photons activate electron-hole recombination and the wide energy gap between conduction and valence band in these materials hinders this photo activation (Li & Li 2001). Various organic dyes have been degraded by different semiconductor materials such as zinc oxide (ZnO) (Daneshvar et al. 2004; Jamil et al. 2017a, 2017b), titanium dioxide (TiO2) (Khataee & Kasiri 2010), tungsten trioxide (WO3) (Kim et al. 2010), cobalt oxide (Co3O4) (Dong et al. 2007; Jamil et al. 2017a, 2017b), ferric oxide (Fe2O3) (Cao & Zhu 2008; Janjua et al. 2017), copper oxide (Cu2O) (Yang et al. 2006) and tin oxide (SnO2) (Khalid et al. 2018). Recently, the focus on tin oxide has increased because of its applications in various fields as a gas sensor (Leite et al. 2000), catalyst (Jiang et al. 2005), electrode (Zhu et al. 2000), transistor (Shin et al. 2011), photovoltaic device (Rajesh et al. 2014) and liquid crystal (Demir-Cakan et al. 2008). Tin oxide is the most important material because of its strong chemical and physical interaction with absorbed species, high transparency in visible spectrum, strong thermal stability in air and low operating temperature. Tin oxide is available in +2 and +4 oxidation states, so two types of oxides are formed (stannous oxide SnO and stannic oxide SnO2) (Ge et al. 2006). However, among these two oxides, SnO2 has received attention due to its n-type semiconductor behavior, wide energy gap of 3.6 eV, chemical and mechanical stability and high mobility of electrons (Tazikeh et al. 2014). Researchers have a great interest in the synthesis of SnO2 nano and micro particles by various methods such as sol gel (Gu et al. 2004), hydrothermal (Chiu & Yeh 2007; Khalid et al. 2018), solvothermal (Anandan & Rajendran 2010), microwave method (Subramanian et al. 2008), mechano-chemical (Kersen 2002) and co-precipitation method (Bouaine et al. 2007). So the co-precipitation method is chosen for the synthesis of SnO2 microparticles because it is inexpensive and simple. This method also gives highly crystalline microparticles without using any surfactant and binding species. Using this method shape and size can be easily controlled by changing the concentration of precipitating reagent and precursor salt, pH of medium and reaction temperature. Khan et al. (2017) synthesized the ferric oxide (Fe2O3) nanorods by a reflux assisted co-precipitation method. Nadaf & Venkatesh (2016) used a co-precipitation technique for the synthesis of SnO2 nanoparticles. Babar et al. (2010) synthesized the nano-crystalline SnO2 by a co-precipitation method. Nejati-Moghadam et al. (2015) fabricated SnO2 nano structures by a co-precipitation approach. In the present work rod like highly crystalline SnO2 microparticles are synthesized via a simple co-precipitation approach from a stannous chloride precursor. The product is analyzed using X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) technique. Photocatalytic application of the synthesized product for degradation of reactive black dye 5 from aqueous water is studied.
Hydrated stannous chloride (SnCl2.2H2O), urea (N2H4CO), reactive black 5 dye (RB5) and hydrogen peroxide (H2O2) were purchased from Sigma-Aldrich, USA. Distilled water was used throughout the synthesis and catalytic study. All the chemicals were used as such without any further purification.
Synthesis of SnO2 microrods
SnO2 microrods were synthesized by the co-precipitation method. The pH of hydrated stannous chloride solution (1 M) was adjusted to 9 using 2 M urea solution under constant stirring at room temperature. The resultant solution was further stirred for 1 h at room temperature. Obtained precipitates were then collected by means of centrifugation and repeatedly washed with ethanol and distilled water and the precipitates were dried in a thermoelectric oven at 80 °C for 24 h. The dried product was ground to powder form and subjected to calcination in an electric furnace at 600 °C for 4 h in the presence of air. Calcined product was subjected to characterization.
Photocatalytic degradation of RB5
RB5 dye, catalyst and H2O2 were added to a beaker in the presence of sunlight. After short time intervals, a small amount of sample mixture was taken in a glass cuvette and its spectra scanned on a Halo DB 20 double beam UV-Visible spectrophotometer at 350–800 nm wavelength range. The photocatalytic reduction of RB5 was studied by changing the amount of catalyst dosage and H2O2 concentration.
A Jeol JDX- 3532 X-ray powder diffractometer was used for XRD analysis. A pattern was recorded in 2θ, range 10–80°. The diffractometer was operated at 45 kV voltage and 40 mA current. A Quanta 250 scanning electron microscope was used for SEM observations at 30 kV voltage. Photocatalytic degradation of RB5 was studied with a Halo DB 20 double beam spectrophotometer.
RESULTS AND DISCUSSION
X-ray powder diffraction analysis
The XRD pattern of SnO2 rods is shown in Figure 1. Strong diffraction peaks are observed at 2-theta values: 26.53, 33.8, 37.89, 51.65, 54.62, 57.76, 61.75, 64.58°. These peaks are associated with different planes (110, 101, 200, 211, 220, 002, 130, 112). The 2-theta values and corresponding planes are characteristic to SnO2 entry no. 96-153-4786. Diffraction peaks are very sharp which demonstrate that the synthesize SnO2 is highly crystalline. One small extra peak is also present at 2-Theta value 44.61 °. This peak is due to the presence of moisture in the sample during analysis. Analysis of SnO2 confirms that it possesses tetragonal crystal lattice with a P 42/m n m space group. The crystal structure of synthesized SnO2 is shown in Figure 2. Figure 2(a) shows that tin atoms are located at the corners and body center positions of the crystal lattice and every tin atom is bonded with six oxygen atoms, four of which are at the face of the lattice and two are at the center. Various planes are also shown in Figure 2(c) to reveal the positions of atoms, bond angles and bond lengths.
Scanning electron microscopic analysis
Figure 3 shows the SEM images of SnO2 microparticles synthesized by a co-precipitation method followed by calcination at 600 °C temperature. It can be seen from Figure 3(a) that the product consists of microrods. The length and diameter of these rods lies in the 6–8 and 0.9–1.7 μm range respectively. Synthesized micro-rods are oriented randomly. Figure 3(b)–3(d) show that the surface of the microrods is not smooth and broken parts of other rods are attached on its surface. Closer analysis of these images reveals that these rod-like structures are formed by self-assembly of needle-like structures. Needle-like structures are arranged parallel to each other and form a rod-like morphology. The diameter of these needle-like structures is around 0.1–0.2 μm. Some irregular shaped small particles are also present along with these rods, which may be the broken parts of other rods.
Figure 4 shows the possible formation mechanism of synthesized product which is suggested by XRD and SEM characterizations. Tin (II) oxidizes into tin (IV) in dilute aqueous medium. On the other hand, urea was added into the water which generates the ammonium ions, hydroxyl ions and carbon dioxide in the solution. There are two possible paths now: (1) carbon dioxide can react with hydroxyl ions and produce carbonate ions and water, then these carbonate ions react with tin ions and produce tin carbonate; (2) carbon dioxide gas escapes from the solvent and hydroxyl ions react with tin ions and produce tin hydroxide. The obtained product was calcined at 600 °C and then its XRD pattern was recorded. XRD analysis confirms that the product is SnO2. It is well known that tin carbonate is stable at 600 °C while tin hydroxide is decomposed into SnO2 at this temperature. Thus XRD analysis confirms that the reaction has progressed through Path-2. The reaction of hydroxyl ions with tin ions is faster than the reaction of hydroxyl ions with carbon dioxide, therefore Path-2 is favored over Path-1.
Photocatalytic degradation of reactive black 5
Azo dyes characterized by azo group (‒N = N‒) are the largest class of dyes that are used for dyeing natural and synthetic materials. Among all azo dyes (metal complex, reactive, direct, acidic, basic and sulfur), the most commonly used dyes are reactive type azo dyes. These reactive azo dyes produce water pollution. This is due to the fact that almost 20% of these dyes drained into water streams during the dyeing processes (Lucas et al. 2013; Ramesh et al. 2016). RB5 is a reactive azo dye and is commonly used in dying industries. RB5 contains highly reactive chromophoric groups such as anthraquinone, azo, oxazine and formazan. These groups can form the covalent bond with fiber. Moreover, due to the complex structure and high chemical stability of RB5, it is very difficult to degrade RB5 from water. RB5 can cause damage to the aquatic environment by preventing the light absorption and photosynthesis process. Furthermore, RB5 is toxic to some organisms and causes the death of these organisms in water. RB5 can be degraded through microorganisms by the breakage of azo bonds but this process causes production of some toxic derivatives (like phenol) which have a carcinogenic and mutagenic effect on human health. Products 2-(ethylthio)-ethyl ester and tert-butyl-N-hydroxycarbamate are produced due to photocatalytic degradation of RB5 (Garg et al. 2016). These products are not carcinogenic and aromatic. Moreover, these products are not resistant to environmental conditions, so they easily break down further into smaller molecules. Lambda max (λmax) of RB5 is 590 nm. Figure 5 possesses only one peak at 590 nm which shows that no product is produced during catalysis which is absorbed in the UV-Visible range. All the aromatic and conjugated compounds absorb in this wavelength range. This favors the results obtained by Garg et al. (2016). Thus toxic dye is converted into less harmful substances via photocatalysis by micro particles, and this technology can be used for water treatment. Photocatalytic reduction of RB5 on SnO2 microrod surfaces was chosen as the model reaction (Zielińska et al. 2001; Kunz et al. 2002; El Bouraie & El Din 2016). The degradation of RB5 was studied by monitoring variation in absorbance at 590 nm wavelength using a UV-Visible spectrophotometer. The UV-Visible spectra of photocatalytic reduction of RB5 in the absence and presence of catalyst is shown in Figure 5(a) and 5(b) respectively. Spectra were scanned at different time intervals. It is observed from Figure 5(a) that absorbance at 590 nm does not decrease, even after 120 min, which indicates that the dye is not degraded, while Figure 5(b) shows that at the start of catalysis, absorbance at 590 nm was 0.48. However, with the passage of time, absorbance at 590 nm decreases. This shows that he concentration of dye is continuously decreasing in the mixture which confirms that the catalyst is performing its role and dye degradation does not occur in the absence of catalyst. The absorbance sharply decreases until 30 min after sunlight irradiation. After 30 min, absorbance at 590 nm decreases slowly with time which indicates that the degradation reaction is near completion. The color of RB5 is changed from dark blue to light blue which also reveals that the concentration of dye is decreased in the reaction mixture.
Effect of catalyst dosage on catalytic degradation of RB5 dye
Figure 6 shows the plot of ln (At/Ao) versus time for various catalyst dosages (0.35, 0.69 and 1.0 mg/mL). In all graphs at the start of the reaction the change in values of ln (At/Ao) is very fast and gives linear behavior. This linear portion of the plots is used to calculate the values of kaap, however, after some time these values decrease slowly. The slope of these plots is different, which suggests that the degradation rate is not the same under different dosages of catalyst. The small value of ln (At/Ao) at 0.35 mg/mL catalyst dosage suggests that the catalysis rate is very slow in the case of low concentrations of catalyst (0.35 mg/mL). The plot between kaap and catalyst dosage is shown in Figure 7. It is seen that the values of kaap increase by increasing the catalyst dosage. The value of kapp is found to be 0.0138, 0.0158 and 0.0175 min−1 at 0.35, 0.69 and 1.00 mg/mL catalyst dosages respectively (Table S1, available with the online version of this paper). It is observed that the value of kapp increases sharply with an increase in catalyst dosage from 0 to 0.35 mg/mL. A further increase in concentration of catalyst from 0.35 to 1.00 mg/mL does not largely affect the value of kaap. Actually, the catalyst provides active sites for the adsorption of dye molecules. As the concentration of catalyst dosage increases, it causes an increase in available active sites, so a greater number of dye molecules can adsorb on the surface of the catalyst and the rate of photocatalytic degradation increases.
Effect of concentration of H2O2 on degradation of RB5
Figure S1 (available online) shows the plot of ln (At/Ao) versus time for different concentrations of H2O2 (0.1, 0.4 and 0.7 M). It can be seen from the plot that at the start of the reaction the value of ln (At/Ao) decreases very fast and shows linear behavior. However, after some time the value of ln (At/Ao) decreases slowly with time in the case of 0.1 M concentration of H2O2. The plot between kaap and various concentrations of H2O2 is shown in Figure 8. This plot shows that the kapp reaction increases by increasing the concentration of H2O2. As H2O2 concentration increases from 0.1 to 0.7 M, the value of kaap also increases from 0.0158 to 0.0266 min−1. Figure 8 also shows that the value of kapp does not linearly increase with an increase in concentration of H2O2. H2O2 donates oxidizing species hydroxyl radical (·OH) which contributes to the degradation process. By increasing the concentration of the H2O2 amount of oxidizing species, ·OH increases which helps to rapidly degrade dye molecules and catalysis rate increases.
Percentage conversion of RB5
The comparison of percentage conversion of RB5 at various concentrations of H2O2 (0.1, 0.4 and 0.7 M) and catalyst (0.35, 0.69 and 1.0 mg/mL) is shown in Figure 9. It is clear from Figure 9 that at low concentrations of H2O2 (0.1 M) and catalyst (0.35 mg/mL) the percentage conversion of RB5 is also low (72%). However, as the concentration of H2O2 increases from 0.1 to 0.4 M, the percentage conversion of dye also increases (76%) and the dye shows maximum percentage conversion (82%) at higher concentrations (0.7 M) of H2O2. The same behavior is seen in the case of catalyst dosage. By increasing the amount of catalyst from 0.35 to 0.69 mg/mL, percentage conversion of RB5 increases to 81% and it reaches the maximum value of 84% at higher catalyst dosages (1.0 mg/mL). The data of percentage conversion of RB5 under studied conditions of H2O2 and catalyst dosage show that maximum degradation occurs when 0.7 M H2O2 and 1.0 mg/mL catalyst is used. This also shows that the catalyst has the ability to rapidly degrade the dye with high percentage conversion. Researchers used various semiconductor materials for the RB5. Lucas et al. (2013) achieved 91% degradation efficiency of RB5 by using TiO2 coated magnetic nanoparticles. Wong et al. (2015) used the reduced graphene oxide for the reduction of RB5 and they achieved 49% decolonization of RB5 under UV irradiation. Soo et al. (2016) used the Fe-doped mesoporous TiO2 for the degradation of RB5. From the literature it is seen that no one used the SnO2 microparticles for the reduction of RB5. So in this work, the catalytic effect of SnO2 microrods for the reduction of RB5 from wastewater is investigated.
SnO2 microrods are synthesized by the co-precipitation method followed by calcination at 600 °C temperature. XRD analysis confirms the formation of high crystalline SnO2. It also reveals that every tin atom is bonded to six oxygen atoms. Oxygen atoms are present at the octahedral position around the tin central atom. Morphological analysis confirms the formation of microrods whose diameter and length are in the range of 6–8 and 0.9–1.7 μm, respectively. Degradation of RB5 is studied to check the photocatalytic behavior of synthesized SnO2 microrods. The apparent rate constant (kapp) values increase by increasing the H2O2 concentration and amount catalyst. Percentage conversion of RB5 is also calculated which revealed that at low H2O2 concentration (0.1 M) and catalyst dosage (0.35 mg/mL), percentage degradation of dye is also small (72%). However, as the dosage of catalyst increases from 0.35 to 1.0 mg/mL, the percentage degradation of dye also increases from 72 to 84%. Similarly, the percentage removal of dye from the reaction mixture increases from 72%, and 82% with an increase in concentration of H2O2 from 0.1 to 0.7 M. These results indicate that synthesized SnO2 microrods possess good photocatalytic properties.
The authors are very grateful to the Department of Chemistry, University of Agriculture, Faisalabad 38000, Pakistan for financial assistance.