Research Article | Open Access
Photocatalytic Oxidation of Carbon Monoxide over Nanocomposites under UV Irradiation
The NiO/SnO2 nanocomposites have been prepared by the simple coprecipitation method and further characterized by the XRD, SEM, TEM, UV-Vis, and BET. X-ray diffraction (XRD) data analyses indicate the exclusive formation of nanosized particles with rutile-type phase (tetragonal SnO2) for Ni contents below 10 mol%. Only above 10 mol% Ni, the formation of a second NiO-related phase has been determined. The particle size is in the range from 12 to 6 nm. It decreases with increasing amounts of doping NiO. The morphology of NiO-doped SnO2 nanocrystalline powders is spherical, and the distribution of particle size is uniform, as seen from transmission electron microscopy (TEM). The photocatalytic oxidation of CO over NiO/SnO2 photocatalyst has been investigated under UV irradiation. Effects of NiO loading on SnO2, photocatalyst loading, and reaction time on photocatalytic oxidation of CO have been systematically studied. Compared with pure SnO2, the 33.3 mol% NiO/SnO2 composite exhibited approximately twentyfold enhancement of photocatalytic oxidation of CO. Our results provide a method for pollutants removal. Due to simple preparation, high photocatalytic oxidation of CO, and low cost, the NiO/SnO2 photocatalyst will find wide application in the coming future of photocatalytic oxidation of CO.
The wide variety of electronic and chemical properties of metal oxides makes them exciting materials for basic research and technological applications alike. Metal oxides span a wide range of electrical and optical properties from wide band gap insulators and lasers to metallic, superconducting, and field-emitting materials. Enormous efforts are being directed towards the development of nanometer-sized metal oxides in studies related to their fundamental mechanisms such as size effect and quantum effect and towards the applications of these materials. Particle sizes in the nanoregime and specific crystal morphologies are expected to enhance the performance and allow the fine tuning of the properties of these materials. The optical properties of semiconducting nanoparticles have recently been a subject of great interest. Tin oxide (SnO2) is a versatile wide band gap (3.6 eV at 300 K) n-type semiconducting oxide with a wide variety of applications.
Semiconductors with wider band gaps exhibit better stability than those with small and medium band gaps, but show lower light-harvesting ability in visible light . Therefore, coupling of semiconductors with different band gaps is good approach to prepare photocatalysts with high activity and good stability. The excited electrons can transfer in coupled semiconductors from the high conduction band to the low one, leading to efficient separation of photogenerated electron-hole pairs.
Enhancements in photocatalytic activity over coupled semiconductors including CdS/TiO2 , ZnS/TiO2 , Cu2O/TiO2 , and CuO/TiO2 [5, 6], WO3/SiCeTiO2 , SrTiO3/TiO2 , SnO2/TiO2 , Ta2O5/TiO2 , CuO/Al2O3/TiO2  have been investigated extensively.
The SnO2 exhibits good activity and stability under irradiation in both acidic and basic media. However, the pure SnO2 shows much lower photocatalytic activity even under UV irradiation due to its large band gap (3.8 eV) .
To improve its photocatalytic activity, it is necessary to couple SnO2 with another semiconductor with lower band gap. The NiO is a p-type semiconductor with a small band gap. If the SnO2 is coupled with the NiO, the n-SnO2/p-NiO heterojunctions can be formed in the interface. The photogenerated electrons from SnO2 can easily migrate to NiO. This favors the separation of photogenerated electrons with holes, leading to enhancement of photocatalytic activity. The CuO/SnO2 nanocomposites have been studied as gas sensor materials [12–14] or electrochemical materials .
To our knowledge, there is a little information regarding photocatalytic oxidation of CO using NiO/SnO2 nanocomposite. This prompts us to synthesize and to investigate the photocatalytic performance of NiO/SnO2 nanocomposite. The objective of this study is to a highly efficient, cost-effective NiO/SnO2 photocatalyst. The NiO/SnO2 photocatalysts have been prepared by simple coprecipitation method and characterized by XRD, SEM, UV-vis, TEM, and BET. Effects of different variables on photocatalytic oxidation of CO have been investigated in detail, including NiO content, photocatalyst loading, and reaction time.
2. Experimental Method
2.1. Preparation of NiO/SnO2 Nanocomposite
The NiO/SnO2 composites have been prepared by the simple coprecipitation method. SnCl4·5H2O and NiSO4·5H2O are used as starting materials. Typically, SnCl4 and NiSO4 with a desired molar ratio are mixed together in distilled water. The obtained solution is continuously stirred at 80°C; meanwhile, the NaOH solution is added with at a rate of 0.02 mL/s until complete precipitation (solution pH value above 11). After continuous stirring for another 2 h, the precipitate is filtered and washed thoroughly with distilled water until free of Cl ion and SO4 ion. Finally, the precipitate is dried at 100°C for 24 h and calcined at 550°C for 5 h to obtain NiO/SnO2 nanocomposite. The NiO content in final composite is calculated by the concentration of NiSO4 solution used during preparation. The NiO/SnO2 is denoted by the molar percentage of NiO in photocatalyst ().
The structure of the catalyst was examined by X-ray diffraction (XRD) on a Rigaku X-ray diffractometer system equipped with as RINT 2000 wide angle Joniometer using Cu K radiation and a power of 40 kV × 30 mA. The intensity data were collected at 25°C over a 2 range of 10–80°. The UV-vis diffuse reflectance absorption spectra were recorded with a Shimadzu UV-2450 at 295 K. N2-adsorption measurement was carried out at 77 K using Nova 2000 series, Chromatech. Prior to analysis, the samples were outgassed at 250°C for 4 h. The morphology and particle size of the prepared samples were examined via a transmission electron microscope (Hitachi H-9500 operated at 300 kv).
2.3. Photocatalytic Activity Tests
In this section, we used quartz photoreactor as shown in Figure 1. UV irradiation was performed by a 150 W medium pressure xenon lamp placed inside a quartz jacket and equipped with a cooling tube. Photooxidation reactions were carried out suspending 0.2 g of the prepared catalyst. A feed gas of ca. 400 ppmV CO was made up of CO (purity 99.99%) and air and was stored in a high-pressure cylinder. Samples were withdrawn at regular intervals from the upper part of the reactor. The Photooxidation rate was determined by measuring the CO consumed using a gas chromatograph Shimadzu GCMS-QP 5050A and a gas-sampling valve. The removal efficiency of CO has been calculated by applying where the original CO content, the retained CO.
3. Results and Discussions
3.1. Characterization of NiO/SnO2 Photocatalyst
3.1.1. XRD Analyses
The typical XRD patterns of the pure and Ni-doped SnO2 samples annealed at 550°C are shown in Figure 2. The peak positions of each sample exhibit the rutile type tetragonal structure of SnO2 which were confirmed from the ICDD card no. 77-0452. Further, no other impurity peak was observed in the XRD pattern showing the single phase sample formation. The crystalline size of all the samples was calculated using Scherer formula , , where is the wavelength of X-ray radiation, is the full width at half maximum (FWHM) of the peaks at the diffracting angle . The calculated particle sizes of each sample are given in Table 1. It can be observed from Table 1 that the crystalline size of SnO2 decreased from 5 nm to 2 nm when Ni2+ content increased from 0% to 50%. The data revealed that the presence of Ni2+ ions in SnO2 prevented the growth of crystal grains. The variations in the lattice parameter have also been studied for different doping concentrations and presented in Figure 3. The ionic radius of Ni2+ is 69 pm, whereas that of Sn4+ is 71 pm. The Ni ions substitute the Sn4+ ions in the crystal due to comparable ionic radius. However, the decrease in the lattice parameter may be due to the smaller ionic radius of Ni ions.
3.1.2. Specific Surface Area Trends
The surface parameters of surface area and the data calculated from the t-plot were estimated by the low-temperature nitrogen adsorption at relative pressures () in the range of 0.05–0.9 and are given in Table 2. The N2 adsorption isotherms for pure and nickel-doped SnO2 nanoparticles (Figure 4) are typical of type II. Specific surface area measurements of SnO2-based powders show that nickel additives drastically influence the morphology of the powders. Table 2 summarizes data and is an evidence that the addition of Ni increases the surface area, because The addition of NiO prevents the densification and growth of SnO2 grain and consequently results in more porous micro structure than pure SnO2 .
|: BET surface area. |
: surface area derived from plots.
: mean pore radius.
: total pore volume.
(a) Pure SnO2
(b) NiO/SnO2 (5 mol%)
(c) NiO/SnO2 (10 mol%)
(d) NiO/SnO2 (33.3 mol%)
(e) NiO/SnO2 (50 mol%)
3.1.3. Optical Properties
Absorption is powerful nondestructive technique to explore the optical properties of semiconducting nanoparticles. The optical absorption spectra of pure and Ni doped SnO2 nanoparticles are shown in Figure 5. The absorption edge of different samples varies, as the concentration of Ni in the SnO2 nanoparticles varies. In order to calculate the direct band gap we used the Tauc relation where is the absorption coefficient, is a constant. for direct band gap semiconductor. An extrapolation of the linear region of a plot of versus gives the value of the optical band gap (Table 1). The measured band gap was found to be 3.63 eV for undoped SnO2 nanoparticles, which is similar to the reported value of the bulk SnO2, that is, 3.6 eV . This can be attributed to the quantum confinement effect of the nanoparticles . On doping with nickel, the band gap energy decreases (Table 1) even though the particle size decreases. This is in contrast to the normal phenomenon of quantum confinement. Chun-Ming et al. have already reported band gap narrowing effect for doped SnO2 nanoparticles . However, there is no clear understanding of this phenomenon. A direct-indirect transition have been proposed by Rakhshani et al.  In order to explain the band gap narrowing effect, many groups have suggested that alloying effect of parent compound with some impurity phases may be responsible for the band gap narrowing [20, 22, 23]. In one hand, the alloying effect from SnO2-NiO can be neglected, because the band gap decreases below the band gap energy of NiO (3.54 eV) . The samples containing up to 10% Ni concentration, SnO2-SnO2-x alloying effect may be responsible for the band gap narrowing effect. For the SnO2 nanoparticles above 10% Ni concentration, there is a huge drop in the band gap. This may be due to the formation of subbands in between the band gap, and the conduction band and subbands are merging with the conduction band to form a continuous band.
3.1.4. SEM and TEM
Figure 6 shows the typical morphology and composition of pure and Ni-doped SnO2 nanoparticles. Figure 6 shows the presence of large spherical aggregates of smaller individual nanoparticles and the presence of Ni is confirmed from the selective area EDAX analysis (Table 3). It can be verified from the results of XRD and EDAX that the Ni is successfully doped in the SnO2 nanocrystals.
Figures 7(a) and 7(b) show TEM images taken for pure and 33.3% Ni-doped SnO2 nanoparticles, respectively. Powder samples were dispersed in ethanol and sonicated in an ultrasonic bath for fifteen minutes for TEM analysis. It is observed from Figure 7 that SnO2 grains have a spherical morphology with an average diameter of 12 nm for pure SnO2 and 6 nm for 5% Ni-doped SnO2, confirming the reduction in particle size as a result of Ni doping in SnO2. Particle size obtained from TEM analysis is slightly higher than the crystallite size calculated from XRD spectra.
3.2. Photocatalytic Activity
3.2.1. Effect of NiO Content on Photocatalytic Oxidation of CO
An increase in the lifetime of photogenerated electron-hole pairs in coupled oxides, due to electrons transfer between the two coupling oxides, seems to be critical to the enhancement for photocatalytic oxidation of CO. In consequence, the NiO content in NiO/SnO2 nanoparticles plays an important role for photocatalytic oxidation of CO. Figure 8 shows effect of NiO content in composites on photocatalytic oxidation of CO. As illustrated in Figure 8, the pure SnO2 exhibited very poor activity for photocatalytic oxidation of CO. With the increment of NiO amount from 0.0 to 10 mol%, the photocatalytic oxidation of CO increases sharply. It increases slightly when NiO content is varied from 10 to 33.3 mol%. However, further increment of NiO to 50 mol% leads to a progressive decrease in photocatalytic oxidation of CO. The optimum NiO content in NiO/SnO2 composites is about 33.3 mol%. Compared with pure SnO2, the 33.3 mol% NiO/SnO2 composite exhibits approximately twentyfold enhancement of photocatalytic oxidation of CO. NiO or SnO2 alone shows poor photocatalytic activity for photocatalytic oxidation of CO, suggesting that the coexistence is responsible for the enhancement of photocatalytic activity. Since the conduction band of NiO is lower than that of SnO2, excited electrons from SnO2 can easily transfer to NiO. Accumulation of excess electrons leads to a negative shift in the Fermil level of NiO [25, 26]. Thus, the NiO on the SnO2 acts as active sites, where the photocatalytic reactions take place. The efficient interface electrons transfer inhibits the quick recombination of the photogenerated electron-hole pairs on SnO2 surface, leading to enhancement of photocatalytic activity of NiO/SnO2 composites. There exists an optimal amount when noble metals or metal oxides are loaded on TiO2 surface [26–28]. Similar dependence has been observed when the NiO is loaded on the SnO2 surface. When NiO content is below optimal level, the number of active trapping sites at NiO/SnO2 interface is enhanced with the increment of NiO amount, consequently resulting in an increase in photocatalytic oxidation of CO. However, when NiO content is above optimal level, the light sensitization of SnO2 is blocked by the excess surrounding NiO, resulting in a lower density of the excited electrons accumulation on NiO . As a result, the photocatalytic oxidation of CO is lowered due to inefficient excitation.
3.2.2. Effect of Photocatalyst Concentration
Apart from NiO content, the photocatalyst concentration is also an important factor for the photocatalytic oxidation of CO. To quantify the dependence of CO oxidation on photocatalyst concentration, the photocatalytic oxidation of CO at various catalyst concentrations have been investigated (Figure 9). As shown in Figure 9, with the increment of photocatalyst concentration from 0.1 to 0.2 g, the photocatalytic CO oxidation is gradually increased. However, it is progressively decreased when photocatalyst concentration is changed from 0.2 to 0.35 g. A similar dependence has been reported with the use of Pt/TiO2 as photocatalyst, which is typical for reaction occurring in suspension [29, 30]. The photocatalytic reactions in suspension are determined by trapping sites on photocatalyst surface and by the light transmission in suspension. At a low photocatalyst concentration, the rate of photocatalytic reactions is mainly limited by the number of trapping sites which is increased with the increment of photocatalyst concentration. However, when photocatalyst concentration is above optimal level, the suspension becomes turbid. The light is unable to penetrate the suspension due to scattering by the suspended photocatalyst particles, leading to a sharp decrement in photocatalytic CO oxidation.
The reaction order with respect to CO was determined by plotting reaction time versus log[CO] according to the following equation for various 33.3 mol% NiO/SnO2 loading: where and represent the concentration (ppm) of the substrate in solution time zero and time of illumination, respectively, and represent the apparent rate constant (min−1). The findings are represented in Figure 10, and the apparent rate constants are summarized in Table 4. The results show that the reaction followed first-order kinetics with respect to CO and the rate constants were ranged from to min−1 by using loading from 33.3 mol% NiO/SnO2 catalyst range from 0.1 to 0.35 gm, respectively.
The first-order rate equation for CO given by:
The NiO/SnO2 nanocomposites have been prepared by the simple coprecipitation method and further characterized by the XRD, SEM, TEM, UV-vis, and BET. The XRD patterns show that the prepared samples are rutile in structure with ≤5 nm in size. No impurity phase has been observed in XRD. The crystallanity, particle size, and lattice constant are decreasing with the increase in nickel concentration. The band gap of the doped samples show a narrowing effect as measured from the Tauc relation. The intensity of visible emission increases as the dopant concentration increases. Thus, the nickel doping can be used as a method to control the band gap and visible luminescence of the SnO2 nanoparticles. The morphology of NiO/SnO2 nanocomposites is spherical, and the distribution of the particle size is uniform as seen from EDX and TEM. The NiO/SnO2 nanocomposites exhibits relatively high activity for photocatalytic oxidation of CO. Effects of NiO content and photocatalyst loading have been studied in detail. The reaction order with respect to CO was determined, The results show that the reaction followed first-order kinetics with respect to CO, and the rate constants were ranged from 134 × 10−4 to 469 × 10−4 min−1 by using loading from 33.3 mol% NiO/SnO2 catalyst range from 0.1 to 0.35 gm, respectively. Due to simple preparation, high photocatalytic oxidation of CO and low cost, the NiO/SnO2 photocatalyst will find wide application in the coming future of photocatalytic removal of pollutants.
The authors acknowledge with pleasure the financial support of this Project no. (210-247/431) by the Deanship of Scientific Research. They also acknowledge with pleasure the Deanship of Scientific Research, King Abdul-Aziz University, for the continuous guidance and support during the period of the project. They acknowledge deeply with pleasure all the partners that helped and encouraged this work at the Department of Chemistry, Faculty of Science and Department of Science, Faculty of Education at King Abdul-Aziz University.
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Copyright © 2012 R. M. Mohamed and Elham S. Aazam. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.