Photocatalytic Oxidation of Carbon Monoxide over NiO/SnO_𝟐 Nanocomposites under UV Irradiation

Abstract

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.

1. Introduction

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 [1]. 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 [2], ZnS/TiO₂ [3], Cu₂O/TiO₂ [4], and CuO/TiO₂ [5, 6], WO₃/SiCeTiO₂ [7], SrTiO₃/TiO₂ [8], SnO₂/TiO₂ [9], Ta₂O₅/TiO₂ [10], CuO/Al₂O₃/TiO₂ [11] 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) [1].

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 [15].

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 (mol%=𝑛_NiO/𝑛_SnO2).

2.2. Characterization

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 K𝛼 radiation and a power of 40 kV × 30 mA. The intensity data were collected at 25°C over a 2𝜃 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%Removalefficiency=𝐶⁰−𝐶𝐶₀×100,(1) where 𝐶₀ the original CO content, 𝐶 the retained CO.

Figure 1 

Schematic representation of photoreactor.

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 [16], 𝐷=0.9𝜆/𝛽cos𝜃, where 𝜆 is the wavelength of X-ray radiation, 𝛽 is the full width at half maximum (FWHM) of the peaks at the diffracting angle 𝜃. 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.

Table 1 

Variation of particle size, lattice parameter and band gap with dopant concentration.

Figure 2 

XRD patterns of pure and nickel-doped SnO2 nanoparticles.

Figure 3 

Variation of particle size and lattice parameter with nickel concentration.

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 (𝑃/𝑃⁰) 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 𝑆_BET 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 [17].

Table 2 

Texture parameters of pure and nickel-doped SnO₂ nanoparticles.

(a) Pure SnO2

(b) NiO/SnO2 (5 mol%)

(c) NiO/SnO2 (10 mol%)

(d) NiO/SnO2 (33.3 mol%)

(e) NiO/SnO2 (50 mol%)

Figure 4 

N2 sorption isotherms of pure and nickel-doped SnO2 nanoparticles.

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 𝛼ℎ𝜈=𝐴ℎ𝜈−𝐸_𝑔^1/2,(2) where 𝛼 is the absorption coefficient, 𝐴 is a constant. 𝑛=1/2 for direct band gap semiconductor. An extrapolation of the linear region of a plot of (𝛼ℎ𝜈)² versus ℎ𝜈 gives the value of the optical band gap 𝐸_𝑔 (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 [18]. This can be attributed to the quantum confinement effect of the nanoparticles [19]. 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 [20]. However, there is no clear understanding of this phenomenon. A direct-indirect transition have been proposed by Rakhshani et al. [21] 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) [24]. 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.

Figure 5 

UV-V is absorption spectrum of pure and nickel-doped SnO2 nanoparticles.

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.

Table 3 

NiO/SnO₂ atomic % ratio of the produced samples measured by EDX Technique at three areas.

(a)

(b)

(c)

(d)

(e)

Figure 6 

SEM images of the typical morphology and composition of pure (a), 5, 10, 33.3, and 50% Ni doped ((b), (c), (d), and (f), resp.) SnO2 nanoparticles.

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.

(a)

(b)

Figure 7 

TEM images of (a) pure and (b) 33.3 mol% nickel-doped SnO2 nanoparticles.

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 [26]. As a result, the photocatalytic oxidation of CO is lowered due to inefficient excitation.

Figure 8 

Photooxidation of CO over pure and nickel-doped SnO2 nanoparticles.

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.

Figure 9 

Photooxidation of CO over different loading from 33.3 mol% NiO/SnO2 calcined at 550°C Figure 10. Kinetic for photooxidation of CO over loading from 33.3 mol% NiO/SnO2 calcined at 550°C.

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: Log[𝐶]_𝑡=−𝑘𝑡+Log[𝐶]_𝑜,(3) where [𝐶]_𝑡 and [𝐶]_𝑜 represent the concentration (ppm) of the substrate in solution time zero and time 𝑡 of illumination, respectively, and 𝑘 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 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.

Table 4 

Apparent rate constant (𝑘) at different loading of 33.3 mol% NiO/SnO₂.

Figure 10 

The first-order rate equation for CO given by: 𝑅=𝑘[CO].(4)

4. Conclusions

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.

Acknowledgments

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.