Research Article | Open Access
Characterization and Photocatalytic Activity of Enhanced Copper-Silica-Loaded Titania Prepared via Hydrothermal Method
TiO2 nanopowder, loaded with SiO2 and Cu-SiO2, was prepared under both acidic and basic environments via the hydrothermal method. The morphology and structure of TiO2 were studied by XRD, TEM, and FT-IR. The photocatalytic activity of samples was studied by monitoring the degradation of methyl orange, using a UV-visible spectrophotometer. The effect of Ti/Si ratio, pH, and Cu2+ addition on the formation of TiO2 and its photocatalytic activity was investigated in detail. The results show that a large surface area and a high surface acidity were important factors to achieve good TiO2 performance. The presence of Ti-O-Si bonding enhanced surface acidity, which improved its ability to adsorb more hydroxyl radicals and increased its surface area. The addition of 0.1 mol% concentration of Cu2+ and 25 mol% SiO2 in TiO2 induced the formation of new states close to the conduction band, which narrowed the band gap energy and enhanced the photodegradation efficiency.
In recent years, cases of air and water pollution have been increasing due to the continuous rise in population and urbanization. Improper waste management has been identified as one of the prime factors that contribute to the pollution. In 2003, the average amount of municipal solid waste (MSW) generated in Malaysia was 0.5–0.8 kg/person/day, which has further increased to 1.7 kg/person/day in major cities . Solid waste management continues to be a major challenge throughout the world, particularly in rapidly growing cities and towns .
Land filling and incineration are two of the most common methods of solid waste disposal. Unlike land filling, incineration requires minimum land, reduces the volume of solid waste by half, and can be operated in any weather. Nevertheless, emission of pollutants, especially dioxin and furan, is the major drawback of incineration. Among the various solutions available, photocatalysis is considered to be a green technique that has great potential to decompose contaminants without leaving harmful intermediates [3–5]. Choi et al. reported that the polychlorinated dibenzo-p-dioxins such as mono-, tetra-, hepta-, and octachlorinated congeners were successfully degraded by a TiO2 film under UV or solar light irradiation .
Titanium dioxide is the most promising photocatalyst due to its superior properties such as low cost, environmental compatibility, and long-term photochemical stability [7–10]. However, its wide band gap energy (3.0 for rutile; 3.2 for anatase) limits its use as a photocatalyst in various applications. The large surface area, high crystallinity, low crystallite size, and crystal structure are important properties that influence the photocatalytic activity of TiO2. In recent years, much effort has been directed toward TiO2 modification to enhance its photocatalytic activity [11–15]. For instance, TiO2 loaded with various secondary oxides, such as ZrO2 , WO3, MnO2, CuO, V2O5, and Al2O3  and transition metals salt such as Cu2+  and Fe2+ [19, 20], has been reported to be a more efficient photocatalyst than pure TiO2. The composite TiO2-SiO2 has attracted great interest because of its ability to prevent the grain growth of TiO2 particles and enhance the thermal stability for the phase transformation of TiO2 from anatase to rutile [21–27]. Xu et al.  studied the effect of a wide range of SiO2 levels added to the microstructure and the photocatalytic activity of TiO2 powder; preparation involved the sol-gel method and calcination at various temperatures. Addition of SiO2 resulted in a marked reduction in grain size and an increase in the surface area of the catalyst. Results of other studies are summarized in Table 1. Although many reports describe the effect of SiO2 on the photocatalytic activity of TiO2, the results do not agree with one another, possibly due to differences in the processing methods or experimental conditions. On the other hand, Chen et al.  has been reported that 0.1 wt% of Cu2+-doped TiO2-SiO2 showed higher photocatalytic activity than that of undoped TiO2-SiO2. The effects of pH value on pure TiO2 formation have been reported by several groups [28, 29]. Yu et al. prepared TiO2 powder at different pH values through the hydrothermal method . They found that the pH value of the starting solution has a significant effect on the crystallinity, crystallite size, phase structure, and photocatalytic activity of the synthesized TiO2 powder. They proposed that basic conditions are favorable for the formation of pure anatase. Samples prepared at pH 9 display higher photocatalytic activity. However, this result contradicts those obtained by Karami , who concluded that TiO2 prepared under acidic conditions (pH 3) through the sol-gel method has higher photocatalytic activity.
|A: anatase, R: rutile, MB: methyl blue, MO: methyl orange.|
Although the sol-gel method is widely used due to its simplicity and low cost, calcination in air is required for the formation of the anatase phase from amorphous TiO2. Li et al.  did a comparative study of the sol-gel and hydrothermal methods for the synthesis of TiO2-SiO2 composite nanoparticles and found that samples prepared through the hydrothermal route still possess a stable anatase phase, a large specific surface area, a small particle size, and a high photocatalytic activity even when calcined at 1000°C. However, they did not report the effect of pH on the formation of TiO2-SiO2 nanoparticles. In the present study, the effect of pH values on photocatalytic activity and the optimum Ti/Si ratio for photocatalytic activity of TiO2 powder prepared by hydrothermal method were examined. The effect of Cu-SiO2 addition on the phase structure and the photocatalytic activity of TiO2 were also studied.
2. Experimental Procedure
Initially, the oxide powder TiO2-SiO2 with various Ti/Si ratios at pH 3 and 9 were prepared, and the photocatalytic performance was evaluated to select the optimum Ti/Si ratio. The Ti/Si ratios and pH values in the experiment are summarized in Table 2. Tetrabutylorthotitanate (TBOT) with a normal purity of 97% (Aldrich, US) and tetraethylorthosilicate (TEOS) with a normal purity of 98% (Merck, Germany) were used as the source of titanium and silicon, respectively. Both were dissolved in equal volumes of ethanol, after which a few drops of hydrochloric acid were added to TEOS as catalyst. Afterward, the mixture was kept in a water bath maintained at 70°C for 2 h. The TEOS and TBOT precursors were then mixed and stirred for 15 minutes before adding the desired amount of copper(II) nitrate. The pH values were adjusted by adding hydrochloric acid for the acidic condition and sodium hydroxide for the basic condition. This solution was stirred for 1 h at room temperature and placed in a Teflon-lined stainless steel autoclave, in which it was heated at 150°C for 24 h, and then cooled to room temperature. The sample obtained was washed and then dried at 100°C. A powder was obtained.
|SA: acid, SB: basic, SN: neutral.|
2.2. Characterization and Photocatalytic Degradation
The powder samples were characterized by X-ray diffraction (XRD) using a Bruker D8 powder diffractometer employing Cu Kα radiation. The accelerating voltage and the applied current were 40 kV and 40 mA, respectively. The average crystallite size of the TiO2-SiO2 oxide powder was calculated using X-ray line broadening methods based on the Scherrer formula. Additionally, the morphology of the powder was observed by transmission electron microscopy (TEM) using a Philips 420 T. The chemical structure information of the particles was obtained by Fourier transform infrared spectroscopy (Spectrum One, Perkin Elmer, US). Photocatalytic degradation studies were performed using 30 mg/L methyl orange (MO) solution. A total of 0.1 g TiO2-SiO2 oxide powder was added to 30 mL of MO solution and photoirradiated for 1 h at room temperature using a TUV 18W UV-C Germicidal light. The concentration of the degraded MO was determined using a UV-visible spectrophotometer (Varian, Cary 50 Conc).
3. Results and Discussion
3.1. Effect of SiO2 Content
XRD was used to analyze the formation of the crystalline phase of the TiO2 powder prepared with various amounts of SiO2 in both acidic and basic environments. Figure 1 illustrates the XRD patterns of TiO2 prepared at pH 3 with various Ti/Si ratios. The TiO2 prepared at various Ti/Si ratios led to formation of the anatase phase. Patterns of samples without SiO2 (SA0) shows peak for anatase and a small peak for the brookite phase at a diffraction angle of~20°. The relative intensities of the anatase diffraction peaks in each sample are different; this suggests that the degree of crystallinity is affected by the SiO2 content. A high crystallinity of TiO2 was obtained with addition of 30 mol% SiO2 (SA30), whereas low crystallinity was observed in samples with 25 mol% SiO2 (SA25). No SiO2 crystal phase was identified in all samples. This indicates that SiO2 existed as an amorphous phase in the TiO2-SiO2 composite.
The formation of brookite phase in sample SA0 (acidic condition) could be explained as follows : Ti(OH)x(OC4H9)4-x forms as a result of hydrolysis reaction between TBOT and HCl, where x was related to the pH value of starting solution. Since the ligand field strength of Cl− ions was larger than that of butoxy group in HCl solution, thus the Cl− ions could substitute the butoxy group in the Ti(OH)x(OC4H9)4-x complex and led to the formation of complex Ti(OH)2Cl2. The complex Ti(OH)2Cl2 actually existed in the form of Ti(OH)2Cl2(H2O)2 in a solution due to the Ti(IV)(3d0) complex ions are all octahedrally coordinated in solution and crystal. It is reported that Ti(OH)2Cl2(H2O)2 could be the precursor of brookite. This mechanism was favored to explain the occurrence of brookite in the sample.
The XRD patterns of TiO2 prepared at pH 9 with various Ti/Si ratios (not shown here) shows samples prepared without SiO2 (SB0) produced peaks characteristic of anatase at 2θ of 24, 34, 39, and 48°, and the peak of rutile at 28°. However, the peak intensity of this sample is very low compared to those produced by samples prepared under acidic conditions; this indicates that the phase transformation to anatase or rutile was not fully achieved at basic conditions. In contrast, samples prepared with SiO2 generally showed amorphous TiO2. In basic conditions, Na+ attacks the Ti-O-Ti bond, which results in the formation of a two-dimensional layered structure with dangling bonds that contribute to the formation of the amorphous phase. This is in good agreement with the TEM analysis which is discussed later.
The crystallite size of the TiO2-SiO2 powders prepared in acidic condition was calculated using Scherrer formula and plotted against SiO2 content. Figure 2 shows a nonlinear relationship between the crystallite size and the amount of added SiO2. The trend is contrary to results in the literature wherein SiO2 addition was found to retard grain growth of TiO2 and, therefore, reduce its crystallite size [23, 24, 26]. In the present study, the lowest crystallite size was 6.3 nm for pure TiO2 (SA0), and increased gradually with further addition of SiO2. However, the crystallite size of TiO2 for sample SA25 decreased drastically as the SiO2 amount reached 25 mol%; this may be attributed to the large surface area of the sample. Binary metal oxides are known to induce surface acidity . The FTIR spectra of the TiO2-SiO2 powder at various SiO2 contents are shown in Figure 3. When the SiO2 content reached 25 mol%, a small peak at 960 cm−1 corresponding to the Ti–O–Si bond was observed. The presence of this oxide might have increased the surface acidity of the sample. Higher surface acidity led to a higher degree of adsorption of the OH radicals (1640 cm−1) and resulted in a larger surface area. The surface area was inversely proportional to grain size. The Ti-O-Si band intensity increased with addition of 30–50 mol% SiO2, and the intensity of bands for the hydroxyl group decreased. These indicate that large amounts of SiO2 (30–50 mol%) in TiO2 cannot effectively improve the surface acidity and prevent the growth of TiO2 grains. Therefore, in sample SA25, SiO2 effectively enhanced the surface acidity, suppressed the grain growth of TiO2,, and produced smaller crystallite size. The peak for the asymmetric stretching of Si-O-Si at 1060 cm−1 clearly increased with addition of SiO2, whereas the band intensity at 400–700 cm−1 (vibration of Ti-O bonds in Ti-O-Ti bonding) decreased. Therefore, the intensity of the peak for the Si-O-Si bond was inversely proportional to the intensity of the peak for the Ti-O-Ti bond.
3.2. Effect of pH Value
Figure 4 shows the XRD pattern of TiO2 powder with 25 mol% SiO2 (TiO2-25mol% SiO2) at various levels of pH. In the presence of SiO2, an acidic condition was favorable to form anatase structure. As the system is shifted from acidic to neutral, the intensity of the anatase peak (2θ = 25°) and the crystallite size increased. Thus, pH value also affected the degree of crystallinity despite the presence of SiO2. At pH 12, the crystal structure of sample SB25-12 could not correspond to anatase, rutile, or brookite. It was similar to that of Na2O7Ti3 , H20O24Si3Ti4, and H4Na4O16Si4Ti2 probably due to their same layered titanate family . Further increase in basic condition (pH 14) resulted in formation of the partial anatase phase, as seen in the TEM images of TiO2-25 mol% SiO2 powders prepared at various pH values in Figure 5. Spherical like particles (Figures 5(a) and 5(b)) were observed in samples prepared in acidic and neutral conditions. At pH 12, a lamellar structure (two-dimensional layered structures) formed as a result of the reaction between the sample and NaOH (Figure 5(c)), which involved attack of Na+ on the Ti-O-Ti bond. The edges of the lamellar structure might have many atoms with dangling bonds with enough energy to destabilize the two-dimensional structures . As the system shifted toward high pH, the lamellar TiO2 deformed to saturate the dangling bonds. At pH 14, the transition three → two → one dimension was almost complete, while the lamellar structure formed tubes (Figure 5(d)); this resulted in a partial anatase phase. Thus, the pH appeared to have a significant effect on TiO2 phase structure and morphology.
Figure 6 illustrates the FTIR transmission spectra of TiO2-25 mol% SiO2 powder prepared at various pH values and autoclaved at 150°C for 24 h. The spectrum of pure TiO2 is also included as reference. The broad peaks at 3400 cm−1 and the peaks at 1640 cm−1 in all spectra are attributed to surface-adsorbed water and the bending mode of hydroxyl groups. The surface-adsorbed water and hydroxyl group decreased slightly as the pH increased; this is possibly due to the reduction in surface area of the sample, which prevented further water vapor absorption. This is consistent with the results wherein samples prepared at pH 3 had higher photocatalytic activity compared with those prepared at pH 7. The peaks, which were absent in pure TiO2, appeared at 960 cm−1 and 1040–1070 cm−1 due to the Ti-O-Si stretching and the asymmetric stretching vibration of Si-O-Si, respectively. These peaks gradually decreased and eventually disappeared at higher pH. This implies that the Ti-O-Si bonds weakened as the pH increased. The stretching vibration of Ti-O bonds in Ti-O-Ti can be observed at 400–700 cm−1. Peaks related to carboxyl groups were observed at the 1340–1470 cm−1 range. The carboxyl groups might be a result of oxidation of organic species during hydrothermal treatment . As the preparation conditions shifted from acidic to basic, the oxidation process was inhibited; this caused the peaks to disappear completely.
3.3. Effect of Cu2+ Addition in TiO2-SiO2 Powder
The XRD patterns of Cu2+-loaded TiO2-SiO2 are shown in Figure 7. The TiO2-SiO2 loaded with various amounts of Cu2+, maintained an anatase phase. The presence of Cu2+ was hardly detected by XRD due to its low content. However, the relative intensities of the anatase peak vary among samples. High TiO2 crystallinity was obtained with the addition of 0.2 mol% Cu2+ and lower degrees of crystallinity were observed in samples containing 0.5 mol% copper, which were due to the poor distribution of Cu2+ in the TiO2 matrix. The TiO2 without Cu2+ produced relatively small diffraction peaks of anatase at 25° (101), in contrast to the patterns of the samples with Cu2+. Therefore, phase transformation was not fully achieved without Cu2+, as the TiO2 retained portions of the inactive amorphous phase.
3.4. Photocatalytic Activity
The photocatalytic activity of the TiO2-SiO2 powder, prepared under acidic and basic conditions was evaluated through the degradation of methyl orange (MO) under ultraviolet light irradiation for 1 h at room temperature. Evaluation was repeated for 3 times, and average values were plotted. Samples prepared in acidic conditions displayed photocatalytic activity higher than that of samples prepared in basic conditions (Figure 8). This is attributed to the presence of anatase in the samples prepared under acidic conditions. Photocatalytic activities of the samples with different Ti/Si ratio vary. Pure TiO2 (SA0) prepared in acidic conditions resulted in high photocatalytic activity. However, addition of 25 and 50 mol% of SiO2 in TiO2 produced the highest photocatalytic activity compared with pure TiO2. Thus, the presence of SiO2 was essential to higher photocatalytic activity. As the SiO2 present in TiO2 promoted a high degree of anatase crystallinity, a lower crystallite size was produced due to suppression of TiO2 grain growth. This might have contributed to the larger surface area of the TiO2 particles. The addition of SiO2 in TiO2 also enhanced the acidity of the binary oxide. A model has been proposed to explain this increase in acidity . In this model, the silicon cation enters the lattice of the host oxide, TiO2, and retains its original coordination number. Since the silicon cation is still bonded to the same number of oxygen atoms despite coordination changes in the oxygen atoms, a charge imbalance is created. The charge imbalance must be satisfied. Thus, Lewis sites are expected to form due to the positive charge in the TiO2-SiO2. The surface with improved acidity can adsorb more OH radicals, thus resulted in a larger surface area. This might enhanced the photocatalytic activity and led to complete degradation of MO. This is consistent with the FTIR results of the sample (pH 3) possessed high intensity of hydroxyl group. The result also indicates that samples prepared at basic conditions showed the poor MO degradation, which is in agreement with the observations of Yu et al. . They revealed that amorphous TiO2 has a lower photocatalytic activity compared with crystalline TiO2. Hence, SiO2-loaded TiO2 powder prepared under basic conditions is inefficient for high photodegradation.
To understand the degradation of MO with time, samples were irradiated and collected every 15 min. Pure TiO2 showed rapid degradation at the beginning, which eventually remained constant after 15 min (Figure 9). This is due to the low anatase concentration of TiO2 and its inherent structure and low surface area. The TiO2 with 25 and 50 mol% SiO2 had low degradation rate during the first 15 min. Afterward, degradation markedly increased; both samples completely degraded MO within 1 h. It is worth while to note that the complete mineralization of MO in sample SA25 was faster than that of sample SA50. The TiO2 with 25 mol% SiO2 exhibited the highest photocatalytic activity, which can be attributed to the combination of several factors including large surface area, high degree of anatase crystallinity, and improved surface acidity. Hence, an optimum amount of SiO2 (25 mol%) added to TiO2 systems may be essential to enhance the photocatalytic activity.
Since SA25 was found to be the best sample for MO degradation, this was used to investigate the effect of pH on photocatalytic activity. The TiO2-SiO2 powder prepared at pH 3 and pH 5 showed the highest photocatalytic activity (Figure 10). Rapid degradation rates were observed and resulted in nearly 100% degradation of MO. In contrast, the photocatalytic activity of samples prepared at pH 7, 9, and 12 were very low. Samples from neutral conditions (pH 7) had the anatase structure and high crystallinity but showed lower photocatalytic activity. As the pH increased from 3 to 7, the crystallite size increased, while the surface area decreased, which resulted in lower photocatalytic activity. These observations also indicate that a neutral environment for the synthesis could not enhance the photocatalytic activity. Overall, TiO2 powder loaded with 25 mol% SiO2 prepared at pH 3 was the sample with the highest photocatalytic activity.
To investigate the effect of Cu2+ on photocatalytic activity, SA25 was loaded with varying amounts of Cu2+. The photocatalytic activity was markedly enhanced in the presence of Cu2+ (Figure 11). About 97% of MO was mineralized with the addition of 0.1 mol% of Cu2+. This is mainly due to the high degree of crystallinity of the samples and the capability of Cu2+ to induce the formation of new states close to the conduction band, which leads to reduction of the band gap energy and an increase in the photodegradation efficiency. However, as the Cu2+ increased, the photocatalytic activity decreased rapidly. The photocatalytic activity dropped to ~21% when the Cu2+ content reached 0.5 mol%. This may be attributed to the low crystallinity and the behavior of Cu2+ at high concentrations, wherein it becomes a recombination center for the photo-induced electrons and holes thereby inhibits photocatalysis . Hence, an optimum amount of Cu2+ in TiO2-SiO2 is essential to improve its photodegradation efficiency.
The effect of SiO2 content and pH value on the TiO2 photocatalytic activity was investigated. The addition of SiO2 into the TiO2 strongly affected the degree of crystallinity of the composite. The phase structure was dependent on pH value in the presence of SiO2. An acidic environment led to anatase phase formation, and samples prepared in basic conditions exhibited the amorphous phase. Addition of SiO2 improved the surface area of TiO2 particles by enhancing the surface acidity; this led to high photocatalytic activity compared with pure TiO2. A higher degree of MO degradation was produced by TiO2 samples with 25 mol% SiO2 prepared at pH 3. The TiO2 loaded with 0.1 mol% Cu2+ and 25 mol% SiO2 exhibited photocatalytic activity higher than that without Cu2+. Therefore, the optimum amount of SiO2 and Cu2+ and the pH during preparation were essential to achieving high photocatalytic TiO2 activity.
The authors would like to thank the Universiti Sains Malaysia Fundamental Research Grant Scheme (Grant no. 6071195) and the USM Fellowship for their sponsorship.
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