The Effects of Doping Copper and Mesoporous Structure on Photocatalytic Properties of TiO2
This paper describes a system for the synthesis of Cu-doped mesoporous TiO2 nanoparticles by a hydrothermal method at relatively low temperatures. The technique used is to dope the as-prepared mesoporous TiO2 system with copper. In this method, the copper species with the form of Cu1+, which was attributed to the reduction effect of dehydroxylation and evidenced by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), was well dispersed in the optimal concentration 1 wt.% Cu-doped mesoporous TiO2. In this as-prepared mesoporous TiO2 system, original particles with a size of approximately 20 nm are aggregated together to shapes of approximately 1100 nm, which resulted in the porous aggregate structure. More importantly, the enhancement of the photocatalytic activity was discussed as effects due to the formation of stable Cu(I) and the mesoporous structure in the Cu-doped mesoporous TiO2. Among them, Cu-doped mesoporous TiO2 shows the highest degradation rate of methyl orange (MO). In addition, the effects of initial solution pH on degradation of MO had also been investigated. As a result, the optimum values of initial solution pH were found to be 3.
Titanium dioxide (TiO2) is the most widely used semiconductor as a stable photocatalyst, and its photocatalytic activity is mainly limited by the number of photogenerated electron-hole pairs and their life. Although a lot of effort is being spent on improving these weaknesses, the efficient and effective method has yet to be developed.
The study found: doping metal ions not only can effectively reduce the band gap of TiO2 to increase the number of photogenerated electron-hole pairs; the doping metal ions can form the corresponding oxide space-charge layer to promote the photogenerated electron-hole pairs effective separation [1, 2]. Sangpour et al.  modified TiO2 by Au, Ag, and Cu incorporation and showed that doping increases the probability of radical formation. The photoenhancement of the studied elements was determined in the following order: Cu : TiO2 Au : TiO2 Ag : TiO2 TiO2. Park et al.  fabricated metal (Co2+, Ni2+, Cu2+, and Zn2+) doped TiO2, Zn-doped TiO2, and Cu-doped TiO2 which were found to be promising materials for the photocatalytic decomposition of methylene blue. Among various metallic doping elements, the copper doping has been proved to be a simple and effective way to increase the visible light absorption.
The construction of TiO2 nano- or microstructures with interesting morphologies and properties has recently attracted considerable attention. It is well known that ordered mesoporous materials with larger specific surface area, pore size, and the relatively regular channel structure are ideal materials for catalyst. Ordered mesoporous materials researched as photocatalysts for the treatment of environmental pollutants are one of the research hot spots in recent years . Zhu et al.  prepared hierarchical mesoporous TiO2 microspheres with high crystallinity and high BET specific surface area. Comparing to P25, the degradation percentages by the mesoporous TiO2 microspheres are more than twice of those by P25. In the work of Xiong et al. , results showed that the polymeric template as well as calcination played an important role in tuning morphology, mesoporosity, and specific surface area of the synthesized mesoporous TiO2.
Considering the two aspects above and utilizing the existing knowledge in the area of nanometer metal oxides preparation, the Cu-doped mesoporous TiO2 was prepared by a hydrothermal method at relatively low temperatures without calcination in this study. The chemical status of the main element was investigated with X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Particle size and pore structure were analyzed with electron microscopy photographs, laser particle size analyzer, and N2 adsorption and desorption isotherms. The light absorption band edge was characterized by means of UV-Vis diffuse reflectance spectroscopy. The photocatalytic activities of the as-prepared Cu-doped mesoporous TiO2 were evaluated by photo degradation of methyl orange (MO).
2.1. Preparation of Catalysts
Herein, all chemicals were used as received without further purification.
The triblock copolymer, Pluronic P123 (M = 5800, Sinopharm Chemical Reagent Co., Ltd.), was used as the template. Tetrabutyl titanate (TBT, Sinopharm Chemical Reagent Co., Ltd., 97%) was used as the titanium source and calculated amounts of Cu(NO3)2·3H2O (M = 241.60, Sinopharm Chemical Reagent Co., Ltd.) were chosen as the precursor of the dopant.
The synthesis of Cu-doped mesoporous TiO2 was carried out with the following procedure according to the report .
7.5 mL tetrabutyl titanate and 3 g triblock copolymer pluronic P123 were ultrasonically dispersed in 10 mL ethanol, labeled as solution A. And then 10 mL ethanol, 5 mL deionized water, 15 mL acetic acid, and 0.2 g Cu(NO3)2·3H2O were mixed to form a homogeneous solution, labeled as solution B. The mixed solution, which was obtained by dropping solution A into solution B, should be stirred at room temperature for 1 h after mixing. Then the mixture was transferred to a 50 mL Teflon-lined stainless steel autoclave, followed by a hydrothermal treatment at 130°C for 24 h. After the reaction, the powder sample was centrifuged, rinsed with ethanol and deionized water, and dried in vacuum at 80°C for 12 h. After grinding, the product (1 wt.% Cu-doped mesoporous TiO2) just desired was obtained without calcination.
For comparison, unsupported catalyst pure TiO2 was also prepared using the same procedure without the addition of P123 or Cu(NO3)2·3H2O.
2.2. Characterization of Catalysts
The crystallite structures of the materials were investigated by analyzing the X-ray diffraction (XRD) patterns obtained with a RIGAKU Ru-200B diffract meter equipped with Cu Ka irradiation with a fixed power source (40 kV, 40 mA). Chemical compositions of the composites were analyzed using X-ray photoelectron spectroscopy (Thermo Scientific, ESCALAB 250Xi). The internal structures of the particles were studied with Transmission Electron Microscopy (TEM, JEOLJEM-2000 FX II). All the powders were ultrasonically dispersed in ethanol for 30 min prior to the measurement. And the surface features and morphologies of the synthesized materials were investigated with Scanning Electron Microscopy (SEM, JEOL JSM-6330F). The particle size distribution of the catalyst was examined using laser particle size analyzer (Malvern mastersizer2000). The specific surface area and pore size distribution were characterized by analyzing the N2 adsorption and desorption isotherms obtained at −196°C using Quantachrome Quadrasorb SI automated surface area and pore size analyzer. All the materials were degassed at 150°C and Pa for 6 h prior to the measurement. UV-Vis diffuse reflectance spectroscopy (Hitachi U3900) with a wavelength scan range of 250–800 nm was used to determine the light absorption of the catalysts.
Throughout the subsequent discussion on catalyst characterization, Cu-doped mesoporous TiO2 refers to 1 wt. % Cu-doped mesoporous TiO2.
2.3. Photocatalytic Activity Test
The photocatalytic activities of the catalysts on the degradation of methyl orange under visible light in aqueous solution were determined with the following procedure .
We used a beaker as the degradation pool, in which 50 mL methyl orange aqueous solution (50 ppm) and 125 mg as-prepared sample were mixed. Prior to irradiation, the mixture solution was shocked in the dark for 30 min until adsorption/desorption equilibrium was reached. The solution was then irradiated under visible light. A certain mixture solution was removed at regular intervals and filtered through a syringe filter (0.45 m). Finally, the degradation of methyl orange was analyzed by detecting the absorption at 462 nm using a UV-7504PC (CANY Shanghai). The xenon lamp (50 W, CEL-HXF300) was used to simulate the solar spectrum. The photocatalytic experiments were performed at pH 7 unless stated otherwise. When required, the initial pH values were adjusted to the desired values using HCl or NaOH solutions.
The photocatalytic activities of the pure TiO2 and mesoporous TiO2 were tested under the same experimental conditions.
3. Results and Discussion
3.1. XRD Characterization
The wide angle X-ray diffraction (XRD) patterns of Cu-doped mesoporous TiO2 with different copper concentration were obtained and compared with pure TiO2 diffraction pattern in Figure 1. All of as-prepared TiO2 had (101), (004), (200), and (211) peaks at 2 values of ca. 25.38°, 37.82°, 48.18°, and 54.4°, indicating that all of the as-prepared TiO2 had an anatase crystal structure according to JCPDS-21-1272.
It must be noted that no copper species (Cu, CuO, Cu2O) were detected in the sample 1 wt.% Cu-doped mesoporous TiO2. The above observations may suggest that Cu was well dispersed in the TiO2. The similarity in the Cu and Ti ionic radii (0.072 nm for Cu and 0.068 nm for Ti) allows the interstitial incorporation of the dopant into the titania network.
In this section, we also provide a simple method for understanding of the copper species in the Cu-doped mesoporous TiO2. The 3 wt.% Cu-doped mesoporous TiO2 with larger copper concentration was investigated, in which Cu2O crystal peaks were detected along with the anatase peaks of anatase TiO2, indicating that Cu phase dispersed in the form of micro-TiO2 crystal with Cu2O small clusters. It leads to the conclusion that the excess copper cannot replace Ti4+ entering into the TiO2 to form stable solid solution. Without special instruction, the optimal concentration of 1 wt.% Cu-doped mesoporous TiO2 was used as the experimental sample in the following results and discussion.
Reading Huang et al.’s work  for reference, the formation of Cu2O may be due to acetic acid and ethanol, reducing Cu2+ to Cu1+. This reduction effect was attributed to the dehydroxylation step, where OH radicals with strong reducing proprieties were produced . The presence of Cu1+ also corroborated with the following XPS results.
3.2. XPS Characterization
X-ray photoelectron spectroscopy (XPS) analysis can provide information about the oxidation states and compositions of the superficial metals, and to identify the chemical status of the main element in the samples.
Figure 2 shows the X-ray photoelectron survey spectra of Cu-doped mesoporous TiO2 composite. As can be seen, the XPS spectra pointed out that the copper has been successfully doped into the titanium dioxide structure.
Figure 3 shows the high resolution XPS spectra of Ti2p region and Cu2p region for the Cu-doped mesoporous TiO2. The peaks for Ti 2p3/2 and Ti 2p1/2, respectively, located at 458.58 eV and 464.38 eV are lower than those reported in the literature for the pure TiO2 at 458.9 eV and 464.6 eV . Thus it means that there are more lattice defects that existed in the Cu-doped mesoporous TiO2 lattice. Lattice defect is active to capture photogenerated electrons and improve catalytic activity.
It is necessary to note that the Cu2p3/2 level binding energy of about 932.2 eV and 933.5 eV is related to Cu2O and CuO, respectively. And Cu2p1/2 of 952.9 eV and 953.6 eV are related to Cu2O and CuO, which are too close to distinguish.
The next steps are useful and innovative to make a distinction between Cu2O and CuO. The high resolution XPS spectra of Cu2p region for uncalcined (blue) and calcined (red) Cu-doped mesoporous TiO2 were analyzed in Figure 4. After calcination in air at 450°C, the Cu2p peak broadened with the emergence of CuO characteristic peak in the red curve, indicating that the original Cu2O was oxidized. It can be concluded that loaded copper existed as oxidation state Cu2O in the Cu-doped mesoporous TiO2 composite.
3.3. Particle Size and Pore Structure
As can be seen in Figure 5(a), the transmission electron microscopy image offers the information that the original particles are quite uniform and their size is about 20 nm but it is difficult to identify the channel structure, which can be attributed to that only a small portion of the sample was observed in the transmission electron microscopy analysis; moreover the sample was ultrasonically dispersed prior to observation. And in the scanning electron microscopy image of Figure 5(b), the aggregation of the original particles was observed and the size of aggregates is about 1000 nm. In addition to electron microscopy analysis, the particle size distribution with statistical significance was analyzed as shown in Figure 6, and the average size of aggregates is about 1100 nm, consistent with the SEM result.
With integrated analysis on results of the electron microscopy image and the particle size distribution, it can be concluded that the spherical nanocrystalline particles with a size of approximately 20 nm are aggregated together to shapes of approximately 1100 nm, which resulted in the porous aggregate structure. More details on pore structure are discussed later in this chapter.
As can be seen in Figure 7, the N2 adsorption-desorption isotherms of pure TiO2, mesoporous TiO2 and Cu-doped mesoporous TiO2 were investigated to evaluate the pore structure and pore size. The isotherms of the samples mesoporous TiO2 and Cu-doped mesoporous TiO2 have classical hysteresis between the adsorption and desorption curves compared with pure TiO2, indicating the presence of pore structures. And the Brunauer Emmet Teller surface area could be ranked as follows: SBET (mesoporous TiO2): 146 m2/g > (Cu-doped mesoporous TiO2): 128 m2/g > SBET (pure TiO2): 57 m2/g. As can be seen, a narrow decrease has been identified for the copper-doped photocatalysts. That means copper doping will lead to the decrease of surface area due to the blockage of the pores by the copper oxide clusters .
3.4. Optical Properties
Cu-doped mesoporous TiO2 and pure TiO2 nanocomposites have been characterized by means of UV-Vis diffuse reflectance spectroscopy as shown in Figure 8. Compared with pure TiO2, absorption band edge of Cu-doped mesoporous TiO2 was noticeably shifted toward the visible light region. According to the work developed by Li and Zhang , the expanded absorption band might be assigned to presence of Cu1+ clusters as well as the bulk Cu2O.
Thus, it can be inferred that doping with a transition metal ion such as copper is effective for visible-light response and will play a significant role for enhancing the photocatalytic activity of the catalysts. However, excessive absorption of visible light and even infrared region will lead to a decline in photocatalytic oxidation ability. So it is important to tune the optical and electronic properties of semiconductor nanocrystals by controlling the type and concentration of dopant.
3.5. Photocatalytic Activity
The photocatalytic activities of the as-prepared TiO2 were evaluated by photo degradation of methyl orange. Figure 9 shows enhanced photocatalytic activity of Cu-doped mesoporous TiO2 as well as that of pure TiO2 and mesoporous TiO2 under visible light irradiation.
The relationship of photocatalytic abilities among the catalysts was as follows: Cu-doped mesoporous TiO2 Mesoporous TiO2 pure TiO2. A discussion of possible reasons for the enhanced photocatalytic activity of composite catalyst under visible light irradiation is summarized as follows. Firstly, the light absorption extended into visible light region due to copper doping. Secondly, Cu1+ acted as electron and hole trappers which could effectively reduce the photogenerated hole-electron recombination rate . Finally, the higher adsorption of methyl orange onto the catalyst surface due to the mesoporous structures also played a role in enhancing the photocatalytic activity of composite catalyst.
Since pH play a vital role in the degradation of dyes and the pH values of the dyestuff waste could be different in practical application, additional experiments were carried out to examine the effects of initial pH values of methyl orange solution on the photocatalytic reaction. The catalyst used was the Cu-doped mesoporous TiO2. The initial pH values of the test solution were in amplitude pH of 3–10 adjusted with HCl or NaOH without any modification during UV irradiation. All the other experimental conditions were identical .
Figure 10 demonstrated the degradation efficiency of methyl orange under different initial pH values of solution, and a higher methyl orange degradation efficiency at acidic pH can be noticed. The reason that the photodegradation efficiency of MO depended on solution pH was due to the absorption/desorption of MO on TiO2 surface. In acidic solution, TiO2 was positively charged and absorbed MO molecules by electrostatic attraction; in alkaline solution, TiO2 was negatively charged and the absorption of MO molecules became weaker due to repulsive forces .
Cu-doped mesoporous TiO2 was prepared by a hydrothermal method. The results confirmed that the high photocatalytic activity of as-prepared sample strongly depended on the stabilization of Cu (I) well dispersed in TiO2. Copper doping greatly enhanced the absorption in visible-light region and played a significant role for enhancing the photocatalytic activity of the catalysts. Moreover, mesoporous structures with larger specific surface area, pore size, and the relatively regular channel structure had better adsorption capacity.
Conflict of Interests
The authors declare that they have no financial and personal relationships with other people or organizations that can inappropriately influence their work, and there is no professional or other personal interest of any nature or kind in any product, service, and/or company that could be construed as influencing the position presented in, or the review of, this paper.
The authors would like to thank Beijing Jiaotong University and the State Key Laboratory of Solid Wastes Resource Utilization and Energy Saving Building Materials in Beijing Building Materials Academy of Sciences Research. This project was supported by the Fundamental Research Funds for the Central Universities (2014JBZ010) and the Project of Beijing Jiaotong University (no. 2012RC002). Yang Wang would also like to express his gratitude to all colleagues who provided assistance.
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