- Nano Commentary
- Open Access
Synthesis of Visible-Light-Responsive Cu and N-Codoped AC/TiO2 Photocatalyst Through Microwave Irradiation
© The Author(s). 2016
- Received: 25 February 2016
- Accepted: 30 May 2016
- Published: 13 June 2016
N–Cu-activated carbon (AC)/TiO2 nanoparticles were prepared by the sol-gel technique through microwave irradiation to modify the visible-light response of TiO2. Their structure, surface chemical composition, and optical absorption properties were characterized. The results showed that the codoped particles had a higher surface area and smaller particle size than pure AC/TiO2 and monodoped AC/TiO2. X-ray photoelectron spectroscopy of N–Cu-AC/TiO2 showed that Cu atoms replaced Ti atom sites, whereas N atoms occupied the O atom sites and interstitial sites in the TiO2 lattice, which changed the electric and band-gap structures of the photocatalyst. N or Cu monodoping of AC/TiO2 reduced the energy band gap of TiO2 from 2.86 eV to 2.81 or 2.61 eV, respectively. In (N, Cu)-codoped AC/TiO2, N and Cu were incorporated into the TiO2 framework and narrowed the band gap of TiO2 to 2.47 eV, causing a large red shift and enhancing visible-light utilization efficiency. Photocatalytic activities were further examined by formaldehyde degradation under visible-light irradiation. N–Cu-AC/TiO2 was found to have the highest activity (ca. 94.4 % formaldehyde degradation efficiency) and to be easily recyclable. These results show an important and innovative method of improving AC/TiO2 activity by modifying the nonmetallic and metallic species.
- Microwave irradiation
Nanocrystalline TiO2 has potential for application to photocatalytic degradation of harmful pollutants dispersed in the environment because of its relatively low cost, nontoxicity, favorable optoelectronic properties, and excellent chemical stability [1, 2]. However, the photocatalytic activity and utilization efficiency of visible light are limited because of the small specific surface area and large band gap (3.2 eV) of pure TiO2 [3–5]. Adsorption can be increased by preparing a porous material-loaded TiO2 photocatalyst, and this has drawn much attention. Given that activated carbon (AC) has a large specific surface area, the reaction rate constant of AC/TiO2 is high for the photocatalytic degradation of organic pollutants . In the work of Pastravanu et al., 92 % methyl orange conversion was achieved after 170 min irradiation using AC/TiO2 with a large surface area of 357 cm2/g .
TiO2 doping with transition metals has been widely investigated to modify the band gap of TiO2 to absorb visible light. Nagaveni et al.  reported that Cu2+-, V5+-, Fe3+-, and Zr4+-doped TiO2 exhibited improved performance in the photodegradation of 4-nitrophenol compared with commercial TiO2. Among the various metals doped into TiO2, Cu has been considered the most important because of the narrow band-gap energies of its oxides (cupric oxide, CuO, possesses 1.4 eV and cuprous oxide, Cu2O, possesses 2.2 eV) and their large light absorption coefficients [9–12]. In addition, Cu doping can reduce the band gap of TiO2 to an appropriate value for visible-light adsorption and can reduce the electron-hole recombination rate during photocatalysis. Wang et al.  reported that a Cu-doped TiO2 thin film exhibited much improved photocatalytic activity compared with pure TiO2 thin film in the degradation of a 10 mg/L methylene blue solution under simulated solar-driven irradiation. In recent years, significant efforts have been made to dope TiO2 with nonmetallic anions, such as N, S, and C, all of which replace O in the TiO2 lattice to generate energy levels just above the top of the TiO2 valence band [14–16]. In terms of performance, nitrogen doping is undoubtedly the most impressive solution for improving the visible-light response of TiO2 [17–20]. Recently, the concept of second-generation TiO2-based materials has been introduced, wherein codoping with two dopant elements produces a synergistic effect to enhance the visible-light absorption efficiency and reduce the recombination processes of the photogenerated charges [21–24]. A Ag/N-codoped TiO2 system was prepared using a sonication-assisted sol-gel method by Mothi et al.  who found that the system was highly active for the photoconversion of 9-(N,N-dimethylaminomethyl) anthracene.
Microwave-assisted preparation of catalytic materials is gaining increased attention . This method is an efficient alternative because it allows swift heating to the required temperature and extremely rapid rates of crystallization, leading to simplification of the preparation procedure [27–29]. However, codoping Cu and N to TiO2 to reduce its band gap and load it onto the AC through a microwave-assisted method has rarely been reported. Thus, an important and innovative method has been attempted to enhance the photodegradation activity of AC/TiO2 with visible-light response via modification with Cu and N through microwave irradiation.
In the present paper, AC/TiO2, N, Cu-monodoped AC/TiO2 and N/Cu-codoped AC/TiO2 nanoparticles were synthesized by a sol-gel method under microwave assistance. The phase structure, morphology, specific surface area, and optical properties were investigated using a variety of techniques. Studies on the use of catalysts for photodegradation of HCHO in aqueous solution under visible-light irradiation are also in progress. The concept used in this research can be further applied to modify other materials for improved photocatalytic performance.
AC was prepared by the means described in a previous study . The obtained AC samples were pretreated by addition to HNO3 solution, then being left for 24 h. The mixture was filtered using distilled water until it became neutral. The pretreated AC was then dried and stored until use. All reagents were of analytical grade. The TiO2 gel/sol was obtained by the conventional sol-gel method . In typical synthesis process, 30 mL of tetrabutyl orthotitanate (TBOT) was dissolved in anhydrous alcohol (EtOH) in proportion of 1:1 (volume ratio). This solution was thoroughly stirred for 40 min and named solution A. Solution B was prepared by mixing 14 mL of glacial acetic acid and 7 mL of distilled water in 35 mL of absolute alcohol. Solution B was added to solution A dropwise and continuously stirred for 1 h. A clear, pale-yellow TiO2 sol was then obtained. Pretreated AC (10 g) was added to TiO2 sol (100 g). The mixture was placed in an air-dry oven at 100 °C for 24 h. After solidification, AC/TiO2 was prepared under microwave irradiation at 700 W for 15 min. To prepare Cu-doped AC/TiO2, 0.44 g of Cu(NO3)2 was mixed with solution B, while for N-doped AC/TiO2, 1.71 g of urea was dissolved in solution B. The doped N and Cu in the samples of N-AC/TiO2 and N–Cu-AC/TiO2 were 0.04 g and 0.01 g, respectively. For N, Cu-codoped AC/TiO2, the sample was noted as 0.04 N-0.01 Cu-AC/TiO2.
The crystal structures of the prepared samples were measured by X-ray diffraction (XRD) on a Rigaku D/Max-2500/PC powder diffractometer. Each sample was scanned using Cu-Kα radiation with an operating voltage of 40 kV and an operating current of 200 mA. The surface micromorphology of the photocatalyst was characterized by scanning electron microscopy (SEM; S4800, Hitachi Ltd.) at an accelerating voltage of 15 kV. Transmission electron microscopy (TEM) was performed on a Tecnai G2 F20 microscope at 100 kV. FTIR spectra were recorded with a Bruker Vertex FTIR spectrometer, resolution of 2 cm−1, in the 4000–400-cm−1 range by the KBr pellet technique. UV-vis diffuse reflectance spectra were obtained with a powder UV-vis spectrophotometer (U-4100, Hitachi Ltd.). The specific surface area (SBET, m2/g) was calculated using the Brunauer–Emmett–Teller (BET) equation. X-ray photoelectron spectroscopy (XPS) analysis of the samples was conducted using a PHI5700 ESCA system equipped with an Mg Kα X-ray source (1253.6 eV) under a vacuum pressure <10−6 Pa. The formation rate of ⋅OH at the photo-illuminated sample/water interface was detected by photoluminescence (PL) using terephthalic acid (TA) as a probe molecule. PL spectroscopy of the synthesized products was undertaken at room temperature on a Hitachi F2500 spectrofluorometer using a Xe lamp with an excitation wavelength of 325 nm.
where C o and C t are the concentrations of HCHO at initial and different irradiation times, respectively.
The rate of ·OH in the photocatalytic reactions was evaluated by adding 50 mL of terephthalic acid in a manner similar to the above photodegradation experiment. For terephthalic acid, to ensure solubility, solutions were prepared in dilute (2 × 10−3 mol/L) NaOH solution. Photocatalyst in the solution was magnetically stirred and illuminated under visible-light irradiation. The photocatalytic experiment was carried out in the presence of 2 mL (10.66 mol/L) t-BuOH, a radical scavenger .
XRD and BET Characterization
Physicochemical properties of different photocatalysts
Ratio of A and R (%)
Morphology of Photocatalysts
UV-vis Diffuse Reflectance Spectroscopy
where α, hν, E g, and A are the absorption coefficient, photon energy, band-gap energy, and a constant, respectively. In accordance with the above equation, a modified plot of the Kubelka–Munk function versus the energy of the exciting light was used to determine the band gap of the samples. A band-gap value of 2.86 eV was obtained for AC/TiO2. According to data reported in literature, it is well-known that bulk anatase and rutile have the direct band-gap energy of 3.2 and 3.0 eV, respectively [3, 5]. This means that the radiation wavelength used for electron excitation of the titania should be smaller at about 387 nm (UV region). The band gap of TiO2-coated on AC samples decreased to 2.86 eV, indicating an influence of the AC support on the optical properties of TiO2 photocatalyst. Coromelci-Pastravanu et al.  also reported that the optical band gap of TiO2-coated mesoporous carbon samples decreases slightly comprised with pure TiO2. The carbon-based composites were found to be able to promote a rapid photoinduced charge separation and a slow charge recombination, by accepting photogenerated electrons from the photocatalytic TiO2 nanoparticles. After N doping, the band gap of TiO2 decreased to 2.81 eV and the band gap of Cu-AC/TiO2 was about 2.61 eV. Meanwhile, significant narrowing of the band gap to approximately 2.47 eV was observed after N, Cu codoping. This significant band-gap reduction is associated with charge-transfer, corresponding to the electronic excitation from the valence band (VB) to the conduction band (CB). Reduction in the band gaps correspondingly reduced the energy required for electron transition from the VB to the CB, thus shifting the optical absorption to a lower energy. This large decrease in band gap for the codoped catalyst may be attributed to the formation of mixed energy levels between the VB and CB. Thus, Cu and N doping promotes the visible absorption of the catalyst and plays a significant role in enhancing the photocatalytic activity of the catalysts.
Photocatalytic Activity of Photocatalysts
The most important factor for the enhancement of photocatalytic activity of doped AC/TiO2 is that Cu substitution of the partial Ti leads to an isolated energy band formation between the VB and CB in the band structure of TiO2. The formation of ⋅OH is attributed to the transition of electrons between the VB and CB. When doped with Cu, a two-step optical transition will occur because of the formation of the newly isolated energy band and the decrease in band-gap energy between the VB and the CB of TiO2, as shown in Fig. 5. This increased the optical response of Cu-doped AC/TiO2. In N-AC/TiO2, the Ti–N linkage may lead to formation of an N1s peak because of substitutional N doping in the TiO2 lattice . Doping of N into TiO2 forms a new state on N1s, just above the VB, leading to strong absorption of visible light and enhancement in the separation efficiency of the photoinduced electrons and holes. The significant enhancement of photocatalytic activity of N, Cu-codoped AC/TiO2 in this study can be attributed to the synergistic effect of N, Cu codoping. Doping of the TiO2 lattice with Cu can lead to O vacancy production, which in turn facilitates N doping. Both Cu and N doped on TiO2 decreases the electronic transition energy and improve the photocatalytic activity of N–Cu-AC/TiO2.
Mechanism of N, Cu-Codoped AC/TiO2
Reusability of the Catalyst
A novel and simple technique for incorporating N and Cu on TiO2 nanoparticles loaded on AC was presented. The nanoscale TiO2 showed anatase and rutile phases with a particle size of 18 nm, an appropriate proportion in codoped AC/TiO2, and a specific surface area of 548 m2/g. Nitrogen and Cu may occupy the internal TiO2 crystal framework, replacing some Ti4+ and O2−, respectively, and extending the band-gap excitation to the visible region. The obtained N–Cu-AC/TiO2 shows a distinctive absorption band in the visible region and presents the lowest band-gap value among all of the samples studied. The N–Cu-AC/TiO2 photocatalyst exhibited good photocatalytic activity for the degradation of HCHO under visible-light irradiation. The incorporation of Cu and N may decrease the required energy for electronic transitions, thereby improving the photocatalytic activity. Furthermore, the produced photocatalyst can be easily recycled and exhibits enhanced stability. Therefore, using N, Cu-doped AC/TiO2 for pollutant photodegradation is a practical method for purifying water under visible light.
This work was supported financially by funding from the National Natural Science Foundation of China (51262025) and the International Scientific and Technological Cooperation Project of Xinjiang Bingtuan (2013BC002).
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