Enhanced Visible Light Photocatalytic Activity of Mesoporous Anatase TiO2 Codoped with Nitrogen and Chlorine

1 Department of Environmental Science and Engineering, Heilongjiang University, Xuefu Road 74, Nangang District, Harbin 150080, China 2 Key Laboratory of Chemical Engineering Process & Technology for High-Efficiency Conversion, College of Heilongjiang Province, Harbin 150080, China 3 State Key Laboratory of Urban Water Resources and Environment (SKLUWRE), Department of Environmental Science and Engineering, Harbin Institute of Technology, Huanghe Road 73, Nangang District, Harbin 150090, China


Introduction
Titanium dioxide (TiO 2 ) is known as a semiconductor with various kinds of application in solar energy conversion, gas sensors, air purification, and waste water treatment due to its cheapness, nontoxicity, and strong oxidation power [1][2][3][4][5][6]. However, anatase TiO 2 can only be excited by UV light with wavelengths less than 388 nm, which only accounts for a small part of the solar spectrum [7]. In order to effectively extend the light absorption to visible region and enhance the photocatalytic activity, many attempts have been made, such as dye sensitization, noble metal deposition, coupling of TiO 2 with a narrow semiconductor and doping of TiO 2 with foreign ions. Among these, doping of TiO 2 with foreign ions has been an effective and feasible approach to improve the visible light response and photocatalytic activity [8][9][10][11][12][13]. Recently, many researches indicate that TiO 2 doping with two different species into the lattice could further enhance the visible light photocatalytic activity. Sun et al. [14] synthesized C-S-codoped TiO 2 by the hydrolysis of tetrabutyl titanate in a mixed aqueous solution containing thiourea and urea. Lv et al. [15] prepared C-N-codoped TiO 2 nanoparticles with visible light photocatalytic activity. Xu and Zhang [16] reported an enhanced photocatalytic activity of the C-Cl-codoped TiO 2 powders towards the degradation of RhB under visible light irradiation. Although such reports demonstrated successful fabrication of relative high photocatalytic activity, they still suffered from some deficiencies due to the discharge of poisonous gases, troublesome procedure during the reaction process.
In this study, N-Cl-codoped TiO 2 photocatalyst (N-Cl-TiO 2 ) has been synthesized through simple one-step sol-gel reactions in the presence of ammonium chloride. N and Cl elements were incorporated into the lattice of TiO 2 through substituting the lattice oxygen atoms. As expected, the N-Clcodoped TiO 2 catalyst exhibited higher visible light response and photocatalytic activity than that of P25 TiO 2 and N-TiO 2 . Further, the activity-enhanced mechanism was also discussed in detail.

Synthesis of Materials.
Typically, a solution consisting of 10 mL absolute EtOH, 12 mL dilute HNO 3 (1 : 5, volume ratio between HNO 3 and DI water), and the desired amount of ammonium chloride was added dropwise into a solution containing 40 mL absolute EtOH and 10 mL Ti(OBu) 4 within 120 min under vigorous stirring. After being aged for 6 h at room temperature, the TiO 2 precursor was received by drying the wet gel in an oven for 36 h. Finally, the N-Cl-codoped TiO 2 sample was obtained by calcining the precursor at 623 K for 4 h with a heating rate of 3 K·min −1 . For comparison, pure TiO 2 and N-TiO 2 catalysts were synthesized as the reference according to the previous work of our groups [17]. X-ray photoelectron spectroscopy (XPS) was performed on a Model PHI-5700 ESCA apparatus with Al Kα X-ray source. All the binding energies (BE) were referenced with the adventitious carbon (binding energy = 284.6 eV). The ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS) of the as-synthesized TiO 2 samples was recorded with a Model Shimadzu UV-2550 spectrophotometer.

Evaluation of Photocatalytic Activity.
The experiments were carried out in a 100 mL quartz photochemical reactor and the visible light source was provided from a side of the reactor, by a 350 W Xe-arc lamp equipped with a UV-cutoff filter (λ > 420 nm). In each run, 20 mg TiO 2 catalyst was added into 20 mL RhB solution of 10 mg·L −1 . Prior to photoreaction, the solution was magnetically stirred in the dark for 30 min to establish the equilibrium of adsorption-desorption. At given time intervals, the samples after filtration and centrifugation were analyzed by a T6 UV-vis spectrometer. In addition, the photodegradation of uncolored phenol was similar to that of RhB, and the concentration was determined by the colorimetric method of 4-aminoantipyrine at the wavelength of 510 nm. In addition, no XRD peaks relate to the dopants were detected. One reason was that the concentration of the dopants was so low that it cannot be detected by XRD. The other was that the dopants were incorporated into the lattice of TiO 2 through substituting oxygen atoms or located in the interstitial sites. Further, as seen from the inset of Figure 1, the diffraction peak position of N-Cl-codoped TiO 2 shifted to lower angle in comparison with the pure TiO 2 , suggesting that oxygen atoms in the lattice of anatase in codoped TiO 2 sample may be substituted by the dopants. Further, the average crystallite sizes of the as-synthesized samples can be calculated by applying the Debye-Scherrer formula [18] on the anatase (101) diffraction peaks:

XRD and TEM Analysis.
where d is the crystallite size, λ is the wavelength of X-ray radiation (in our test, λ = 0.15418 nm), K is a constant (K = 0.89), β is the full width at half-maximum, and θ is the diffraction angle. The calculated d values were 10 and 5 nm for pure and N-Cl-codoped TiO 2 , respectively. Thus, we can conclude that codoping with N and Cl elements could effectively retard the phase transformation of TiO 2 from anatase to rutile and growth of the crystallite size. The crystal structure and grain size of N-Cl-codoped TiO 2 sample were also evaluated with TEM and HRTEM. Figure 2 showed the TEM and HRTEM images of mesoporous N-Cl-TiO 2 photocatalyst. As shown in Figure 2(a), many spherical aggregates of nanoparticles were observed with the nanoparticles at about 5∼10 nm in crystallite sizes. However, an obvious agglomerative phenomenon was observed. As shown in Figure 2  was in good accordance with the anatase (101) lattice fringes of 0.352 nm [19].

XPS Analysis.
In order to investigate the chemical states of the N-Cl-codoped TiO 2 sample, XPS was conducted and shown in Figure 3. As seen from Figure 3(a), a single peak at binding energy of 399.6 eV was observed, which was attributed to the presence of substitutional N in O-Ti-N bond [20,21], indicating that some lattice oxygen were substituted by nitrogen atoms, which correlated with the visible light photocatalytic activity of doped TiO 2 . As shown in Figure 3(b), two peaks at 197.7 and 199.3 eV seen from the Cl2p core-level XPS spectrum were observed. The strong peak at 197.7 eV was assigned to Cl − ions physically adsorbed on the surface of codoped TiO 2 [22], while the minor peak located at 199.3 eV might be assigned to the Cl incorporated into the lattice of TiO 2 [16], indicating that nitrogen and chlorine were incorporated into the lattice of TiO 2 through substituting the lattice oxygen atoms.

DRS Analysis.
Generally speaking, narrower band gap of a semiconductor corresponds to a higher photocatalytic activity. In order to investigate the optical absorbance property of pure and N-Cl-codoped TiO 2 samples, ultravioletvisible diffuse reflection spectra were conducted and shown in Figure 4. In addition, band gap energies were also calculated according to the Kubelka-Munk function [23], which was shown in the insert in Figure 4. Noticeably, both as-synthesized TiO 2 samples showed a typical absorbance spectrum with an intense transition in the UV region of the spectra, corresponding to the band gap energy of 3.2 eV from the intrinsic band gap of pure anatase. However, the light absorption edge of N-Cl-codoped TiO 2 sample was greatly red-shifted to the visible light region, demonstrating a decrease in the band gap energy, which was responsible for the impurity level (formed from hybridization of N2p and Cl2p levels) in the band gap of TiO 2 . After calculating, the band gap energy of N-Cl-codoped TiO 2 was 2.12 eV. As a result, the N-Cl-codoped TiO 2 should most probably possess excellent visible light photocatalytic activity for organic degradation.

Mechanism
Analysis. Based on the above analysis, a possible photocatalytic mechanism was proposed and shown in Figure 5. Nitrogen and chlorine were incorporated into the lattice of TiO 2 through substituting lattice oxygen atoms. Thus, a new impurity level was formed from the hybridization of N2p and Cl2p levels between the valence band and conduction band of TiO 2 . As shown in Figure 5, pure TiO 2 exhibited a relatively low visible photocatalytic activity for the degradation of pollutants due to its wide band gap (3.2 eV, process A). After codoping with nitrogen and chlorine elements, electrons can be promoted from the impurity level to the conduction band of TiO 2 (process B), thereby enhancing the separation efficiency of photoinduced charge carriers. As a result, more departed photoinduced electrons and holes can participate in the photocatalytic reactions. Further, the process of visible light photocatalytic oxidation of phenol can be described as followed Firstly, electrons and holes were generated under visible irradiation.

N-Cl-TiO 2 + hν
Then, the departed electrons and holes can react with the absorbed O 2 and H 2 O molecules, respectively, forming the main active species (such as ·O − 2 and ·OH) responsible for the degradation of organic pollutants, such as RhB and phenol in our case. Eventually, organic pollutants were mineralized into small molecules, such as CO 2 Figure 6 showed the photocatalytic degradation rates of RhB and phenol on different TiO 2 samples. As a comparison, the photocatalytic activity of P25 TiO 2 and N-TiO 2 was also measured and shown in Figure 6. As seen from Figure 6, N-Cl-codoped TiO 2 sample exhibited higher photocatalytic activities than that of P25 TiO 2 and N-TiO 2 catalysts, suggesting that the visible-induced photocatalytic reactions can contribute to the degradation of the colored RhB and uncolored phenol. Under the visible light irradiation for 120 min, 95.7% and 82% degradation rates could be achieved for colored RhB and uncolored phenol, respectively. The enhanced photocatalytic activity of N-Cl-codoped TiO 2 was attributed to the small crystallite size, intense light absorption in visible region, and narrow band gap energy.

Conclusions
Based on the above analysis, the following conclusions can be drawn: (1) N-Cl-codoped TiO 2 catalyst with high visible light photocatalytic activity has been successfully synthesized through simple one-step sol-gel reactions in the presence of ammonium chloride. (2) TiO 2 codoping with N and Cl could effectively retard the phase transformation of TiO 2 from anatase to rutile and growth of crystallite size. (3) N and Cl elements were incorporated into the lattice of TiO 2 through substituting some oxygen atoms, which could form a new impurity level between the valence band and conduction band of TiO 2 . (4) Codoping with N and Cl could greatly improve the photoresponse of TiO 2 , thereby reducing the band gap. (5) The enhanced activity of N-Cl-codoped TiO 2 was mainly attributed to the small crystallize size, intense light absorption in visible region, and narrow band gap.