TiO 2 / Halloysite Composites Codoped with Carbon and Nitrogen from Melamine and Their Enhanced Solar-Light-Driven Photocatalytic Performance

Carbon (C) and nitrogen (N) codoped anatase TiO 2 /amorphous halloysite nanotubes (C+N-TiO 2 /HNTs) were fabricated using melamine as C and N source. The samples prepared by different weight ratios of melamine and TiO 2 were investigated by X-ray diffraction (XRD) and UV-vis diffuse reflectance spectrometer. It is shown that the doping amounts of C and N could influence the photocatalytic performance of as-prepared composites. When the weight ratio of melamine/TiO 2 is 4.5, the C+N-TiO 2 /HNTs exhibited the best photocatalytic degradation efficiency of methyl blue (MB) under solar light irradiation. The obtained C+NTiO 2 /HNTs were characterized by transmission electron microscopy (TEM), N 2 adsorption-desorption isotherm (BET), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectrum (FT-IR). The results showed that the aggregation was effectively reduced, and TiO 2 nanoparticles could be uniformly deposited on the surface of HNTs.This leads to an increase of their specific surface area. XPS and FT-IR analyses indicated TiO 2 particles were doped successfully with C and N via the linkage of the Ti–O–N, O–Ti–N, and Ti–O–C. Photocatalytic experiments showed that C+N-TiO 2 /HNTs had higher degradation efficiency of MB than TiO 2 /HNTs. This makes the composite a potential candidate for the photocatalytic wastewater treatment.


Introduction
Industrial dyes are one of the main sources of water contamination, which are enormously harmful to ecological environment and human beings [1,2].Numerous representative methods, including Fenton oxidation, biological treatment, photocatalytic degradation, membrane filtration, and adsorption [3][4][5][6][7], have been employed to remove the organic dyes from polluted wastewater.Photocatalysis has been commonly deemed to be a mature and reliable technique for the wastewater treatment.In the past decades, enormous efforts have been devoted to researching oxide semiconductor photocatalysts with high activities for environmental protection [8][9][10].As a promising solar-driven photocatalyst, anatase titania (TiO 2 ) has attracted tremendous attentions for water cleaning.However, some malpractices of anatase TiO 2 are still under concern: (a) the agglomerates, composed of the primary small particles, increase the size of TiO 2 ; (b) the ability of visible-light response is not satisfactory.Therefore, much effort has been made on anatase to improve its visible-light photocatalytic capability by controlling its microstructure (morphology, size, crystallinity, and facets) and by tuning its band structure near the valence maximum and conduction band minimum (with element doping, oxygen vacancies, etc.) [11,12].Among them, element doping impurities may be alternative for the extension of photocatalytic activity of TiO 2 into the visible region compared to other methods because the doping element states are near the valence band edge.The several nonmetal codoped TiO 2 materials, such as nitrogen/fluorine [9], sulfur/ nitrogen [13], and carbon/nitrogen [14,15], mainly based on nitrogen doping effect, could result in higher visiblelight responses as compared to the TiO 2 doped with single element.

International Journal of Photoenergy
As a sort of available aluminosilicate clay, halloysite nanotubes (HNTs) have been intensively investigated in the treatment of dye wastewater [16][17][18] due to their well-defined hollow tubular structure with ca.15 nm diameter lumen and 600-1500 nm length averagely [19], which owns a large specific surface area and more complicated pore distribution.Furthermore, the clay nanotubes possess the advantages of large surface area, high porosity, and tunable surface chemistry, which enable this nanomaterial to be utilized as an attractive support for the assembly of small metal and metal oxide.Thus, depositing the TiO 2 nanoparticles onto HNTs is a promising method to block their aggregation.Then HNTs can be directly used to support the TiO 2 nanoparticles because of these hydroxyl groups.The combination of TiO 2 and HNTs is promising to simultaneously possess excellent photocatalytic activity and absorptivity, which could deliver exceptional performances in photocatalytic degradation of organics.
In this work, TiO 2 /amorphous halloysite composites were facilely fabricated by a "precipitation-dissolution-recrystallization" route at a low temperature, and then C+N codoped TiO 2 /amorphous halloysite photocatalysts were obtained using melamine as C+N source at a high temperature.The performance of TiO 2 /amorphous halloysite composites incorporating C+N codoped TiO 2 /amorphous halloysite photocatalysts on the photocatalytic degradation of methyl blue (MB) under solar light is studied.

Preparation of C+N Codoped TiO 2 /Amorphous Halloysite
Composite Catalysts (C+N-TiO 2 /HNTs).TiO 2 /amorphous halloysite composites were fabricated by a "precipitationdissolution-recrystallization" route.The concrete procedures were as follows: titanium tetrachloride (TiCl 4 ) solution (2.3 M, 60 mL) was put into the flask with four necks, and then NaOH solution (2.5 M, 165 mL) was added dropwise into the above TiCl 4 solution with vigorous agitation.The reaction temperature was controlled and set to 10 ∘ C. The turbid solution was obtained at the end of reaction.Subsequently, the system temperature was controlled to 80 ∘ C, halloysite dispersion (0.068 g/mL, 320 mL) was added rapidly into the above solution when the turbid solution began to clarify, and finally the whole mixture was kept at 80 ∘ C for 4 h.The obtained TiO 2 /amorphous halloysite dispersion was filtrated and washed with deionized water, and the asprepared TiO 2 /amorphous halloysite composites were again mixed with melamine (MA), and the obtained uniform slurry mixture was placed into a muffle furnace at 550 ∘ C for 4 h.With this, the yellow powders C+N-TiO 2 /HNTs were completely formed.For comparison, C+N-TiO 2 and TiO 2 /HNTs catalysts were prepared under similar conditions.

Characterizations. X-ray diffraction (XRD) patterns
were recorded on D/Max 2500 PC X-ray diffractometer (Rigaku Corporation, Japan) with Cu K radiation of the X-ray wavelength 0.15418 nm over a 2 range (5∼80 ∘ ).The N 2 adsorption-desorption isotherms and pore distribution (Brunauer-Emmett-Teller, BET, method) were determined by Micromeritics Corporation ASAP2010C surface area and porosimetry system.The morphologies of the as-obtained samples were observed by JEOL Corporation (Japan) JEM-2100 transmission electron microscopy (TEM).X-ray photoelectron spectroscopy (XPS) measurement was carried out by VG Corporation (UK) ESCALAB MKII with an Al K X-ray source.Fourier transform infrared (FT-IR) spectrum was performed by a Nicolet Avatar 370 (Thermo Corporation, USA) from 4000 to 400 cm −1 .The UV-vis absorption spectra were measured under the diffused reflection mode using the integrating sphere (UV2401/2, Shimadu, Japan) attached to a Shimadu 2550 UV-vis spectrometer.

Evaluation of Photocatalytic Activity.
The photocatalytic activities of the obtained photocatalysts were tested by the degradation of methylene blue (MB) under simulated solar light irradiation in the vessel with Xeon lamp (300 W). 0.03 g of catalyst powders were dispersed in aqueous solution of MB (500 mL, 20 mg/L) by ultrasonication and oscillation, and the obtained mixture was sonicated.Prior to irradiation, the dispersions were magnetically stirred in dark for 30 min.During the MB photodecomposition, samples were withdrawn at regular intervals (10 min) and centrifuged to separate solid particles for analysis.The concentration of aqueous MB was determined using 722 vis spectrophotometer by measuring its absorbance at the range of 664 nm.The MB degradation was calculated by Lambert-Beer equation (1), where  0 is the initial absorbance of the MB solution ( 0 = 1.525),   is the absorbance of MB solution after irradiation, and  is photodegradation yield.Photoactivities for MB in the dark in the presence of the photocatalyst and under solar light irradiation in the absence of the photocatalyst were also evaluated as follows:

XRD Analysis.
To ascertain the structures of the products, XRD patterns of HNTs and TiO 2 /HNTs with different mass ratios of MA and TiO 2 are shown in Figure 1.The XRD reflections of HNTs at 2 = 12.1 ∘ , 19.9 ∘ , and 24.8 ∘ in accordance with reflection planes (001), (020), (110), and (002) [20] disappear after calcination, suggesting that the crystal structure of HNTs has been destroyed.The result is in agreement with the previous report [21].However, the tube-like morphology of HNTs can be still maintained from the following TEM in Figure 3. Besides, it can be seen that calcined TiO  with those of original TiO 2 /HNTs, indicating that the doping of C and N modifies the bandgap energy of TiO 2 .In addition, the red shift of C+N-TiO 2 /HNTs is slightly different with the increasing dosage of MA, demonstrating that the MA dosage shows important impact on the properties of TiO 2 .In addition, the samples of C+N-TiO 2 /HNTs show the highest red shift when the mass ratio of MA and TiO 2 is 4.5, in which the wavelength is extended to approximately 470 nm, further shifting to the visible region.This result is in accordance with the theoretical electronic structure calculation that the C+N codoped TiO 2 presents strong visible-light absorption in the range of 400-600 nm [22].The observation implies that C+N-TiO 2 /HNTs (mass ratio of MA and TiO 2 is 4.5, C+N-TiO 2 /HNTs (4.5)) may have preferable photocatalytic performance.

TEM Analysis.
The morphologies of HNTs, TiO 2 , TiO 2 / HNTs, and C+N-TiO 2 /HNTs (4.5) are analyzed by TEM as shown in Figure 3. Pristine halloysite is a cylindrical-shaped tube with multilayer walls.Generally, the HNTs contain agglomerates of nanotubes with some irregularities in diameter, wall thickness, and morphology [23] (Figure 3(a)).Serious aggregation of the oblate-like C+N-TiO 2 particles obtained under similar conditions can be found (Figure 3(b)), and the length of particles is ∼100 nm and the width is ∼30 nm.However, TiO 2 particles are uniformly deposited on the surface of HNTs, indicating that aggregation is effectively prevented by introducing HNTs (Figure 3 The phenomena could be attributed to the presence of more interparticle pores of the composites and a better TiO 2 distribution on HNTs [24], which may be beneficial to dark adsorption for dye.The above results are in good agreement with those of TEM.

XPS and FT-IR Analysis.
Chemical states of incorporated dopants in the as-prepared materials are determined by XPS (Figure 5).The binding energy (BE) distribution of Ti 2p for calcined TiO 2 /HNTs shifts to high binding energy direction compared with that of pristine TiO 2 , suggesting the formation of Ti-O-Si bond (Figure 5(a)).The electronegativity of Ti is less than that of Si, which makes the BE of Ti 2p of Ti-O-Ti be lower than that of Ti-O-Si.For C+N-TiO 2 /HNTs, the lattice incorporation of N generates Ti-N bonds by the partial replacement of O 2− with N − (Figure 5(a)).This gives rise to an increase in the electron density on Ti due to the fact that the electronegativity of the N atom is smaller than the O atom and partial reduction of Ti 4+ to Ti 3+ occurs, which manifests as a slightly decrease in Ti 2p binding energy [25].The core level of N 1s for C+N-TiO 2 /HNTs can be fitted into two peaks at 398.5 eV and 400.1 eV, respectively (Figure 5(b)).The first major peak at 398.5 eV is assigned to the substituted N in the form of O-Ti-N [26,27], indicating that partial of O atoms in the lattice of TiO 2 is substituted by N − anions.The analysis is in agreement with that of Ti 2p result.The latter minor peak is attributed to the interstitial N-doping or the formation of Ti-O-N species [28].Furthermore, XPS spectra of O 1s for calcined TiO 2 /HNTs and C+N-TiO 2 /HNTs are analyzed.The O 1s peak of calcined TiO 2 /HNTs (Figure 5(c)) can be fitted into two components centered at 530.1 eV and 532.0 eV.The first component is attributed to lattice oxygen in TiO 2 , while the second one can be assigned to oxygen atoms of Si-O/Al-O bonds.For C+N-TiO 2 /HNTs, the O 1s spectra can be fitted into three components.The first two components at 530.0 eV and 532.2 eV are similar to those of calcined TiO 2 /HNTs.The newly emerged component is centered at 532.8 eV; it may be ascribed to O 1s originated from Ti-O-C (Ti-O=C) or Ti-O-N groups due to the substitution of carbon for some of the lattice titanium atom [12].These findings agree well with the results of UV-vis analyses, which are responsible for the high photocatalytic activity.
In order to provide additional evidence of the codoping C and N, FT-IR spectra are performed (Figure 6).The broadband at 3424 cm −1 could be ascribed to the stretch vibration of O-H, whereas the peaks at 1634 cm −1 and 1401 cm −1 could be assigned to the bending vibrations of O-H formed by adsorbed water molecules and N-H groups [29].It has been reported that codoping with C and N increased the amount of surface adsorbed water and hydroxyl groups [30].Clearly, the intensity of hydroxyl groups from C+N-TiO 2 /HNTs noticeably increases compared to that of the pure TiO 2 /HNTs, and a new peak at 2050 cm −1 from C+N-TiO 2 /HNTs can be observed, which indicated that TiO 2 may be codoped with C and N after calcination.7(a), inset).The two catalysts exhibit satisfactory photodegradation of MB under solar light irradiation.When MB is exposed for 1 h, approximately 85% and 95% of MB are removed by TiO 2 /HNTs and C+N-TiO 2 /HNTs, respectively.However, C+N-TiO 2 /HNTs display more excellent performance in comparison with TiO 2 /HNTs (Figure 7(a)).The better adsorption and photodegradation of MB by C+N-TiO 2 /HNTs are ascribed to the larger BET surface and the better doping of TiO 2 .To evaluate its usefulness, the two catalysts are reused five times for photodegradation (Figure 7(b)).After five cycles, the degradation effectiveness decreases rapidly for TiO 2 /HNTs, which indicates that photodegradation by C+N-TiO 2 /HNTs remain steady and more effective than TiO 2 /HNTs under successive solar light.

Conclusions
C and N codoped anatase TiO 2 /HNT photocatalysts with a series of mass ratios of melamine and TiO 2 have been successfully synthesised.It is found that the photoactivity of C+N-TiO 2 /HNTs can be clearly improved when the mass ratio of melamine and TiO 2 is 4.5.The anatase TiO 2 nanoparticles can be uniformly deposited on the surface of HNTs, and their particle size decreases compared with the analogous TiO 2 .Consequently, the C+N-TiO 2 /HNTs exhibit steadier and more effective adsorption and photodegradation  due to the larger BET surface and the better doping than TiO 2 /HNTs.Experiments prove that C+N-TiO 2 /HNTs can be employed repeatedly as a promising candidate for the treatment of dye wastewater.