Synthesis of highly water-dispersible N-doped anatase titania based on low temperature solvent-thermal method

Wenyi Huang; Hao Cheng; Jun Feng; Zhipeng Shi; Dawei Bai; Lijun Li

doi:10.1016/j.arabjc.2017.12.028

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11 (

); 871-879

doi:

10.1016/j.arabjc.2017.12.028

Synthesis of highly water-dispersible N-doped anatase titania based on low temperature solvent-thermal method

Wenyi Huang¹, Hao Cheng¹, Jun Feng, Zhipeng Shi, Dawei Bai, Lijun Li^⁎

Guangxi Key Laboratory of Green Processing of Sugar Resources, College of Biological and Chemical Engineering, Guangxi University of Science and Technology, Liuzhou 545006, China

⁎Corresponding author. lilijun0562@sina.com (Lijun Li)

Received: 2017-8-30, Accepted: 2017-12-21, Published: 2018-09-01

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

1 These authors contributed equally to this work and should be considered co-first authors.

Abstract

In this paper, the high-concentration aqueous dispersion N-doped anatase TiO₂ is prepared based on low temperature non-aqueous solvent-thermal method by using TCl₄ and NH₄Cl as titanium source and nitrogen source respectively. From TEM image, we can learn that the particle is composed of spindle-shaped particles with the size from 8 to 15 nm. According to the XRD and XPS spectra of N-doped TiO₂, it can be seen that, during the synthesis process, N atoms permeate into the TiO₂ lattice, and the impurity level can be formed within the band gap of TiO₂. The impurity level can extend to absorb UV and the visible light. As for N-doped TiO₂ nanoparticle of XPS spectrum, ethanol is used as solvent to form carbonaceous species on the surface of TiO₂. The carbonaceous species in visible light can excite electrons towards the TiO₂ conduction band, so as to increase the number of electrons in the conduction band under visible light to improve the catalytic activity of TiO₂ which possesses the feature of high catalytic activity under UV–visible light.

Keywords

TiO2 nanoparticle

Highly aqueous dispersion

Visible light photocatalytic activity

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1 Introduction

Semiconductor-based photocatalysts are a class of advanced materials because of important applications to cleaning environmental pollution and producing new energy, such as degradation of organic pollutants and H₂ evolution from water-splitting under UV or visible-light irradiation. Due to its non-toxic and inexpensive, Titanium dioxide (TiO₂), as a semiconductor-based photocatalysts, has aroused extensive attention ( Kumar et al., 2012; Katoh et al., 2010; Dette et al., 2014; Periyat et al., 2009; Sun et al., 2012; Sukor et al., 2017; Hassan et al., 2017 ). TiO₂ has a wide band gap with 3.0–3.2 eV, which makes it has strong absorption capacity in the UV range and little absorption capacity in the visible range. This property restricts catalytic activity in the visible light range. At the same time, in order to obtain excellent photocatalysis properties, TiO₂ materials are usually made into products with nano scale. TiO₂ nanoparticles have high specific surface area and own strong surface energy, which makes it agglomerate in water easily and arouse photocatalysis activity to decrease greatly. These aforementioned shortcomings have often prevented TiO₂ materials from further being applied to different fields.

To enhance the catalytic activity of TiO₂ under visible light, doping of TiO₂ with different elements, including wildly being explored both metallic and nonmetallic elements. Doping will help bring impurity level in the band gap, and then the impurity level can create splitting of energy levels of TiO₂. In this way, absorption under visible light can be improved. At the same time, doping also helps form crystal defects, which can reduce the rate of electron-hole recombination, thereby improving the efficiency of photocatalytic reaction. Kubacka’s group prepared N,W-codoped TiO₂ using ammonium tungsten oxide as nitrogen source and tungsten source, Triton X-100 as surfactant and hexanol as cosurfactant, and then maintaining calcine under 500 °C. The N,W-codoped TiO₂ has high photocatalytic activity, but it can’t disperse in water (Periyat et al., 2009; Halim et al., 2017; Rahman et al., 2017 ). Elaine's group uses tungstic acid and melamine borate as dopants to prepare C, W co-doped TiO₂ under the 400 °C calcination, and the result shows that W atoms can diffuse into the TiO₂ lattice, which instead increases in visible-light activity of TiO₂. However, high-temperature calcination would lead TiO₂ to agglomerate. The agglomeration of TiO₂ greatly decreases the dispersion in aqueous solution (Elaine et al., 2012; Basheer et al., 2017; Ismail and Hanafiah, 2017 ). Di Li’s group had used high-temperature treatment device for atomizing to prepare F,N-codoped TiO₂ (Li et al., 2005; Aziz and Hanafiah, 2017) _. This F,N-codoped TiO₂ has some capacity for absorbing visible light, but the dispersibility in water is poor. Sakthivel’s group synthesized N doped TiO₂ using thiourea as N source. The band gap of obtained TiO₂ can decrease to 2.91 eV, but it would agglomerate after calcination at 400–600 °C. In this way, the dispersibility becomes worse (Sakthivel et al., 2004; Khan et al., 2017). Doping can more or less improve the photocatalysis activity of TiO₂ under visible light. However, high temperature calcination in these methods often lead the obtained TiO₂ can hardly disperse in water (Elaine et al., 2012; Jobbagy et al., 2008; Kiraz et al., 2011; Sakthivel et al., 2004; Tachikawa et al., 2004; Chen et al., 2007; Serpone, 2006; Li et al., 2005; Shamsudin et al., 2017; Ghafar et al., 2017 ).

To improve the dispersible property of TiO₂ nanomaterials in water, surface modification with organic chelating ligands, such as oleic acid, oleate, and dopamine is a common method. Surface modification can reduce the number of surface hydroxyl groups, thereby reducing the TiO₂ surface energy and increasing the dispersibility of TiO₂. For instance, tetrabutylammonium modified TiO₂ nanoparticles dispersed both in the water and ethanol, but tetramethylammonium modified TiO₂ nanoparticles can only disperse in water (Chen et al., 2007; Razali and Said, 2017). Surface modification of dopamine is responsible for controlling the morphology, crystallinity, phase selection, and water dispersion of TiO₂ (Tahir et al., 2006; Halim and Phang, 2017). However, the TiO₂ catalytic reaction is mainly carried out in the particles surface. Surface modification may increase the thickness of the TiO₂ surface and reduce the activity of TiO₂ response to light. Efficiency of the catalytic reduction of TiO₂, therefore, is lower. Thus, how to greatly improve TiO₂ dispersion without surfactant modification is a challenging and meaningful task (Ai et al., 2014).

For study, we choose NH₄Cl as the nitrogen source to synthesize N-doped TiO₂ nanoparticle. Since the radius of an oxygen atom and a nitrogen atom are similiar, in the preparation process, the nitrogen atoms can directly dop into the TiO₂ lattice without calcination process. Meanwhile, by avoiding the heat treatment and surface modification, the obtained TiO₂ not only can disperse easily in water, but also has significant catalytic activity under visible light. Most importantly, this method is simple with no strict requirements for equipment. It can easily achieve large-scale industrial production with high dispersion and significant catalytic activity by this method.

2 Experimental section

2.1

2.1 Synthesis of TiO₂ nanoparticles

Firstly, 0.3 g NH₄Cl is added to 50 ml of ethanol, stirring at 60 °C for 30 min. After 30 min, part of NH₄Cl solid has been dissolved and the mixture solution, filtrated by filter paper to obtain saturated NH₄Cl solution. Then, 30 ml TiCl₄ is pumped into the saturated NH₄Cl solution carefully by peristaltic pump under constantly stirring at room temperature, and the solution would change from colorless to yellow color. After TiCl₄ is pumped, the yellow solution is stirred for another 5 min, then moved into reaction kettle (Teflon), and heated at 150 °C for 5 h. Next, the reaction kettle is cooled at room temperature and a white solid is obtained. The solid is washed three times with 50 ml of ethanol each time, and subsequently dried at 55 °C for 45 min. The resulting product is powdered for further experiment and characterizing. Without adding NH₄Cl, the undoped TiO₂ nanoparticles can be prepared in the same method.

2.2

2.2 Characterization

The crystal phases of TiO₂ nanoparticles were identified in accordance with X-ray diffraction (XRD) patterns on a Bruker D8 Advance X-ray diffractometer with Cu Ka radiation. The morphology of TiO₂ nanoparticles was observed by high-resolution transmission electron microscopy (HRTEM) on a JEOL JEM-2010 electron microscope at 200 kV. The TEM samples were prepared by dispersed TiO₂ nanoparticles powder in water and then dropped on carbon–copper grid. Finally, the copper grid deposited with TiO₂ nanoparticles were observed by HRTEM. X-ray photoelectron spectroscopy (XPS) was recorded with a Kratos ASIS-HS X-ray photoelectron spectroscope equipped with a standard and monochromatic source (AlKR) at 150 W (15 kV, 10 mA). The binding energies obtained in the XPS analysis were corrected for specimen charging by referencing the C 1s line to 284.5 eV. The optical absorption spectra of the samples were recorded by UV–vis spectra at room temperature with the UV–visible spec-trophotometer equipped with an integrated sphere. Thermal gravimetry analysis was determined on STA 409 PC Luxx differential scanning calorimetry-thermo-gravimetric (DSC-TG) at the heating rate of 10 ^◦C min⁻¹ under nitrogen condition.

2.3

2.3 Study on photocatalytic properties

For the photocatalytic property of prepared TiO₂ nanoparticle, solar light and LED lamp are used as light source to degrade methylene blue. Firstly, 6.7 × 10⁻⁵ M methylene blue solution was prepared by dissolving 0.01 g methylene blue in 400 ml deionized water. Next, 0.25 g prepared TiO₂ nanoparticle powder was added to 50 ml of methylene solution with blue color and the powder and solution were mixed uniformly. According to the method, the desired concentration (0.5 g/L) of TiO₂-methylene blue solution can be obtained. Finally, to study the photocatalytic activity of prepared TiO₂ nanoparticle, TiO₂-methylene blue solution was divided into two parts and placed under the sunlight and LED light respectively. To monitor the reaction process for photocatalytic degradation, the UV–vis spectra of TiO₂-methylene blue solution was measured in the same time interval during the photocatalytic activity experiments.

3 Results and discussion

3.1

3.1 Characterizations of TiO₂ nanocrystals

Fig. 1 refers to the TEM image of N-doped TiO₂ nanoparticle in different amplifications. TEM image analysis reveals that the as-prepared N-doped TiO₂ nanoparticle is spindle-shaped particles with the size from 8 nm- to 15 nm. Fig. 1(c) further displays the higher magnification of the nanocrystal lattice fringes of (1 0 1) crystallographic planes with lattice space of 0.36 nm in Fig. 1(d).

Fig. 2 shows the XRD spectra of N-doped TiO₂ drying at different temperatures. The peaks with the position of 25.3, 37.68, 47.82, 53.74, 55.16, and 62.57, are represented to (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), (2 0 4) planes, respectively. The XRD result proves that as-prepared N-doped TiO₂ nanoparticles dried in different temperature are all anatases (Ai et al., 2014; Yurdakal et al., 2008; Du et al., 2013 ). Based on Scherrer formula, it can be calculated that the average particle size of the prepared TiO₂ are 8.72 nm, 9.00 nm and 12.33 nm at 45 °C, 55 °C and 65 °C respectively, and the calculated diameter is similar to that of electron microscopy. It can be easily concluded that the average particle size of TiO₂ nanoparticles gradually increases with the drying temperature increasing also. Meanwhile, the intensity peaks of XRD also increases with the drying temperature rising. Hence, the degree of crystallinity of TiO₂ also increases with the drying temperature increasing. The peak intensity of TiO₂ nanoparticles drying under 45 °C is lower than that of the standard anatase TiO₂, that is, the degree of crystallinity of TiO₂ drying under 45 °C is insufficient. When the temperature is 55 °C and 65 °C, the intensity of peak is higher than that of standard anatase TiO_2. From 55 °C to 65 °C, the increasing scale of the peak intensity is not obvious. It indicates that the degree of crystallinity TiO₂ nanoparticles reaches a maximum drying degree under 55 °C.

XRD spectra of N-dioped TiO2 dried at different temperatures. (a) Standard anatase TiO2, (b) N-dioped TiO2 dried under 45 °C, (c) N-dioped TiO2 dried under 55 °C, and (d) N-dioped TiO2 dried under 65 °C.

Fig. 3a shows the XPS spectra of N-doped TiO₂ nanoparticle. In this spectrum, it can be found easily that the peaks of O1s, Ti2p, N1s, C1s, and Cl2p. in Fig. 3b is the XPS spectra of Ti2p. From the figure, the peaks at 458.98 eV and 464.68 eV represent Ti2p_3/2 and Ti2p_1/2, respectively (Yu et al., 2009; Zhang et al., 2015; Asahi et al., 2014; Zhu et al., 2009 ). Fig. 3c shows the XPS spectra of N1s. The XPS spectra of N1s can be fitted by two peaks centered at 399.78 eV and 400.25 eV respectively. The peak of 400.25 eV can be ascribed to Ti—N—O linkage. And the peak of 399.78 eV mainly comes from the NH₄Cl which is adsorbed on the surface of TiO₂ nanoparticle (Li et al., 2005; Asahi et al., 2001; Irie et al., 2003; Kumar et al., 2013; Cong et al., 2006; Yu et al., 2009; Zhu et al., 2009 ). Comparing the intensity of two peaks, it can be inferred that the doped nitrogen atoms of TiO₂ nanoparticle mainly come from the nitrogen atoms which enter the crystal lattice of the TiO₂ nanoparticle to form Ti—N—O linkage, and a small of nitrogen atoms from the NH₄Cl which is absorbed on the surface of TiO₂ nanoparticle. Fig. 3d refers to the XPS spectra of C2s, the three peaks of C2s at 284.66 eV, 286.11 eV and 288.78 eV, respectively. The peak at 284.66 eV is attributed to pure carbon (C—C). It results from the adsorbed carbon on the surface of the sample during the XPS sample pretreatment. Also, the other two peaks can be ascribed to carbon bound to oxygen (C—O) (Huang et al., 2008). It is indicated that carbon atom can be introduced into the system when ethanol is used as the solvent for preparation of N-doped TiO₂ nanoparticle.

XPS spectra of TiO2 nanoparticle. (a) the full XRD spectrum of TiO2 nanoparticle, (b) XRD spectra of Ti2p, (c) XRD spectra of N1s, and (d) XRD spectra of C2s.

XPS data shows that NH₄Cl is used as the nitrogen source, ethanol as the solvent, except for a part of nitrogen doped into the space lattice of titanium dioxide. At the same time, epibiotic ammonium chloride and carbon can be adsorbed onto the surface of TiO_2. The O replaced by N will form defects, which facilitate to introduce impurity level to the forbidden band. Introducing impurity level helps decrease the recombination of electric-hole pairs and increase the utilization rate of light.

Fig. 4 is the TG of N-doped TiO₂ nanoparticle. As shown in step1, when the temperature increases from 30 °C to 161.8 °C under N₂ flow, the weight loss of N-doped TiO₂ is 0.8 mg, and the weight loss rate, 7.06%. It can be the water absorbed on the surface of TiO₂ nanoparticle, and the boiling point of water is 100 °C. Therefore, the weight losses in step 1 probably mainly come from the water evaporation. From 161.8 °C to 700 °C, the loss rate begins to decrease and quality stabilizes to approximating 700 °C. In this step, the mass loss is 0.68 mg. It is speculated that the weight loss results from carbonizing carbon components on the surface of TiO₂.

Fig. 5 is the FT-IR spectrum of N-doped TiO₂ nanoparticle. Above 2000 cm⁻¹, the FT-IR spectrum is dominated by the broad bands of O—H and N—H centered at 3300 cm⁻¹, which is greatly determined by C—OH groups or N—OH groups. The C—H stretching vibrations of the —CH₂— groups of the hydrocarbon at 2921.3 cm⁻¹ and 2851.6 cm⁻¹ are observed. The peak at 1629.2 cm⁻¹ comes from N—H bending vibration, and the peak at 1065.9 cm⁻¹ is attributable to C—N stretching vibration (Tian et al., 2011; Shan and Demopoulos, 2010; Han et al., 2012 ). FT-IR spectrum confirms that carbonaceous species and nitrogenous species exist, including C—H, C—O, C—N, N—H, and N—O. From the aforementioned XPS, TG and FT-IR characterizations, the saturated NH₄Cl ethanol solutions can introduce carbonaceous species and nitrogenous species. By introducing carbonaceous species and nitrogenous species, the surface defect states of TiO₂ can be changed and agglomeration can be prevented. Also, it helps endow with the property of highly water dispersiblility and photocatalytic properties.

FT-IR spectrum of N-doped TiO2 nanoparticle.

Fig. 6 shows the UV–vis spectrum of undoped TiO₂ nanoparticle and N-doped TiO₂ nanoparticle in different concentrations of NH₄Cl. With the increasing amount of NH₄Cl in ethyl alcohol, in the visible region, the absorbance rate of TiO₂ gradually increases. In the ultraviolet region from 350 to 400 nm, the change is significant. However, regularity remained unchanged. From 400 to 600 nm, with the increase in the amount of NH₄Cl in ethyl alcohol, the absorbance of N-doped TiO₂ nanoparticle gradually helps increase regularity. The result shows that under visible light, doping N atoms into TiO₂ can improve the absorbance of TiO₂. Fig. 7 shows the curve of energy gap calculated based on Kubelka-Munk function [F(R∞)E]² vs the energy of absorbed light. From the extension cord of the curve, the energy band gap can be calculated as 3.33 eV, 3.30 eV, 3.21 eV and 3.15 eV for un-doped TiO₂, 0.6 g/L NH₄Cl N-doped TiO₂, 3.0 g/L NH₄Cl N-doped TiO₂, and 6.0 g/L NH₄Cl N-doped TiO₂ respectively . It is obvious that with the increasing amount of N atoms in TiO₂ nanoparticle, the band gap of TiO₂ nanoparticle becomes smaller.

UV–vis spectrum of N-doped TiO2 nanoparticle prepared in different concentration of NH4Cl. (a) Undoped TiO2 nanoparticle, (b) N-doped TiO2 nanoparticle prepared in 0.6 g/L NH4Cl EtOH solution, (c) N-doped TiO2 nanoparticle prepared in 3.0 g/L NH4Cl EtOH solution, and (d) N-doped TiO2 nanoparticle prepared in 6.0 g/L NH4Cl EtOH solution.

The energy gap of N-doped TiO2 nanoparticle prepared in different concentrations of NH4Cl. (a) Undoped TiO2 nanoparticle, (b) N-doped TiO2 nanoparticle prepared with 0.6 g/L NH4Cl EtOH solution, (c) N-doped TiO2 nanoparticle prepared with 3.0 g/L NH4Cl EtOH solution, and (d) N-doped TiO2 nanoparticle prepared with 6.0 g/L NH4Cl EtOH solution. The illustration shows the tendency of the energy gap between different N-doped TiO2s.

3.2

3.2 Photocatalytic degradation of methylene blue with highly water dispersible N-TiO2 nanoparticle

In order to assess the photocatalytic activity of water-dispersible N-doped TiO₂ nanoparticle under visible light, LED was used as the visible light source to degrade methylene blue. Figs. 8 and 9 are the UV–vis spectrum of methylene blue solution after degraded by different TiO₂ nanoparticles. With the comparison with the absorbance at 650 nm, it can be found that N-doped TiO₂ nanoparticle can completely degrade methylene blue by irradiating 5 h under LED light. However, commercially available undoped TiO₂ nanoparticle cannot be degraded completely. This suggests that as-preparation N-doped TiO₂ nanoparticle has high light response to visible light, and it can further prove that N-doped TiO₂ nanoparticle has a smaller energy level comparing with undoped TiO₂ nanoparticle.

UV spectrum of N-doped TiO2 nanoparticle degrading methylene blue under LED light.

UV spectrum of commercially available undoped TiO2 nanoparticle degrading methylene blue under LED light.

As Fig. 10a shows, it is clear that the degradation rate of methylene blue by N-doped TiO₂ nanoparticle is faster than that of commercially available undoped TiO₂ nanoparticle under the LED lights irradiation. Fig. 10b shows the kinetic equation for N-doped TiO₂ naoparticle and undoped TiO₂ nanoparticle. The ln (Co/C) and the value of the time T are in accordance with the first-order kinetic equation y = Kx fitting. From the figure, we can see that the N-doped TiO₂ nanoparticle and commercially available undoped TiO₂ nanoparticle follows the first-order kinetic equation. The first-order kinetic equation of N-doped TiO₂ nanoparticle is y = 0.20579x, R² = 0.97454, and the reaction rate constant K_p = 0.20579. The first-order kinetic equation of commercially available undoped TiO₂ nanoparticle is y = 0.09583x, R² = 0.92471, and the reaction rate constant K_c is 0.09583. Comparing K_p and K_c, it is clear that the reaction of N-doped TiO₂ nanoparticle requires less time than that of commercially available undoped TiO₂ nanoparticle. Fig. 11 just shows the digital image of methylene blue solution after being degraded by undoped and N-doped TiO₂ nanoparticle under LED light with 5 h, respectively.

(a) Comparison with degradation of N-doped TiO2 nanoparticle and commercially available undoped TiO2 nanoparticle and (b) degradation kinetic equation.

Digital image of degradating methylene blue under LED light for 5 h. N-doped TiO2 nanoparticle (on the left) and commercially available undoped TiO2 nanoparticle (on the right).

Based on the results, a explanation mechanism has been proposed. In the preparation of N-doped TiO₂ nanoparticle, since the hydrothermal reaction, NH₄Cl can be broken down into NH₃ and HCl during the reaction 1. And TiO₂ will react with NH₃ following the reaction Eq. (2). The fact that low-level XPS N doping was observed indicates that NH₄Cl decomposition is very important for the N doping.

(1)

{NH}_{4} Cl \to {NH}_{3} + HCl

(2)

{TiO}_{2} + {NH}_{3} \to {TiO}_{2 - x} N_{x} + H_{2} O

(3)

{TiCl}_{4} + {nH}_{2} O \to Ti {(OH)}_{n} {Cl}_{4 - n} + nHCl

The water generated in reaction 2 will promote hydrolysis of TiCl₄. This is also true for reaction 3. The Ti(OH)_nCl_4-n generated in reaction 3 will condensate further and produce TiO₂. The produced TiO₂ turns to react with NH₃ as reaction 2 shows. This cycle will generate the N-doped TiO₂ nanoparticle, according to Fig. 12.

Preparing flowchart of N-doped TiO2 nanoparticle.

Since TiO₂ is a wide band gap semiconductor, to generate electron-hole pairs under the visible light irradiation the electron in valence band cannot jump directly to the conduction band. To solve this problem, impurity level must be introduced into the band gap of TiO₂ by doping N atom. The reaction mechanism is shown in Fig. 13. In N-doped TiO₂ nanoparticle, the doped N atoms form impurity level, which is located above the valence band, so it helps reduce the energy band gap of TiO₂. When visible light irradiates to the N-doped TiO₂ nanoparticle, the electrons are excited from N2p states to transmit to the conduction band of TiO₂ (N2p to Ti3d) with holes left in N2p state. Based on quantum effects, the electron and hole generated by light irradiation move quickly to the surface of TiO₂. The only exception is that they are recombined or trapped. The excited electrons at the conduction band can transfer to new Ti³⁺ states and then to deeper states of lower potential below conduction band. Ti³⁺ and surface trapped electrons contribute to the photocatalytic of N-doped TiO₂ nanoparticle. Simultaneously, some of the generated photoelectrons will be recombined with the hole located at the N2p state (Kumar et al., 2012; Katoh et al., 2010; Berger et al., 2005 ). However, as the conduction band position of TiO₂ is −0.5 V vs NHE, more negative than the reduction potential (−0.33 V vs NHE) of O₂/^•O₂⁻, the electrons on the conduction band will react with the surface chemisorbed O₂ to yield the superoxide anion radicals (^•O₂⁻) and further produce hydroxyl radicals (^•OH) through multi-electron reactions. Meanwhile, carbon on the surface of TiO₂ may serve as photosensitizer, which can be excited under visible light irradiation. These excited electrons are injected into the TiO₂ conduction band to play the role of sensitization (Ai et al., 2014; Chen et al., 2012). These excited electrons will react with O₂ absorbed on the surface of TiO₂ to produce ^•O₂⁻ and ^•OH subsequently. This process can increase the number of hydroxyl radicals, thereby improving the efficiency of photocatalytic. Thus, by doping N aotm into TiO₂ to reduce the band gap, the yield of free electron in conduction band under visible light can be increased. Meanwhile, through sensitization of C atom, the number of free electrons in the conduction band can be increased to enhance the efficiency of photocatalytic. The two methods coordinate to improve the efficiency of photocatalytic for TiO₂ under visible light.

Photocatalytic mechanism of N-doped TiO2 nanoparticle.

4 Conclusions

In this paper, N-doped TiO₂ nanoparticle was prepared by non-aqueous solvent-thermal method under low temperature. The obtained TiO₂ not only has high photocatalytic activity in visible light, but also shows high dispersibility in water. According to TEM and XRD characterizations, the obtained N-doped TiO₂ is spindle-shaped particles with the size of 8–15 nm. Based on XRD and XPS spectra analysis, it can be revealed that after N elements are doped into the TiO₂ lattice, the impurity level can be formed in the band gap of TiO₂. In this way, TiO₂ can extend to absorb lights from UV to visible light. The photocatalytic degradation of methylene blue experiment confirms that the photocatalytic activity of TiO₂ is enhanced obviously under visible light after doping with N atom. At the same time, ethanol is used as a solvent to form carbonaceous species on the surface of TiO₂. Then, the carbonaceous species can excite electrons into the conduction band of TiO₂ under visible light and increase the number of electrons in the conduction band. In this way, the visible light catalytic activity of TiO₂ can be increased. It is through the N doping and sensitized C that the surface state of TiO₂ can be changed, thereby increasing its visible light activity and high water-dispersiblity.

Acknowledgments

This work is financially supported by the fund of the Key Laboratory for Processing of Sugar Resources of Guangxi Higher Education Institutes (2015TZYKF02), the Scientific Research and Technology Development Project of Guangxi Province (1348018-3), and the National Natural Science Foundation of Guangxi Province (2014GXNSFAA118402).

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Introduction

Experimental section

Synthesis of TiO2 nanoparticles
Characterization
Study on photocatalytic properties

Results and discussion

Characterizations of TiO2 nanocrystals
Photocatalytic degradation of methylene blue with highly water dispersible N-TiO2 nanoparticle

Conclusions

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[23] Khan, I.U., Sajid, S., Javed, A., Sajid, S., Shah, S.U., Khan, S.N., Ullah, K., 2017. Comparative Diagnosis Of Typhoid Fever By Polymerase Chain Reaction And Widal Test In Southern Districts (Bannu, Lakki Marwat And D.I. Khan) Of Khyber Pakhtunkhwa, Pakistan. Acta Scientifica Malaysia, 1(2), 12–15.

[24] Kiraz N., Burunkaya E., Kesmez O., Camurlu H., Asilturk M., Yesil Z., Ertugrul A., . Preparation of Sn doped nanometric TiO₂ powders by reflux and hydrothermal syntheses and their characterization. J. Sol-Gel Sci. Technol.. 2011;59:381-386.
[Google Scholar]

[25] Kumar A., Patel A.S., Mohanty T., . Correlation of photodegradation efficiency with surface potential of silver-TiO₂ nanocomposite thin films. J. Phys. Chem. C. 2012;116:20404-20408.
[Google Scholar]

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[Google Scholar]

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[28] Periyat P., McCormack D.E., Hinder S.J., Pillai S.C., . One-pot synthesis of anionic (nitrogen) and cationic (sulfur) codoped high-temperature stable, visible light active, anatase photocatalysts. J. Phys. Chem. C. 2009;113:3246-3253.
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[43] Zhu J., Zhang D., Bian Z., Li G., Huo Y., Lu Y., Li H., . Aerosol-spraying synthesis of SiO₂/TiO₂ nanocomposites and conversion to porous TiO₂ and single-crystalline TiOF₂. Chem. Commun.. 2009;36:5394.
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Synthesis of highly water-dispersible N-doped anatase titania based on low temperature solvent-thermal method