A series of nitrogen-doped mesoporous TiO2 nanomaterials and nitrogen-doped mesoporous TiO2/reduced graphene oxide (RGO) composites were successfully prepared by hydrothermal method using triammonium citrate as the nitrogen source. The effects of nitrogen and graphene oxide (GO) dopping on the photocatalytic properties of the TiO2 were investigated to optimize preparation conditions. The results showed that all prepared samples were mainly composed of the anatase phase and possessed a mesoporous structure. The use of the triammonium citrate not only significantly increased the specific surface area of the synthesized samples but also caused the partial reduction of GO to RGO, leading to further increase of the specific surface area and the improvement of quantum efficiency of the photogenerated electrons. All synthesized samples showed superior photocatalytic performance for methyl orange solution. Among them, the NMT/RGO-1.8-10 was found to be the best; the degradation rate of methyl orange solution on the sample reached 100% in 30 minutes under visible light irradiation.
As extensively reported in the literature, the TiO2 is widely used in many areas, such as environmental pollution control, new energy and biopharmaceuticals, due to its low-cost and nonpollutant properties, and strong chemical stability [
Graphene is a new two-dimensional honeycomb carbon material. It is composed of a single layer of carbon atoms. It has a specific surface area up to 2630 m2·g-1 and has very high electron mobility, which can rapidly transfer electrons and is beneficial for accelerating many catalytic reactions. Meanwhile, its periodically arranged two-dimensional planar structure makes it become an ideal catalyst carrier [
In this study, a series of nitrogen-doped mesoporous TiO2 nanomaterials and nitrogen-doped mesoporous TiO2/RGO composites were prepared by a hydrothermal method using triammonium citrate (TC) as the nitrogen source. Here, the TC not only played a good structure the directing agent role but also acted as a reducing agent, allowing the formation of partially reduced GO, which further increased the specific surface area and photogenerated electron efficiency of the resulting composite. Meanwhile, utilizing triammonium citrate also provides a nitrogen source for the composite and thus can effectively improve the photoresponse range and visible light photocatalytic performance.
Figure
Schematic illustration of photocatalytic enhancement mechanism of N-doped mesoporous TiO2/RGO composites.
Natural graphite scales (599 mesh) were purchased from Jinhua Biotechnology Co. Ltd., Inner Mongolia, China. Potassium permanganate (AR) was purchased from Yonghui Chemical Co. Ltd., Tianjin, China. Titanium sulfate (AR) and hydrogen peroxide (AR) were purchased from Beilian Fine Chemicals Development Co. Ltd., Tianjin, China. Polyethylene glycol (AR) was purchased from Guangfu Fine Chemical Research Institute, Tianjin, China. TC (AR) was purchased from Damao Chemical Reagent Factory, Tianjin, China. Anhydrous ethanol (AR) was purchased from Chemical Factory, Beijing, China. Concentrated hydrochloric acid (AR), methyl orange (AR), and concentrated sulfuric acid (AR) were purchased from Yongsheng Fine Chemical Co. Ltd., Tianjin, China. They were used without further purification.
GO was prepared via a modified Hummers method reported by Hummers and Offeman [
0.028 g of polyethylene glycol was dissolved in 10 mL of distilled water, and then 1.2 g of titanium sulfate was added into the solution, followed by stirring for 20 min. After that, 0.24 g of TC was added and stirred for 10 min and the solution was hydrothermally reacted at 110°C for 24 h. The obtained solid was washed with absolute ethanol and distilled water and dried at 50°C for 6 h. In order to investigate the effect of TC on the properties of the nitrogen-doped TiO2 photocatalyst, a series of samples were prepared using different dosages of TC. Thus, prepared N-doped mesoporous TiO2 (NMT) are hereafter referred to as NMT-MR, where MR is the mass ratio of TC to TiO2. The following five samples were prepared in the same way mentioned above: NMT-0, NMT-0.6, NMT-1.2, NMT-1.8, and NMT-2.4.
A series of nitrogen-doped mesoporous TiO2/RGO composites were prepared by adding different amounts of GO under the optimum amount (
The prepared samples were characterized by XRD (X-ray powder diffraction) (Ultima IV, Rigaku Corporation, Japan), N2 adsorption/desorption analysis (iQ, Quantachrome Corporation, America), TEM (transmission electron microscopy) (Tecnai G2F20, FEI Electron Microscopy Corporation, America), XPS (X-ray photoelectron spectroscopy) (Axis Ultra, Shimadzu Corporation, Japan), Raman (LabRAM HR Evolution, Horichang Group Co. Ltd., Longremo city, France), FTIR (Fourier transform infrared spectroscopy, Nicolet 6700, Nicolet Corporation, America), PL (photoluminescence) (F4500, Hitachi Limited, Japan), and UV–vis spectroscopy (UV-Vis 2550) (Shimadzu Corporation, Japan).
To prepare the solution for photocatalysis characterization, 0.25 g of the catalyst was dispersed in 250 mL (20 mg/L) of methyl orange (MO) solution. Then the resultant mixture was stirred in darkness for 30 min for adsorption-desorption equilibrium at constant temperature, followed by the irradiation with a 300W BELSRI/UV-type high-voltage xenon lamp in a XPA-type photochemical reactor. Samples were withdrawn at 10 min intervals, and the supernatant was separated by centrifugation at 10000 rpm for 20 min. The concentration of the MO in the supernatant was measured by a 722G visible spectrophotometer at 464 nm. To elucidate which active species play a key role in photodegradation of MO under visible light irradiation, control experiments were also carried out by adding different radical scavengers with the same concentration of 3 mmol/L.
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XRD patterns of (a) NMT-MR and (b) NMT/RGO-MR-m composites.
Figure
Figures
N2 adsorption-desorption isotherms (a) and pore size distribution plot (b) of NMT-0, NMT-1.8, and NMT/RGO-1.8-10.
The grain size, most probable aperture, specific surface area, mean pore size, and total pore volume of NMT-0, NMT-1.8, and NMT/RGO-1.8-10.
Samples | NMT-0 | NMT-1.8 | NMT/RGO-1.8-10 |
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Grain size (nm) | 9.2 | 4.4 | 4.0 |
Most probable aperture (nm) | 17.3 | 21.6 | 19.3 |
Specific surface area (m2·g-1) | 154 | 325 | 412 |
Mean pore size (nm) | 13.5 | 8.9 | 5.6 |
Total pore volume (cm3·g-1) | 8.6 | 11.9 | 8.5 |
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TEM images of NMT-1.8 nanophase material (a) and NMT/RGO-1.8-10 composite with different magnification of 200 nm (b) and 20 nm (c).
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(a) The survey XPS spectra of NMT-1.8. (b) High-resolution C 1s core level XPS spectra of NMT-1.8. (c) High-resolution N 1s core level XPS spectra of NMT-1.8. (d) High-resolution S 2p core level XPS spectra of NMT-1.8.
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The spectra of different samples: (a) Raman spectra, (b) FT-IR spectra, (c) PL spectra, and (d, e) UV-vis patterns.
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The PL spectra of NMT-0, MT/GO-0-10 and NMT/RGO-1.8-10 are shown in Figure
Figures
In the process of photocatalytic testing, samples were stirred in a dark room for 30 min to achieve adsorption equilibrium and then irradiated by visible light. Figure
The photocatalytic degradation curves of MO by N-doped mesoporous TiO2 (a) and N-doped mesoporous TiO2/RGO composites (b) under visible light irradiation.
The quasi-second-order dynamic model equation can be expressed as follows:
After the integral, the linear form is expressed as follows:
Quasi-second-order dynamic plot for MO degradation on NMT/RGO-1.8-10.
Result of data fitting for the quasi-second-order dynamic model.
Sample | Quasi-second-order kinetic equation | Linearly dependent coefficient | Rate constant |
---|---|---|---|
NMT/RGO-1.8-10 | 0.9997 | 0.914 |
In order to further study the photocatalytic stability and reusability of the composites, the following study was carried out on sample NMT/RGO-1.8-10. The photocatalytic activity of the sample against MO solution under visible light irradiation for a long time (3 h) was tested, and the experiment was repeated three times at the same condition. It can be seen in Figure
The photocatalytic degradation curve of MO on NMT/RGO-1.8-10 under long time irradiation (a), the XRD patterns of the NMT/RGO-1.8-10 before and after the photocatalytic reactions (b), and UV-visible absorption spectra of the MO solution during the visible light irradiation in the presence of NMT/RGO-1.8-10 (c).
Figure
The differential thermal analysis curve (a) and thermogravimetric curve (b) of NMT/RGO-1.8-10.
Figure
In order to clarify the photodegradation mechanism of NMT/RGO-1.8-10 for MO under visible light, the effect of several scavengers was investigated. In general, the photodegradation reaction of dyes on the TiO2-based catalyst not only produces the e−/h+ pair but also generates different reactive species such as
Effect of different radical scavengers (3 mmol/L) on the photodegradation of MO by NMT/RGO-1.8-10.
A series of nitrogen-doped mesoporous TiO2 nanomaterials and nitrogen-doped mesoporous TiO2/RGO composites were prepared by hydrothermal method. For this, TC was used as a nitrogen source and reducing agent, which enabled the prepared samples to have good dispersibility, increased specific surface area, and mesoporous structure. The RGO-doping can further increase the specific surface area of nitrogen-doped mesoporous TiO2 and effectively reduce the recombination rate of the photogenerated electron hole pairs, leading to the marked enhancement of visible photocatalytic performance. Among all the prepared samples, NMT/RGO-1.8-10 exhibited the best performance, with a grain size of 3.7 nm, most probable aperture of 19.3 nm, and a surface area of 412 m2·g-1. It exhibited a strong adsorption-photocatalytic double effect. And the degradation rate of MO on the sample can reach 100% within 30 min under visible light irradiation.
The data (XRD, BJH, TEM, XPS (C 1s, N 1s, and S 2p), Raman, FT-IR, PL, UV-vis, and photocatalytic degradation curve relative to composites) used to support the findings of this study are included within the article.
The authors declare that there is no conflict of interest regarding the publication of this paper.
This project is supported by the National Natural Science Foundation of China (21367020), Natural Science Foundation of Inner Mongolia Autonomous Region (2016MS0226), Inner Mongolia Autonomous Region Key Projects of Colleges and Universities (NJZZ19017), and Graduate Innovation Foundation of Inner Mongolia Normal University (CXJJS17082).