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Photocatalytic Properties of Nitrogen-Doped Bi12TiO20 Synthesized by Urea Addition Sol-Gel Method
Undoped and nitrogen-doped Bi12TiO20 materials were synthesized by urea addition sol-gel method. By adding urea, undoped, and N-doped gel-type precursors were synthesized by low-temperature dehydrolyzation. Nitrogen-doped and undoped nanocrystalline Bi12TiO20 were prepared by annealing at 600∘C for 30 minutes. From UV-Vis absorption and diffuse reflection spectrum, the absorbing band shifted from 420 to 500 nm by nitrogen doping. The bonds of Ti–N and N–O were identified by XPS spectra from the prepared materials, and the enhancement of visible light absorption was attributed to nitrogen's substitution of oxygen. Photocatalytic properties of prepared materials were characterized by the decomposition of Rhodamine B illuminated by whole spectra of 300 W Xe light. The photocatalyst () with N/(N+O) mole ratio about 3% shows better performance than that of heavily doped (), undoped Bi12TiO20, and light-doped () photocatalysts due to its better crystalline morphology.
Environmental and energetic problems are great challenges for human beings now. The photosensitized electrolytic oxidation  and the electrochemical photolysis of water  on TiO2 electrode performed by Fujishima and Honda offered some solutions. In recent years, TiO2 and other photocatalysts have been extensively studied for environmental pollutant treatment  such as water disinfection .
However, wide bandgap semiconductor photocatalysts, such as TiO2, can only absorb UV light, which only take 4% of the whole solar spectra owing to the wide bandgap. In order to expand the absorption spectra region of TiO2, doping with nonmetallic atoms, such as nitrogen, fluorine and sulfur has been developed [5–18]. The conventional doping processes reported were commonly by the annealing of prepared TiO2 materials in NH3 or other dopant atmospheres [5–8]. Rengifo-Herrera et al. have reported N, S codoped commercial TiO2 powders  which were doped by the decomposition of thiourea as a nitrogen and sulfur source. Effects of calcination temperatures , morphologies , and composite  on photocatalytic activity were also studied.
Recently, photocatalysts containing ion units of (Bi2O2)2+, such as Bi2WO6 [22, 23] and bismuth titanate were extensively studied for the photocatalytic splitting of water and the photodegradation of organic pollutants. Bismuth titanate was a wide bandgap semiconductor with several crystal phases: ferroelectric perovskite phase (Bi4Ti3O12), dielectric pyrochlorite phase (Bi2Ti2O7), refractive sillenite phase (Bi12TiO20), and so forth. Kudo and Hijii have studied bismuth titanate as potential photocatalyst . Yao et al. [24–27] and other groups [28–36] explored nano crystalline materials of Bi12TiO20 as visible light photocatalysts, and the as-prepared materials showed high photocatalytic activity for decomposing organic dyes under ultraviolet light irradiation. The further expansion of Bi12TiO20 material’s absorption to visible region is very important for the visible light responsive photocatalytic activity, because Bi12TiO20 can only absorb light with wavelength below 420 nm (about 2.9 eV) [21–24]. Little work has been performed for nonmetallic (such as nitrogen and carbon) doping of bismuth titanate crystals, though the doping of Bi12TiO20 nano- or single crystals with metal element [25, 26] and nonmetallic element such as P for the influence of refractive optical properties has been studied .
In this paper, nitrogen-doped bismuth titanate powders and films were prepared by sol-gel method and dip-coating method, respectively, the nitrogen dopant was introduced by the adding of urea in precursor solutions as additives, which is widely used as fertilizer. Urea can be dissolved in water and other polar solvents, and can release free radicals as nitrogen sources during thermal decomposition. After preparation of urea included sol-gel precursors, high-temperature annealing was applied to undoped and doped Bi12TiO20 precursors for crystallization and also for the in-place nitrogen doping by urea decomposing. The results of whole-spectra photocatalytic activities of nitrogen-doped and undoped Bi12TiO20 were presented by the photodegradation of rhodamine b (RB) solutions.
2. Experimental Details
Undoped and nitrogen-doped Bi12TiO20 nanocrystalline powders and films were prepared by sol-gel method and dip coating. Chemical agents of analytical grade were used in the experiments, such as acetic acid, Bi(NO3)3·5H2O and Ti(OC4H9)4. Reagents were dissolved in ethylene glycol monomethyl ether solution while urea was used as nitrogen additive. Crystalline materials of undoped and nitrogen-doped Bi12TiO20 were prepared by annealing of the sol-gel precursors at temperature of 600°C for about 30 minutes.
2.1. Preparation of Bi12TiO20 and Nitrogen-Doped Bi12TiO20 Crystalline Materials
The mole ratio of reagents Bi(NO3)3·5H2O and Ti(OC4H9)4 was 12 : 1 in the precursor solution for preparation of Bi12TiO20 and , , and sol-gel precursors, with additional urea added for nitrogen-doped materials (molecule ratio of : Bi3+ about 0 : 1, 1 : 1, 2 : 1, and 3 : 1 in sol-gel). The reagents were blended in acetic acid and ethylene glycol monomethyl ether solutions, and were dried by infrared light to get the gel. Bi12TiO20 and , , and sillenite phase crystalline powders were obtained by annealing the gel precursors at 600°C for about 30 minutes.
2.2. Preparation of Bi12TiO20 Crystalline Films for UV-Vis Absorption Characterization
Crystalline films of Bi12TiO20, , , and B were prepared by dipping glass chips into the prepared sol-gel precursor solutions, and then were dried by infrared light, and annealed at 600°C for 30 minutes in nitrogen atmosphere.
2.3. Characterization Methods
The crystal phases of prepared powders were identified by X-ray diffraction (XRD, Cu Kα, D/max-ra X-ray). Morphology of the doped and undoped Bi12TiO20 nanocrystals was characterized by scanning electron microscopy (SEM, JEOL JSM6700F). The compositions and the electron bonding states of nitrogen-doped Bi12TiO20 were characterized by X-ray photoelectron spectroscopy (XPS, ESCALAB 250 of Thermal Fisher Scientific). To study the red shift of absorption wavelength of nitrogen-doped materials, UV-Vis absorption spectra of Bi12TiO20 crystalline films were performed by UV-Vis spectrophotometer (U-3500, 187 nm–3500 nm). The UV-Vis diffuse reflection spectra of doped and undoped Bi12TiO20 powders were characterized by UV/Vis spectroscopy (UV-2550, Shimadzu). Photocatalytic activity of undoped and nitrogen-doped Bi12TiO20 was characterized by the photodegradation of rhodamine b (RhB) solutions under the irradiation of 300 W Xe arc lamp (focused through a shutter window). The efficiency of the degradation processes was evaluated by monitoring the dye decolorization at the maximum absorption around 557 nm as a function of irradiation time in the separated RhB solution with a UV-vis spectrophotometer (UV-7502PC, Xinmao, Shanghai).
3. Results and Discussion
3.1. X-Ray Characterization
The crystal phases of synthesized doped and undoped nano crystalline materials were identified by X-ray diffraction. The XRD spectra of annealed Bi12TiO20, , , and were shown in Figure 1. No phase transformation of Bi12TiO20 structure (JCPDS no.78–1158) was observed by adding of urea as nitrogen additive. The slight shift and broadening of XRD peaks showed the effect of nitrogen doping in Bi12TiO20 structure materials. From Figure 1, the sharp diffraction peak and strong intensity of Bi12TiO20 indicates good crystallinity; while after nitrogen doping, the diffraction peak intensity became weaker and the peaks broadened slightly. The (310) and (222) peak split into two peaks and shifted to lower angles with the increase of nitrogen dopant, which shows us there must be an increase for the interlamellar spacing of (310) and (222) crystal facets as nitrogen is doped in Bi12TiO20.
3.2. Morphology of Nanocrystalline Bismuth Titanate Materials
Morphologies of undoped and nitrogen-doped Bi12TiO20 materials prepared by sol-gel synthesis and annealing were characterized by SEM as show in Figure 2. The undoped Bi12TiO20 microgel like bulks dotted with nanoflakes were observed in Figure 2(a), with few holes due to the decomposition and elution of sol-gel precursor. With addition of urea in precursors, the decomposition eluted more gas and the microbulk became fractal bulks piled up as shown in Figures 2(b), 2(c), and 2(d); while with too much urea added, there are only fractal bulks and few nanoflakes, which may decrease the surface and refrain the photocatalytic performance of heavily nitrogen-doped . The morphology of proper nitrogen-doped material is , as shown in Figure 2(c), which is composed of separated nano bulks dotted with nanosheet-like crystals. The morphology of has more nano crystalline facets and larger surface areas than other samples, which would show better photocatalytic performance than the other samples.
3.3. Element Content and Doping of Nanocrystalline Bismuth Titanate Materials
The chemical bonding states of as-prepared materials were characterized by XPS as shown in Figure 3. As shown in Figure 3(a), the XPS peaks of Bi4f, C 1s, N 1s, and Ti 2p were detected. The split of nitrogen 1s peak and carbon 1s peak shown in Figures 3(b) and 3(c) indicate the variety of bonding states. The split of N1s peaks in Figure 3(b) was assigned to N 1s peaks of N–O bond (401.6 eV)  and N–Ti bond (396.4 eV) [39, 40]. The N/(N+O) molar ratio calculated from the XPS peak is about 3%. The broadening and split of C 1s peak shown in Figure 3(c) were assigned to CO/Bi2O3 (286.6 eV) and CO/TiO2 (288.5 eV) physical absorb circumstances . So most carbon in the sample is absorbed CO, and the visible light absorption enhancement is attributed to the doping of nitrogen.
3.4. UV-Vis Absorption Enhancement of Nitrogen-Doped Bismuth Titanate Nanocrystalline Films
The UV-Vis absorption spectra of bismuth titanate nano crystalline films with different nitrogen doping ratios were characterized in Figure 4(a). The absorption spectra of undoped bismuth titanate were broadened successfully by urea-assisted sol-gel growth of nitrogen doping as shown in the UV-Vis absorption spectra of Bi12TiO20 and and films by sol-gel deposition with and without nitrogen doping. The UV-Vis absorption spectra band edge of Bi12TiO20 shifted from 400 nm to about 470 nm of nitrogen-doped ( with : Bi3+ about 1 : 1 solution, and N/(N+O) mole ratios about 3%). The absorption spectra’s broadening of nitrogen-doped Bi12TiO20 was attributed to the doping of nitrogen’s substitution of oxygen site that induced the narrowing of the bandgap. There was an additional absorption peak at 580 nm wavelength that was attributed to the deep-level nitrogen dopant.
The UV-Vis diffuse reflection spectra of nitrogen-doped and undoped Bi12TiO20 powders were characterized as shown in Figure 4(b), P25 was also used as a reference. TiO2-P25 materials have intensive UV absorption and the absorption edge line at about 380 nm, while all Bi12TiO20 of nitrogen-doped and undoped have less UV absorption but longer bandgap-related wavelength about 450 nm. The bandgap of nitrogen-doped Bi12TiO20 versus undoped Bi12TiO20 in DRS spectra varied about 20 nm, from 440 nm to 460 nm. The red shift of absorption edge wavelength of nitrogen-doped Bi12TiO20 powders in DRS spectra is less than that in absorption spectra of nitrogen-doped Bi12TiO20 films, which we attribute to the variation of N/(N+O) ratios from inner to the outer layer. As known, the absorption spectra of films show the absorption of whole bulk materials, while the DRS spectrum more attribute to materials’ surfaces. While annealing, there may be more nitrogen dopant in inner part and more oxygen in surfaces, so the absorption spectra edge of whole film materials will show more red shift than the diffuse reflection spectra edge obtained only from powders’ surface.
The absorption band edge of Bi12TiO20 films and UV-Vis diffuse reflection band edge of Bi12TiO20 powders were successfully expanded to visible light region of solar spectra by urea-assisted sol-gel method, which would greatly enhance the utilizing ratio of visible light region of solar spectra in photocatalysis and solar cell researches.
3.5. Influence of Photocatalytic Capability of Bi12TiO20 by Nitrogen Doping
To study the relationship of photocatalytic property with nitrogen doping of bismuth titanate, photocatalysis of rhodamine b (RhB) by undoped and nitrogen-doped bismuth titanate was performed. The photocatalytic activity was characterized by the decomposition of 10 mg/L rhodamine b (RhB) solution, with 100 mg of photocatalyst added to 100 mL of rhodamine b (RhB) solution. The reaction system was irradiated by 300 W Xe lamps.
The photocatalytic ability of undoped Bi12TiO20 materials had been studied by Yao et al. [24–26] and other groups [28, 29, 31, 33, 34]. Here we are concerned with relationship of photocatalytic ability of undoped and nitrogen-doped Bi12TiO20 materials with different amount of urea added in precursors (: Bi3+ about 0 : 1, 1 : 1, 2 : 1, and 3 : 1). Different amount of urea added in precursor solution deduced to the different amount of nitrogen dopant in the Bi12TiO20 photocatalysts. All samples were annealed at 600°C for 30 minutes.
As shown in Figure 5, the performance of catalyst Bi12TiO20 and have better performance than other catalysts. The undoped and nitrogen-doped catalysts with N/(N+O) mole ratio about 0% and 3% were prepared by : Bi3+ about 0 : 1 and 2 : 1 in the precursor solutions, respectively.
The velocity constants of catalyst Bi12TiO20 and catalyst (; ) are about 1.5-times the velocity constant of catalyst (; ). So the nitrogen-doped photocatalyst with N/(N+O) mole ratio about 3% have better performance than undoped Bi12TiO20, light-doped , and heavily doped photocatalysts.
The variation of whole spectra photocatalytic performance of undoped and nitrogen-doped Bi12TiO20 was attribute to the morphologies as observed by SEM in Figure 2. have more crystalline nano crystal facets and larger surface areas than other samples, which lead to better photocatalytic performance than other samples. But a high level of nitrogen-doped (above 6% of N/(N+O)) would lead to the formation of more defects in nano crystals of prepared catalysts, and would influence the transportation of photogenerated electrons and holes. Though high level of nitrogen doping can effectively expand the visible light absorption, a high defects density would lead to poor photocatalytic activity.
In conclusion, by urea-assisted sol-gel method, undoped and nitrogen-doped Bi12TiO20, , , and powders were prepared. After annealing, the morphologies of as-synthesized samples exhibit sheet-like morphologies. And by controlling the amount of urea added in the precursor solutions, photocatalyst with sheet-like morphologies with good crystalline facet and larger surface area can be synthesized.
The absorption spectra of nitrogen-doped , , and films were successfully expanded from about 400 nm of Bi12TiO20 to about 500 nm, also from DRS spectra the band edge were successfully expanded from about 420 nm to about 460 nm, which would greatly enhance the utilization ratio of visible light in solar spectra. The expanded visible light can be attributed to the doping of nitrogen substituting of oxygen in Bi12TiO20 crystal lattice. Photocatalytic performance of nitrogen-doped with N/(N+O) mole ratio about 3% is better photocatalytic activity than that of undoped, light-doped (below 1%), and heavily doped photocatalysts.
This paper was financially supported by research grants from the National Basic Research Program of China (973 Program, no. 2007CB613302), and the National Science Foundation of China (nos. 20973102 and 51021062). The authors also express our thanks for the editor’s patience.
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