Hydrothermal derived nitrogen doped SrTiO3 for efficient visible light driven photocatalytic reduction of chromium(VI)
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In this work, we report on the synthesis of nitrogen doped SrTiO3 nanoparticles with efficient visible light driven photocatalytic activity toward Cr(VI) by the solvothermal method. The samples are carefully characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, UV–Vis diffuse reflectance spectroscopy and photocatalytic test. It is found that nitrogen doping in SrTiO3 lattice led to an apparent lattice expansion, particle size reduction as well as subsequent increase of Brunner–Emmet–Teller surface area. The visible light absorption edge and intensity can be modulated by nitrogen doping content, which absorption edge extends to about 600 nm. Moreover, nitrogen doping can not only modulate the visible light absorption feature, but also have consequence on the enhancement of charge separation efficiency, which can promote the photocatalytic activity. With well controlled particle size, Brunner–Emmet–Teller surface area, and electronic structure via nitrogen doping, the photocatalytic performance toward Cr(VI) reduction of nitrogen doped SrTiO3 was optimized at initial hexamethylenetetramine content of 2.
KeywordsDoping Photocatalysis Chromium(VI) reduction Strontium titanate
Hexavalent chromium (Cr(VI)) is a common pollutant detected in groundwater originated from excessively released of electroplating, pigment production and tanning of leather, etc. (Wang et al. 2013a). Cr(VI) has raised considerable attention because of its high toxic, intense mobility and strong teratogenic activity to human organisms. The World Health Organization (WHO) has stipulated that Cr(VI) concentration in drinking water should be below 0.05 ppm (Chen et al. 2011). Precipitation (Gheju and Balcu 2011), adsorption (Sun et al. 2014a, b), ion exchange (Edebali and Pehlivan 2010) and membrane separation (Hsu et al. 2013) as conventional techniques are commonly used to eliminate Cr(VI) from wastewater. Precipitation and adsorption processes are economic and effective, but only efficient when the Cr(VI) concentration is relatively high (Abyaneh and Fazaelipoor 2016; Hokkanen et al. 2016). Ion exchange is high-efficiency in general, but it is rather expensive to maintain and operate (Ali et al. 2015). Some even cause secondary pollution. For example, solvent extraction method could bring in organic pollutants, sulfide precipitator as common precipitator may be residual and generate hydrogen sulfide (H2S). In general, conventional techniques are either low efficiency or cost too much when they are applied to low Cr(VI) concentration in wastewater (Wang et al. 2012a; Huang and Huang 1996).
Semiconductor photocatalytic reduction technology has attracted a lot of attention in recent years (Miseki et al. 2008; Kato and Kudo 2002; Wang et al. 2009; Mu et al. 2011; Nakhjavan et al. 2012; Duo et al. 2015; Zhang et al. 2009). Semiconductor photocatalytic technology has a promising prospect for wastewater Cr(VI) removing because it is efficiency and inexpensive to maintain and operate without secondary pollution (Hu et al. 2014; Meichtry et al. 2014; Gherbi et al. 2013; Alanis et al. 2013). Strontium titanate (SrTiO3) could be applied in Cr(VI) ion contaminant reduction with excellent photocatalyst performance, but it is only effective under ultraviolet irradiation which is about 4 % of the sunlight (Zheng et al. 2011). That is to say, strontium titanate is ineffective under visible light irradiation when applied to photocatalysis because it has a 3.2 eV band gap energy (Dong et al. 2012). Many leading groups also take advantage of the high bandgap. Li Ji group and Ib Chorkendorff group use SrTiO3 and TiO2 as protective window layers for Si photocathode during water splitting, and they achieve good result (Ji et al. 2015; Bae et al. 2016). Doping with nonmetal atoms to SrTiO3 material could hoist the valence band edge and extend its optical absorption edge towards the visible light range, resulting in visible light driven photocatalytic activity (Sulaeman et al. 2011; Zou et al. 2012). The perovskite phases materials characteristics depend on the anionic composition to a large extent. Therefore, replacing oxygen with other anions, take nitrogen for example, can greatly influence the physicochemical property of the material. There are many reports about doping action including anionic dopant species and metals ions, and anionic doping could narrow the desired semiconductor band gap better than cation ions doping (Khan et al. 2002; Chen and Burda 2008).
In our work, nitrogen-doped SrTiO3 powders are synthesized by hydrothermal method reaction. We take hexamethylenetetramine as doping sources and KOH as mineralizer to obtain the fine particles with excellent photocatalytic activity. The nitrogen doping effects on SrTiO3 nanoparticles are fully studied in an attempt to investigate the microstructure, optical properties and the relevance to the improved photocatalytic activity toward chromium(VI) reduction.
Synthesis of nitrogen doped SrTiO3 samples
Titanium tetraisopropoxide Ti(OC3H7)4 and strontium nitrate Sr(NO3)2·4H2O were used as starting materials, hexamethylenetetramine (HMT) as nitrogen source, and KOH as mineralizer. All of them were reagent grade and used without further purification. SrTiO3 was prepared by hydrothermal method. Ti(OC3H7)4 was dissolved in 10 mL 2-propanol firstly, Sr(NO3)2 aqueous solution was added to Ti(OC3H7)4 propanol solution dropwise with continuously stirring. Then, 0–8 g of HMT and 20 mL of 2 M KOH aqueous solution were added to the suspension in turn. The solution was placed into a Teflon container with a stainless steel autoclave outside and then the solution was heated at 200 °C for 3 h in an oven. After that, the autoclave was cooled to room temperature naturally, the obtained powder was washed with distilled water and alcohol three times and dried in vacuum at 60 °C overnight (Sulaeman et al. 2010). The final samples were labeled as pure SrTiO3, N-SrTiO3(0.5), N-SrTiO3(1), N-SrTiO3(2), N-SrTiO3(3), N-SrTiO3(4), N-SrTiO3(5), N-SrTiO3(6) and N-SrTiO3(8) with the increased HMT content.
X-ray power diffraction (XRD) was applied to characterize the purity and crystallinity of all our samples (D8 Advance Bruker X-ray diffractometer, CuKα radiation, 2θ = 20–80o). Transmission electron microscopy (TEM) was used to determine the morphology of the as-prepared samples (JEM-2010 apparatus, 200 kVA acceleration voltage). Diffusive reflectance UV–Vis spectrophotometer (Perkin-Elmer Lambda35) was employed to measure the samples UV–Vis absorption, BaSO4 was taken as the reference sample. Barrett–Emmett–Teller (BET) technique was taken to determine the specific surface areas (Micromeritics ASAP 2000 Surface Area and Porosity Analyzer). X-ray photo spectrometer (XPS) analysis was employed for sample element state (ESCALab220i-XL). PGSTAT302 N potentiostat galvanostat Autolab electrochemical working station using a standard three-compartment cell was used for photoelectrochemical characteristics under 300 W Xe arc lamp (≥420 nm). The fluorine-doped tin oxide (FTO) glasses (0.6 cm2) were washed for 30 min using absolute ethanol with ultrasonication. 0.1 g sample mixed with 0.01 g Polyvinylidene fluoride (PVDF) and 0.5 mL N-methyl pyrrolidinone (NMP) were placed in an glass bottle under magnetic stirring for at least 8 h. Then the obtained mixtures were coated on the FTO glasses. Photocatalyst solution was coated onto the FTO glasses substrate by drop casting using 5 μL pipette tip, and 3 drops were enough. Then we use the pipette tip to smooth the film at room temperature in the air. Lastly, the coated FTO glasses were dried for 4 h at 60 °C in the air. Photocatalyst coated FTO glass, a piece of Pt sheet, an Ag/AgCl electrode and 0.5 M sodium sulfate were used as the working electrode, counter electrode, reference electrode and electrolyte, respectively.
Photocatalytic reactivity test
Results and discussion
In summary, nitrogen doped SrTiO3 nanoparticles with controlled particle size, electronic structure and efficient visible light driven photocatalytic activity toward Cr(VI) were successfully prepared by a solvothermal method. XRD, BET and TEM analyses indicated that nitrogen doped SrTiO3 nanoparticles with cube-like morphology exhibited an apparent lattice expansion, particle size reduction as well as subsequent increase of BET surface area via nitrogen doping. The visible light absorption edge and intensity can be modulated by nitrogen doping content, which absorption edge extends to about 600 nm. Moreover, nitrogen doping can not only modulate the visible light absorption feature, but also have consequence on the enhancement of charge separation efficiency, which can promote the photocatalytic activity. With well controlled particle size, BET surface area, and electronic structure via nitrogen doping, the visible light driven photocatalytic performance toward Cr(VI) reduction of nitrogen doped SrTiO3 was optimized at initial HMT content of 2. Such a finding may help to provide hints for developing and designing new photocatalytic semiconductors.
The manuscript was conceived and designed by SYG and XGJ. ZLX and ST performed acquisition of data. WXJ made some revisions of the manuscript. All authors read and approved the final manuscript.
This work is financially supported by the National Natural Science Foundation of China (Grants 21267041, 21367018, 21563021), the Project of Scientific and Technological Innovation Team of Inner Mongolia University (12110614).
The authors declare that they have no competing interests.
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