Contents lists available at ScienceDirect Catalysis Communications journal homepage: www.elsevier.com/locate/catcom Short communication Nitric acid-assisted one-step solvothermal synthesis of visible-light-active N- doped ThO 2 for use as a potential photocatalyst in the reduction of Cr(VI) Hongtao Wei a , Yaoting Wen b , Yongcai Zhang b,⁎ a College of Chemistry and Pharmaceutical Science, Qingdao Agricultural University, Qingdao 266109, China b School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, China ARTICLE INFO Keywords: N-doped ThO 2 Homogenous catalysis Visible-light photocatalyst Cr(VI) reduction ABSTRACT Nanostructure N-doped ThO 2 (which was named as ThO 2 -HNO 3 ) was synthesized by solvothermal treatment of Th(NO 3 ) 4 ·4H 2 O in the mixed solution of absolute ethanol (92.5 vol%) and nitric acid (7.5 vol%) at 180 °C for 20 h. ThO 2 -HNO 3 exhibited significant visible-light absorption and high photocatalytic activity in the reduction of aqueous Cr(VI) under visible-light (wavelength > 420 nm) irradiation. By contrast, the commercial ThO 2 and the ThO 2 synthesized when ammonia replaced nitric acid exhibited only negligible visible-light photo- catalytic activity. Thus, ThO 2 -HNO 3 is a new alternative visible-light photocatalyst for treating Cr(VI)-polluted wastewaters. 1. Introduction Photocatalytic reduction is an economical and environmental- friendly method for the treatment of aqueous Cr(VI) [1–10], which is a common highly toxic and intractable pollutant in the wastewaters from electroplating, leather tanning, chromate manufacturing and me- tallurgy, etc. The top priority for achieving the industrial application of photocatalytic reduction technology is to develop efficient visible-light photocatalyst. N-doped wide bandgap metal oxides have great potential for use as photocatalysts, because they have visible-light-driven pho- tocatalytic activity and good stability [11–19]. So far, N-doped metal oxides were synthesized mostly through two successive steps [11–19]: (i) first sol-gel or coprecipitation synthesis of nitrogenous precursors, then the precursors were calcined at high temperatures to generate N- doped metal oxides; or (ii) first synthesis of metal oxides, then the metal oxides were doped with nitrogen by ion implantation or treatment in nitrogenous atmospheres at high temperatures. Nevertheless, the two- step methods were often ineffective and uneconomical to obtain effi- cient visible-light photocatalysts. Furthermore, the nitrogen sources employed in solution synthesis of N-doped metal oxides were usually ammonia, hydrazine, urea or other organic amines [11–19]. The alka- line nitrogen sources can promote the hydrolysis of metal ions to form hydroxide precipitates, which is unfavorable for the homogeneous ni- trogen doping process. Nitric acid also has the potential for use as a source of nitrogen because of the following features [20]: (i) nitric acid has thermal unstability and strong oxidation ability, thus it can be thermally decomposed or reduced to NO 2 , NO, N 2 O or NH 4 + , which have been previously used as the nitrogen sources for N-doped metal oxides; and (ii) nitric acid possesses strong acidity, thus it can facilitate the homogeneous nitrogen doping by inhibiting the hydrolysis of metal ions. Nonetheless, to our knowledge, nitric acid has not yet been em- ployed as a nitrogen source in synthesizing N-doped ThO 2 . ThO 2 is a wide bandgap semiconductor material [21]. It cannot absorb visible light, so it has no visible-light-driven photocatalytic ac- tivity. To date, the report on the study of ThO 2 as photocatalyst is scarce. Here, we attempt to synthesize N-doped ThO 2 (ThO 2 -HNO 3 ) by a nitric acid-assisted one-step solvothermal method, aiming to explore an alternative new efficient visible-light photocatalyst. The photo- catalytic activity of ThO 2 -HNO 3 was evaluated for the reduction of aqueous Cr(VI) under visible-light (wavelength > 420 nm) irradiation, and compared with those of ThO 2 -NH 3 (which denotes the ThO 2 syn- thesized when ammonia replaced nitric acid) and the commercial ThO 2 . 2. Experimental First, 5 mmol Th(NO 3 ) 4 ·4H 2 O was dissolved into 37 mL of absolute ethanol in a 50 mL Teflon jar. Afterwards, 3 mL of nitric acid (14.36–15.16 mol/L) or ammonia (13.32–14.44 mol/L) was added into the above solution. The mixture was stirred for 20 min. Then, the Teflon jar was sealed into a stainless steel autoclave, and heated at 180 °C for 20 h. After the autoclave cooled naturally to room temperature, the product was centrifuged, washed with water and ethanol, and finally dried in vacuum at 90 °C for 6 h. XRD spectra were acquired using a Bruker AXS D8 ADVANCE X-ray http://dx.doi.org/10.1016/j.catcom.2017.05.030 Received 20 March 2017; Received in revised form 10 May 2017; Accepted 30 May 2017 ⁎ Corresponding author. E-mail address: zhangyc@yzu.edu.cn (Y. Zhang). Catalysis Communications 99 (2017) 66–70 Available online 30 May 2017 1566-7367/ © 2017 Elsevier B.V. All rights reserved. MARK