•FBC and OBC activated differential expressions of three main groups of target genes.•FBC and OBC induced different OS, inflammation and autophagy related genes.•No significant difference was observed between the toxic effects of FBC and OBC.•Oxidation process changed the initial cellular responses of BC.Epidemiological studies have showed an association between black carbon (BC) exposure and adverse health effects. This study intends to investigate the influence of oxidation processes in atmosphere on the initial cellular responses of BC. The changes of gene expressions induced by fresh BC (FBC) and ozone-oxidized BC (OBC) in human lung epithelial A549 cells were analyzed. And their toxic effects presented by viability, LDH release and DNA damage were compared. Totally 47, 000 genes in A549 cells were examined using Affymetrix Human U133 plus 2.0 chips. Some of the differentially expressed genes were verified by reverse transcription-quantitative polymerase chain reaction (RT-qPCR). The results showed that 1446 genes (including 756 up-regulated and 690 down-regulated) and 1594 genes (including 788 up-regulated and 806 down-regulated genes) were significantly changed by FBC and OBC respectively. Only 4 of 14 (FBC)/15 (OBC) oxidative stress related genes, up- or down-regulated by FBC and OBC, were identical; 13 of 29 (FBC)/31 (OBC) inflammation related genes, and 6 of 20 (FBC)/18 (OBC) autophagy related genes were identical. No obvious differences were observed between the toxic effects of FBC and OBC. The cytotoxicity of OBC and FBC in A549 cells is at least partially induced by oxidative stress and consequent inflammation or autophagy process. Previous studies indicated that OBC may be more toxic than FBC. However, our results suggested that FBC and OBC might lead to diverse toxic endpoints through activating different molecular pathways.

•PEC reduction of Cr(VI) was realized with no semiconducting photocatalysts.•The citric acid, Ti anode, and Cr(VI) formed an energy-relay cascade structure.•PEC reduction of Cr(VI) with Ti anode was 4 times faster than PC reduction.•PEC reduction of Cr(VI) using Ti anode was 2 times faster than ITO/TiO2 anode.Highly efficient photoelectrocatalytic (PEC) reduction of Cr(VI) has been realized in the absence of semiconducting photocatalysts. In the novel-configuration cell using Ti anode, Pt cathode, Cr(VI), and citric acid, the rate constant of the PEC reduction of Cr(VI) at a bias of 1.5 V was almost 3 times than that in the conventional-configuration cell using ITO/TiO2 anode, Ti cathode, Cr(VI), and citric acid. It was mostly because the citric acid, Ti anode, and Cr(VI) formed an energy-relay cascade structure, in which the photogenerated electrons in the citric acid were transferred to the positively biased Ti and then from anode to Cr(VI), leading to the very efficient Cr(VI) reduction. We develop a simple PEC method to reduce Cr(VI) over the Ti anode sensitized by photoexcited organic dye in no need of metal oxide photocatalysts, which can be considered as an important advance towards the cost-effective, environmentally friendly treatment of waste water.The photoinduced cascade energy transfer between the citric acid, Ti anode, and Cr(VI) has been demonstrated. The photoelectrocatalytic (PEC) reduction of Cr(VI) using Ti anode was two times faster than using ITO/TiO2 anode. We provide a novel PEC concept of Cr(VI) removal towards using no semiconducting photocatalysts.

Anodization is a popular method of preparing TiO2 nanotube array films (TiNTs) by using direct current (DC) power as the driving voltage. In this study, three driving voltage modes, namely, the sine alternating current (sine) mode, the full-wave rectification of sine waves via four diodes (sine-4D, where D means diode) mode, and the DC mode, were used to prepare TiNTs by anodization. At 20 V, TiNTs were formed under sine-4D mode but only irregular porous TiO2 films were formed under DC mode. At 50 V, TiNTs formed under both the sine-4D and DC modes. No TiNTs formed in the sine mode anodization at either 20 or 50 V. Compared with the DC mode, the sine-4D mode required a lower oxidation voltage for TiNT formation, which suggests that sine-4D is an economical, convenient, and efficient driving voltage for TiNT preparation by anodization. The morphologies and structures of TiNT samples anodized at 50 V in the sine-4D and DC modes at different oxidation time (1, 5, 10, 30, 60, and 120 min) were analyzed. TiNT growth processes were similar between the studied modes. However, the growth rate of the films was faster under the sine-4D mode than the DC mode during the first 30 min of anodization.

•Cr(VI) reduction on TiO2 photoanode is dominant with the addition of citric acid.•Cr(VI) is reduced on photocathode with Fermi level above Cr(VI) chemical potential.•Photogenerated holes can hamper Cr(VI) photoreduction over TiO2.Photocatalytic and photoelectrocatalytic (PEC) reductions of Cr(VI) based on TiO2 thin films were investigated under various conditions. Photogenerated electrons transferred from TiO2 thin film to cathode can contribute to PEC reduction of Cr(VI) only when the Fermi level of cathode lies above the chemical potential of Cr(VI), almost independent on the applied voltage of the direct current. In addition, the TiO2-coated anode is the major site that accommodates the PEC reduction of Cr(VI) with hole scavenger citric acid, regardless of the Fermi level of the cathode. Although electron transfer from TiO2 to Cr(VI) is an exothermic process, the photogenerated holes in TiO2 can markedly hamper Cr(VI) reduction over the TiO2 thin film by oxidizing the lower-valence Cr back to Cr(VI), which may be counteracted by the citric acid. This research provides some in-depth insights on developing photocatalysts which enable highly efficient PEC reduction of Cr(VI) in the future.

•Half-rectified square wave is first applied as potential for photoelectrocatalysis.•HR-SW is much more efficient in PEC degradation of rhodamine B than direct current.•HR-SW may enhance RhB adsorption and TiO2 exciton dissociation alternately.•HR-SW of 3V, 50 Hz and 50% duty ratio is optimal for RhB PEC degradation.The photoelectrocatalytic (PEC) degradations of rhodamine B (RhB) over titanium oxide (TiO2) thin films have been investigated under three applied potential modes of direct current (DC), square wave (SW), and half-rectified square wave (HR-SW). The synergetic effect between the light and electricity under the SW mode (~1.0) was found much lower than those under the DC (~9.0) and HR-SW (~25.9) modes, because the flow direction of the photoelectrons was periodically varied under the SW, leading to almost no improvement in the exciton dissociation. Compared to the DC mode, the HR-SW mode might offer much enhanced adsorption of RhB on the TiO2 surface and exciton dissociation in the TiO2 anode during the low and high bias potential periods, respectively, thereby giving rise to significantly improved PEC degradation of RhB. The current research provides a low-cost method to achieve the efficient PEC removal of liquid-phase organic pollutants, which is useful for advancing the photocatalytic technique toward the large-scale commercial application.

We report the efficient photoelectrocatalytic (PEC) reduction of Cr(VI) using TiO2 nanotubes (TNTs) as the photoanode and a large-area Ti mesh as the photocathode. Short-length TNTs (S-TNTs) show much greater PEC activity than either long-length TNTs (L-TNTs) or sol–gel-prepared TiO2 film, due to the fact that the TNT structure is advantageous of trapping light energy over the thin-film structure and the S-TNTs enable the more efficient electron transfer into the substrate than L-TNTs. More importantly, increasing the surface area of the photocathode (Ti mesh) can greatly accelerate the PEC reduction of Cr(VI), presumably due to the increased number of the active reduction sites on the larger-surface Ti mesh. In the PEC reduction of Cr(VI), Cr(V) is identified as a reaction intermediate using the electroparamagnetic resonance technique, whereby the process for the Cr(VI) evolution is proposed. The S-TNTs have been confirmed to be stable over many repetitive cycles of use, indicating their suitability for wide-scale use.Graphical abstractNearly complete PEC reduction of Cr(VI) to Cr(III) was achieved using TNTs photoanode and large-area Ti mesh photocathode.Research highlights▶ TNT arrays photoanode combine with Ti mesh photocathode for PEC reduction of Cr(VI). ▶ The complete PEC reduction of Cr(VI) to Cr(III) is achieved. ▶ Large-area Ti mesh photocathode is essential for the efficient reduction of Cr(VI).

Heterogeneous reactions of SO2 on ZnO particle surfaces were studied using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The influences of relative humidity (RH) and UV radiation (λ ≈ 365 nm) were investigated. In the absence of UV radiation, sulfite was the prominent product on the particle surfaces, and a negative relationship between RH and sulfite production was observed. In the presence of UV radiation, infrared (IR) absorption of sulfite and sulfate was evident in the spectra. With increasing RH or UV intensity, sulfite was gradually transformed into sulfate. UV intensity and RH exhibited a synergistic effect on the heterogeneous oxidation of SO2 on ZnO. On dry particles and with no UV radiation, the reaction order of SO2 on ZnO particles was 1.6. The initial uptake coefficient for the formation of sulfite, using the Brunauer-Emmett-Teller (BET) area as the reactive surface area of SO2, was 4.87 × 10−6. At 40% RH and with UV radiation, the reaction order was 0.91, and the initial uptake coefficient was 2.29 × 10−5.

The photophysical processes for three nanosized photocatalytic oxides, titanium dioxide (TiO2) thin film, tin dioxide (SnO2) thin film, and layered TiO2/SnO2 thin film, have been examined in planar solid-state devices. It is found that, for SnO2 thin film, the dissociation of the photogenerated excitons can take place both on the film surface and inside the film, while for TiO2 thin film, almost all excitons dissociate on the film surface. Such a difference is proposed to account for the higher photocatalytic activity of TiO2 over SnO2, since it is experimentally shown that the excitons dissociate in SnO2 thin film as efficiently as they do in TiO2 thin film. For layered TiO2/SnO2 thin film, when it is illuminated by a beam of UV light, it is suggested that there exists a local electrostatic field at the SnO2 side of the interface, mainly formed by those holes efficiently photogenerated and then localized in SnO2 thin film. The photo-induced local electrostatic field is believed to facilitate hole–electron separation on TiO2 thin film and therefore increase the photocatalytic activity of the layered thin film over single TiO2 thin film.

•High efficient TiHAP and TiFHAP films were prepared for Cr(VI) reduction.•TiHAP film has better PC and PEC activities than TiO2 for Cr(VI) reduction.•PC and PEC activities of TiFHAP were enhanced compared to TiHAP film.•PEC reduction of Cr(VI) is mainly occurred on the surface of TiHAP photoanode.Ti-doped hydroxyapatite (TiHAP) is a novel photocatalyst with high adsorption capacity. It shows high photocatalytic activity for the degradation of organic pollutants and sterilization than conventional semiconductors. In this study, highly efficient TiHAP film and fluorine-doped TiHAP (TiFHAP) film were prepared on ITO glass by sol-gel method to investigate their photocatalytic (PC) and photoelectrocatalytic (PEC) reduction towards hexavalent chromium (Cr(VI)). Field emission scanning electronic microscope (FESEM), UV–vis spectrophotometry, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques were used to characterize the morphologies, structure and composition of the samples. In the conditions of 0.5 mol/L Na2SO4, 0.5 mmol/L citric acid, 0.34 mmol/L Cr(VI) and pH = 3.4, the PC and PEC conversion rates of Cr(VI) over TiHAP film can achieve 70% and 84% under 90 min UV illumination, respectively, which were both higher than TiO2 film. Fluorine-doped TiHAP film (TiFHAP) showed enhanced PC and PEC activities than TiHAP film, with 5.6 wt% F-doping being the best. A two-compartment system was employed to study the reaction mechanism of the PEC process. Different from the traditional knowing that Cr(VI) would be reduced on photocathode by photoinduced electrons transferred from anode photocatalyst, the result suggested that it was on the surface of TiHAP photoanode where Cr(VI) was mainly reduced.Download full-size image

Ozone (O3) is an important atmospheric oxidant. Black carbon (BC) particles released into the atmosphere undergo an aging process via O3 oxidation. O3-aged BC particles may change their uptake ability toward trace reducing gases such as SO2 in the atmosphere, leading to different environmental and health effects. In this paper, the heterogeneous reaction process between O3-aged BC and SO2 was explored via in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Combined with ion chromatography (IC), DRIFTS was used to qualitatively and quantitatively analyze the sulfate product. The results showed that O3-aged BC had stronger SO2 oxidation ability than fresh BC, and the reactive species/sites generated on the surface had an important role in the oxidation of SO2. Relative humidity or 254 nm UV (ultraviolet) light illumination enhanced the oxidation uptake of SO2 on O3-aged BC. The oxidation potentials of the BC particles were detected via dithiothreitol (DTT) assay. The DTT activity over BC was decreased in the process of SO2 reduction, with the consumption of oxidative active sites.Changes of DTT activity and sulfate production of ozonized UBC particles in the reaction with SO2: sulfate formation was negatively correlated with DTT activity as a function of exposure time of ozonized UBC to SO2, which is due to the consumption of the active sites on the surface of the particles during the reaction.Download full-size image