•SiNW covered with controllable amount of nanopores was successfully prepared.•The photoelectrochemical stability of SiNW enhanced with the increase of the amount of nanopores.•The introduction of nanopores increased special surface area of SiNW.•Nanopores on SiNW exhibited improved photoelectrocatalytic reduction of Cr(VI).Herein, we reported a method to inhibit the oxidation passivation of Si nanomaterials (taking Si nanowire array (SiNW, about 100 nm of diameter) as a representative) via structuring nanopores (less than 10 nm) on their surface. SiNW array was prepared through metal-assisted chemical etching. After 30 min of etching, nanopores were formed on the surface of SiNW and the amount of nanopores increased with the prolonging of the etching duration. Due to the generation of the nanopores, the decrease of optical reflection was observed. The photoelectrochemical performance of samples prepared after 30, 60 and 90 min etching were evaluated. The sample etched for 60 min displayed the highest and stable photocurrent and was used in the photoelectrocatalytic testing. The SiNW with nanopores exhibited significant photoelectrocatalytic stability than that of SiNW. The kinetic constant of Cr(VI) reduction over SiNW with nanopores was 0.081 min−1, which was 1.8 times as much as that over SiNW (0.045 cm−1) and maintained above 0.080 min−1 after three repeated experimental runs. These results demonstrate that constructing surface nanopores on the surface of SiNW is an effective approach to realize the use of Si materials as photoelectrodes in the aqueous solution.

Although Si is a successful photovoltaic material, its application in the photoelectrochemical (PEC) field is limited because an insulated SiO2 layer always extends quickly from surface to depth once bare Si contacts with an aqueous solution. To inhibit this oxidation passivation, Si nanowires (SiNWs) were coated with graphitic-C3N4 quantum dots (g-C3N4 QDs) to form SiNWs@g-C3N4 QDs, in which g-C3N4 QDs acted as a protection layer to isolate Si from water and to improve the photogenerated charge transfer. Observed by SEM and TEM, it was confirmed that dispersive g-C3N4 QDs anchored on the surface of SiNWs. PEC performance indicated that the photocurrent of bare SiNWs declined obviously while the photocurrent of SiNWs@g-C3N4 QDs was stable. The photocurrent of SiNWs@g-C3N4 QDs reached 6.7 mA cm−2 at −1.5 V (vs. SCE) which was 1.6 times higher than that of pristine SiNWs (4.2 mA cm−2). Taking 4-chlorophenol as a target pollutant to investigate the photoelectrocatalytic capability of the SiNWs@g-C3N4 QDs, more than 85% of 4-chlorophenol was successfully removed in 120 min, while the value for SiNWs was only 52.0%. The pseudo-first-order kinetic constant of 4-chlorophenol degradation on SiNWs@g-C3N4 QDs was 2.3 times as great as that on pristine SiNWs. The improved degradation efficiency benefited from the improved stability as well as the enhanced photo-generated charge transfer and separation driven by the built-in electric field at the interface between g-C3N4 QDs and SiNWs. The SiNWs@g-C3N4 QDs will also be useful in other research areas such as water splitting, sensors, etc.

Quantum-sized CdS-coated TiO2 nanotube array (Q-CdS-TiO2 NTA) was fabricated by the modified successive ionic layer absorption and reaction method. Scanning electron microscope and transmission electron microscope images showed the regular structure of TiO2 NTA, where quantum-sized CdS (diameter <10 nm) deposited on both the inside and outside of TiO2 nanotube wall. Fabrication conditions including immersing cycles, calcination temperature and drying process were well optimized, and the Q-CdS-TiO2 NTA and its photoelectrochemical (PEC) properties were characterized by X-ray fluorescence spectrometer, UV–Vis diffuse reflectance spectra and photovoltage. Distinct increases in visible light absorption and photocurrent were observed as the immersing cycle was increased from 5 to 20 times. The additional drying process accelerated the CdS crystal growth rate, and thus, the fabrication time could be shortened accordingly. Calcination temperature influenced the PEC property of Q-CdS-TiO2 NTA deeply, and the optimized calcination temperature was found as 500 °C. As the Q-CdS-TiO2 NTA was fabricated under such condition, the visible photocurrent density increased to 2.8 mA/cm and the photovoltage between 350 and 480 nm was enhanced by 2.33 times than that without calcination. This study is expected to optimize Q-CdS-TiO2 NTA fabrication conditions for the purpose of improving its PEC performance.本文采用改进的连续离子层吸附反应法(SILAR)制备了CdS量子点包覆的TiO2纳米管阵列(Q-CdS-TiO2 NTA)。扫描电子显微镜和透射电子显微镜结果表明,TiO2纳米管为有序阵列结构,CdS颗粒沉积在TiO2纳米管的内壁和外壁上,粒径小于10 nm。改变浸渍循环次数、焙烧温度以及干燥方法,并用X射线荧光光谱仪、紫外-可见分光光度计和光电压谱仪测试Q-CdS-TiO2 NTA样品的光电化学(PEC)性能,从而获得最优制备参数。浸渍循环次数从5次增加到20次时,Q-CdS-TiO2 NTA可见光吸附和光电流明显增大。干燥过程能够加快CdS晶体生长速度,缩短制备时间。与未焙烧的Q-CdS-TiO2 NTA相比,500 °C焙烧制备的Q-CdS-TiO2 NTA可见光电流密度增大至2.8 mA/cm2,350–480 nm间的光电压提高了2.33倍。上述研究结果为制备此类量子点包覆阵列材料提供了参考。

Not only for utilizing both visible and UV light but also for enhancing photogenerated charge separation capability, a hybrid photocatalyst composed of graphite-like carbon nitride (g-C3N4, mainly response to visible light) and TiO2 (response to UV light) was fabricated by a hydrolysis approach. The TEM images of g-C3N4/TiO2 samples displayed that TiO2 nanoparticles dispersed well on the surface of g-C3N4 sheet and the average size of TiO2 particles on g-C3N4 sheet was much smaller than that of TiO2 samples without g-C3N4 sheet, meaning the inhibiting TiO2 aggregation function of g-C3N4 sheet. The UV–Vis diffuse reflectance spectra displayed that the optical absorption range of g-C3N4/TiO2 hybrid was from 300 to 450 nm, including both UV and visible light. The lower intensity of photoluminescence for g-C3N4/TiO2 than those for pristine g-C3N4 and TiO2, indicated that the recombination of photogenerated charge was inhibited effectively. Profiting from the above mentioned advantages, g-C3N4/TiO2 showed good photocatalysis capability. The pseudo-first-order kinetic constant of phenol degradation on g-C3N4/TiO2 was 2.41 and 3.12 times those on pristine g-C3N4 and TiO2, respectively. We also proposed the scheme for electron–hole separation and transport at the light-driven g-C3N4/TiO2 hybrid photocatalyst interface as well as the phenol degradation mechanism under full spectrum and visible light irradiation.Highlights► g-C3N4/TiO2 can superpose visible light response of g-C3N4 and UV response of TiO2. ► g-C3N4/TiO2 hybrid displays good photogenerated charge separation capability. ► g-C3N4/TiO2 photocatalyst is used to decompose toxic pollutants for the first time.

Not only for utilizing both visible and UV light but also for enhancing photogenerated charge separation capability, a hybrid photocatalyst composed of graphite-like carbon nitride (g-C3N4, mainly response to visible light) and TiO2 (response to UV light) was fabricated by a hydrolysis approach. The TEM images of g-C3N4/TiO2 samples displayed that TiO2 nanoparticles dispersed well on the surface of g-C3N4 sheet and the average size of TiO2 particles on g-C3N4 sheet was much smaller than that of TiO2 samples without g-C3N4 sheet, meaning the inhibiting TiO2 aggregation function of g-C3N4 sheet. The UV–Vis diffuse reflectance spectra displayed that the optical absorption range of g-C3N4/TiO2 hybrid was from 300 to 450 nm, including both UV and visible light. The lower intensity of photoluminescence for g-C3N4/TiO2 than those for pristine g-C3N4 and TiO2, indicated that the recombination of photogenerated charge was inhibited effectively. Profiting from the above mentioned advantages, g-C3N4/TiO2 showed good photocatalysis capability. The pseudo-first-order kinetic constant of phenol degradation on g-C3N4/TiO2 was 2.41 and 3.12 times those on pristine g-C3N4 and TiO2, respectively. We also proposed the scheme for electron–hole separation and transport at the light-driven g-C3N4/TiO2 hybrid photocatalyst interface as well as the phenol degradation mechanism under full spectrum and visible light irradiation.Highlights► g-C3N4/TiO2 can superpose visible light response of g-C3N4 and UV response of TiO2. ► g-C3N4/TiO2 hybrid displays good photogenerated charge separation capability. ► g-C3N4/TiO2 photocatalyst is used to decompose toxic pollutants for the first time.