Photocatalytic Reduction of CO2 to Methanol by Cu2O/TiO2 Heterojunctions

The conversion of CO2 to low-carbon fuels using solar energy is considered an economically attractive and environmentally friendly route. The development of novel catalysts and the use of solar energy via photocatalysis are key to achieving the goal of chemically reducing CO2 under mild conditions. TiO2 is not very effective for the photocatalytic reduction of CO2 to low-carbon chemicals such as methanol (CH3OH). Thus, in this work, novel Cu2O/TiO2 heterojunctions that can effectively separate photogenerated electrons and holes were prepared for photocatalytic CO2-to-CH3OH. More visible light-active Cu2O in the Cu2O/TiO2 heterojunctions favors the formation of methanol under visible light irradiation. On the other hand, under UV-Vis irradiation for 6 h, the CH3OH yielded from the photocatalytic CO2-to-CH3OH by the Cu2O/TiO2 heterojunctions is 21.0–70.6 µmol/g-catalyst. In contrast, the yield of CH3OH decreases with an increase in the Cu2O fraction in the Cu2O/TiO2 heterojunctions. It seems that excess Cu2O in Cu2O/TiO2 heterojunctions may lead to less UV light exposure for the photocatalysts, and may decrease the conversion efficiency of CO2 to CH3OH.

Highly Improved Photocatalytic Activity of Magnetically Separable Ferroferric Oxide@Zinc Oxide/Carbon Nitride Nanocomposites and Interface Mechanism

To develop an efficient and recyclable photocatalyst, ternary magnetic Fe3O4@ZnO/g-C3N4 nanocomposites were synthesized for the photodegradation of methylene blue (MB). The microstructures, magnetic response and photocatalytic activity of the as-prepared nanocomposites were characterized by using X-ray diffraction (XRD), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM), N2 adsorption–desorption isotherms and spectrophotometer. All results indicate that ZnO nanoparticles anchor on the surface of Fe3O4 nanoparticles and Fe3O4@ZnO exists on the surface of g-C3N4 to form Fe3O4@ZnO/g-C3N4 nanocomposites. The photocatalytic activity to MB of Fe3O4@ZnO/g-C3N4 nanocomposites is significantly higher than those of pristine g-C3N4 and Fe3O4@ZnO. Owing to the heterojunctions between the interface of g-C3N4 and ZnO, the high separation efficiency of the photogenerated electrons and holes increases the radicals [Formula: see text]OH and [Formula: see text]O[Formula: see text] to photodegrade MB. Fe3O4@ZnO/g-C3N4 (20%) presents the highest MB removal of 93.74% and could be easily separated from solution with magnetic separation method.

Wavelength Dependence of the Transformation Mechanism of Sulfonamides Using Different LED Light Sources and TiO2 and ZnO Photocatalysts

The comparison of the efficiency of the commercially available photocatalysts, TiO2 and ZnO, irradiated with 365 nm and 398 nm light, is presented for the removal of two antibiotics, sulfamethazine (SMT) and sulfamethoxypyridazine (SMP). The •OH formation rate was compared using coumarin, and higher efficiency was proved for TiO2 than ZnO, while for 1,4-benzoquinone in O2-free suspensions, the higher contribution of the photogenerated electrons to the conversion was observed for ZnO than TiO2, especially at 398 nm irradiation. An extremely fast transformation and high quantum yield of SMP in the TiO2/LED398nm process were observed. The transformation was fast in both O2 containing and O2-free suspensions and takes place via desulfonation, while in other cases, mainly hydroxylated products form. The effect of reaction parameters (methanol, dissolved O2 content, HCO3− and Cl−) confirmed that a quite rarely observed energy transfer between the excited state P25 and SMP might be responsible for this unique behavior. In our opinion, these results highlight that “non-conventional” mechanisms could occur even in the case of the well-known TiO2 photocatalyst, and the effect of wavelength is also worth investigating.

NiO-TiO2 p-n Heterojunction for Solar Hydrogen Generation

Photocatalytic water splitting for hydrogen production has been widely recognized as a promising strategy for relieving the pressure from energy crisis and environmental pollution. However, current efficiency for photocatalytic hydrogen generation has been limited due to a low separation of photogenerated electrons and holes. p-n heterojunction with a built-in electric field emerges as an efficient strategy for photocatalyst design to boost hydrogen evolution activities due to a spontaneous charge separation. In this work, we investigated the effect of different preparation methods on photocatalytic hydrogen production over NiO-TiO2 composites. The results demonstrated that a uniform distribution of NiO on a surface of TiO2 with an intimate interfacial interaction was formed by a sol-gel method, while direct calcination tended to form aggregation of NiO, thus leading to an uneven p-n heterojunction structure within a photocatalyst. NiO-TiO2 composites fabricated by different methods showed enhanced hydrogen production (23.5 ± 1.2, 20.4 ± 1.0 and 8.8 ± 0.7 mmolh−1g−1 for S1-20%, S2-20% and S3-10%, respectively) as compared with pristine TiO2 (6.6 ± 0.7 mmolh−1g−1) and NiO (2.1 ± 0.2 mmolh−1g−1). The current work demonstrates a good example to improve photocatalytic hydrogen production by finely designing p-n heterojunction photocatalysts.

Au@CoS-BiVO4 {010} Constructed for Visible-Light-Assisted Peroxymonosulfate Activation

A visible-light-Fenton-like reaction system was constructed for the selective conversion of peroxymonosulfate to sulfate radical. Au@CoS, when doped on monoclinic BiVO4 {010} facets, promoted spatial charge separation due to the different energy band between the m-BiVO4 {010} and {110} facets. The visible-light response of m-BiVO4 was enhanced, which was attributed to the SPR effect of Au. And the photogenerated electrons were transferred from the m-BiVO4 {010} facet to Au via a Schottky junction. Owing to higher work function, CoS was able to capture these photoelectrons with acceleration of the Co(Ⅱ)/Co(Ⅲ) redox, enhancing peroxymonosulfate conversion to sulfate radical (Co2+ + HSO5−→ Co3+ + •SO4− + OH−). On the other hand, holes accumulated on m-BiVO4 {110} facets also contributed to organics oxidation. Thus, more than 95% of RhB was degraded within 40 min, and, even after five cycles, over 80% of RhB could be removed. The radical trapping experiments and EPR confirmed that both the sulfate radical and photogenerated hole were the main species for organics degradation. UV-vis DRS, photoluminescence (PL) and photoelectrochemical analyses also confirmed the enhancement of the visible-light response and charge separation. In a pilot scale experiment (PMS = 3 mM, initial TOC = 151 mg/L, reaction time = 4 h), CoS-Au-BiVO4 loaded on glass fiber showed a high mineralization rate (>60%) of practical wastewater.

Photocatalytic Reduction of Cr (VI) over g-C3N4 Photocatalysts Synthesized by Different Precursors

Graphitic carbon nitride (g-C3N4) photocatalysts were synthesized via a one-step pyrolysis process using melamine, dicyandiamide, thiourea, and urea as precursors. The obtained g-C3N4 materials exhibited a significantly different performance for the photocatalytic reduction of Cr(VI) under white light irradiation, which is attributed to the altered structure and occupancies surface groups. The urea-derived g-C3N4 with nanosheet morphology, large specific surface area, and high occupancies of surface amine groups exhibited superior photocatalytic activity. The nanosheet morphology and large surface area facilitated the separation and transmission of charge, while the high occupancies of surface amine groups promoted the formation of hydrogen adsorption atomic centers which were beneficial to Cr(VI) reduction. Moreover, the possible reduction pathway of Cr(VI) to Cr(III) over the urea-derived g-C3N4 was proposed and the reduction process was mainly initiated by a direct reduction of photogenerated electrons.

Application of doping graphene quantum dots and gold nanoparticles on dye-sensitized solar cells

The doctor blade coating method is used to prepare dye-sensitized solar cells (DSSCs) and dope the original titanium dioxide (TiO2, P25) photoanode (PA) with single-layer graphene (G), graphene quantum dots (GQDs), and gold (Au) nanoparticles in this research. The results show that doping PAs with G, GQDS, and Au effectively increases the short-circuit current density [Formula: see text], conversion efficiency [Formula: see text], and decreases the internal structure impedance [Formula: see text] of DSSCs. [Formula: see text] increases from 13.62 to 17.02, 15.22, 16.05 mA/cm2, while [Formula: see text] (%) increases from 6.36 to 7.50, 7.08, 7.04% when doping G, GQDs, and Au, respectively. The analysis of Electrochemical Impedance Spectroscopy (EIS) reveals that the doping decreases [Formula: see text] from 11.28 to 8.36, 8.78, 8.54 [Formula: see text], respectively. Then, the titanium dioxide (TiO2)-doped G-GQDs, G-Au, and QDs-Au on DSSCs influence [Formula: see text] that increases to 5.45, 15.37, and 15.31 mA/cm2, respectively. In this case, the values of [Formula: see text] are found to be 7.21%, 7.35%, and 7.00%, while those of [Formula: see text] are 8.44, 8.63, and 9.18 [Formula: see text]. The values of [Formula: see text] and [Formula: see text] are highest but that of [Formula: see text] are lowest when doping with G, which proves that the photoanode of the DSSC effectively activates the photogenerated electrons in the film by doping single-layer graphene and TiO2 captures its electrons through graphene. The decreasing electron–hole recombination rate allows the photogenerated electrons to be quickly transferred to the external circuit. As a result, the efficiency of DSSCs is improved.

Photocatalytic Reduction of CO2 over Iron-Modified g-C3N4 Photocatalysts

Pure g-C3N4 sample was prepared by thermal treatment of melamine at 520 °C, and iron-modified samples (0.1, 0.3 and 1.1 wt.%) were prepared by mixing g-C3N4 with iron nitrate and calcination at 520 °C. The photocatalytic activity of the prepared materials was investigated based on the photocatalytic reduction of CO2, which was conducted in a homemade batch reactor that had been irradiated from the top using a 365 nm Hg lamp. The photocatalyst with the lowest amount of iron ions exhibited an extraordinary methane and hydrogen evolution in comparison with the pure g-C3N4 and g-C3N4 with higher iron amounts. A higher amount of iron ions was not a beneficial for CO2 photoreduction because the iron ions consumed too many photogenerated electrons and generated hydroxyl radicals, which oxidized organic products from the CO2 reduction. It is clear that there are numerous reactions that occur simultaneously during the photocatalytic process, with several of them competing with CO2 reduction.

Dual-Modified Cu2S with MoS2 and Reduced Graphene Oxides as Efficient Photocatalysts for H2 Evolution Reaction

Noble metal-free cocatalysts have drawn great interest in accelerating the catalytic reactions of metal chalcogenide semiconductor photocatalyst. In particular, great efforts have been made on modifying a semiconductor with dual cocatalysts, which show synergistic effect of a fast transfer of exciton and energy simultaneously. Herein, we report the dual-modified Cu2S with MoS2 and reduced graphene oxides (Cu2S-MoS2/rGO). The in situ growth of Cu2S nanoparticles in the presence of MoS2/rGO resulted in high density of nanoscale interfacial contacts among Cu2S nanoparticles, MoS2, and rGO, which is beneficial for reducing the photogenerated electrons’ and holes’ recombination. The Cu2S-MoS2/rGO system also demonstrated stable photocatalytic activity for H2 evolution reaction for the long term.