Photocatalytic H_2 production on trititanate nanotubes coupled with CdS and platinum nanoparticles under visible light: revisiting H_2 production and material durability

The photocatalytic production of molecular hydrogen (H_2) on ternary composites of Pt, CdS, and sodium trititanate nanotubes (Na_xH_(2−x)Ti_3O_7, TNTs) is examined in an aqueous 2-propanol (IPA) solution (typically 5 vol%) at a circum-neutral pH under visible light (λ > 420 nm). The H2 production rates are dependent on the Pt-loading level, and the optimum production rate in the Pt/CdS/TNTs is approximately six times higher than that in Pt/CdS/TiO_2. A D2O solution containing 5 vol% IPA leads only to the production of D_2 molecules, whereas increasing the IPA amount to 30 vol% leads to the production of DH molecules. This indicates that the Pt/CdS/TNTs composites enable H_2 production via true water splitting under our typical experimental conditions. X-ray photoelectron spectroscopy (XPS) analyses of the as-synthesized Pt/CdS/TNTs and those used for 6 and 12 h show that metallic Pt on the CdS/TNTs is less susceptible to oxidation than Pt on CdS/TiO_2. In addition, photocorrosion of CdS (i.e., sulfate formation) is significantly inhibited during the photocatalytic H_2 production reactions in the Pt/CdS/TNTs because of the efficient charge transfer via the TNTs framework. The Pt/CdS/TNTs samples are thermally more stable than Pt/CdS/TiO_2 and CdS/TNTs, effectively inhibiting the formation of CdO during the thermal synthesis. Detailed surface characterizations of the as-synthesized ternary composites are performed using X-ray diffraction, transmission electron microscopy, energy dispersive spectroscopy, and XPS

Photoelectrochemical Water Oxidation Using Cobalt Phosphate‐Modified Nitrogen‐Doped Titania Nanotube Arrays

The synthesis of cobalt phosphate (CoPi)‐modified, nitrogen‐doped TiO2 nanotube arrays (N‐TNAs) for photoelectrochemical (PEC) oxygen evolution under visible light is reported. Because of the nitrogen doping, the N‐TNAs exhibit enhanced visible‐light activity toward the PEC water oxidation reaction. However, the performance is diminished as a result of self‐oxidation by photogenerated holes in the valence band of the N‐TNAs. Compared with the N‐TNAs, the CoPi/N‐TNAs prepared under optimal conditions show a twofold improvement in photocurrent generation and are also sensitive to light with wavelengths as long as 580 nm. Stable oxygen evolution with a Faradaic efficiency approaching unity is demonstrated using the CoPi/N‐TNA photoanodes under simulated sunlight for at least 2 h of operation

Photocatalytic conversion of carbon dioxide to methane on TiO_2/CdS in aqueous isopropanol solution

The photocatalytic conversion of CO_2 to CH_4 on hybrid TiO_2/CdS catalysts in water in the presence of isopropanol (IPA) was explored. As compared to TiO_2, TiO_2/CdS increased the production of CH_4 by a factor of ∼10, whereas the production of H_2 and CO remained comparable. The amount of CdS loaded on the TiO_2 was not observed to significantly affect the yields and distributions of the products. An electron impact time-of-flight mass spectrometry (TOF-MS) study revealed ^(13)CH_4 to be a dominant product in the early stages of the photocatalysis under ^(13)CO_2 atmosphere, whereas only ∼25% of the total observed methane accounted for ^(13)CH_4 resulting from ^(13)CO_2 in the prolonged photocatalytic reaction over 6 h. Although the remainder of the methane originated from unlabeled carbons (e.g., from ^(12)C-IPA and ^(12)C-organic contaminants), the use of deuterated IPA in the TOF-MS study did not provide evidence for the contribution of the methyl groups of IPA. Furthermore, the diffuse reflectance infrared Fourier transform analysis showed the adsorption of aquated CO_2 species (e.g., (bi)carbonate via mono- and bi-dentate modes at pH ∼4.5) to be enhanced by the coupling of CdS to TiO_2, which was found to significantly weaken after the reactions. On the other hand, the IPA-associated IR bands were influenced to a lesser extent by the photoreaction

Standalone photoconversion of CO2 using Ti and TiOx-sandwiched heterojunction photocatalyst of CuO and CuFeO2 films

Artificial photoconversion of carbon dioxide to value-added chemicals remains a challenge. In this work, we synthesize heterojunction copper oxide and copper iron oxide (CuO/CuFeO2; CFO) films on transparent conducting substrates with metallic Ti layers, and deposit disordered TiOx particles on the CFO films. The Ti/CFO/TiOx samples convert CO2 into formate with ∼100% selectivity and drive O2 evolution under simulated sunlight in the absence of any electrical and chemical biases, achieving the overall solar-to-chemical energy conversion efficiency of ∼5%. The primary roles of the Ti underlayer are to create a robust contact between CFO and substrates and facilitate hole transfer. The TiOx (with unsaturated Ti atoms) delocalizes the electron density of the CFO and enhances electron transfer to the adsorbed CO2. Density functional theory calculations reveal that Ti/CFO/TiOx is the most suitable among the bare and modified CFO samples for efficient CO2 adsorption, formiloxyl intermediate formation, and formate desorption.- International Research Collaboration Co-fund (IRCC) grant #IRCC-2020-007. - Kyungpook National University (KNU). - National Research Foundation of Korea (NRF) grant #2018R1A6A1A03024962, 2019M1A2A2065616, 2019R1A2C2002602, 2020R1I1A1A01061380

TiO2 complexed with dopamine-derived polymers and the visible light photocatalytic activities for water pollutants

Visible light-induced chemical transformation using inexpensive photocatalytic materials has been proposed as an eco-friendly method for energy and environmental applications. In this work, we employed polymers of environmentally benign derivatives of dopamine (DA) as low-cost sensitizers of titania and systematically investigated their properties for the visible light photocatalytic transformation in aquatic environment. DA and its derivatives (norepinephrine and nitrodopamine) were chosen as monomers, and their polymers (pDA, pNE, and pNDA) were synthesized and subsequently complexed with TiO2. Visible light-induced catalytic transformations were successfully demonstrated for the reduction of Cr(VI) to Cr (III), dechlorination of CCI4, oxidation of As(III) to As(V), and H2O2 production via dioxygen reduction using polymer-complexed TiO2. pDA-TiO2 exhibited the highest activities, much higher than those of DA-TiO2 in all tested cases, which indicates that the polymerized form of DA forms a stronger and more efficient surface complex on the TiO2 surface for visible light sensitization. DA-derived polymers could efficiently transfer electrons to the TiO2 conduction band under visible light to initiate reductive transformations, whereas the oxidative transformation of organic substrates was largely inhibited because the organic polymer layer on TiO2 should scavenge any oxidizing radical species. pDA and pNE exhibited far higher activity than pNDA due to the extensive it electron delocalization induced by the 5,6-dihydroxyindole structure. This was also supported by the higher photon-to-current conversion and lower charge transfer resistance obtained with pDA-TiO2 and pNE-TiO2 (compared with pNDA-TiO2), which was observed with photoelectrochemical measurements. pDA should be an attractive visible light sensitizer for aquatic transformations. (C) 2016 Elsevier Inc. All rights reserved.111711sciescopu

Activation of Hematite Photoanodes for Solar Water Splitting: Effect of FTO Deformation

The sintering at 800 °C is found to induce the diffusion of Sn from the F-doped SnO₂ (FTO) into the hematite lattice, enhancing the photoelectrochemical cell (PEC) properties of the hematite photoanodes, but this diffusion also has detrimental effects on the conductivity of the FTO substrate. In the present research we examined the role of FTO deformation during the activation of hematite photoanodes synthesized on FTO substrates. The incorporation of Sn dopants from the FTO substrates in the hematite lattice was confirmed by X-ray photoelectron spectroscopy and was found to increase with sintering time. Further from the extended X-ray absorption fine structure analysis, it was found that the diffused Sn atoms affected the metal sites of the hematite lattice. Increased diffusion of Sn into the hematite lattice caused structural disordering of the FTO, but optimum sintering time compensated for the structural disordering and improved the ordering. Under high-temperature annealing at 800 °C, the FTO substrates underwent a stoichiometric change that directly affected their electrical conductivity; their resistivity was doubled after 20 min of sintering. Activation of hematite photoanodes by high-temperature sintering entails a kinetic competition between Sn dopant diffusion from the FTO substrate into the hematite and the resulting thermal deformation and conductivity loss in the FTO substrates