Band-Gap Engineering of Layered Perovskites by Cu Spacer Insertion as Photocatalysts for Depollution Reaction

A multi-step ion-exchange methodology was developed for the fabrication of Cu(LaTa2O7)2 lamellar architectures capable of wastewater depollution. The (001) diffraction line of RbLaTa2O7 depended on the guest species hosted by the starting material. SEM and TEM images confirmed the well-preserved lamellar structure for all intercalated layered perovskites. The UV–Vis, XPS, and photocurrent spectroscopies proved that Cu intercalation induces a red-shift band gap compared to the perovskite host. Moreover, the UV–Vis spectroscopy elucidated the copper ions environment in the Cu-modified layered perovskites. H2-TPR results confirmed that Cu species located on the surface are reduced at a lower temperature while those from the interlayer occur at higher temperature ranges. The photocatalytic degradation of phenol under simulated solar irradiation was used as a model reaction to assess the performances of the studied catalysts. Increased photocatalytic activity was observed for Cu-modified layered perovskites compared to RbLaTa2O7 pristine. This behavior resulted from the efficient separation of photogenerated charge carriers and light absorption induced by copper spacer insertion.

Cu2O/SrTi1-xCrxO3 Heterojunction Photocatalyst for the Efficient Degradation of Isopropanol under Visible Light Irradiation

A heterojunction of Cu₂O and Cr-doped SrTiO₃ (SrTi_1-xCr_xO₃) was designed for selective photocatalytic isopropanol (IPA) oxidation under visible light irradiation. The photocatalytic oxidation of IPA was measured in a fixed-bed reactor. Cr dopants can increase the light absorption and improve the activity of the catalyst. The formation of the Cu₂O/SrTi_1-xCr_xO₃ heterojunction can further broaden the absorption range of lights and dramatically increase the photocatalytic activity for selective oxidation of IPA. The 3% Cu₂O/SrTi_0.99Cr_0.01O₃ catalyst can fully convert ∼1000 ppm IPA under illumination in 2 h. The selectivity of acetone is ∼100%. The yield is 83 and 4 times higher than that using SrTiO₃ and SrTi_0.99Cr_0.01O₃ as catalysts, respectively. By measuring the ultraviolet-visible absorption spectra and Mott-Schottky plots, we obtained the band structure of the heterojunction, which shows that the conduction and valence bands of Cu₂O are higher than those of SrTi_1-xCr_xO₃, therefore facilitating the separation and transfer of photogenerated electrons and holes. In addition, electron paramagnetic resonance spectroscopy and radical trapping tests reveal that the generation of hydroxyl and superoxide leads to photocatalytic oxidation of IPA by the heterojunction photocatalyst.

Cu2O/TiO2 decorated on cellulose nanofiber/reduced graphene hydrogel for enhanced photocatalytic activity and its antibacterial applications

Photocatalysis has gained attention as a viable wastewater remediation technique. However, the difficulty of recovering powder-based photocatalyst has often become a major limitation for their on-site practical application. Herein, we report on the successful in-situ preparation of a novel three-dimensional (3D) photocatalyst consisting of Cu₂O/TiO₂ loaded on a cellulose nanofiber (CNF)/reduced graphene hydrogel (rGH) via facile hydrothermal treatment and freeze-drying. The 3D macrostructure not only provides a template for the anchoring of Cu₂O and TiO₂ but also provides an efficient electron transport pathway for enhanced photocatalytic activity. The results showed that the Cu₂O and TiO₂ were uniformly loaded onto the aerogel framework resulting in the composites with large surface area with exposed actives sites. As compared to bare rGH, CNF/rGH, Cu₂O/CNF/rGH and TiO₂/CNF/rGH, the Cu₂O/TiO₂/CNF/rGH showed improved photocatalytic activity for methyl orange (MO) degradation. MO degradation pathway is proposed based on GC-MS analysis. The enhanced photoactivity can be attributed to the charge transfer and electron-hole separation from the synergistic effect of Cu₂O/TiO₂ anchored on CNF/rGH. In terms of their anti-bacterial activity towards Staphylococcus aureus and Escherichia coli, the synergistic effect of the Cu₂O/TiO₂ anchored on the CNF/rGH framework showed excellent activity towards the bacteria.

Enhanced solar water splitting using plasmon-induced resonance energy transfer and unidirectional charge carrier transport

Solar water splitting by photoelectrochemical (PEC) reactions is promising for hydrogen production. The gold nanoparticles (AuNPs) are often applied to promote the visible response of wideband photocatalysts. However, in a typical TiO₂/AuNPs structure, the opposite transfer direction of excited electrons between AuNPs and TiO₂ under visible light and UV light severely limits the solar PEC performance. Here we present a unique Pt/TiO₂/Cu₂O/NiO/AuNPs photocathode, in which the NiO hole transport layer (HTL) is inserted between AuNPs and Cu₂O to achieve unidirectional transport of charge carriers and prominent plasmon-induced resonance energy transfer (PIRET) between AuNPs and Cu₂O. The measured applied bias photon-to-current efficiency and the hydrogen production rate under AM 1.5G illumination can reach 1.5% and 16.4 μmol·cm^-2·h^-1, respectively. This work is original in using the NiO film as the PIRET spacer and provides a promising photoelectrode for energy-efficient solar water splitting.

Enhancement of Photoelectrochemical Cathodic Protection of Copper in Marine Condition by Cu-Doped TiO2

Photochemical cathodic protection (PEC) efficiency was enhanced by doping TiO2 with Cu (Cu/TiO2) through impregnation and reduction under hydrogen. The Cu loading was vaired from 0.1 to 1.0 mol% (0.1 Cu/TiO2, 0.5 Cu/TiO2, 1 Cu/TiO2). Then, up to 50 wt% Cu/TiO2 was mixed with TiO2 to form nanocomposite films. The film photocurrent and photopotential were measured under 1 mW/cm2 UV irradiation. The Cu/TiO2 film with 10 wt% of 0.5 Cu/TiO2 exhibited the highest photocurrent of 29.0 mA/g, which was three times higher than the TiO2 film. The underlying reason for the high photocurrent was the lower photopotential of film than the corrosion potential of copper for PEC. This film was also applied on copper terminal lug for anti-corrosion measurement by Tafel polarization in 3.5 wt% NaCl solution. The results showed that the photopotential of terminal lug coated with the film was −0.252 V vs. Ag/AgCl, which was lower than the corrosion potential of copper (−0.222 V vs. Ag/AgCl). Furthermore, the film can protect the corrosion of copper in the dark with 86.7% lower corrosion current (icorr) than that of bare copper.

Photoreduction route for Cu2O/TiO2 nanotubes junction for enhanced photocatalytic activity

Here, we synthesized copper(i) oxide and titanium dioxide nanotubes (TNTs) heterojunctions (HJs) by a photoreduction method using a low-power UV lamp without involving any additional steps, such as chemical reduction, surfactant, or protection agents. Transmission electron microscopy, X-ray diffraction, Raman scattering, X-ray photoelectron spectroscopy, diffuse reflectance spectra, and photoluminescence spectroscopy were carried out to verify the formation of a HJ between the Cu₂O nanoparticles (Cu₂O NPs) and TNTs. The efficiency and the rate of methylene blue photo-degradation over the Cu₂O/TNTs HJ were found to be nearly double and triple compared to the isolated TNTs. The enhanced efficiency is attributed to the narrow band gap and defect states caused by the oxygen vacancies in the vicinity of HJs. Moreover, the type II band alignment of Cu₂O NPs and TNTs naturally separates the photo-generated carriers and constrains the recombination process owing to the internal electric field across the Cu₂O/TNTs interface.

Good photocatalytic performance is demonstrated by Cu₂O NPs directly loaded onto hydrothermally synthesized TiO₂ NTs via photoreduction method conducted with a lower power UV lamp at room temp. This is due to narrowing bandgap and forming an internal electric field.

Preparation of mesoporous SiO2/Bi2O3/TiO2 superhydrophilic thin films and their surface self-cleaning properties

Mesoporous SiO₂/Bi₂O₃/TiO₂ triple-layered thin films were prepared on glass slides using a simple sol–gel/spin-coating method.

Facet-controlled anatase TiO2 nanoparticles through various fluorine sources for superior photocatalytic activity

Reactive surface-exposed anatase TiO2 (a-TiO2) is highly desirable for applications requiring superior photocatalytic activity. In order to obtain a favorable surface, morphology control of the a-TiO2 using capping agents has been widely investigated. Herein, we systematically study the effects of different F sources (HF, TiF4, and NH4F) as the capping agent on the morphology control and photocatalytic activities of a-TiO2 in a hydrothermal process. When either HF or TiF4 was added, large truncated bipyramids formed with the photocatalytically active {001} facet, whereas the NH4F was not effective for facet control, yielding nanospheres similar to the pure a-TiO2. The morphology changes were related to the decomposition behaviors of the F sources in the solvent material: HF and TiF4 decomposed and supplied F(-) ions before a-TiO2 nucleation, which changed the nucleation rate and growth direction, leading to the resultant a-TiO2 morphology. On the other hand, NH4F supplied F(-) ions after a-TiO2 nucleation and could not change the growth behavior. In terms of the photocatalytic effect, the HF- and TiF4-treated a-TiO2 effectively decomposed ∼90% and ∼80% of methylene blue, respectively, in 1 h, while ∼60% was decomposed for the NH4F-treated a-TiO2. Note that pure a-TiO2 photocatalytically decomposed only ∼10% of methylene blue over the same time. These results pave the way to precise control of the facet of TiO2 through using different capping agents.

Design of Novel Visible Light Active Photocatalyst Materials: Surface Modified TiO2

Work on the design of new TiO2 based photocatalysts is described. The key concept is the formation of composite structures through the modification of anatase and rutile TiO2 with molecular-sized nanoclusters of metal oxides. Density functional theory (DFT) level simulations are compared with experimental work synthesizing and characterizing surface modified TiO2 . DFT calculations are used to show that nanoclusters of metal oxides such as TiO2 , SnO/SnO2 , PbO/PbO2 , ZnO and CuO are stable when adsorbed at rutile and anatase surfaces, and can lead to a significant red shift in the absorption edge which will induce visible light absorption; this is the first requirement for a useful photocatalyst. The origin of the red shift and the fate of excited electrons and holes are determined. For p-block metal oxides the oxidation state of Sn and Pb can be used to modify the magnitude of the red shift and its mechanism. Comparisons of recent experimental studies of surface modified TiO2 that validate our DFT simulations are described. These nanocluster-modified TiO2 structures form the basis of a new class of photocatalysts which will be useful in oxidation reactions and with a correct choice of nanocluster modified can be applied to other reactions.