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Critical catalysis studies often lack elucidation of the mechanistic role of defect equilibria in solid solubility and charge compensation. This approach is applied to interpret the physicochemical properties and catalytic performance of a free‐standing 2D–3D CeO2−x scaffold, which is comprised of holey 2D nanosheets, and its heterojunctions with MoO3−x and RuO2. The band gap alignment and structural defects are engineered using density functional theory (DFT) simulations and atomic characterization. Further, the heterojunctions are used in hydrogen evolution reaction (HER) and catalytic ozonation applications, and the impacts of the metal oxide heteroatoms are analyzed. A key outcome is that the principal regulator of the ozonation performance is not oxygen vacancies but the concentration of Ce3+ and Ce vacancies. Cation vacancy defects are measured to be as high as 8.1 at% for Ru‐CeO2−x. The homogeneous distribution of chemisorbed, Mo‐oxide, heterojunction nanoparticles on the CeO2−x holey nanosheets facilitates intervalence charge transfer, resulting in the dominant effect and resultant ≈50% decrease in overpotential for HER. The heterojunctions are tested for aqueous‐catalytic ozonation of salicylic acid, revealing excellent catalytic performance from Mo doping despite the adverse impact of Ce vacancies. The present study highlights the use of defect engineering to leverage experimental and DFT results for band alignment. [ABSTRACT FROM AUTHOR]

Highly-transparent TiO 2 thin films with varying W-dopant levels (0.010–0.100 mol%) were deposited on soda-lime-silica glass substrates by spin coating, followed by annealing in air at 450 °C for 2 h. Characterization was by GAXRD, AFM, SIMS, XPS, UV–vis, and MB degradation. The films were comprised of single-crystal anatase grains. Increasing doping levels increased the crystallite sizes, grain sizes, and film thicknesses; they also changed the grain morphology from anhedral to euhedral. W 6+ ions formed a substitutional solid solution in anatase and probably exhibited charge compensation by Ti vacancy formation, which led to lattice contraction. Contamination from the glass substrates was homogeneously distributed on the grain boundaries. Doping decreased the optical indirect band gap. However, at the highest doping level, it is not certain if this was an artificial effect of precipitation from overdoping or a true effect of lattice stress. The 0.010 mol% W sample showed the best performance probably owing to its formation of a homogeneous solid solution within the range of semiconducting properties, which could have narrowed the band gap or introduced mid-gap states. The 0.100 mol% W sample may have been overdoped and caused WO 3 precipitation. If so, the presence of the TiO 2 –WO 3 couple may have enhanced charge separation and hence photocatalytic performance. [ABSTRACT FROM AUTHOR]

Bismuth-based metal oxide compounds are promising candidates as novel photocatalysts for environmental remediation. However, their low quantum efficiencies and rapid electron-hole recombination times have hindered their uptake for photocatalytic applications. The present work reports an investigation of the effects of variable Bi/Ti ratio during hydrothermal synthesis on the phase assemblage, nanostructure, surface chemistry, band gap, and photocatalytic performance. Increasing Ti concentration resulted in variations in characteristics and properties according to the sequential formation of the principal phases α-Bi 2 O 3 (BO), Bi 4 Ti 3 O 12 (BTO), and (Na 0.5 Bi 0.5)TiO 3 (NBT). Samples fabricated with precursors having a Bi/Ti ratio of 12/12 exhibited the highest methylene blue (MB) photodegradation rate of 92% under UV irradiation for 2 h. This excellent photocatalytic performance is attributed to the formation of an NBT/BTO heterojunction with advantageous band alignment. [Display omitted] • The phase assemblage shifted from Bi 2 O 3 to BTO to NBT with increasing Bi/Ti ratio. • The photocatalytic performance was dominated by the heterojunctions of BTO on NBT. • The importance of the heterojunctions was verified by the improved charge transfer. [ABSTRACT FROM AUTHOR]

The present work applies a focal point of materials-related issues to review the major case studies of electron transport layers (ETLs) of metal halide perovskite solar cells (PSCs) that contain graphene-based materials (GBMs), including graphene (GR), graphene oxide (GO), reduced graphene oxide (RGO), and graphene quantum dots (GQDs). The coverage includes the principal components of ETLs, which are compact and mesoporous TiO2, SnO2, ZnO and the fullerene derivative PCBM. Basic considerations of solar cell design are provided and the effects of the different ETL materials on the power conversion efficiency (PCE) have been surveyed. The strategy of adding GBMs is based on a range of phenomenological outcomes, including enhanced electron transport, enhanced current density-voltage (J-V) characteristics and parameters, potential for band gap (Eg) tuning, and enhanced device stability (chemical and environmental). These characteristics are made complicated by the variable effects of GBM size, amount, morphology, and distribution on the nanostructure, the resultant performance, and the associated effects on the potential for charge recombination. A further complication is the uncertain nature of the interfaces between the ETL and perovskite as well as between phases within the ETL. [ABSTRACT FROM AUTHOR]

Uniformly codoped anatase TiO 2 thin films of varying (equal) Mo and Cr concentrations (≤1.00 mol% for each dopant) were fabricated using sol-gel spin coating and deposited on fused silica substrates. All films were annealed at 450 °C for 2 h to recrystallise anatase. Undoped anatase films have been subjected to dual ion implantation for the first time, using Mo, Cr, and sequential Mo + Cr at 1 × 1014 atoms/cm2. The films were characterised by GAXRD, AFM, SIMS, XPS, and UV–Vis and the performance was assessed by dye degradation. Despite the volumetric doping by sol-gel and the directional doping by ion implantation, neither method resulted in homogeneous dopant distributions. Both methods caused decreasing crystallinities and associated partial amorphisation. The XPS signal of the uniformly codoped films is dominated by undissolved dopant ions, which is not the case for the ion-implanted films. Increasing Ti valences are attributed to the fully oxidised condition of the Ti4+ ions that diffuse to the surface from Ti vacancy formation compared to the Ti valence of the bulk lattice, which contains Ti3+. Increasing O valence is attributed to the electronegativity of O2−, which is higher than that of Ti4+. Detailed structural mechanisms for the solubility and energetics mechanisms involve the initial formation of Mo and Cr interstitials that fill the two voids adjacent to the central Ti ion in the TiO 6 octahedron, followed by integrated solid solubility (ISS) and intervalence/multivalence charge transfer (IVCT/MVCT). The sequential order of the last two is reversed for the two different doping methods. These two effects are likely to be the source of synergy, if any, between the two dopant ions. The photocatalytic performances of the uniformly codoped films are relatively poor and correlate well with the band gap (E g). The performances of the ion - implanted films do not correlate with the E g , where TiO 2 –Mo performs poorly but TiO 2 –Cr and TiO 2 –Mo–Cr outperform the undoped film. These results are interpreted in terms of the competition between the effects of Mo doping, which causes partial amorphisation and/or blockage of active sites, and Cr doping, which may cause Mo–Cr synergism, Cr-based heterojunction formation, and/or improved charge-carrier separation owing to the surface-deposition nature of ion implantation. •Mo-Cr synergism responsible for improved catalytic behaviour of implanted films. •Mo implantation caused blockage of active sites. •Cr implantation formed heterojunction and improved charge-carrier separation. •Mo-Cr codoped films showed lower performance from band gap increase. •Structural models explained varying dopant solubility scenarios. [ABSTRACT FROM AUTHOR]

Thin films of anatase codoped with 0.01 mol% Co and 0.01 mol% Mo up to 5.00 mol% Co and 5.00 mol% Mo were fabricated by spin coating on soda-lime-silica glass substrates. The films subsequently were calcined in air at 450 °C for 2 h. The mineralogical, chemical, morphological, topographical, optical, and photocatalytic properties of the films were investigated in order to assess the factors and mechanisms affecting the photocatalytic properties. The analytical techniques included glancing angle X-ray diffraction, X-ray photoelectron spectrometry, atomic force microscopy, UV–Vis spectrophotometry, and methylene blue photodegradation. A key determinant of the data was the solubility limit of the dopants, particularly Co. When this was exceeded (≥0.30 mol%), the formation of undetectable precipitates commenced blockage of the active sites, thereby reducing the photocatalytic performance. The initial precursor cation valences altered from Co2+ and Mo5+ to Co2+/Co3+ and Mo4+/Mo5+ upon interstitial solid solution. The absence of charge-compensating Ti3+/Ti4+ redox indicates that charge compensation for the codoping was by IVCT through the reaction Co2+ + Mo5+ ↔ Co3+ + Mo4+. While the three dominant factors affecting the band gap were crystallinity, grain size, and precipitation, the band gap did not dominate the photocatalytic performance or its kinetics. These were reduced by blockage of the active sites by dopant precipitates and, to a lesser extent, from contamination from the cations from the substrate. These phases were the dominant factor in decreasing the optical transmission and thus increasing the band gap through the absorption, reflection, and scattering mechanisms of these grains; the crystallinity and grain size were of secondary importance. • Two dominant effects influenced the photocatalytic properties of codoped TiO 2 films. • Decreased transmittance resulted from increased diffuse scattering and reflectivity. • Dopant precipitation resulted in the greatest extent of blockage of active sites. [ABSTRACT FROM AUTHOR]

A simple template-free method was used to fabricate unique undoped, Cr-doped, and S-doped mesoporous 2D-3D ceria (CeO 2-x) nanostructures by using triethanolamine (TEA) to exfoliate electrodeposited cerium coordination polymer (Ce-CP), followed by heat treatment at 450 °C. For undoped CeO 2-x fabrication, varying the amount of TEA impacted the exfoliation extent and the resultant microstructural and defect characteristics, with the optimal Ce-CP to TEA ratio determined to be 30 mg:0.35 mL. For doped CeO 2-x , the introduction of dopants (substitutionally and/or interstitially) and heterojunction formation resulted in the creation of defects from charge compensation (ionic, electronic, and redox) which influenced charge carrier concentrations for reactions. The precipitation of excess dopants on the surface to create heterojunctions allowed for reduced crystallite sizes, lowering of the optical band gaps, and increased charge carrier separation, which in turn minimised charge carrier diffusion distance, ultimately increasing photocatalytic efficiency. Dye degradation tests showed that 10 mol% Cr-doped CeO 2-x revealed superior photocatalytic performance with a reaction rate of 15.88 × 10−3 min−1 (87% degradation after 160 min) compared to both 2.5 mol% S-doped CeO 2-x and undoped CeO 2-x (68% and 57% after 160 min, respectively). With sulphur addition, although the defect concentration increased and the crystallite sizes and band gaps were lowered, the degradation performance declined with increasing dopant amount owing to the reduced porosity from liquid-phase microstructural densification. Thus, the positive changes to the electronic and microstructural features from dopant-related heterojunction formations were observed to be the most critical factors impacting the photocatalytic performance of these unique nanostructures. [Display omitted] • Exfoliation with TEA and annealing generates 2D-3D ceria nanostructures. • Defect creation by doping and charge compensation mechanisms. • Cr-doping induces heterojunction formation, enhancing photocatalytic efficiency. • S-doping cause microstructural densification and reduces active sites. [ABSTRACT FROM AUTHOR]