Modification of Charge Trapping at Particle/Particle Interfaces by Electrochemical Hydrogen Doping of Nanocrystalline TiO2

Substrate-Enabled Room-Temperature Electrochemical Deposition of Crystalline ZnMnO3

Mixed transition metal oxides have emerged as promising electrode materials for electrochemical energy storage and conversion. To optimize the functional electrode properties, synthesis approaches allowing for a systematic tailoring of the materials' composition, crystal structure and morphology are urgently needed. Here we report on the room-temperature electrodeposition of a ternary oxide based on earth-abundant metals, specifically, the defective cubic spinel ZnMnO₃ . In this unprecedented approach, ZnO surfaces act as (i) electron source for the interfacial reduction of MnO₄ ^- in aqueous solution, (ii) as substrate for epitaxial growth of the deposit and (iii) as Zn precursor for the formation of ZnMnO₃ . Epitaxial growth of ZnMnO₃ on the lateral facets of ZnO nanowires assures effective electronic communication between the electroactive material and the conducting scaffold and gives rise to a pronounced 2-dimensional morphology of the electrodeposit forming - after partial delamination from the substrate - twisted nanosheets. The synthesis strategy shows promise for the direct growth of different mixed transition metal oxides as electroactive phase onto conductive substrates and thus for the fabrication of binder-free nanocomposite electrodes.

Impact of Nanoparticle Consolidation on Charge Separation Efficiency in Anatase TiO2 Films

Mesoporous films and electrodes were prepared from aqueous slurries of isolated anatase TiO₂ nanoparticles. The resulting layers were annealed in air at temperatures 100°C ≤ T ≤ 450°C upon preservation of internal surface area, crystallite size and particle size. The impact of processing temperature on charge separation efficiency in nanoparticle electrodes was tracked via photocurrent measurements in the presence of methanol as a hole acceptor. Thermal annealing leads to an increase of the saturated photocurrent and thus of the charge separation efficiency at positive potentials. Furthermore, a shift of capacitive peaks in the cyclic voltammograms of the nanoparticle electrodes points to the modification of the energy of deep traps. Population of these traps triggers recombination possibly due to the action of local electrostatic fields attracting photogenerated holes. Consequently, photocurrents saturate at potentials, at which deep traps are mostly depopulated. Charge separation efficiency was furthermore investigated for nanoparticle films and was tracked via the decomposition of hydrogen peroxide. Our observations evidence an increase of charge separation efficiency upon thermal annealing. The effect of particle consolidation, which we associate with minute atomic rearrangements at particle/particle contacts, is attributed to the energetic modification of deep traps and corresponding modifications of charge transport and recombination, respectively.

Paramagnetic electron centers in BaTiO3 nanoparticle powders

Knowledge about the emergence and depletion of point defects in BaTiO3 (BTO) nano-structures during materials processing is key to our understanding of their later activity as components in functional dielectric devices or as photocatalysts. In this electron paramagnetic resonance (EPR) study we investigated BaTiO3 nanoparticle powders produced by flame spray pyrolysis (FSP) with powders of TiO2 anatase nanocrystals of comparable size as reference system. Paramagnetic Ti3+ ions located at regular lattice sites and with well-defined EPR signatures were measured in vacuum annealed BaTiO3 nanoparticles, which convert upon further annealing in the temperature range between 873 K and 1173 K from monocrystalline grains with an average size of d = 12 nm, BTO (873 K), to polycrystalline particles with d = 70 nm, BTO (1173 K). Whereas the starting material hosts predominantly polaron-type Ti3+ ions being surrounded by compressed O2- ion octahedra, barium-oxygen divacancy complexes, , become susceptible to electron trapping in polycrystalline and tetragonal BTO (1173 K) particles after pre-annealing at temperatures T > 873 K. The insights obtained provide a base for the detection of local distortion effects, for the identification of charge trapping sites and for the elucidation of their impact on spontaneous polarization in BaTiO3 nanoparticles as photocatalysts or dielectric components.

Surface polaron states on single-crystal rutile TiO2 nanorod arrays enhancing charge separation and transfer

Polaron states on TiO2 photoanodes provide an important electron transfer pathway at the electrode-electrolyte interface. Here, we electrochemically doped single-crystal rutile TiO2 nanorod arrays with exposed (110) facets to produce surface polaron states, Ti3+-OH, which greatly contributed to charge separation and transfer. Our results experimentally clarified the previously confused understanding of the origin of improved photoelectrochemical (PEC) water splitting performance and verified that the enhanced PEC effects mainly arise from surface polaron states instead of grain boundary passivation.

Investigation of Well-Defined Pinholes in TiO2 Electron Selective Layers Used in Planar Heterojunction Perovskite Solar Cells

The recently introduced perovskite solar cell (PSC) technology is a promising candidate for providing low-cost energy for future demands. However, one major concern with the technology can be traced back to morphological defects in the electron selective layer (ESL), which deteriorates the solar cell performance. Pinholes in the ESL may lead to an increased surface recombination rate for holes, if the perovskite absorber layer is in contact with the fluorine-doped tin oxide (FTO) substrate via the pinholes. In this work, we used sol-gel-derived mesoporous TiO2 thin films prepared by block co-polymer templating in combination with dip coating as a model system for investigating the effect of ESL pinholes on the photovoltaic performance of planar heterojunction PSCs. We studied TiO2 films with different porosities and film thicknesses, and observed that the induced pinholes only had a minor impact on the device performance. This suggests that having narrow pinholes with a diameter of about 10 nm in the ESL is in fact not detrimental for the device performance and can even, to some extent improve their performance. A probable reason for this is that the narrow pores in the ordered structure do not allow the perovskite crystals to form interconnected pathways to the underlying FTO substrate. However, for ultrathin (~20 nm) porous layers, an incomplete ESL surface coverage of the FTO layer will further deteriorate the device performance.

Photoelectrochemical Properties of SnO2 Photoanodes Sensitized by Cationic Perylene-Di-Imide Aggregates for Aqueous HBr Splitting

Perylene-sensitized mesoporous SnO₂ films were used as electrodes for photoelectrochemical HBr splitting in aqueous solution. Upon AM 1.5 G illumination a 3-4 fold increase of the saturated photocurrent was observed when decreasing the pH of the aqueous solution from pH 3 to pH 0 (j ^max = 0.05 ± 0.01 mAcm^-2 at pH 3 and 0.17 ± 0.02 mAcm^-2 at pH 0, respectively). A detailed spectroscopic and electrochemical analysis of the hybrid material was carried out in order to address the impact of interfacial energetics on charge separation dynamics. UV/Vis spectroelectrochemical measurements showed that the energy of semiconductor states in such systems can be adjusted independently from the molecular levels by varying proton concentration. Photoelectrochemical measurements and ns-μs transient absorption spectroscopy reveal that pH-related changes of the interfacial energetics have only a minor impact on the charge injection rate. An increase of the proton concentration improves charge collection mainly by retarding recombination, which in the case of Br^- oxidation is in critical competition with perylene regeneration. Control of the back recombination appears to be a key feature in heterogeneous molecular systems tasked to drive energetically demanding redox reactions.

Particle Consolidation and Electron Transport in Anatase TiO2 Nanocrystal Films

A sequence of chemical vapor synthesis and thermal annealing in defined gas atmospheres was used to prepare phase-pure anatase TiO₂ nanocrystal powders featuring clean surfaces and a narrow particle size distribution with a median particle diameter of 14.5 ± 0.5 nm. Random networks of these nanocrystals were immobilized from aqueous dispersions onto conducting substrates and are introduced as model systems for electronic conductivity studies. Thermal annealing of the immobilized films at 100 °C < T < 450 °C in air was performed to generate particle-particle contacts upon virtual preservation of the structural properties of the nanoparticle films. The distribution of electrochemically active electronic states as well as the dependence of the electronic conductivity on the Fermi level position in the semiconductor films was studied in aqueous electrolytes in situ using electrochemical methods. An exponential distribution of surface states is observed to remain unchanged upon sintering. However, capacitive peaks corresponding to deep electron traps in the nanoparticle films shift positive on the potential scale evidencing an increase of the trapping energy upon progressive thermal annealing. These peaks are attributed to trap states at particle-particle interfaces in the random nanocrystal network (i.e., at grain boundaries). In the potential region, where the capacitive peaks are detected, we observe an exponential conductivity variation by up to 5 orders of magnitude. The potential range featuring the exponential conductivity variation shifts positive by up to 0.15 V when increasing the sintering temperature from 100 to 450 °C. Importantly, all films approach a potential- and sintering-temperature-independent maximum conductivity of ∼10^-4 Ω^-1·cm^-1 at more negative potentials. On the basis of these results we introduce a qualitative model, which highlights the detrimental impact of electron traps located on particle-particle interfaces on the electronic conductivity in random semiconductor nanoparticle networks.

A Photoresponsive Rutile TiO2 Heterojunction with Enhanced Electron-Hole Separation for High-Performance Hydrogen Evolution

Rutile titanium dioxide (TiO₂ ) is a promising photocatalyst due to its high thermodynamic stability and few intragrain defects. However, it has not yet achieved photocatalytic activity comparable to that of anatase TiO₂ owing to its higher recombination rate of electron-hole pairs. To effectively separate the electron-hole pairs in rutile TiO₂ , a facet heterojunction (FH) structure to prolong the lifetime of the photogenerated electrons is proposed. Ultrathin TiO₂ nanosheets with different facets are coated in situ onto TiO₂ nanorod (NR) substrates, where FHs are built among the nanosheets as well as between the nanosheets and NR substrates. The as-prepared rutile TiO₂ , with an FH structure (FH-TiO₂ ), serves as an effective photocatalyst for water splitting. More than 45 and 18 times higher photogenerated current density and H₂ production rate, respectively, are obtained compared to those of pure rutile TiO₂ NRs. Moreover, FH-TiO₂ delivers a 0.566 mmol g^-1 h^-1 H₂ production rate even in pure water. This study offers important insights into the rational design of rutile TiO₂ structures for highly efficient photocatalytic reactions.

Grain Boundary Facilitates Photocatalytic Reaction in Rutile TiO2 Despite Fast Charge Recombination: A Time-Domain ab Initio Analysis

TiO₂ is an excellent photocatalytic and photovoltaic material but suffers low efficiency because of deep trap states giving rise to fast charge and energy losses. Using a combination of time-domain density functional theory and nonadiabatic molecular dynamics, we demonstrate that grain boundaries (GBs), which are common in polycrystalline TiO₂, accelerate nonradiative electron-hole recombination by a factor of 3. Despite GBs increase the band gap without creating deep trap states, and accelerate coherence loss, they enhance nonadiabatic electron-phonon coupling, and facilitate the relaxation. Importantly, electrons accumulated at the boundaries together with the relatively long-lived excite state favor photocatalytic reaction. Our study rationalizes the experimental observations and provides valuable perspectives for improving the device performance by defect engineering.

Localization and Stabilization of Photogenerated Electrons at TiO2 Nanoparticle Surface by Oxygen at Ambient Temperature

Understanding the mechanism by which oxygen adsorption influences the separation behavior of charge carriers is important in photocatalytic removal of air pollutants. In this study, we performed steady-state surface photovoltage and surface photocurrent spectroscopy combined with an atmosphere control system to determine the effect of oxygen on the charge separation behavior at the surface of anatase TiO₂ nanoparticles at ambient temperature. Results showed that photogenerated electrons were movable in N₂ atmosphere but were localized in O₂ atmosphere. O₂ obviously enhanced the stabilization of the localized photogenerated electrons when the surface defects of TiO₂ were fully occupied by adsorbed O₂. Moreover, O₂ adsorption increased the energy demand for exciting electrons from the valence band to localized surface defect states and reduced the density of band tail states. These findings suggest us that the effect of gaseous species on the mobility and stability of charge carriers should be considered to understand the photocatalytic degradation of air pollutants.

Cd/In-Codoped TiO2 nanochips for high-efficiency photocatalytic dye degradation

Titanium dioxide has been widely investigated in the field of photocatalysis research. However, the wide bandgap (3.2 eV) greatly limits its practical applications because only ultraviolet light can be absorbed by bare TiO2. Herein, we report a facile approach to prepare Cd/In-codoped TiO2 nanochips with the capability of visible light absorption. Such bimetallic-doped TiO2 was synthesized through a two-step process: Cd/In/S-TiO2 gels were first synthesized by mixing the preformed Cd-In-S supertetrahedral nanoclusters with a titanium source, and the subsequent pyrolytic process effectively converted the gels into Cd/In-TiO2 nanochips with a thickness of ∼2.19 nm and a uniform diameter of ∼10.60 nm. Interestingly, the absorption band of Cd/In-TiO2 nanochips was adjusted by pyrolysis temperature, which further regulated the photocatalytic efficiency of dye degradation under visible light. Current research demonstrates that doping TiO2 by multimetallic sulfide nanoclusters opens up a new door to further enrich the dopants in TiO2 and broaden their potential applications.

Delocalized Impurity Phonon Induced Electron-Hole Recombination in Doped Semiconductors

Semiconductor doping is often proposed as an effective route to improving the solar energy conversion efficiency by engineering the band gap; however, it may also introduce electron-hole (e-h) recombination centers, where the determining element for e-h recombination is still unclear. Taking doped TiO₂ as a prototype system and by using time domain ab initio nonadiabatic molecular dynamics, we find that the localization of impurity-phonon modes (IPMs) is the key parameter to determine the e-h recombination time scale. Noncompensated charge doping introduces delocalized impurity-phonon modes that induce ultrafast e-h recombination within several picoseconds. However, the recombination can be largely suppressed using charge-compensated light-mass dopants due to the localization of their IPMs. For different doping systems, the e-h recombination time is shown to depend exponentially on the IPM localization. We propose that the observation that delocalized IPMs can induce fast e-h recombination is broadly applicable and can be used in the design and synthesis of functional semiconductors with optimal dopant control.

Tubular nitrogen-doped TiO2 samples with efficient photocatalytic properties based on long-lived charge separation under visible-light irradiation: synthesis, characterization and reactivity

A nitrogen-doped TiO₂ sample was prepared at 413 K by direct hydrothermal treatment of titanium isopropoxide in an aqueous solution of NH₃. This new material has a large specific surface area of ca. 220 m² g^-1 because of its tubular structure and it exhibits a prominent absorption feature in the region between 400 and 650 nm. It responds strongly to light in the visible region, which is key to its potential performance as a photocatalyst that may improve the efficiency for utilization of solar energy. Actually, this sample exhibits very efficient activity in the decomposition of CH₃COOH under visible light among the samples prepared. This effective photocatalysis of the present sample was substantiated by characteristic spectroscopic features, such as: (1) an optical absorption band with λ > 400 nm because of the doped nitrogen species; (2) the formation of EPR-active, long-lived N˙ and O₂^- species, as well as N₂^- species, under visible-light irradiation in the O₂ or N₂ adsorption process at 300 K by way of the monovalent nitrogen ions in the bulk (both substitutional and interstitial); (3) the existence of IR-active O₂ species adsorbed on the nitrogen-doped TiO₂ sample even without light irradiation; and (4) an XPS N_1s band around 399.6 eV that is assignable to the N^- species. The amounts of N˙ and O₂^- species formed in the nitrogen-doped TiO₂ sample under visible-light irradiation correlated well with the levels of reactivity observed in the decomposition of CH₃COOH on the samples with varying amounts and types of doped nitrogen species. We conclude that the photoactive N˙ and O₂^- species created in the present sample are responsible for the decomposition of organic materials assisted by visible light irradiation. These features may be attributable to the interface between the sample's tubular structure and anatase with poor crystallinity, which probably causes the resistance to the recombination of electron-hole pairs formed by irradiation.

Engineering the Surface/Interface Structures of Titanium Dioxide Micro and Nano Architectures towards Environmental and Electrochemical Applications

Titanium dioxide (TiO₂) materials have been intensively studied in the past years because of many varied applications. This mini review article focuses on TiO₂ micro and nano architectures with the prevalent crystal structures (anatase, rutile, brookite, and TiO₂(B)), and summarizes the major advances in the surface and interface engineering and applications in environmental and electrochemical applications. We analyze the advantages of surface/interface engineered TiO₂ micro and nano structures, and present the principles and growth mechanisms of TiO₂ nanostructures via different strategies, with an emphasis on rational control of the surface and interface structures. We further discuss the applications of TiO₂ micro and nano architectures in photocatalysis, lithium/sodium ion batteries, and Li-S batteries. Throughout the discussion, the relationship between the device performance and the surface/interface structures of TiO₂ micro and nano structures will be highlighted. Then, we discuss the phase transitions of TiO₂ nanostructures and possible strategies of improving the phase stability. The review concludes with a perspective on the current challenges and future research directions.

Effects of spatial topologies and electron Fermi-level gradient on the photocatalytic efficiency of nano-particulate semiconductors

Nanocrystalline (nc) semiconductor materials are important in photocatalysis. The nanoparticle (NP) topologies and electron Fermi-level (E_F) gradient along the interconnected NPs affect the photocatalytic efficiency (η) of the nc-materials because of the charge carrier interparticle transport (IPT). However, the detailed physiochemical kinetic mechanism remains unclear. Based on the kinetic analysis and the numerical Monte-Carlo simulation of random walks, the statistical probability distributions p_Rec(t) and p_it(t) for the recombination time and interfacial transfer (IT) time have been proposed in this study. The recombination lifetime (τ_Red) and IT lifetime (τ_IT) were calculated by averaging p_Rec(t) and p_it(t). The characteristic time τ_e of the entire electron kinetics was defined using τ_Re and τ_IT, and η was calculated by dividing τ_e by τ_IT. The simulation results show that the p_Rec(t) clearly shows the IPT of electrons. Both the kinetic factors (NP spatial topologies and boundary barrier) and the thermodynamic factor (electron E_F gradient) can affect the IPT. It was observed that the increase in IPT cannot lead to a monotonous increase in η although it can prohibit recombination. Whether the IPT can increase the η is dependent on ratio of the back IPT for recombination and the forward IPT for IT. The existence of an electron E_F gradient from the electron generation site to the active site can increase η by promoting the forward IPT.

Charge Transfer Reductive in situ Doping of Mesoporous TiO2 Photoelectrodes - Impact of Electrolyte Composition and Film Morphology

Some material properties depend not only on synthesis and processing parameters, but may furthermore significantly change during operation. This is particularly true for high surface area materials. We used a combined electrochemical and spectroscopic approach to follow the changes of the photoelectrocatalytic activity and of the electronic semiconductor properties of mesoporous TiO₂ films upon charge transfer reductive doping. Shallow donors (i.e. electron/proton pairs) were introduced into the semiconductor by the application of an external potential or, alternatively, by band gap excitation at open circuit conditions. In the latter case the effective open circuit doping potential depends critically on electrolyte composition (e.g. the presence of electron or hole acceptors). Transient charge accumulation (electrons and protons) in nanoparticle electrodes results in a photocurrent enhancement which is attributed to the deactivation of recombination centers. In nanotube electrodes the formation of a space charge layer results in an additional decrease of charge recombination at positive potentials. Doping is transient in nanoparticle films, but turns out to be stable for nanotube arrays.