Primary intermediates of oxygen photoevolution reaction on TiO2 (Rutile) particles, revealed by in situ FTIR absorption and photoluminescence measurements

Complex Roles of Solution Chemistry on Graphene Oxide Coagulation onto Titanium Dioxide: Batch Experiments, Spectroscopy Analysis and Theoretical Calculation

Although graphene oxide (GO) has been used in multidisciplinary areas due to its excellent physicochemical properties, its environmental behavior and fate are still largely unclear. In this study, batch experiments, spectroscopy analysis and theoretical calculations were addressed to promote a more comprehensive understanding toward the coagulation behavior of GO onto TiO₂ under various environmental conditions (pH, co-existing ions, temperature, etc.). The results indicated that neutral pH was beneficial to the removal of GO due to the electrostatic interaction. The presence of cations accelerated GO coagulation significantly owing to the influence of electrical double layer compression. On the contrary, the presence of anions improved the stability of GO primarily because of electrostatic repulsion and steric hindrance. Results of XRD, FTIR and XPS analysis indicated that the coagulation of GO on TiO₂ was mainly dominated by electrostatic interactions and hydrogen bonds, which were further evidenced by DFT calculations. The high binding energy further indicated the stability of GO + TiO₂ system, suggesting that TiO₂ can be used as an effective coagulant for the efficient elimination and coagulation of GO from aqueous solutions. These findings might likely lead to a better understanding of the migration and transformation of carbon nanomaterials in the natural environment.

Using a chitosan nanolayer as an efficient pH buffer to protect pH-sensitive supramolecular assemblies

We propose that chitosan can be used as an efficient pH-responsive protective layer for pH sensitive soft materials.

Catalytic role of bridging oxygens in TiO2 liquid phase photocatalytic reactions: analysis of H216O photooxidation on labeled Ti18O2

TiO₂ surface lattice oxygens are actively involved in the photocatalytic oxidation of water as demonstrated by isotopic tracing experiments with Ti¹⁸O₂.

Frontiers of water oxidation: the quest for true catalysts

Development of advanced analytical techniques is essential for the identification of water oxidation catalysts together with mechanistic studies.

Following the Reduction of Oxygen on TiO2 Anatase (101) Step by Step

We have investigated the reaction between O2 and H2O, coadsorbed on the (101) surface of a reduced TiO2 anatase single crystal by scanning tunneling microscopy, density functional theory, temperature-programmed desorption, and X-ray photoelectron spectroscopy. While water adsorbs molecularly on the anatase (101) surface, the reaction with O2 results in water dissociation and formation of terminal OH groups. We show that these terminal OHs are the final and stable reaction product on reduced anatase. We identify OOH as a metastable intermediate in the reaction. The water dissociation reaction runs as long as the surface can transfer enough electrons to the adsorbed species; the energy balance and activation barriers for the individual reaction steps are discussed, depending on the number of electrons available. Our results indicate that the presence of donor dopants can significantly reduce activation barriers for oxygen reduction on anatase.

A solar light driven dual photoelectrode photocatalytic fuel cell (PFC) for simultaneous wastewater treatment and electricity generation

In this paper, a novel dual heterojunction Photocatalytic Fuel Cell (PFC) system based on BiVO4/TiO2 nanotubes/FTO photoanode and ZnO/CuO nanowires/FTO photocathode has been designed. Compared with the electrodes in PFCs reported in earlier literatures, the proposed heterojunction not only enhances the visible light absorption but also offers a higher photoconversion efficiency. In addition, the nanostructured heterojunction owns a large surface area that ensures a large amount of active sites for organics degradation. The performance of the PFC base on the dual photoelectrodes was also studied herein. The results indicated that the PFC in ths paper exhibits a superior performance and its JV(max) reached 0.116 mw cm(-2), which is higher than that in most of reported PFCs with a Pt-free photocathode. When hazardous organic compounds such as methyl orange, Congo red and methylene blue were decomposed, the degradation rates obtained is to be 76%, 83%, and 90% respectively after 80 mins reaction. The proposed heterojunction photoelectrodes provided great potential for cost-effective and high-efficiency organic pollutants degradation and electricity generation in a PFC system.

Determination of photoelectrochemical water oxidation intermediates on haematite electrode surfaces using operando infrared spectroscopy

Semiconductor electrodes capable of using solar photons to drive water-splitting reactions, such as haematite (α-Fe2O3), have been the subject of tremendous interest over recent decades. The surface has been found to play a significant role in determining the efficiency of water oxidation with haematite; however, previous works have only allowed hypotheses to be formulated regarding the identity of relevant surface species. Here we investigate the water-oxidation reaction on haematite using infrared spectroscopy under photoelectrochemical (PEC) water-oxidation conditions. A potential- and light-dependent absorption peak at 898 cm(-1) is assigned to a Fe(IV)=O group, which is an intermediate in the PEC water-oxidation reaction. These results provide direct evidence of high-valent iron-oxo intermediates as the product of the first hole-transfer reaction on the haematite surface and represent an important step in establishing the mechanism of PEC water oxidation on semiconductor electrodes.

Modeling and Simulations in Photoelectrochemical Water Oxidation: From Single Level to Multiscale Modeling

This review summarizes recent developments, challenges, and strategies in the field of modeling and simulations of photoelectrochemical (PEC) water oxidation. We focus on water splitting by metal-oxide semiconductors and discuss topics such as theoretical calculations of light absorption, band gap/band edge, charge transport, and electrochemical reactions at the electrode-electrolyte interface. In particular, we review the mechanisms of the oxygen evolution reaction, strategies to lower overpotential, and computational methods applied to PEC systems with particular focus on multiscale modeling. The current challenges in modeling PEC interfaces and their processes are summarized. At the end, we propose a new multiscale modeling approach to simulate the PEC interface under conditions most similar to those of experiments. This approach will contribute to identifying the limitations at PEC interfaces. Its generic nature allows its application to a number of electrochemical systems.

Pertinent parameters in photo-generation of electrons: Comparative study of anatase-based nano-TiO2 suspensions

In the field of solar fuel cells, the development of efficient photo-converting semiconductors remains a major challenge. A rational analysis of experimental photocatalytic results obtained with material in colloïdal suspensions is needed to access fundamental knowledge required to improve the design and properties of new materials. In this study, a simple system electron donor/nano-TiO2 is considered and examined via spin scavenging electron paramagnetic resonance as well as a panel of analytical techniques (composition, optical spectroscopy and dynamic light scattering) for selected type of nano-TiO2. Independent variables (pH, electron donor concentration and TiO2 amount) have been varied and interdependent variables (aggregate size, aggregate surface vs. volume and acid/base groups distribution) are discussed. This work shows that reliable understanding involves thoughtful combination of interdependent parameters, whereas the specific surface area seems not a pertinent parameter. The conclusion emphasizes the difficulty to identify the key features of the mechanisms governing photocatalytic properties in nano-TiO2.

A mechanism for the hole-mediated water photooxidation on TiO2 (1 0 1) surfaces

The mechanism of water photooxidation on TiO2 surfaces is still controversial. Here we report a first-principles density functional study based on a hybrid functional method in which an adsorbed water molecule is found to directly interact with a self-trapped hole at a bridging oxygen site and to transform into an OH(•) radical species through a concerted proton/hole transfer. This study analyzes both the thermodynamics and kinetics of this step of the reaction, which is generally considered to be the rate determining one. The fate of the OH(•) radical is then investigated in terms of its reactivity with different surface species, with a second OH(•) radical, or with a second water molecule coming from the environment. We find that OH(•) radicals can either acquire a hydrogen from surrounding water molecules or, if they meet, couple to form hydrogen peroxide with highly associated energy gain.

A heterojunction photocatalyst composed of zinc rhodium oxide, single crystal-derived bismuth vanadium oxide, and silver for overall pure-water splitting under visible light up to 740 nm

We have prepared a solid-state heterojunction photocatalyst, which can split pure water under red light up to 740 nm.

Elementary photocatalytic chemistry on TiO2surfaces

In this article, we review the recent advances in the photoreactions of small molecules with model TiO₂surfaces, and propose a photocatalytical model based on nonadiabatic dynamics and ground state surface reactions.

Enhanced Dispersion of TiO2 Nanoparticles in a TiO2/PEDOT:PSS Hybrid Nanocomposite via Plasma-Liquid Interactions

A facile method to synthesize a TiO₂/PEDOT:PSS hybrid nanocomposite material in aqueous solution through direct current (DC) plasma processing at atmospheric pressure and room temperature has been demonstrated. The dispersion of the TiO₂ nanoparticles is enhanced and TiO₂/polymer hybrid nanoparticles with a distinct core shell structure have been obtained. Increased electrical conductivity was observed for the plasma treated TiO₂/PEDOT:PSS nanocomposite. The improvement in nanocomposite properties is due to the enhanced dispersion and stability in liquid polymer of microplasma treated TiO₂ nanoparticles. Both plasma induced surface charge and nanoparticle surface termination with specific plasma chemical species are proposed to provide an enhanced barrier to nanoparticle agglomeration and promote nanoparticle-polymer binding.

A nucleus-coupled electron transfer mechanism for TiO2-catalyzed water splitting

Based on first-principles calculations, we reveal that in the photocatalytic oxygen evolution reaction (OER) at the TiO2/water interface, the formation of an O-O bond always involves the anti-bonding σ2p* state elevated from the valence band into the conduction band of TiO2 regardless of a detailed reaction pathway. The role of photoholes is to deplete this anti-bonding state once it emerges into the band gap. The reaction barrier is thus determined by the onset where photoholes enter the reaction. This process represents a new reaction mechanism, termed nucleus-coupled electron transfer (NCET), where electron transfer is enabled by the movement of nuclei that promotes the reactive orbital to become the frontier orbital. The NCET mechanism for the OER is shown to exhibit an overall kinetic barrier surmountable at room temperature.

Electronic, magnetic structure and water splitting reactivity of the iron-sulfur dimers and their hexacarbonyl complexes: A density functional study

The iron sulfide dimers (FeS)2 and their persulfide isomers with S-S bonds are studied with the B3LYP density functional as bare clusters and as hexacarbonyls. The disulfides are more stable than the persulfides as bare clusters and the persulfide ground state lies at 3.2 eV above the global minimum, while in the hexacarbonyl complexes this order is reversed: persulfides are more stable, but the energy gap between disulfides and persulfides becomes much smaller and the activation barrier for the transition persulfide → disulfide is 1.11 eV. Carbonylation also favors a non-planar Fe2S2 ring for both the disulfides and the persulfides and high electron density in the Fe2S2 core is induced. The diamagnetic ordering is preferred in the hexacarbonyls, unlike the bare clusters. The hexacarbonyls possess low-lying triplet excited states. In the persulfide, the lowest singlet-to-triplet state excitation occurs by electron transition from the iron centers to an orbital located predominantly at S2 via metal-to-ligand charge transfer. In the disulfide this excitation corresponds to ligand-to-metal charge transfer from the sulfur atoms to an orbital located at the iron centers and the Fe-Fe bond. Water splitting occurs on the hexacarbonyls, but not on the bare clusters. The singlet and triplet state reaction paths were examined and activation barriers were determined: 50 kJ mol(-1) for HO-H bond dissociation and 210 kJ mol(-1) for hydrogen evolution from the intermediate sulfoxyl-hydroxyl complexes Fe2S(OH)(SH)(CO)6 formed. The lowest singlet-singlet excitations in the hexacarbonyls, the water adsorption complexes and in the reaction intermediates, formed prior to dihydrogen release, fall in the visible light region. The energy barrier of 210 kJ mol(-1) for the release of one hydrogen molecule corresponds to one visible photon of 570 nm. The dissociation of a second water molecule, followed by H2 and O2 release via hydro-peroxide intermediate is a two-step process, with activation barriers of 218 and 233 kJ mol(-1), which also fall in the visible light region. A comparison of the full reaction path with that on diiron dioxide hexacarbonyls Fe2O2(CO)6 is traced.

Rate law analysis of water oxidation on a hematite surface

Water oxidation is a key chemical reaction, central to both biological photosynthesis and artificial solar fuel synthesis strategies. Despite recent progress on the structure of the natural catalytic site, and on inorganic catalyst function, determining the mechanistic details of this multiredox reaction remains a significant challenge. We report herein a rate law analysis of the order of water oxidation as a function of surface hole density on a hematite photoanode employing photoinduced absorption spectroscopy. Our study reveals a transition from a slow, first order reaction at low accumulated hole density to a faster, third order mechanism once the surface hole density is sufficient to enable the oxidation of nearest neighbor metal atoms. This study thus provides direct evidence for the multihole catalysis of water oxidation by hematite, and demonstrates the hole accumulation level required to achieve this, leading to key insights both for reaction mechanism and strategies to enhance function.

Exploring the important role of nanocrystals orientation in TiO₂ superstructure on photocatalytic performances

Efficient charge separation has been widely accepted as one of the important factors responsible for the photocatalytic water splitting, organic oxidation, and solar cell, etc. TiO2 mesocrystal is a superstructure which could largely enhance charge separation, where TiO2 nanocrystals with parallel crystallographic alignment assemble in a form of oriented aggregation. Here, the intercrystal misorientation in TiO2 superstructure was first concerned and evaluated on the influence of photocatalytic efficiency. Our results showed that the intercrystal misorientation in TiO2 superstructures had a harmful effect on the charge separation efficiency.

Photodesorption of water from rutile(110): ab initio calculation of five-dimensional potential energy surfaces of ground and excited electronic states and wave packet studies

In this paper, we report on our results concerning the interaction of water with titanium dioxide in its rutile modification. The (110) surface is modelled by an embedded Ti9O18Mg7(14+) cluster. We present up to five-dimensional potential energy surfaces for the water molecule on this surface and include the dissociation of one hydrogen atom. The electronic ground state as well as one electronically excited state is included. To deal with the multi-configurational character of the wave function, we use the complete active space self-consistent field (CASSCF) approach. The resulting potential energy surfaces are fitted by means of an artificial neural network. As a first example of quantum dynamical studies based on our potential surfaces, we present results on the photodesorption of water from rutile(110).

Identifying the distinct features of geometric structures for hole trapping to generate radicals on rutile TiO2(110) in photooxidation using density functional theory calculations with hybrid functional

Using density functional theory calculations with HSE 06 functional, we obtained the structures of spin-polarized radicals on rutile TiO₂(110), which is crucial to understand the photooxidation at the atomic level, and furthermore the thermodynamic stability of the radicals and their promotion effect on water photooxidation are also investigated.

Highly efficient and stable photocatalytic reduction of CO2 to CH4 over Ru loaded NaTaO3

An efficient and stable photocatalytic activity was obtained over NaTaO₃ by introducing electron donor (H₂) into the CO₂ reduction process with water and loaded Ru as cocatalysts. The main effect of the electron donor was found to release the peroxide intermediates of the half reaction for water oxidation.

UO22+-amino hybrid materials: structural variation and photocatalysis properties

We have synthesized four new uranyl complexes. Their photoluminescence was characterized and their photocatalytic properties were studied in detail.

Light-induced water splitting by titanium-tetrahydroxide: a computational study

Light induced water splitting by Ti(OH)₄ following the hydroxyl radical generation mechanism. Subsequent reactions lead to O₂ and H₂ production.

Sol–gel synthesis of nanocrystal-constructed hierarchically porous TiO2 based composites for lithium ion batteries

Hierarchically porous TiO₂ based composites have been synthesized by a facile sol–gel method. As anode materials for lithium-ion batteries (LIBs), which exhibit excellent cycling stability and superior rate capability.

The potential versus current state of water splitting with hematite

This review describes the potential of hematite as a photoanode material for photoelectrochemical (PEC) water splitting.

The pH dependence of OH radical formation in photo-electrochemical water oxidation with rutile TiO2 single crystals

The experimental results in photoelectrolysis with rutile (100) and (110) TiO₂ single crystals support a plausible reaction mechanism that the surface Ti–O–O–Ti structure is an intermediate of water oxidation process, by which mechanism the O₂ production becomes favorable in alkaline solution.

Improved photoelectrochemical water oxidation kinetics using a TiO2 nanorod array photoanode decorated with graphene oxide in a neutral pH solution

Opposite contributions of graphene oxide are obtained in TiO₂ nanorod array photoanodes, in electrolytes of varying pH, for the water splitting application. The photocurrent of TiO₂ photoanode is improved, after graphene oxide coating, in a neutral pH electrolyte, while it decreases in a basic electrolyte.

Difference in TiO2 photocatalytic mechanism between rutile and anatase studied by the detection of active oxygen and surface species in water

A short Ti–Ti distance on a rutile surface leads to the formation of Ti–OO–Ti, which is accountable for the production of O₂ and lower photocatalytic reactivity.