XPS and UPS Characterization of the TiO2/ZnPcGly Heterointerface: Alignment of Energy Levels

Size transformation of Au nanoclusters for enhanced photocatalytic hydrogen generation: Interaction behavior at nanocluster/semiconductor interface

Recently, atomically precise metal nanoclusters (NCs) become a new class of photosensitizer for light energy conversion in metal-cluster-sensitized semiconductor (MCSS) system. However, fundamental understanding for the suitable combination of NCs and semiconductor is still unclear. Aside from aspects of light harvesting, energy level alignment and catalytic activity, interfacial interaction behavior at NCs/semiconductor interface is also crucial due to its important influence in charge transportation. In this work, the interface interaction between Au NCs and TiO2 is examined by precise transformation of Au NCs from Au22(SG)18 to Au18(SG)14, as well as its effect on photocatalytic hydrogen production activity. From the optical, charge transport and solid-states spectroscopy analyses, it is able to display that precisely tuning the number of core atoms from Au22(SG)18 to Au18(SG)14 results in the strong interface interaction between Au NCs and TiO2, reflecting in high difference of work function and modified surface band bending of TiO2, therefore promoting the injection of electrons from NCs to TiO2 and reducing interfacial charges recombination. As a result, Au18(SG)14/TiO2 shows higher hydrogen generation rate than Au22(SG)18/TiO2 under light irradiation. This work would provide new insights into rational combination of metal NCs with semiconductor and highlights the overlooked effect of interfacial interaction behavior on light energy conversion.

Elucidating the mechanism of photocatalytic reduction of bicarbonate (aqueous CO2) into formate and other organics

The photocatalytic reduction of CO2 under solar irradiation is an ideal approach to mitigating global warming, and reducing aqueous forms of CO2 that interact strongly with a catalyst (e.g., HCO3-) is a promising strategy to expedite such reductions. This study uses Pt-deposited graphene oxide dots as a model photocatalyst to elucidate the mechanism of HCO3- reduction. The photocatalyst steadily catalyzes the reduction of an HCO3- solution (at pH = 9) containing an electron donor under 1-sun illumination over a period of 60 h to produce H2 and organic compounds (formate, methanol, and acetate). H2 is derived from solution-contained H2O, which undergoes photocatalytic cleavage to produce •H atoms. Isotopic analysis reveals that all of the organics formed via interactions between HCO3- and •H. This study proposes mechanistic steps, which are governed by the reacting behavior of the •H, to correlate the electron transfer steps and product formation of this photocatalysis. This photocatalysis achieves overall apparent quantum efficiency of 27% in the formation of reaction products under monochromatic irradiation at 420 nm. This study demonstrates the effectiveness of aqueous-phase photocatalysis in converting aqueous CO2 into valuable chemicals and the importance of H2O-derived •H in governing the product selectivity and formation kinetics.

Plasmonic Nanocrystal Assembly-Semiconductor Hybrids for Boosting Visible to Near-Infrared Photocatalysis

Plasmonic metal-semiconductor hybrid photocatalysts have received much attention because of their wide light harvesting range and efficient charge carrier generation capability originating from plasmon energy transfer. Here, we introduce a plasmonic metal-semiconductor hybrid nanostructure consisting of a Au core-satellite assembly and crystalline TiO2. The formation of Au@TiO2-Au core-satellite assemblies using TiO2 as a spacer and the subsequent growth of outer TiO2 shells on the core-satellite assemblies, followed by calcination, successfully generated Au core-satellite assembly@TiO2 nanostructures. Exquisite control over the growth of the TiO2 interlayer enabled the regulation of the gap distance between the core and satellite Au nanocrystals within the same hybrid morphology. Due to the structural controllability of the present approach, the gap-distance-dependent plasmonic and photocatalytic properties of the hybrid nanostructures could be explored. The nanostructures possessing the most closely arranged Au nanocrystals showed high photocatalytic activity under visible to near-infrared light irradiation, which can be attributed to strong plasmon coupling between the core and satellite Au nanocrystals that can expedite the formation of intense plasmon energy and its transfer to TiO2.

Boosting Tetracycline Degradation with an S-Scheme Heterojunction of N-Doped Carbon Quantum Dots-Decorated TiO2

N-doped carbon quantum dots (N-CQDs) derived from the Rumex crispus L. plant were incorporated into TiO₂ via a facile hydrothermal method. As-prepared materials were characterized and used in the photocatalytic tetracycline (TC) degradation under UVA light irradiation by examining several operational parameters involving the N-CQDs amount, initial TC concentration, pH, and photocatalytic reaction time. XRD analysis revealed the conversion of the rutile phase to the anatase phase after the incorporation of N-CQDs into the TiO₂ structure. The results revealed that the N-CQDs/TiO₂ photocatalysts demonstrated the highest efficiency in TC degradation compared to other processes of adsorption, photolysis (UVA), and photocatalysis with TiO₂ (TiO₂/UVA). Under optimized conditions, 10 mg/L TC at pH 5.15 with 0.2 g/L N-CQDs/TiO₂ catalyst showed 97.7% photocatalytic degradation for 120 min under UVA irradiation. The formation of an S-scheme heterojunction between N-CQDs and TiO₂ provided enhanced charge separation and strong redox capability, causing significant improvement in the photocatalytic performance of N-CQDs/TiO₂. Trapping experiments showed that O₂^•- and h⁺ are the predominant reactive species for the TC elimination in an aqueous solution.

Effect of Titanium and Molybdenum Cover on the Surface Restructuration of Diamond Single Crystal during Annealing

Diamond is an important material for electrical and electronic devices. Because the diamond is in contact with the metal in these applications, it becomes necessary to study the metal-diamond interaction and the structure of the interface, in particular, at elevated temperatures. In this work, we study the interaction of the (100) and (111) surfaces of a synthetic diamond single crystal with spattered titanium and molybdenum films. Atomic force microscopy reveals a uniform coating of titanium and the formation of flattened molybdenum nanoparticles. A thin titanium film is completely oxidized upon contact with air and passes from the oxidized state to the carbide state upon annealing in an ultrahigh vacuum at 800 °C. Molybdenum interacts with the (111) diamond surface already at 500 °C, which leads to the carbidization of its nanoparticles and catalytic graphitization of the diamond surface. This process is much slower on the (100) diamond surface; sp²-hybridized carbon is formed on the diamond and the top of molybdenum carbide nanoparticles, only when the annealing temperature is raised to 800 °C. The conductivity of the resulting sample is improved when compared to the Ti-coated diamond substrates and the Mo-coated (111) substrate annealed at 800 °C. The presented results could be useful for the development of graphene-on-diamond electronics.

Selective Nitridation Crafted a High-Density, Carbon-Free Heterostructure Host with Built-In Electric Field for Enhanced Energy Density Li-S Batteries

To achieve both high gravimetric and volumetric energy densities of lithium-sulfur (Li-S) batteries, it is essential yet challenging to develop low-porosity dense electrodes along with diminishment of the electrolyte and other lightweight inactive components. Herein, a compact TiO₂ @VN heterostructure with high true density (5.01 g cm^-3 ) is proposed crafted by ingenious selective nitridation, serving as carbon-free dual-capable hosts for both sulfur and lithium. As a heavy S host, the interface-engineered heterostructure integrates adsorptive TiO₂ with high conductive VN and concurrently yields a built-in electric field for charge-redistribution at the TiO₂ /VN interfaces with enlarged active locations for trapping-migration-conversion of polysulfides. Thus-fabricated TiO₂ @VN-S composite harnessing high tap-density favors constructing dense cathodes (≈1.7 g cm^-3 ) with low porosity (<30 vol%), exhibiting dual-boosted cathode-level peak volumetric-/gravimetric-energy-densities nearly 1700 Wh L^-1 _cathode /1000 Wh kg^-1 _cathode at sulfur loading of 4.2 mg cm^-2 and prominent areal capacity (6.7 mAh cm^-2 ) at 7.6 mg cm^-2 with reduced electrolyte (<10 µL mg^-1 _sulfur ). Particular lithiophilicity of the TiO₂ @VN is demonstrated as Li host to uniformly tune Li nucleation with restrained dendrite growth, consequently bestowing the assembled full-cell with high electrode-level volumetric/gravimetric-energy-density beyond 950 Wh L^-1 _{cathode+anode} /560 Wh kg^-1 _{cathode+anode} at 3.6 mg cm^-2 sulfur loading alongside limited lithium excess (≈50%).

Combinatorial Screening of Cu-W Oxide-Based Photoanodes for Photoelectrochemical Water Splitting

Metal oxide libraries for photoanodes for the oxygen evolution reaction (OER) were generated by printing a metal salt solution in an array layout, followed by calcination to yield 22 ternary metal oxide systems. The libraries included a ternary metal cation system based on CuWO₄ with one out of eight transition or posttransition metal ions Cr, Mn, Fe, Co, Ni, Zn, Bi, and Ga in different overall atomic ratios. The photocatalyst libraries were screened by scanning photoelectrochemical microscopy for the highest anodic photocurrents. Array elements that showed promising performance were printed in another set of eight libraries with smaller increments of overall composition. Improved performance with respect to CuWO₄ was found for Ga, Co, and Ni as the third element. A comparison of the most active composition of those arrays within one library showed the highest activity for Cu₄₈Ga₃W₄₉O_x. Printing spots of identical composition (Cu₄₈Ga₃W₄₉O_x, Cu₄₄Ni₉W₄₇O_x, and Cu₄₄Co₉W₄₇O_x) over a larger area facilitated further characterization by X-ray photoelectron spectroscopy ultraviolet photoelectron spectroscopy (UPS), X-ray diffraction, scanning electron microscopy, chopped light voltammetry, and scanning electrochemical microscopy for the OER. High and stable steady-state photocurrents were generated in a photoelectrochemical cell for all three electrodes even at a low constant bias voltage. The best overall photoanode composition Cu₄₈Ga₃W₄₉O_x showed currents that were 36 times higher than the currents of the binary Cu₅₀W₅₀O_x system. Significant n-doping was found by UPS valence band spectra for Ga-containing materials.

Controllable oxidation of cyclohexanone to produce sodium adipate in an electrochemical reactor with a Pt NPs/Ti membrane electrode

An electrocatalytic membrane reactor (ECMR) with an anode consisting of Pt nanoparticles (NPs) loaded on a Ti membrane electrode (Pt NPs/Ti) was designed to oxidize cyclohexanone (K) to produce sodium adipate (SA) under mild conditions. The effects of residence time, reaction temperature, current density and initial K concentration on K conversion were investigated. Optimization experiments were conducted to determine the effects of and interactions between different operating parameters on K conversion using a central composite design within the response surface methodology. A 88.3% conversion of K and 99% selectivity to SA were obtained by the ECMR under the optimum conditions of reaction temperature = 30.8 °C, K concentration = 22.54 mmol L−1, residence time = 25 min and current density = 2.07 mA cm−2. The high performance of the ECMR is attributed to electrocatalytic oxidation (at the Pt NPs/Ti electrode), convection-enhanced mass transfer, and the timely removal of the desired products.

Influence of TiO2 surface defects on the adsorption of N719 dye molecules

Surface defects influence the dye adsorption on TiO₂ used as a substrate in dye-sensitized solar cells (DSSCs). In this study, we have used different Ar⁺ sputtering doses to create a controlled density of defects on a TiO₂ surface exposed to different pre-heating temperatures in order to analyse the influence of defects on the N719 dye adsorption. TiO₂ was pre-treated using two different treatments. The first treatment involved heating to 200 °C with subsequent sputtering at different doses. The second treatment included heating only, but at four different temperatures starting at 200 °C. After the pre-treatments, the TiO₂ samples were immersed into an N719 dye solution for 24 hours at room temperature to dye the TiO₂ substrates. The amount of Ti³⁺ surface defects introduced by the different pre-treatments and their influence on dye adsorption onto the TiO₂ surface were examined by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and metastable induced electron spectroscopy (MIES). Neutral impact collision ion scattering spectroscopy (NICISS) was used to determine the coverage of the TiO₂ surface by adsorbed dye molecules. It was found that Ti³⁺ surface defects were formed by Ar⁺ sputtering but not by pre-treatment through heating alone. MIES analysis of the outer-most layer and density of states calculations show that the thiocyanate ligand of the N719 dye becomes directed away from the TiO₂ surface. Both XPS and NICISS results indicate that the amount of adsorbed N719 dye decreases with increasing density of Ti³⁺ surface defects. Thus, the generation of surface defects reduces the ability of the TiO₂ surface to adsorb the dye molecules. Heating alone as pre-treatment of the TiO₂ substrates instead increases the dye adsorption, without causing detectable defects on the TiO₂ surface.

Creating self-assembled arrays of mono-oxo (MoO3)1 species on TiO2(101) via deposition and decomposition of (MoO3)n oligomers

Hierarchically ordered oxides are of critical importance in material science and catalysis. Unfortunately, the design and synthesis of such systems remains a key challenge to realizing their potential. In this study, we demonstrate how the deposition of small oligomeric (MoO3)1-6 clusters-formed by the facile sublimation of MoO3 powders-leads to the self-assembly of locally ordered arrays of immobilized mono-oxo (MoO3)1 species on anatase TiO2(101). Using both high-resolution imaging and theoretical calculations, we reveal the dynamic behavior of the oligomers as they spontaneously decompose at room temperature, with the TiO2 surface acting as a template for the growth of this hierarchically structured oxide. Transient mobility of the oligomers on both bare and (MoO3)1-covered TiO2(101) areas is identified as key to the formation of a complete (MoO3)1 overlayer with a saturation coverage of one (MoO3)1 per two undercoordinated surface Ti sites. Simulations reveal a dynamic coupling of the reaction steps to the TiO2 lattice fluctuations, the absence of which kinetically prevents decomposition. Further experimental and theoretical characterizations demonstrate that (MoO3)1 within this material are thermally stable up to 500 K and remain chemically identical with a single empty gap state produced within the TiO2 band structure. Finally, we see that the constituent (MoO3)1 of this material show no proclivity for step and defect sites, suggesting they can reliably be grown on the (101) facet of TiO2 nanoparticles without compromising their chemistry.

Green energy by recoverable triple-oxide mesostructured perovskite photovoltaics

Perovskite solar cells have developed into a promising branch of renewable energy. A combination of feasible manufacturing and renewable modules can offer an attractive advancement to this field. Herein, a screen-printed three-layered all-nanoparticle network was developed as a rigid framework for a perovskite active layer. This matrix enables perovskite to percolate and form a complementary photoactive network. Two porous conductive oxide layers, separated by a porous insulator, serve as a chemically stable substrate for the cells. Cells prepared using this scaffold structure demonstrated a power conversion efficiency of 11.08% with a high open-circuit voltage of 0.988 V. Being fully oxidized, the scaffold demonstrated a striking thermal and chemical stability, allowing for the removal of the perovskite while keeping the substrate intact. The application of a new perovskite in lieu of a degraded one exhibited a full regeneration of all photovoltaic performances. Exclusive recycling of the photoactive materials from solar cells paves a path for more sustainable green energy production in the future.

Improving the Open-Circuit Voltage of Sn-Based Perovskite Solar Cells by Band Alignment at the Electron Transport Layer/Perovskite Layer Interface

Organic-inorganic lead halide perovskites are promising materials for realization of low-cost and high-efficiency solar cells. Because of the toxicity of lead, Sn-based perovskite materials have been developed as alternatives to enable fabrication of Pb-free perovskite solar cells. However, the solar cell performance of Sn-based perovskite solar cells (Sn-PSCs) remains poor because of their large open-circuit voltage (V_OC) loss. Sn-based perovskite materials have lower electron affinities than Pb-based perovskite materials, which result in larger conduction band offset (CBO) values at the interface between the Sn-based perovskite and a conventional electron transport layer (ETL) material such as TiO₂. Herein, the relationship between the V_OC and the CBO in these devices was studied to improve the solar cell performances of Sn-PSCs. It was found that the band offset at the ETL/perovskite layer interface affects the V_OC of the Sn-PSCs significantly but does not affect that of the Pb-PSCs because the Sn-based perovskite material is a p-type semiconductor, unlike the Pb-based perovskite. It was also found that Nb₂O₅ has the CBO that is closest to zero for Sn-based perovskite materials, and the V_OC values of Sn-PSCs that use Nb₂O₅ as their ETL are higher than those of Sn-PSCs using TiO₂ or SnO₂ ETLs. This study indicates that control of the energy alignment at the ETL/perovskite layer interface is an important factor in improving the V_OC values of Sn-PSCs.

Adsorption of Polarized Molecules for Interfacial Band Engineering of Doped TiO2 Thin Films

Owing to their chemical and mechanical stability, metal-oxides have emerged as potential alternatives for conventional pure-metal and organic molecule-based solid-state electronic devices. Traditionally, band engineering of these metal-oxides has been performed to improve the efficiency of solar cells and transistors. However, recent advancements in the field of oxide-based electronic devices demand reversible band structure engineering for applications in next-generation adaptive electronics and memory devices. Therefore, this work aims to reversibly engineer the surface band structure of doped metal-oxides using stable organic ligands with weak dipoles. Para-substituted benzoic acid (BZA) ligands with positive and negative dipole moments were adsorbed in situ on the surface of TiO₂:Ni²⁺ thin film to modify the interfacial dipole moment, and the valence band structure was probed using surface-sensitive ultraviolet photoelectron spectroscopy (UPS). UPS, paired with density functional theory (DFT) simulations, demonstrate the ability to selectively tune interfacial electronic/chemical landscapes with ligand-dependent dipole moment. The unique ability to reversibly tune the band bending at the organic-inorganic interface of doped metal-oxide semiconductors using molecular dipoles is expected to play a key role in the development of metal-oxide-based adaptive electronics that outperform the conventional polymer-based and Si-based devices.

Electron injection effect in In2O3 and SnO2 nanocrystals modified by ruthenium heteroleptic complexes

In this work, the optical characteristics and conductivity under photoactivation with visible light of hybrids based on nanocrystalline SnO2 or In2O3 semiconductor matrixes and heteroleptic Ru(ii) complexes were studied. The heteroleptic Ru(ii) complexes were prepared based on 1H-imidazo[4,5-f][1,10]phenanthroline and 2,2'-bipyridine ligands. Nanocrystalline semiconductor oxides were obtained by chemical precipitation with subsequent thermal annealing and characterized by XRD, SEM and single-point BET methods. The heteroleptic Ru(ii) complexes as well as hybrid materials were characterized by time-resolved luminescence and X-ray photoelectron spectroscopy. The results showed that the surface modification of SnO2 nanoparticles with heteroleptic ruthenium complexes led to an increase in conductivity upon irradiation with light appropriate for absorption by organometallic complexes. In the case of In2O3, the deposition of Ru(ii) complexes resulted in a decrease in conductivity, apparently due to the special structure of the surface layer of the semiconductor.

g-C3N4/Pt/BiVO4 nanocomposites for highly efficient visible-light photocatalytic removal of contaminants and hydrogen generation

Inspired by natural photosynthesis, artificial heterojunction photocatalysts have been extensively studied. Herein, a novel ternary graphitic carbon nitride/platinum/bismuth vanadate (g-C₃N₄/Pt/BiVO₄) photocatalytic system was successfully synthesized, where Pt/BiVO₄ nanosheet is anchored on the surface of layered g-C₃N₄, as evidenced by structural observations. Ultraviolet photoelectron spectroscopy and ultraviolet-visible diffuse reflectance spectroscopy are carried out to identify the position of the conduction band and valence band. A Z-scheme is used to interpret the superior photocatalytic performance of g-C₃N₄/Pt/BiVO₄ and further verified by the capture of free radicals and terephthalic acid photoluminescence experiments. Compared with the g-C₃N₄/BiVO₄ binary system, the Z-scheme g-C₃N₄/Pt/BiVO₄ photocatalyst not only possesses enhanced carrier separation efficiency but also maintains sufficient redox properties, thus inducing superior photocatalytic activity. More importantly, the novel Z-scheme photocatalyst exhibits excellent recycle stability, which could provide inspiration for the rational design of efficient and practical photocatalysts for environmental pollution treatment. The ternary photocatalyst also exhibits significantly enhanced visible-light photocatalytic hydrogen production performance.

Charge Transfer in Mixed-Phase TiO2 Photoelectrodes for Perovskite Solar Cells

In mesoscopic perovskite solar cells (PSCs) the recombination processes within the TiO2 photoelectrode and at the TiO2/perovskite interface limit power conversion efficiency. To overcome this challenge, we investigated the effect of TiO2 phase composition on the electronic structure of TiO2 photoelectrodes, as well as on PSCs performance. For this, a set of PSCs based on TiO2 thin films with different content of anatase and rutile particles was fabricated under ambient conditions. X-ray diffraction, optical spectroscopy and scanning electron microscopy were used to study the structural, morphological and optical characteristics of TiO2 powders and TiO2-based thin films. X-ray photoelectron spectroscopy (XPS) analysis of anatase revealed a cliff conduction band alignment of 0.2 eV with respect to the rutile. Energy band alignment at the anatase/rutile/perovskite interfaces deduced from the XPS data provides the possibility for interparticle electron transport from the rutile to anatase phase and the efficient blocking of electron recombination at the TiO2/perovskite interface, leading to efficient electron-hole separation in PSCs based on mixed-phase TiO2 photoelectrodes. PSCs based on TiO2 layers with 60/40 anatase/rutile ratio were characterized by optimized charge extraction and low level of recombination at the perovskite/TiO2 interface and showed the best energy conversion efficiency of 13.4% among the studied PSCs. Obtained results provide a simple and effective approach towards the development of the next generation high efficiency PSCs.

Ba-addition induced enhanced surface reducibility of SrTiO3: implications on catalytic aspects

Surface reducibility engineering is one of the vital tools to enhance the catalytic activity of materials. A heavy redox treatment can be utilized to affect the structure and surface of catalytic materials. Here, we choose SrTiO₃ (STO) with a cubic perovskite structure as a system to induce oxygen vacancies by using nascent hydrogen from NaBH₄ leading to a heavily reduced version of SrTiO₃ (RSTO). To further understand the surface reduction and its dependence on foreign-ion (Ba) incorporation into SrTiO₃, Sr_0.5Ba_0.5TiO₃ (SBTO) and BaTiO₃ (BTO) are synthesized using a facile hydrothermal method. The reduced version of the pristine and mixed oxide shows distinct optical absorptions, indicating oxygen vacancy-mediated reducibility engineering. Detailed CO oxidation experiments suggest the order of activity over the as-prepared and reduced supports as STO > SBTO > BTO and RSBTO > RSTO > RBTO, respectively. The interesting observation of reversal of CO oxidation activity over STO and SBTO after reduction negates the assumption of a similar intensity of reduction on the surfaces of these oxide supports. The fundamental aspect of surface reducibility is addressed using temperature programmed reduction/oxidation (TPR/TPO) and XPS.

Plasmon-induced charge separation at the interface between ITO nanoparticles and TiO2 under near-infrared irradiation

Plasmon-induced charge separation (PICS) by continuous electron injection from plasmonic compound nanoparticles to a semiconductor was achieved by using solid-state cells based on tin-doped indium oxide (ITO) nanoparticles with a short capping agent and a TiO2 film. The cells extended the PICS range to longer wavelengths and exhibited photoresponses to 1500-2200 nm near-infrared light.

Multisensory Gas Chromatography for Field Analysis of Complex Gaseous Mixtures

A novel approach to analysis of complex gaseous mixtures is presented. The approach is based on the utilization of a compact gas chromatograph in combination with an array of highly integrated and selective metal oxide (MOX) sensors. Thanks to the implementation of a multisensory detector, the device collects multiple chromatograms in a single run. The sensors in the integrated MEMS platform are very distinct in their catalytic properties. Hence, the time separation by chromatographic column is complemented by catalytic separation by a multisensory detector. Furthermore, the device can perform the analysis in a broad range of concentrations, from ppb to hundreds of ppm. Low ppb and even sub-ppb levels of detection for some analytes were achieved. As a part of this effort, nanocomposite gas sensors were synthesized for selective detection of hydrogen sulfide, mercaptans, alcohols, ketones, and heavy hydrocarbons.

High efficiency dye-sensitized solar cells with VOC–JSC trade off eradication by interfacial engineering of the photoanode|electrolyte interface

Interfacial modification of the photoanode|electrolyte interface using oleic acid (OA) is thoroughly investigated in this present study. The overall photoconversion efficiency of 11.8% was achieved under the illumination of 100 mW cm⁻² with an optical filter of AM 1.5 G. OA molecules were meant to be adsorbed on to the vacant areas of the TiO₂ and the OA moieties leached out the aggregated C106 dye molecules from the TiO₂ surface. There was a strong spectral overlap between the absorption spectrum of donor (OA) and the emission spectrum of acceptor (C106), leading to effective Förster Resonance Energy Transfer (FRET) between OA and C106 and suggested an excellent opportunity to improve the photovoltaic performances of DSSCs. UV-vis DRS and UPS analysis revealed that OA molecules created new surface (mid-gap energy) states (SS) in TiO₂ and these SS played a major role in the electron transport kinetics. Mott–Schottky analysis of DSSCs under dark conditions was carried out to find the shift in the flat band potential of TiO₂ upon OA modification. Surprisingly, no trade off between V_OC and J_SC was observed after interfacial modification with OA. The dynamics of charge recombination and electron transport at the photoanode|electrolyte interface were studied in detail using electrochemical impedance spectroscopy.

V_OC–J_SC trade off is eliminated. Newly created surface states by OA in TiO₂ facilitated the charge transfer kinetics.

Investigation of the kinetics and mechanism of Z-scheme Ag3PO4/WO3 p–n junction photocatalysts with enhanced removal efficiency for RhB

The activity of an Ag₃PO₄/polyhedron-like WO₃ photocatalyst could be enhanced 27 times compared to P25 in the visible light-driven region towards RhB.

Strategic Design of Vacancy-Enriched Fe1- xS Nanoparticles Anchored on Fe3C-Encapsulated and N-Doped Carbon Nanotube Hybrids for High-Efficiency Triiodide Reduction in Dye-Sensitized Solar Cells

A new class of hybrids with the unique electrocatalytic nanoarchitecture of Fe_{1- x}S anchored on Fe₃C-encapsulated and N-doped carbon nanotubes (Fe_{1- x}S/Fe₃C-NCNTs) is innovatively synthesized through a facile one-step carbonization-sulfurization strategy. The efficient synthetic protocols on phase structure evolution and dynamic decomposition behavior enable the production of the Fe_{1- x}S/Fe₃C-NCNT hybrid with advanced structural and electronic properties, in which the Fe vacancy-contained Fe_{1- x}S showed the 3d metallic state electrons and an electroactive Fe in +2/+3 valence, and the electronic structure of the CNT was effectively modulated by the incorporated Fe₃C and N, with the work function decreased from 4.85 to 4.63 eV. The meticulous structural, electronic, and compositional control unveils the unusual synergetic catalytic properties for the Fe_{1- x}S/Fe₃C-NCNT hybrid when developed as counter electrodes (CEs) for dye-sensitized solar cells (DSSCs), in which the Fe₃C- and N-incorporated CNTs with reduced work function and increased charge density provide a highway for electron transport and facilitate the electron migration from Fe₃C-NCNTs to ultrahigh active Fe_{1- x}S with the electron-donating effect, and the Fe vacancy-enriched Fe_{1- x}S nanoparticles exhibit ultrahigh I₃^- adsorption and charge-transfer ability. As a consequence, the DSSC based on the Fe_{1- x}S/Fe₃C-NCNT CE delivers a high power conversion efficiency of 8.67% and good long-term stability with a remnant efficiency of 8.00% after 168 h of illumination, superior to those of traditional Pt. Furthermore, the possible catalytic mechanism toward I₃^- reduction is creatively proposed based on the structure-activity correlation. In this work, the structure engineering, electronic modulation, and composition control opens up new possibilities in constructing the novel electrocatalytic nanoarchitecture for highly efficient CEs in DSSCs.

Significantly enhanced dielectric and hydrophobic properties of SiO2@MgO/PMMA composite films

In this article, we provide a feasible method to prepare high-dielectric-constant hydrophobic composite films, which show potential application as the insulator layer in electrowetting devices.

Titanium-oxo clusters functionalized with catecholate-type ligands: modulating the optical properties through charge-transfer transitions

Charge transfer transitions effectively modulated the optical properties of the catecholate-type ligand functionalized titanium-oxo clusters [Ti₈(μ₃-O)₂(μ₂-O)₂(μ₂-OiPr)₄(OiPr)₈(O₃PC₆H₅)₄(cat)₂] and [Ti₈(μ₃-O)₂(μ₂-O)₂(μ₂-OiPr)₄(OiPr)₈(O₃PC₆H₅)₄(O₂C₁₀H₆)₂].

UPS and UV spectroscopies combined to position the energy levels of TiO2 anatase and rutile nanopowders

Positioning of absolute energy levels and the quantitative description of occupied levels obtained for TiO₂ nanopowders, combining UPS and UV-Vis spectroscopies.

TiO2 Electron Transport Bilayer for Highly Efficient Planar Perovskite Solar Cell

In planar perovskite solar cells, it is vital to engineer the extraction and recombination of electron-hole pairs at the electron transport layer/perovskite interface for obtaining high performance. This study reports a novel titanium oxide (TiO₂ ) bilayer with different Fermi energy levels by combing atomic layer deposition and spin-coating technique. Energy band alignments of TiO₂ bilayer can be modulated by controlling the deposition order of layers. The TiO₂ bilayer based perovskite solar cells are highly efficient in carrier extraction, recombination suppression, and defect passivation, and thus demonstrate champion efficiencies up to 16.5%, presenting almost 50% enhancement compared to the TiO₂ single layer based counterparts. The results suggest that the bilayer with type II band alignment as electron transport layers provides an efficient approach for constructing high-performance planar perovskite solar cells.

A Novel Highly Sensitive NO2 Sensor Based on Perovskite Na0.5+xBi0.5TiO3-δ Electrolyte

NO_x is one of dangerous air pollutants, and the demands for reliable sensors to detect NO_x are extremely urgent recently. Conventional fluorite-phase YSZ used for NO_x sensor requires higher operating temperature to obtain desirable oxygen ion conductivity. In this work, perovskite-phase Na_0.5Bi_0.5TiO₃ (NBT) oxygen conductor was chosen as the solid electrolyte to fabricate a novel highly sensitive NO₂ sensor with CuO as the sensing electrode and Pt as reference electrode. Na dopped Na_0.5Bi_0.5TiO₃ greatly improved the sensing performance of this sensor. The optimal sensor based on Na_0.51Bi_0.50TiO_3−δ exhibited good response-recovery characteristics to NO₂ and the response current values were almost linear to NO₂ concentrations in the range of 50–500 ppm at 400–600 °C. The response current value towards NO₂ reached maximum 11.23 μA at 575 °C and the value on NO₂ is much higher than other gases (CH₄, C₂H₄, C₃H₆, C₃H₈, CO), indicating good selectivity for detecting NO₂. The response signals of the sensor were slightly affected by coexistent O₂ varying from 2 to 21 vol% at 575 °C. The response current value decreased only 4.9% over 2 months, exhibiting the potential application in motor vehicles.

A ferrocenecarboxylate-functionalized titanium-oxo-cluster: the ferrocene wheel as a sensitizer for photocurrent response

A ferrocenecarboxylate-functionalized titanium-oxo cluster [Ti₆(μ₃-O)₆(OiPr)₆(O₂CFc)₆] was synthesized, in which the ferrocene wheel performs as a sensitizer for photocurrent response.

Effects of the large distribution of CdS quantum dot sizes on the charge transfer interactions into TiO2 nanotubes for photocatalytic hydrogen generation

Hydrogen fuels generated by water splitting using a photocatalyst and solar irradiation are currently gaining the strength to diversify the world energy matrix in a green way. CdS quantum dots have revealed a hydrogen generation improvement when added to TiO2 materials under visible-light irradiation. In the present paper, we investigated the performance of TiO2 nanotubes coupled with CdS quantum dots, by a molecular bifunctional linker, on photocatalytic hydrogen generation. TiO2 nanotubes were obtained by anodization of Ti foil, followed by annealing to crystallize the nanotubes into the anatase phase. Afterwards, the samples were sensitized with CdS quantum dots via an in situ hydrothermal route using 3-mercaptopropionic acid as the capping agent. This sensitization technique permits high loading and uniform distribution of CdS quantum dots onto TiO2 nanotubes. The XPS depth profile showed that CdS concentration remains almost unchanged (homogeneous), while the concentration relative to the sulfate anion decreases by more than 80% with respect to the initial value after ∼100 nm in depth. The presence of sulfate anions is due to the oxidation of sulfide and occurs in greater proportion in the material surface. This protection for air oxidation inside the nanotubular matrix seemingly protected the CdS for photocorrosion in sacrificial solution leading to good stability properties proved by long duration, stable photocurrent measurements. The effect of the size and the distribution of sizes of CdS quantum dots attached to TiO2 nanotubes on the photocatalytic hydrogen generation were investigated. The experimental results showed three different behaviors when the reaction time of CdS synthesis was increased in the sensitized samples, i.e. similar, deactivation and activation effects on the hydrogen production with regard to TiO2 nanotubes. The deactivation effect was related to two populations of sizes of CdS, where the population with a shorter band gap acts as a trap for the electrons photogenerated by the population with a larger band gap. Electron transfer from CdS quantum dots to TiO2 semiconductor nanotubes was proven by the results of UPS measurements combined with optical band gap measurements. This property facilitates an improvement of the visible-light hydrogen evolution rate from zero, for TiO2 nanotubes, to approximately 0.3 μmol cm(-2) h(-1) for TiO2 nanotubes sensitized with CdS quantum dots.

Constructing Bi24O31Cl10/BiOCl heterojunction via a simple thermal annealing route for achieving enhanced photocatalytic activity and selectivity

This work reports on the construction of a Bi₂₄O₃₁Cl₁₀/BiOCl heterojunction via a simple thermal annealing method. The X-ray diffraction (XRD) results indicated that the phase transformation from BiOCl to Bi₂₄O₃₁Cl₁₀ could be realized during the thermal annealing process. The high-resolution transmission electron microscopy (HRTEM) images, X-ray photoelectron spectroscopy (XPS) binding energy shifts, Raman spectra and Fouier transform infrared spectroscopy (FT-IR) spectra confirmed the formation of the Bi₂₄O₃₁Cl₁₀/BiOCl heterojunction. The obtained Bi₂₄O₃₁Cl₁₀/BiOCl photocatalyst showed excellent conversion efficiency and selectivity toward photocatalytic conversion of benzyl alcohol to benzaldehyde under visible light irradiation. The radical scavengers and electron spin resonance (ESR) results suggested that the photogenerated holes were the dominant reactive species responsible for the photocatalytic oxidation of benzyl alcohol and superoxide radicals were not involved in the photocatalytic process. The in-situ generation of Bi₂₄O₃₁Cl₁₀/BiOCl heterojunction may own superior interfacial contact than the two-step synthesized heterojunctions, which promotes the transfer of photogenerated charge carriers and is favorable for excellent photocatalytic activities.

Probing the energy levels of perovskite solar cells via Kelvin probe and UV ambient pressure photoemission spectroscopy

We present a study of the energy levels present in a perovskite solar cell using Kelvin probe and UV air photoemission measurements. By constructing a detailed map of the energy levels in the system we are able to predict the maximum open circuit voltage of the solar cell.

Constructing Fe2O3/TiO2 core-shell photoelectrodes for efficient photoelectrochemical water splitting

In this study, plasma enhanced chemical vapor deposition (PECVD) was utilized to co-axially modify hydrothermally grown Fe2O3 nanorod arrays by depositing a TiO2 overlayer to create Fe2O3/TiO2 core-shell photoelectrodes. Comprehensive structural (XRD, SEM, TEM) and compositional (XPS) analyses were performed to understand the effects of the TiO2 shell on the PEC activities of the Fe2O3 core. It was revealed that the heterojunction structure formed between TiO2 and Fe2O3, significantly improved the separation efficiency of photo-induced charge carriers and the oxygen evolution kinetics. A maximum photocurrent density of ∼900 μA cm(-2) at 0.6 V vs. saturated calomel electrode (SCE) was obtained for the Fe2O3/TiO2 photoelectrodes, which was 5 and 18 times higher when compared to that of hydrothermally synthesized Fe2O3 and PECVD synthesized TiO2 electrodes, respectively. Moreover, the Fe2O3/TiO2 core-shell nanorod arrays displayed superior stability for PEC water splitting. During 5000 s PEC measurements, a steady decrease of the photocurrent was observed, mainly attributed to the evolution of oxygen bubbles adsorbed on the working electrodes. This observation was verified by the complete recovery of the PEC performance demonstrated for a second 5000 s PEC measurement carried out after a brief time interval (10 min) that allowed the electrode surface to regenerate.

Very low amount of TiO2 on N-doped carbon nanotubes significantly improves oxygen reduction activity and stability of supported Pt nanoparticles

Highly dispersed TiO₂ on nitrogen-doped carbon nanotubes is used to tackle the carbon corrosion problem when used as support for Pt in electrocatalysis.

Band offset at TiO2/MDMO PPV and TiO2/PEDOT PSS interfaces studied using photoelectron spectroscopy

We report band alignment and band offset studies across the interfaces of hetero-structures of TiO₂ with MDMO PPV and PEDOT PSS using photoelectron spectroscopy.

High performance of P(VDF-HFP)/Ag@TiO2 hybrid films with enhanced dielectric permittivity and low dielectric loss

Ag@TiO₂ core–shell nanoparticles were synthesized as fillers using a simple vapor-thermal method and P(VDF-HFP)/Ag@TiO₂ composites exhibited enhanced dielectric properties.

Spatial charge separation in asymmetric structure of Au nanoparticle on TiO2 nanotube by light-induced surface potential imaging

Both enhancing the excitons' lifetime and ingeniously controlling the spatial charge transfer are the key to the realization of efficiently photocatalytic and artificially photosynthetic devices. Nanostructured metal/metal-oxide interfaces often exhibit improved energy conversion efficiency. Understanding the surface potential changes of nano-objects under light illumination is crucial in photoelectrochemical cells. Under ultraviolet (UV) illumination, here, we directly observed the charge separation phenomena at the Au-nanoparticle/TiO2-nanotube interfaces by using Kelvin probe force microscopy. The surface potential maps of TiO2 nanotubes with and without Au nanoparticles were compared on the effect of different substrates. We observed that in a steady state, approximately 0.3 electron per Au particle of about 4 nm in diameter is effectively charged and consequently screens the surface potential of the underlying TiO2 nanotubes. Our observations should help design improved photoelectrochemical devices for energy conversion applications.

Potassium iodate assisted synthesis of titanium dioxide nanoparticles with superior water-dispersibility

In this paper, we report a novel polyol process to synthesize highly water-dispersible anatase titanium dioxide (TiO2) nanoparticles (∼5 nm) by the introduction of inorganic oxidizing agent--KIO3. The obtained TiO2 nanoparticles are well dispersible in water at pH≥5.0 and the resulting aqueous dispersion remains stable over months. The superior water-dispersibility of as-formed TiO2 is ascribed to the electrostatic repulsion from carboxylic acid group modified on TiO2 nanoparticles, which is the oxidation product of solvent diethylene glycol (DEG) by KIO3. Based on the characterization results, the formation processes of water-dispersibility TiO2 nanoparticles are proposed. Meanwhile, the synthesized TiO2 nanoparticles are found to be doped by iodine and exhibit excellent photocatalytic activity on degradation of rhodamine-B (RhB) under visible-light irradiation. The further tests demonstrate that the O(2-) is the main active species during photodegradation of RhB.

High voltage and efficient bilayer heterojunction solar cells based on an organic–inorganic hybrid perovskite absorber with a low-cost flexible substrate

A low temperature (<100 °C), flexible solar cell based on an organic–inorganic hybrid CH₃NH₃PbI₃perovskite–fullerene planar heterojunction (PHJ) is successfully demonstrated.