Holes In TiO2 Research Articles

Self-powered SnSx/TiO2 photodetectors (PDs) with dual-band binary response and the applications in imaging and light-encrypted logic gates

In this work, we report the design and fabrication of self-powered binary response PDs based on II-type heterostructures consisting of SnSx nanoflakes (NFs) and rutile TiO2 nanorod arrays (NRs). The TiO2 NRs effectively block light with wavelengths below 400 nm from reaching SnSx. Under 385 nm light, the photoelectrons in TiO2 recombine with holes in SnSx at the interface due to the energy band bending, resulting in a positive photocurrent. Under 410 nm light, the photoelectrons in SnSx and the photogenerated holes in TiO2 accumulate at the interface, overcoming the interfacial potential barriers induced by the higher Fermi levels of SnSx and inducing a negative photocurrent. Based on the bipolar response, the dual-band imaging capability without external filters and the light-encrypted OR, AND, and NOT logic gates using a single device are demonstrated. This work provides a blueprint for the development of multifunctional self-powered PDs that can simplify system architecture, reduce the energy consumption, and improve accuracy for applications, such as visual systems, light-controlled logic circuits, and encrypted optical communications.

Role of Ag Loading on the Concentration of Surface-Reaching Photoexcited Holes in TiO2 Nanoparticles

Role of Ag Loading on the Concentration of Surface-Reaching Photoexcited Holes in TiO₂ Nanoparticles

Photocatalytic Production of Ethanolamines and Ethylenediamines from Bio‐Polyols over a Cu/TiO2 Catalyst

AbstractValorization of biomass‐derived polyols into high‐value‐added ethanolamines and ethylenediamines is highly attractive. Herein, we report a one‐step photocatalytic protocol to convert bio‐polyols into a 60 % yield of ethanolamines and ethylenediamines over a multifunctional Cu/TiO2 catalyst. This catalyst enables a tandem process of photocatalytic polyol C−C bond cleavage and reductive amination in one pot at room temperature, and also allows the selective conversion of various bio‐polyols and amines. Mechanistic studies revealed that photogenerated holes in TiO2 promote the retro‐aldol C−C bond cleavage or oxidative dehydrogenation of polyols, and photogenerated electrons accumulate on small‐sized Cu clusters, which facilitate the reductive amination via hydrogen transfer and prevent the H2 generation. This strategy provides new opportunities for the development of non‐noble metal photocatalysts and methods of biomass conversion under mild conditions.

Photocatalytic Production of Ethanolamines and Ethylenediamines from Bio-Polyols over a Cu/TiO2 Catalyst.

Valorization of biomass-derived polyols into high-value-added ethanolamines and ethylenediamines is highly attractive. Herein, we report a one-step photocatalytic protocol to convert bio-polyols into a 60 % yield of ethanolamines and ethylenediamines over a multifunctional Cu/TiO2 catalyst. This catalyst enables a tandem process of photocatalytic polyol C-C bond cleavage and reductive amination in one pot at room temperature, and also allows the selective conversion of various bio-polyols and amines. Mechanistic studies revealed that photogenerated holes in TiO2 promote the retro-aldol C-C bond cleavage or oxidative dehydrogenation of polyols, and photogenerated electrons accumulate on small-sized Cu clusters, which facilitate the reductive amination via hydrogen transfer and prevent the H2 generation. This strategy provides new opportunities for the development of non-noble metal photocatalysts and methods of biomass conversion under mild conditions.

Photoelectrocatalytic C\u2013H halogenation over an oxygen vacancy-rich TiO2 photoanode

Photoelectrochemical cells are emerging as powerful tools for organic synthesis. However, they have rarely been explored for C–H halogenation to produce organic halides of industrial and medicinal importance. Here we report a photoelectrocatalytic strategy for C–H halogenation using an oxygen-vacancy-rich TiO2 photoanode with NaX (X=Cl−, Br−, I−). Under illumination, the photogenerated holes in TiO2 oxidize the halide ions to corresponding radicals or X2, which then react with the substrates to yield organic halides. The PEC C–H halogenation strategy exhibits broad substrate scope, including arenes, heteroarenes, nonpolar cycloalkanes, and aliphatic hydrocarbons. Experimental and theoretical data reveal that the oxygen vacancy on TiO2 facilitates the photo-induced carriers separation efficiency and more importantly, promotes halide ions adsorption with intermediary strength and hence increases the activity. Moreover, we designed a self-powered PEC system and directly utilised seawater as both the electrolyte and chloride ions source, attaining chlorocyclohexane productivity of 412 µmol h−1 coupled with H2 productivity of 9.2 mL h−1, thus achieving a promising way to use solar for upcycling halogen in ocean resource into valuable organic halides.

Planar heterojunction boosts solar-driven photocatalytic performance and stability of halide perovskite solar photocatalyst cell

The excellent optoelectronic properties of metal halide perovskites (MHPs) have been employed in various photocatalytic applications, but their poor water stability is considered as the main bottleneck for further development. Herein, we protect the light-absorbing CsPbBr3 MHP with a NiOx and TiO2 hole and electron extracting layer. This planar NiOx/CsPbBr3/TiO2 architecture can easily be fabricated through solution-processing. When applied to selective photocatalytic oxidation of benzyl alcohol, this system presents a 7-fold enhancement of photoactivity and an improved stability for over 90 h compared to CsPbBr3 counterpart. Interestingly, we find that trace amounts of water improve photoactivity. Through experimental and theoretical analyses, this improvement could be attributed to water-induced structural reorganization of MHP, leading to improved crystal quality and decreased effective masses of charge carriers. This work indicates planar heterojunction helps improve the photoactivity and stability of MHP photocatalyst, and our findings provide insights into the effect of water on MHPs.

Rational design of {0 0 1}-faceted TiO2 nanosheet arrays/graphene foam with superior charge transfer interfaces for efficient photocatalytic degradation of toxic pollutants

Exposing high-energy active facets for nanocrystals has received increasing attentions due to its intrinsic surface-dependent properties in extensiveapplications. Unfortunately, most anatase TiO2 crystals were dominated by the thermodynamically stable {101} facets instead of the much more catalytically active {001} facets. Meanwhile, the photo-generated electrons and holes in TiO2 might experience a rapid recombination, which significantly diminishes the efficiency of the photocatalytic reaction. Herein, we integrated {001} facets TiO2 nanosheet arrays into a free-standing graphene foam through a facile vacuum filtration process combined with a solvothermal method. Uniformly distributed TiO2 nanosheet arrays were in situ intercalated into interlayers of flexible graphene foam and were simultaneously anchored on its surface to form 3D hierarchical porous architecture, which was highly appreciated for its easy separation and recycling after photocatalytic process. The multifunctional TiO2/graphene foam exhibited superior photocatalytic activity, and the rate constants (k) of the TiO2/graphene foam towards degradation of methyl orange (MO) and phenol were 4, 8.7 times higher than commercial P25, respectively. Inspiringly, the TiO2/graphene foam also achievedhigh efficiency in Cr(VI) photoreduction with 92% removal yield. The remarkably photocatalytic capability of the TiO2/graphene foam was attributed to the synergistic effects arising from intrinsic single crystalline TiO2 with high percentage exposure of {001} facets, as well as a good linkage between TiO2 and graphene, which could facilitate the surface catalytic reaction by providing abundant reactive sites for an effective molecular oxygen activation and strengthening the interfacial charge transfer both in-plane and vertical transportation. The recycling experiments presented these flexible foam catalysts were stable and could be reused with high efficiency after 12 runs. This work would provide a feasible strategies to design outstanding TiO2-based photocatalysts with highly reactive {001} facet, and also offer more extensive possibilities in a broad application range.

Oxidized titanium carbide MXene-enabled photoelectrochemical sensor for quantifying synergistic interaction of ascorbic acid based antioxidants system

Antioxidants can protect organization from damage by scavenging of free radicals. When two kinds of antioxidants are consumed together, the total antioxidant capacity might be enhanced via synergistic interactions. Herein, we develop a simple, direct, and effective strategy to quantify the synergistic interaction between ascorbic acid (AA) and other different antioxidants by photoelectrochemical (PEC) technology. MXene Ti3C2–TiO2 composites fabricated via hydrogen peroxide oxidation were applied as sensing material for the antioxidants interaction study. Under excitation of 470 nm wavelength, the photogenerated electrons transfer from the conduction band of TiO2 nanoparticles to the Ti3C2 layers, and the holes in TiO2 can oxidize antioxidants, leading to an enhanced photocurrent as the detection signal. This PEC sensor exhibits a good linear range to AA concentrations from 12.48 to 521.33 μM as well as obvious antioxidants capability synergism. In particular, the photocurrents of AA + gallic acid (GA) and AA + chlorogenic acid (CHA) mixtures at 476.19 μM increase 1.95 and 2.35 times respectively comparing with the sum of photocurrents of AA and GA or CHA. It is found that the synergistic effect is mainly depending on the fact that AA with the low redox potential (0.246 V vs NHE) can reduce other antioxidants radical to promote regeneration, improving the overall antioxidant performance. Moreover, it is proved that the greater redox potential of antioxidants, the more obvious the synergistic effect. In addition, the sensor was used to real sample assay, which provides available information towards food nutrition analysis, health products design and quality inspection.

Charge accumulation kinetics in multi-redox molecular catalysts immobilised on TiO2.

Multi-redox catalysis requires the accumulation of more than one charge carrier and is crucial for solar energy conversion into fuels and valuable chemicals. In photo(electro)chemical systems, however, the necessary accumulation of multiple, long-lived charges is challenged by recombination with their counterparts. Herein, we investigate charge accumulation in two model multi-redox molecular catalysts for proton and CO2 reduction attached onto mesoporous TiO2 electrodes. Transient absorption spectroscopy and spectroelectrochemical techniques have been employed to study the kinetics of photoinduced electron transfer from the TiO2 to the molecular catalysts in acetonitrile, with triethanolamine as the hole scavenger. At high light intensities, we detect charge accumulation in the millisecond timescale in the form of multi-reduced species. The redox potentials of the catalysts and the capacity of TiO2 to accumulate electrons play an essential role in the charge accumulation process at the molecular catalyst. Recombination of reduced species with valence band holes in TiO2 is observed to be faster than microseconds, while electron transfer from multi-reduced species to the conduction band or the electrolyte occurs in the millisecond timescale. Finally, under light irradiation, we show how charge accumulation on the catalyst is regulated as a function of the applied bias and the excitation light intensity.

Color-changing refractive index sensor based on Fano-resonant filtering of optical modes in a porous dielectric Fabry-Pérot microcavity.

Refractometry is a ubiquitous technique for process control and substance identification in the chemical and biomedical fields. Herein, we present an all-dielectric, wafer-scalable, and compact Fabry-Pérot microcavity (FPMC) device for refractive index (RI) sensing. The FPMC consists of a highly porous SiO2 microcavity capped with a thin, quasi-periodically patterned TiO2 hole array partial reflector that enables rapid, nanoliter-scale analyte transport to and from the sensor. Liquid (alcohols) or condensed-vapor (water from human breath) infiltration resulted in spectral redshifts up to 100 nm, highly apparent visible color change, rapid recovery (< 20 s), and RI sensitivity of up to 680 nm/RIU. The sensor can also be used in spectral or single-wavelength detection modes. Effective-medium and finite-difference time-domain optical simulations identified that Fano-resonant scattering modes induced by the quasi-periodic TiO2 outcoupling layer effectively filter higher-order Fabry-Pérot cavity modes and thereby confer an easily identifiable red-to-green color transition during analyte infiltration.

Quasiordered, subwavelength TiO2 hole arrays with tunable, omnidirectional color response

Fabrication, optical characterization, and simulation of subwavelength TiO2 hole arrays exhibiting geometry-tunable, omnidirectional color response across the visible spectrum is described. Partially suspended TiO2 grating “membranes” (hole arrays supported by a high void-fraction, low-index underlayer) with quasiperiodic hexagonal order were created on an Si substrate using colloidal lithography, metal mask, plasma-based pattern transfer, and XeF2 etching. Optical measurements under specular and diffuse reflection conditions, along with finite-difference time-domain simulations, indicated that the omnidirectional color response of the hole arrays emerges from a broad distribution of Fano resonance states formed by coupling between guided and internal Fabry–Pérot (FP) cavity modes of the TiO2 layer. Higher-order FP resonances from the external cavity formed between the TiO2 layer and substrate control the apparent color when devices are viewed in direct light. The simulated modal behavior of arrays was found to be very sensitive to the degree of Si removal from the underlayer, in agreement with experimental observations. The fabrication methodology presented herein is substrate-agnostic and can be employed to fabricate suspended, subwavelength hole arrays in many material systems, with potential application to optical filters and reflectors, photocatalytic electrodes, photovoltaics, and sensors.

Synthesis, characterization and photocatalytic properties of PDI/TiO2/activated carbon nanocomposite

1-Hydroxy perylene diimide-doped TiO2 loaded on the activated carbon (HO-PDI/TiO2/AC) nanocomposite was prepared by hydrothermal method. The nanocomposite was characterized by X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, BET surface area, UV–Vis diffuse reflectance spectroscopy, and photoluminescence spectroscopy. The photocatalytic activity of the nanocomposite was investigated by degradation of AMX and EBT under visible light irradiation. The results indicated that the photocatalytic activity of the TiO2 increased under visible light irradiation in the presence of the HO-PDI and activated carbon. AC could disperse the HO-PDI/TiO2 nanoparticles and inhibit their agglomeration to improve the photoreactivity, while HO-PDI could be excited to generate electrons and holes under visible-light illumination. Photogenerated holes in TiO2 could be captured by AC, and the holes can be easily moved to the surface of AC, directly initiating the photocatalytic reaction. Based on these results, a dual catalytic mechanism of AC-TiO2 involving dye sensitization and electron–hole separation was proposed.

Sunlight-Triggered Dye Degradation and Antibacterial Activity of Graphene-Iron Oxide-Titanium Dioxide Heterostructure Nanocomposites.

Iron oxide (Fe₂O₃)-titanium dioxide (TiO₂) heterostructure nanocomposite has been used for contaminant decomposition and antibacterial application. However, both the photogenerated electrons and holes of TiO₂ may transfer to Fe₂O₃ due to straddling band alignment in type-I heterostructure, which is not helpful to more efficiently inhibit the electron-hole recombination. In this paper graphene-Fe₂O₃-TiO₂ (GFT) heterostructure nanocomposites (NCs) were fabricated to facilitate the separation of photo-induced electrons and holes according to their staggered energy level, further improve dye degradation efficiency and antibacterial activity. GFT NCs were fabricated through a simple hydrothermal method. X-ray diffraction patterns and transmission electron microscopy images indicated that Fe₂O₃ and TiO₂ were successfully loaded on graphene. UV-Vis spectra showed that GFT NCs had higher absorption against sunlight. Under simulated sunlight irradiation, GFT NCs could effectively degraded dyes and inhibit bacterial growth.

Phosphorus-doped graphene quantum dots loaded on TiO2 for enhanced photodegradation

Reducing the direct recombination of the photogenerated electron-hole pairs in a photocatalytic system is important to improve the photocatalytic efficiency. Transferring the photogenerated carriers to other adsorbates is a usual way to reduce the direct recombination probability. By synthesizing p-type phosphorus-doped graphene quantum dots (P-GQDs) to form p-n junctions with n-type TiO2 nanocomposites, the photogenerated holes of TiO2 can be effectively transferred to P-GQDs, thus the direct carrier recombination of TiO2 is decreased, resulting in the conspicuous improvement of the Methyl Orange (MO) photodegradation efficiency to 95.5% in 14 min. This work provides a new type of p-n junction in highly efficient photodegradation system.

MXene Ti3C2 derived Z–scheme photocatalyst of graphene layers anchored TiO2/g–C3N4 for visible light photocatalytic degradation of refractory organic pollutants

Highly photoactive Z–scheme of graphene layers anchored TiO2/g–C3N4 (GTOCN) photocatalyst was synthesized from MXene Ti3C2 through one–step in–situ calcination, and used for tetracycline (TC) and ciprofloxacin (CIP), bisphenol A (BPA) and rhodamine B (RhB) degradation under visible light irradiation. The crystal phases, chemical compositions, morphological structures and photoelectrochemical properties were characterized. The results demonstrated that hierarchical structured GTOCN photocatalysts exhibited significant improvement in visible light adsorption and photogenerated carriers separation. The GTOCN3 presented the best visible light photocatalytic synergism of degradation rate towards TC (0.02442 min−1), CIP (0.01675 min−1), BPA (0.01935 min−1) and RhB (0.05586 min−1), which was much higher than that of graphene layers anchored TiO2, g–C3N4 and g–C3N4/TiO2(P25), benefitting from solid interactions among graphene, TiO2 and g–C3N4 for separation and accumulation of g–C3N4 electrons with high reduction capability and TiO2 holes with high oxidation capability. Furthermore, the main species of O2− and OH for pollutants decomposition was confirmed by free radical capture and electron spin resonance experiments. Besides, the acceptable reusability and stability indicated the great application potential of GTOCN in pharmaceuticals, endocrine disruptors and colourants contained wastewater disposal and environmental protection.

What Is the Transfer Mechanism of Photoexcited Charge Carriers for g-C3N4/TiO2 Heterojunction Photocatalysts? Verification of the Relative p–n Junction Theory

Investigation of photoexcited charge transfer mechanism has always been one of the hotspots of the photocatalysis field. In our recent studies, the relative p–n junction was proposed as a new concept, and the built-in electric field formed in the heterojunction is the inner impetus for driving photoexcited charge transfer. Here, a series of g-C3N4/TiO2 samples with different mass percentage contents were synthesized and further characterized by physical and chemical techniques for the investigation of the charge transfer mechanism and internal natural law. The results state clearly that the migration of photoexcited charges belongs to Z-scheme mechanism, which is suitable for as-synthesized g-C3N4/TiO2 samples, whether the main part of the g-C3N4/TiO2 is TiO2 or g-C3N4. The photoexcited electrons enriched in g-C3N4 with a higher negative conduction band (CB) potential have reduction ability to convert O2 into superoxide radicals (•O2–). Meanwhile, the photoexcited holes in TiO2 with a higher positive valence band (VB) potential have oxidation ability to activate H2O or hydroxyl ions (OH–) to hydroxyl radicals (•OH). Furthermore, the g-C3N4/TiO2 photocatalyst exhibits better photocatalytic performance than TiO2 and g-C3N4. It is encouraging that the abovementioned Z-scheme mechanism of photoexcited charge transfer can also be explained and confirmed by the relative p–n junction theory. The built-in electric field promotes the migration of the photoexcited charges in the heterojunction, and its migration direction is opposite to that of the photoexcited charge in the CB and VB of g-C3N4 and TiO2. Therefore, the relative p–n junction theory not only is used to explicate the migration mechanism and internal natural law of the photoexcited charge in the heterojunction photocatalysts but also has crucial guiding significance for the theoretical design and practical construction of composite photocatalysts.

Low-temperature solution synthesis and characterization of enhanced titanium dioxide photocatalyst on tailored mesoporous γ-Al2O3 support

Abstract TiO2/γ-Al2O3 composite powder was prepared by the low-temperature hydrothermal synthesis in an alcohol solution without subsequent high-temperature calcination. When the as-prepared mesoporous γ-Al2O3 particles were added into the reaction solution, their surfaces with a negative potential were adsorbed by Ti(OH)4, forming mesoporous structure with the ink bottle type. These mesopores can absorb varisized organic molecules into their cavities, so that the mesoporous γ-Al2O3 is highly suitable for use as the adsorption and degradation material. Lots of oxygen defects formed during the synthesis process can hinder the recombination of the excited electrons and holes in TiO2. More free excited holes and electrons react with dissolved oxygen and water molecules in the solution to form the key functional groups of superoxide anion radicals (·O2−) and hydroxyl radicals (·OH), making the organic dye completely degrade to CO2 and H2O.

Optical control of selectivity of high rate CO2 photoreduction via interband- or hot electron Z-scheme reaction pathways in Au-TiO2 plasmonic photonic crystal photocatalyst

Photonic crystals consisting of TiO2 nanotube arrays (PMTiNTs) with periodically modulated diameters were fabricated using a precise charge-controlled pulsed anodization technique. The PMTiNTs were decorated with gold nanoparticles (Au NPs) to form plasmonic photonic crystal photocatalysts (Au-PMTiNTs). A systematic study of CO2 photoreduction performance on as-prepared samples was conducted using different wavelengths and illumination sequences. A remarkable selectivity of the mechanism of CO2 photoreduction could be engineered by merely varying the spectral composition of the illumination sequence. Under AM1.5 G simulated sunlight (pathway#1), the Au-PMTiNTs produced methane (302 μmol gcat.-1 h−1) from CO2 with high selectivity (89.3 %). When also illuminated by a UV-poor white lamp (pathway#2), the Au-PMTiNTs produced formaldehyde (420 μmol gcat.-1 h−1) and carbon monoxide (323 μmol gcat.-1 h−1) with almost no methane evolved. We confirmed the photoreduction results by 13C isotope labeling experiments using GCMS. These results point to optical control of the selectivity of high-rate CO2 photoreduction through selection of one of two different mechanistic pathways. Pathway#1 implicates electron-hole pairs generated through interband transitions in TiO2 and Au as the primary active species responsible for reducing CO2 to methane. Pathway#2 involves excitation of both TiO2 and surface plasmons in Au. Hot electrons produced by plasmon damping and photogenerated holes in TiO2 proceed to reduce CO2 to HCHO and CO through a plasmonic Z-scheme.

Enhanced solar light photocatalytic performance based on a novel Au-WO3@TiO2 ternary core–shell nanostructures

Based on difference in work function, novel Au nanoparticle-decorated WO3@TiO2 ternary core–shell rod-like nanocomposites (Au-WO3@TiO2) are designed and constructed as solar-light-driven photocatalysts. The photocatalytic activities were evaluated by monitoring the concentration change of methylene blue (MB) under simulated solar light irradiation. The results show that the Au-WO3@TiO2 ternary core–shell nanocomposites exhibited higher photocatalytic performance than the pure WO3 nanorods, the Au decorated WO3 nanorods and the WO3@TiO2 core–shell nanorods. The highest photocatalytic activity was found in the 1 wt% Au-WO3@TiO2 ternary core–shell nanocomposites, whose degradation efficiency and apparent rate constant (k) are 94.5% in 240 min under simulated solar light irradiation and 0.0295 min−1, respectively. The high performance of the ternary composites can be attributed to high specific area by coating of TiO2, the high utilization of solar light achieved by the surface plasmon resonance (SPR) effect of Au nanoparticles, and the low recombination rate of electrons and holes caused by electron transfer from TiO2 to Au and WO3. This study may offer a demonstration to make use of WO3 based ternary structure to help suppress the recombination of photo-induced electrons and holes in TiO2 and ultimately improve the photocatalytic efficiencies for multiple energy and environmental application.

Preparation of Al–O-Linked Porous-g-C3N4/TiO2-Nanotube Z-Scheme Composites for Efficient Photocatalytic CO2 Conversion and 2,4-Dichlorophenol Decomposition and Mechanism

The photocatalytic activity of TiO2 nanotubes can be improved through the construction of a Z-scheme composite, where photogenerated electrons from TiO2 recombine with the photogenerated holes in a coupled semiconductor. This arrangement allows for improved oxidation due to the residual holes of TiO2 and better chemical reduction due to the greater availability of the photogenerated electrons of the coupled semiconductor. Efficient Z-scheme porous-g-C3N4/TiO2-nanotube (PCN/TNTs) composites were developed here using a solid sublimation and transition approach, with Al–O links added by an impregnation method to increase interfacial linkages between the PCN and TNTs. The best results for photocatalytic CO2 conversion were obtained using 0.7PCN/0.4Al/TNTs, as shown by production of 54.9 ± 0.70 mg L–1 h–1 of acetic acid, 42.7 ± 0.54 mg L–1 h–1 of formic acid, and 45.4 ± 0.55 mg L–1 h–1 of methanol, which were about 3.8, 4.3, and 4.2 times that produced with bare TNTs. Photocatalytic 2,4-dichlorophenol decompos...