Phase Junction Research Articles

Probing the charge recombination in rGO decorated mixed phase (anatase-rutile) TiO2 multi-leg nanotubes

Recombination of photo-generated charges is one of the most significant challenges in designing efficient photo-anode for photo electrochemical water oxidation. In the case of TiO2, mixed phase (anatase-rutile) junctions often shown to be more effective in suppressing electron-hole recombination compared to a single (anatase or rutile) phase. Here, we report the study of bulk and surface recombination process in TiO2 multi-leg nanotube (MLNTs) anatase-rutile (A-R) junctions decorated with reduced graphene oxide (rGO) layers, through an analysis of the photo-current and impedance characteristics. To quantify the charge transport/transfer process involved in these junctions, holes arriving at the interface of semiconductor/electrolyte were collected by adding H2O2 to the electrolyte. This enabled us to interpret the bulk and surface recombination process involved in anatase/rutile/rGO junctions for photo-electrochemical water oxidation. We correlated this quantification to the electrochemical impedance spectroscopy (EIS) measurements, and showed that in anatase/rutile junction the increase in PEC performance was due to suppression in electron-hole recombination rate at the surface states that effectively enhances the hole transfer rate to the electrolyte. On the other hand, in rGO wrapped A-R MLNTs junction it was due to both phenomenon i.e decrease in bulk recombination rate as well as increase in hole transfer rate to the electrolyte at the semiconductor/electrolyte interface.

Enhanced Photoresponse of FeS2 Films: The Role of Marcasite-Pyrite Phase Junctions.

The beneficial role of marcasite in iron-sulfide-based photo-electrochemical applications is reported for the first time. A spectacular improvement of the photoresponse observed experimentally for mixed pyrite/marcasite-FeS2 films can be ascribed to the presence of p/m phase junctions at the interface. Density functional theory calculations show that the band alignment at the phase boundary contributes to enhanced charge separation and transfer across the interface.

Hydrilla derived ZnIn2S4 photocatalyst with hexagonal-cubic phase junctions: A bio-inspired approach for H2 evolution

The nature provides inspiration for many special structures due to biological diversity. In this report ZnIn2S4 with hexagonal-cubic phase junctions was prepared through a bio-inspired approach using hydrilla as a template and can significantly enhance photocatalytic H2 evolution under visible-light in comparison to those hexagonal or cubic single-phase structures. The enhancement in photocatalytic performance has been shown to be attributed to efficient charge separation and transfer across the hexagonal-cubic phase junction and light harvesting of special hydrilla leaves structure.

Hexagonal@Cubic CdS Core@Shell Nanorod Photocatalyst for Highly Active Production of H2 with Unprecedented Stability.

A highly effective, low-cost strategy for improved photocatalytic efficiency and stability of CdS is described. Based on the integration of hexagonal-cubic core-shell architecture with nanorod morphology, the concentric CdS nanorod phase junctions (NRPJs) obtained demonstrate extremely high H2 production rate and unprecedented photocatalytic stability.

Facile regulation of crystalline phases and exposed facets on Ti3+ self-doped TiO2 for efficient photocatalytic hydrogen evolution

The control of the photocatalytic activity of TiO2 by tailoring their crystalline structure and electronic structure is a current topic of great interest. In the present study, the phase composition and exposed facets on Ti3+ self-doped TiO2 were regulated by simply changing the fluorinion concentration during the hydrothermal synthesis process of TiO2. It was found that the phase composition can be tuned facilely, and the phase transformation process was analyzed by XRD and UV Raman spectroscopy. The introducing fluorinion during the hydrothermal preparation of TiO2 resulted in F-doping and phase transformation. And the F-doping also affects the amount of Ti3+ ions in the TiO2. SEM analysis indicated that the morphology and exposed facets changed with the change in fluorinion concentration, and the ratios of exposed facets are adjustable. Through systematically optimizing these parameters, the TiO2 prepared under fluorinion concentration of 17.5 mM showed the excellent photocatalytic performance under full spectrum illumination. The good photocatalytic performance was mainly attributed to the more phase junctions and surface heterojunctions for enhancing charge separation.

Construction of α-β Phase Junction on Bi4V2O11 via Electrospinning Retardation Effect and Its Promoted Photocatalytic Performance.

The creation of a phase junction structure in photocatalysts is a wise approach to promote photocatalytic performance, as phase junctions possess the potential to inhibit the recombination of photoinduced charge carriers. Here, Bi4V2O11 nanofibers with an α-β phase junction are fabricated via electrospinning with subsequent calcination. Electrospinning offers the opportunity to keep α-Bi4V2O11 from transforming into β-Bi4V2O11 completely due to an electrospinning retardation effect, leading to the formation of an α-β Bi4V2O11 phase junction. Furthermore, the α-β Bi4V2O11 phase junction realizes a well-established type-II band alignment. Photoelectrochemical measurements and photoluminescence spectroscopic investigations demonstrate that the phase junction structure has a significant impact on the separation and transfer of photogenerated electrons and holes. Thus, the α-β phase junction on Bi4V2O11 holds the key to achieving promoted efficiency in the photocatalysis process.

Fabrication of a monoclinic/hexagonal junction in WO3 and its enhanced photocatalytic degradation of rhodamine B

AbstractA series of WO3 samples with different crystalline phases were prepared by the thermal decomposition method from ammonium tungstate hydrate. X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy, and N2 adsorption-desorption were used to characterize the crystalline phase, morphology, particle size, chemical composition, and surface area of the WO3 samples. The formation of hexagonal (h-WO3) and monoclinic (m-WO3) crystal structures of WO3 at different temperatures or different times was confirmed by XRD. m-WO3 is formed at 600 °C, while m-WO3 starts to transform into h-WO3 at 800 °C. However, h-WO3, which forms at 800 °C, may transform into m-WO3 by increasing the calcination temperature to 1000 °C. SEM results indicate that m-WO3 particles exhibit a bulky shape with heavy aggregates, while h-WO3 particles exhibit a rod-like shape. Moreover, m-WO3 crystals are sporadically patched on the surface of the h-WO3 rod-like particles, resulting in the exposure of both m-WO3 and h-WO3 on the surface. It is observed that the monoclinic phase (m-WO3)/hexagonal phase (h-WO3) junction was fabricated by tuning the calcination temperature and calcination time. The relative ratios between m-WO3 and h-WO3 in the phase junction can readily be tailored by control of the calcination time. The photocatalytic activities of WO3 with different crystalline phases were evaluated by the photocatalytic degradation of rhodamine B as a model pollutant. A higher photocatalytic activity was observed in the WO3 sample with the m-WO3/h-WO3 junction as compared with the sample with only m-WO3. The improvement of photocatalytic activity can be attributed to the reduction of the electron-hole recombination rate owing to the formation of the phase junction, whose presence has been confirmed by HRTEM and photoluminescence spectra.

Self-induced synthesis of phase-junction TiO2 with a tailored rutile to anatase ratio below phase transition temperature.

The surface phase junction of nanocrystalline TiO2 plays an essential role in governing its photocatalytic activity. Thus, facile and simple methods for preparing phase-junction TiO2 photocatalysts are highly desired. In this work, we show that phase-junction TiO2 is directly synthesized from Ti foil by using a simple calcination method with hydrothermal solution as the precursor below the phase transition temperature. Moreover, the ratio of rutile to anatase in the TiO2 samples could be readily tuned by changing the ratio of weight of Ti foil to HCl, which is used as the hydrothermal precursor, as confirmed by the X-ray diffraction analysis. In the photocatalytic reaction by the TiO2 nanocomposite, a synergistic effect between the two phases within a certain range of the ratio is clearly observed. The results suggest that an appropriate ratio of anatase to rutile in the TiO2 nanocomposite can create more efficient solid-solid interfaces upon calcination, thereby facilitating interparticle charge transfer in the photocatalysis.

Local Interfacial Structure Influences Charge Localization in Titania Composites: Beyond the Band Alignment Paradigm

The phase junction of nanocomposite materials is key to enhanced performance but is largely ignored in recent theoretical examinations of photocatalytic interactions in titania-based composites. Computational advances now allow more precise modeling of the electronic and optical properties of composites, and focusing on mixed-phase TiO2 as a model, we use density functional theory (DFT) to interrogate the essential structural feature, namely, the rutile–anatase interface, and its relationship to photogenerated charge localization, bulk band alignments, and defect formation. The interfacial region is disordered and distinct from rutile and anatase and contains low coordinated Ti sites and oxygen vacancies, both drivers of charge localization. The relaxations of the interface upon formation of excited electrons and holes determine the final location of charges which cannot always be predicted from bulk band alignments. A detailed understanding of the interfacial phase junction lays the foundation for direct...

Heterostructured TiO2 Nanorod@Nanobowl Arrays for Efficient Photoelectrochemical Water Splitting.

Heterostructured TiO2 nanorod@nanobowl (NR@NB) arrays consisting of rutile TiO2 nanorods grown on the inner surface of arrayed anatase TiO2 nanobowls are designed and fabricated as a new type of photoanodes for photoelectrochemical (PEC) water splitting. The unique heterostructures with a hierarchical architecture are readily fabricated by interfacial nanosphere lithography followed by hydrothermal growth. Owing to the two-dimensionally arrayed structure of anatase nanobowls and the nearly radial alignment of rutile nanorods, the TiO2 NR@NB arrays provide multiple scattering centers and hence exhibit an enhanced light harvesting ability. Meanwhile, the large surface area of the NR@NB arrays enhances the contact with the electrolyte while the nanorods offer direct pathways for fast electron transfer. Moreover, the rutile/anatase phase junction in the NR@NB heterostructure improves charge separation because of the facilitated electron transfer. Accordingly, the PEC measurements of the TiO2 NR@NB arrays on the fluoride-doped tin oxide (FTO) substrate show significantly enhanced photocatalytic properties for water splitting. Under AM1.5G solar light irradiation, the unmodified TiO2 NR@NB array photoelectrode yields a photocurrent density of 1.24 mA cm(-2) at 1.23 V with respect to the reversible hydrogen electrode, which is almost two times higher than that of the TiO2 nanorods grown directly on the FTO substrate.

Understanding the anatase-rutile phase junction in charge separation and transfer in a TiO2 electrode for photoelectrochemical water splitting.

New insight into junction-based designs for efficient charge separation is vitally important for current solar energy conversion research. Herein, an anatase-rutile phase junction is elaborately introduced into TiO2 films by rapid thermal annealing treatment and the roles of phase junction on charge separation and transfer are studied in detail. A combined study of transient absorption spectroscopy, electrochemical and photoelectrochemical (PEC) measurements reveals that appropriate phase alignment is essential for unidirectional charge transfer, and a junction interface with minimized trap states is crucial to liberate the charge separation potential of the phase junction. By tailored control of phase alignment and interface structure, an optimized TiO2 film with an appropriately introduced phase junction shows superior performance in charge separation and transfer, hence achieving ca. 3 and 9 times photocurrent density enhancement compared to pristine anatase and rutile phase TiO2 electrodes, respectively. This work demonstrates the great potential of phase junctions for efficient charge separation and transfer in solar energy conversion applications.

Nitrogen-doped TiO2 microspheres with hierarchical micro/nanostructures and rich dual-phase junctions for enhanced photocatalytic activity

We successfully synthesized microspherical nitrogen-doped TiO2 with hierarchical nano/microstructures, rich anatase TiO2–TiO2(B) phase junctions and a reduced band gap for the photo-degradation of dyes with improved photocatalytic activity.

Tuning Phase Composition of TiO2 by Sn(4+) Doping for Efficient Photocatalytic Hydrogen Generation.

The anatase-rutile mixed-phase photocatalysts have attracted extensive research interest because of the superior activity compared to their single phase counterparts. In this study, doping of Sn(4+) ions into the lattice of TiO2 facilitates the phase transformation from anatase to rutile at a lower temperature while maintaining the same crystal sizes compared to the conventional annealling approach. The mass ratios between anatase and rutile phases can be easily manipulated by varying the Sn-dopant content. Characterization results reveal that the Sn(4+) ions entered into the lattice of TiO2 by substituting some of the Ti(4+) ions and distributed evenly in the matrix of TiO2. The substitution induced the distortion of the lattice structure, which realized the phase transformation from anatase to rutile at a lower temperature and the close-contact phase junctions were consequently formed between anatase and rutile, accounting for the efficient charge separations. The mixed-phase catalysts prepared by doping Sn(4+) ions into the TiO2 exhibit superior activity for photocatalytic hydrogen generation in the presence of Au nanoparticles, relatively to their counterparts prepared by the conventional annealling at higher temperatures. The band allignment between anatase and rutile phases is established based on the valence band X-ray photoelectron spectra and diffuse reflectance spectra to understand the spatial charge separation process at the heterojunction between the two phases. The study provides a new route for the synthesis of mixed-phase TiO2 catalysts for photocatalytic applications and advances the understanding on the enhanced photocatalytic properties of anatase-rutile mixtures.

Nano-PCMs for passive electronic cooling applications

The present work aims at investigating a new challenging use of oxide (TiO2, Al2O3, etc.) nanoparticles to enhance the thermal properties: thermal conductivity, specific heat, and latent heat of pure paraffin waxes to obtain a new class of Phase Change Materials (PCMs), the so-called nano-PCMs. The nano-PCMs were obtained by seeding different amounts of oxide nanoparticles in a paraffin wax having a melting temperature of 45°C. The thermophysical properties such as latent heat and thermal conductivity were then measured to understand the effects of the nanoparticles on the thermal properties of both the solid and liquid PCM. Finally, a numerical comparison between the use of the pure paraffin wax and the nano-PCM in a typical electronics passive cooling device was implemented. Numerical simulations were carried out using the Ansys-Fluent 15.0 code. Results in terms of solid and liquid phase temperatures, melting time and junction temperature were reported. Moreover, a comparison with experimental results was also performed.

Relation between the Photocatalytic and Photoelectrocatalytic Performance for the Particulate Semiconductor-Based Photoconversion Systems with Surface Phase Junction Structure

Although the surface phase junction can efficiently boost photocatalytic (PC) reactions, its role in photoelectrocatalytic (PEC) reactions has not been well-understood yet. In this study, we investigated the effect of the surface phase junction on the PEC performance of photoelectrodes fabricated with Ga2O3 and TiO2 particles. The surface phase junctions beneficial for PC reactions show a negative effect on PEC performance, which is mainly due to the significant influence of charge transportation between semiconductor particles by the surface phase junction. Photogenerated charge separation is promoted by the surface phase junction for both PC and PEC reactions, but the much more severe interfacial recombination occurs in the PEC reaction when charge carriers migrate across semiconductor particles to reach a conducting substrate. The opposite effects of the surface phase junction on PC and PEC performance reported here indicates that the fabrication of a heterojunction or phase junction in the right struc...

Charge separation promoted by phase junctions in photocatalysts

Since the 1980s, photocatalysis research has expanded at an unexpected rate. Fabrication of phase junctions has proved to be an effective method to enhance photocatalytic performance. As a model photocatalyst, titanium dioxide (TiO2) has been extensively studied. This feature article mainly reviews the study on TiO2 surface phase junctions, including the characterization of the surface phases of TiO2, the use of anatase: rutile TiO2 phase junctions in photocatalytic hydrogen production, and the current understanding of how TiO2 phase junctions work in photocatalysis. The surface structure of TiO2 can be well characterized by ultraviolet (UV) Raman spectroscopy, unlike X-ray diffraction and visible Raman spectroscopy. Based on these results, the mechanism of phase transformation processes of TiO2 was clearly identified. The infrared (IR) spectra of probe molecules CO and CO2 on TiO2 further characterized the surface structure of TiO2, strongly supporting the UV Raman results. Furthermore, the typical visible emission of anatase and near-infrared emission of the rutile phase of TiO2 make photoluminescence (PL) a suitable technique to characterize the surface phase structure of TiO2. PL can also provide information about the carrier dynamics of TiO2 in photocatalysis. Because of the surface phase junction formed between anatase and rutile, the mixed-phase structure of TiO2 exhibits a superior H2 production activity to those of pure anatase or rutile phase. The activity of Degussa P25 TiO2 can be further increased by three to five times by tuning the phase structure through thermal treatment. Moreover, the phase transformation of TiO2 from anatase to rutile can be controlled by surface modification with Na2SO4, resulting in catalysts with activity six times higher than that of P25. High-resolution transmission electron microscopy provided a clear phase-junction image of TiO2, which showed atomic contact at the interface of the phase junction. The mechanism of phase junctions improving photocatalytic performance was investigated by time-resolved spectroscopic techniques. The charge transfer process across the anatase: rutile phase junction was confirmed by the results of time-resolved IR measurements, and electron transfer from anatase to rutile phases is proposed to occur in mixed-phase TiO2. These studies on the phase junctions of TiO2 improve our understanding of photocatalysis and may inspire new ideas for the design of promising photocatalytic systems.

Facile synthesis and enhanced visible-light photocatalytic activity of Ti3+-doped TiO2 sheets with tunable phase composition

Ti3+-doped TiO2 nanosheets with tunable phase composition (doped TiO2 (A/R)) were synthesized via a hydrothermal method with high surface area anatase TiO2 nanosheets TiO2 (A) as a substrate, structure directing agent, and inhibitor; the activity was evaluated using a probe reaction-photocatalytic CO2 conversion to methane under visible light irradiation with H2 as an electron donor and hydrogen source. High-resolution transmission electron microscope (HRTEM), field emission scanning electron microscope, UV-Vis diffuse reflectance spectra, and X-ray diffraction (XRD) etc., were used to characterize the photocatalysts. XRD and HRTEM measurements confirmed the existence of anatase-rutile phase junction, while Ti3+ and single-electron-trapped oxygen vacancy in the doped TiO2 (A/R) photocatalyst were revealed byelectron paramagnetic resonance (EPR) measurements. Effects of hydrothermal synthesis temperature and the amount of added anatase TiO2 on the photocatalytic activity were elucidated. Significantly enhanced photocatalytic activity of doped TiO2 (A/R) was observed; under the optimized synthesis conditions, CH4 generation rate of doped TiO2 (A/R) was 2.3 times that of Ti3+-doped rutile TiO2. Open image in new window

Effect of Phase Junction Structure on the Photocatalytic Performance in Overall Water Splitting: Ga2O3 Photocatalyst as an Example

The fabrication of surface phase junctions has proven to be an efficient strategy for the enhancement of photocatalytic activity; however, some questions about these systems are not yet well understood. In this study, the photocatalytic overall water splitting reaction was achieved on photocatalysts with pure and mixed phase compositions of cubic γ-Ga2O3 and monoclinic β-Ga2O3. All the Ga2O3 photocatalysts can split water stoichiometrically into H2 and O2; however, the phase-mixed γ/β-Ga2O3 photocatalyst with a small amount of β phase shows the lowest activity. This is opposite from that of the reported α/β-Ga2O3 system in which the phase-mixed α/β-Ga2O3 photocatalyst with a small amount of β phase shows much higher photocatalytic activity than the individual phases. Much more disordered structure is found between the γ and β phases in the γ/β-Ga2O3 photocatalyst with low content of β phase due to the defective spinel structure of γ phase. Spectroscopic characterization and theoretical calculations indica...

A novel thermophotocatalyst of mixed-phase cerium oxide (CeO2/Ce2O3) homocomposite nanostructure: Role of interface and oxygen vacancies

A mixed phase metal oxide system is considered as a highly active and stable photocatalyst due to extended absorption spectrum and reduced recombination of the photogenerated carriers as a result of their efficient separation at the phase interface. In ceria such a mixed phase has two different oxidation states wherein the oxygen deficient state has a complementary characteristic of high charge carrier mobility. Further due to the direct dependence of mobility on temperature in such a system, and its synergy with reduced recombination in mixed phase and extended absorption spectrum of solar radiation, it suits a solar concentrator based photo-catalytic reactor which normally works at high temperature. Accordingly a mixed phase (biphasic) homo-composite (same material with different oxidation state) of low temperature engineered cerium oxide (CeO2/Ce2O3) nanostructures was prepared by hydrothermal method. The physico-chemical characterization of the vacuum annealed disordered nanostructure material revealed the presence of biphasic homocomposite with extended absorption spectrum and also the presence of oxygen vacancies at the junction of phases. The mixed phase Ce-oxide with Ce3+/Ce4+ introduced disorder in the surface layers of nanostructure as well. The photoactivity of CeO2 nanoparticles was evaluated using methyl orange as a probe pollutant both under UV and visible irradiation over a wide range of temperature. The photocatalytic activity and its temperature dependence were found to be high compared to monophasic Ce-oxide. This work provides a rational design and development procedure for high-performance thermo-photocatalysts.

Defect Engineering and Phase Junction Architecture of Wide-Bandgap ZnS for Conflicting Visible Light Activity in Photocatalytic H₂ Evolution.

ZnS is among the superior photocatalysts for H2 evolution, whereas the wide bandgap restricts its performance to only UV region. Herein, defect engineering and phase junction architecture from a controllable phase transformation enable ZnS to achieve the conflicting visible-light-driven activities for H2 evolution. On the basis of first-principle density functional theory calculations, electron spin resonance and photoluminescence results, etc., it is initially proposed that the regulated sulfur vacancies in wurtzite phase of ZnS play the key role of photosensitization units for charge generation in visible light and active sites for effective electron utilization. The symbiotic sphalerite-wurtzite phase junctions that dominate the charge-transfer kinetics for photoexciton separation are the indispensable configuration in the present systems. Neither ZnS samples without phase junction nor those without enough sulfur vacancies conduct visible-light photocatalytic H2 evolution, while the one with optimized phase junctions and maximum sulfur vacancies shows considerable photocatalytic activity. This work will not only contribute to the realization of visible light photocatalysis for wide-bandgap semiconductors but also broaden the vision on the design of highly efficient transition metal sulfide photocatalysts.