Photocatalytic activity and adsorption performance of p-CuBi2O4/n-TiO2 p–n heterojunction composites prepared by in situ sol–gel coating method

The CuBi₂O₄/TiO₂ p–n heterojunction composites were obtained by in situ sol–gel coating process using prepared CuBi₂O₄ powders and titania colloid. The obtained CuBi₂O₄/TiO₂ composites were characterized by XRD, SEM, BET and UV–vis diffuse reflectance spectroscopy. Their photocatalytic and adsorption activities were evaluated by degradations and adsorptions of methyl orange and congo red. Results showed that some compositions of prepared p–n heterojunction composites exhibited much higher photocatalytic activity and absorption performance than pure CuBi₂O₄ or pure TiO₂, demonstrating that the formation of p–n heterojunction between the two materials plays an important role in improving photocatalytic activity and adsorption performance.     

3D hierarchical structure collaborating with 2D/2D interface interaction in BiVO4/ZnCr-LDH heterojunction with superior visible-light photocatalytic removal efficiency for tetracycline hydrochloride

Fabrication of heterojunction with specialized geometry shape could improve the photodegradation efficiency of catalysts more efficiently based on the synergistic effect of components. Herein, the BiVO₄/ZnCr-LDH type-II heterojunction was successfully synthesized by the growth of two-dimensional (2D) ZnCr-LDH on 2D BiVO₄, forming a unique three-dimensional (3D) hierarchical structure. The 3D hierarchical BiVO₄/ZnCr-LDH type-II heterojunction with intimate 2D/2D heterointerfaces enlarges the interfacial contact areas and shortens the transfer distance of carriers simultaneously, which could promote the separation and transfer of photoinduced carriers and prolong the lifetime of carriers. Additionally, the 3D structure could enhance the photon utilization efficiency caused by the multiple reflections of incident light, and provide more active sites to promote the adsorption of pollutants. Therefore, the visible light catalytic performance of the optimal heterostructure for the removal of tetracycline hydrochloride is 3.47 times and 22.43 times higher than that of BiVO₄ and ZnCr-LDH, respectively. Superoxide radical (O₂^?) is confirmed as the primary active species, whereas hole (h⁺) is the secondary active species in the photodegradation process. This work provides a facile strategy to enhance the visible light catalytic behavior of BiVO₄ through the fabrication of 3D hierarchical architecture heterojunction for the removal of organic pollutant.     

CdTe/CdS共敏化TiO2电极材料的制备与光电性能

将电沉积法和化学浴沉积法结合,分别将CdTe和CdS量子点纳米晶材料引入到TiO₂纳米管阵列上制备CdTe/CdS量子点共敏化TiO₂光电极。利用扫描电镜、X射线衍射和X射线能量色散光谱等测试手段对所得样品的形貌、晶型和组分进行表征。在模拟太阳光照射条件下,通过电化学工作站测试其光电化学性能。研究结果表明,相对于单一量子点敏化CdS/TiO₂和CdTe/TiO₂光电极而言,共敏化CdTe/CdS/TiO₂光电极表现出更好的光电转化性能,短路电流密度和光电转换效率分别可以达到3.1 mA·cm^-2和1.85%。此外,采用电化学阻抗测试技术对材料性能提升的原因进行深入的探究。     

CdTe/CdS共敏化TiO2电极材料的制备与光电性能

将电沉积法和化学浴沉积法结合,分别将CdTe和CdS量子点纳米晶材料引入到TiO_2纳米管阵列上制备CdTe/CdS量子点共敏化TiO_2光电极。利用扫描电镜、X射线衍射和X射线能量色散光谱等测试手段对所得样品的形貌、晶型和组分进行表征。在模拟太阳光照射条件下,通过电化学工作站测试其光电化学性能。研究结果表明,相对于单一量子点敏化CdS/TiO_2和CdTe/TiO_2光电极而言,共敏化CdTe/CdS/TiO_2光电极表现出更好的光电转化性能,短路电流密度和光电转换效率分别可以达到3.1 m A·cm~(-2)和1.85%。此外,采用电化学阻抗测试技术对材料性能提升的原因进行深入的探究。     

The preparation,and applications of g-C<Subscript>3</Subscript>N<Subscript>4</Subscript>/TiO<Subscript>2</Subscript> heterojunction catalysts—a review

In recent years, more and more attention has been paid in the research of heterojunction catalysts, due to their better catalytic ability than that of single component catalysts. Up to now, many kinds of heterojunction catalysts have been reported, such as Bi₂O₃/Bi₂WO₆, WO₃/BiVO₄, SnO₂/TiO₂, CdS/TiO₂, Ta₃N₅/Pt/IrO₂ and so on, among which the heterojunction catalyst composed of g-C₃N₄ and TiO₂ has been studied tremendously recently, due to the high activity, high thermal and chemical stability, and well matched energy structure of them. Up to now, many methods have been explored for the synthesis of g-C₃N₄/TiO₂ heterojunction catalysts, such as ball milling of g-C₃N₄ and TiO₂, hydrothermal growth of TiO₂ on g-C₃N₄ and so on. In this review, the recent researches on the synthesis of g-C₃N₄/TiO₂ catalysts were summarized. Moreover, the applications of g-C₃N₄/TiO₂ catalysts in the field of photocatalysis were detailedly introduced.     

BiVO4/TiO2 core-shell heterostructure: wide range optical absorption and enhanced photoelectrochemical and photocatalytic performance

In the present study, pristine BiVO₄, TiO₂ and BiVO₄/TiO₂ core-shell heterostructured nanoparticles are prepared by hydrothermal methods and studied for structural, morphological, optical, photoelectrochemical water splitting and photocatalytic degradation of methylene blue as an organic pollutant. Both pristine BiVO₄ and TiO₂ exhibit poor PEC and PC performance under visible light illumination. However, an enhanced PEC and PC activity in BiVO₄/TiO₂ core-shell heterostructure is observed due to high solar energy absorption and superior charge separation properties in core-shell nanoparticles. The photoelectrode prepared using BiVO₄/TiO₂ core-shell nanoparticles exhibit a photocathode behavior and produced cathodic photocurrent, however, the pristine BiVO₄ and TiO₂ photoelectrodes act as photoanode and produced anodic photocurrent. This behavior of change in current direction is also observe in the Mott-Schottky analysis where the BiVO₄/TiO₂ core-shell nanoparticles photoelectrode exhibits the positive slow showing p-type semiconducting behavior. The change in cathodic photoresponse in core-shell nanoparticles in comparison to anodic photoresponse of BiVO₄ and TiO₂ nanoparticles is explained in terms of the variations in the work function values. These results highlight the advantages of core-shell nanoparticle of suitable materials for photocatalytic and photoelectrochemical applications.     

BiVO4/ZnFe2O4同型异质结光阳极的构筑及其光电催化分解水性能

光生电子-空穴对的复合被认为是限制BiVO₄材料光电催化转换效率的重要原因之一。基于此,通过简单的水热-煅烧方法构筑了 BiVO₄/ZnFe₂O₄同型异质结光阳极,BiVO₄/ZnFe₂O₄复合光阳极在 1.23 V(vs RHE)下的光电流密度为 3.33 mA·cm^-2,较纯BiVO₄提升了2倍 (1.20 mA·cm^-2)。相关的结构及性能测试表明,BiVO₄和ZnFe₂O₄形成了带隙错开的n-n异质结,使得光生载流子得到有效分离,更有效地参与水氧化过程,进而提高了BiVO₄的光电催化水分解性能。     

非晶TiO2修饰CuBi2O4光阴极增强其光电化学产氢活性

光催化作为太阳能利用领域的研究热点引起了广泛的关注.其中,光电化学技术能够通过分解水提供清洁的氢能源,因此被认为是一种潜在的新能源制造方式.在光电化学分解水产氢的过程中,最重要的是高效光电极的制备.一系列n型半导体材料已被广泛地报道并用作光阳极,如BiVO4,ZnO,Fe2O3等.然而对于光阴极材料,其可选择性则较少.CuBi2O4是一种天然矿物,具有廉价易得以及化学性质稳定的特性,而且是一种p型半导体材料,因此能够用于制备光阴极;另外因为其强的可见光响应(1.70 eV),所以具有广泛的应用前景.目前对于CuBi2O4光阴极研究主要集中在合成和理论计算方面,而对于如何促进界面处的载流子分离研究较少.本文通过一种简单的电沉积方法成功制备出CuBi2O4光阴极,然后利用非晶TiO2和助催化剂Pt进行修饰后将其用于光电化学产氢.由于形成了CuBi2O4/TiO2 p-n结,因此其光阴极活性得到增强.新的Pt/TiO2/CuBi2O4光阴极在0.60 V偏压处的光电流为0.35 mA/cm2,其数值约为Pt/CuBi2O4光阴极的两倍.XRD结果表明,我们制备的CuBi2O4为纯相且结晶性较好,其表面修饰的TiO2为非晶相的.SEM结果表明,CuBi2O4电极层由100-150 nm的颗粒构成.紫外-可见吸收光谱表明,制备的CuBi2O4光电极拥有良好的可见光吸收性质,而且TiO2修饰未对CuBi2O4的光吸收产生明显的影响.XPS结果表明,修饰TiO2并未对CuBi2O4电极造成成分上的破坏.光电化学测试表明,修饰TiO2层厚度和结晶性会影响光电极的最终活性.修饰四层TiO2和退火200 oC的样品具有最好的活性.另外稳定性测试也表明,修饰非晶TiO2的CuBi2O4光阴极具有良好的稳定性.在IPCE测试中,Pt/TiO2/CuBi2O4光阴极在其光响应范围内均比Pt/CuBi2O4光阴极表现出更高的效率.阻抗结果测试中Pt/TiO2/CuBi2O4光阴极具有更小的阻抗,这表明其载流子传输更加高效.在Mott-Shetty测试中,Pt/TiO2/CuBi2O4和Pt/CuBi2O4光阴极都表现出p型半导体性质,但是Pt/TiO2/CuBi2O4具有更负的平带电位,这表明修饰的TiO2仍具有n型半导体材料的特性,并与p型的CuBi2O4形成p-n结,从而促进了载流子分离效率.     

Water oxidation kinetics of nanoporous BiVO4 photoanodes functionalised with nickel/iron oxyhydroxide electrocatalysts

In this work, spectroelectrochemical techniques are employed to analyse the catalytic water oxidation performance of a series of three nickel/iron oxyhydroxide electrocatalysts deposited on FTO and BiVO₄, at neutral pH. Similar electrochemical water oxidation performance is observed for each of the FeOOH, Ni(Fe)OOH and FeOOHNiOOH electrocatalysts studied, which is found to result from a balance between degree of charge accumulation and rate of water oxidation. Once added onto BiVO₄ photoanodes, a large enhancement in the water oxidation photoelectrochemical performance is observed in comparison to the un-modified BiVO₄. To understand the origin of this enhancement, the films were evaluated through time-resolved optical spectroscopic techniques, allowing comparisons between electrochemical and photoelectrochemical water oxidation. For all three catalysts, fast hole transfer from BiVO₄ to the catalyst is observed in the transient absorption data. Using operando photoinduced absorption measurements, we find that water oxidation is driven by oxidised states within the catalyst layer, following hole transfer from BiVO₄. This charge transfer is correlated with a suppression of recombination losses which result in remarkably enhanced water oxidation performance relative to un-modified BiVO₄. Moreover, despite similar electrocatalytic behaviour of all three electrocatalysts, we show that variations in water oxidation performance observed among the BiVO₄/MOOH photoanodes stem from differences in photoelectrochemical and electrochemical charge accumulation in the catalyst layers. Under illumination, the amount of accumulated charge in the catalyst is driven by the injection of photogenerated holes from BiVO₄, which is further affected by the recombination loss at the BiVO₄/MOOH interface, and thus leads to deviations from their behaviour as standalone electrocatalysts.

Elucidating the role of charge accumulation and reaction kinetics in governing the performance of Ni/Fe oxyhydroxides as electrocatalysts and as co-catalysts on BiVO₄ photoanodes water oxidation.     

Visible-light-promoted gas-phase water splitting using porous WO3/BiVO4 photoanodes

We recently described the use of Ti(0) microfibers as an anodization substrate for the preparation of TiO₂ nanotubes arrays as porous photoanodes. Here, we report the use of these fibers as a scaffold to build porous photoanodes based on a WO₃/BiVO₄ heterojunction. The obtained photoelectrodes show promising results under visible light irradiation for water oxidation both in a typical liquid-phase photoelectrochemical setup and in a gas phase reactor (developed in-house) based on a polymeric electrolyte membrane.     

Boosting the photoelectrochemical water oxidation performance of bismuth vanadate by ZnCo2O4 nanoparticles

Due to the involvement of four-electron transfer process at photoanode,water oxidation is the ratelimiting step in water splitting reaction.To settle this dilemma,ZnCo₂ O₄ nanoparticles are combined with BiVO₄ to form a p-n ZnCo₂ O₄/BiVO₄ heterojunction photoanode,which is proved by an input voltage-output current test.The built-in electric field formed within the heterojunction structure promotes the effective separation of elect...     

Near infrared-driven photoelectrochemical water splitting: Review and future prospects

Photoelectrochemical (PEC) water splitting supplies an environmentally friendly, sustainable approach to generating renewable hydrogen fuels. Oxides semiconductors, e.g. TiO₂, BiVO₄, and Fe₂O₃, have been widely developed as photoelectrodes to demonstrate the utility in PEC systems. Even though significant effort has been made to increase the PEC efficiency, these materials are still far from practical applications. The main issue of metal oxides is the wide bandgap energy that hinders effective photons harvesting from sunlight. In solar spectrum, over 40% of the energy is located in the near-infrared (NIR) region. Developing sophisticated PEC systems that can be driven by NIR illumination is therefore essential. This review gives a concise overview on PEC systems based on the use of NIR-driven photoelectrodes. Promising candidates as efficient yet practical NIR-responsive photoelectrodes are suggested and discussed. Future outlooks on the advancement of PEC water splitting are also proposed.     

Low-temperature deposition and crystallization of RuO2/TiO2 on cotton fabric for efficient solar photocatalytic degradation of o-toluidine

A green low-temperature deposition and crystallization method was developed to uniformly coat RuO₂/TiO₂ nanocomposite onto cotton fabrics for efficient solar photocatalysis. The sequential growth of anatase TiO₂ and rutile RuO₂ on the surface of the cotton was confirmed by XRD, Raman and XPS characterizations. After the deposition of RuO₂, the optical properties of RuO₂/TiO₂/Cotton revealed better visible light absorption and higher charge mobility, and XPS spectra showed that the peaks of Ti 2p_3/2 and O 1 s shifted towards the lower binding energies due to the interfacial charge transfer at the robust RuO₂/TiO₂ mediated with Ti–O–Ru bonding. The photocatalytic performances of the RuO₂/TiO₂/Cotton were evaluated towards the photodegradation of o-toluidine (o-TD), an aromatic amine widely used in the chemical industry. Compared with TiO₂/Cotton, RuO₂/TiO₂/Cotton exhibited a remarkable improvement in the photocatalytic activity. The presence of RuO₂ on the surface of TiO₂/Cotton narrowed the band gap and improved the absorption of visible light. Moreover, the successful formation of a robust heterogeneous interface between TiO₂ and RuO₂ suppressed the charge carrier (e^–/h⁺) recombination effectively. With the RuO₂/TiO₂ coating chemically bound to the cotton fibers, RuO₂/TiO₂/Cotton delivered long-term stability in photocatalytic activity and high mechanical durability even after 20 washing times. Our facile and scalable synthesis strategy paved a universal route to efficient immobilization of visible-light-responsible TiO₂-based photocatalysts on the low-heat-resistant substrates for various applications.

Graphical abstract

     

Phase relations in the pseudobinary system TiO2Ga2O3

Phase relations and microstructures in the TiO₂-rich part of the TiO₂Ga₂O₃ pseudobinary system have been determined at temperatures between 1373 and 1623°K using X-ray diffraction and electron and optical microscopy. The phases occurring in the system are TiO₂ (rutile), β-Ga₂O₃, a series of oxides Ga₄Ti_m?4O_2m?2 (m odd) which exist above 1463°K, and Ga₂TiO₅, which exists above 1598°K. The width of the phase region occupied by the Ga₄Ti_m?4O_2m?2 phases varies with temperature. At 1473°K it is narrow, and has limits of Ga₄Ti₂₅O₅₆ to Ga₄Ti₂₁O₄₈ while at higher temperatures it broadens to limits of from Ga₄Ti₂₇O₆₀ to Ga₄Ti₁₁O₂₈ at 1623°K. These phases are often disordered and crystals frequently contain partially ordered intergrowths of oxides with various values of m. On the TiO₂-rich side of the phase region there is a continuous change in texture from rutile to the end members of the Ga₄Ti_m?4O_2m?2 structures. The findings are summarized on a phase diagram.     

CdSe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>S<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>/CdS-cosensitized 3D TiO<Subscript>2</Subscript> hierarchical nanostructures for efficient energy conversion

Three-dimensional TiO₂ hierarchical nanostructures (3D-TiO₂-HNs) containing TiO₂ nanotrees and nest-like hollow spheres were synthesized and used as backbones for CdSe _x S_1?x quantum dot (QD) loading. These CdSe _x S_1?x QD-sensitized 3D-TiO₂-HNs were then used as photoelectrodes in the preparation of quantum-dot-sensitized solar cells. As revealed by TEM images, the highly porous 3D-TiO₂-HNs represent an excellent framework on which to deposit a large number of CdSe _x S_1?x QDs in order to form homogeneous and compact CdSe _x S_1?x -sensitized layers in photoelectrodes using a spin-assisted successive ionic layer adsorption and reaction method (spin-SILAR). Following careful adjustment of the molar ratio of Se^2? to S^2?, the number of spin-SILAR cycles, and the thickness of the CdS passivation layer used, the best-performing QDSC was shown to yield a short-circuit current density of 18.22 mA cm^?2, an open-circuit voltage of 0.520 V, a fill factor of 0.510, and a power conversion efficiency of 4.83%. This high performance is possible because the device is able to absorb a relatively broad range of wavelengths and because charge recombination is suppressed in the device.     

TiO2/BiVO4, a Heterojuncted Microfiber with Enhanced Photocatalytic Performance for Methylene Blue under Visible Light Irradiation

Novel TiO₂/BiVO₄ microfiber heterojunctions were constructed using cotton as biomorphic templates. The as-synthesized samples were characterized by scanning electron microscope, X-ray diffraction, X-ray photoelectron spectroscopy, UV-Vis diffuse reflectance spectra and photocatalytic experiment. The morphology of the as-synthesized TiO₂/BiVO₄ composites was consisted of a large quantity of microfiber structures with diameter from 2.5 μm to 5 μm, and the surface of samples became more coarse and compact with the increase of weight ratio of TiO₂. The TiO₂/BiVO₄ samples with proper content (10.00wt%) showed the highest pho-tocatalytic degradation activity for methylene blue (MB) degradation among all the samples under visible light, and 88.58%MB could be degraded within 150 min. The enhancement of photocatalytic activity was mainly attributed to the formation of n-n heterojunction at the contact interface of TiO₂ and BiVO₄, which not only narrowed the band gap of BiVO₄ for extending the absorption range of visible light, but also promoted the transfer of charge carriers across interface. A possible photodegradation mechanism of MB in the presence of TiO₂/BiVO₄ microfibrous photocatalyst was proposed.     

Tunnel and intergrowth structures in the gallia-rich gallium titanate system

Phase analysis studies of the homologous series Ga₄Ti_m?4O_2m?2 by electron microscopy and diffraction reveals that m = 5 and 7 may occur but they are metastable, appearing only in incompletely reacted specimens. Thus m = 9 is the lowest stable member of the series. The observation of and structure determination of a new tetragonal tunnel structure, containing hollandite-type tunnels separated by elements of the β-gallia and rutile structure types, are also reported. The stoichiometry range M_xGa_10+2xTi_6?2xO₂₇ (M = Al₂O²⁺; 0.80 < x < 1.20) is suggested for this phase, since Al³⁺ is necessary to stabilize this phase. Lattice image studies of Ga₂TiO₅ reveal that it does not have the pseudobrookite structure, as assumed by previous authors, but instead adopts the “low-temperature” Ti₃O₅ structure type.     

BiVO4/ZnFe2O4同型异质结光阳极的构筑及其光电催化分解水性能

光生电子-空穴对的复合被认为是限制BiVO₄材料光电催化转换效率的重要原因之一。基于此,通过简单的水热-煅烧方法构筑了BiVO₄/ZnFe₂O₄同型异质结光阳极,BiVO₄/ZnFe₂O₄复合光阳极在1.23 V(vs RHE)下的光电流密度为3.33 mA·cm^-2,较纯BiVO₄提升了2倍(1.20 mA·cm^-2)。相关的结构及性能测试表明,BiVO₄和ZnFe₂O₄形成了带隙错开的n-n异质结,使得光生载流子得到有效分离,更有效地参与水氧化过程,进而提高了BiVO₄的光电催化水分解性能。     

Comparison study of BiVO4 heterojunctions photocatalyst with different carbon materials

MWCNT/BiVO₄ and RGO/BiVO₄ heterojunctions were synthesized by the sol–gel method, and the photocatalytic activity of MWCNT/BiVO₄ and RGO/BiVO₄ were evaluated by monitoring the degradation of RhB in a heterogeneous photocatalytic reactor. The catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy, scanning electron microscopy, Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy. The results indicated that a series of heterojunction structure composites were successfully synthesized, the heterojunction structure playing a crucial role in the electron–hole recombination. The crystal structure of BiVO₄ was still monoclinic after the doping of MWCNT and RGO according to the XRD analysis, while the active sites of MWCNT/BiVO₄ and RGO/BiVO₄ were more than pure BiVO₄. The RGO with a two-dimensional carbon sheet expressed higher performance than MWCNT with a one-dimensional carbon sheet for BiVO₄ because the layered structure of the RGO had a stronger electron-trapping capability than MWCNT. The optimal MWCNT content was 2%, and the 6% RGO/BiVO₄ showed the highest photocatalytic degradation efficiency of RhB. The photocatalytic degradation efficiency remained above 80% after five cycle tests.

     

Synergy effects in mixed Bi2O3, MoO3 and V2O5 catalysts for selective oxidation of propylene

In this work, the possible synergy effects between Bi₂O₃, MoO₃ and V₂O₅, and between Bi₂Mo₃O₁₂ and BiVO₄, were investigated. The catalytic activity of the ??mechanical mixture?? of these compounds was measured. The mixture containing 36.96?mol% Bi₂O₃, 39.13?mol% MoO₃ and 23.91?mol% V₂O₅ (21.43?mol% Bi₂Mo₃O₁₂ and 78.57?mol% BiVO₄), corresponding to the compound Bi_1?x/3V_1?x Mo _x O₄ with x?=?0.45 (Bi_0.85V_0.55Mo_0.45O₄), exhibited the highest activity for the selective oxidation of propylene to acrolein. The mixed sample prepared chemically by a sol?Cgel method possessed higher activity than that of mechanical mixtures.