Influence of MoS2 deposition time on the photocatalytic activity of MoS2/ V,N co-doped TiO2 heterostructure thin film in the visible light region

Electron-hole separation and a narrow band-gap are essential steps to obtain efficient photocatalysis, towards which the use of co-catalysts or co-doped-TiO₂ photocatalysts has become a widely used strategy. In this article, the combination of MoS₂ and co-doping of V, N is the goal to achieve high performance photocatalysts. We synthesized MoS₂/V, N co-doped TiO₂ heterostructure thin film by sol-gel and chemical bath deposition methods. Herein, we investigated the influence of deposition time of MoS₂ layer on visible-photocatalytic activity of the obtained samples. The thin films were characterized by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and UV–vis spectroscopy techniques. Visible-photocatalytic activity of these samples were evaluated on the removal of methylene blue (MB) under visible light irradiation. The results show that the aforementioned heterostructure thin films have better photocatalytic activities than those of TiO₂, MoS₂ and V, N co-doped TiO₂ counterparts in visible light region. The mechanism for increasing visible-photocatalytic property of the heterostructure thin films is discussed in detail. We find that MoS₂/V, N co-doped TiO₂ heterostructure thin film at MoS₂ deposition time of 45-min shows the highest photocatalytic performance in the visible light region with MB photodegradation rate about 99% for 150 min and the degradation rate constant is 2.06 times higher than that of V and N co-doped TiO₂ counterpart.     

Enhanced photocatalytic activity and stability of nano-scaled TiO2 co-doped with N and Fe

Titanium dioxide (TiO₂) nanoparticles co-doped with N and Fe were prepared via modified sol-gel process. The products were characterized by transmission electron microscopy (TEM), N₂ adsorption, X-ray diffraction (XRD), Raman spectroscopy, UV-vis spectroscopy, photoluminescence (PL), and X-ray photoelectron spectroscopy (XPS). It is shown that the prepared TiO₂ particles were less than 10 nm with narrow particle size distribution. The addition of MCM-41 caused the formation of Ti-O-Si bond which fixed the TiO₂ on MCM-41 surface, thus restricted the agglomeration and growth of TiO₂ particles. The photocatalytic performance in the degradation of methylene blue showed that N, Fe co-doped TiO₂ exhibited much higher photocatalytic activity than doped sample with nitrogen or Fe³⁺ alone under both UV and visible light. N, Fe co-doping decreased the loss of doping N during the degradation reaction, thus increased the photocatalytic stability. It was also found that the nitridation time had significant influence on the photocatalytic activity of prepared TiO₂ catalysts.     

N,Fe, La三掺杂锐钛矿型TiO2能带调节的理论与实验研究

采用基于密度泛函理论(DFT)的平面波超软赝势方法(PWPP), 利用Material studio 计算N, Fe, La三种元素掺杂引起的锐钛矿TiO₂晶体结构、能带结构和态密度变化. 并通过溶胶-凝胶法制得锐钛矿型本征TiO₂, N, Fe共掺杂TiO₂和N, Fe, La共掺杂TiO₂; 用X射线衍射和扫描电镜表征结构; 紫外-可见分光光度计检测TiO₂对甲基橙的降解效率变化. 计算结果表明, 由于N, Fe, La三掺杂TiO₂的晶格体积、键长等发生变化, 导致晶体对称性下降, 光生电子-空穴对有效分离, 同时在导带底和价带顶形成杂质能级, TiO₂禁带宽度由1.78 eV变为1.35 eV, 减小25%, 光吸收带边红移, 态密度数增加, 电子跃迁概率提升, 光催化能力增加. 实验结果表明: 离子掺杂使颗粒变小, 粒径大小: 本征TiO₂>N/Fe_TiO₂>N/Fe/La_TiO₂, 并测得N/Fe/La_TiO₂发光峰425 nm, 能隙减小, 光催化能力比N/Fe_TiO₂强, 增强原因是杂质能级和电子态数量增加引起.     

Synergetic Photocatalytic Nanostructures Based on Au/TiO2/Reduced Graphene Oxide for Efficient Degradation of Organic Pollutants

Recently, there is crucial interest in the design and fabrication of nanocatalysts for efficient decomposition of organic pollutants in wastewater using visible light. This work reports the assembling fabrication of synergetic photocatalytic Au/TiO₂/RGO nanostructures by utilizing the reduced graphene oxide (RGO) as substrate material and efficient separator for electrons and holes. The Au/TiO₂ nanostructures with a ≈7 nm TiO₂ particles size are dispersed uniformly on RGO nanosheets. UV–vis diffuse reflectance spectroscopy verifies that Au/TiO₂/RGO nanocomposites show strong absorption of visible light. The degradation efficiency after 1 h for hydroquinone under visible light and UV light is ≈77% and ≈90%, respectively. Under visible light, the calculated apparent rates (k ) of the Au/TiO₂/RGO nanocomposites are 0.0112 and 0.0174 min^?1 for decomposition of methylene blue and hydroquinone. That are five times greater than that of bare TiO₂. The high photocatalytic activity is mainly attributed to the synergy between RGO and Au/TiO₂ nanostructure. The strategy of composite nanostructures assembling on RGO is ensured to have a great practicable potential for the designing of high efficient multielement composite nanoparticles catalysts.     

Visible-light-activated Ce-Si co-doped TiO2 photocatalyst

To enhance the visible photocatalytic activity and thermal stability of TiO₂, Ce-Si co-doped TiO₂ materials were synthesized through a nonaqueous method of which the purpose was to reduce the aggregation between TiO₂ particles. The obtained materials maintained anatase phase and large surface area of 103.3 m² g⁻¹ even after calcined at 800 °C. The XPS results also indicated that Si was weaved into the lattice of TiO₂, and Ce mainly existed as oxides on the surface of TiO₂ particles. The doped Si might enhance surface area and suppress transformation from anatase to rutile, while the doped Ce might cause visible absorption and inhibit crystallite growth during heat treatment. Evaluated by decomposing dye Rhodamine B, visible photocatalytic activity of Ce-Si co-doped TiO₂ was obviously higher than that of pure TiO₂ and reached the maximum at Ce and Si contents of 0.5 mol% and 10 mol%.     

Synthesis, structure and photocatalytic activity of nano TiO2 and nano Ti1-xMxO2-δ (M = Cu, Fe, Pt, Pd, V, W, Ce, Zr)

We have synthesized 5–7 nm size, highly crystalline TiO₂ which absorbs radiation in the visible region of solar spectrum. The material shows higher photocatalytic activity both in UV and visible region of the solar radiation compared to commercial Degussa P25 TiO₂. Transition metal ion substitution for Ti⁴⁺ creates mid-gap states which act as recombination centers for electron-hole induced by photons thus reducing photocatalytic activity. However, Pt, Pd and Cu ion substituted TiO₂ are excellent CO oxidation and NO reduction catalysts at temperatures less than 100° C.     

First principles calculations of electronic and optical properties of Mo and C co-doped anatase TiO2

Using the first principles calculations, the electronic and optical properties of C, Mo and Mo-C-doped anatase TiO₂ are studied. For the Mo mono-doped TiO₂, the band gap reduces little, and the largest perturbation occurs at the CBM of TiO₂. C mono-doping suppresses the effective band gap, but the partially occupied subbands in the gap probably also serve as the recombination centers for electrons and holes. Therefore, the Mo-C co-doping is investigated for the charge compensation consideration. We discuss six doped configurations and find that the total energy of the system is increased with increasing distance of C and Mo. It is found that co-doped configurations with C nearest to Mo possess the lowest total energy. Then, we focus on discussing three possible Mo-C adjacent co-doped configurations. The subbands mainly induced by C-2p states in the band gap become fully occupied because the Mo atom contributes sufficient electrons to C anion for compensation. At the same time, the effective band gap is narrowed about 0.9 eV and the perturbation at the CBM occurred in Mo mono-doped TiO₂ disappears, which means the band edges of doped system still straddle the redox potentials of water. Furthermore, the optical properties of the compensated Mo-C adjacent co-doped TiO₂ and pure TiO₂ are calculated. The optical absorption edges of the Mo-C co-doped TiO₂ shift towards the visible light region.     

A facile method to synthesize nitrogen and fluorine co-doped TiO2 nanoparticles by pyrolysis of (NH4)2TiF6

The nitrogen and fluorine co-doped TiO₂ (N-F-TiO₂) nanoparticles of anatase crystalline structure were prepared by a facile method of (NH₄)₂TiF₆ pyrolysis, and characterized by thermogravimetry-differential thermal analysis (TG-DTA), X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and ultraviolet visible (UV-Vis) spectroscopy etc. With the increase of calcination temperature, (NH₄)₂TiF₆ decomposed into TiOF₂ and NH₄TiOF₃ at first, and then formed anatase-type TiO₂ with thin sheet morphology. H₃BO₃ as oxygen source can promote the formation of anatase TiO₂, but decrease the F content in the N-F-TiO₂ materials due to the formation of volatile BF₃ during the precursor decomposition. The photocatalytic activity of the obtained N-F-TiO₂ samples was evaluated by the methylene blue degradation under visible light, and all the samples exhibited much higher photocatalytic activity than P25. Moreover, the merits and disadvantages of this proposed method to prepare doped TiO₂ are discussed.     

铈和磷钨酸共掺杂纳米二氧化钛中空纤维的制备及其光催化活性

以棉花纤维为模板,以钛酸四正丁酯、硝酸铈铵和磷钨酸为原料采用模板法制备了一系列铈和磷钨酸共掺杂的、具有中空纤维结构的TiO₂光催化材料, 利用扫描电子显微镜、X射线衍射、BET和紫外-可见光谱等技术对其形貌、晶体结构及表面结构、光吸收特性等进行了表征. 以苯酚溶液的光催化降解为模型反应,考察了不同掺杂量的样品在紫外和可见光下的光催化性能. 结果表明,用模板法制备的TiO₂纤维材料具有中空结构,共掺杂的TiO₂纤维在紫外和可见光条件下较纯TiO₂纤维和单掺杂TiO₂纤维对苯酚溶液具有更好的光催化降解效果, 且铈和磷钨酸的掺杂量显著影响该纤维材料的催化性能;当铈掺杂量为0.3mol%和磷钨酸掺杂量为2mol%,在500 oC焙烧2 h所得中空纤维材料的催化性能最佳,4 h即可使苯酚溶液的降解率达98.5%;重复使用4次仍可使苯酚溶液的降解率保持在87%以上,且该催化剂材料易于离心分离去除.     

Optical and structural properties of TiO<Subscript>2</Subscript> nanopowders with Ce/Sn doping at various calcination temperature and time

In this paper, the influence of calcination temperature and time has been investigated on the structural, morphological, optical, and photo-catalytic properties of Sn/Ce co-doped TiO₂ nano-powders prepared via sol–gel process. They were calcined at the temperatures in the range of 475–975 °C for 1 and 2 h. The photocatalytic ability of TiO₂ powders was evaluated by means of methylene blue degradation experiments conducted under the irradiation of simulated solar light. The X-ray diffraction results showed that the tensile strain in the host lattice was present. The comparison with undoped and Sn or Ce-doped TiO₂, co-doped TiO₂ shows an obviously higher catalytic activity under visible light irradiation. The absorbance spectrum of Ce and Sn co-doped TiO₂ exhibited significant red-shift to visible region. The red-shift is caused by the appearance of a new electronic state in the middle of the TiO₂ band-gap. FESEM images show the formation of doped TiO₂ nanoparticles with small size in structure and spherical in shape. The FTIR spectra exhibited peaks corresponding to the anatase and rutile structure phases of TiO₂.     

Synthesis and characterization of the Pd/InVO4-TiO2 co-doped thin films with visible light photocatalytic activities

Novel Pd/InVO₄-TiO₂ thin films with visible light photocatalytic activity were synthesized from the Pd and InVO₂ co-doped TiO₂ sol via sol-gel method. The photocatalytic activities of Pd/InVO₄-TiO₂ thin films were investigated based on the oxidative decomposition of methyl orange in aqueous solution. The as-prepared samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS) and UV-vis absorption spectroscopy (UV-vis). The results indicate that the Pd/InVO₄-TiO₂ thin films are compact, uniform and consist of sphere nanoparticles with diameters about 80-100 nm. The UV-vis spectra show that the Pd/InVO₄-TiO₂ thin films extend the light absorption spectrum toward the visible region. XPS results reveal that doped Pd exist in the form of metallic palladium. The photocatalytic experiments demonstrate that Pd doping can effectively enhance the photocatalytic activities of InVO₄-TiO₂ thin films in decomposition of aqueous methyl orange under visible light irradiation. It has been confirmed that Pd/InVO₄-TiO₂ thin films could be excited by visible light (E < 3.2 eV) due to the existence of the Pd and InVO₄ doped in the films.     

Hydrothermal synthesis,characterization and optical absorption property of nanoscale WS2/TiO2 composites

Nanosized tungsten disulfide (WS₂) sensitized titanium dioxide (TiO₂) was successfully prepared via a simple yet facile hydrothermal process. The nanocomposite exhibited a wide and intensive absorption in the visible light region of 400–700 nm, and may have a potential application as a visible photocatalyst. In addition, the sensitization mechanism of the nano-WS₂ was proposed to elaborate the wide visible light absorption of the WS₂/TiO₂ nanocomposites.     

Preparation and characterization of TiO2 photocatalysts co-doped with iron (III) and lanthanum for the degradation of organic pollutants

Titanium dioxide photocatalysts co-doped with iron (III) and lanthanum were prepared by a facile sol-gel method. The structure of catalysts was characterized by X-ray diffraction (XRD), Raman spectroscopy, UV-vis diffuse reflectance spectroscopy and X-ray photoelectron spectroscopy (XPS). The photocatalytic activities of the samples were evaluated by the degradation of methylene blue in aqueous solutions under visible light (λ > 420 nm) and UV light irradiation. Doping with Fe³⁺ results in a lower anatase to rutile (A-R) phase transformation temperature for TiO₂ particles, while doping with La³⁺ inhibits the A-R phase transformation, and co-doping samples indicate that Fe³⁺ partly counteracts the effect of La³⁺ on the A-R transformation property of TiO₂. Fe-TiO₂ has a long tail extending up the absorption edges to 600 nm, whereas La-TiO₂ results in a red shift of the absorption. However, Fe and La have synergistic effect in the absorption of TiO₂. Compared with Fe³⁺ and La³⁺ singly doped TiO₂, the co-doped simple exhibits excellent visible light and UV light activity and the synergistic effect of Fe³⁺ and La³⁺ is responsible for improving the photocatalytic activity.     

Control of optical absorption edge of TiO2 through co-doped acceptors: The chemical trend

Tuning the optical adsorption edge of TiO₂ is attracting increasing attention as a potential solution to the worldwide energy shortage. A possible strategy to achieve high efficiency photocatalysis with TiO₂ is through dopants to modulate chemical composition. Based on first-principles calculations, we propose a hole-strain-mediated coupling mechanism between co-doped acceptors in anatase TiO₂. When the dopant complex on neighboring oxygen sites contains a large radius atom, and the doped system has at least one net hole, the dopants will strongly couple to form a pair through the local lattice strain induced by the large dopant. The coupling results in bandgap narrowing due to the appearance of the fully occupied mid-gap states, leading to a much more effective band gap reduction than that induced by mono-doping or conventional donor–acceptor codoping. The calculated absorption spectra show that acceptor–acceptor codopings could shift the absorption edge to the visible light region.     

The C-V characteristics of TiO2/p-Si/Ag,GNR doped TiO2/p-Si/Ag and MWCNT doped TiO2/p-Si/Ag heterojunction devices

The TiO₂/p-Si/Ag, graphene (GNR) doped TiO₂/p-Si/Ag and multi-walled carbon nanotube (MWCNT) doped TiO₂/p-Si/Ag heterojunction devices were fabricated by electrospinning technique at same conditions. Their structural, morphological properties, thermal analyses (TGA), and capacitance voltage characteristics were studied and compared. The undoped, GNR and MWCNT doped TiO₂ structures obtained successfully according to XRD measurements. Morphological properties of the undoped, GNR and MWCNT doped TiO₂ composite structures have rod or ribbon like structures. The TGA result confirmed the GNR and MWCNT doped TiO₂ structures. The C-V and G-V measurements were employed for electrical characterization of the TiO₂/p-Si/Ag, GNR doped TiO₂/p-Si/Ag and MWCNT doped TiO₂/p-Si/Ag devices for various frequencies at room temperatures. The results imparted that the capacitance and conductance behaviors of all devices are strong functions of the frequency and voltage. The electrical parameters were calculated from C⁻²-V plots of the heterojunction devices and compared for three devices. The transient photocapacitance plots revealed that the devices can be employed for optical communication applications.     

Visible-light-driven titania photocatalyst co-doped with nitrogen and ferrum

Nitrogen and ferrum co-doped titania photocatalyst was prepared by the sol-gel route. The prepared photocatalyst was characterized by various techniques including X-ray diffraction (XRD), UV-vis diffusive reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), and N₂ adsorption-desorption isotherm. The photocatalytic activity of the co-doped titania photocatalyst was evaluated by the degradation of methyl orange (MO) from aqueous solution under visible light irradiation and was compared with that of the commercial TiO₂ photocatalyst (Degussa P25). The results revealed that the nitrogen doping could lead to the response to visible light and that the ferrum doping could improve the photocatalytic performance. The effects of the component and the annealing temperature of the co-doped titania photocatalyst on the photocatalytic activity were investigated.     

Modified g-C3N4/TiO2 nanosheets/ZnO ternary facet coupled heterojunction for photocatalytic degradation of p-toluenesulfonic acid (p-TSA) under visible light

Novel ternary nanocomposites with facet coupled structure were synthesized by using modified g-C₃N₄, TiO₂ nanosheets and nano-ZnO. Nanosheet/nanosheet heterojunction structure was investigated by TEM, XPS and XRD. FT–IR and Nitrogen adsorption were illustrated for chemical/physical structure analyses. Solution of p-Toluenesulfonic acid (p-TSA) was chosen as target pollutant for visible light photodegradation and the excellent removal efficiency was achieved by this structurally modified g-C₃N₄/TiO₂/ZnO hybrid. The visible light absorption improvement and quantum efficiency enhancement, which were testified by UV–vis DRS, PL and p-TSA photodegradation measurements, due to the facet coupled structure and appropriate quantity of modified g-C₃N₄ in the nanocomposites.     

TiO2–carbon nanotube composites for visible photocatalysts – Influence of TiO2 crystal structure

We investigated the influence of the crystal structure of TiO₂ and the use of different TiO₂ precursors on the properties and photocatalytic activity of carbon nanotube (CNTs)–titania composites. We found that the crystal structure and properties of starting TiO₂ nanomaterial significantly affected the effect of CNTs incorporation on the photocatalytic activity under simulated solar and visible light illumination (simulated solar illumination with UV-blocking filter). In case of significant photocatalytic activity under visible light illumination (anatase TiO₂), likely due to the presence of native defects, composites exhibited lower activity under visible illumination only, but higher activity under simulated solar illumination. The opposite trends were observed for P25 (anatase + rutile) and rutile TiO₂, where incorporation of CNTs resulted in a significant increase of photocatalytic activity under visible illumination. Thus, control over crystal structure and native defects is essential for the development of efficient visible light activated photocatalysts.     

Thickness effect of the TiO2 nanofiber scattering layer on the performance of the TiO2 nanoparticle/TiO2 nanofiber-structured dye-sensitized solar cells

TiO₂ nanofibers (NFs) were fabricated by an electrospinning process and were used as scattering layers in dye-sensitized solar cells (DSSCs). The NF-coated photoanodes of the DSSCs were prepared with a variety of scattering layer thicknesses. The thickness effect of the scattering layer on the double-layered TiO₂ nanoparticle (NP)/TiO₂ NF structure was investigated through structural, morphological, and optical measurements. In the double-layered photoanode, the TiO₂ NP layer plays a major role in dye adsorption and light transmission, and the TiO₂ NF scattering layer improves the absorption of visible light due to the light scattering effects. The scattering effect of TiO₂ NFs layer was examined by the incident monochromatic photon-to-electron conversion efficiency (IPCE) and UV–Vis spectrometry. The conversion efficiency for the 12 μm-thick photoanode composed of a 2 μm-thick TiO₂ NF layer and 10 μm-thick TiO₂ NP layer was higher than that of DSSCs with only TiO₂ NPs photoanode by approximately 33%.     

Electrostatic assembly of CdTe quantum dots with different charged ligands into TiO2 porous film for solar cells

We have demonstrated an approach for the electrostatic assembly of CdTe quantum dots (QDs) with different charged ligands as sensitizers, achieving high coverage and good dispersion in TiO₂ porous films. The CdTe QD-sensitized TiO₂ porous films were subjected to thermal annealing in a high vacuum chamber to remove the ligand linker, resulting in the formation of direct heterojunctions between the bare CdTe QDs and TiO₂ for a favorable charge transfer. The as-received CdTe QD-sensitized TiO₂ porous films were employed as photoanodes for quantum dot-sensitized solar cells (QSSCs), and the photocurrent density reached as high as 4.69 mA/cm² under a standard illumination condition of simulated AM 1.5G (100 mW/cm²).