Nano-zirconia supported by graphitic carbon nitride for enhanced visible light photocatalytic activity

Graphitic carbon nitride (g-C₃N₄) was prepared by high-temperature calcination of urea. A mixture of g-C₃N₄ and nano-ZrO₂ precursor was directly calcined to prepare g-C₃N₄/ZrO₂ hybrid photocatalysts. The photocatalytic properties of the sample were characterized by degradation of rhodamine B (RhB) under visible light. The g-C₃N₄/ZrO₂ hybrid photocatalysts have better degradation performance than the pure g-C₃N₄ and ZrO₂. The prepared catalysts were characterized by various techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-vis diffuse reflectance spectroscopy (DRS), Fourier transform infrared spectroscopy (FT-IR), and photoluminescence spectroscopy (PL) and electrochemical tests. The reasons for the improvement of catalytic activity were investigated from the aspects of crystal structure, surface morphology and photoelectric properties, and the catalytic mechanism were studied. The results show that the ZrO₂ nanoparticles were coated with g-C₃N₄ to form a heterostructure. Compared with the pure g-C₃N₄ and ZrO₂, the g-C₃N₄/ZrO₂ hybrids reduce the charge transfer resistance and inhibit the recombination of electron–holes well. In addition, it affects the band structure and improves the absorption of visible-light. At the same time, the study found that the main active species in the catalytic process were h⁺ and ·O₂⁻.

Graphitic carbon nitride (g-C₃N₄) was prepared by high-temperature calcination of urea.     

Hierarchically ordered macro–mesoporous anatase TiO2 prepared by pearl oyster shell and triblock copolymer dual templates for high photocatalytic activity

Hierarchically ordered macro–mesoporous anatase TiO₂ is prepared by combining the supramolecular-templating self-assembly of amphiphilic triblock copolymer P123 with a natural pearl oyster shell in a hard-templating process by a facile sol–gel reaction. The obtained materials are characterized by Raman spectroscopy, X-ray diffraction, N₂ adsorption–desorption analysis, scanning electron microscopy, and transmission electron microscopy. The results demonstrate that all TiO₂ materials obtained after calcination at various temperatures are in the anatase phase, and interestingly the resultant ordered structure of both macropores and mesopores are well-preserved after calcination at 350 °C or 450 °C, with the walls of macropores composed of ordered mesopores. However, upon calcination at 550 °C or 650 °C, while the ordered macroporous structures remain well-preserved, the mesoporous structures collapse. The photocatalytic activities of the resulting TiO₂ materials are also evaluated by photodegradation of rhodamine B under UV light irradiation. The prepared TiO₂ calcined at 450 °C shows the highest photocatalytic activity.

Hierarchically ordered macro–mesoporous anatase TiO₂ with photocatalytic activity was prepared using triblock copolymer P123 and natural pearl oyster shell as dual templates.     

Synthesis and thermal stability of ZrO2@SiO2 core–shell submicron particles

ZrO₂@SiO₂ core–shell submicron particles are promising candidates for the development of advanced optical materials. Here, submicron zirconia particles were synthesized using a modified sol–gel method and pre-calcined at 400 °C. Silica shells were grown on these particles (average size: ∼270 nm) with well-defined thicknesses (26 to 61 nm) using a seeded-growth Stöber approach. To study the thermal stability of bare ZrO₂ cores and ZrO₂@SiO₂ core–shell particles they were calcined at 450 to 1200 °C. After heat treatments, the particles were characterized by SEM, TEM, STEM, cross-sectional EDX mapping, and XRD. The non-encapsulated, bare ZrO₂ particles predominantly transitioned to the tetragonal phase after pre-calcination at 400 °C. Increasing the temperature to 600 °C transformed them to monoclinic. Finally, grain coarsening destroyed the spheroidal particle shape after heating to 800 °C. In striking contrast, SiO₂-encapsulation significantly inhibited grain growth and the t → m transition progressed considerably only after heating to 1000 °C, whereupon the particle shape, with a smooth silica shell, remained stable. Particle disintegration was observed after heating to 1200 °C. Thus, ZrO₂@SiO₂ core–shell particles are suited for high-temperature applications up to ∼1000 °C. Different mechanisms are considered to explain the markedly enhanced stability of ZrO₂@SiO₂ core–shell particles.

Silica encapsulation dramatically enhances the thermal stability of zirconia submicron particles by grain growth inhibition and tetragonal phase stabilization.     

Fabrication,characterization and high photocatalytic activity of Ag–ZnO heterojunctions under UV-visible light

Pure ZnO and Ag–ZnO nanocomposites were fabricated via a sol–gel route, and the obtained photocatalysts were characterized by XRD, SEM, TEM, BET, XPS, PL and DRS. The results showed that Ag⁰ nanoparticles deposit on the ZnO surface and Ag modification has negligible impact on the crystal structure, surface hydroxyl group content and surface area of ZnO. However, the recombination of photogenerated electrons and holes was suppressed effectively by Ag loading. The photocatalytic activity was investigated by evaluating the degradation of MB under xenon lamp irradiation as the UV-visible light source, and the results show that the photocatalytic activity of ZnO significantly improved after Ag modification. Ag–ZnO photocatalysts exhibit higher photocatalytic activity than commercial photocatalyst P25. The degradation degree of MB for 1%Ag–ZnO was 97.1% after 15 min. ˙O₂⁻ radicals are the main active species responsible for the photodegradation process, and Ag–ZnO heterojunctions generate more ˙O₂⁻ radicals, which is the primary reason for the improved photocatalytic performance.

Ag–ZnO heterojunction promotes the separation of photogenerated pairs and thus exhibits high catalytic activity under UV-visible light.     

The deactivation of a ZnO doped ZrO2–SiO2 catalyst in the conversion of ethanol/acetaldehyde to 1,3-butadiene

A deactivation study on the ethanol/acetaldehyde conversion to 1,3-butadiene over a ZnO promoted ZrO₂–SiO₂ catalyst prepared by a sol–gel method was performed. The samples were characterized by N₂ adsorption–desorption isotherms, scanning electron microscopy (SEM), NH₃ temperature programmed desorption (NH₃-TPD), X-ray powder diffraction characterization (XRD), thermogravimetric analyses (TGA), Fourier transform infrared resonance (FT-IR), ¹³C magic-angle spinning nuclear magnetic resonance (¹³C NMR) and X-ray photoelectron spectroscopy (XPS). The pore structure characteristics and surface acidity of Zn_0.5–Zr–Si catalysts were largely decreased with time-on-stream and no crystal structure was formed in the used catalyst, indicating that the deactivation was caused by carbon deposition. Two main types of carbon deposition were formed, namely low-temperature carbon deposition with the oxidation temperature of around 400 °C and high-temperature carbon deposition with the oxidation temperature of 526 °C. The carbon species were mainly composed of graphitized carbon, amorphous carbon, carbon in C–O bonds and carbonyls. The deactivated catalyst could be regenerated by a simple oxidation process in air. Adding a certain amount of water into the feed had a positive effect on reducing the carbon deposition.

Deactivation study on the ethanol/acetaldehyde conversion to 1,3-butadiene over a ZnO–ZrO₂–SiO₂ catalyst.     

Mesoporous TiO2 coated ZnFe2O4 nanocomposite loading on activated fly ash cenosphere for visible light photocatalysis

Several activated fly ash cenosphere (AFAC) supporting TiO₂ coated ZnFe₂O₄ (TiO₂/ZnFe₂O₄/AFAC) photocatalysts were prepared by sol–gel and hydrothermal methods. These photocatalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), UV-vis diffuse reflectance spectroscopy (UV-DRS) and nitrogen adsorption analyses for Brunauer–Emmett–Teller (BET) specific surface area measurements. We found that the main components of spherical AFAC were mullite (Al₆Si₂O₁₃) and SiO₂; the crystallite size of the TiO₂/ZnFe₂O₄ nanocomposite was less than 10 nm and its specific surface area was 162.18 m² g⁻¹. The TiO₂/ZnFe₂O₄ nanocomposite had a band-gap of 2.56 eV, which would photodegrade 95% of rhodamine B (RhB) under visible light within 75 min. When hybridized with 0.02 g AFAC, the TiO₂/ZnFe₂O₄/0.02 g AFAC photocatalyst with a band-gap of 2.50 eV could remove 97.1% of RhB and be reused three consecutive times with minor decrease in photocatalytic performance. However, the photocatalytic performance decreased to 91.0% on increasing the dosage of AFAC to 0.30 g. The mesoporous structure of all the photocatalysts and the strong adsorption ability of AFAC accounted for the notable performance.

Mesoporous TiO₂ coating ZnFe₂O₄ nanocomposite loading on different amounts of activated fly ash cenosphere (AFAC) for visible light photocatalysis of RhB were successfully synthesized by sol–gel and hydrothermal methods.     

The three-dimensional ordered macroporous structure of the Pt/TiO2–ZrO2 composite enhanced its photocatalytic performance for the photodegradation and photolysis of water

Using polystyrene (PS) spheres as a template, three-dimensional ordered macroporous Pt/TiO₂–ZrO₂ (3DOM Pt/TiO₂–ZrO₂) composites were prepared by vacuum impregnation combined with photoreduction. The crystal structure, composition, morphology, optical absorption, and surface physicochemical properties of the as-synthetized samples were characterized by X-ray diffraction (XRD), UV-visible diffuse reflectance spectroscopy (UV-vis/DRS), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and N₂ adsorption–desorption analyses. The results showed that the 3DOM Pt/TiO₂–ZrO₂ composites were mainly composed of anatase TiO₂ and tetragonal ZrO₂ crystal phases, in which Pt mainly existed as a single species. In addition, the as-synthesized composites had open, three-dimensionally ordered macroporous structures that could enhance their multi-mode photocatalytic degradation performance under UV, visible light, simulated solar light, and microwave-assisted irradiation. Moreover, the 3DOM Pt/TiO₂–ZrO₂ composites exhibited the best photocatalytic water splitting performance as compared to other systems.

Using polystyrene spheres as templates, 3DOM Pt/TiO₂–ZrO₂ composites were prepared by the vacuum impregnation combined with photoreduction method, which exhibited an enhanced photodegradation and water splitting performance.     

Photocatalytic activity of Ag/Al2O3–Gd2O3 photocatalysts prepared by the sol–gel method in the degradation of 4-chlorophenol

The photocatalytic activity in the degradation of 4-chlorophenol (4-ClPh) in aqueous medium (80 ppm) using 2.0 wt% Ag/Al₂O₃–Gd₂O₃ (Ag/Al–Gd-x; where x = 2.0, 5.0, 15.0, 25.0 and 50.0 wt% of Gd₂O₃) photocatalysts prepared by the sol–gel method was studied under UV light irradiation. The photocatalysts were characterized by N₂ physisorption, X-ray diffraction, SEM, HRTEM, UV-Vis, XPS, FTIR and fluorescence spectroscopy. About 67.0% of 4-ClPh was photoconverted after 4 h of UV light irradiation using Ag/γ-–Al₂O₃. When Ag/Al–Gd-x photocatalysts were tested, the 4-ClPh photoconversion was improved and more than 90.0% of 4-ClPh was photoconverted after 3 h of UV light irradiation in the materials containing 15.0 and 25.0 wt% of Gd₂O₃. Ag/Al–Gd-25 was the material with the highest efficacy to mineralize dissolved organic carbon, mineralizing more than 85.0% after 4 h of UV light irradiation. Silver nanoparticles and micro-particles of irregular pentagonal shape intersected by plane nanobelts of Al₂O₃–Gd₂O₃ composite oxide were detected in the Ag/Al–Gd-25 photocatalyst. This material is characterized by a lowest recombination rate of electron–hole pairs. The low recombination rate of photo-induced electron–hole pairs in the Ag/Al–Gd-x photocatalysts with high Gd₂O₃ contents (≥15.0 wt%) confirmes that the presence of silver nanoparticles and microparticles interacting with Al₂O₃–Gd₂O₃ composite oxide entities favors the separation of photo-induced charges (e⁻ and h⁺). These materials could be appropriate to be used as highly efficient photocatalysts to eliminate high concentrations of 4-ClPh in aqueous medium.

Ag/Al₂O₃–Gd₂O₃ showed high efficacy to photodegradate 4-chlorophenol, the strong interaction between silver nano-particles and micro-particles and Al₂O₃–Gd₂O₃ entities favors the decrease in the recombination rate.     

Ionic liquid/TiO2 nanoparticles doped with non-expensive metals: new active catalyst for phenol photodegradation

TiO₂ nanoparticles were synthesized using 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMI·BF₄) ionic liquid and doped with non-expensive metals Cu²⁺ and Fe³⁺ by the sol–gel method. The new generated photocatalysts had their morphological, textural and structural characteristics analysed by scanning electron microscopy and dispersive X-ray spectroscopy (SEM/EDS), transmission electron microscopy (TEM), Brunauer–Emmett–Teller analysis (BET), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and diffuse reflectance spectroscopy (DRS). The results showed two phases by XRD analysis, anatase (majority) and rutile (minority). The SEM micrographs exposed spherical TiO₂ NPs/BMI·BF₄ IL and compact layers for Cu²⁺ and Fe³⁺-doped TiO₂ NPs in BMI·BF₄ IL, the EDX confirmed only the presence of Ti, O, Fe and Cu. The BET and BJH analyses exhibited high porous TiO₂ NPs/BMI·BF₄ IL. The BET and BJH analyses confirmed that the pore diameter of mesoporous materials was between 12 and 16 nm with similar values for surface area (55–63 m² g⁻¹). The TEM images exhibited spherical shape nanoparticles with mean diameter of 20–22 nm. The DRS analysis and Tauc equation were applied to estimate the optical energy band gap of the photocatalysts. The energy band gap values of 3.1 eV, 3.32 eV, and 2.78 eV were obtained for TiO₂ NPs/BMI·BF₄ IL, 1% Fe³⁺-doped TiO₂ NPs/BMI·BF₄ IL and 1% Cu²⁺-doped TiO₂ NPs/BMI·BF₄ IL, respectively. Phenol photodegradation was realized using Cu²⁺ and Fe³⁺-doped TiO₂ NPs/BMI·BF₄ IL under UV/visible irradiation and quantified by HPLC-FLD. The phenol photodegradation was investigated by different concentrations of metal-doped TiO₂ NPs/BMI·BF₄ IL. The new active photocatalysts 1% Cu²⁺-doped TiO₂ NPs and 1% Fe³⁺-doped TiO₂ NPs/BMI·BF₄ IL exhibited high catalytic activity (99.9% and 96.8%, respectively). The photocatalysts 1% Cu²⁺ and 1% Fe³⁺-doped TiO₂ NPs/BMI·BF₄ IL were also evaluated using industrial wastewater from the tobacco industry. The results showed 56.7% phenol photodegradation, due to the complexity of the tobacco matrix wastewater.

TiO₂ nanoparticles were synthesized using 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMI·BF₄) ionic liquid and doped with non-expensive metals Cu²⁺ and Fe³⁺ by the sol–gel method.     

The effect of heat treatment on the anatase–rutile phase transformation and photocatalytic activity of Sn-doped TiO2 nanomaterials

Sn-doped TiO₂ nanomaterials with different amounts of Sn (1, 2.5, 5, 10, and 15 at%) were prepared by a sol–gel method and characterized by XRD, TG, DTA, EDS, XPS, DRS, SEM, BET, and PL. The photocatalytic activity of the prepared samples was investigated by measuring the degradation of rhodamine B in aqueous solution under UV light. The experimental results indicate that doping with Sn promotes phase transformation from anatase to rutile. The photocatalytic activity of TiO₂ is influenced by both the heat treatment temperature and the Sn doping concentration. 1% Sn–TiO₂ exhibits the highest degradation rate at 350 °C and 5% Sn–TiO₂ exhibits the best photocatalytic activity at 500 °C and 650 °C. The enhancement of the photocatalytic activity can be ascribed to a larger surface area and a better hydration ability, as well as less recombination of the photogenerated pairs.

Sn incorporation into TiO₂ lattices promotes anatase/rutile transformation and Sn–TiO₂ exhibits better photocatalytic activity at different temperatures.     

New titania-based photocatalysts for hydrogen production from aqueous-alcoholic solutions of methylene blue

A series of CuO_x–TiO₂ photocatalysts were prepared using fresh and thermally activated Evonik Aeroxide P25 titanium dioxide. The photocatalysts were characterized by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, XANES, diffuse reflectance spectroscopy, and N₂ adsorption technique. Photocatalytic activity of the samples was tested in hydrogen production from aqueous-alcoholic solutions of methylene blue under UV radiation (λ = 386 nm). It was found for the first time the synergistic effect of hydrogen production from two substrates—dye and ethanol. The maximum hydrogen production rate in the system water–ethanol–methylene blue was 1 μmol min⁻¹, which is 25 times higher than a value measured in a 10% solution of ethanol in water. The thermal activation of titania also leads to a change in the rate of hydrogen production. The highest catalytic activity was observed for a CuO_x–TiO₂ photocatalyst based on titania thermally-activated at 600 °C in air. A mechanism of the photocatalytic reaction is discussed.

Simultaneous presence of ethanol and methylene blue was shown to provide the most efficient hydrogen production and methylene blue removal.     

Solution-processed amorphous ZrO2 gate dielectric films synthesized by a non-hydrolytic sol–gel route

Solution-processed zirconium oxide (ZrO₂) dielectrics were formed via a non-hydrolytic sol–gel route at low-temperature, and are suitable for flexible thin film transistor (TFT) devices. Precursor solutions with equimolar zirconium halide and zirconium alkoxide were prepared, and amorphous ZrO₂ films were obtained by spin-coating and annealing at 300 °C through the direct condensation reaction between them. The ZrO₂ films exhibited a high dielectric constant near 10, and a low leakage current density of 5 × 10⁻⁸ A cm⁻² at a field of 1 MV cm⁻¹. High mobility p-type pentacene TFTs were fabricated using the ZrO₂ dielectrics, with a saturation field-effect mobility of 3.7 cm² V⁻¹ s⁻¹, a threshold voltage of −2.7 V, an on/off ratio of 1.1 × 10⁶ and a subthreshold swing of 0.65 V dec⁻¹.

Solution-processed amorphous zirconium oxide (ZrO₂) dielectrics were formed via a non-hydrolytic sol–gel route at low-temperature. The ZrO₂ films exhibited a high dielectric constant and high mobility p-type pentacene TFTs were fabricated using them.

High dielectric constant (high-k) materials have received great interest as alternatives to conventional dielectric materials such as silicon dioxide (SiO₂). These high-k dielectrics can be used as key components of the semiconductor and display industry in the form of capacitor dielectrics and transistor gate insulators.¹ In the semiconductor industry, as the traditional gate dielectric becomes extremely thin (∼a few atomic layer thick), leakage currents are generated due to direct tunneling of electrons through the dielectric itself, resulting in power dissipation and heat emission, which is a serious problem to be solved. High-k dielectrics are expected to reduce the leakage currents since thicker dielectric layers can be used without altering the required dielectric properties.^1,2 In the display industry, high-k dielectrics are considered core materials that allow high on-currents in low voltage operation resulting in low-power consumption devices. Among many high-k materials, zirconium oxide (ZrO₂) was chosen because of its outstanding physical and chemical properties such as high intrinsic dielectric constants, high melting points, and great chemical stability.³ ZrO₂ is an important metal oxide and has been applied to several technologies including gate dielectrics.^2–4Compared with vacuum-based processes such as chemical vapor deposition (CVD) and sputtering, solution-based processes (especially sol–gel methods) have distinguished advantages: low-cost and ease of large area deposition. However, solution-processes based on conventional hydrolytic sol–gel methods involve hydrolysis and condensation reactions, which require high thermal energy to form three-dimensional solid networks of pure components.⁵ In the past, several groups have reported on the synthesis of inorganic oxides via non-hydrolytic sol–gel routes.^6–12 The basis of these methods is the reaction between a metal halide and an oxygen donor in the absence of water. The oxygen donor may be an alkoxide, an ether or an alcohol. The reaction formulae using an alkoxide or an ether as an oxygen donor are as follows.^8–12MX_n + M(OR)_n → 2MO_n/2 + nRX (alkoxide route)1MX_n + (n/2)ROR → MO_n/2 + nRX (ether route)2In the above reaction scheme, the ether route is finally followed by the alkoxide route, in which the alkoxide generation is preceded by a substitution reaction between an alkoxide and an ether. M–X + R–O–R → M–OR + R–X3Finally, the metal oxide network formation is completed through the condensation reaction between the metal halide and the alkoxide as described in eqn (1).By direct condensation without any hydroxylation of the conventional hydrolytic sol–gel route, the above reactions complete at lower temperatures than the hydrolytic ones.¹² The advantages of low temperature process are variously known, including flexible and low-cost processes. The most important characteristic we thought of is the ability to form amorphous films. Amorphous gate insulators are generally preferred because of their smooth surfaces, which eliminates some of problems associated with grain boundaries in crystalline films. The crystalline rough surfaces induce charge trapping and leakage currents, deteriorating the channel mobility and the reliability of thin film devices.^13–16 Even though ZrO₂ crystallizes at much lower temperatures than SiO₂,² the crystallinity of thin ZrO₂ films can be controlled by adjusting the process temperature of the non-hydrolytic sol–gel method.In this letter, we present the solution-processed amorphous ZrO₂ thin films by a non-hydrolytic sol–gel route that can be applied as gate insulators for TFTs. To demonstrate advantages of the high dielectric quality of the ZrO₂ films, we selected a pentacene as channel material for TFT devices. Pentacene was thermally evaporated, which minimizes secondary effects that may influence the properties of the gate insulator, either chemically or thermally. To grow the ZrO₂ thin films, the precursor solution was spin-coated onto Si wafers and glass substrates and then annealed at low temperature (∼300 °C). Also, to investigate the physical properties of the thin ZrO₂ films including morphological and electrical characteristics, we carried out several analyses, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), current–voltage (IV) and TFT characterization. The ZrO₂ films exhibited a high dielectric constant near 10 and p-type pentacene TFTs with high mobility were fabricated using the ZrO₂ dielectrics, with a saturation field-effect mobility of 3.7 cm² V⁻¹ s⁻¹.Materials such as zirconium(iv) isopropoxide isopropanol complex (Zr[OCH(CH₃)₂]₄(CH₃)₂CHOH), zirconium(iv) chloride (ZrCl₄) and 2-methoxy ethanol were used as received without further purification. We prepared equimolar amounts of ZrCl₄ and Zr[OCH(CH₃)₂]₄(CH₃)₂CHOH precursor solutions with concentrations of 5 wt% and 16 wt% in 2-methoxyethanol respectively. The prepared solutions were then spin-coated onto silicon and glass substrates at 500 rpm. Finally, the coated substrates were baked at 100 °C on a hot plate for 1 min for solvent evaporation and then post-annealed at 300 °C and 400 °C on a hot plate and 600 °C in a furnace for 1 h for the curing of the thin films to confirm their morphology at each temperature.To study the electrical properties of the ZrO₂ thin films, we fabricated metal–insulator–metal (MIM) structures (Fig. 1(a)). The dielectric constants, the leakage currents, and the breakdown voltages of the ZrO₂ thin films were measured. TFT devices were also fabricated as shown in Fig. 1(b) to investigate the high-k effect of ZrO₂ gate dielectrics with pentacene channel layers. Pentacene channel layers were deposited by thermal evaporation. In both devices, we used shadow masks for patterning aluminum (Al) top electrodes, pentacene channel layer, and gold (Au) source/drain electrodes. In the TFT device structure, molybdenum/tungsten alloy (MoW) gate electrodes were patterned by photolithographic processes.Open in a separate windowFig. 1(a) Metal–insulator–metal (MIM) structure used to measure the dielectric constants, the leakage currents, and the breakdown voltages of the ZrO₂ films (Al diameter = 1 mm). (b) TFT structure with a 700 Å pentacene channel and a 1500 Å ZrO₂ gate dielectric layer (L (channel length) = 160 μm; W (channel width) = 1 mm).The thickness of ZrO₂ thin films were measured with a spectroscopic ellipsometer (M-2000V, J. A. Woollam. Co. Inc.). The film crystallinity was measured at 40 kV and 30 mA using a Phillips X''pert Pro X-ray diffractometer equipped with a Cu Kα source. The surface morphology were measured using a Hitachi S4800 scanning electron microscopy. The dielectric constants of ZrO₂ thin films were measured using an Agilent 4284A precision LCR meter in the frequency range between 20 Hz and 100 kHz. Leakage currents, breakdown voltages and TFT characteristics were measured by a Keithley 4200, semiconductor characterization system.Through the spin-coating and annealing process, we have obtained thin ZrO₂ films by the non-hydrolytic sol–gel route. The film thicknesses were 420 Å and 1500 Å with concentration of 5 wt% and 16 wt%, respectively. We chose the alkoxide route mentioned in reaction (1), where zirconium chloride and zirconium isopropoxide act as a metal halide and a metal alkoxide (an oxygen donor), respectively. XRD and SEM analyses were performed to confirm the possibility of controlling the film morphology by varying the annealing temperature. Fig. 2 shows the X-ray diffraction patterns of the thin ZrO₂ films with post-annealing temperatures of 300 °C, 400 °C, and 600 °C. An amorphous phase is observed for the samples with 300 °C and 400 °C. Only the sample annealed at 600 °C exhibits a crystalline structure. ZrO₂ exists in three major crystal phases: cubic, tetragonal, and monoclinic polymorphs.¹⁷ In our experiment, the thin films annealed at 600 °C showed the tetragonal phase. From the full width at half maximum height of the peak, the crystalline domain size can be estimated by the Scherrer relation (L_s = nλ/β cos θ, where n is unity, λ the wavelength of incident X-rays and θ the diffracted angle). The crystalline domain size was calculated to be about 160 Å, which means that grain boundaries are caused by the nanocrystals in the amorphous matrix. Through SEM analysis (Fig. S1^†), the morphology of the thin film surface could be confirmed intuitively, and a comprehensive conclusion could be made comparing with the crystallographic analysis of XRD. As expected, the conditions of 300 °C and 400 °C show microscopically flat surfaces, and at 600 °C, it is a rough surface due to the crystal growth as confirmed by XRD. To avoid deleterious effects of the grain boundaries and take advantage of the various features of the low temperature processes, we adopted the low temperature processed (∼300 °C) films as gate insulators for TFTs fabrication. In addition, because of the tendency of binary oxides to crystallize at low thermal energy, multi-metal oxides are commonly used to induce amorphous structures.¹³ One of the important facts to consider in this work is that reliable amorphous gate insulators can be obtained using only binary oxides at relatively low temperatures by virtue of the non-hydrolytic sol–gel route.Open in a separate windowFig. 2XRD patterns of the spin-coated thin ZrO₂ films post-annealed at 300 °C, 400 °C, and 600 °C, respectively.In order to investigate the potential of thin ZrO₂ films as gate insulators of TFTs, the dielectric constants, the leakage currents, and the breakdown voltages of the ZrO₂ thin films were measured. Fig. 3(a) shows the measured dielectric constants of a ZrO₂ film over the predetermined frequency range, indicating a high dielectric constant of about 10 at 100 kHz. This value is generally less than the known value, over 20.^2–4 Our solution-processed thin ZrO₂ films are amorphous and inevitably contain pores as a consequence of the sol–gel process.⁵ This, in turn, produces a film of lower density than a vacuum-deposited crystalline film, and as a result the dielectric constant of the film must be small. Even though the non-hydrolytic sol–gel ZrO₂ films have relatively small dielectric constants compared to their vacuum deposited counterparts, the amorphous nature could offset this negative effect by minimizing the grain boundary-induced device instability, and they still have a much higher dielectric constant value than silicon dioxide (∼3.9). Fig. 3(b) shows the leakage current characteristics of the ZrO₂ films. The leakage current at an electric field of 1 MV cm⁻¹ is observed at about 5 × 10⁻⁸ A cm⁻² and the breakdown voltage is greater than 4 MV cm⁻¹, which demonstrates the applicability of gate dielectrics. Besides, the completion of the ZrO₂ non-hydrolytic sol–gel reaction at a temperature of 300 °C or higher can also be confirmed by the electrical characteristics at 300 °C. Similar dielectric constants were measured according to frequencies. Generally, when the reaction is insufficient, abnormal dielectric constant values appear at high frequencies due to the mobile charges in the films. In addition, low leakage currents also represent a stable ZrO₂ thin film state.Open in a separate windowFig. 3(a) Dielectric constants of a thin ZrO₂ films as a function of frequency, (b) leakage current across a thin ZrO₂ film.For the pentacene TFT, a representative output curve of the drain current (I_DS) versus the drain voltage (V_DS) at various gate voltages (V_GS) is shown in Fig. 4(a). The TFT demonstrates a typical transistor behaviour. Current saturation is observed at high V_DS as the accumulation layer of the pentacene channel is pinched off near the drain electrode. A transfer curve of I_DSversus V_GS with a V_DS of −10 V is plotted in Fig. 4(b). By taking the square root of both sides in the following equation, TFT characteristics including the field effect mobility (μ) were obtained in the saturation regime.¹⁸4where, C_i is the areal capacitance of the ZrO₂ insulator, W is the channel width, L is the channel length, V_T is the threshold voltage. The TFTs exhibit excellent device characteristics: a field-effect mobility (μ_sat) of 3.7 cm²V⁻¹s⁻¹, a threshold voltage of −2.7 V, a on/off ratio of 1.1 × 10⁶, and a subthreshold swing of 0.65 V dec⁻¹. The field-effect mobility of 3.7 cm² V⁻¹ s⁻¹ at low V_GS is one of the highest results reported so far. Compared with pentacene TFTs with SiO₂ gate insulators, the superiority of ZrO₂ high-k dielectrics of this study can be demonstrated. In general, for pentacene TFTs with SiO₂ gate insulators, the field effect mobility is less than 1 cm² V⁻¹ s⁻¹, which is compared with the mobility of 3.7 cm² V⁻¹ s⁻¹ of this study.¹⁹ This excellent result is attributed to the high-k effect and smooth amorphous morphology of the ZrO₂ film. High-k dielectrics can induce more charge carriers in the semiconductor channel region. Unlike the traditional metal-oxide-semiconductor field-effect transistor (MOSFET) theory,²⁰ several studies have shown that mobility depends on N (accumulated carriers in the channel) in organic TFTs and support high mobility effect of our experiments.^21–24 This claim is based on the multiple trapping and release (MTR) model, which is widely used for the amorphous silicon (a-Si : H) TFT modeling.^22–24 Lee et al. have demonstrated the same relevance for oxide semiconductor TFTs using the MTR model and noted generalization for disordered semiconductor TFTs including the organic TFTs.²⁵ They modelled the relationship between mobility and capacitance and presented a generalized equation.²⁵ Another remarkable feature of our results is a very small subthreshold swing. It determines the voltage swing required to switch the device from the “off” to “on” state. The small subthreshold swing allows TFTs with low operating voltage to be implemented.Open in a separate windowFig. 4Device characteristics of spin-coated ZrO₂/pentacene TFT devices. (a) Output characteristics: V_GS varied from 0 V to −8 V, (b) Transfer characteristics: V_DS = −10 V.     

Synthesis and photocatalytic properties of three dimensional laminated structure anatase TiO2/nano-Fe0 with exposed (001) facets

Three dimensional laminated structure anatase TiO₂/nano-Fe⁰ with exposed (001) facets used as photocatalysts were synthesized by a two-step solvothermal route and a liquid phase reduction deposition method. The resulting samples were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy, scanning electron microscopy, transmission electron microscopy and selected-area electron diffraction. Characterization and experimental results indicated that the three dimensional laminated structure of anatase TiO₂ was assembled by two dimensional TiO₂-sheets with a thickness of approximately 30 nm. The three dimensional laminated structure anatase TiO₂/nano-Fe⁰ photocatalysts with improved visible-light responsive capability, high charge-hole mobility, and low electron–hole recombination exhibited higher photocatalytic performance in the photocatalytic degradation of methylene blue. The composite of nano-Fe⁰ and TiO₂ could effectively promote the generation of hydroxyl radicals (˙OH) with a synergistic effect and Photo-Fenton theory. This study provided new insights into the fabrication and practical application of high-performance photocatalysts in degrading organic pollutants.

Three dimensional laminated structure anatase TiO₂/nano-Fe⁰ with exposed (001) facets were successfully synthesized, which exhibited higher photocatalytic performance in the photocatalytic degradation of methylene blue.     

Goethite–titania composite: disinfection mechanism under UV and visible light

Goethite–titania (α-FeOOH–TiO₂) composites were prepared by co-precipitation and mechanical milling. The structural, morphological and optical properties of as-synthesized composites were characterized by X-ray powder diffraction, scanning electron microscopy and UV-Vis diffuse reflectance spectroscopy, respectively. α-FeOOH–TiO₂ composites and TiO₂-P25, as reference, were evaluated as photocatalysts for the disinfection of Escherichia coli under UV or visible light in a stirred tank reactor. α-FeOOH–TiO₂ exhibited better photocatalytic activity in the visible region than TiO₂-P25. The mechanical activation increased the absorption in the visible range of TiO₂-P25 and the photocatalytic activity of α-FeOOH–TiO₂. In the experiments with UV light and α-FeOOH–TiO₂, mechanically activated, a 5.4 log-reduction of bacteria was achieved after 240 min of treatment. Using visible light the α-FeOOH–TiO₂ and the TiO₂-P25 showed a 3.1 and a 0.7 log-reductions at 240 min, respectively. The disinfection mechanism was studied by ROS detection and scavenger experiments, demonstrating that the main ROS produced in the disinfection process were superoxide radical anion, singlet oxygen and hydroxyl radical.

A photocatalytic mechanism for FeOOH–TiO₂ composite is proposed under UV-Vis light, the FeOOH–TiO₂ composite showed higher photocatalytic activity than TiO₂-P25.     

Z-scheme LaCoO3/C3N5 for efficient full-spectrum light-simulated solar photocatalytic hydrogen generation

The development of photocatalysts with high activity and low cost is still a major challenge. Since its synthesis in 2019, C₃N₅ has become an emerging photocatalytic material and has been widely studied. In this work, we report on the preparation of LaCoO₃/C₃N₅ nanosheets and the use of LaCoO₃ instead of precious metals to improve photocatalytic hydrogen production activity. First, LaCoO₃ was successfully prepared by the sol–gel method and then a series of high-efficiency Z-type LaCoO₃/C₃N₅ heterojunction photocatalysts were synthesized by the solvothermal method. Various characterization techniques (XRD, FT-IR, SEM, TEM, EDS, XPS, UV-Vis DRS, BET, ESR) confirmed the formation between LaCoO₃ nanoparticles and C₃N₅ nanosheet heterostructures and interface interactions. In the photocatalytic water split test, 50 wt% LaCoO₃/C₃N₅ showed the highest photocatalytic activity of 956.11 μmol h⁻¹ g⁻¹, which was 3.21 and 1.59 times that of LaCoO₃ and C₃N₅, respectively. This work not only designs an inexpensive and efficient LaCoO₃/C₃N₅ photocatalytic system for water splitting or other photocatalytic applications, but also provides ideas for constructing new material photocatalytic systems.

An inexpensive and efficient LaCoO₃/C₃N₅ photocatalytic system for water splitting or other photocatalytic applications was designed. The photocatalytic reaction and mechanism of C₃N₅ and its complexes was verified.     

Activation of ZrO2–WO3 solid acid catalysts in a Friedel–Crafts reaction through post-hydrothermal treatment

ZrO₂–WO₃ mixed oxide plays an essential role in the chemical and petroleum industries. So far, very little work has paid attention to the activation of the low activity of ZrO₂–WO₃ catalysts. In this work, poorly reactive ZrO₂–WO₃ was prepared as a model catalyst by a sol–gel method and it was accompanied by post-hydrothermal treatment with various solutions. The catalytic results in the Friedel–Crafts reaction of anisole and benzyl alcohol showed that the post-hydrothermal treatment with ethylenediamine or ammonium hydroxide solutions dramatically improved the activity of ZrO₂–WO₃, while the hydrothermal treatments with water or ammonia chloride solution resulted in poorer activity and selectivity. The former treatments were found to induce a huge transformation of the ZrO₂ crystal from monoclinic to tetragonal as well as a significant increase in acidic WO_x clusters that anchored onto ZrO₂. The generation of the WO_x clusters was responsible for the activation of ZrO₂–WO₃.

The high pH value of the post-hydrothermal treatment induces the generation of acidic WO_x cluster active sites.     

Enhanced photocatalytic performance of rhodamine B and enrofloxacin by Pt loaded Bi4V2O11: boosted separation of charge carriers,additional superoxide radical production,and the photocatalytic mechanism

Photocatalytic performance is influenced by two contradictory factors, which are light absorption range and separation of charge carriers. Loading noble metals with nanosized interfacial contact is expected to improve the separation and transfer of photo-excited charge carriers while enlarging the light absorption range of the semiconductor photocatalyst. Therefore, it should be possible to improve the photocatalytic performance of pristine nontypical stoichiometric semiconductor photocatalysts by loading a specific noble metal. Herein, a series of novel Pt–Bi₄V₂O₁₁ photocatalysts have been successfully prepared via a surface reduction technique. The crystal structure, morphology, and photocatalytic performance, as well as photo-electron properties of the as-synthesized samples were fully characterized. Moreover, the series of Pt–Bi₄V₂O₁₁ samples were evaluated to remove typical organic pollutants, rhodamine B and enrofloxacin, from aqueous solutions. The photoluminescence, quenching experiments and the electron spin resonance technique were utilized to identify the effective radicals during the photocatalytic process and understand the photocatalytic mechanism. The photocatalytic performance of Pt–Bi₄V₂O₁₁ was tremendously enhanced compared with pristine Bi₄V₂O₁₁, and there was additional ˙O²⁻ produced during the photocatalytic process. This study deeply investigated the relation between the separation of charge carriers and the light harvesting, and revealed a promising strategy for fabricating efficient photocatalysts for both dyes and antibiotics.

The Effect of Pt for producing additional superoxide radicals, and the photocatalytic mechanism.     

Gas phase dehydration of glycerol to acrolein over WO3-based catalysts prepared by non-hydrolytic sol–gel synthesis

Solid acid catalysts based on WO₃–SiO₂ and WO₃–ZrO₂–SiO₂ were prepared by one-pot non-hydrolytic sol–gel method and tested in the gas phase glycerol dehydration to acrolein. Their structural and textural characteristics were determined by X-ray diffraction (XRD), N₂ adsorption, X-ray energy dispersive spectroscopy (XEDS), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Their acid characteristics were studied by both temperature programmed desorption of ammonia (NH₃-TPD) and FTIR of adsorbed pyridine. Under our operating conditions, all the catalysts were active and selective in the transformation of glycerol to acrolein, which was always the main reaction product. The high selectivity to acrolein is achieved on catalysts presenting a higher proportion of Brønsted acid sites. In addition, the role of oxygen in the feed on catalytic performance of these catalysts is also discussed.

Active and selective W–Si–(Zr)–O catalysts for glycerol dehydration to acrolein have been successfully prepared by non-hydrolytic sol gel method.     

Microscale flower-like magnesium oxide for highly efficient photocatalytic degradation of organic dyes in aqueous solution

Flower-like MgO microparticles with excellent photocatalytic performance in degradation of various organic dyes (e.g., methylene blue, Congo red, thymol blue, bromothymol blue, eriochrome black T, and their mixture) were synthesized by a facile precipitation method via the reaction between Mg²⁺ and CO₃²⁻ at 70 °C. The reaction time was found to be crucial in determining the final morphology of flower-like MgO. After studying the particles from time-dependent experiments, scanning electron microscope observation, Fourier transform infrared spectra and thermogravimetric analyses demonstrated that the formation of flower-like particles involved a complex process, in which agglomerates or rod-like particles with a formula of xMgCO₃·yH₂O (x = 0.75–0.77 and y = 1.87–1.96) were favorably formed after the initial mixture of the reactants. Owing to the chemical instability, they would turn into flower-like particles, which had a composition of xMgCO₃·yMg(OH)₂·zH₂O (x = 0.84–0.86, y = 0.13–0.23, and z = 0.77–1.15). After calcination, the generated product not only possessed a superior photocatalytic performance in degradation of organic dyes (100 mg L⁻¹) under UV light irradiation in contrast to other morphologies of MgO and other related state-of-the-art photocatalysts (e.g., N-doped TiO₂, Degussa P25 TiO₂, ZnO, WO₃, α-Fe₂O₃, BiVO₄, and g-C₃N₄), but also could be used for five cycles, maintaining its efficiency above 92.2%. These capacities made the flower-like MgO a potential candidate for polluted water treatment. Also, the underlying photocatalysis mechanism of MgO was proposed through radical trapping experiments.

Flower-like MgO microparticles with excellent photocatalytic performance in degradation of various organic dyes were synthesized by a facile precipitation method via the reaction between Mg²⁺ and CO₃²⁻ at 70 °C.     

Synergetic effect of photocatalysis and peroxymonosulfate activated by MFe2O4 (M = Co,Mn, or Zn) for enhanced photocatalytic activity under visible light irradiation

Nanosized MFe₂O₄ (M = Co, Mn, or Zn) photocatalysts were synthesized via a simple sol–gel method. MFe₂O₄ photocatalysts exhibited lower photocatalytic activity for the degradation of levofloxacin hydrochloride under visible light irradiation. For enhancement of photocatalytic activity, MFe₂O₄ was used to activate peroxymonosulfate and degrade levofloxacin hydrochloride under visible light irradiation. The influences of peroxymonosulfate dosage, levofloxacin hydrochloride concentration, pH value, and temperature on peroxymonosulfate activation to degrade levofloxacin hydrochloride were investigated in detail. The mechanism of activation of peroxymonosulfate by MFe₂O₄ was proposed and proved by radical quenching experiments, electron spin resonance analysis, X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy, and transient photocurrent responses. The combined activation effects of photogenerated e⁻/h⁺ and transition metals on peroxymonosulfate to produce sulfate radical clearly enhanced the degradation efficiency.

The combined activation effects of photogenerated e⁻/h⁺, Fe, Co, Mn, and Zn on peroxymonosulfate to produce SO₄˙⁻ clearly enhanced the degradation efficiency.