Preparation of the visible light responsive TiO2 thin film photocatalysts by the RF magnetron sputtering deposition method

TiO₂ thin film photocatalysts which could induce photoreactions under visible light irradiation were successfully developed in a single process by applying an ion engineering technique, i.e., the radio frequency (RF) magnetron sputtering deposition method. The TiO₂ thin films prepared at temperatures greater than 773 K showed the efficient absorption of visible light; on the other hand, the TiO₂ thin films prepared at around 573 K were highly transparent. This clearly means that the optical properties of TiO₂ thin films, which absorb not only UV but also visible light, can be controlled by the preparation temperatures of the RF magnetron sputtering deposition method. These visible light responsive TiO₂ thin films were found to exhibit effective photocatalytic reactivity under visible light irradiation (λ > 450 nm) at 275 K for the reductive decomposition of NO into N₂ and N₂O. From various characterizations, the orderly aligned columnar TiO₂ crystals could be observed only for the visible light responsive TiO₂ thin films. This unique structural factor is expected to modify the electronic properties of a TiO₂ semiconductor, enabling the efficient absorption of visible light.     

Preparation of nitrogen-substituted TiO2 thin film photocatalysts by the radio frequency magnetron sputtering deposition method and their photocatalytic reactivity under visible light irradiation

Nitrogen-substituted TiO2 (N-TiO2) thin film photocatalysts have been prepared by a radio frequency magnetron sputtering (RF-MS) deposition method using a N2/Ar mixture sputtering gas. The effect of the concentration of substituted nitrogen on the characteristics of the N-TiO2 thin films was investigated by UV-vis absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and scanning electron microscopy (SEM) analyses. The absorption band of the N-TiO2 thin film was found to shift smoothly to visible light regions up to 550 nm, its extent depending on the concentration of nitrogen substituted within the TiO2 lattice in a range of 2.0-16.5%. The N-TiO2 thin film photocatalyst with a nitrogen concentration of 6.0% exhibited the highest reactivity for the photocatalytic oxidation of 2-propanol diluted in water even under visible (lambda > or = 450 nm) or solar light irradiation. Moreover, N-TiO2 thin film photocatalysts prepared on conducting glass electrodes showed anodic photocurrents attributed to the photooxidation of water under visible light, its extent depending on wavelengths up to 550 nm. The absorbed photon to current conversion efficiencies reached 25.2% and 22.4% under UV (lambda = 360 nm) and visible light (lambda = 420 nm), respectively. UV-vis and photoelectrochemical investigations also confirmed that these thin films remain thermodynamically and mechanically stable even under heat treatment at 673 K. In addition, XPS and XRD studies revealed that a significantly high substitution of the lattice O atoms of the TiO2 with the N atoms plays a crucial role in the band gap narrowing of the TiO2 thin films, enabling them to absorb and operate under visible light irradiation as a highly reactive, effective photocatalyst.     

Effect of the sputtering parameters on the physical properties and photocatalytic reactivity of TiO2 thin films prepared by an RF magnetron sputtering deposition method

Visible light-responsive TiO₂ (Vis-TiO₂) thin films were successfully developed by applying a radio-frequency magnetron sputtering deposition method by controlling various sputtering parameters such as the substrate temperature, Ar gas pressure, and the target-to-substrate distance. UV–Vis, XRD and SEM investigations revealed that optical property, the crystal structure, and photocatalytic activity of Vis-TiO₂ are strongly affected by the sputtering parameters during the deposition step. Vis-TiO₂ was found to act as an efficient photocatalyst for the H₂ and O₂ evolution from water under visible light irradiation (λ ≥ 420 nm). SIMS investigations have revealed that a slight decrease in the O/Ti ratio of the TiO₂ thin films plays an important role in the modification of the electronic properties of Vis-TiO₂ thin films, enabling them to absorb visible light.     

Separate evolution of H2 and O2 from H2O on visible light-responsive TiO2 thin film photocatalysts prepared by an RF magnetron sputtering method

Visible light-responsive TiO₂ thin film photocatalysts (Vis-TiO₂) have been prepared on Ti metal foil (Vis-TiO₂/Ti) or ITO glass (Vis-TiO₂/ITO) substrates by a radio-frequency magnetron sputtering (RF-MS) method. The UV–Vis spectra as well as photoelectrochemical performance of Vis-TiO₂ were affected by various calcination treatments such as calcination in air or NH₃. Calcination treatment in NH₃ (1.0 × 10⁴ Pa, 673 K) was particularly effective in increasing the visible light absorption of Vis-TiO₂ as well as in enhancing its photoelectrochemical performance and photocatalytic activity. A novel Vis-TiO₂ thin film photocatalyst (Vis-TiO₂/Ti/Pt) was prepared by an RF-MS method where Vis-TiO₂ was deposited on one side of a Ti metal foil substrate and nanoparticles of Pt were deposited on the other side. The separate evolution of H₂ and O₂ from H₂O could be successfully achieved by using an H-type glass cell consisting of two aqueous phases separated by Vis-TiO₂/Ti/Pt and a proton-exchange membrane. It was found that the rate of the separate evolution of H₂ and O₂ was also dramatically enhanced by calcination treatment of Vis-TiO₂ in NH₃.     

A facile preparation and visible light-induced photocatalysis of indium sulfide superstructure

β-In₂S₃ superstructure comprised of nanoflakes with the sizes of 200–450 nm has been synthesized by a facile reflux method with the assistance of the sodium dodecylsulfate at low temperature (80 °C). XRD, SEM, TEM, HRTEM, and UV–vis spectra were used to characterize the In₂S₃ superstructure. Prepared In₂S₃ superstructures exhibited high photocatalytic activities in the degradation of rhodamine B under visible light irradiation (λ > 420 nm), in which 97.5% RhB was photodegraded after 60 min.     

Characterization of TiO2 nanotube arrays prepared via anodization of titanium films deposited by DC magnetron sputtering

Highly ordered TiO₂ nanotube arrays were fabricated on a conducting glass substrate in NH₄HF₂/glycol electrolyte via anodization of titanium film which was deposited by direct current magnetron sputtering (DCMS) at different temperatures. The results showed that Ti films with good homogeneity and high denseness could be formed under the conditions Ar pressure 0.35 Pa, direct current 3.5 A, and 2 h at 300 °C. Characterization of the TiO₂ nanotube arrays was investigated comparatively, by altering anodization time. The surface morphology of the samples changed as the anodization time was prolonged from 10 to 150 min at 30 V. The TiO₂ thin film was amorphous and could be transformed into anatase by annealing at 450 °C. On the basis of UV?Cvisible transmission spectra the bandgap of the thin film was calculated to be 3.12 eV, and its tail extended to 2.6 eV.     

N-doped NaTaO3: novel visible-light-driven photocatalysts synthesised by a sol–gel method

N-doped NaTaO₃ catalysts were synthesized via a sol–gel method followed by a subsequent calcination process under NH₃ atmosphere. The as prepared samples were characterized by XPS, XRD, UV–Vis DRS, and BET analyses. All XRD peaks of the sample calcined at 900 °C matched with pure perovskite NaTaO₃ while peaks of TaON and Na₂Ta₄O₁₁ were found for that calcined at 1,000 °C. The DRS of samples shown cutoff edge has red shifted, from 315 nm of pure to 391 nm of N-doped NaTaO₃. N-doping helps to narrow the band gap, and the prepared sample was visible light sensitive. The XPS spectrum of Ta4p3&N1s shown two new peaks at 398.3 and 401.4 eV appear in the N-doped sample corresponding to Ta–N bonds and adsorption nitride, respectively. Photocatalytic activity of the catalysts was evaluated using Rhodamine B dye. The result demonstrated that the sample calcined under NH₃ had a higher photocatalytic activity than that of P25 under visible light. The NaTaO₃/TaON heterojunction played an important role on promoting photoactivity.     

Prolonged Hot Electron Dynamics in Plasmonic‐Metal/Semiconductor Heterostructures with Implications for Solar Photocatalysis

Ideal solar‐to‐fuel photocatalysts must effectively harvest sunlight to generate significant quantities of long‐lived charge carriers necessary for chemical reactions. Here we demonstrate the merits of augmenting traditional photoelectrochemical cells with plasmonic nanoparticles to satisfy these daunting photocatalytic requirements. Electrochemical techniques were employed to elucidate the mechanics of plasmon‐mediated electron transfer within Au/TiO₂ heterostructures under visible‐light (λ>515 nm) irradiation in solution. Significantly, we discovered that these transferred electrons displayed excited‐state lifetimes two orders of magnitude longer than those of electrons photogenerated directly within TiO₂ via UV excitation. These long‐lived electrons further enable visible‐light‐driven H₂ evolution from water, heralding a new photocatalytic paradigm for solar energy conversion.     

Highly Selective Photoreduction of CO2 with Suppressing H2 Evolution over Monolayer Layered Double Hydroxide under Irradiation above 600 nm

Although progress has been made to improve photocatalytic CO₂ reduction under visible light (λ>400 nm), the development of photocatalysts that can work under a longer wavelength (λ>600 nm) remains a challenge. Now, a heterogeneous photocatalyst system consisting of a ruthenium complex and a monolayer nickel‐alumina layered double hydroxide (NiAl‐LDH), which act as light‐harvesting and catalytic units for selective photoreduction of CO₂ and H₂O into CH₄ and CO under irradiation with λ>400 nm. By precisely tuning the irradiation wavelength, the selectivity of CH₄ can be improved to 70.3 %, and the H₂ evolution reaction can be completely suppressed under irradiation with λ>600 nm. The photogenerated electrons matching the energy levels of photosensitizer and m‐NiAl‐LDH only localized at the defect state, providing a driving force of 0.313 eV to overcome the Gibbs free energy barrier of CO₂ reduction to CH₄ (0.127 eV), rather than that for H₂ evolution (0.425 eV).     

Photocatalytic activity of visible light-responsive TiO<Subscript>2</Subscript> thin films deposited on various anodized Ti-metal substrates by a RF magnetron sputtering method

Visible light-responsive TiO₂ (Vis-TiO₂) thin film has been deposited on two different kinds of anodized Ti-metal substrates, i.e., plate-type Titanystar (P-Titanystar) or non-woven fabric Titanystar (N-Titanystar) by a radio-frequency magnetron sputtering (RF-MS) method. These Vis-TiO₂ thin film photocatalysts (Vis-TiO₂/P-Titanystar and Vis-TiO₂/N-Titanystar) showed high activity for the oxidation of 2-propanol in water under UV light irradiation while Vis-TiO₂/N-Titanystar showed higher activity than Vis-TiO₂/P-Titanystar. Furthermore, photocatalytic activity of Vis-TiO₂/N-Titanystar was enhanced by the Pt deposition by an RF-MS deposition method. Pt loaded Vis-TiO₂/N-Titanystar (Pt-Vis-TiO₂/N-Titanystar) acted as an efficient photocatalyst for the oxidation of methanol vapor under UV light irradiation even in a flow system. Moreover, it was found that Pt-Vis-TiO₂/N-Titanystar is an effective photocatalyst for the oxidation of various organic compounds in gas phase or ammonia gas even under visible light irradiation (λ > 420 nm) at 293 K.     

Solvent effects on the photoelectrochemical properties of WO<Subscript>3</Subscript> and its application as dopamine sensor

Nanostructured WO₃ films were produced by a simple method using ammonium tungstate dissolved in different solvents: ethanol, PEG 300, and a mixture of ethylene glycol with PEG 300. The suspensions were deposited on an FTO substrate by drop casting method and calcined at 500 °C in air atmosphere. The films were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), UV–Vis diffuse reflectance spectroscopy, and photoelectrochemistry measurements. FTO substrates were fully covered by a thin and adherent WO₃ film, which presented a nanostructure with particle diameter of 30–80 nm. XRD confirms the monoclinic structure of WO₃. Ethanol samples presented higher photocurrent for water oxidation, compared to other solvents. However, these electrodes showed high fragility and the response did not present repeatability. High adhesion was obtained with PEG as solvent (by itself or mixed with ethylene glycol). In addition, WO₃ was applied as a photoelectrochemical sensor to detect dopamine under visible light irradiation. The developed sensor showed photosensitivity to dopamine with reproducibility, stability, wide linear range, and low detection limit (0.30 μM).     

Control of Surface Plasmon Resonance of Au/SnO2 by Modification with Ag and Cu for Photoinduced Reactions under Visible‐Light Irradiation over a Wide Range

Gold particles supported on tin(IV) oxide (0.2 wt % Au/SnO₂) were modified with copper and silver by the multistep photodeposition method. Absorption around λ=550 nm, attributed to surface plasmon resonance (SPR) of Au, gradually shifted to longer wavelengths on modification with Cu and finally reached λ=620 nm at 0.8 wt % Cu. On the other hand, the absorption shifted to shorter wavelength with increasing amount of Ag and reached λ=450 nm at 0.8 wt % Ag. These Cu‐ and Ag‐modified 0.2 wt % Au/SnO₂ materials (Cu‐Au/SnO₂ and Ag‐Au/SnO₂) and 1.0 wt % Au/SnO₂ were used for mineralization of formic acid to carbon dioxide in aqueous suspension under irradiation with visible light from a xenon lamp and three kinds of light‐emitting diodes with different wavelengths. The reaction rates for the mineralization of formic acid over these materials depend on the wavelength of light. Apparent quantum efficiencies of Cu‐Au/SnO₂, Au/SnO₂, and Ag‐Au/SnO₂ reached 5.5 % at 625 nm, 5.8 % at 525 nm, and 5.1 % at 450 nm, respectively. These photocatalysts can also be used for selective oxidation of alcohols to corresponding carbonyl compounds in aqueous solution under visible‐light irradiation. Broad responses to visible light in formic acid mineralization and selective alcohol oxidation were achieved when the three materials were used simultaneously.     

Ultrathin Metal–Organic Framework Nanosheets with Ultrahigh Loading of Single Pt Atoms for Efficient Visible‐Light‐Driven Photocatalytic H2 Evolution

A surfactant‐stabilized coordination strategy is used to make two‐dimensional (2D) single‐atom catalysts (SACs) with an ultrahigh Pt loading of 12.0 wt %, by assembly of pre‐formed single Pt atom coordinated porphyrin precursors into free‐standing metal–organic framework (MOF) nanosheets with an ultrathin thickness of 2.4±0.9 nm. This is the first example of 2D MOF‐based SACs. Remarkably, the 2D SACs exhibit a record‐high photocatalytic H₂ evolution rate of 11 320 μmol g^?1 h^?1 via water splitting under visible light irradiation (λ>420 nm) compared with those of reported MOF‐based photocatalysts. Moreover, the MOF nanosheets can be readily drop‐casted onto solid substrates, forming thin films while still retaining their photocatalytic activity, which is highly desirable for practical solar H₂ production.     

Ion engineering techniques for the preparation of the highly effective TiO<Subscript>2</Subscript> photocatalysts operating under visible light irradiation

The successful application of ion engineering techniques for the development of TiO₂ photocatalysts operating under visible and/or solar light irradiations has been summarized in this review article. First, we have physically doped various transition metal ions within a TiO₂ lattice on an atomic level by using an advanced metal ion implantation method. The metal ion implanted TiO₂ could efficiently work as a photocatalyst under visible light irradiation. Some field tests under solar light irradiation clearly revealed that the Cr or V ions implanted TiO₂ samples showed 2–3 times higher photocatalytic reactivity than the un-implanted TiO₂. Second, we have developed the visible light responsive TiO₂ thin film photocatalyst by a single process using an RF-magnetron sputtering (RF-MS) deposition method. The vis-type TiO₂ thin films showed high photocatalytic reactivity for various reactions such as reduction of NOx, degradation of organic compounds, and splitting of H₂O under visible and/or solar light irradiations.     

Construction of Pt complex within Zr-based MOF and its application for hydrogen production under visible-light irradiation

The present article deals with Pt complex construction within Zr-based MOF having bipyridine units in the framework (Zr-MOF-bpy-PtCl₂) and its photocatalytic activity for hydrogen production under visible-light irradiation (λ > 420 nm). Zr-MOF-bpy-PtCl₂ is prepared by the construction of a Zr-based MOF using 2,2′-bipyridine-5,5′-dicarboxylic acid (Zr-MOF-bpy), and subsequent complexation reaction with K₂PtCl₄. XRD and N₂ adsorption–desorption measurements have revealed that both Zr-MOF-bpy and Zr-MOF-bpy-PtCl₂ have a UiO-type structure. From the results of UV–Vis and XAFS measurements, the incorporated Pt species has been proven to be in square planar geometry involving two N atoms and two Cl atoms as a result of the Pt coordination with bipyridine units in the framework. Zr-MOF-bpy-PtCl₂ has been employed for a hydrogen production reaction from water containing a sacrificial electron donor under visible-light irradiation (λ > 420 nm). Zr-MOF-bpy-PtCl₂ realizes steady hydrogen production, and the amount of evolved hydrogen reaches 8.3 μmol after the 9 h reaction period, while Zr-MOF-bpy exhibits no photocatalytic activity under the same conditions. It has also been found that the activity of Zr-MOF-bpy-PtCl₂ is superior to that of the corresponding homogeneous complex analogue (bpy)PtCl₂.     

Visible‐Light‐Induced Electron Transport from Small to Large Nanoparticles in Bimodal Gold Nanoparticle‐Loaded Titanium(IV) Oxide

A key to realizing the sustainable society is to develop highly active photocatalysts for selective organic synthesis effectively using sunlight as the energy source. Recently, metal‐oxide‐supported gold nanoparticles (NPs) have emerged as a new type of visible‐light photocatalysts driven by the excitation of localized surface plasmon resonance of Au NPs. Here we show that visible‐light irradiation (λ>430 nm) of TiO₂‐supported Au NPs with a bimodal size distribution (BM‐Au/TiO₂) gives rise to the long‐range (>40 nm) electron transport from about 14 small (ca. 2 nm) Au NPs to one large (ca. 9 nm) Au NP through the conduction band of TiO₂. As a result of the enhancement of charge separation, BM‐Au/TiO₂ exhibits a high level of visible‐light activity for the one‐step synthesis of azobenzenes from nitrobenzenes at 25 °C with a yield greater than 95 % and a selectivity greater than 99 %, whereas unimodal Au/TiO₂ (UM‐Au/TiO₂) is photocatalytically inactive.     

Comparison of fullerene and large argon clusters for the molecular depth profiling of amino acid multilayers

A major challenge regarding the characterization of multilayer films is to perform high-resolution molecular depth profiling of, in particular, organic materials. This experimental work compares the performance of C₆₀ ⁺ and Ar₁₇₀₀ ⁺ for the depth profiling of model multilayer organic films. In particular, the conditions under which the original interface widths (depth resolution) were preserved were investigated as a function of the sputtering energy. The multilayer samples consisted of three thin δ-layers (~8 nm) of the amino acid tyrosine embedded between four thicker layers (~93 nm) of the amino acid phenylalanine, all evaporated on to a silicon substrate under high vacuum. When C₆₀ ⁺ was used for sputtering, the interface quality degraded with depth through an increase of the apparent width and a decay of the signal intensity. Due to the continuous sputtering yield decline with increasing the C₆₀ ⁺ dose, the second and third δ-layers were shifted with respect to the first one; this deterioration was more pronounced at 10 keV, when the third δ-layer, and a fortiori the silicon substrate, could not be reached even after prolonged sputtering. When large argon clusters, Ar₁₇₀₀ ⁺, were used for sputtering, a stable molecular signal and constant sputtering yield were achieved throughout the erosion process. The depth resolution parameters calculated for all δ-layers were very similar irrespective of the impact energy. The experimental interface widths of approximately 10 nm were barely larger than the theoretical thickness of 8 nm for the evaporated δ-layers.

Sol–gel synthesis of ZnO–SiO2 thin films: impact of ZnO contents on its photonic efficiency

Highly crystalline ZnO–SiO₂ films obtained by a sol–gel method at different ZnO contents were deposited on silicon substrate (P(100)) using spin coating process. The XRD results revealed that the strong ZnO(100) peak is grown with highly c-axis oriented film and the crystallinity is progressively improved with increasing ZnO contents. SEM micrographs of the films deposited on silicon substrate show a homogeneous and uniformity structure at different ZnO content. The prepared ZnO–SiO₂ films are compared with either a film prepared from a commercial photocatalysts Hombikat UV-100 or Pilkington Glass Activ? by the determination of their photonic efficiencies for degradation of methylene blue. The photocatalytic efficiency of the 10 wt% ZnO–SiO₂ film was found to be about four times higher than film prepared from UV-100 or Pilkington Glass Activ?. The photocatalytic efficiencies of ZnO–SiO₂ films are increased with increasing ZnO content from 1 wt% to 10 wt% ZnO and then decreased at 15 wt% ZnO. The order of photocatalytic efficiencies of ZnO–SiO₂ films at different ZnO content and commercial photocatalysts after 6 h illumination were as following: 10 wt% ZnO > 15 wt% ZnO > 1 wt% ZnO > as-prepared 10 wt% ZnO–SiO₂ film > UV-100 > Pilkington Glass Activ?, which suggested that the ZnO–SiO₂ films are photoactive than commercial photocatalysts. The improved efficiency and potentially the low-cost synthesis suggest that this material might be practically useful as a photocatalyst film.     

Effect of Pt loading on the photocatalytic reactivity of titanium oxide thin films prepared by ion engineering techniques

Platinum-loaded titanium oxide thin-film photocatalysts were prepared by using an ionized cluster beam (ICB) deposition method and a RF magnetron sputtering (RF-MS) deposition method as dry processes. From the results of the photocatalytic oxidation of acetaldehyde with O₂ under UV light irradiation, small amounts of Pt loading (less than 10 nm film thickness) were found to dramatically enhance the photocatalytic reactivity. However, when TiO₂ thin films were loaded with relatively larger amounts of Pt (more than 30 nm as the film thickness), the photocatalytic reactivity became lower than for the pure TiO₂ thin films. Moreover, investigations of the ratio of Pt loaded onto the surface of the thin film catalysts by XPS measurements revealed that the small amounts of Pt loaded exist as very small clusters working to efficiently enhance the charge separation, whereas, large amounts of Pt covers the entire surface of the TiO₂ thin films, resulting in a decrease of the photocatalytic reactivity.     

Optical, mechanical, and dielectric properties of Bi1/2Na1/2TiO3 thin film synthesized by sol–gel method

The Bi_1/2Na_1/2TiO₃ (BNT) thin film has been researched as an excellent candidate of lead-free ferroelectric materials. We synthesized BNT thin film on Si wafers or quartz glass by sol–gel spin coating method. The homogeneous and crack-free BNT thin film was synthesized by cost effective solution sol gel coating method. The main crystal phase of the film was identified as Bi_1/2Na_1/2TiO₃. The BNT thin film which was coated 3 times and heat-treated at 700 °C had about 70% of transmittance in the ultra-violet visible (UV–VIS) light wavelength region. The calculated band gap energies from the UV transmittance spectra were 3.0 and 3.5 eV for indirect and direct transition, respectively and the refractive index of BNT thin film was 2.16 at 898 nm of wavelength. The hardness and elastic modulus of the film were about 9 and 136 GPa at 10 mN load, where the penetration depth was about 220 nm. BNT thin film showed the diffuse type of dielectric properties due to its Na⁺ and Bi³⁺ ions in A′_1/2A″_1/2BO₃-type perovskite structure and the dielectric constant was about 10 until 300 °C and showed maximum value at 550 °C, 450 at 1 kHz.