Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals

<div class="section abstract"><div class="htmlview paragraph">In a photocatalytic air purifier system, the catalyst that cleans the air is typically titanium dioxide and it is energized by ultraviolet (UV) light. When UV light shines on the titanium dioxide, electrons (negatively charged particles inside atoms) are released at its surface. The electrons interact with water molecules (H<sub>2</sub>O) in the air, breaking them up into hydroxyl radicals (OH·), 9which are highly reactive, short-lived, uncharged forms of hydroxide ions (OH−). These small, agile hydroxyl radicals then attack bigger organic (carbon-based like virus) pollutant molecules, breaking apart their chemical bonds and turning them into harmless substances such as carbon dioxide and water.</div><div class="htmlview paragraph">Current investigation uses the above principle to kill living organic germs, bacteria; pathogen, etc. from the cabin air in recirculation mode. A HVAC system has been developed by using a filter impregnated by titanium di-oxide (TiO<sub>2</sub>) with UV lights to improve and maintain cabin air quality. The developed system has been developed to kill virus, germs, pathogens and bacteria that typically exist in a conditioned space. The designed system can be used for conventional vehicles, EVs, ride sharing and for autonomous vehicles. Tests were conducted at a certified laboratory with MS2, a bacteriophage size of 0.027 microns. MS2 is a proxy for SARS-CoV-2, the virus that causes COVID-19 with a size of 0.125 microns. Effectiveness of the destruction rate was determined for the developed system. Detailed summary will be presented in the paper.</div></div>

Black TiO2 attracts enormous attention due to its large solar absorption and induced excellent photocatalytic activity. Herein, a new approach assisted by hydrogen plasma to synthesize unique H‐doped black titania with a core/shell structure (TiO2@TiO2‐xHx) is presented, superior to the high H2‐pressure process (under 20 bar for five days). The black titania possesses the largest solar absorption (≈83%), far more than any other reported black titania (the record (high‐pressure): ≈30%). H doping is favorable to eliminate the recombination centers of light‐induced electrons and holes. High absorption and low recombination ensure the excellent photocatalytic activity for the black titania in the photo‐oxidation of organic molecules in water and the production of hydrogen. The H‐doped amorphous shell is proposed to play the same role as Ag or Pt loading on TiO2 nanocrystals, which induces the localized surface plasma resonance and black coloration. Photocatalytic water splitting and cleaning using TiO2‐xHx is believed to have a bright future for sustainable energy sources and cleaning environment.

Hydrogen is the ideal candidate to fulfill the growing energy demand in a sustainable manner because of its high energy content and no emission of greenhouse gases from its combustion. Currently most of hydrogen generation techniques involve the employment of fossil fuels, with consequent production of toxic greenhouse gases. The possibility to produce hydrogen by means of photocatalytic processes using the solar radiation as energy source fits in perfectly with the switch to a more sustainable energy production. The solar photocatalytic hydrogen generation can be achieved by reforming organic substances contained in civil or industrial wastewaters. This could allow to combine water decontamination with production of an energy carrier starting from a renewable source, the solar radiation. Hydrogen production through photoreforming of organic species using copper-modified TiO2 photocatalysts is attracting a considerable attention during last years. It is reported that the doping of TiO2 with copper species helps enhance to separate the electron-hole pairs, thus reducing the occurrence of the recombination reaction, and extend the light absorption to the visible range of the solar spectrum. The choice of copper is supported by its low-cost and abundance in Earth’s crust. In particular, the use of catalysts prepared by in situ photodeposition processes, with nanometric size, could represent a straightforward promising strategy to improve the process efficiency. In this study, the production of hydrogen by photocatalytic reforming of oxygenated organic species was investigated using metal copper-modified TiO2 nanoparticles, prepared “in situ” by reduction of cupric ions. The behavior of different alcohols and organic acids to undergo photoreforming with hydrogen production was investigated and compared. A characterization of the catalysts recovered at the end of the runs revealed the formation of zero-valent copper nanoparticles on the catalysts surface. The effect of adopting different crystallographic phases of TiO2 was also assessed. In particular, three TiO2 commercial samples of different crystalline phases (mixed-phase P25, pure anatase and pure rutile) were employed to prepare Cu-doped TiO2 materials by in situ copper photo-deposition. The resulting samples were extensively characterized by several complementary techniques and tested as photocatalysts for hydrogen production through photoreforming of alcohols. Correlations between hydrogen production rates and physical-chemical properties (structural, compositional and optical properties) of the samples are discussed. The analyses highlighted the major roles played by physical sizes and surface properties of TiO2 particles in determining the morphology, the dispersion of zero-valent copper nanoparticles on TiO2 surface and, ultimately, the photocatalytic performances. A modeling investigation was performed through the development of a simplified kinetic model taking into account the mass balance equations for the main reactive species involved in the photocatalytic system. The kinetic model was tested to predict hydrogen generation rates for experimental runs carried out at different initial concentrations of sacrificial agent (methanol and glycerol) and at varying photocatalyst load. The modeling investigation allowed to estimate for the first time the equilibrium adsorption constants and the kinetic constant for the hole-capture by sacrificial agents, as well as the quantum yield and the rate constant of electron-hole recombination for the copper modified-TiO2 nano-photocatalyst. The simultaneous presence in the aqueous matrix of an inorganic ion, that is chloride, was also investigated when formic acid was adopted as sacrificial agent. The effect on hydrogen generation rate of the initial concentrations of formic acid, chloride and cupric ion, and pH values was evaluated. These experimental outcomes were rationalized within a consistent reaction mechanism able to predict the system behavior under different operating conditions. Therefore, this critical literature review has been performed with the aim of providing a complete and reliable approach to promote new competitive processes able to use waste organic streams for hydrogen generation through photacatalytic system based on solar energy.

Semiconductor photocatalysis has been intensively studied in recent decades for a wide variety of application such as hydrogen production from water splitting and water and air treatment. The majority of photocatalysts are, however, wide band-gap semiconductors which are active only under UV irradiation. In order to effectively utilize visible solar radiation, this thesis investigates various types of visible-light active photocatalysts including metal ion-doped TiO₂, nanocomposites of potassium niobate (KNbO₃) and CdS with Ni co-catalyst, and a mixed-phase CdS matrix interlinked with elemental Pt deposits. Thirteen different metal ion-doped TiO₂ nanoparticles are synthesized. I compare the effects of individual dopants on the resulting physicochemical properties and corresponding photocatalytic activities with respect to the catalysis of several reactions under visible-light irradiation. I found several metal ion-doped Ti₂ nanoparticles such as Pt, Cr, and V had visible-light photocatalytic activities and the presence of rutile phase in these metal ion-doped TiO₂ may affect their photoreactivities. In addition, visible-light photocatalytic activities of TiO₂ are enhanced by co-doping with two metal ions. Hybrid nanocomposite photocatalysts based on CdS nanoparticles (e.g., Ni(0)/NiO/ KNbO₃/CdS, Zeolite/CdS, and nanocomposites of Q-sized cubic phase CdS and bulk-phase hexagonal CdS interlinked with elemental Pt deposits) are also studied. Different types of CdS nanocomposite photocatalysts are synthesized, optimized, and characterized using various analytical techniques. It is shown that these nanocomposites can enhance inherent photocatalytic activity of bulk-phase CdS for hydrogen production via effective charge separation of photogenerated electrons and holes in CdS under visible-light irradiation. Additionally, a sub-pilot size hybrid electrochemical system with Bi-doped TiO₂ anodes and SS cathodes for the degradation of organic pollutants and simultaneous hydrogen production has been developed to make the electrochemical system more economically viable. This system degrades a variety of organic pollutants and real wastewater with simultaneous production of hydrogen at the current efficiencies of 50~70%. Furthermore, it is demonstrated that this electrochemical system can be driven by a photovoltaic (PV) cell.

BACKGROUND: Nanoparticulate titanium dioxide (TiO2) has the advantages of high chemical stability, high photocatalytic activity to oxidise pollutants in air and water, relatively low price and non‐toxicity. However, its high surface energy leads to the aggregation of nanoparticles. In addition, the wide band gap of TiO2 (3.2 eV) only allows it to absorb ultraviolet (UV) light (<387 nm), which represents just a small fraction (3–5%) of the solar photons. These factors have limited its use in many fields. In this study, nanoparticulate TiO2 was modified by polyaniline (PANI) in order to enhance its photoactivity under UV light and sunlight illumination.RESULTS: TiO2 nanoparticles were modified by PANI via a chemical oxidative method. The introduction of small amounts of PANI enhanced the dispersion of TiO2 nanoparticles and improved the photocatalytic activity under UV light. In addition, the band gap energies of all PANI/TiO2 nanocomposites were lower than that of neat TiO2 nanoparticles, so the PANI/TiO2 nanocomposites can be excited to produce more electron–hole pairs under sunlight, which could result in higher photocatalytic activities.CONCLUSION: The modification of nanoparticulate TiO2 by PANI can increase its photoactivity in the process of phenol degradation under UV light and sunlight illumination. Copyright © 2008 Society of Chemical Industry

The influence of TiO 2 morphology and doped Pt on hydrogen production has been investigated. TiO 2 nanotubes (TiO 2 NT) were obtained by using combination of sonication and hydrothermal methods. Pt was doped on the surface of TiO 2 NT by using photo-deposition method. TiO 2 Degussa P25 nanoparticle was employed as comparison. Sonication was performed using ultrasonic cleaner for 60min then followed by hydrothermal treatment in a teflon lined stainless steel autoclave for 12 hours at 13 °C. TiO 2 nanotubes were characterized by means of X-Ray Powder Diffractometer (XRD), Scanning Electron Microscope (SEM), UV-Vis Diffuse Reflectane Spectroscopy (UV-Vis DRS) and Brunauer-Emmet-Teller (BET) technique. A pyrex reactor was employed to conduct hydrogen production while methanol was used as sacrificial agent. The result shows by using Pt/TiO 2 NT increased hydrogen production about 18 times than that of TiO 2 NT without Pt doped. Keywords: TiO 2 nanotubes, Platina, Hydrogen, Photocatalytic.

The role of co ₂ in improving sonic hydrogen production

Titanium dioxide (TiO2) coating was prepared through dipping stainless steel net into titanium dioxide sol and then extracting it. The photocatalytic activities for all titanium dioxide coatings were tested by methyl orange degradation under ultraviolet and visible light irradiation. The photo-absorption property was determined by UV-Vis spectrophotometer. The titanium dioxide coating is photo-catalytically reactive for the degradation of methyl orange. The photo-catalytic activity is influenced by extraction times, degradation time, doping element and light source. La-doped titanium dioxide exhibits the best photocatalytic activity in comparison with undoped, V-doped and La-V-codoped ones. The degradation rate of methyl orange by La-doped titanium dioxide coating reaches 92% after 70 minutes irradiation under ultraviolet light.

Evaluation of Pd-TiO2/ZSM-5 catalysts composition effects on hydrogen production by photocatalytic water splitting

A modified sol-gel process has been found to significantly improve the photocatalytic activity of TiO2 nanoparticle in the process of solar hydrogen production. The surface of TiO2 nanoparticles were modified by the optimization of solvent of titanium precursor (acetic acid and/or ethanol) in the sol-gel method. A multi technique approach (SEM, XRD, FTIR, UV-DRS and TGA) was used to characterize the prepared TiO2 nanoparticles. The photocatalytic hydrogen production was tested using a suspension of photocatalyst TiO2 at 10 vol. % methanol under natural solar light. The produced hydrogen was subjected to gas chromatography with a continuous flow of N2 in the photoreactor system. It was found that the TiO2 nanoparticles synthesized with acetic acid as the solvent of titanium precursor, TiO2-AA, have a better photocatalytic activity for hydrogen production compared to nanoparticles synthesized with ethanol, TiO2-EA. The obtained results showed that the better crystallinity, small size and proper surface properties of TiO2-AA nanoparticles is due to higher photoactivity.

Avoiding Sabatier’s conflict in bifunctional heterogeneous catalysts for the WGS reaction

Photocatalysis on semiconductor metal oxide surfaces has attracted considerable attention as a sustainable environmentally friendly method for water/air purification and hydrogen production by water splitting. Among semiconducting metal oxides TiO2 has been intensively investigated as a promising photocatalyst candidate. However, despite many efforts, its photocatalytic activity is far from a practical level mainly due to inefficient charge carrier separation and resulting charge carrier recombination. An advantageous strategy to address this issue is the development of heterostructures by coupling to a metal to form a Schottky junction or to metal oxides to create a p-n junction at their interface in order to prevent the recombination by vectorial charge carrier separation at these energy junctions. On the other hand it was revealed over the past decade that crystal facets play a decisive role in trapping of charge carriers and thus photocatalytic redox reactions. Thus, selective deposition of metal or metal oxides onto specific facets would enhance the photocatalytic activity by improving charge separation. To achieve higher activities, two methods, the supercritical fluid chemical deposition route and the photodeposition method, were investigated to deposit selectively p-type NiO onto specific facets of ntype TiO2 single crystalline nanoparticles to establish a p-n junction. The resulting NiO/TiO2 nanocrystals were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), N2 sorption measurements, UV-visible diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The heterojunction photocatalysts showed higher photocatalytic efficiency than pure TiO2 for the decomposition of organic dyes. Particularly, 0.1-0.25 wt % of NiO was the optimal loading amount, showing the highest activity. To elucidate the role of crystal facets of TiO2 and the effect of selective deposition of NiO, rutile (001), rutile (110), anatase (001), and anatase (101) surfaces with different surface states were prepared and their electronic properties were systematically compared by XPS and UPS measurements. Furthermore, water adsorption onto the different surfaces were also investigated. Regardless of surface stoichiometry, the Fermi level position of the anatase (001) surface is situated higher than that of the anatase (101) surface in energy while that of the rutile (001) surface is located lower than that of the rutile (110) surface. This can explain why photo-generated electrons and holes preferentially migrate to the (101) and (001) facets on TiO2 anatase crystals, respectively. Work function values of these oriented surfaces vary depending upon the surface states related to distribution and amount of oxygen vacancies as well as adsorbed oxygen peroxo species on the surface. In order to experimentally determine energy band alignments, interface experiments were performed by stepwisely depositing NiO onto above well-defined oriented TiO2 surfaces. The enhanced photocatalytic activity of NiO/TiO2 heterostructure nanoparticles were rationalized on the basis of the obtained band alignments. The information of electronic properties of different oriented TiO2 under various surface states would provide a new insight to construct the optimal energy band alignment of the heterostructure system with TiO2. In addition, the concept of heterojuction nanocrystals where co-catalysts are selectively deposited should find practical application to purify the environment and to sustainably produce renewable hydrogen.

When ultraviolet (UV) light comes into contact with titanium dioxide (TiO 2 ), a variety of free radicals are released to provide a potent oxidizing power. Few reports are available, however, evaluating the bactericidal effects of TiO 2 particle under UV light and fluorescent light (FL) in the same line of research for clinical applications. In the present study, we set out to evaluate the in vitro photocatalytic bactericidal effects on Staphylococcus aureus , which is one of the most common pathogens of infectious disease, in an aqueous system of TiO 2 particles irradiated by UV and FL. A TiO 2 particle mixture containing 0.019 mg/ml of TiO 2 was prepared. A bacterial solution was added dropwise to the mixture, and the resulting product was irradiated by UV or FL light. The colony‐forming units were counted and the bacterial survival rate was calculated. Control samples maintained a relatively high bacterial survival rate. In the TiO 2 mixture group, however, the bacterial survival rate decreased steadily, reaching 9.4% after 60 min of exposure to UV light and 10.9% after 60 min of FL irradiation. Distributing the TiO 2 particles in a water mixture produces highly efficient light absorption and enables greater and more frequent adhesion with bacteria, allowing a high degree of photocatalytic antibacterial action. Although the quantity was inferior to UV, our TiO 2 particles were able to show effective bactericidal activity even under FL. The TiO 2 particle mixture is expected to prove effective in preventing postoperative infection. Copyright © 2009 John Wiley & Sons, Ltd.

Hydrogen is a clean and renewable green energy source. Hydrogen is also widely considered to be the fuel of future. Over past decades, generation of hydrogen by low cost and environmental friendly ways became a popular and urgent issue for the world. As previously reported, hydrogen produced from photocatalytic or photoelectrochemical (PEC) routes have showed its strongly predominant compatibility for future developments. Titanium dioxide has been generally recognized as one of the most promising material candidate used in photocatalytic and PEC. However, this material, TiO2 was limited in the solar energy utilization due to its wide bandgap (3.2 eV). Therefore, it became great importance to loading/doping metals or other substances into TiO2 in order to enhance the hydrogen production by photocatalytic or photoelectrochemical (PEC) technologies. In this work, CuxO/TiO2 nanotubes were prepared and used as photoanodes in PEC splitting of water for hydrogen generation. Experimentally, highly ordered nanotubular TiO2/Ti substrates were firstly synthesized by the anodization technique. The incipient wetness impregnation was then employed to load copper oxides on the surface of TiO2 nanotubes. The effects of preparation conditions including calcination temperature and concentration of impregnation solution on the properties of CuxO nanoparticles such as crystalline structure, particles size distribution and loading amount were investigated. Furthermore, the bandgap and photoactivity of the CuxO/TiO2 photoanodes were charaterized by using reflectance UV/Vis analysis and PEC reaction. The experimental results showed that the final product was CuO with tenorite structure. The optimum preparation conditions occured at 0.01 M of initial Cu(NO3)2 concentration with impregnating for three times, and the optimized calcination temperature was 450°C in air. The Cu loading of this sample was determined as 0.6 μmole. In the PEC water spliting by using methanol as the sacrificial agent under xenon lamp illumination (1000W, 100mW/cm2), it was found that the maximum photoconversion efficiency, e.g., 0.48%, could be obtained, and the hydrogen generation rate was at 0.343 μmol/cm2.hr. Alternatively, electrodeposition method was in advance employed to deposit CuxO on the TiO2 surface. Effects of deposition variables including copper concentration in the deposition bath, applied voltage and deposition time on the morphology and microstructure of the resulting CuxO/TiO2 nanotubes were investigated. Besides, several alcohol-water mixtures were served as electrolytes for the PEC study. From the experimental results, it revealed that the optimal deposition conditions occured at applied voltage of 1V for 15 s with a 10.45 mM CuSO4 solution. In this case, the maximum photoconversion efficiency was 0.50%, and the hydrogen generation rate was 0.384 μmol/cm2.hr. The results concluded that maximum photoconversion efficiencies, as well as hydrogen generation rates, obtained by the two routes in this study were quite close. However, their performances were still far away from the industrial criteria. Many efforts should be paid to find more effective materials for hydrogen generation by PEC water splitting in future.

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