Formal Quantum Efficiency Research Articles

Treatment of waste using waste-derived materials and free energy: A practical concept of circular-economy

Controlling industrial effluent and treating it with improvised technologies continues to be a dominant research topic in the scientific community. In this study, the concepts of circular-economy and control-of-waste-by-waste were applied to the synthesis of a photocatalyst composite composed entirely of waste resources, namely waste rust (Fe2O3) and seashells (CaO). The ratio of these two waste materials was optimized to yield a composition 10% Fe2O3/90% CaO (10-FC) that was effective in decolorizing 85% of methylene blue in 60 min, with a formal quantum efficiency of 0.27% in direct sunlight. The heterogeneous photocatalyst was characterized by UV–Vis/NIR, SEM and XRD and is proposed to follow Langmuir-Hinshelwood kinetics and Z-scheme mechanism of charge transfer. Here, waste products were integrated and utilized to create a resource that is recommended for the treatment of dye effluent using a renewable energy source. This research aims to identify elements present in waste dumps and create a valuable resource from them to promote sustainable practices.

A TiO2 nanotubes film with excellent antireflective and near-perfect self-cleaning performances

SiO2/TiO2 composite films have been frequently used to realize the functions of self-cleaning and antireflection. Increasing the TiO2 volume ratio in SiO2/TiO2 composite film is beneficial to enhance the self-cleaning effect, while high TiO2 content leads to a strong Rayleigh scattering and depresses the antireflective performance, resulting in a bottleneck problem for the dual-functional application. Here, we have achieved a high-quality TiO2 nanotubes film with excellent antireflective and near-perfect self-cleaning performances. Ultrasound assisted pickling method has been developed to effectively prepare the well-dispersed protonated titanate nanotubes colloid. After spin-coating and annealing treatment, glass substrate coated with double-side TiO2 nanotubes film has a peak transmittance of 99.2 % and average transmittance of 97.4 % at 400−800 nm. Ultra-high porosity of TiO2 nanotubes film (80 %) and ultra-fine size of TiO2 nanotubes (8.6 nm in outer diameter) lead to excellent antireflective performance. With high UV absorptivity (80 % at 254 nm) and formal quantum efficiency of stearic acid (10.9 × 10−3), TiO2 nanotubes film shows near-perfect self-cleaning performance. A persistent anti-fogging ability is also presented. This study demonstrates the feasibility to fabricate pure TiO2 antireflective coating for glass substrate, extends application field of the classic TiO2 nanotubes, and sheds lights on the practical applications of high-powered TiO2 nanotube-based multi-functional films.

Zn and N Codoped TiO2 Thin Films: Photocatalytic and Bactericidal Activity.

We explore a series of Zn and N codoped TiO2 thin films grown using chemical vapor deposition. Films were prepared with various concentrations of Zn (0.4-2.9 at. % Zn vs Ti), and their impact on superoxide formation, photocatalytic activity, and bactericidal properties were determined. Superoxide (O2•-) formation was assessed using a 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-5-[(phenylamino)carbonyl]-2H-tetrazolium sodium salt (XTT) as an indicator, photocatalytic activity was determined from the degradation of stearic acid under UVA light, and bactericidal activity was assessed using a Gram-negative bacterium E. coli under both UVA and fluorescent light (similar to what is found in a clinical environment). The 0.4% Zn,N:TiO2 thin film demonstrated the highest formal quantum efficiency in degrading stearic acid (3.3 × 10-5 molecules·photon-1), while the 1.0% Zn,N:TiO2 film showed the highest bactericidal activity under both UVA and fluorescent light conditions (>3 log kill). The enhanced efficiency of the films was correlated with increased charge carrier lifetime, supported by transient absorption spectroscopy (TAS) measurements.

Catalytic and photocatalytic water gas shift reaction (WGSR) using a continuous flow, gas phase reactor

A series of different M/TiO2 catalysts are prepared, where M = Au, Pt, Pd and Ag, with a 0.2 wt% loading, and tested for activity as catalysts and photocatalysts for the water gas shift reaction, WGSR, using a continuous flow, gas phase reactor. For both the thermal (dark) and photocatalytic (UV irradiated) WGSR the best catalyst/photocatalyst is Au/TiO2. The kinetics of the WGSR is studied as a function of reactor temperature, Au loading and concentrations of CO and H2O. Langmuir-Hinshelwood kinetics are observed for the photocatalytic WGSR with non-competitive adsorption of the CO and H2O, whereas those for the thermal WGSR fit a slightly more complex, previously reported, reaction mechanism. At 125 °C, with a 0.2 wt% loading, the Au/TiO2 photocatalysis of the WGSR is very efficient, with a formal quantum efficiency for 1/2H2 production of 57 %. The potential scale up of the relatively efficient photocatalytic WGSR is discussed briefly.

Nanostructured titanium dioxide coatings prepared by Aerosol Assisted Chemical Vapour Deposition (AACVD)

Titanium dioxide is a compound of great interest, due to its functional properties; one of its most important uses is as a photocatalyst. TiO2 coatings can be deposited using different techniques. Aerosol Assisted Chemical Vapour Deposition (AACVD) is particularly interesting, as high temperature or pressure are not necessary to generate the gaseous precursors. Furthermore, by carefully choosing the deposition conditions (i.e. deposition temperature, solvent), it is possible to obtain deposits with different morphology and, consequently, different functional properties. In this paper we present the synthesis of titanium dioxide coatings with AACVD using complexes between titanium isopropoxide (TIPP) and acetyl acetone (acac) as precursors. Deposition experiments were performed using different ratios of TIPP to acac, to assess the effect on the composition of the coatings, their morphology and photocatalytic activity. Results showed that the use of acac led to nanostructured titanium dioxide (nanoparticles of about 10−25 nm diameter). Raman analysis showed the presence of both anatase and rutile phases. XPS analysis indicated the presence of residual carbonaceous species in the coatings; despite this, they displayed photocatalytic properties similar or superior to AACVD films without carbon. Photocatalytic tests, performed measuring the Formal Quantum Efficiency (FQE) and the Formal Quantum Yield (FQY) in the degradation of resazurin, showed that a acac:TIPP ratio equal to 1 led to the material with the highest performance, as the FQE value was about three times higher than that for the coating prepared with TIPP alone. Overall the complexes between TIPP and acac are promising precursors for the AACVD technique, leading to nanostructured coatings with enhanced performance.

Photoelectrochemical reduction of CO2 with TiNT

In order to reduce CO2 emissions and utilise CO2 as a useful by-product, artificial photosynthesis is being explored for carbon capture and utilisation. Semiconductor photocatalysts excited by solar energy may be used to convert CO2 to fuels or useful chemicals, e.g. CO, CH4, CH3OH. The photocatalytic reduction of CO2 to useful products has been widely studied in order to overcome the greenhouse effect and the current energy necessities. However, this reaction has proved to be extremely low efficiency when compared to other processes. In order to improve these yields, photoelectrochemical reduction of CO2 has been considered, since it combines photocatalysis and electrocatalysis. To this end, a two compartment photoelectrochemical cell (PEC) has been designed and fabricated for the reduction of CO2. This custom built reactor consists of dual phases, where gas phase CO2 is feed into the cathode compartment and an aqueous phase in the anode compartment. The anode and cathodes for a sandwich were the anode perforated foil on exposing aligned titania nanotubes to electrolyte bonded to nafion; which forms a bridge to Pt deposited carbon cloth cathode electrode in gas phase compartment. CO2 reduction products were detected with GC connected to the reactor. The PEC reactor improved the yield and the formal quantum efficiency compared to our previous studies using photocatalytic reactors.

Growth stages of nano-structured mixed-phase titania thin films and effect on photocatalytic activity

The development of nano-structured mixed-phase titania thin films and the relationship to photocatalytic activity (PCA) is of interest for pollution reduction and antimicrobial applications. A set of films grown by pulsed pressure metallorganic chemical vapour deposition using titanium tetraisopropoxide precursor was studied. The growth method is single-stage, scalable and produces high-quality, adherent films. Film thicknesses were 101 nm to 4.0 μm. Three growth stages were identified. All films were a mixture of anatase and rutile. Early stage films had rounded morphologies, low surface roughness and low PCA. Transition stage films developed columnar [110] orientated anatase dendrites, contained amorphous carbon and had reduced UV transmittance. Late stage films had strong anatase (220) and rutile (200) textures, contained amorphous carbon and exhibited highly branched anatase dendrites with nanoscale secondary plates. PCA was determined from degradation of stearic acid and correlated with increasing surface roughness. The most active film had formal quantum efficiency of (6.62 ± 0.64) × 10−4 molecules/photon, 59 times higher than a commercially available control. The performance is attributed to the combination of phases yielding polymorphic phase boundaries and amorphous carbon enhancing the adsorption of organic molecules, the unusual (220) anatase texture yielding nanostructured anatase dendrites in combination with nanocrystalline rutile and hierarchical porosity.

Low-Cost One-Step Fabrication of Highly Conductive ZnO:Cl Transparent Thin Films with Tunable Photocatalytic Properties via Aerosol-Assisted Chemical Vapor Deposition.

Low-cost, high-efficiency, and high quality Cl-doped ZnO (ZnO:Cl) thin films that can simultaneously function as transparent conducting oxides (TCOs) and photocatalysts are described. The films have been fabricated by a facile and inexpensive solution-source aerosol-assisted chemical vapor deposition technique using NH4Cl as an effective, cheap, and abundant source of Cl. Successful ClO substitutional doping in the ZnO films was evident from powder X-ray diffraction, X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry results, while scanning electron microscopy reveals the impact of Cl doping on the ZnO thin film morphology. All ZnO:Cl films deposited were transparent and uncolored; optical transmittance in the visible region (400–700 nm) exceeded 80% for depositions using 5–20 mol % Cl. Optimal electrical properties were achieved when using 5 mol % Cl with a minimum measured resistivity of (2.72 ± 0.04) × 10–3 Ω·cm, in which the charge carrier concentration and mobility were measured at (8.58 ± 0.16) × 1019 cm–3 and 26.7 ± 0.1 cm2 V–1 s–1 respectively, corresponding to a sheet resistance (Rsh) of 41.9 Ω□–1 at a thickness of 650 nm. In addition to transparent conducting properties, photocatalytic behavior of stearic acid degradation in the ZnO:Cl films was also observed with an optimal Cl concentration of 7 mol % Cl, with the highest formal quantum efficiency (ξ) measured at (1.63 ± 0.03) × 10–4 molecule/photon, while retaining a visible transparency of 80% and resistivity ρ = (9.23 ± 0.13) × 10–3 Ω·cm. The dual functionality of ZnO:Cl as both a transparent conductor and an efficient photocatalyst is a unique combination of properties making this a particularly unusual material.

TiO₂ Films Modified with Au Nanoclusters as Self-Cleaning Surfaces under Visible Light.

In this study, we applied cluster beam deposition (CBD) as a new approach for fabricating efficient plasmon-based photocatalytic materials. Au nanoclusters (AuNCs) produced in the gas phase were deposited on TiO2 P25-coated silicon wafers with coverage ranging from 2 to 8 atomic monolayer (ML) equivalents. Scanning Electron Microscopy (SEM) images of the AuNCs modified TiO2 P25 films show that the surface is uniformly covered by the AuNCs that remain isolated at low coverage (2 ML, 4 ML) and aggregate at higher coverage (8 ML). A clear relationship between AuNCs coverage and photocatalytic activity towards stearic acid photo-oxidation was measured, both under ultraviolet and green light illumination. TiO2 P25 covered with 4 ML AuNCs showed the best stearic acid photo-oxidation performance under green light illumination (Formal Quantum Efficiency 1.6 × 10−6 over a period of 93 h). These results demonstrate the large potential of gas-phase AuNCs beam deposition technology for the fabrication of visible light active plasmonic photocatalysts.

Photocatalytic treatment of dye wastewater and parametric study using a novel Z-scheme Ag2CO3/SiC photocatalyst under natural sunlight

Ag2CO3 is a known photocatalyst active in the visible region of solar spectrum. In the present study, a series of novel hybrid Ag2CO3/SiC nanostructures have been successfully synthesised through a simple precipitation route. The photocatalytic performance was evaluated by the degradation of MB under natural solar irradiation. It was observed that heterojunction with SiC improves its photoactivity by inducing a charge transfer between SiC and Ag2CO3 mimicking the Z-scheme in photosynthesis, proved by scavenger studies. The photocatalysts were characterized by XRD, SEM with EDX, TEM, TGA-DTG and UV-Vis/NIR. The best photocatalytic results and formal quantum efficiency (FQE) under natural sunlight were obtained with SCSC-12 nano-composite. The percentage photo-discolouration of MB over SCSC-12 was 98% against 90% with conventional TiO2. Factors viz., photocatalyst dosage, solution pH, solar intensity, substrate and its initial concentration and speed of agitation were found to influence photo-treatment. Corresponding optimum parametric values have been found and reported in terms of FQE. As a case study, performance of the photocatalyst was investigated on a real industrial effluent and rate expression developed in terms of TOC and [TOC]0. The present work, which includes a parametric study, has been carried out in direct solar light and is expected to be useful in real-time solar photocatalytic design of reactors and other solar applications for environmental remediation.

Order of magnitude increase in photocatalytic rate for hierarchically porous anatase thin films synthesized from zinc titanate coatings.

In this paper we report on the use of aerosol assisted chemical vapour deposition (AACVD) to form thin films of the zinc titanate phases using zinc acetate and titanium isopropoxide as precursors in methanol solution. Analysis by XRD and XPS found that through variation in experimental conditions we have been able to synthesize films of zinc titanate with composition of Zn2TiO4 or Zn0.3Ti2.7O4.94, which adopt the spinel and pseudobrookite structure respectively. In addition, we have also formed hybrid films of Zn2TiO4 with either ZnTiO3 or ZnO. Using a technique previously reported with powders, the mixed ZnO and Zn2TiO4 films were treated with acid to produce porous Zn2TiO4 which, through reduction and vapour leaching of zinc, were converted to hierarchically porous thin films of anatase TiO2. This conversion was monitored by XRD. Analysis of photocatalytic activity of the hierarchically porous titania, using dye and stearic acid degradation tests, found a factor of 12 to 14 increase in rates of photocatalysis over conventional TiO2 thin films. Finally we are able to report a maximum formal quantum efficiency for stearic acid degradation of 1.76 × 10-3 molecules per photon.

Highly Photocatalytically Active Iron(III) Titanium Oxide Thin films via Aerosol‐Assisted CVD

AbstractThis paper presents, for the first time, the synthesis, via aerosol‐assisted (AA)CVD followed by annealing at 620 °C for 5 h, of pure Fe2TiO5 thin films on glass. The thin film is deposited from a one pot solution containing titanium isopropoxide and tris(acetylacetonato)iron in an ethyl acetate solvent. The film is characterized using a range of techniques including powder X‐ray diffraction(XRD), wavelength dispersive X‐ray (WDX) spectroscopy, X‐ray photoelectron spectroscopy(XPS), scanning electron microscopy (SEM), and UV‐vis spectroscopy. The photocatalytic activity of the film under UVA and visible light irradiation is also tested. The results show that Fe2TiO5 is able to degrade resazurin redox dye under UVA illumination at a rate much higher than Pilkington NSG ActivTM, with a formal quantum efficiency (FQE) an order of magnitude superior.

Chemically grown TiO2 on glass with superior photocatalytic properties

TiO2 anatase and Degussa (Enovik) P25 thin films were deposited on glass using the sol–gel technique at 600 and 80°C, respectively. The structural, morphological and optical properties of the as-grown TiO2 films were studied against their thickness, for both of the experimental procedures described. Moreover, the photocatalytic activity of the TiO2 samples was investigated regarding the degradation of stearic acid and the decolorization of methylene blue, under UV-A light illumination. In specific, the 90nm-thick (4×) Degussa P25 TiO2 films deposited on glass at 80°C is by far the more photocatalytically active against methylene blue decolorization, reaching an apparent rate constant of 0.094min−1, while the photocatalytic activity of the 110nm-thick (4×) TiO2 thin films deposited on glass at 600°C by sol–gel show the highest stearic acid disappearance rate, i.e., 9.46×10−8mol/min (formal quantum efficiency=10.02×10−3) at 15min of UV-A light illumination.

Photocatalytic and electrooxidation properties of TiO2 thin films deposited by sol–gel

Anatase TiO2 thin films are deposited on glass using alcoholic solutions at 400–600°C. The effect of film thickness, grain size and annealing temperature on the catalytic activity of the TiO2 thin films is investigated regarding the degradation of octadecanoic (stearic) acid under UV-A light illumination. Specifically, the photocatalytic activity of 150-nm-thick TiO2 thin films deposited on glass at 500°C is quite remarkable, showing the highest octadecanoic acid disappearance rate, i.e., 6.28×10−8mol/min (formal quantum efficiency=6.65×10−3) at 30min of UV-A light illumination. Moreover, the electrochemical performance of the same sample is evaluated by studying the electrocatalytic oxidation of methanol in an alkaline medium in dark and under UV illumination. Under UV illumination, the methanol electrooxidation is significantly improved, as evidenced by the current–potential measurements presenting a satisfactory incident photon to charge carrier efficiency of 88.3% with a fill factor of 0.51.

PbO-Modified TiO2 Thin Films: A Route to Visible Light Photocatalysts

PbO clusters were deposited onto polycrystalline titanium dioxide (anatase) films on glass substrates by aerosol-assisted chemical vapor deposition (AACVD). The as-deposited PbO/TiO2 films were then tested for visible light photocatalysis. This was monitored by the photodegradation of stearic acid under visible light conditions. PbO/TiO2 composite films were able to degrade stearic acid at a rate of 2.28 × 10(15) molecules cm(-2) h(-1), which is 2 orders of magnitude greater than what has previously been reported. The PbO/TiO2 composite film demonstrated UVA degradation of resazurin redox dye, with the formal quantum yield (FQY) and formal quantum efficiency (FQE) exceeding that of a TiO2 film grown under the same conditions and Pilkington Activ, a commercially available self-cleaning glass. This work correlates with computational studies that predicted PbO nanoclusters on TiO2 form active visible light photocatalysts through new electronic states through PbO/TiO2 interfacial bonds resulting in new electronic states above the valence band maximum in TiO2, shifting the valence band upward as well as more efficient electron/hole separation with hole localization on PbO particles and electron on the TiO2 surface.

The use of pulsed laser deposited seed layers for the aqueous solution growth of highly oriented ZnO nanowires on sapphire substrates at 95 °C: Study of their photocatalytic activity in terms of octadecanoic (stearic) acid degradation

The photocatalytic activity of ZnO nanowires grown via an aqueous solution approach at 95°C onto sapphire substrates, which were first pre-coated with ZnO seed layers deposited by pulsed laser deposition at various temperatures (400–700°C), was investigated against the degradation of octadecanoic (stearic) acid under UV-A light illumination. The crystallinity of the ZnO seed layers, as well as their growth orientation along the c-axis, increases with deposition temperature. All chemically grown ZnO samples show narrower (002) reflections with significantly higher intensity than the seed layers deposited by pulsed laser deposition. The grain size, as well as the seed layer roughness, gradually increases with substrate deposition temperature. The nanowires’ diameter also rises with seed layer's deposition temperature, ranging from ∼40±15nm for 400°C to ∼155±65nm for 700°C. The chemically grown ZnO nanowires exhibit relatively high transmittance values, especially those grown on sapphire substrates, which were pre-coated with seed layers deposited by pulsed laser deposition at 400 and 500°C. All ZnO samples, including seed layers and nanowires’ arrays, show very good photocatalytic activity regarding the degradation of octadecanoic (stearic) acid under UV-A light exposure. In specific, the photocatalytic activity of ZnO nanowires’ arrays grown on top of a seed layer deposited by pulsed laser deposition at 600°C is quite remarkable, showing the highest stearic acid disappearance rate, i.e., 4.22×10−8mol/min (formal quantum efficiency=4.48×10−3) at 30min of UV-A light illumination.

Aerosol assisted chemical vapour deposition of hydrophobic TiO2–SnO2 composite film with novel microstructure and enhanced photocatalytic activity

Aerosol assisted chemical vapour deposition (AACVD) was used to synthesise a TiO2–SnO2 composite film onto a glass substrate. For comparison a TiO2 film and a SnO2 film were also prepared. All films were characterised by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and wavelength dispersive X-ray spectroscopy (WDX). XPS and WDX of the composite film revealed a TiO2 rich film with a high level of SnO2 segregation at the surface. Highly structured pyramid-like features gave rise to hydrophobic films with static water contact angles of 134°. Photocatalytic activities were determined by monitoring the degradation of intelligent ink (containing Resazurin redox dye) via UV-visible spectroscopy. Under UVA irradiation, the TiO2 film only began to degrade the dye after being irradiated in excess of 100 minutes, whereas the composite TiO2–SnO2 film required only 6 minutes of irradiation before degradation was observed. The formal quantum efficiency (FQE) for the TiO2–SnO2 composite was determined to be 1.01 × 10−2 molecules per incident photon and the formal quantum yield (FQY) was 1.17 × 10−2 molecules per absorbed photon. This is an order of magnitude superior to Pilkington Activ™ self-cleaning glass a commercial self-cleaning TiO2 coating on glass. This improved photocatalytic activity is attributed to the presence of electron scavenging SnO2 sites that increase charge separation and the increased surface area due to the highly structured morphology.

Photocatalytic removal of taste and odour compounds for drinking water treatment

Photocatalytic degradation of geosmin and 2-methylisoborneol (MIB), which are two taste and odour compounds commonly found in drinking water supply sources, was investigated using an immobilised TiO2 photoreactor. It was found that the degradation of geosmin and MIB followed similar pseudo-first-order kinetics with reaction rate constants being approximately 0.025 min−1 for typical geosmin and MIB concentrations of 250 and 500 ng/L. The normalised formal quantum efficiency was calculated to be in the range of 162–182 L/mol. Influence of additives (i.e. sodium bicarbonate and alcohols) on the degradation process was also investigated. It was found that there was a small reduction in the degradation rate constants of geosmin and MIB with increasing sodium bicarbonate concentration. At 50 mg/L sodium bicarbonate the degradation rate constants decreased by approximately 5%. Similarly, for methanol and ethanol concentrations up to 35 and 50 mg/L, respectively, these constants were found to also decrease. While addition of sodium bicarbonate and alcohols was seen to have relatively small negative effects on the photocatalytic degradation performance, the magnitude of their influence was consistent with the hypothesis that the degradation mechanism of geosmin and MIB was predominately that of attack involving HO∙ radicals.

Photocatalytic oxidation of soot by P25 TiO 2 films

The photomineralisation of soot by P25 titania films is studied using FT-IR and the process shown to involve the oxidation of carbon to CO 2 exclusively. The efficiency of this process is low, however, with a formal quantum efficiency of 1.1 × 10 −4 molecules of carbon oxidized per incident photon of UVA light. The cause of this low efficiency is attributed largely to the less than intimate contact between the fibrous soot layer and the surface of the photocatalyst.

The photo-oxidation of water by sodium persulfate, and other electron acceptors, sensitised by TiO 2

A number of different electron acceptors are tested for efficacy in the oxidation of water to oxygen, photocatalysed by titanium dioxide. The highly UV-absorbing metal ion electron acceptors, Ce 4+ and Fe 3+, appear ineffective at high concentration (10 −2 M), due to UV-screening, but more effective at lower concentrations (10 −3 M). The metal-depositing electron acceptor, Ag +, is initially effective, but loses activity upon prolonged irradiation due to metal deposition which promotes electron–hole recombination as well as UV-screening the titania particles. Most striking of the electron acceptors tested is persulfate, particularly in alkaline solution (0.1 M NaOH). The kinetics of the photo-oxidation of water by persulfate, photocatalysed by titania are studied as a function of pH, [S 2O 8 2−] and incident light intensity ( I). The initial rate of water oxidation increases with pH, is directly proportional to the concentration of persulfate present and depends upon I 0.6. The TiO 2/alkaline persulfate photosystem is robust and shows very little evidence of photochemical wear upon repeated irradiation. The results of this work are discussed with regard to previous work in this area and current mechanistic thinking. The formal quantum efficiency of the TiO 2/alkaline persulfate photosystem was estimated as ca. 2%.