Publications

Journal article

Heiba HF, Bullen JC, Kafizas A, Petit C, Fearn S, Skinner SJ, Weiss DJet al., 2024,

Engineered Sn-TiO2@SnO2 and SnO2@Sn-TiO2 heterophotocatalysts for enhanced As(III) remediation: a comprehensive bulk and surface characterization and precise photocatalytic oxidation rates determination

, Colloids and Surfaces A: Physicochemical and Engineering Aspects, Vol: 702, ISSN: 0927-7757

Arsenite, As(III), is a highly toxic form of arsenic that poses a significant risk to human health if present in drinking water. Oxidation of As(III) to the less toxic As(V) using TiO2 as photocatalyst is an attractive solution in water treatment applications but challenged the high bandgap energy. In this study, we investigate the potential of doping TiO2 with Sn to reduce the bandgap and hence to improve the photocatalytic oxidation (PCO). To this end, we studied first the effect of varying Sn:TiO2 molar doping on the structure of the newly synthesized SnO2@TiO2 and Sn-TiO2@SnO2 hetero photocatalysts. We found that at low Sn:TiO2 doping ratios (0.1Sn:1TiO2), SnO2 tends to float on the surface and form a coat around the TiO2 (SnO2@Sn-TiO2), whereas at higher doping ratio (1Sn:1TiO2) a Sn-TiO2 coat forms alongside SnO2 clusters in the core of the catalyst (Sn-TiO2@SnO2). We assessed the PCO and observed significant shifts to lower conduction and valence band edge energies and a reduction of the bandgap at higher doping ratios. The smallest bandgap was 2.87 eV with a doping ratio of 1Sn:1TiO2. Sn-TiO2@SnO2 and as SnO2@Sn-TiO2 improved the PCO of TiO2 by ∼30 and 46 %, respectively. We finally determined the rate constant (k) for the As(III) oxidation using a combination of spectrochemical and surface sensitive techniques and determined for a 1Sn:1TiO2 (i.e. Sn-TiO2@SnO2) catalyst a value of 0.055 ±0.002 min−1, i.e., 78 folds faster than using only TiO2. We conclude that Sn doping of TiO2 is a very promising approach for improving the PCO of As(III) in water treatment.

Journal article

Zhao S, Jia C, Shen X, Li R, Oldham L, Moss B, Tam B, Pike SD, Harrison N, Ahmad EA, Kafizas Aet al., 2024,

The aerosol-assisted chemical vapour deposition of Mo-doped BiVO4 photoanodes for solar water splitting: an experimental and computational study

, Journal of Materials Chemistry A, ISSN: 2050-7488

BiVO4 is one of the most promising light absorbing materials for use in photoelectrochemical (PEC) water splitting devices. Although intrinsic BiVO4 suffers from poor charge carrier mobility, this can be overcome by Mo-doping. For Mo-doped BiVO4 to be applied in commercial PEC water splitting devices, scalable routes to high performance materials need to be develop. Herein, a scalable aerosol-assisted chemical vapour deposition (AA-CVD) route to high performance Mo-doped BiVO4 is developed. The materials were characterised using X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM), UV-visible absorption spectroscopy, and a range of PEC tests. By studying a range of Mo-precursor doping levels (0 to 12% Mo: V), an optimum precursor doping level was found (6% Mo: V); substituting V5+ sites in the host structure as Mo6+. In PEC water oxidation the highest performing material showed an onset of photocurrent (Jon) at ~0.6 VRHE and a theoretical solar photocurrent (TSP) of ~1.79 mA.cm-2 at 1.23 VRHE and 1 sun irradiance. Importantly, Mo-doping was found to induce a phase change from monoclinic clinobisvanite (m-BiVO4), found in undoped BiVO4, to tetragonal scheelite (t-BiVO4). The effect of Mo-doping on the phase stability, structural and electronic properties was examined with all-electron hybrid exchange density functional theory (DFT) calculations. Doping into V and Bi sites at 6.25 and 12.5 at.% was performed for t-BiVO4 and m-BiVO4 phases. In accord with our observations, 6.25 at.% Mo doping into the V sites in t-BiVO4 is found to be energetically favoured over doping into m-BiVO4 (by 2.33 meV / Mo atom inserted). The computed charge density is consistent with n-doping of the lattice as Mo6+ replaces V5+ generating an occupied mid-gap state ~0.4 eV below the conduction band minimum (CBM) which is primarily of Mo-4d character. Doubling this doping level to 12.5 at.% in t-B

Journal article

Itskou I, Kafizas A, Nevjestic I, Carrero SG, Grinter DC, Azzan H, Kerherve G, Kumar S, Tian T, Ferrer P, Held G, Heutz S, Petit Cet al., 2024,

Effects of phosphorus doping on amorphous boron nitride’s chemical, sorptive, optoelectronic, and photocatalytic properties

, The Journal of Physical Chemistry C, Vol: 128, Pages: 13249-13263, ISSN: 1932-7447

Amorphous porous boron nitride (BN) represents a versatile material platform with potential applications in adsorptive molecular separations and gas storage, as well as heterogeneous and photo-catalysis. Chemical doping can help tailor BN’s sorptive, optoelectronic, and catalytic properties, eventually boosting its application performance. Phosphorus (P) represents an attractive dopant for amorphous BN as its electronic structure would allow the element to be incorporated into BN’s structure, thereby impacting its adsorptive, optoelectronic, and catalytic activity properties, as a few studies suggest. Yet, a fundamental understanding is missing around the chemical environment(s) of P in P-doped BN, the effect of P-doping on the material features, and how doping varies with the synthesis route. Such a knowledge gap impedes the rational design of P-doped porous BN. Herein, we detail a strategy for the successful doping of P in BN (P-BN) using two different sources: phosphoric acid and an ionic liquid. We characterized the samples using analytical and spectroscopic tools and tested them for CO2 adsorption and photoreduction. Overall, we show that P forms P–N bonds in BN akin to those in phosphazene. P-doping introduces further chemical/structural defects in BN’s structure, and hence more/more populated midgap states. The selection of P source affects the chemical, adsorptive, and optoelectronic properties, with phosphoric acid being the best option as it reacts more easily with the other precursors and does not contain C, hence leading to fewer reactions and C impurities. P-doping increases the ultramicropore volume and therefore CO2 uptake. It significantly shifts the optical absorption of BN into the visible and increases the charge carrier lifetimes. However, to ensure that these charges remain reactive toward CO2 photoreduction, additional materials modification strategies should be explored in future work. These strategies could include the

Journal article

Jeong SB, Heo KJ, Yoo JH, Kang D-G, Santoni L, Knapp CE, Kafizas A, Carmalt CJ, Parkin IP, Shin JH, Hwang GB, Jung JHet al., 2024,

Photobiocidal activity of TiO2/UHMWPE composite activated by reduced graphene oxide under white light

, Nano Letters: a journal dedicated to nanoscience and nanotechnology, Vol: 24, Pages: 9155-9162, ISSN: 1530-6984

Herein, we introduce a photobiocidal surface activated by white light. The photobiocidal surface was produced through thermocompressing a mixture of titanium dioxide (TiO2), ultra-high-molecular-weight polyethylene (UHMWPE), and reduced graphene oxide (rGO) powders. A photobiocidal activity was not observed on UHMWPE-TiO2. However, UHMWPE-TiO2@rGO exhibited potent photobiocidal activity (>3-log reduction) against Staphylococcus epidermidis and Escherichia coli bacteria after a 12 h exposure to white light. The activity was even more potent against the phage phi 6 virus, a SARS-CoV-2 surrogate, with a >5-log reduction after 6 h exposure to white light. Our mechanistic studies showed that the UHMWPE-TiO2@rGO was activated only by UV light, which accounts for 0.31% of the light emitted by the white LED lamp, producing reactive oxygen species that are lethal to microbes. This indicates that adding rGO to UHMWPE-TiO2 triggered intense photobiocidal activity even at shallow UV flux levels.

Journal article

Reddick C, Sotelo-Vazquez C, Tam B, Kafizas A, Reynolds K, Stanley S, Creasey G, Hankin A, Pablos C, Marugán Jet al., 2024,

Photoelectrochemical disinfection efficiency of WO3-based photoanodes: development of multifunctional photoelectrocatalytic materials

, Catalysis Today, Vol: 437, ISSN: 0920-5861

Access to safe water is a growing global concern, with millions lacking acceptable water sources. Photocatalysis offers eco-friendly water remediation, yet its combination with electrocatalysis for both water treatment and hydrogen production remain underexplored. This study investigates UVA LED photoelectrocatalysis using WO3-based photoanodes, alone or in heterojunction with BiVO4, to purify wastewater and co-produce hydrogen. Tests on polluted water streams containing 105 PFU mL−1 of MS2 bacteriophage virus and 106 CFU mL−1 of E. coli reveal that nanostructured WO3 achieves rapid MS2 disinfection within 5 min. (k= 0.80 min−1), with enhanced efficiency over flat counterparts. However, nanostructuring does not improve E. coli inactivation due to bacterium size constraints. These findings advance the design of tandem photoreactors for dual wastewater purification and energy generation.

Journal article

Lin Z-P, Li Y, Haque SA, Ganose AM, Kafizas Aet al., 2024,

Insights from experiment and machine learning for enhanced TiO₂ coated glazing for photocatalytic NOₓ remediation

, Journal of Materials Chemistry A, Vol: 12, Pages: 13281-13298, ISSN: 2050-7488

In this study, 58 distinct TiO2-coated glass samples were synthesized via Atmospheric Pressure Chemical Vapour Deposition (APCVD) under controlled synthesis conditions. The crystal properties, optical properties, surface properties and photogenerated charge carrier behaviour of all synthesized samples were characterized by X-ray diffraction (XRD), UV-visible spectroscopy, atomic force microscopy (AFM), and transient absorption spectroscopy (TAS), respectively. The photocatalytic activity of all coatings was systematically assessed against NO gas under near-ISO (22 197-1:2016) test conditions. The most active TiO2 coating showed ∼22.3% and ∼6.6% photocatalytic NO and NOx conversion efficiency, respectively, with this being ∼60 times higher than that of a commercial self-cleaning glass. In addition, we compared the accuracy of different machine learning strategies in predicting photocatalytic oxidation performance based on experimental data. The errors of the best strategy for predicting NO and NOx removal efficiency on the entire data set were ±2.20% and ±0.92%, respectively. The optimal ML strategy revealed that the most influential factors affecting NO photocatalytic efficiency are the sample surface area and photogenerated charge carrier lifetime. We then successfully validated our ML predictions by synthesising a new, high-performance TiO2-coated glass sample in accordance with our ML simulated data. This sample performed better than commercially available self-cleaning glass under a new metric, which comprehensively considered the visible light transmittance (VLT), NO degradation rate and NO2 selectivity of the material. Not only did this research provide a panoramic view of the links between synthesis parameters, physical properties, and NOx removal performance for TiO2-coated glass, but also showed how ML strategies can guide the future design and production of more effective photocatalytic coatings.

Journal article

Tam B, Babacan O, Kafizas A, Nelson Jet al., 2024,

Comparing the net-energy balance of standalone photovoltaic-coupled electrolysis and photoelectrochemical hydrogen production

, Energy and Environmental Science, Vol: 17, Pages: 1677-1694, ISSN: 1754-5692

Photovoltaic-coupled electrolysis (PV-E) and photoelectrochemical (PEC) water splitting are two options for storing solar energy as hydrogen. Understanding the requirements for achieving a positive energy balance over the lifetime of facilities using these technologies is important for ensuring sustainability. While neither technology has yet reached full commercialisation, they are also at very different technology readiness levels and scales of development. Here, we model the energy balance of standalone large-scale facilities to evaluate their energy return on energy invested (ERoEI) over time and energy payback time (EPBT). We find that for average input parameters based on present commercialised modules, a PV-E facility shows an EPBT of 6.2 years and ERoEI after 20 years of 2.1, which rises to approximately 3.7 with an EPBT of 2.7 years for favourable parameters using the best metrics amongst large-scale modules. The energy balance of PV-E facilities is influenced most strongly by the upfront embodied energy costs of the photovoltaic component. In contrast, the simulated ERoEI for a PEC facility made with earth abundant materials only peaks at 0.42 after 11 years and about 0.71 after 20 years for facilities with higher-performance active materials. Doubling the conversion efficiency to 10% and halving the degradation rate to 2% for a 10-year device lifetime can allow PEC facilities to achieve an ERoEI after 20 years of 2.1 for optimistic future parameters. We also estimate that recycling the materials used in hydrogen production technologies improves the energy balance by 28% and 14% for favourable-case PV-E and PEC water splitting facilities, respectively.

Journal article

Yang G, Zhou Y, Wang M, Murawski J, Oldham L, Tian T, Stephens IEL, Kafizas Aet al., 2023,

Elucidating the effect of nitrogen occupancy on the hydrogen evolution reaction for a series of titanium oxynitride electrocatalysts

, ChemCatChem, Vol: 15, ISSN: 1867-3880

Titanium nitride (TiN) shows desirable properties for use as an electrocatalyst and catalyst support, as it possesses high electrical conductivity and excellent corrosion resistance. Any oxygen and humidity present or incomplete nitridation during the synthesis process of nitrides can lead to an increasing oxygen content. However, the role of oxygen contents or nitrogen occupancies in the bulk of the nitrides during the electrocatalytic reactions is not well understood. In this work, we have synthesised a series of titanium oxynitrides with varied bulk nitrogen occupancies by ammonolysis at different temperatures. Higher ammonolysis temperatures will give a higher nitrogen occupancy but result in a lower surface area. The geometric activities towards the hydrogen evolution reaction (HER) have been normalised by the electrochemically active surface areas (ECSA) and the BET surface areas to get the specific activities. Their specific activity towards the HER is found to be strongly correlated with the bulk nitrogen occupancy and a higher bulk nitrogen occupancy is beneficial to the specific HER activities.

Journal article

Wilson AA, Shalvey TP, Kafizas A, Mumtaz A, Durrant JRet al., 2023,

Analysis of charge trapping and long lived hole generation in SrTiO<sub>3</sub> photoanodes

, SUSTAINABLE ENERGY & FUELS, Vol: 7, Pages: 5066-5075, ISSN: 2398-4902

Charge carrier dynamics studies of SrTiO3 under applied bias offer the opportunity to gain unique insights into what underpins its state-of-the-art photocatalytic water splitting activity. Herein, time resolved spectroscopic measurements are employed, to investigate the impact of applied bias on the transient and steady state charge carrier dynamics of SrTiO3 across μs–s timescales, and simultaneously measure charge extraction kinetics. A high density of Ti3+ defect states in SrTiO3 photoanodes are identified and associated with prevalent electron trapping into deep states, which is in competition with electron extraction and limits the photocurrent. Despite the high density of trapped electrons, an intrinsically long lifetime for photogenerated holes in SrTiO3 photoanodes is observed using transient absorption spectroscopy, even in the absence of applied bias. This is important for overcoming the slow kinetics and hole accumulation associated with the water oxidation reaction, and for enabling good performance in photocatalytic systems where bias cannot be applied.

Journal article

Quan Y, YiO MHN, Li Y, Myers RJ, Kafizas Aet al., 2023,

Influence of Bi co-catalyst particle size on the photocatalytic activity of BiOI microflowers in Bi/BiOI junctions - a mechanistic study of charge carrier behaviour

, Journal of Photochemistry and Photobiology A: Chemistry, Vol: 443, ISSN: 1010-6030

Herein, we investigate the effect of Bi particle size in BiOI/Bi junctions on their photocatalytic function towards NO gas. BiOI microflowers (BiOI) and BiOI microflowers decorated with micron-sized Bi particles (BiOI/Bi MPs) were produced by a solvothermal method. BiOI decorated with nano-sized Bi particles (BiOI/Bi NPs) were produced by a reduction process. All samples were physically characterised by XRD, FT-IR, SEM, HR-TEM coupled with EDX analysis, DR-UV–visible and PL spectroscopy and functionally characterised by photocatalytic testing towards NO gas, TAS and EPR spectroscopy.Their photocatalytic activity towards NO gas was measured following ISO protocol (ISO 22197–1:2016). The best performing BiOI-based sample was BiOI/Bi NPs, showing NO and NOx conversion efficiencies of ∼33 and ∼11% under UVA light, and ∼26 and ∼8.1% under visible light, respectively. The BiOI and BiOI/Bi MPs samples showed significantly lower activities, displaying overall NOx conversion efficiencies of ∼3.5 and ∼0.8% under UVA light, respectively. Importantly, the best performing BiOI/Bi NPs samples showed visible light activity that was at least 6 times higher than that of a commercial TiO2 benchmark (CristalACTiVTM PC-S7). TAS measurements showed that charge carriers were significantly longer lived in the BiOI/Bi NPs sample (t50% from 10 μs of ∼90 μs) than the BiOI and BiOI/Bi MPs samples (t50% from 10 μs of ∼50 μs). This was attributed to the significant degree of interfacial contact formed between Bi and BiOI in the BiOI/Bi NPs sample, which enhanced charge carrier separation. EPR studies showed that this interfacial contact between BiOI and Bi likely promoted the formation of VO, which may have contributed to enhancement seen in photocatalytic activity in the BiOI/Bi junction.

Journal article

Meng Z, Pastor E, Selim S, Ning H, Maimaris M, Kafizas A, Durrant JR, Bakulin AAet al., 2023,

Operando IR optical control of localized charge carriers in BiVO4 photoanodes

, Journal of the American Chemical Society, Vol: 145, Pages: 17700-17709, ISSN: 0002-7863

In photoelectrochemical cells (PECs) the photon-to-current conversion efficiency is often governed by carrier transport. Most metal oxides used in PECs exhibit thermally activated transport due to charge localization via the formation of polarons or the interaction with defects. This impacts catalysis by restricting the charge accumulation and extraction. To overcome this transport bottleneck nanostructuring, selective doping and photothermal treatments have been employed. Here we demonstrate an alternative approach capable of directly activating localized carriers in bismuth vanadate (BiVO4). We show that IR photons can optically excite localized charges, modulate their kinetics, and enhance the PEC current. Moreover, we track carriers bound to oxygen vacancies and expose their ∼10 ns charge localization, followed by ∼60 μs transport-assisted trapping. Critically, we demonstrate that localization is strongly dependent on the electric field within the device. While optical modulation has still a limited impact on overall PEC performance, we argue it offers a path to control devices on demand and uncover defect-related photophysics.

Journal article

Wang M, Kafizas A, Sathasivam S, Blunt MO, Moss B, Gonzalez-Carrero S, Carmalt CJet al., 2023,

ZnO/BiOI heterojunction photoanodes with enhanced photoelectrochemical water oxidation activity

, Applied Catalysis B: Environmental, Vol: 331, ISSN: 0926-3373

ZnO/BiOI heterojunction photoanode thin films were prepared by aerosol-assisted chemical vapour deposition, and the impact of growth temperature and film thickness on the water oxidation functionality was systematically investigated. A top ZnO layer with a thickness of 120 nm (deposited at 350 °C) and a 390 nm thick BiOI layer (deposited at 300 °C) were found to achieve the best photoelectrochemical performance of the heterojunction. The ZnO/BiOI heterojunction exhibited a significant increase in photoelectrochemical activity, with a photocurrent of 0.27 mA·cm−2 observed at 1.1 VRHE (350 nm, 2.58 mW·cm−2), which is ~ 2.2 times higher than that of single-layer ZnO and far higher than that of BiOI. Photoluminescence spectroscopy and transient absorption spectroscopy measurements showed that there was effective charge transfer across the heterojunction which spatially separated charge carriers and increased their lifetime and ability to drive photoelectrochemical water oxidation.

Conference paper

Creasey G, McCallum T, O'Neill L, Rodriguez Acosta J, Kafizas A, Hankin Aet al., 2023,

Materials and reactor development for photoelectrochemical hydrogen production

, Materials for Sustainable Development Conference (MATSUS), Publisher: Fundació de la comunitat valenciana SCITO

Journal article

Nasser SMT, Rana AA, Doffinger R, Kafizas A, Khan TA, Nasser Set al., 2023,

Elevated free interleukin-18 associated with severity and mortality in prospective cohort study of 206 hospitalised COVID-19 patients

, Intensive Care Medicine Experimental, Vol: 11, ISSN: 2197-425X

BackgroundDivergence between deterioration to life-threatening COVID-19 or clinical improvement occurs for most within the first 14 days of symptoms. Life-threatening COVID-19 shares clinical similarities with Macrophage Activation Syndrome, which can be driven by elevated Free Interleukin-18 (IL-18) due to failure of negative-feedback release of IL-18 binding protein (IL-18bp). We, therefore, designed a prospective, longitudinal cohort study to examine IL-18 negative-feedback control in relation to COVID-19 severity and mortality from symptom day 15 onwards.Methods662 blood samples, matched to time from symptom onset, from 206 COVID-19 patients were analysed by enzyme-linked immunosorbent assay for IL-18 and IL-18bp, enabling calculation of free IL-18 (fIL-18) using the updated dissociation constant (Kd) of 0.05 nmol. Adjusted multivariate regression analysis was used to assess the relationship between highest fIL-18 and outcome measures of COVID-19 severity and mortality. Re-calculated fIL-18 values from a previously studied healthy cohort are also presented.ResultsRange of fIL-18 in COVID-19 cohort was 10.05–1157.7 pg/ml. Up to symptom day 14, mean fIL-18 levels increased in all patients. Levels in survivors declined thereafter, but remained elevated in non-survivors. Adjusted regression analysis from symptom day 15 onwards showed a 100 mmHg decrease in PaO2/FiO2 (primary outcome) for each 37.7 pg/ml increase in highest fIL-18 (p < 0.03). Per 50 pg/ml increase in highest fIL-18, adjusted logistic regression gave an odds-ratio (OR) for crude 60-day mortality of 1.41 (1.1–2.0) (p < 0.03), and an OR for death with hypoxaemic respiratory failure of 1.90 [1.3–3.1] (p < 0.01). Highest fIL-18 was associated also with organ failure in patients with hypoxaemic respiratory failure, with an increase of 63.67 pg/ml for every additional organ supported (p < 0.01).ConclusionsElevated free IL-

Journal article

Anagnostopoulou M, Zindrou A, Cottineau T, Kafizas A, Marchal C, Deligiannakis Y, Keller V, Christoforidis KCet al., 2023,

MOF-Derived Defective Co3O4 Nanosheets in Carbon Nitride Nanocomposites for CO2 Photoreduction and H2 Production

, ACS APPLIED MATERIALS & INTERFACES, Vol: 15, Pages: 6817-6830, ISSN: 1944-8244

Journal article

Jiamprasertboon A, Kafizas A, Hawkins E, Singsen S, Butburee T, Wannapaiboon S, Sangkhun W, Nijpanich S, Eknapakul T, Chanlek N, Waehayee A, Suthirakun S, Siritanon Tet al., 2023,

Photocatalytic NOₓ oxidation of BiOCl nanostructure-based films grown using aerosol-assisted chemical vapor deposition

, ACS Applied Nano Material, Vol: 6, Pages: 738-749, ISSN: 2574-0970

Coating of photocatalytic nanomaterials on various surfaces enables interesting applications. This work demonstrates the ability of the aerosol-assisted chemical vapor deposition (AACVD) approach to prepare high-quality BiOCl nanostructure-based films and also to tune the nanostructure and photocatalytic properties of the films by varying the solvent and carrier gas. Solvents have a dramatic impact on the surface morphologies and crystallite size. X-ray diffraction (XRD) and grazing incidence X-ray diffraction (GIXRD) analyses indicate that BiOCl crystals displayed preferential growth in the (101) plane in most samples, while both the (101) and (102) planes were favored in films deposited using ethyl acetate and methanol. Surface energy and adsorption energy calculation reveal that the preferred growth depends on the interaction between the Bi atom and solvent molecules. X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDS) characterizations showed that all films did not contain any impurity elements but did contain some oxygen vacancies. The obtained nanostructured BiOCl films show good photocatalytic properties. The highest photocatalytic NOx removal efficiency is achieved in the film prepared using ethyl acetate and air, which we attribute to the large crystallite size and therefore high mobility of the carriers. Herein, we show that different crystal morphologies and sizes of BiOCl have strong impacts on the photocatalytic activity toward NO oxidation, and both factors can be effectively tuned in the AACVD process. Such knowledge may be useful for future research on coating materials for resolving environmental problems.

Journal article

Wong Y, Li Y, Lin Z, Kafizas Aet al., 2022,

Studying the effects of processing parameters in the aerosol-assisted chemical vapour deposition of TiO2 coatings on glass for applications in photocatalytic NOx remediation

, Applied Catalysis A: General, Vol: 648, Pages: 1-12, ISSN: 0926-860X

Herein, we employ an aerosol-assisted method (AA-CVD) to produce TiO2 on window glass and study how the process parameters affect their photocatalytic activity towards NOx (NO + NO2) remediation. A range of process parameters are explored to produce 50 unique TiO2 coatings with wide ranging physicochemical properties. The physicochemical properties were examined using X-ray diffraction (XRD), atomic force microscopy (AFM), UV–visible transmission spectroscopy and transient absorption spectroscopy (TAS), and the photocatalytic activity towards NO gas was measured using protocol akin to the ISO (22197-1:2016). The most active sample showed an NO removal of ∼14.4 ± 1.7 % and NOx removal of ∼5.4 ± 0.77 %, which was ∼40 and ∼25 times higher than that of a commercially available self-cleaning window. The links between the process parameters, physicochemical properties and photocatalytic activity were studied in depth, where it was seen that the three most influential physicochemical properties on the observed activity were surface roughness, charge carrier population and charge carrier lifetime. Therefore, we recommend that these properties be targeted in the rational design of more active coatings for applications in photocatalytic NOx remediation.

Journal article

Schukraft GEM, Moss B, Kafizas AG, Petit Cet al., 2022,

Effect of band bending in photoactive MOF-based heterojunctions.

, ACS Applied Materials and Interfaces, Vol: 14, Pages: 19342-19352, ISSN: 1944-8244

Semiconductor/metal-organic framework (MOF) heterojunctions have demonstrated promising performance for the photoconversion of CO2 into value-added chemicals. To further improve performance, we must understand better the factors which govern charge transfer across the heterojunction interface. However, the effects of interfacial electric fields, which can drive or hinder electron flow, are not commonly investigated in MOF-based heterojunctions. In this study, we highlight the importance of interfacial band bending using two carbon nitride/MOF heterojunctions with either Co-ZIF-L or Ti-MIL-125-NH2. Direct measurement of the electronic structures using X-ray photoelectron spectroscopy (XPS), work function, valence band, and band gap measurements led to the construction of a simple band model at the heterojunction interface. This model, based on the heterojunction components and band bending, enabled us to rationalize the photocatalytic enhancements and losses observed in MOF-based heterojunctions. Using the insight gained from a promising band bending diagram, we developed a Type II carbon nitride/MOF heterojunction with a 2-fold enhanced CO2 photoreduction activity compared to the physical mixture.

Journal article

Bullen JC, Heiba HF, Kafizas A, Weiss DJet al., 2022,

Parasitic light absorption, rate laws and heterojunctions in the photocatalytic oxidation of arsenic(III) using composite TiO2/Fe2O3

, Chemistry: A European Journal, Vol: 28, ISSN: 0947-6539

Composite photocatalyst-adsorbents such as TiO2/Fe2O3 are promising materials for the one-step treatment of arsenite contaminated water. However, no previous study has investigated how coupling TiO2 with Fe2O3 influences the photocatalytic oxidation of arsenic(III). Herein, we develop new hybrid experiment/modelling approaches to study light absorption, charge carrier behaviour and changes in the rate law of the TiO2/Fe2O3 system, using UV-Vis spectroscopy, transient absorption spectroscopy (TAS), and kinetic analysis. Whilst coupling TiO2 with Fe2O3 improves total arsenic removal by adsorption, oxidation rates significantly decrease (up to a factor of 60), primarily due to the parasitic absorption of light by Fe2O3 (88% of photons at 368 nm) and secondly due to changes in the rate law from disguised zero-order kinetics to first-order kinetics. Charge transfer across this TiO2-Fe2O3 heterojunction is not observed. Our study demonstrates the first application of a multi-adsorbate surface complexation model (SCM) towards describing As(III) oxidation kinetics which, unlike Langmuir-Hinshelwood kinetics, includes the competitive adsorption of As(V), and we further highlight the importance of parasitic light absorption and catalyst fouling when designing heterogeneous photocatalysts for As(III) remediation.

Journal article

Heiba HF, Bullen JC, Kafizas A, Petit C, Skinner SJ, Weiss Det al., 2022,

The determination of oxidation rates and quantum yields during the photocatalytic oxidation of As(III) over TiO2

, Journal of Photochemistry and Photobiology A: Chemistry, Vol: 424, Pages: 113628-113628, ISSN: 1010-6030

The determination of reaction rates for the photocatalytic oxidation (PCO) of arsenite (As(III)) using TiO2 under UV radiation is challenging due to the numerous experimental processes. This includes chemical processes running simultaneously with PCO (e.g. adsorption of arsenic species, direct UV photolysis of As(III)) and the analytical approach used (e.g. whether As(III) or As(V) are measured and used in the calculation of the PCO rate). The various experimental approaches used to date have led to oxidation rates and rate constants which vary by orders of magnitude and contradicting information on rate laws. Here we present the results of a critical examination of possible controls affecting the experimental determination of PCO rates. First, we demonstrate that the choice of analytical technique is not critical, provided that the rate constants are calculated based on the depletion of As(III) after correction of the directly adsorbed As(III). Second, we show the correction of the directly adsorbed As(III) at each time interval is best done by running two parallel experiments (one under UV and the other in dark) instead of running sequential experiment (i.e. running the experiment in the dark then turning on the UV lamp). These findings are supported by XPS analysis of the oxidation state of TiO2-sorbed As. Third, we demonstrate that photolysis by the light source itself, as well as the chemical composition of the solution (i.e. the effect of HEPES and the ionic strength), can significantly increase As(III) oxidation rates and need to be corrected. Finally, to determine the quantum yield of As(III) oxidation, we measured the photon absorption by the TiO2 photocatalyst. Our results showed that the quantum yield (Ø) for this oxidation reaction was low, and in the region of 0.1 to 0.2 %.

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Materials and reactor development for photoelectrochemical hydrogen production

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