Symposium FJ
Advanced Photocatalytic Materials for Energy and Chemistry in Transition and for the Environment


FJ-1:IL03  Nanostructure Design for Solar Water Splitting
F. Caddeo, Z. Durmus, B. Mahmoudi, F. Himmelstein, D. Eberhart, T. Lindenberg, H. Zhang, R. Naumann, A.W. Maijenburg, Center for Innovation Competence SiLi-Nano, Martin-Luther-University Halle-Wittenberg, Halle (Saale), Germany

The research of my group is focused on the design and synthesis of different nanostructures for solar water splitting. Within the group, we are currently investigating several different synthesis procedures (e.g. electrodeposition, anodization, electrospinning, surfactant-driven self-assembled soft templates, co-evaporation and atomic layer deposition) in order to obtain optimized nanostructures of several different materials (e.g. Metal-Organic Frameworks (MOFs), Cu2O, CuBi2O4, BiVO4, Si, CuGaSe2/CuGa3Se5, g-C3N4 and CoP). Furthermore, next to the standard structural characterization techniques like SEM and XRD, we are performing photoelectrochemical (PEC) and gas chromatography (GC) measurements (and a combination of the two) for the functional characterization of our photo-active samples. In addition, we are also using techniques like (Photo-)Electrochemical Impedance Spectroscopy ((P)EIS) in order to understand the physical phenomena that take place in our semiconductor materials and nanostructures. Within this presentation, I will focus on several of our recent achievements, namely (i) the synthesis of core-shell Cu2O@MOF nanowires, (ii) the development of g-C3N4/MOF heterostructures and (iii) the successful application of CuGaSe2/CuGa3Se5 films for solar water splitting.

FJ-2:IL02  Semiconductor Induced Nitrate Radicals Formation and Applications for Selective Photocatalytic Syntheses
F. Parrino, S. Dirè, R. Ceccato, Department of Industrial Engineering, University of Trento, Trento, Italy; L. Palmisano, Department of Engineering, University of Palermo, Palermo, Italy; S. Livraghi, E. Giamello, Dipartimento di Chimica and NIS, University of Torino, Torino, Italy

The possibility of generating nitrate radicals by means of heterogeneous photocatalysis has been often neglected due to the photolabile nature of these species and to the negligible influence of nitrate ions on the photocatalytic degradation of many organic pollutants. In the present paper some evidences of the formation of nitrate radicals in irradiated TiO2 suspensions are presented and, on this basis, it has been proposed that nitrate radicals can be generated by hole induced oxidation of nitrate ions. The oxidizing power of nitrate radicals is similar to that of OH radicals so that it is not surprising that both these species can induce similar degradation efficiency of some pollutants. However, nitrate radicals also possess a highly specific chemistry which can be exploited for novel organic and inorganic photocatalytic green syntheses. In particular, the presence of nitrate radicals selectively addresses the oxidation of olefine compounds toward the formation of alkyl esters of nitrate through the highly specific interaction with the double bond. Moreover, nitrate radicals effectively scavenge bromide ions, selectively producing elemental bromine. These reactions can be considered as paradigmatic examples opening the route to novel photocatalytic syntheses.

FJ-2:IL04  Charge Carriers Dynamics in WO3/BiVO4 Heterojunction Photoanodes
I. Grigioni, M.V. Dozzi, E. Selli, Dipartimento di Chimica, Università degli Studi di Milano, Milano, Italy

Efficient photoanodes can be obtained by combining bismuth vanadate with tungsten trioxide in a WO3/BiVO4 heterojunction, by merging the excellent visible light harvesting properties of BiVO4 with the superior conductivity of WO3. Under selective BiVO4 excitation, electron transfer from photoexcited BiVO4 to WO3 occurs immediately after excitation, as demonstrated by the increase of the trapped holes lifetime in BiVO4 and confirmed by tracking electron transfer processes by transient absorption mid infrared spectroscopy. A recombination channel opens instead when both oxides are simultaneously excited, as confirmed by the lower photocurrent enhancement attained upon short wavelength excitation in incident photon to current efficiency measurements. The efficiency of the heterojunction is also discussed in relation to the thickness of the two oxide layers and of different doping extents. Transient absorption measurements in operando conditions reveal that the bias dependent alteration of intra band gap states in BiVO4, evidenced by electrochromic measurements, play a key role under an applied anodic potential.

FJ-2:IL05  Antibacterial Properties of Ca2Fe2O5 Brownmillerite for Water Disinfection
A. Šutka1, M. Vanags1, L. Mezule2, 1Institute of Materials and Surface Engineering, Faculty of Materials Science and Applied Chemistry, Riga Technical University, Riga, Latvia; 2Water Research and Environmental Biotehnology Laboratory, Faculty of Civil Engineering, Riga Technical University, Riga, Latvia

Brownmillerite Ca2Fe2O5 has been reported as active visible light photocatalyst. Here we demonstrate that the Ca2Fe2O5 in water is an efficient radical generation material under dark conditions. Effecient hydroxyl radical generation in water provide seven order of magnitude decrease in bacterial concentration over 10 min. Hydroxil radical generation is attributed to the structural transformations and release of interlayer Ca2+ from the layered structure of Ca2Fe2O5. The efficacy of Ca2Fe2O5 is demonstrated by disinfecting turbid sewage sludge. The results indicate an obvious disinfection of the sludge in the presence of Ca2Fe2O5 nanoparticles, with a 3.47 log decrease in E. coli CFU over 40 min. The identification of this cheap, abundant, and nontoxic antibacterial material may open an opportunity for broad scale clean water generation globally.

FJ-2:IL06  Quantifying Charge Transport and Recombination in Photoelectrodes
P. Cendula, A. Sancheti, P. Simon, G. Cibira, E. Service, T. Moehl, University of Zilina, Zilina, Slovakia

The efficiency of most semiconductor photoelectrodes for water splitting is limited by a slow reaction with electrolyte and a fast recombination. Among the recombination pathways, trap-mediated recombination due to doping and material processing is often the dominant process. Impedance spectroscopy under illumination is a popular and powerful method to investigate time constants of recombination and reaction. Traditionally, an equivalent circuit is involved in the analysis of impedance spectroscopy, nevertheless, it provides limited direct information about the microscopic quantities related to diffusion, reaction and recombination. These quantities can be related to the impedance data with a physical model, which we developed and validated on TiO2-protected silicon and Cu2O photoelectrodes. Different interfaces can be attributed to voltage windows of the impedance data. Time constant extracted from impedance data is related to recombination lifetime if depletion capacitance is not dominant, otherwise it lacks physical meaning due to the influence of capacitive decay time of spatially separated carriers.
References: 10.1021/acs.jpcc.9b07244, 10.1002/aenm.202003569

FJ-2:IL07  Molybdenum Doped CuWO4-based Photoanodes for Solar Energy Conversion
M.V. Dozzi, A. Polo, C. Nomellini, I. Grigioni, E. Selli, Dipartimento di Chimica, Università degli Studi di Milano, Milano, Italy

CuWO4 (band gap = 2.3 eV) has recently gained increasing interest in water oxidation, as a much better visible light absorber and more stable oxide alternative to WO3. However, an efficient use of CuWO4 as photoanode material requires to overcome its severe internal charge recombination, as ascertained in our recent PEC investigation on this material coupled with ultrafast transient absorption analysis. In this work, this crucial issue of CuWO4 photoanodes is addressed by adopting a partial Mo6+ for W6+ substitution strategy resulting in CuW1-xMoxO4 electrodes with a greatly enhanced visible light-induced photoactivity compared to pure CuWO4. We optimized both the film thickness in the wide 250-700 nm range and the Mo6+ for W6+ substitution degree, by adopting an aqueous-based deposition procedure onto fluorine-doped tin oxide (FTO) glass substrates. Furthermore, aiming at enhancing the charge carrier separation within the photoanodes, the best performing Mo-doped films were combined either in heterojunctions with BiVO4 or with both WO3 and BiVO4. The optimal conditions ensuring a better performance of the coupled systems is discussed in relation to the competitive cross-back electron-hole recombination occurring at the semiconductor/semiconductor heterojunctions.

FJ-2:IL09  Multiredox Catalysis on Metal Oxides for Solar Water Splitting
C.A. Mesa, Institute of Advanced Materials (INAM), Universitat Jaume I, Castellón, Spain

Artificial photosynthesis, inspired by natural photosynthesis, is considered a promising technology to store solar energy into chemical bonds (e.g., hydrogen or carbon-based fuels) via (photo)electrochemical water splitting or CO2 reduction. In this process, the oxygen evolution reaction (OER) acts as electron donor and it is considered to be the bottleneck of the process when using metal-oxide (photo)anodes. The efficiency of these (photo)catalysts does not only depend on the nature of the metal oxide, but also on their physical characteristics such as composition, doping variations, defects density, amongst others. However, the mechanism of the OER on metal oxides as well as the nature of the efficiency loses remains elusive. In this talk, I will present recent advances on the understanding of the kinetics of OER on different metal oxide photo(electro)catalysts, focusing particularly on Earth-abundant metal oxides. A detailed mechanistic analysis of the OER on hematite photoanodes as well as the effect of intragap states from oxygen vacancies in the reaction kinetics. These findings suggest that understanding the underlaying mechanisms of charge accumulation are key to further developments in the artificial photosynthesis field.

FJ-2:IL10  Tailoring the Electron and Energy Transfer Mechanisms in Heterogeneous Photocatalysis by TiO2 Silanization: A Promising Opportunity for Emerging Technological Applications
M. D'ARIENZO1, F. Parrino2, R. Scotti1, L. Palmisano3, S. Dirè2, M. Bellardita3, R. Ceccato2, S. Mostoni1, B. Di Credico1, 1Department of Materials Science (INSTM), University of Milano-Bicocca, Milano, Italy; 2Department of Industrial Engineering (DII), University of Trento, Trento, Italy; 3Department of Engineering, University of Palermo, Palermo, Italy

Photocatalytic transformations in the presence of TiO2 are generally considered in terms of interfacial electron transfer. However, more elusive energy transfer driven reactions have been also hypothesized, mainly referring to detected reaction products, whose existence could not be simply justified by electron transfer. In this context, recent studies pointed out that 1O2 production occurs at a higher extent upon TiO2 surface modification with metal complexes, fluoride, and other organic molecules, envisaging a leading role for energy-transfer-produced 1O2 in the mechanism. Prompted by this background, we investigated and compared the fate of photogenerated charge carriers in naked and organosilane-modified TiO2 (TiO2@Si) by tracking, in particular, the formation of superoxide radicals as a result of electron transfer using Electron Spin Resonance (ESR). Results revealed that the surface functionalization, besides hindering recombination phenomena, effectively hampers the O2- stabilization in TiO2@Si, corroborating a pivotal contribution of energy-transfer-produced 1O2 to the photocatalytic process. This foreshadows the chance to switch from electron to energy transfer procedures for several emerging applications, like photocatalytic syntheses or photoactivated nanocomposites.

FJ-2:IL11  Post-excitation Transient IR Phenomena in a-Fe2O3 Films
A. Suligoj, D. Grinberg, Y. Paz, Department of Chemical Engineering, Technion, Haifa, Israel

Hematite (α-Fe2O3) is one of the most studied materials for electrochemical water splitting and photovoltaic applications. The common concept is based on formation of small electron polarons within a few picoseconds, having a lifetime of up to a few nanoseconds. In this work, Step Scan Transient IR spectroscopy (SS-TRIR) was used to follow transient spectral changes in the IR spectrum of hematite following pulsed excitation. The transient spectrum resembled the spectrum of maghemite (-Fe2O3) suggesting similar local distortion following excitation. The most pronounced change was the appearing of an absorption peak at 640 cm-1, whose intensity was highest 40-50 ns from excitation and its lifetime was found to be in the order of a few hundreds of ns, i.e. considerably longer than what is usually considered as carriers' lifetime in hematite. The intensity of the 640 cm-1 peak was found to change with the film thickness in a manner that correlated with the photoinduced current measured by linear sweep voltammetry. This correlation demonstrates that transient IR spectroscopy in the nanoseconds range may be useful as a tool for studying photoinduced phenomena in photoactive materials.

FJ-2:IL12  EPR Analysis as a Tool for the Characterization of Photofunctional Materials
S. LIVRAGHI, Dipartimento di Chimica and NIS, Università di Torino, Torino, Italy

The photochemistry of semiconducting oxides is related firstly to the charge carriers separation, due to the light absorption of appropriate wavelength, and secondly, to the fate of the charge carriers (localization, recombination, surface transfer ...). Since almost all of these phenomena involve paramagnetic states, the Electron Paramagnetic Resonance (EPR) technique represents a powerful tool to deeply analyse such phenomena. In this communication, some example will be reported in order to show: i) the potentiality of this technique to point out photoinduced radical processes in heterogeneous photocatalysis using the spin trapping technique. Spin trapping technique in fact allows to increase the lifetime of highly reactive radical species making possible the detection. ii) How this technic can be employed, coupled with other characterization techniques such as UV-Vis-NIR diffuse reflectance (DR-UV-Vis-NIR), to elucidate the mechanism of charge carriers generation and transfer under both visible or UV light, in bare or doped oxides.

FJ-3:L02  Graphene as a 2D Support Shuttle for Separating Photo-generated Charges: an Example combining Copper and TiO2 Nanotubes
E. Zghab, M. Hamandi, F. Dappozze, C. Guillard, G. Berhault, IRCELYON, Villeurbanne, France; E. Zghab, M. Saïd Zina, Univ. Tunis El Manar, Laboratoire de Chimie des Matériaux et Catalyse, Tunis, Tunisia; H. Kochkar, Basic & Applied Scientific Research Center, Imam Abdulrahman Bin Faisal University, Dammam, Saudi Arabia

TiO2 nanotubes (NTs), even if better photocatalysts than classical 3D particles, still suffer from recombination of electron-hole pairs. In a previous study, we found that adding graphene oxide (GO) to NTs help to improve the separation of photogenerated charges. However, the driving force for the transfer of electrons from TiO2 to GO was also found to come from the reduction of functional oxygen groups on graphene layers leading to instability under irradiation. Herein, an alternative is proposed through the addition of Cu NPs onto GO without contact with TiO2 NTs leading to a physical separation of charges with GO acting here only as a 2D support shuttle without inactivation. Comparison was performed between Cu/TiO2 NTs and Cu/(R)GO/NTs systems. On Cu/TiO2 NTs, results show a stabilization of Cu NPs at a +I oxidation state due to a strong interaction with TiO2 NTs leading to photocatalytic activity under visible light for the formic acid (FA) photodegradation. On Cu/(R)GO/TiO2 systems, strong activity was observed using non-reduced GO due to a physical separation of Cu NPs and TiO2 nanotubes confirming our assumption while reducing GO leads to a relocation of Cu NPs onto TiO2 NTs and loss of any beneficial effect due to GO. Application for H2 production will also be presented.

FJ-3:IL03  Artificial Photosynthesis of C1-C6 using Copper and Iron Mixed Oxide Films
Hyunwoong Park, School of Energy Engineering, Kyungpook National University, Daegu, South Korea

The photocatalytic conversion of CO2 and water into value-added chemicals remains a great challenge. Most photocatalysts and their systems suffer from poor efficiency, mixed products, and short durability, while requiring auxiliary chemicals and electrical energy. However, uniformly mixed copper and iron oxide (CuO/CuFeO2; CFO) bulky heterojunction films are capable of converting CO2 and water at circum-neutral pH into C1-C6 aliphatic acid anions and O2 at a solar-to-chemical energy conversion (STC) efficiency close to 3% under simulated sunlight (AM 1.5; 100 mW/cm2) in the absence of any sacrificial chemicals or electrical biases. When the CFO film is simply wired to a Pt foil, C1 (formate) and O2 are produced at a near stoichiometric ratio at an STC efficiency of ~5%. The CFO films are durable over a week and recyclable over five weeks under continuous irradiation. The addition of chloride significantly enhances formate production, with an STC efficiency of 10%, while inhibiting the deformation of CFOs. Density functional theory computations support the observed selectivity and durability.

FJ-3:L05  Al & Ga doped ZnO Nanowires for Water Remediation by Photocatalysis
A. BAILLARD1, E. Appert1, T. Dedova3, P. Gaffuri1, 2, M. Danilson4, M. Webber1, O. Chaix-Pluchery1, I. Oja-Acik3, V. Consonni1, 1Université Grenoble Alpes, CNRS, Grenoble INP, LMGP, Grenoble, France; 2Université Grenoble Alpes, CNRS, Grenoble INP, Institut NEEL, Grenoble, France; 3Laboratory of Thin Film Chemical Technologies, Department of Materials and Environmental Technology, Tallinn Univ. of Technology, Tallinn, Estonia; 4Laboratory of Optoelectronic Materials Physics, Department of Materials and Environmental Technology, Tallinn Univ. of Technology, Tallinn, Estonia

In one century, fresh water world demand was multiply by six and hence new accessible and eco-efficient water treatment processes are needed. Advanced oxidation processes including heterogeneous photocatalysis are high of interest to address this challenge. Zinc oxide (ZnO) is a widely studied material because it is non-toxic, bio-compatible, and offers a wide band gap energy (3.37 eV) that is favourable for electron and hole generation under UV light. To extend the surface of ZnO, nanowires grown by chemical bath deposition (CBD), as an inexpensive, low-temperature and green chemistry compatible method, have emerged as highly promising. Extrinsic doping using group-III elements is a strategy to improve photocatalytic activity through the generation of new energy levels in the bandgap to limit the electron and hole pair recombination. In this work, we investigate the doping of ZnO nanowires with Al and Ga by CBD to assess its effects on the photocatalytic activity. Several characterization techniques including XRD, XPS, Raman spectroscopy, and UV-visible absorption are performed to study the incorporation of Al and Ga into ZnO nanowires and to deeply understand how it affects the photocatalytic reaction kinetics and mechanisms during the organic dye degradation under UV light.


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