Earth-Abundant Tin Sulfide-Based Photocathodes for Solar Hydrogen Production.

Tin-based chalcogenide semiconductors, though attractive materials for photovoltaics, have to date exhibited poor performance and stability for photoelectrochemical applications. Here, a novel strategy is reported to improve performance and stability of tin monosulfide (SnS) nanoplatelet thin films for H2 production in acidic media without any use of sacrificial reagent. P-type SnS nanoplatelet films are coated with the n-CdS buffer layer and the TiO2 passivation layer to form type II heterojunction photocathodes. These photocathodes with subsequent deposition of Pt nanoparticles generate a photovoltage of 300 mV and a photocurrent density of 2.4 mA cm-2 at 0 V versus reversible hydrogen electrode (RHE) for water splitting under simulated visible-light illumination (λ > 500 nm, Pin = 80 mW cm-2). The incident photon-to-current efficiency at 0 V versus RHE for H2 production reach a maximum of 12.7% at 575 nm with internal quantum efficiency of 13.8%. The faradaic efficiency for hydrogen evolution remains close to unity after 6000 s of illumination, confirming the robustness of the heterojunction for solar H2 production

Galvanic restructuring of exsolved nanoparticles for plasmonic and electrocatalytic energy conversion

There is a growing need to control and tune nanoparticles (NPs) to increase their stability and effectiveness, especially for photo‐ and electrochemical energy conversion applications. Exsolved particles are well anchored and can be re‐shaped without changing their initial location and structural arrangement. However, this usually involves lengthy treatments and use of toxic gases. Here, the galvanic replacement/deposition method is used, which is simpler, safer, and leads to a wealth of new hybrid nanostructures with a higher degree of tailorability. The produced NiAu bimetallic nanostructures supported on SrTiO3 display exceptional activity in plasmon‐assisted photoelectrochemical (PEC) water oxidation reactions. In situ scanning transmission electron microscopy is used to visualize the structural evolution of the plasmonic bimetallic structures, while theoretical simulations provide mechanistic insight and correlate the surface plasmon resonance effects with structural features and enhanced PEC performance. The versatility of this concept in shifting catalytic modes to the hydrogen evolution reaction is demonstrated by preparing hybrid NiPt bimetallic NPs of low Pt loadings on highly reduced SrTiO3 supports. This powerful methodology enables the design of supported bimetallic nanomaterials with tunable morphology and catalytic functionalities through minimal engineering

Ni-doped A-site excess SrTiO3 thin films modified with Au nanoparticles by a thermodynamically-driven restructuring for plasmonic activity

Plasmonically active nanoparticles offer a promising pathway to extend the absorption range of photocatalysts. While not necessarily catalytically active themselves, these particles allow the absorption of lower energy photons in wide band gap photocatalysts. Here, we present A-site excess SrTiO3 thin films, doped with Ni, where through a subsequent exsolution process we created well socketed Ni nanoparticles in the surface of SrTiO3. These were galvanically replaced by Au, resulting in well-socketed Au nanoparticles with variable size on the surface, depending on the galvanic replacement time. Photoelectrochemical measurements and electron energy loss spectroscopy revealed the improved photoresponse of the thin films by plasmonic activity of the nanoparticles. The energy of the plasmon peak suggests that the main improvement results from the injection of hot charge carriers. Our study opens new avenues for the design and synthesis of the next generation of photocatalytic materials

Hydrogen production using the photoelectrochemical-photosynthetic methods

The scope of the PhD thesis is the development of a new method for simultaneous hydrogen production and organic pollutants’ reduction. For the purposes of this project, a novel photoelectrocatalytic-enzymatic hybrid system is presented for the first time, which is able to produce hydrogen under the simultaneous destruction of toxic and detrimental pollutants present in wastewaters. The main aim of the project is to replace the complex natural photosynthetic systems, such as PSII, PSI, FDX etc., with a semiconducting anode, which under illumination it will be able to oxidize organic pollutants, simultaneously. In addition, this system utilizes the cells of the green photosynthetic algae Chlamydomonas Reinhardtii, strain CC-124, which induce the hydrogenase enzymes and not purified hydrogenase enzymes, for the reduction of the H⁺ to H₂. It consists of a 2 compartments photoelectrochemical cell, where in the anodic compartment, the activation of a TiO₂ electrode by appropriate light is able to degrade most of the organic pollutants present in wastewaters. In the cathodic compartment and under anaerobic conditions, the green photosynthetic algae induce the hydrogenase enzymes, which have the ability to reduce hydrogen cations and forms molecular hydrogen. More specific, the destruction of the antibiotic Chloramphenicol, which acts as a model pollutant, will take place in the anodic compartment, while the photogenerated electrons in the TiO₂ anode are transferred to the cathode, where the hydrogenase enzymes catalyze the reduction of the H⁺ species to H₂. Parameters like the growing medium, detergent, electron relay and algae concentration have been optimized. Fifty percent reduction in the organic carbon content and almost complete destruction of the Chloramphenicol molecule is possible at the anode under photoelectrocatalytic conditions (light intensity 3.9 mW cm⁻²). At the same time, in the cathode compartment and in the presence of the algae culture Chlamydomonas Reinhardtii, 110 μmol H₂ are produced over a reaction time of 450 min, using 0.4 mM Triton X 100 for the cell membranes rapture and 0.01 mM Methyl Viologen (MV⁺²), which acts as an electron relay, in a Tris-Acetate-Phosphate (TAP) sulphur free medium. The enzymatic hydrogen production rate is equal to 0.24 μmol H₂ min⁻¹, while the yield of the system defined as IPCH2E is 1.56%. This hybrid system, which represents a method of artificial photosynthesis, has the potential to lead to an innovative and effective operational system under solar light for hydrogen production and wastewater treatment, simultaneously. The understanding and the mimic of Nature’s mechanisms are the “keys” for the development of environmentally friendly methods that will contribute to a viable and sustainable future for the planet.Ο σκοπός της Διδακτορικής Διατριβής είναι η παραγωγή υδρογόνου, ως καύσιμο νέας γενιάς, ταυτόχρονα με την αποικοδόμηση ρύπων που βρίσκονται σε υγρά απόβλητα. Για την εκπλήρωση του στόχου αυτού αναπτύχτηκε μια νέα μεθοδολογία, η οποία συνδυάζει την ενζυματική παραγωγή υδρογόνου με τη φωτοηλεκτροχημική διάσπαση οργανικών ρύπων. Για την υλοποίηση της μεθόδου κατασκευάστηκε και παρουσιάστηκε, για πρώτη φορά στη βιβλιογραφία ένα υβριδικό φωτοηλεκτροχημικό-ενζυματικό σύστημα, στο οποίο, ταυτόχρονα με την παραγωγή υδρογόνου στο καθοδικό του τμήμα από τα ένζυμα υδρογενάσης, στο ανοδικό τμήμα λαμβάνει χώρα η φωτοηλεκτροκαταλυτική οξείδωση οργανικών ρύπων. Ο κύριος στόχος της εργασίας ήταν η αντικατάσταση των πολύπλοκων φυσικών συστημάτων (PSII, PSI, FDX κ.α.) που λειτουργούν κατά τη φωτοσύνθεση με μια ημιαγώγιμη άνοδο, η οποία επιπλέον, υπό την επίδραση ακτινοβολίας, μπορεί ταυτόχρονα και να οξειδώνει οργανικές ενώσεις. Επίσης, καινοτομία αποτελεί το γεγονός πως στο καθοδικό τμήμα δε χρησιμοποιήθηκαν καθαρά ένζυμα, αλλά ολόκληρα τα ευκαριωτικά κύτταρα Chlamydomonas Reinhardtii το στέλεχος CC-124, τα οποία μπορούν και εκφράζουν τα ένζυμα υδρογενάσης σε συνθήκες έλλειψης θείου και οξυγόνου. Tα σημαντικότερα αποτελέσματα που προέκυψαν από την εφαρμογή της μεθόδου στην πρότυπη φωτοηλεκτροχημική-ενζυμική υβριδική κυψέλη συνοψίζονται ως εξής: Η οξείδωση και η ανοργανοποίηση 10 mg L⁻¹ αντιβιοτικού Χλωραμφενικόλη, το οποίο χρησιμοποιήθηκε ως ρύπος μοντέλο, ήταν 100% και 50%, αντίστοιχα, ύστερα από 8 ώρες φωτισμού (ένταση φωτός 3.9 mW cm⁻²) της ανόδου TiO₂, σε δυναμικό πόλωσης +0.6 V vs Ag/AgCl και πυκνότητα φωτορεύματος περίπου 28 μA cm⁻². Την ίδια στιγμή, στο καθοδικό τμήμα της κυψέλης παράχθηκαν σε θρεπτικό υλικό TAP-SF, παρουσία ~1.9 107 κυττάρων άλγεων mL⁻¹ (δραστικότητα ενζύμων υδρογενάσης ~1.75 10⁻² units), 0.4 mM Triton X 100 και 0.010 mM MV⁺², περίπου 110 μmol H₂ σε 7.5 ώρες φωτισμού της ανόδου, δηλαδή ρυθμός ενζυμικής παραγωγής υδρογόνου ίσος με 0.24 μmol H₂ min⁻¹. Η απόδοση του συστήματος που ορίζεται ως ο αριθμός των φωτονίων που προσπίπτει στην επιφάνεια της ανόδου και μετατρέπονται εμμέσως σε υδρογόνο ορίζεται ως IPCH2E και βρέθηκε ίσος με 1.56%. Ο ρόλος του απορρυπαντικού Triton X 100 είναι η διαλυτοποίηση των μεμβρανών των κυττάρων, ενώ το Methyl Viologen (MV⁺²) έχει το ρόλο του διαμεσολαβητή φορτίου από το ηλεκτρόδιο της καθόδου (ράβδος γραφίτη) στα ένζυμα υδρογενάσης. Πρόκειται για μια μέθοδο τεχνητής φωτοσύνθεσης, η οποία έχει τη δυνατότητα μακροπρόθεσμα να αξιοποιήσει την ηλιακή ακτινοβολία για την αντιμετώπιση της περιβαλλοντικής ρύπανσης, με ταυτόχρονη παραγωγή ενός καθαρού καυσίμου, όπως είναι το υδρογόνο. Η κατανόηση και η αντιγραφή των φυσικών συστημάτων, τα οποία η φύση έχει αναπτύξει εδώ και δισεκατομμύρια χρόνια, αποτελούν το «κλειδί» για την ανάπτυξη μεθόδων φιλικών και σε αρμονία με το φυσικό περιβάλλον, πράγμα που θα συμβάλλει σε μια βιώσιμη οικονομική και κοινωνική εξέλιξη της ανθρωπότητας

Recent Advances in the Use of Black TiO2 for Production of Hydrogen and Other Solar Fuels

Black TiO2 has emerged as one of the most promising photocatalysts recently discovered. The reason behind its catalytic activity is considered to be due to the presence of defects and Ti3+ species at the surface of black TiO2 nanostructures, which are crucial for its diverse applications. Moreover, disordered/crystalline surface layers and bulk regions have been identified and appear to influence the intrinsic properties of the material. Here, we present the latest studies on the use of black TiO2 for metal free hydrogen production, as well as for CO2 photoreduction and N2 photofixation. After highlighting the structure/property relations, we conclude with some critical questions and suggest further topics of research in order to better understand the underlying mechanisms of light absorption in black TiO2, especially towards solar fuels production

Preparation of TiO2 rutile nanorods decorated with cobalt oxide nanoparticles for solar photoelectrochemical activity

The present paper investigates the preparation of titanium dioxide (TiO2) nanorods decorated with cobalt oxide (Co3O4) nanoparticles and its application as solar photoelectrocatalyst. TiO2 rutile nanorods have been prepared on conductive glass via a hydrothermal reaction in acidic media, shaped as squared rods with an average length of 1.7 µm and thicknesses between 50 and 120 nm. Co3O4 nanoparticles with an average diameter of 30 nm were synthesized using a simple precipitation method. Spin coating the nanoparticles on the TiO2 nanorods shows enhanced photocurrents under simulated solar light up to 1.3 mA/cm2 at 1.6 V vs. SHE.acceptedVersio

Solid-state photoelectrochemical cell with TiO2 nanotubes for water splitting

We have fabricated and tested a photoelectrochemical (PEC) cell where the aqueous electrolyte has been replaced by a proton conducting hydrated Nafion® polymer membrane. The membrane was sandwiched between a TiO2-based photoanode and a Pt/C-based cathode. The performance was tested with two types of photoanode electrodes, a thermally prepared TiO2 film on Ti foil (T-TiO2) and a nanostructured TiO2 films in the form of highly ordered nanotubes (TNT) of different lengths. Firstly, photovoltammetry experiments were conducted under asymmetric conditions, where the anode was immersed in deionized water, while the cathode was kept in ambient air. The results showed a high incident photon-to-current efficiency (IPCE) of 19% under unassisted conditions (short-circuit, 0 V vs. cathode) with short TNT (ca. 1 μm) under 4 mW cm−2 illumination with UV-A rich light. Secondly, the deionized water was replaced by 0.5 M Na2SO4 and now the performance was higher with longer nanotubes, assigned to increased ionic conductivity inside the tubes. An unassisted (0 V) IPCE of 33% was achieved with nanotubes of ca. 8 μm. The presented solid-state PEC cell minimizes the electrode distance and volume of the device, and provides a way towards compact practical applications in solar water splitting

Photocatalytic generation of gas phase reactive oxygen species from adsorbed water: Remote action and electrochemical detection

The improvement of indoor environments is of great importance as it can significantly improve human health, comfort and productivity. Herein, different forms of TiO2 nanorods were used as the photocatalyst for generation of reactive oxygen species (ROS) in a gas phase photoreactor under controlled humidity. Several parameters were investigated by monitoring the remote decolourisation of Methylene Blue (MB) embedded in a Nafion film. A decolourisation of 26% under 80% relative humidity was observed when the MB film was 0.5 cm away from the photocatalyst. The length and ratio of light/dark intervals have major impacts on the efficiency of the gas phase photocatalytic process, which we link to the amount of water adsorbed on the photocatalyst, as the source for hydroxyl radicals. Furthermore, the photocatalytic production of ROS was quantified through a polyaniline electrochemical sensor and a rate of 1 · 10*12 of ROS molecules s−1 was estimated. This study contributes to the efficacy of the gas phase photocatalytic method in air decontamination, for the development of efficient air cleaning devices