TiO2-based Photoanodes Research Articles

Synthesis, characterization and application of quinoxaline-based organic dyes as anodic sensitizers in photoelectrochemical cells

Three new D-A-π-A organic dyes (1a-c) containing a 2,3-diphenylquinoxaline core as the main chromophore were designed to act as sensitizers for the TiO2-based photoanode of dye-sensitized photoelectrochemical cells (DS-PEC) for water splitting. The dyes structures featured a cyanoacrylic acid group as the terminal acceptor/anchoring moiety and three different donor groups of moderate strength, namely mono- and dialkoxy-substituted benzenes, which were introduced to modulate the energies of the respective HOMO levels and enable the electron transfer from a Ru-based molecular water oxidation catalyst (WOC). Dyes 1a-c were synthesized following a C–H activation strategy and fully characterized. In addition, the previously known catalyst Ru(bda)(PyP)2 (bda = 2,2′-bipyridine-6,6′-dicarboxylate; PyP = pyridin-4-methyl phosphonic acid) was also prepared by modification of a reported procedure. The dynamics of charge transfer processes involving the dyes adsorbed on nanocrystalline TiO2 films were investigated by means of transient absorption spectroscopy. Photo-electrochemical experiments carried out on photoanodes functionalized with 1a-c and Ru(bda)(PyP)2 in 0.1 M aq. Na2SO4 electrolyte showed the production of photocurrents up to ca. 0.15 mA cm−2 at + 0.5 V vs. NHE, and highlighted how modifications of the TiO2 staining procedure can lead to significant differences in cell performances.

A review of recent modification strategies of TiO2-based photoanodes for efficient photoelectrochemical water splitting performance

To address concerns about greenhouse gas emissions and sea level rise brought on by global climate change, hydrogen (H2) has the capacity to substitute fossil fuels as a carbon-free source of energy. The most reliable method of producing H2 is photoelectrochemical (PEC) water splitting, which uses water and solar as renewable energy sources. Titanium dioxide (TiO2) is one of the excellent semiconductors utilized as an effective photoanode in PEC water splitting due to its efficient light absorption, cost-effectiveness and excellent photostability. The primary emphasis of this review was on the various techniques used to modify TiO2-based photoanodes and presents a comprehensive overview of the latest progress made towards improving their effectiveness for PEC water splitting. It involves different preparation methods of TiO2-based photoanodes such as hydrothermal, chemical vapor, physical vapor, electrochemical deposition, and other methods which plays a crucial role in determining their structural, morphological, and surface properties. These properties, in turn, influence the light absorption, charge transport, and PEC water splitting activity, ultimately affecting the effectiveness and efficiency of PEC water splitting. Furthermore, elaboration on the outcomes of modification on the physical-chemical-optical features of TiO2-based photoanodes that result from preparation methods is included in this review. The review provides valuable understanding regarding the creation and advancement of high-performing TiO2-based photoanodes in the sustainable energy generation.

Increasing the photoelectric conversion efficiency of dye-sensitized solar cells by doping SrAl2O4:Eu+2,Dy+3 to TiO2-based photoanodes

Dye-sensitized solar cells (DSSCs) are among the third-generation solar cell technologies that can improve power conversion efficiency by modifying the photoanode. In this study, TiO2 nanoparticles were synthesized by hydrothermal method and SrAl2O4:Eu+2,Dy+3 was doped to this matrix at different rates. XRD results confirmed the formation of anatase TiO2. SEM images reveal aggregation morphology in all samples. TEM images shows that the produced TiO2-based samples are tight rather than scattered. It was understood by BET studies that doped samples had a larger specific surface area than pure TiO2. I–V characterization indicated an increase in efficiency of about 47% with doping.

CuS–CdS@TiO2 multi-heterostructure-based photoelectrode for highly efficient photoelectrochemical water splitting

A three-dimensional hierarchical CuS–CdS@TiO2 multi-heterostructure has been fabricated as a highly promising photoanode for PEC water splitting. The CdS@TiO2 structure was synthesized by a two-step hydrothermal method, therein TiO2 nanorods (TiO2 NRs) were used as the template to grow the CdS branches (CdS BRs). Then the surface of the heterostructure was sequentially coated with a p-type CuS semiconductor via a simple SILAR method. The optimized CuS–CdS@TiO2 photoelectrode exhibits a remarkably high photocurrent density of 12.6 mA cm−2 at 1.23 V vs RHE under standard AM 1.5 light illumination. The enhancement in the PEC performance could attribute to the wide range of solar spectrum absorption in CdS BRs, which can generate more photo charge carriers. Meanwhile, the high conductivity of TiO2 NRs serves as an efficient pathway to transfer photogenerated electrons from the CdS BRs to the FTO substrate. In addition, the p-n junction formed between CuS and CdS@TiO2 heterostructure could improve the separation of electron-hole on the surface of the photoanode. This study demonstrates an efficient pathway to improve the photoelectrochemical performance of TiO2-based photoanodes.

Ultrathin TiO2 Blocking Layers via Atomic Layer Deposition toward High-Performance Dye-Sensitized Photo-Electrosynthesis Cells

The collection of solar energy in chemical bonds via dye-sensitized photoelectrosynthesis cells (DSPECs) is a reliable solution. Herein, atomic layer deposition (ALD) introduced ultrathin blocking layers (BLs) between a mesoporous TiO2 membrane and fluorine-doped tin oxide (FTO), and much improved photoelectrochemical water oxidation performance was well documented. Samples with different BL thicknesses deposited on FTO were obtained by ALD. In the photoanode, polypyridyl Ru(II) complexes were used as photosensitizers, and Ru(bda)-type was used as a catalyst during water oxidation. Under one sun irradiation, the BL (i) increased the photocurrent density; (ii) slowed down the open-circuit voltage decay (OCVD) by electrochemical measurement; (iii) increased the photo-generated electron lifetime roughly from 1 s to more than 100 s; and (iv) enhanced the water oxidation efficiency from 25% to 85% with 0.4 V of applied voltage bias. All this pointed out that the ALD technique-prepared layers could greatly hinder the photogenerated electron–hole pair recombination in the TiO2-based photoanode. This study offers critical backing for the building of molecular films by the ALD technique to split water effectively.

In-situ cation-exchange strategy for engineering single-atomic Co on TiO2 photoanode toward efficient and durable solar water splitting

Single-atomic catalysts (SACs) have been emerging as one of potential candidates in catalysts, owing to their unique merits with extremely high specific surface area as well as remarkably exposed active sites. Herein, we develop an in-situ gas-phase cation exchange strategy for engineering single-atomic Co on the surface of TiO2 photoanode toward solar water splitting. It is verified that the atomically-dispersed Co with Co-O coordination could optimize the surface electronic structures, enhance the light absorption, promote the photoinduced charge transfer, lower the reaction barrier and accelerate the reaction kinetics, which consequently enable the overall improved photoelectrochemical (PEC) behaviors for photoanodes. As a proof of concept, the as-constructed TiO2-based photoanodes deliver robust stability up to 100 h and a photocurrent density up to 1.47 mA cm−2 at 1.23 V vs. RHE, which are superior to those of pristine TiO2, representing their significance for potential applications.

Recent progress of applied TiO2 photoelectrocatalysis for the degradation of organic pollutants in wastewaters

Photoelectrocatalysis (PEC) is a promising powerful electrochemical advanced oxidation process (EAOP) that combines photocatalysis and electrolysis. Upon light irradiation over a semiconductor photocatalyst, electrons of its valence band are promoted to its conduction band giving rise to positively charged holes. The external bias potential applied to the photoanode extracts the photoexcited electrons toward the cathode of the electrolytic cell. Organics can then be oxidized directly by the holes or with hydroxyl radical formed by their reaction with H2O/OH−, and at lesser extent with other reactive oxygen species formed. The extensive study of TiO2 photoanodic nanomaterials and their applicability in environmental remediation makes necessary a further analysis of the state of the art and the actual understanding of potentials and shortcomings. This paper presents a comprehensive and critical review on the application of PEC with TiO2 photoanodes to remove organic pollutants from wastewaters from 2017 to May 2022. Fundamentals of this technique with pristine TiO2 photoanodes are explained, with special emphasis on the understanding of ROS origination mechanisms and the analytical techniques used for their characterization. Typical PEC systems and key operating variables related to the effectiveness of the process are examined. Recent advances obtained by TiO2 and TiO2-based photoanodes treatment of chemicals, dyes, pharmaceuticals, and pesticides are described and discussed. The preparation of TiO2 nanocomposites and their heterojunction mechanism, as well as the application of hybrid processes are detailed. Finally, prospects are proposed for the future research of PEC with perspectives to industrial application at higher technology readiness level.

AuNRs attached TiO2 NPs modified by cobalt-imidazolate frameworks as exceptional materials to improve the energy conversion efficiency of dye-sensitized solar cells.

This work reports a significant improvement in both the open-circuit voltage (VOC) and current density (J) of dye-sensitized solar cells (DSSCs) using gold nanorod-modified TiO2 nanoparticles (TiO2/AuNRs) together with a cobalt-imidazolate framework (ZIF-67) as an efficient photoanode. It was demonstrated that adding ZIF-67 (8 wt%) to TiO2 NPs increased the VOC by 160 mV and J by 2.5 times. This observation was described based on the significant increase in the amount of adsorbed dye in the presence of highly porous ZIF-67, which boosts the photoanode's light harvesting. Modifying TiO2 NPs with AuNRs also caused a remarkable enhancement in J (∼ 2.8 times), which can be explained via electron transfer between the TiO2 conduction band and AuNRs. It can result in a more efficient inhibiting effect on the interfacial charge recombination processes in TiO2/AuNRs/ZIF-67 because of the formation of a Schottky barrier at the interface between TiO2 and Au. These effects were confirmed by the reduction in the photoluminescence intensity of TiO2 in the presence of AuNRs. More reduction in the photoluminescence intensity was observed when ZIF-67 was added. The prepared photoanode showed an outstanding improvement in the overall efficiency of the DSSC (η) to 8.38% compared to the bare TiO2-based photoanode (1.83%). The notable improvement in the TiO2/AuNRs/ZIF-67 performance confirmed its practicality for high-efficiency DSSCs.

Understanding Wavelength-Dependent Synergies between Morphology and Photonic Design in TiO2-Based Solar Powered Redox Cells.

Solar powered redox cells (SPRCs) are promising for large-scale and long-term storage of solar-energy, particularly when coupled with redox flow batteries (RFBs). While efforts have primarily focused on heterostructure engineering, the potential of synergistic morphology and photonic design has not been carefully studied. Here, we investigate the wavelength-dependent effects of light-absorption and charge transfer characteristics on the performance of gold decorated TiO2-based SPRC photoanodes operating with RFB-compatible redox couples. Through an in-depth optical and photoelectrochemical characterization of three complementary TiO2 microstructures, namely nanotubes, honeycombs, and nanoparticles, we elucidate the combined effects of nanometer-scale semiconductor morphology and plasmonic design across the visible spectrum. In particular, thin-walled TiO2 nanotubes exhibit a ∼ 50% increase in solar-to-chemical efficiency (STC) compared to thick-walled TiO2 honeycombs thanks to improved charge transfer. Au nanoparticles both increase generation and interfacial charge transfer (above bandgap) and promote hot carrier injection (below bandgap) leading to a further 25% increase in STC. Overall, Au/TiO2 nanotubes achieve a high photocurrent at 0.098 mA/cm2 and an excellent STC of 0.06%, among the highest with respect to the theoretical limit. The incident photon to current efficiency and internal quantum efficiency are also superior to those of bare TiO2 showing maximum values of 54.7% and 67%, respectively. Overall, nanophotonic engineering that synergistically combines morphology optimization and plasmonic sensitization schemes offer new avenues for improving rechargeable solar-energy technologies such as solar redox flow batteries.

Black TiO2-Based Dual Photoanodes Boost the Efficiency of Quantum Dot-Sensitized Solar Cells to 11.7.

Quantum dot-sensitized solar cells (QDSSC) have been regarded as one of the most promising candidates for effective utilization of solar energy, but its power conversion efficiency (PCE) is still far from meeting expectations. One of the most important bottlenecks is the limited collection efficiency of photogenerated electrons in the photoanodes. Herein, we design QDSSCs with a dual-photoanode architecture, and assemble the dual photoanodes with black TiO2 nanoparticles (NPs), which were processed by a femtosecond laser in the filamentation regime, and common CdS/CdSe QD sensitizers. A maximum PCE of 11.7% with a short circuit current density of 50.3 mA/cm2 is unambiguously achieved. We reveal both experimentally and theoretically that the enhanced PCE is mainly attributed to the improved light harvesting of black TiO2 due to the black TiO2 shells formed on white TiO2 NPs.

Pengaruh Penambahan Nanopartikel ZnO Terhadap Morfologi Nanokomposit TiO2/ZnO

The photoanode layer which is generally an oxide semiconductor material plays an important role in dye-sensitized solar cells. Photoanode facilitate electron transfer from dye to external circuit and electron injection from the dye. Photoanode should have a high surface area as well as high pore size to accommodate the dye sensitizer capacity optimally. To improve the function of TiO2-based photoanode, it can be done by adding ZnO nanoparticles. The characterization results showed that the addition of nanoparticles did not change the structure of each component, ZnO had a hexagonal wurtzite structure while TiO2 had a tetragonal structure in the anatase phase. The surface morphology of ZnO, TiO2 and TiO2/ZnO nanocomposites is quite homogeneous with spherical particles. The addition of ZnO nanoparticles affects the specific surface area and pore volume of TiO2/ZnO nanocomposites. Compared to the specific surface area and pore volume of TiO2, the addition of ZnO nanoparticles increased the specific surface area by 68% and the specific pore volume to three times the TiO2 pore volume

Photovoltaics and its magnetic-field promotion effect in dye-sensitized solar cells with BiFeO3 + TiO2 composite photoanodes

In dye-sensitized solar cells (DSSCs), the light absorbance of photoanode is the most important factor in power conversion efficiency (PCE). Here the authors report on an alternative modified TiO2-based photoanode for obtaining enhanced photovoltaic (PV) properties in DSSCs. Multiferroics BiFeO3, which possesses excellent magnetic, electric, and photocatalytic performances, was employed to construct BiFeO3 + TiO2 composite photoanode for DSSCs. In this type of DSSCs with BiFeO3 + TiO2 composite photoanodes, BiFeO3 with suitable bandgap may also act as photoelectric conversion medium, which is generally acted by dye in DSSCs, and increases photogenerated carrier transport channels, which improves the PCE of DSSCs with BiFeO3 + TiO2 composite photoanodes. More importantly, magnetic-field enhanced PV properties in abovementioned DSSCs was firstly obtained, which may result from magnetic-field-suppression the recombination of charge carriers and magnetoresistance effect in BiFeO3. The investigated results offer both an alternative method to improve PV properties in DSSCs, and a referential train of thought to manipulate photoelectric properties of solar cells.

Regulating the surface of anion-doped TiO2 nanorods by hydrogen annealing for superior photoelectrochemical water oxidation

Dedications to achieve the highly efficient metal oxide semiconductor for the photoelectrochemical water splitting system have been persisted to utilize the TiO2 as the promising photoanode material. Herein, we report notable progress for nanostructured TiO2 photoanodes using facile sequential one-pot hydrothermal synthesis and annealing in hydrogen. A photocurrent density of 3.04 mA·cm−2 at 1.23 V vs. reversible hydrogen electrode was achieved in TiO2 nanorod arrays annealed in hydrogen ambient, which is approximately 4.25 times higher than that of pristine TiO2 annealed in ambient air. 79.2% of incident photon-to-current efficiency at 380 nm wavelength demonstrates the prominence of the material at the near-UV spectral range region and 100 h chronoamperometric test exhibits the stability of the photoanode. Detailed studies regarding crystallinity, bandgap, and elemental analysis provide the importance of the optimized annealing condition for the TiO2-based photoanodes. Water contact angle measurement displays the effect of hydrogen annealing on the hydrophilicity of the material. This study clearly demonstrates the marked improvement using the optimized hydrogen annealing, providing the promising methodologies for eco-friendly mass production of water splitting photoelectrodes.Graphical

3D coral-like Fe doped NiCo2S4 cathode for stable high-efficiency photoelectrochemical tetracycline degradation and simultaneous energy recovery

Herein, 3D coral-like Fe doped NiCo2S4 cathode are fabricated to construct a photocatalytic fuel cell (PFC) system with TiO2-based Si photovoltaic cell combined photoanode for tetracycline (TC) wastewater treatment and simultaneous energy recovery. The results demonstrated that Fe doping significantly changed the morphology of NiCo2S4 and increased the specific surface area, thereby increasing the active sites and promoting the electron transfer rate. The established PFC system exhibited a high TC removal ratio of 95.9% after 90 min with a stable short-circuit current density output of 0.65 mA cm−2 in the degradation process. Quenching experiments and ESR test confirmed that radicals including ·O2–, 1O2 and ·OH played a vital role in TC degradation. The PFC system can operate efficiently in a wide pH range of 3–11. The synergistic effect between Fe2+, Co3+ and Ni2+ maintained the stability of the cathode so as to ensure the long-term stable operation of the system.

Nickel-modified polymeric carbon nitride for improving TiO2-based photoanode: photoelectrocatalytical evaluation and mechanistical insights

Here, we report for the first time the synthesis of a TiO2-based photoanode containing nickel-modified poly (heptazine imide) (Ni–PHI), a polymeric carbon nitride, as well as its application for the methanol oxidation reaction (MOR) under solar simulated and UV illumination. The photoanode was obtained initially as a thin film and the photocurrent response was evaluated in solutions containing Na2SO4 and methanol. For the MOR under illumination in an acidic medium, an optimal composition of the catalyst was found depositing 30-layers of TiO2–Ni–PHI, which yielded a photocurrent of 11 mA cm−2 using UV radiation. The material also showed excellent stability, with a decay of approximately 5% of the photocurrent after 100 min. The improvement in the performance with Ni–PHI is likely due to the formation of a heterojunction between TiO2 and the graphitic carbon nitride, while nickel sites act through a co-catalysis process possibly driven by in situ generation of nickel active species for MOR like Ni(OH)2|NiOOH. Finally, mechanistic insights into in situ infrared Fourier transform spectroscopy showed that TiO2–Ni–PHI can selectively oxidize methanol to formaldehyde in the dark, while under the irradiation of UV light, the oxidized products are CO2 and formic acid, a product with high added value.

Bifacial dye-sensitized solar cells utilizing green-colored NIR sensitive unsymmetrical squaraine dye

Dye-sensitized solar cells (DSSCs) with the capability of photon harvesting from both front and rear sides, consisting of newly developed NIR dye (SQ-140), iodine-based redox electrolyte, and transparent counter electrode with high bifaciality factor (BFF) and good cumulated photoconversion efficiency (PCE), are successfully fabricated and characterized. The use of transparent TiO2-based photoanode leads to improved BFF as compared to its opaque TiO2 counterparts: from 74% to 86% under similar experimental conditions and device parameters. The high molar extinction coefficient of the NIR dye SQ-140, needing a thinner TiO2 layer, was found to be advantageous to maintaining the high BFF even in thicker TiO2 films without serious compromise in the PCE. The lower efficiency of the bifacial DSSC under rear illumination was attributed to the hampered optical penetration as well as absorption of light by the intense colored iodine-based redox electrolyte.

Influence of Bio-Based Surfactants on TiO2 Thin Films as Photoanodes for Electro-Photocatalysis

Photocatalytic water splitting into hydrogen is considered as one of the key solutions to the current demand for eco-responsible energy. To improve the efficiency and sustainability of this process, the development of a TiO2-based photoanode by adding bio-sourced surfactants to the sol–gel preparation method has been considered. Three different polymeric biosurfactants (GB, GC, and BIO) have been tested, giving rise to three different materials being structurally and morphologically characterized by XRD, Rietveld refinement, BET, SEM, AFM, and XPS, which was completed by light absorption, photocatalytic (Pilkington test), electronic (EIS and C-AFM), and photoelectrochemical (cyclic voltammetry) measurements. Correlations between the structure/morphology of materials and their functional properties have been established. One specific surfactant has been proven as the most suitable to lead to materials with optimized photoelectrochemical performance in direct relation with their photocatalytic properties essentially controlled by their specific surface area.

MOF-derived Co2+-doped TiO2 nanoparticles as photoanodes for dye-sensitized solar cells

Facile synthesis and application of nano-sized semiconductor metal oxides for optoelectronic devices have always affected fabrication challenges since it involves multi-step synthesis processes. In this regard, semiconductor oxides derived directly from metal–organic frameworks (MOFs) routes have gained a great deal of scientific interest owing to their high specific surface area, regular and tunable pore structures. Exploring the application potential of these MOF-derived semiconductor oxides systems for clean energy conversion and storage devices is currently a hot topic of research. In this study, titanium-based MIL-125(Ti) MOFs were used as a precursor to synthesize cobalt-doped TiO2-based dye-sensitized solar cells (DSSCs) for the first time. The thermal decomposition of the MOF precursor under an air atmosphere at 400 °C resulted in mesoporous anatase-type TiO2 nanoparticles (NPs) of uniform morphology, large surface area with narrow pore distribution. The Co2+ doping in TiO2 leads to enhanced light absorption in the visible region. When used as photoanode in DSSCs, a good power conversion efficiency (PCE) of 6.86% with good photocurrent density (Jsc) of 13.96 mA cm−2 was obtained with the lowest recombination resistance and the longest electron lifetime, which is better than the performance of the pristine TiO2-based photoanode.

Fabrication of multilayer 1D TiO2/CdS/ZnS with high photoelectrochemical performance and enhanced stability

Nano TiO2 is a basic photoanode material in the application of new type solar cell for its appropriate bandgap and photoemission. However, traditionally TiO2-based photoanode films are limited in weak photoresponse and effortless recombination. Herein, we increase light absorption range through combined the uniform CdS quantum dots (QDs) by successive ionic layer adsorption reaction (SILAR) method and decrease the recombination of photo-generated electrons/holes through ZnS passivation layer overlaying on TiO2/CdS. Photocurrent response (i-t), photocurrent density-voltage (J-V) curves of TiO2/CdS/ZnS demonstrated the photovoltaic conversion efficiency (η) has been reaching 3.43%, which has 230% and 33% improvement than pure TiO2 and TiO2/CdS.

A novel photoelectrochemical system to disrupt microalgae for maximizing lipid-extraction efficiency

Microalgae biodiesel is a candidate to provide renewable, C-neutral energy. However, microalgal cells possess strong cell walls and cell membranes with high selective permeability, both of which hinder the extraction of lipids from the microalgae. Optimizing lipid extraction involves disrupting the microalgae’s cell wall and membrane. The hydroxyl free radical (•OH) can attack the cell wall and cell membrane in the microalgae, making the microalgae more susceptible to lipid extraction. Introduced here is a PhotoElectroChemical (PEC) with TiO2-based photoanode utilized UV–Vis light to produce •OH at a phosphate-palladium cathode. Until now, PEC has not been utilized to disrupt microalgal cells to improve the extraction of microalgae lipids. Here, a novel PEC system -- comprised of two nitrogen-doped TiO2 nanotube photoanodes, a bipolar membrane, and a phosphorus-palladium cathode -- was demonstrated to disrupt microalgal biomass to improve lipid extraction. PEC utilizes ultraviolet and visible light as the only input energy for the effective disruption of the microalgal cell. Pretreatment of Chlorella using the PEC system resulted in 96% oil-extraction efficiency which was approximately 10 times higher than that achieved without the pretreatment and an increases in desirable C16:0. Also explored were the mechanisms underlying •OH generation and microalgae disruption. PEC promises to be an eco-friendly and low-energy technology that can promote the development of microalgae industries.