Nanostructured TiO2 Electrodes Research Articles

Interplay Between Charge Accumulation and Oxygen Reduction Catalysis in Nanostructured TiO2 Electrodes Functionalized with a Molecular Catalyst

AbstractThe catalytic reduction of O2 by a manganese(III) porphyrin immobilized in a nanostructured semiconductive transparent TiO2 electrode is here investigated by UV‐Vis spectroelectrochemistry in an aqueous buffered medium. Analysis of the operando spectroelectrochemical data, collected for both the immobilized catalyst and the TiO2 matrix, demonstrates the coexistence of two faradaic electrochemical processes, namely (i) irreversible interfacial electron transfer from TiO2 to the immobilized porphyrin triggering the catalytic reduction of O2, and (ii) reversible proton‐coupled electrochemical reduction of TiO2 leading to the accumulation of electrons in the TiO2 bulk. The competition between these two processes is modulated by the local concentration of O2, which itself varies with the rate of the catalysis. Indeed, when O2 is locally strongly depleted by catalysis, the process switches from catalysis to charge storage, like a battery. As a result, the electrons stored in TiO2 were observed to pursue the catalysis even after the electrode polarization was switched‐off (i. e., under open circuit). This is an overlooked phenomenon that we believe is important to consider in applications relying on metal oxide‐based photoelectrodes operating in aqueous media.

Tetrazolium anchor porphyrin-based self-assembly for supramolecular solar cells

We have synthesized tetrazolium anchor porphyrin (MPTz) with different metal ions (M, M = Zn2+, Cd2+ and Co2+ ) and triphenylamine group antenna molecule L2 on the nanostructured TiO2 electrode surfaces, resulting in a ZnPTz + L2 assembled approach, based on self-assembled dye ZnPTz + L2 for highly efficient dye-sensitized solar cells (DSSCs). The photoelectric conversion efficiency of solar cells sensitized by the dyes with antenna molecule (L2) were enhanced when compared to that sensitized by the dye with monolayer anchor porphyrin (ZnPTz). We have carefully examined their optical and electrochemical properties and device performance. In addition, the modes of the assemblies immobilized on TiO2 electrode surfaces were also verified by transmission electron microscopy (TEM).

Synthesis and Modification of TiO2 nanostructured Materials for Energy Storage

Increasing energy demand is inexorably linked to the need for efficient energy storage techniques. For practical applications, it is highly desirable to decrease the size of electrical components and to increase the storage capacities while maintaining power, stability, and charging-discharging speeds. Much focus has been directed towards the development of supercapacitors. These are often fabricated from carbonaceous or metal oxide materials with high surface areas to maximize electrode/electrolyte interactions. The use of nanostructured TiO2 electrodes has been explored for this application due to their low cost, high stability, and highly tunable morphology.Here we present the fabrication of nanostructured TiO2 films via a facile anodic process. Characterization of the films was carried out by scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. Electrochemical reduction of the formed TiO2 film was further performed to increase its capacitance, which was confirmed by cyclic voltammetry and electrochemical impedance spectroscopy. The performance of the symmetric capacitor constructed with the modified TiO2 films will be presented.

Efficient Nanostructured TiO2/SnS Heterojunction Solar Cells

AbstractTin sulfide (SnS) is one of the most promising solar cell materials, as it is abundant, environment friendly, available at low cost, and offers long‐term stability. However, the highest efficiency of the SnS solar cell reported so far remains at 4.36% even using the expensive atomic layer deposition process. This study reports on the fabrication of SnS solar cells by a solution process that employs rapid thermal treatment for few seconds under Ar gas flow after spin‐coating a precursor solution of SnCl2 and thiourea dissolved in dimethylformamide onto a nanostructured thin TiO2 electrode. The best‐performing cell exhibits power conversion efficiency (PCE) of 3.8% under 1 sun radiation conditions (AM1.5G). Moreover, secondary treatment using SnCl2 results in a significant improvement of 4.8% in PCE, which is one of the highest efficiencies among SnS‐based solar cells, especially with TiO2 electrodes. The thin film properties of SnS after SnCl2 secondary treatment are analyzed using grazing‐incidence wide‐angle X‐ray scattering, and high‐resolution transmittance electron microscopy.

On the unsuspected role of multivalent metal ions on the charge storage of a metal oxide electrode in mild aqueous electrolytes.

Insertion mechanisms of multivalent ions in transition metal oxide cathodes are poorly understood and subject to controversy and debate, especially when performed in aqueous electrolytes. To address this issue, we have here investigated the reversible reduction of nanostructured amorphous TiO2 electrodes by spectroelectrochemistry in mild aqueous electrolytes containing either a multivalent metal salt as AlCl3 or a weak organic acid as acetic acid. Our results show that the reversible charge storage in TiO2 is thermodynamically and kinetically indistinguishable when carried out in either an Al3+- or acetic acid-based electrolyte, both leading under similar conditions of pH and concentrations to an almost identical maximal charge storage of ∼115 mA h g-1. These observations are in agreement with a mechanism where the inserting/deinserting cation is the proton and not the multivalent metal cation. Analysis of the data also demonstrates that the proton source is the Brønsted weak acid present in the aqueous electrolyte, i.e. either the acetic acid or the aquo metal ion complex generated from solvation of Al3+ (i.e. [Al(H2O)6]3+). Such a proton-coupled charge storage mechanism is also found to occur with other multivalent metal ions such as Zn2+ and Mn2+, albeit with a lower efficiency than Al3+, an effect we have attributed to the lower acidity of [Zn(H2O)6]2+ and [Mn(H2O)6]2+. These findings are of fundamental importance because they shed new light on previous studies assuming reversible Al3+-insertion into metal oxides, and, more generally, they highlight the unsuspected proton donor role played by multivalent metal cations commonly involved in rechargeable aqueous batteries.

Full Parametric Impedance Analysis of Photoelectrochemical Cells: Case of a TiO2 Photoanode

Issues in the electrical characterization of semiconducting photoanodes in a photoelectrochemical (PEC) cell, such as the cell geometry dependence, scan rate dependence in DC measurements, and the frequency dependence in AC measurements, are addressed, using the example of a TiO2 photoanode. Contrary to conventional constant phase element (CPE) modeling, the capacitive behavior associated with Mott-Schottky (MS) response was successfully modeled by a Havriliak-Negami (HN) capacitance function-which allowed the determination of frequency-independent Schottky capacitance parameters to be explained by a trapping mechanism. Additional polarization can be successfully described by the parallel connection of a Bisquert transmission line (TL) model for the diffusion-recombination process in the nanostructured TiO2 electrode. Instead of shunt CPEs generally employed for the non-ideal TL feature, TL models with ideal shunt capacitors can describe the experimental data in the presence of an infinite-length Warburg element as internal interfacial impedance - a characteristic suggested to be a generic feature of many electrochemical cells. Fully parametrized impedance spectra finally allow in-depth physicochemical interpretations. Key words: Photoanodes, Impedance, TiO2, Havriliak-Negami equation, Bisquert TL model

Surface states as electron transfer pathway enhanced charge separation in TiO2 nanotube water splitting photoanodes

Surface states on TiO2 photoanodes are usually considered to hinder the photoelectrochemical (PEC) water splitting via slowing charge transport and increasing charge recombination. Here, we found that electrochemically doping the TiO2 nanotubes electrode at a potential negative than its flat band potential will induce the insertion of protons into the lattice of TiO2via a reaction of TiIVO2 + e− + H+ = TiIIIO(OH) to form the surface states. Photoelectrochemical/electrochemical tests confirmed that the surface states (Ti-OH) are an important photogenerated electron transfer route, significantly increasing charge separation efficiency, and hence contributing to the PEC performance enhancement. In addition, we have made a self-consistent explanation on Mott-Schottky (M–S) plots of the TiO2 nanotubes electrode, demonstrating that the M–S measurement is able to accurately describe electron filling and extraction into/from surface states and not suitable to determine the flat band potential of nanostructured TiO2 electrodes. Our results offer a new insight to understanding the role of surface states during the charge separation, transfer, and injection processes of the water splitting reaction.

(Invited) Influence of Nanoscale Surface Structure of TiO2 Single Crystal Electrode on Water Photooxidation Reaction Process

The oxygen photoevolution reaction on TiO2 and related metal oxides has attracted strong attention from the point of view of solar water splitting. In our previous study, we have proposed a new reaction model in which generated holes are consumed not only by the O2 evolution reaction but also by three competitive paths possibly occur at different surface sites; photoluminescence, surface roughening (i.e. photocorrosion), and nonradiative recombination [1,2]. Actually, we found that the branch ratio for these paths on TiO2 (110) changed by the density and the direction of steps (i.e. atomic leveled surface structure) [3]. On the other hand, it is expected that the nano-sized structure of the TiO2 surface also affect the branch ratio of 4 competitive photooxidation reactions. Thus, in this study, we investigated oxygen photoevolution efficiency on a nanostructured TiO2 electrode that exposes different facets. A Pt wire and an Ag/AgCl/sat.KCl electrode were used as a counter and a reference electrode, respectively. An n-type TiO2 (110) doped with 0.05wt% Nb was used as a working electrode. The j−U curves were measured in 0.1 M HClO4 (pH 1.1) under UV irradiation. Nanostructured TiO2(110) surfaces were prepared by photoetching in 0.05 M H2SO4 [4]. We obtained the nanostructured surfaces on which shallow grooves are regularly arranged. We should note that formed surface of the nanostructure is composed by (100) facet faces [4]. Photocurrent generated along with the O2 evolution from H2O on the nanostructured TiO2 (110) was apparently different from that on the flat TiO2 (100). The onset potential of nanostructured TiO2 electrode was shifted toward negative voltage, indicating that the activation energy of oxygen evolution reaction was smaller than that on the flat surface. In addition, the consumption ratio of the photogenerated holes by above mentioned 4 competitive reactions was largely different from that on the atomically flat (100) surface. Considering the fact that the photoetched TiO2(110) surface is composed by (100) microfacets[4], this result can be attributed to the nanostructuring effect. We also carried out the same experiment using the some other TiO2 electrode those have different nanostructure on their surfaces, and found that the nanostructuring effect (1. Branch ratio of 4 competitive photooxidation reactions, 2. Activation energy of O2 evolution reaction) depends on the nano-sized surface morphology of the TiO2 electrode. [1] A. Imanishi, T. Okamura, N. Ohashi, R. Nakamura, Y. Nakato, J. Am. Chem. Soc., 129 (2007) 11569. [2] A. Imanishi, K. Fukui, J. Phys. Chem. Lett., 5 (2014) 2108. [3] M. Kadono, Master’s thesis, Osaka Univ. (2013). [4] A. Imanishi, H. Suzuki, K. Murakoshi, Y. Nakato, J. Phys. Chem. B, 110 (2006) 21050.

Tunability of the Photogenerated Charge Carrier Density on Semiconductors By in-Situ Electrochemical Treatments

Nanostructured semiconductors are widely studied materials due to their wide range of potential applications, such as solar energy conversion. In the latter, a determining characteristic for semiconductors is the generated photocurrent, which is greatly influenced by the synthetic route and subsequent treatments. In this work, we present an in-situ potentiostatic and potentiodynamic approach to modify the photoelectrocatalytic properties of nanostructured TiO2 electrodes. The effect of the morphology was studied by comparing a nanotubular and nanorod particulate TiO2. A potentiodynamic (cyclic voltammetry) and potentiostatic (differential pulsed amperometry) were used to modify the electrodes in 0.5 M H2SO4. The photogenerated charge carriers separation was studied by CV, LSV and CA. Self-doping can tune the electronic and band structures of semiconductor photocatalysts like binary metal oxides. Thus, a change in the capacitance was observed after the reductive self-doping (SD) treatment that was studied by recording Mott-Schottky plots. The morphological, structural, and optical properties were characterized by SEM, XRD/Rietveld refining, XPS, respectively. The observed behaviors from electrochemical measurements suggested that morphology has an important in the capacitive properties. In the meantime, nanorod reacted quickly to light. Experimental results confirmed that self-doping could change the electronic structures to intrinsically improve the optical absorption property and charge transfer ability, thus enhancing the photocatalytic activity of semiconductors. This successful band structure tailoring example of semiconductors suggests the electrochemical treatments represent a facile and systematic technique to be general to develop novel visible light driven photoelectrocatalysts with enhanced performances.

Integration of Enzymes in Polyaniline-Sensitized 3D Inverse Opal TiO2 Architectures for Light-Driven Biocatalysis and Light-to-Current Conversion.

Inspired by natural photosynthesis, coupling of artificial light-sensitive entities with biocatalysts in a biohybrid format can result in advanced photobioelectronic systems. Herein, we report on the integration of sulfonated polyanilines (PMSA1) and PQQ-dependent glucose dehydrogenase (PQQ-GDH) into inverse opal TiO2 (IO-TiO2) electrodes. While PMSA1 introduces sensitivity for visible light into the biohybrid architecture and ensures the efficient wiring between the IO-TiO2 electrode and the biocatalytic entity, PQQ-GDH provides the catalytic activity for the glucose oxidation and therefore feeds the light-driven reaction with electrons for an enhanced light-to-current conversion. Here, the IO-TiO2 electrodes with pores of around 650 nm provide a suitable interface and morphology needed for the stable and functional assembly of polymer and enzyme. The IO-TiO2 electrodes have been prepared by a template approach applying spin coating, allowing an easy scalability of the electrode height and surface area. The successful integration of the polymer and the enzyme is confirmed by the generation of an anodic photocurrent, showing an enhanced magnitude with increasing glucose concentrations. Compared to flat and nanostructured TiO2 electrodes, the three-layered IO-TiO2 electrodes give access to a 24-fold and 29-fold higher glucose-dependent photocurrent due to the higher polymer and enzyme loading in IO films. The three-dimensional IO-TiO2|PMSA1|PQQ-GDH architecture reaches maximum photocurrent densities of 44.7 ± 6.5 μA cm-2 at low potentials in the presence of glucose (for a three TiO2 layer arrangement). The onset potential for the light-driven substrate oxidation is found to be at -0.315 V vs Ag/AgCl (1 M KCl) under illumination with 100 mW cm-2, which is more negative than the redox potential of the enzyme. The results demonstrate the advantageous properties of IO-TiO2|PMSA1|PQQ-GDH biohybrid architectures for the light-driven glucose conversion with improved performance.

High Surface Area NanoCOT Catalysts on Ni Foam Framework for Efficient Water Electrolysis

Carbon doped nanostructured TiO2 electrodes (NanoCOT) are prepared by anodizing titanium in a fluoride-based electrolyte followed by thermal annealing in an atmosphere of methane and hydrogen in the presence of Fe precursors. NanoCOT are highly conductive and contain more than 1 × 10^9 /cm2 of nanowires or nanotubes to enhance their double layer charge capacitance and electrochemical stability. This electrode material also exhibits enhanced oxygen evolution reaction (OER) catalytic activity in alkaline solution, providing a current density of 1.33 mA/cm2 at an overpotential of 0.42 V. This OER current density of a NanoCOT electrode is about 4 times higher than an oxidized Ir electrode and 15 times higher than a Pt electrode because of its nanostructured high surface area and favorable OER kinetics. The enhanced catalytic activity of NanoCOT is attributed to the presence of a continuous energy band of the titanium oxide electrode with predominantly reduced defect states of Ti (e.g., Ti^1+, Ti^2+, and Ti^3+) formed by chemical reduction with hydrogen and carbon. The OER performance of NanoCOT can also be further enhanced by decreasing its overpotential by 150 mV at a current density of 1.0 mA/cm2after coating its surface electrophoretically with 2.0 nm IrOx nanoparticles. Further improve can be obtained by using porous substrate such as Ni and Ti foam.

A dye-sensitized solar cell containing an anchoring porphyrin

We have designed a self-assembly ZnP-ZnPA, based on a porphyrin ZnP bearing 1,3,5-triazine-2,4-diamine unit, and anchoring porphyrin ZnPA. The assembly with ZnP-ZnPA was immobilized on nanostructured TiO2 electrode surfaces. The assembled structures were characterized by transmission electron microscopy. The optical, photovoltaic, electrochemical impedance spectroscopy, and incident photon-to-current conversion efficiency were measured. The results revealed that the ZnP-ZnPA device had better photovoltaic performance than ZnPA and possessed a higher shortcircuit photocurrent density (JSC = 6.04) but a lower open-circuit photovoltage (VOC = 0.51) than anchoring ZnPA. Moreover, our previously reported assembly (ZnP-A1) as reference, the assembly device ZnP-ZnPA had better η (2.24%) and FF (72.7%).

Electron trapping induced electrostatic adsorption of cations: a general factor leading to photoactivity decay of nanostructured TiO2

In this work, a mechanism of electron trapping induced electrostatic adsorption of electrolyte cations (ETIEA) is proposed to explain the general photoactivity decay of nanostructured TiO2 electrodes, usually occurring during the initial several minutes of photoelectrochemical (PEC) processes.

Flame-made ultra-porous TiO2 layers for perovskite solar cells

We report methyl ammonium lead iodide (MAPbI3) solar cells with an ultra-porous TiO2 electron transport layer fabricated using sequential flame aerosol and atomic layer depositions of porous and compact TiO2 layers. Flame aerosol pyrolysis allows rapid deposition of nanostructured and ultra-porous TiO2 layers that could be easily scaled-up for high-throughput low-cost industrial solar cell production. An efficiency of 13.7% was achieved with a flame-made nanostructured and ultra-porous TiO2 electrode that was coated with a compact 2 nm TiO2 layer. This demonstrates that MAPbI3 solar cells with a flame-made porous TiO2 layer can have a comparable efficiency to that of the control MAPbI3 solar cell with the well-established spin-coated porous TiO2 layer. The combination of flame aerosol and atomic layer deposition provides precise control of the TiO2 porosity. Notably, the porosity of the as-deposited flame-made TiO2 layers was 97% which was then fine-tuned down to 87%, 56% and 35% by varying the thickness of the subsequent compact TiO2 coating step. The effects of the decrease in porosity on the device performance are discussed. It is also shown that MAPbI3 easily infiltrates into the flame-made porous TiO2 nanostructure thanks to their high porosity and large pore size.

A novel self-assembly with two acetohydrazide zinc porphyrins coordination polymer for supramolecular solar cells

Two new acetohydrazide zinc porphyrin with different donor units based Mn (Ⅱ) ion coordination polymers (CPsx, x=1,2) have been designed, synthesized, and well-characterized. Two coordination polymers and anchor porphyrin (ZnPA) self-assembly by metal-ligand axial coordination to modify the nano-structured TiO2 electrode surface has been investigated in photoelectrochemical device. Our results reveal that the self-assemblies devices show significantly improved photocurrent conversion efficiency. Particularly, CPs1 based solar cell showed higher short circuit current density and conversion efficiency. Their optical performance, electrochemical impedance spectroscopy and transient photovoltage decay were also investigated to further understand the photoelectrochemical results. In addition, the assembled modes of the assemblies immobilized on TiO2 electrode surfaces were also verified by transmission electron microscopy (TEM), theory calculations and energy dispersive X-ray spectroscopy (EDX).

Influence of the Adsorption of Phycocyanin on the Performance in DSS Cells: and Electrochemical and QCM Evaluation

The influence of some coadsorbents and different pH values on the efficiency of DSS cells assembled with phycocyanin was evaluated using quartz crystal microbalance (QCM) and electrochemical techniques as impedance spectroscopy (EIS) and cyclic voltammetry (CV). Chlorophyll, heptadecanoic acid and 7.5 or 8.5 pH values were applied when nanostructured TiO2 electrode was dipped in the dye solution. Best efficiency conversion values were obtained when using fatty acids as coadsorbents, reaching a conversion efficiency of 0.04 % for open cells.

(Invited) Plasmon-Induced Deposition of Conductive Polymer on Au Nanostructured TiO2 Electrode

The plasmon-induced photo-polymerization of molecules was observed at the gold nano structures on TiO2 electrodes in an electrolyte. Spatially confined electromagnetic fields generated by the excitation of localized surface plasmon resonance leads to highly localized deposition of conductive polymer at the edge of Au nanostructures on TiO2 electrodes. Characteristics on the deposition rate depending on the Au nanostructures provide the information on the mechanism of plasmon-exciton conversions.

Surface Rutilization of Anatase TiO2 Nanorods for Creation of Synergistically Bridging and Fencing Electron Highways

In a photoelectrochemical cell, the most concerned issue in the nanostructured TiO2 electrode is the charge transport, which consists of the internal movement of electrons in TiO2 nanostructures and the intergrain charge transfer. Here, inspired by electrochemical studies on different polymorphs of TiO2, it is proposed to bridge the adjacent building blocks and fence the electron transport highways in TiO2 electrodes by surface rutilization of anatase nanorods. The ultrathin rutilized layer completely coated on the anatase surface has a slightly higher conduction band edge than that of anatase. The obtained surface rutilized anatase nanorods can not only improve the intergrain charge transfer while maintaining fast electron transport within anatase but also minimize the internal energy consumption and protect the electrons in TiO2 electrodes from recombination, which are beneficial to the charge collection and can significantly improve the photovoltaic performance of photoelectrochemical cells.

Novel Potentiometric Sensors Based on Nanostructured TiO2 Electrodes for Selective Determination of Biologically Relevant Transition Metals

The purpose of this work is to design and develop sensitive and selective nanostructured electrochemical sensors for the analyses of chromium (III) and iron (III) ions in different samples, such as drinking water, seawater, pharmaceutical products, soil and food. Chromium is an essential metal to life in small amounts while toxic in larger quantities. Recommended chromium intakes are provided in the dietary reference intakes (DRIs) developed by the Institute of Medicine of the National Academy of science and serum chromium levels normally range from less than 0.05 up to 0.5 micrograms/milliliter (mcg/mL) in humans. For medical and environmental reasons, it is of great importance to determine how much of this metal ion is present in such media [1]. To date, many Cr (III) selective electrodes with PVC membrane, based on various ionophores, have been introduced; however, these sensors suffer from the disadvantages of a) significant interferences from other cations, b) deviation from Nernstian behavior, c) small linear range and d) narrow pH range of operation. In addition, iron is an essential metal especially for biological systems since it plays an important role in many metabolic pathways, has an effective role in oxygen transport, storage and also in electron transport. The concentration of Fe(III) in biological systems has to be efficiently balanced as both its deficiency and excess can cause various biological disorders. Also iron is a vital element in environment systems as well as industries. The same iron limit-importance holds for environmental systems, such as fresh and seawaters, in which the iron concentration is claimed to be of crucial relevance [2]. It is therefore very important for biological, clinical, environmental and industrial purposes to efficiently detect Fe3+ion. Several methods such as AAS, ICP-MS have been reported for the determination of iron. However, these methods suffer from being time consuming as they involve multiple sample manipulations, instability if a large number of samples analysis is needed, and high cost. The proposed potentiometric sensors were prepared by anchoring glyoxalbis (2-hydroxyanil) (GBHA) as well as Desferal ionophores onto TiO2/FTO glass as Cr (III) and Fe (III)-selective electrodes, respectively. These analytical devices based on nanostructured titanium dioxide were highly sensitive due to the large surface-to-volume ratio of the nanostructure, and additionally showed excellent selectivity as they did not have the disadvantages of the above-mentioned transition metal-determination methods. One of the most important components of the proposed potentiometric sensor is the working electrode. TiO2/FTO glass substrate was chosen as the working electrode material. The TiO2 film was prepared following a standard method described elsewhere [3]. The morphology of the TiO2 films was investigated by scanning electron microscopy (SEM) to assure that the TiO2 particles formed a homogenous porous film and to monitor film thickness. The TiO2/FTO substrates were subsequently modified with the ionophores of interest (GBHA and Desferal) and the potentials of the varying concentration solutions of Cr (III) and Fe (III) were read form the voltmeter upon increasing the concentration of the test solutions. Our results demonstrated that the proposed potentiometric sensors exhibit a nernstian response for Cr (III) as well as Fe (III) ions over a wide concentration range (1.0×10-7 M-1.0×10-2 M for Cr (III)), and (1.0×10-7 - 2.2×10-1 for Fe (III)). They also showed a fast response time (< 1 min), and their potential response remained unaffected of PH in a wide range (2.5-6.3 for the Cr (III)-selective nanosensor, and 2.5-7.2 for the Fe (III)-selective nanosensor). Moreover, the performance of the sensors is discussed in terms of stability, response time, and possible interferences from other ions. For the latter studies, potentiometric selectivity coefficients were determined by the Matched Potential Method (MPM) for the alkali, transition metal and other heavy metal ions as interfering ions. According to our results, the interfering ions could not disturb the functioning of the proposed sensor electrodes significantly, indicating negligible interference in the performance of the nanostructured sensor assemblies. The fabricated sensors are to be tested for the analysis of some water samples and food materials for the determination of Cr (III) and Fe (III) ions. The designed TiO2-based nanosensors offered simplicity, rapidity, and reliability as an analytical tool.

Toward High-Efficiency Solution-Processed Planar Heterojunction Sb2S3 Solar Cells.

Low‐cost hybrid solar cells have made tremendous steps forward during the past decade owing to the implementation of extremely thin inorganic coatings as absorber layers, typically in combination with organic hole transporters. Using only extremely thin films of these absorbers reduces the requirement of single crystalline high‐quality materials and paves the way for low‐cost solution processing compatible with roll‐to‐roll fabrication processes. To date, the most efficient absorber material, except for the recently introduced organic–inorganic lead halide perovskites, has been Sb2S3, which can be implemented in hybrid photovoltaics using a simple chemical bath deposition. Current high‐efficiency Sb2S3 devices utilize absorber coatings on nanostructured TiO2 electrodes in combination with polymeric hole transporters. This geometry has so far been the state of the art, even though flat junction devices would be conceptually simpler with the additional potential of higher open circuit voltages due to reduced charge carrier recombination. Besides, the role of the hole transporter is not completely clarified yet. In particular, additional photocurrent contribution from the polymers has not been directly shown, which points toward detrimental parasitic light absorption in the polymers. This study presents a fine‐tuned chemical bath deposition method that allows fabricating solution‐processed low‐cost flat junction Sb2S3 solar cells with the highest open circuit voltage reported so far for chemical bath devices and efficiencies exceeding 4%. Characterization of back‐illuminated solar cells in combination with transfer matrix‐based simulations further allows to address the issue of absorption losses in the hole transport material and outline a pathway toward more efficient future devices.