Pitfalls in Photochemical and Photoelectrochemical Reduction of CO2 to Energy Products

The photochemical and photoelectrochemical reduction of CO2 is a promising approach for converting carbon dioxide into valuable chemicals (materials) and fuels. A key issue is ensuring the accuracy of experimental results in CO2 reduction reactions (CO2RRs) because of potential sources of false positives. This paper reports the results of investigations on various factors that may contribute to erroneous attribution of reduced-carbon species, including degradation of carbon species contained in photocatalysts, residual contaminants from synthetic procedures, laboratory glassware, environmental exposure, and the operator. The importance of rigorous experimental protocols, including the use of labeled 13CO2 and blank tests, to identify true CO2 reduction products (CO2RPs) accurately is highlighted. Our experimental data (eventually complemented with or compared to literature data) underline the possible sources of errors and, whenever possible, quantify the false positives with respect to the effective conversion of CO2 in clean conditions. This paper clarifies that the incidence of false positives is higher in the preliminary phase of photo-material development when CO2RPs are in the range of a few 10s of μg gcat-1 h-1, reducing its importance when significant conversions of CO2 are performed reaching 10s of mol gcat-1 h-1. This paper suggests procedures for improving the reliability and reproducibility of CO2RR experiments, thus validating such technologies.

Construction of TiO2/starch nanocomposite cryogel for ethylene removal and banana preservation

Perishability caused by natural plant hormone ethylene has attracted great attention in the field of fruit and vegetable (F&V) preservation. Various physical and chemical methods have been applied to remove ethylene, but the eco-unfriendliness and toxicity of these methods limit their application. Herein, a novel starch-based ethylene scavenger was developed by introducing TiO₂ nanoparticles into starch cryogel and applying ultrasonic treatment to further improve ethylene removal efficiency. As a porous carrier, the pore wall of cryogel provided dispersion space, which increased the area of TiO₂ exposed to UV light, thereby endowing starch cryogel with ethylene removal capacity. The photocatalytic performance of scavenger reached the maximum ethylene degradation efficiency of 89.60 % when the TiO₂ loading was 3 %. Ultrasonic treatment interrupted starch molecular chains and then promoted their rearrangement, increasing the material specific surface area from 54.6 m²/g to 225.15 m²/g and improving the ethylene degradation efficiency by 63.23 % compared with the non-sonicated cryogel. Furthermore, the scavenger exhibits good practicability for removing ethylene as a banana package. This work provides a new carbohydrate-based ethylene scavenger, utilizing as a non-food contact inner filler of F&V packaging in practical applications, which exhibits great potential in F&V preservation and broadens the application fields of starch.

Highly customized porous TiO2-PANI nanoparticles with excellent photocatalytic efficiency for dye degradation

The present work encompasses a simple strategy to synthesize highly porous TiO₂ by incorporating PANI polymer into the sol-gel chemistry of Titanium dioxide/Titanium (IV)-iso-propoxide (TiO₂/TTIP). A series of TiO₂ samples by varying wt.% of PANI, have been synthesized. A probable growth mechanism has been presented for the formation of a porous ginger-like nanostructure of TiO₂-PANI (TP). HRTEM images reveal that the particle size range is 6-16 nm for pristine TiO₂ and 5-13 nm for TP samples. XPS analysis confirms the presence of the hydrogen bonds in-between surface hydroxyl groups (Ti-OH) of TiO₂ and the protonated nitrogen of PANI. UV-visible absorption study reveals a small shift towards longer wavelength for TP8 sample than that of pristine TiO₂ (λ_max = 314 nm) as well as reduction in E_g value from 3.02 eV to 2.89 eV. FTIR results confirm the successful interaction of PANI and TiO₂. BJH and BET analysis confirms an increase of porosity in TP8 sample with an average pore volume of 0.36 cm³ g^-1. High photocatalytic activity (98.77%) towards Methylene blue dye degradation is observed for TP8 sample having 8 wt% of PANI and it is explained through the combined effect of structural porosity of TiO₂ and synergic effect of PANI. The K_appa value at pH 11 (0.01372 min^-1) is found to be 7.84-folds higher than that of the photocatalytic reaction at pH 3 (K_appa = 0.00175 min^-1). While pristine TiO₂ exhibits the minimum removal efficiency (89.57%) with K_appa of 0.00756 min^-1. K_appa value of catalysis reaction for TP8 is found to be almost 2-fold higher than pristine TiO₂. Quantum Yield value for TP8 is found to be 3.59 × 10^-4 molecules photon^-1. This high Quantum Yield value of present photocatalytic system explicates the low energy consumption for the treatment of textile dye pollutant. Additionally, STY value (1.79 × 10^-5 molecules photon^-1 mg^-1) confirms the outstanding mineralization strength of TP8 by a unit mass for high amounts of MB dye per unit time. Thus, the present study offers an excellent photocatalyst i.e., TP8 having 8 wt% of PANI for the degradation of MB dye.

Structures and electric properties of PANI/polymorphic-ZrO2 composites

A polyaniline/zirconia (PANI/ZrO₂) composite has been successfully synthesized using an in situ method with HCl used as a doping agent, SDBS as a surfactant and APS as an oxidant. The filler variations were comprised of various phases, which are amorphous, tetragonal, monoclinic and a mixture of tetragonal–monoclinic and varied volumes of 0, 2.5, 5.0, 7.5 and 10%. Amorphous, tetragonal and a mixture of tetragonal–monoclinic ZrO₂ were obtained from purifying natural zircon sand (ZrSiO₄), while monoclinic ZrO₂ was obtained from commercial materials. The structure characterisation was performed using X-ray diffraction and Fourier Transform Infrared (FTIR). The morphology was observed using a Scanning Electron Microscope (SEM). The electrical properties of impedance, dielectric constant and dielectric losses, and electrical conductivity were characterised using Electrochemical Impedance Spectroscopy (EIS). The research results showed that the filler phase had an effect towards the electrical properties of PANI/ZrO₂. The impedance of PANI as well as PANI/ZrO₂ showed a constant phase element (CPE) behaviour with impedance values of 9.87, 9.16, 13.31, and 79.59 kΩ for the composites with amorphous filler, monoclinic filler, tetragonal filler, and tetragonal–monoclinic filler, respectively. The dielectric constant and dielectric losses decreased with the increasing amount of ZrO₂ filler. The AC conductivity of PANI/ZrO₂ was found to be lower when compared to PANI for all of the filler phases and volume fractions.

A polyaniline/zirconia (PANI/ZrO₂) composite has been successfully synthesized using an in situ method with HCl used as a doping agent, SDBS as a surfactant and APS as an oxidant.

Stable and Efficient Photoinduced Charge Transfer of MnFe2O4/Polyaniline Photoelectrode in Highly Acidic Solution

Tailoring conductive polymers with inorganic photocatalysts, which provide photoinduced electron-hole generation, have significantly enhanced composites leading to excellent photoelectrodes. In this work, MnFe2O4 nanoparticles prepared by a hydrothermal method were combined with polyaniline to prepare mixed (hybrid) slurries, which were cast onto flexible FTO to prepare photoelectrodes. The resulting photoelectrodes were characterized by XRD, FESEM, HRTEM and UV-VIS. The photoelectrochemical performance was investigated by linear sweep voltammetry and chronoamperometry. The photocurrent achieved by MnFe2O4/Polyaniline was 400 μA/cm2 at 0.8 V vs. Ag/AgCl in Na2SO4 (pH = 2) at 100 mW/cm2, while polyaniline alone achieved only 25 μA/cm2 under the same conditions. The best MnFe2O4/Polyaniline displayed an incident photon-to-current conversion efficiency (IPCE) and applied bias photon-to-current efficiency (ABPE) of 60% at 405 nm wavelength, and 0.17% at 0.8 V vs. Ag/AgCl, respectively. High and stable photoelectrochemical performance was achieved for more than 900 s in an acidic environment.

Advanced Photocatalysts Based on Conducting Polymer/Metal Oxide Composites for Environmental Applications

Photocatalysts provide a sustainable method of treating organic pollutants in wastewater and converting greenhouse gases. Many studies have been published on this topic in recent years, which signifies the great interest and attention that this topic inspires in the community, as well as in scientists. Composite photocatalysts based on conducting polymers and metal oxides have emerged as novel and promising photoactive materials. It has been demonstrated that conducting polymers can substantially improve the photocatalytic efficiency of metal oxides owing to their superior photocatalytic activities, high conductivities, and unique electrochemical and optical properties. Consequently, conducting polymer/metal oxide composites exhibit a high photoresponse and possess a higher surface area allowing for visible light absorption, low recombination of charge carriers, and high photocatalytic performance. Herein, we provide an overview of recent advances in the development of conducting polymer/metal oxide composite photocatalysts for organic pollutant degradation and CO2 conversion through photocatalytic processes.

Efficient non-metal based conducting polymers for photocatalytic hydrogen production: comparative study between polyaniline, polypyrrole and PEDOT

Incorporation of conducting polymers (CPs) with TiO2 is considered a promising pathway toward the fabrication of highly efficient non-metal based photocatalysts. Herein, we report the fabrication of TiO2@polyaniline, TiO2@polypyrrole, and TiO2@poly(3,4-ethylenedioxythiophene) photocatalyst heterostructures via the facile wet incipient impregnation method. The mass ratios of CPs in the composites were optimized. The structure, morphology, optical and surface texture of the samples were characterized by XRD, TEM, TGA, DRS, and N2-physisorption techniques. The TiO2@2PEDOT, TiO2@2PPy, and TiO2@5PAn composites were found to exhibit the highest H2 evolution rate (HER) of 1.37, 2.09, and 3.1 mmol h-1 g-1, respectively. Compared to bare TiO2, the HER was significantly enhanced by 16, 24, and 36-fold, respectively. Photoelectrochemical measurements (CV, CA and EIS) were conducted, to evaluate the photoelectric properties of the synthesized composites and assist in understanding the photocatalytic mechanism. The deposition method plays a key-role in forming the photocatalyst/CP interface. This simple impregnation route was found to provide an excellent interface for charge transfer between composite components compared to chemisorption and in situ polymerization methods. This study sheds light on the promising effect of CP incorporation with semiconductor photocatalysts, as a cheap and efficient matrix, on photocatalytic performance.

Hierarchical PANI/ZnO nanocomposite: synthesis and synergistic effect of shape-selective ZnO nanoflowers and polyaniline sensitization for efficient photocatalytic dye degradation and photoelectrochemical water splitting

Hierarchical nanoflowers (NFs) of zinc oxide (ZnO) have been synthesized in the hexagonal wurtzite structure by a facile hydrothermal method. Polyaniline (PANI) has been prepared by the chemical oxidative polymerization method and incorporated with ZnO NFs by the chemisorption method. The potential of the synthesized nanostructures has been demonstrated for efficient photocatalytic degradation of methylene blue (MB) and photoelectrochemical water splitting. The PANI/ZnO nanocomposite has exhibited the enhanced photocatalytic activity which is ∼9 fold higher in comparison to pristine ZnO NFs and enhanced photocurrent density which is ∼16 fold higher than the ZnO photoanode. Importantly, ∼4 fold increment in the incident photon-to-current conversion efficiency (IPCE) is exhibited by PANI/ZnO, than that of ZnO photoanode. The remarkably enhanced photocatalytic and photoelectrochemical performance of PANI/ZnO nanocomposite is attributed to the availability of more interfacial sites facilitated by the hierarchical ZnO NFs, improved overall photoresponse due to its photosensitization with PANI and the resulting type-II heterojunction between them, which helps in the efficient separation of photogenerated charge carriers at the interface. A plausible reaction mechanism for the substantially improved performance of nanostructured PANI/ZnO towards MB degradation and water splitting has also been elucidated.

Polaron and bipolaron induced charge carrier transportation for enhanced photocatalytic H2 production

Photocatalysis is one of the facile approaches for efficient solar energy conversion and storage. However, rapid charge carrier recombination considerably decreases solar to energy conversion efficiency. Herein, polaron and bipolaron rich polypyrrole (PPy) has been utilized as a solid support for effective photogenerated charge carrier separation. Simple oxidative polymerization using a high concentration of ammonium persulfate (APS) induces radical cation/bipolaron formation in PPy due to the cleavage of π-bonds as confirmed by electron paramagnetic resonance spectroscopy (EPR). The formation of radical cations led to an increase of the dielectric constant which retards the charge carrier recombination and thereby enhances the conductivity. Moreover, the polarons and bipolarons induced charge carrier separation in photocatalytic H₂ production was studied with the well-known g-C₃N₄ photocatalyst. It is worth mentioning that compared to bare g-C₃N₄, the PPy supported system showed a drastically enhanced photocatalytic H₂ production rate. A maximum H₂ production rate of 1851 μmoles per g is achieved, which is ∼51 times higher than that of the bare g-C₃N₄ catalyst due to efficient charge carrier separation assisted by radical cations/bipolarons. Thus, utilizing this simple polaron and bipolaron rich PPy solid support could be an effective strategy and alternative for using noble metal cocatalysts to enhance charge carrier separation.

Preparation and Photocatalytic Properties of Heterostructured Ceria/Polyaniline Nanoparticles

Cerium dioxide (CeO2, ceria), a promising and abundant catalytic material with high-efficiency, nontoxicity, photochemical stability, and affordability, can be used as a photocatalyst to photocatalytically degrade organics and split water for hydrogen production under ultraviolet (UV) irradiation (about 5% of solar energy). However, the applications of the CeO2 photocatalyst are limited due to low photocatalytic efficiency under sunlight irradiation. In this study, a nanosized CeO2 powder was prepared by the precipitation method. Subsequently, various amounts of polyaniline (PANI) nanoparticles were deposited onto the surface of the CeO2 nanoparticles to form a heterostructure by the polymerization method. The crystal structure, morphology, surface and optical properties of the CeO2/PANI nanoparticles were investigated by X-ray diffractometry (XRD), scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), ultraviolet-visible (UV-Vis) absorption spectroscopy, and photoluminescence (PL). Experimental results demonstrated that PANI deposition improved the light absorption of CeO2 nanoparticles in the visible light region. The heterostructured CeO2/PANI nanoparticle with 4 wt % PANI deposition exhibited optimal photocatalytic activities with a hydrogen production rate of 462 μmolg−1 within 6 h and a methyl orange (MO) degradation rate of 45% within 4 h under visible light irradiation. The photocatalytic mechanisms of the composite powder are also proposed in this report.

Green synthesis of conductive polyaniline by Trametes versicolor laccase using a DNA template

The green synthesis of highly conductive polyaniline by using two biological macromolecules, i.e laccase as biocatalyst, and DNA as template/dopant, was achieved in this work. Trametes versicolor laccase B (TvB) was found effective in oxidizing both aniline and its less toxic/mutagenic dimer N-phenyl-p-phenylenediamine (DANI) to conductive polyaniline. Reaction conditions for synthesis of conductive polyanilines were set-up, and structural and electrochemical properties of the two polymers were extensively investigated. When the less toxic aniline dimer was used as substrate, the polymerization reaction was faster and gave less-branched polymer. DNA was proven to work as hard template for both enzymatically synthesized polymers, conferring them a semi-ordered morphology. Moreover, DNA also acts as dopant leading to polymers with extraordinary conductive properties (∼6 S/cm). It can be envisaged that polymer properties are magnified by the concomitant action of DNA as template and dopant. Herein, the developed combination of laccase and DNA represents a breakthrough in the green synthesis of conductive materials.

The design of a polyaniline-decorated three dimensional W18O49 composite for full solar spectrum light driven photocatalytic removal of aqueous nitrite with high N2 selectivity

Photocatalysis using solar energy is the most promising green technology for nitrite removal. However, effective photocatalytic performance is often challenged by the limited light absorption, utilization of expensive noble metals and undesired products (nitrate and ammonium). Here, we report for the first time that a full solar light response polyaniline-decorated three dimensional W₁₈O₄₉ composite (PANI@W₁₈O₄₉), a noble metal-free photocatalyst, possesses excellent photocatalytic activity for aqueous nitrite removal with high N₂ selectivity. The prepared sample was thoroughly identified via XRD, Raman, FTIR, SEM, TEM, UV-vis DRS and PL. The catalytic results demonstrated that over 80% N₂ selectivity (initial concentration 1.0 mM) was achieved through the PANI@W₁₈O₄₉ without sacrificial agent under 300 W Xe lamp irradiation for 60 min. Such advantages were attributed to the built-in junction between n-type W₁₈O₄₉ and p-type PANI, offering suitable redox levels of electron-hole pairs for NO₂^- reaction. The modification of PANI also benefited the light harvesting ability and activated carriers migration, the calculated rate constant of PANI@W₁₈O₄₉ is about four times as high as that of W₁₈O₄₉. The current study not only prepared a promising photocatalyst, but also provides new insights into improving the photocatalytic activity and N₂ selectivity for nitrite treatment.

Nafion®-coated mesoporous Pd film toward remarkably enhanced detection of lactic acid

Mesoporous metal films can detect biomarkers with high sensitivity. Further coating the mesoporous metal with polymers enhances sensing selectivity by favoring specific biomarkers against other interferents. In the present study, we report the fabrication of a Nafion®-coated mesoporous Pd film to filtrate interferents present in sweat during non-invasive biosensing. By using a Nafion®-coated mesoporous Pd film, lactic acid, a metabolite present in sweat, can be successfully detected with high sensitivity.

Visible light-induced photocatalytic degradation of Reactive Blue-19 over highly efficient polyaniline-TiO2 nanocomposite: a comparative study with solar and UV photocatalysis

Polyaniline-TiO₂ (PANI-TiO₂) nanocomposite was prepared by in situ polymerisation method. X-ray diffractogram (XRD) showed the formation of PANI-TiO₂ nanocomposite with the average crystallite size of 46 nm containing anatase TiO₂. The PANI-TiO₂ nanocomposite consisted of short-chained fibrous structure of PANI with spherical TiO₂ nanoparticles dispersed at the tips and edge of the fibres. The average hydrodynamic diameter of the nanocomposite was 99.5 nm. The band gap energy was 2.1 eV which showed its ability to absorb light in the visible range. The nanocomposite exhibited better visible light-mediated photocatalytic activity than TiO₂ (Degussa P25) in terms of degradation of Reactive Blue (RB-19) dye. The photocatalysis was favoured under initial acidic pH, and complete degradation of 50 mg/L dye could be achieved at optimum catalyst loading of 1 g/L. The kinetics of degradation followed the Langmuir-Hinshelhood model. PANI-TiO₂ nanocomposite showed almost similar photocatalytic activity under UV and visible light as well as in the solar light which comprises of radiation in both UV and visible light range. Chemical oxygen demand removal of 86% could also be achieved under visible light, confirming that simultaneous mineralization of the dye occurred during photocatalysis. PANI-TiO₂ nanocomposites are promising photocatalysts for the treatment of industrial wastewater containing RB-19 dye.

3D micro-structured arrays of ZnΟ nanorods

The fabrication of nanostructures with controlled assembly and architecture is very important for the development of novel nanomaterial-based devices. We demonstrate that laser techniques coupled with low-temperature hydrothermal growth enable complex three-dimensional ZnO nanorod patterning on various types of substrates and geometries. This methodology is based on a procedure involving the 3D scaffold fabrication using Multi-Photon Lithography of a photosensitive material, followed by Zn seeded Aqueous Chemical Growth of ZnO nanorods. 3D, uniformly aligned ZnO nanorods are produced. The increase in active surface area, up to 4.4 times in the cases presented here, provides a dramatic increase in photocatalytic performance, while other applications are also proposed.

Synthesis, Property Characterization and Photocatalytic Activity of the Polyaniline/BiYTi₂O₇ Polymer Composite

A new polyaniline/BiYTi₂O₇ polymer composite was synthesized by chemical oxidation in-situ polymerization method for the first time. The effect of polyaniline doping on structural and catalytic properties of BiYTi₂O₇ was reported. The structural properties of novel polyaniline/BiYTi₂O₇ have been characterized by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and UV-Vis DRS. The results showed that BiYTi₂O₇ crystallized well with the pyrochlore-type structure, stable cubic crystal system by space group Fd3m. The lattice parameter or band gap energy of BiYTi₂O₇ was found to be a = 10.2132 Å or 2.349 eV, respectively. The novel polyaniline/BiYTi₂O₇ polymer composite possessed higher catalytic activity compared with BiYTi₂O₇ or nitrogen doped TiO₂ for photocatalytic degradation of Azocarmine G under visible light irradiation. Additionally, the Azocarmine G removal efficiency was boosted from 3.0% for undoped BiYTi₂O₇ to 78.0% for the 10% polyaniline-modified BiYTi₂O₇, after only 60 min of reaction. After visible light irradiation for 330 min with polyaniline/BiYTi₂O₇ polymer composite as photocatalyst, complete removal and mineralization of Azocarmine G was observed. The photocatalytic degradation of Azocarmine G followed first-order reaction kinetics. Ultimately, the promoter action of H₂O₂ for photocatalytic degradation of AG with BiYTi₂O₇ as catalyst in the wastewater was discovered.

Photodegradation of diethyl phthalate with PANi/CNT/TiO2 immobilized on glass plate irradiated with visible light and simulated sunlight-effect of synthesized method and pH

Diethyl phthalate (DEP) is one of the most common phthalates for industrial use and has widely spread in environment. A series of PANi/CNT/TiO₂ potocatalysts immobilized on glass plate irradiated with visible light were presented to degrade DEP in this study. The PANi/CNT/TiO₂ potocatalysts were fabricated by co-doping with polyaniline (PANi) and two functionalized CNT (CNT-COCl and CNT-COOH) onto TiO₂ followed by a hydrothermal synthesis and a sol-gel hydrolysis. Doping of PANi resulted in the absorption edge of the fabricated potocatalysts shifting to 421-437nm and the most distinguished red-shift effect was found in hydrothermal synthesized photocatalysts. The best DEP degradation of 41.5-59.0% and 44.5-67.4% was found in the simulated sunlight system irradiated for 120min for sol-gel hydrolysis PANi/CNT/TiO₂ photocatalysts and hydrothermal synthesized ones, respectively. The optimum pH was determined at 5.0 and 7.0 for the two PANi/CNT/TiO₂ photocatalysts mentioned above, respectively. The reusability of the sol-gel hydrolyzed photocatalysts up to 5 times was observed no decline in the photodegradation efficiency but less photocatalytic stability of the hydrothermal synthesized ones was found. Meanwhile, the active species of OH radicals generated in the DEP degradation system was identified by free radical scavenging experiments.

Facile route to a conducting ternary polyaniline@TiO2/GN nanocomposite for environmentally benign applications: photocatalytic degradation of pollutants and biological activity

A polyaniline@TiO₂/graphene (Pani@TiO₂/GN) nanocomposite was prepared by the in situ oxidative polymerization of aniline in the presence of TiO₂ and GN nanoparticles.