Atomic Layer Deposition of Pd Nanoparticles on TiO₂ Nanotubes for Ethanol Electrooxidation: Synthesis and Electrochemical Properties

Enhanced CO Functionality on Carbon Papers Ensures Lowering Nucleation Delay of ALD for Ru towards Unprecedented Alkaline HER Activity

The success in lowering the nucleation delay for Atomic Layer Deposition (ALD) of Ru on carbon surfaces is mitigated by constructive pretreatments resulting enhancement of CO functionality. Treatment of the carbon papers (CP) allowed Ru species deposition for minimum number of ALD cycles (25 cycles) with good conformality. The development of electrocatalysts from single atoms to nanoparticles (NPs) on conductive supports with low metal loadings, thus improving performance, is essential in electrocatalysis. For alkaline hydrogen evolution reaction, ALD decorated CPs with Ru exhibit low onset potentials of ≈4.7 mV versus reversable hydrogen electrode (RHE) (at 10 mA cm-2 ) and a high turnover frequency of 1.92 H2 s-1 at 30 mV versus RHE. The Ru decorated CPs show comparable to higher catalytic activity than of Platinum (Pt) decorated CP also developed by ALD. The current representation of unfamiliar catalytic activities of Ru active centers developed by ALD, pave a bright and sustainable path for energy conversion reactions.

CoFe-Layered Double Hydroxide Coupled with Pd Particles for Electrocatalytic Ethanol Oxidation

Electrocatalytic efficiency and stability have emerged as critical issues in the ethanol oxidation reaction (EOR) of direct ethanol fuel cells. In this paper, Pd/Co₁Fe₃-LDH/NF as an electrocatalyst for EOR was prepared by a two-step synthetic strategy. Metal-oxygen bonds formed between Pd nanoparticles and Co₁Fe₃-LDH/NF guaranteed structural stability and adequate surface-active site exposure. More importantly, the charge transfer of the formed Pd-O-Co(Fe) bridge could effectively modulate the electrical structure of hybrids, improving the facilitated absorption of OH^- radicals and oxidation of CO_ads. Benefiting from the interfacial interaction, exposed active sites, and structural stability, the observed specific activity for Pd/Co₁Fe₃-LDH/NF (17.46 mA cm^-2) was 97 and 73 times higher than those of commercial Pd/C (20%) (0.18 mA cm^-2) and Pt/C (20%) (0.24 mA cm^-2), respectively. Besides, the j_f/j_r ratio representing the resistance to catalyst poisoning was 1.92 in the Pd/Co₁Fe₃-LDH/NF catalytic system. These results provide insights into optimizing the electronic interaction between metals and the support of electrocatalysts for EOR.

Review of the Electrospinning Process and the Electro-Conversion of 5-Hydroxymethylfurfural (HMF) into Added-Value Chemicals

Electrochemical converters (electrolyzers, fuel cells, and batteries) have gained prominence during the last decade for the unavoidable energy transition and the sustainable synthesis of platform chemicals. One of the key elements of these systems is the electrode material on which the electrochemical reactions occur, and therefore its design will impact their performance. This review focuses on the electrospinning method by examining a number of features of experimental conditions. Electrospinning is a fiber-spinning technology used to produce three-dimensional and ultrafine fibers with tunable diameters and lengths. The thermal treatment and the different analyses are discussed to understand the changes in the polymer to create usable electrode materials. Electrospun fibers have unique properties such as high surface area, high porosity, tunable surface properties, and low cost, among others. Furthermore, a little introduction to the 5-hydroxymethylfurfural (HMF) electrooxidation coupled to H₂ production was included to show the benefit of upgrading biomass derivates in electrolyzers. Indeed, environmental and geopolitical constraints lead to shifts towards organic/inorganic electrosynthesis, which allows for one to dispense with polluting, toxic and expensive reagents. The electrooxidation of HMF instead of water (OER, oxygen evolution reaction) in an electrolyzer can be elegantly controlled to electro-synthesize added-value organic chemicals while lowering the required energy for CO₂-free H₂ production.

Decoration of titanium dioxide nanotubes with silver nanoparticles using the photochemical deposition method and their application as an electrocatalyst to determine tinidazole

In this study, titanium nanotube electrodes were decorated with silver nanoparticles (AgNPs/TiO₂NTs) and used as an electrocatalyst for the reduction of tinidazole. AgNPs/TiO₂NTs are constructed by anodization of titanium sheet metal and photochemical deposition of AgNPs on TiO₂NTs. The structural and elemental analysis characteristics of the AgNPs/TiO₂NT electrode have been studied by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) methods. Based on the cyclic voltammetric data, it has been confirmed that the AgNPs/TiO₂NT electrode has good electrocatalytic activity to reduce tinidazole. Two liner concentration ranges of 0.2-55.0 μM and 55.0-111.2 μM were obtained by amperometric method. A detection limit of 60.9 nM was obtained for measuring tinidazole at the AgNPs/TiO₂NT electrode surface. In addition, the designed sensor has been successfully used for quantitative measurement of tinidazole in pharmaceutical and human urine samples.

Atomic/molecular layer deposition for energy storage and conversion

Energy storage and conversion systems, including batteries, supercapacitors, fuel cells, solar cells, and photoelectrochemical water splitting, have played vital roles in the reduction of fossil fuel usage, addressing environmental issues and the development of electric vehicles. The fabrication and surface/interface engineering of electrode materials with refined structures are indispensable for achieving optimal performances for the different energy-related devices. Atomic layer deposition (ALD) and molecular layer deposition (MLD) techniques, the gas-phase thin film deposition processes with self-limiting and saturated surface reactions, have emerged as powerful techniques for surface and interface engineering in energy-related devices due to their exceptional capability of precise thickness control, excellent uniformity and conformity, tunable composition and relatively low deposition temperature. In the past few decades, ALD and MLD have been intensively studied for energy storage and conversion applications with remarkable progress. In this review, we give a comprehensive summary of the development and achievements of ALD and MLD and their applications for energy storage and conversion, including batteries, supercapacitors, fuel cells, solar cells, and photoelectrochemical water splitting. Moreover, the fundamental understanding of the mechanisms involved in different devices will be deeply reviewed. Furthermore, the large-scale potential of ALD and MLD techniques is discussed and predicted. Finally, we will provide insightful perspectives on future directions for new material design by ALD and MLD and untapped opportunities in energy storage and conversion.

Preparation of ZnO/two-layer self-doped black TiO2 nanotube arrays and their enhanced photochemical properties

Highly efficient TiO₂ photoanodes can be achieved by enhancing electrical conductivity and improving charge separation and transfer. In this paper, Ti foils were used to fabricate TiO₂ nanotubes by anodic oxidation and ZnO/two-layer self-doped black TiO₂ nanotubes were prepared by electrochemical reduction and a hydrothermal method. The formed black TiO₂ nanotubes have a better photoconversion efficiency and the maximum photoconversion efficiency increased by 59% compared with the pure nanotubes. The deposition of ZnO further improves the maximum photoconversion efficiency to 456% based on black TiO₂. The photocurrent responses also increase by about 5 times in our results. This work is instructive for the development of highly robust and efficient photoanode materials in fields including photoelectrochemistry and photocatalysis.

Highly efficient TiO₂ photoanodes can be achieved by enhancing electrical conductivity and improving charge separation and transfer.

Silver-Decorated TiO2 Inverse Opal Structure for Visible Light-Induced Photocatalytic Degradation of Organic Pollutants and Hydrogen Evolution

TiO₂ inverse opal (TIO) structures were prepared by the conventional wet chemical method, resulting in well-formed structures for photocatalytic activity. The obtained structures were functionalized with liquid flame spray-deposited silver nanoparticles (AgNPs). The nanocomposites of TIO and AgNPs were extensively characterized by various spectroscopies such as UV, Raman, X-ray diffraction, energy-dispersive spectroscopy, and X-ray photoelectron spectroscopy combined with microscopic methods such as scanning electron microscopy, transmission electron microscopy (TEM), and high-resolution TEM. The characterization confirmed that high-quality heterostructures had been fabricated with evenly and uniformly distributed AgNPs. Fabrication of anatase TiO₂ was confirmed, and formation of AgNPs was verified with surface plasmon resonant properties. The photocatalytic activity results measured in the gas phase showed that deposition of AgNPs increases photocatalytic activity both under UVA and visible light excitation; moreover, enhanced hydrogen evolution was demonstrated under visible light.

The Effect of Laser Re-Solidification on Microstructure and Photo-Electrochemical Properties of Fe-Decorated TiO2 Nanotubes

Fossil fuels became increasingly unpleasant energy source due to their negative impact on the environment; thus, attractiveness of renewable, and especially solar energy, is growing worldwide. Among others, the research is focused on smart combination of simple compounds towards formation of the photoactive materials. Following that, our work concerns the optimized manipulation of laser light coupled with the iron sputtering to transform titania that is mostly UV-active, as well as exhibiting poor oxygen evolution reaction to the material responding to solar light, and that can be further used in water splitting process. The preparation route of the material was based on anodization providing well organized system of nanotubes, while magnetron sputtering ensures formation of thin iron films. The last step covering pulsed laser treatment of 355 nm wavelength significantly changes the material morphology and structure, inducing partial melting and formation of oxygen vacancies in the elementary cell. Depending on the applied fluence, anatase, rutile, and hematite phases were recognized in the final product. The formation of a re-solidified layer on the surface of the nanotubes, in which thickness depends on the laser fluence, was shown by microstructure studies. Although a drastic decrement of light absorption was recorded especially in UV range, laser-annealed samples have shown activity under visible light even 20 times higher than bare titania. Electrochemical analysis has shown that the improvement of photoresponse originates mainly from over an order of magnitude higher charge carrier density as revealed by Mott-Schottky analysis. The results show that intense laser light can modulate the semiconductor properties significantly and can be considered as a promising tool towards activation of initially inactive material for the visible light harvesting.

TiO2 Nanotubes with Pt and Pd Nanoparticles as Catalysts for Electro-Oxidation of Formic Acid

In the present work, the magnetron sputtering technique was used to prepare new catalysts of formic acid electrooxidation based on TiO₂ nanotubes decorated with Pt (platinum), Pd (palladium) or Pd + Pt nanoparticles. TiO₂ nanotubes (TiO₂ NTs) with strictly defined geometry were produced by anodization of Ti foil and Ti mesh in a mixture of glycerol and water with ammonium fluoride electrolyte. The above mentioned catalytically active metal nanoparticles (NPs) were located mainly on the top of the TiO₂ NTs, forming 'rings' and agglomerates. A part of metal nanoparticles decorated also TiO₂ NTs walls, thus providing sufficient electronic conductivity for electron transportation between the metal nanoparticle rings and Ti current collector. The electrocatalytic activity of the TiO₂ NTs/Ti foil, decorated by Pt, Pd and/or Pd + Pt NPs was investigated by cyclic voltammetry (CV) and new Pd/TiO₂ NTs/Ti mesh catalyst was additionally tested in a direct formic acid fuel cell (DFAFC). The results so obtained were compared with commercial catalyst-Pd/Vulcan. CV tests have shown for carbon supported catalysts, that the activity of TiO₂ NTs decorated with Pd was considerably higher than that one decorated with Pt. Moreover, for TiO₂ NTs supported Pd catalyst specific activity (per mg of metal) was higher than that for well dispersed carbon supported commercial catalyst. The tests at DFAFC have revealed also that the maximum of specific power for 0.2 Pd/TiO₂ catalyst was 70% higher than that of the commercial one, Pd/Vulcan. Morphological features, and/or peculiarities, as well as surface composition of the resulting catalysts have been studied by scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and chemical surface analytical methods (X-ray photoelectron spectroscopy-XPS; Auger electron spectroscopy-AES).

Combining nanoparticles grown by ALD and MOFs for gas separation and catalysis applications

Supported metallic nanoparticles (NPs) are essential for many important chemical processes. In order to implement precisely tuned NPs in miniaturized devices by compatible processes, novel nanoengineering routes must be explored. Atomic layer deposition (ALD), a scalable vapor phase technology typically used for the deposition of thin films, represents a promising new route for the synthesis of supported metallic NPs. Metal–organic frameworks (MOFs) are a new exciting class of crystalline porous materials that have attracted much attention in the recent years. Since the size of their pores can be precisely adjusted, these nanomaterials permit highly selective separation and catalytic processes. The combination of NPs and MOF is an emerging area opening numbers of applications, which still faces considerable challenges, and new routes need to be explored for the synthesis of these NPs/MOF nanocomposites. The aim of this paper is double: first, it aims to briefly present the ALD route and its use for the synthesis of metallic NPs. Second, the combination of ALD-grown NPs and MOFs has been explored for the synthesis of Pd NPs/MOF ZIF-8, and several selected examples were ALD-grown NPs and MOFs have been combined and applied gas separation and catalysis will be presented.

Bimetallic alloy and semiconductor support synergistic interaction effects for superior electrochemical catalysis

The design and fabrication of economically viable anode catalysts for the methanol oxidation reaction (MOR) have been challenging issues in direct methanol fuel cells (DMFCs) over the decades. In this work, a composite electrochemical catalyst of Pd-coupled Ag and ZnO for the possible replacement of expensive Pt catalysts in DMFCs is successfully prepared. The as-made Pd@Ag/ZnO exhibits specific activity, which is 1.8-fold, 2.8-fold, and 4.6-fold higher than that of a Pd/ZnO catalyst, 20% Pd/C catalyst and Pd black, respectively. The improvement of the catalytic mechanism is likely due to the synergistic interaction between Pd@Ag and ZnO. The density functional theory (DFT) calculation results confirm that Ag doped into Pd weakens the adsorption of CO, dramatically improving the capability to resist CO poisoning.

Recent advances in the electrooxidation of biomass-based organic molecules for energy, chemicals and hydrogen production

The recent developments in biomass-derivative fuelled electrochemical converters for electricity or hydrogen production together with chemical electrosynthesis have been reviewed.

Dual application of Shewanella oneidensis MR-1 in green biosynthesis of Pd nanoparticles supported on TiO2 nanotubes and assisted photocatalytic degradation of methylene blue

Biosynthesised nanocomposites have attracted growing interests attributed to their 'green' synthesis nature in recent years. Shewanella oneidensis MR-1, a dissimilatory metal-reducing bacterium, was used to reduce palladium (II) nitrate to palladium (0) nanoparticles (Pd NPs) under anaerobic conditions, resulting in the in situ formation of Pd NPs immobilised on TiO₂ nanotubes (TNTs) (Pd/TNTs nanocomposites). The Pd/TNTs nanocomposites were characterised by transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, energy dispersive X-ray, and electron spin resonance, respectively. The results indicated that Pd NPs are successfully grown on the TNTs without aggregation. Photocatalytic degradation of methylene blue (MB) by Pd/TNTs nanocomposites under simulated sunlight was also investigated. Pd/TNTs nanocomposites had photocatalytic efficiency superior to that of single TiO₂ nanomaterials. The photocatalytic activity of Pd/TNTs nanocomposites can be enhanced by S. oneidensis MR-1. The results showed that after only 10 min, the degradation ratio of MB reached 98.7% by Pd/TNTs nanocomposites when simultaneously assisted with S. oneidensis MR-1.

Galvanic Replacement-Mediated Synthesis of Ni-Supported Pd Nanoparticles with Strong Metal-Support Interaction for Methanol Electro-oxidation

Herein, well-defined Pd nanoparticles (NPs) developed on Ni substrate (Pd NPs/Ni) are synthesized via a facile galvanic replacement reaction (GRR) route performed in ethaline-based deep eutectic solvent (DES). For comparison, a Pd NPs/Ni composite is also prepared by the GRR method conducted in an aqueous solution. The Pd NPs/Ni obtained from the ethaline-DES is catalytically more active and durable for the methanol electro-oxidation reaction (MOR) than those of the counterpart derived from conventional aqueous solution and commercial Pd/C under alkaline media. Detailed kinetic analysis indicates that the unique solvent environment offered by ethaline plays vital roles in adjusting the reactivity of the active species and their mass transport properties to control over the genesis of the Pd NPs/Ni nanocomposite. The resulting Pd NPs/Ni catalyst possesses a homogeneous dispersion of Pd NPs with a strong Pd (metal)-Ni (support) interaction. This structure enhances the charge transfer between the support and the active phases, and optimizes the adsorption energy of OH^- and CO on the surface, leading to superior electrocatalytic performance. This work provides a novel GRR strategy performed in ethaline-DES to the rational design and construction of advanced metal/support catalysts with strong interaction for improving the activity and durability for MOR.

Boron Nitride as a Novel Support for Highly Stable Palladium Nanocatalysts by Atomic Layer Deposition

The ability to prepare controllable nanocatalysts is of great interest for many chemical industries. Atomic layer deposition (ALD) is a vapor phase technique enabling the synthesis of conformal thin films and nanoparticles (NPs) on high surface area supports and has become an attractive new route to tailor supported metallic NPs. Virtually all the studies reported, focused on Pd NPs deposited on carbon and oxide surfaces. It is, however, important to focus on emerging catalyst supports such as boron nitride materials, which apart from possessing high thermal and chemical stability, also hold great promises for nanocatalysis applications. Herein, the synthesis of Pd NPs on boron nitride (BN) film substrates is demonstrated entirely by ALD for the first time. X-ray photoelectron spectroscopy indicated that stoichiometric BN formed as the main phase, with a small amount of BNxOy, and that the Pd particles synthesized were metallic. Using extensive transmission electron microscopy analysis, we study the evolution of the highly dispersed NPs as a function of the number of ALD cycles, and the thermal stability of the ALD-prepared Pd/BN catalysts up to 750 °C. The growth and coalescence mechanisms observed are discussed and compared with Pd NPs grown on other surfaces. The results show that the nanostructures of the BN/Pd NPs were relatively stable up to 500 °C. Consequent merging has been observed when annealing the samples at 750 °C, as the NPs' average diameter increased from 8.3 ± 1.2 nm to 31 ± 4 nm. The results presented open up exciting new opportunities in the field of catalysis.

High-Performance Nanowire Hydrogen Sensors by Exploiting the Synergistic Effect of Pd Nanoparticles and Metal-Organic Framework Membranes

Herein, we report the fabrication of hydrogen gas sensors with enhanced sensitivity and excellent selectivity. The sensor device is based on the strategic combination of ZnO nanowires (NWs) decorated with palladium nanoparticles (Pd NPs) and a molecular sieve metal-organic framework (MOF) nanomembrane (ZIF-8). The Pd NPs permit the sensors to reach maximal signal responses, whereas the ZIF-8 overcoat enables for an excellent selectivity. Three steps were employed for the fabrication: (i) coating of a miniaturized sensor with vapor-grown ZnO NWs, (ii) decoration of these NWs with Pd NPs by atomic layer deposition, and (iii) partial solvothermal conversion of the tuned NWs surface to ZIF-8 nanomembrane. The microstructure and composition investigations of the ZIF-8/Pd/ZnO nanostructured materials confirmed the presence of both metallic Pd NPs and uniform ZIF-8 thin membrane layer. The integration of these nanomaterials within a miniaturized sensor device enabled the assessment of their performance for H₂ detection at concentrations as low as 10 ppm in the presence of various gases such as C₆H₆, C₇H₈, C₂H₅OH, and CH₃COCH₃. Remarkably high-response signals of 3.2, 4.7, and 6.7 ( R_a/ R_g) have been measured for H₂ detection at only 10, 30, and 50 ppm, whereas no noticeable response toward other tested gases was detected, thus confirming the excellent H₂ selectivity obtained with such a sensor design. The results obtained showed that the performance of gas sensors toward H₂ gas can be greatly increased by both the addition of Pd NPs and the use of ZIF-8 coating, acting as a molecular sieve membrane. Furthermore, the presented strategy could be extended toward the sensing of other species by a judicious choice of both the metallic NPs and MOF materials with tuned properties for specific molecule detection, thus opening a new avenue for the preparation of highly selective sensing devices.

A Review on the Electrochemically Self-organized Titania Nanotube Arrays: Synthesis, Modifications, and Biomedical Applications

Titania nanotubes grown by anodic oxidation have intrigued the material science community by its many unique and potential properties, and the synthesis of technology is merging to its mature stage. The present review will focus on TiO2 nanotubes grown by self-organized electrochemical anodization from Ti metal substrate, which critically highlights the synthesis of this type of self-organized titania nanotube layers and the means to influence the size, shape, the degree of order, and crystallized phases via adjusting the anodization parameters and the subsequent thermal annealing. The relationship between dimensions and properties of the anodic TiO2 nanotube arrays will be presented. The latest progress and significance of the research on formation mechanism of anodic TiO2 nanotubes are briefly discussed. Besides, we will show the most promising applications reported recently in biomedical directions and modifications carried out by doping, surface modification, and thermal annealing toward improving the properties of anodically formed TiO2 nanotubes. At last, some unsolved issues and possible future directions of this field are indicated.

Enhanced Electrocatalytic Activity of Ethanol Oxidation Reaction on Palladium-Silver Nanoparticles via Removable Surface Ligands

This work developed a facile colloidal route to synthesize BH₄^--capped Pd_xAg_y nanoparticles (NPs) in water using the reducing ionic liquids of [C_nmim]BH₄, and the resulting NPs were prone to form the nanocomposites with [amim]⁺-modified reduced graphene (RG). The removal of the metal-free inorganic ions of BH₄^- can create the profoundly exposed interfaces on the Pd_xAg_y NPs during the electrooxidation, and favor the ethanol oxidation reaction (EOR) in lowering energy barrier. The counterions of [C_nmim]⁺ can gather ethanol, OH^- ions, and the reaction intermediates on catalysts, and synergistically interact with RG to facilitate the charge transfer in nanocomposites. The interface-modified RG nanosheets can effectively segregate the Pd_xAg_y NPs from aggregation during the EOR. Along with the small size of 4.7 nm, the high alloying degree of 60.2%, the large electrochemical active surface area of 64.1 m² g^-1, and the great peak current density of 1501 mA cm^-2 mg^-1, Pd₁Ag₂@[C₂mim]BH₄-amimRG nanocomposite exhibits the low oxidation potentials, strong poison resistance, and stable catalytic activity for EOR in alkaline media, and hence can be employed as a promising anodic catalyst in ethanol fuel cells.

Niobium-doped titanium dioxide on a functionalized carbon supported palladium catalyst for enhanced ethanol electro-oxidation

Pd nanoparticles anchored on Nb-doped TiO₂ with functionalized carbon support (denoted as Pd/Nb–TiO₂–C) is synthesized through a controllable hydrolysis and impregnation method.

Preparation and Characterization of Zirconia-Coated Nanodiamonds as a Pt Catalyst Support for Methanol Electro-Oxidation

Zirconia-coated nanodiamond (ZrO₂/ND) electrode material was successfully prepared by one-step isothermal hydrolyzing from ND-dispersed ZrOCl₂·8H₂O aqueous solution. High-resolution transmission electron microscopy reveals that a highly conformal and uniform ZrO₂ shell was deposited on NDs by this simple method. The coating obtained at 90 °C without further calcination was mainly composed of monoclinic nanocrystalline ZrO₂ rather than common amorphous Zr(OH)₄ clusters. The ZrO₂/NDs and pristine ND powder were decorated with platinum (Pt) nanoparticles by electrodeposition from 5 mM chloroplatinic acid solution. The electrochemical studies indicate that Pt/ZrO₂/ND catalysts have higher electrocatalytic activity and better stability for methanol oxidation than Pt/ND catalysts in acid.

Atomic Layer Deposition for Coating of High Aspect Ratio TiO2 Nanotube Layers

We present an optimized approach for the deposition of Al₂O₃ (as a model secondary material) coating into high aspect ratio (≈180) anodic TiO₂ nanotube layers using the atomic layer deposition (ALD) process. In order to study the influence of the diffusion of the Al₂O₃ precursors on the resulting coating thickness, ALD processes with different exposure times (i.e., 0.5, 2, 5, and 10 s) of the trimethylaluminum (TMA) precursor were performed. Uniform coating of the nanotube interiors was achieved with longer exposure times (5 and 10 s), as verified by detailed scanning electron microscopy analysis. Quartz crystal microbalance measurements were used to monitor the deposition process and its particular features due to the tube diameter gradient. Finally, theoretical calculations were performed to calculate the minimum precursor exposure time to attain uniform coating. Theoretical values on the diffusion regime matched with the experimental results and helped to obtain valuable information for further optimization of ALD coating processes. The presented approach provides a straightforward solution toward the development of many novel devices, based on a high surface area interface between TiO₂ nanotubes and a secondary material (such as Al₂O₃).