Thin Films Fabricated by Pulsed Laser Deposition for Electrocatalysis

Abstract

Open AccessRenewablesREVIEWS20 Jan 2023Thin Films Fabricated by Pulsed Laser Deposition for Electrocatalysis Hainan Sun†, Hyunseung Kim†, Xiaomin Xu, Liangshuang Fei, WooChul Jung and Zongping Shao Hainan Sun† Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141 †H. Sun and H. Kim contributed equally to this work.Google Scholar More articles by this author , Hyunseung Kim† Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141 †H. Sun and H. Kim contributed equally to this work.Google Scholar More articles by this author , Xiaomin Xu WA School of Mines: Minerals, Energy and Chemical Engineering (WASM-MECE), Curtin University, Perth, Western Australia 6845 Google Scholar More articles by this author , Liangshuang Fei State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, Nanjing 211816 Google Scholar More articles by this author , WooChul Jung *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141 Google Scholar More articles by this author and Zongping Shao *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] WA School of Mines: Minerals, Energy and Chemical Engineering (WASM-MECE), Curtin University, Perth, Western Australia 6845 Google Scholar More articles by this author https://doi.org/10.31635/renewables.022.202200002 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Advances in energy conversion and storage technologies, such as water electrolyzers, rechargeable metal-air batteries, and fuel cells, have enabled a renewable and sustainable future. The efficiency and effectiveness of these technologies largely relies on the physicochemical properties of the functional materials used, specifically electrocatalysts. Pulsed laser deposition (PLD) is a powerful technique for the synthesis of thin film materials, offering a unique platform for understanding electrochemical reaction mechanisms and searching for low-cost and high-performance electrocatalysts. In this mini-review, we present the latest studies in which thin film materials (mainly focused on perovskite oxide thin films) via PLD have been actively utilized in the field of electrocatalysis. The fundamentals and advantages of PLD in the synthesis of thin films are discussed first. Then, emerging types of thin films associated with electrochemical applications are presented. Special emphasis is placed on material design methods to reveal the reaction mechanisms and establish the structure–performance relationships by understanding structural variations in precatalysts and surface reconstruction under reaction conditions. Finally, we discuss remaining challenges and future perspectives. Download figure Download PowerPoint Introduction Renewable energy storage and conversion technologies call for highly active electrocatalysts for multiple electrochemical reactions.1–6 For example, electrochemical water splitting has been demonstrated as one of the most promising technologies for the efficient production of green hydrogen.7–10 However, the overall device efficiency is hindered by the low reaction kinetics of two half-reactions, especially the oxygen evolution reaction (OER) at the anode.11–16 Thus, considerable research efforts have focused on developing high-performance oxygen evolution electrocatalysts. Exploring the reactivity descriptors is a highly efficient way to predict the structure–performance relationships of a designed electrocatalyst, bypassing the traditional trial-and-error approach.17–21 Remarkably, the d-band center, the Fermi softness, the eg occupancy, the position of the O 2p band center, the transition-metal oxygen-bond covalency, and the density of coordinatively unsaturated metal centers have all been widely used for both designing and evaluating catalytic materials toward oxygen activation.18,22,23 Notably, the electrochemical reactions typically involve the surface region of the catalysts. For example, it was reported that a reaction depth of ∼14 nm was applied for La0.5Sr0.5CoO3-δ and SrCoO3-δ during the alkaline OER process.24 However, the above-mentioned reactivity descriptors are mainly based on the bulk electronic structure of powder materials. Thus, the gap between the bulk and surface properties of the catalysts, including the composition, electronic structure, and geometric structure, hinders the accurate unraveling of structure–performance relationships.25–27 Although great efforts have been devoted to developing strategies for the synthesis of nanomaterials, it remains a major challenge to realize precise control of the composition, particle size, surface termination, crystallinity, defect density, and electrochemical performance of the obtained nanomaterials, especially by conventional chemical synthesis routes. To elucidate the nature of active sites, a well-defined model surfaces without the influence of polymer binders, carbon additives, or uncertainty in the estimations of the electrochemical surface areas of powder materials are needed.28–31 Such model surfaces can be single crystals or thin films epitaxially grown [e.g., by molecular beam epitaxy (MBE) or pulsed laser deposition (PLD)].32–34 In particular, as the use of well-defined thin films can explicitly regulate some key material properties that influence the electrocatalytic activity of the material, such as the crystal orientation, strain, and stoichiometry, model thin-film samples can be ideal model systems to establish and understand the structure–activity relationships.30,31,35–40 PLD is a type of physical vapor deposition technique that is used to fabricate thin films.41–44 However, studies on well-defined PLD thin films compared to powder ink-based films are rare in the field of renewables and catalysts. A timely and systematic review of electrocatalysis based on thin films synthesized by PLD is also highly desired. This focused review aims to summarize the recent advances in thin films based on the PLD technique for promoting electrocatalysis. Herein, we begin with a brief introduction of the fundamentals and advantages of PLD for the synthesis of thin film materials. The recent developments of thin films toward their applications in electrocatalysis are summarized. In addition, we emphasize design strategies, aiming to provide an in-depth understanding of reaction mechanisms and the designs of advanced electrocatalysts. Finally, the remaining challenges and perspectives regarding future research directions are also provided. Fundamentals and Advantages of PLD in the Synthesis of Thin Film Materials PLD, a type of physical vapor deposition technique, has some obvious advantages with regard to the synthesis of thin film nanomaterials, including ferroelectrics, ferrites, amorphous diamonds, and other ultrahard phases, biocompatible and tribological coatings, polymers, compound semiconductors, and nanocrystalline materials.38,45,46 The advantages of applying PLD to study various forms of thin films include the ease in creating artificial structures and metastable phases and the systematic control over their properties. Working Mechanism The working mechanism of PLD lies in the pulsed laser ablation process. The underlying ablation process is complex but the concept of PLD is simple. The focused beam on the polycrystalline target induces rapid heating and vaporization of the target, forming a dense plasma consisting of neutral atoms, molecules, ions, and energetic electrons. In the low-pressure oxygen atmosphere, oxide molecules are also formed. After propagation toward the heated substrate, which typically is only a few centimeters away, thin films with various morphologies are deposited (Figure 1).42 Figure 1 | Schematic of a PLD setup used for fabrication of thin film materials, including the PLD chamber and laser path. Download figure Download PowerPoint Advantages of Thin Film Fabrication by PLD To date, different techniques, including PLD, atomic layer deposition (ALD), chemical vapor deposition (CVD), MBE, sputtering, and thermal/e-beam evaporation, have been developed to fabricate thin films (Table 1).38,47–52 Compared to other thin film growth methods, PLD has various advantages. First, the rapid heating and highly nonthermal erosion caused by the high-energy laser enable the stoichiometric transfer of the multiple components by PLD. Furthermore, because there is no need for the use of electron beams or hot filaments, ambient gases can be used. With manipulation of the gas atmosphere, combined with the congruent-transfer capability, PLD can deposit high-quality thin films of multicomponent materials at a rapid deposition rate. Table 1 | Comparisons of Different Thin Film Techniques. Reproduced with Permission Ref 47. Copyright 2017 Elsevier. Property PLD ALD CVD MBE Sputtering Thermal/e-Beam Evaporation Deposition rate Good Poor Good Fair Good Good Film density Good Good Good Good Good Fair Lack of pin-holes Fair Good Good Good Fair Fair Thickness uniformity Fair Good Good Fair Good Fair Sharp dopant profile Varies Good Fair Good Poor Good Step coverage Poor Good Varies Poor Poor Poor Sharp interfaces Varies Good Fair Good Poor Good Low substrate temperature Good Good Varies Good Good Good Smooth interfaces Varies Good Varies Good Varies Good No plasma damage Fair Good Varies Good Poor Good Selection of Target Materials Perovskite oxides (ABO3s) as representative metal oxides have a wide range of applications, such as in (photo)electrocatalysis, batteries, and ferroelectrics, due to their mixed electronic and ionic conductivity and superior electrochemical catalytic properties.3,53–59 Moreover, the physicochemical properties can be finely tuned by tailoring the composition, crystal structure, defect, strain, and other aspects.60–63 In terms of structure, perovskite oxides have a large family, including single perovskites, double perovskites, triple perovskites, quadruple perovskites, and layered perovskites.4,53,64–66 For example, Jung and Tuller67 used PLD in an investigation of SrTi1-xFexO3-δ thin films and their surface oxygen exchange kinetics, while surface cation segregation or ex-solution characteristics were studied via strain engineering while varying the lattice constants of the substrates. Here, we focus on the deposition of perovskite oxide thin films as electrocatalysts. Other simple metal oxides such as undoped CeO2 or doped CeO2 with dense/flat or porous columnar morphologies; nanoparticles of ZnO, ZnO2, SnO2, and Bi2O3, and nanocrystalline or epitaxial stabilized Bi2O3 have been reported.68–72 Even the deposition of sulfide, selenide, or carbide thin films is possible through the PLD technique.73–75 Selection of Substrates Single crystals The choice of substrate is another important factor determining the properties of the thin film. Depending on the lattice constants of the target material and the substrate, thin films can be deposited onto either single-crystalline or polycrystalline types. Figure 2 introduces various types of single-crystalline oxide substrates and the representative thin films onto which they can be epitaxially grown. With the rational selection of the substrate and precise control of the deposition conditions, including the temperature, pressure, and deposition rate, either epitaxial or polycrystalline film growth is achievable. With an appropriate selection of the single-crystal substrate to achieve epitaxial growth of the thin film, a very well-defined surface with nanometer-scale roughness can be prepared. Because the epitaxially-grown thin film with small thicknesses are in tensile or compressive strain following the lattice parameter of the substrate, the effect of strain on the surface properties of high-temperature electrocatalysts was also studied with PLD.31,32 With this strain engineering strategy, given by epitaxial growth on single-crystal substrates, the rational design of a high-performance electrocatalyst is expected. Figure 2 | Pseudocubic lattice parameters of perovskite and perovskite-related films and substrates. Download figure Download PowerPoint Conductive frameworks Besides single-crystal substrates, conductive frameworks can also serve as substrates for the synthesis of nanoscale thin film materials.76–81 The commonly used conductive frameworks for electrocatalyst design include metal substrates (e.g., metal foils, metal meshes, and metal foams) and carbon-based substrates (e.g., carbon cloth, carbon nanofiber, carbon fiber paper, carbon nanotubes, and graphene).82–84 In particular, metal foams such as Ni foam, NiFe foam, Fe foam, and Cu foam with a three-dimensional structure endow the characteristics of high conductivity, porosity, and mechanical strength, which enable high catalyst loading and efficient mass/charge transfer ability during the reaction process (Figure 3a).85 The abundant surface active sites and strong interaction between the thin film and conductive frameworks may jointly contribute to superior catalytic activity. In this regard, Du et al.77 fabricated perovskite oxide SrCo0.85Fe0.1P0.05O3-δ (SCFP) thin films deposited on Ni foam (NF; Figure 3b). Bulk SrCo0.95P0.05O3-δ has been demonstrated as a good OER electrocatalyst due to its high electrical conductivity and large amount of surface oxidative oxygen O22−/O−.86 More importantly, abundant oxygen vacancies can be created on the surface of perovskite nano-films by the PLD method (Figure 3c). Calculation results have also proved the positive role of surface oxygen vacancies in promoting the OER kinetics, as demonstrated by the remarkably decreased overpotentials (0.72 and 0.34 V for SCFP and SCFP with oxygen vacancies, respectively) (Figure 3d). SCFP-NF exhibits greatly enhanced OER activity and good stability (Figure 3e,f). In another work reported by Wang et al.,87 it was found that amorphous MoS2 films deposited on Au-coated carbon cloth (MoS2/Au/CC) by the PLD technique conducted at room temperature can serve as an efficient hydrogen evolution reaction (HER) electrocatalyst in an acidic environment. Of note, the introduction of Au facilitated the electron transfer between the conductive substrate and MoS2. Remarkably, MoS2/Au/CC exhibited robust HER performance after 5000 cycles of cyclic voltammetry (CV). However, the reported electrocatalysts involved in conductive substrates based on the PLD technique are still limited. More related works are expected to enrich the electrocatalysis family. Figure 3 | (a) Illustration of metal foam-derived materials as self-supported electrodes. Reproduced with permission ref 85. Copyright 2020 American Chemical Society. (b) Micrographs of bare nickel foam and SCFP-NF. High- and low-magnification scanning electron microscopy (SEM) images of SCFP-NF. (c) X-ray photoelectron spectroscopy spectra of O 1s for SCFP and SCFP-NF. (d) The OER Gibbs free energy profiles at a Fe atom on surface without an O vacancy (left) and a Co atom on surface with an O vacancy (right) at U = 0 and 1.23 V. (e) Linear sweep voltammetry (LSV) curves of SCFP-NF, SCFP + NF, bulk SCFP, NF, and RuO2 toward the OER in a 1 M KOH alkaline solution. (f) Galvanostatic measurements of SCFP-NF and RuO2 at a constant current density of 10 mA cm−2. Reproduced with permission ref 77. Copyright 2020 Elsevier. Download figure Download PowerPoint Key factors that influence the electrocatalytic performance The final composition, surface morphology, and crystal structure of deposited thin films are influenced by various adjustable parameters, such as the laser fluence, background gas pressure, gas environment, deposition temperature, substrate type, target composition, and substrate-to-target distance.28,40,88–92 Thus, adjusting these parameters allows for rational designs of advanced electrocatalysts. For instance, Boucly et al.90 investigated the influences of different deposition temperatures (from 350 to 650 °C) on the morphology, crystalline structure, and surface composition of La0.2Sr0.8CoO3-δ thin films. It was found that a high temperature facilitated Sr segregation in the topmost layers, which can be easily removed after immersion in ultrapure water, leading to a cobalt-rich surface. When the temperature ranged from 450 to 550 °C, the OER activities were positively correlated with the temperature because Sr segregation was promoted by the temperature within this range, suggesting that the amount of the surface active element also increased with the temperature. In addition, Zeng et al.68 also depicted that substrate temperature, laser fluence, and oxygen ambient pressure largely affect the optical and electrical properties for highly c-axis oriented ZnO films, while Ramana et al.42 observed the grain size change of pulsed-laser-deposited V2O5 thin films with the deposition temperature. In general, high temperature conditions result in the deposition of highly crystalline films with low defect density and large grain sizes. Oxygen ambient pressure is also a frequently controlled laser fluence and substrate-to-target distance can affect the deposition rate, which will also be a factor in determining the film morphology, where a rule of thumb is that a slower deposition rate will result in lower roughness. Thus, it is necessary to present detailed information to ensure reproducibility by different groups. In addition to summarizing the catalytic activity and main findings of advanced thin film electrocatalysts, related growth conditions are also presented. Applications of Thin Films by PLD in Electrocatalysis OER electrocatalysts in alkaline media Electrochemical water splitting has been demonstrated as one of the most promising technologies to produce green hydrogen.7,8,93 In the two half-reactions, in this case the OER at the anode and the HER at the cathode, the OER is regarded as the bottleneck in the overall water splitting process due to a four-electron transfer process with sluggish reaction kinetics.11,14 Particularly, earth-abundant transition metal-based electrocatalysts enable high efficiency in the alkaline water electrolysis process. Therefore, great efforts have been devoted to designing nonprecious metal-based electrocatalysts, especially OER electrocatalysts for alkaline water electrolysis.16,94–97 For example, Co–B is a popular electrocatalyst in the form of powders for the application of water splitting. However, the report of nanostructured Co–B thin film by PLD as the anode catalyst for water splitting is rare. Patel et al. synthesized Co–B thin film by PLD. After the electrochemical reaction, the bulk of the thin film is metallic, while the surface is enriched with CoOOH species as the active sites. As a result, the Co–B thin film can generate a current density of 10 mA cm−2 at a small overpotential of 280 mV, showing a remarkable activity advantage compared to the powder catalyst.98 Mixed transition metal-based materials also show great potential for electrocatalysis. Later, the same group synthesized Co–Fe–B–O thin film deposited on a fluorine tin oxide substrate. Particularly, they evaluated the morphology and crystallinity by tuning the annealing temperatures (room temperature, 200, 400, and 600 °C in air). The sample annealed at 400 °C exhibited good OER catalytic activity toward the OER with an overpotential of 315 mV at 10 mA cm−2 and a low Tafel slope of 31.5 mV dec−1. It was found that B facilitated the formation of Co3+ active sites (CoOOH) by preventing the complete oxidation of Co. Meanwhile, Fe helped to reduce Co3+ to Co2+, thus completing the Co2+/Co3+ redox cycle.99 OER electrocatalysts in acidic media Proton exchange membrane water electrolyzers have been regarded as a more promising hydrogen production technique (e.g., higher voltage efficiency and hydrogen purity) compared to alkaline water electrolyzers.15,66,100–102 It remains a great challenge to use transition metal-based materials as high-performance OER electrocatalysts in acidic conditions, mainly due to the serious dissolution of transition metal ions under corrosive conditions.66,94,103,104 At present, state-of-the-art acidic OER electrocatalysts still rely on Ru- and Ir-based materials.15,105–109 It should be noted that applications of metal-based materials toward the OER in acidic environments always involve surface reconstruction, which will be also highlighted as an effective strategy to design advanced electrocatalysts.110 Thin films represent useful model systems to study operational stability during the electrochemical reaction. Herein, we focus the discussion on recent developments related to SrIrO3 thin films to characterize and fundamentally study the acidic OER process. Iridium-based perovskite oxides are highly active acidic OER electrocatalysts, associated with the selective leaching of A-site cations under OER conditions.105,106,111 Seitz et al.112 used the PLD technique to deposit SrIrO3 thin film on SrTiO3 (100) substrates. The electrochemically induced IrOx/SrIrO3 exhibited excellent OER performance in 0.5 M H2SO4 (maintaining OER activity at a current density of 10 mA cm−2 with an overpotential of 270–290 mV for 30 h), exceeding that of conventional IrOx and RuOx catalysts (Figure 4a,b). The active site was ascribed to the surface Ir-rich overlayer due to Sr leaching at the surface (Figure 4c). Figure 4 | (a) Galvanostatic measurements of IrOx/SrIrO3 thin films. (b) Tafel plots of IrOx/SrIrO3 thin films compared with several reported OER electrocatalysts in an acidic electrolyte. (c) Theoretical overpotential volcano plot with O* and OH* binding energies as descriptors. Reproduced with permission ref 112. Copyright 2016 American Association for the Advancement of Science. (d) HR-TEM image of the as-prepared SrIrO3 and STO substrates. (e) HR-TEM image of SrIrO3 and STO substrates after OER testing. Surface sensitive images for the helium ion microscope with secondary ion mass spectrometry (HIM-SIMS) characterization of (f) Sr+ mapping and (g) IrO2− mapping of SrIrO3|Nb∶STO after high-potential testing. Reproduced with permission ref 113. Copyright 2021 Wiley-VCH. Visual summaries of (h) X-ray thicknesses analyzed from the XRR results, (i) Sr concentration information, (j) Ir formal oxidation state, and (k) crystalline-to-amorphous transformation processes in SrIrO3. Reproduced with permission ref 114. Copyright 2021 American Association for the Advancement of Science. Download figure Download PowerPoint Later, Jaramillo et al.113 investigated catalyst degradation mechanisms for the same material system (crystalline SrIrO3 (001) thin films grown epitaxially onto SrTiO3 (001) substrates) using advanced nanoscale microscopy techniques, including high-resolution transmission electron microscopy (HR-TEM) for the identification of the crystal structure and homogeneity as well as a helium ion microscope with secondary ion mass spectrometry for chemical imaging at a nanometer level and elemental specificity. Furthermore, with the assistance of other physical and chemical characterization methods (e.g., X-ray photoelectron spectroscopy, grazing-incidence X-ray absorption spectroscopy, and atomic force microscopy), a three-dimensional picture of the catalyst before and after the OER process revealed that the initial degradation mechanism was layer-wise dissolution, wherein Sr leaching and Ir dissolution lead to an IrOx-rich active layer and a bulk structure (Figure 4d–g). As a result, a layer-by-layer degradation mechanism was proposed. Compared to high-performance catalysts in the form of nanoparticles and porous supports, thin films of SrIrO3 provide a good platform to study surface reconstruction at the nanoscale. Wan et al.114 studied the amorphization mechanism of SrIrO3 with a thin film model of SrIrO3 grown on a DyScO3 (110) substrate along the (001) orientation. In particular, X-ray diffraction (XRD) and X-ray reflection (XRR) methods were used to identify the crystalline-to-amorphous transformation process (Figure 4h). Furthermore, combined with grazing incidence X-ray absorption near-edge spectroscopy, extended X-ray absorption fine structure, and soft X-ray absorption spectroscopy, local geometry/coordination and electronic structure changes in the amorphous and crystalline layers of SrIrO3 were tracked (Figure 4i–k). As a result, the initial formation of amorphous SryIrOx with primarily Ir square-planar motifs and a second-stage transformation of a highly disordered structure consisting of Ir4+ octahedra were revealed, providing a standpoint from which to understand how oxygen redox initiates ionic diffusion and surface reconstruction processes. HER electrocatalysts The reaction rate of the HER at the cathode during the electrochemical water splitting process also has a significant effect on the device efficiency.13,115,116 Among non-noble metal-based HER catalysts, transition-metal dichalcogenides have been the subject of much attention due to their remarkable catalytic activities.117 Particularly, molybdenum sulfide is considered to be a promising alternative to Pt-based materials toward the HER.118 Giuffredi et al.88 fabricated MoSx nanostructured films by PLD, which exhibited the distinguishing features of highly defective clusters with a metastable structure. In this work, the relationship between the final morphology and structure of the synthesized materials and the tailored parameters related to PLD set-up was described. The new phase formation of a conductive MoO2 shell, amorphous molybdenum oxysulfide, and an optimal mesostructure jointly contributed to the superior HER activity with a small Tafel slope of 35 mV dec−1 and low overpotentials at high current densities (e.g., overpotentials of 126, 170, and 550 mV to obtain 10, 100, and 500 mA cm−2 current densities, respectively). Remarkably, the hierarchical nanoarchitectures are capable of exposing considerable active sites and improving the conductivity to more readily facilitate the HER process. Another work by Wang et al.119 reported the direct growth of high-ratio 1T MoS2 thin films by PLD, showing enhanced HER activity. Different amounts of sulfur were added to a MoS2 target to modulate the 1T MoS2 ratio in the synthesized thin films. The introduction of sulfur facilitated the formation of pores on the films. Moreover, the optimized thin film provided more active sites and exhibited high metallic conductivity. Benefiting from these advantages, HER activity in 0.5 M H2SO4 with a low Tafel slope of 38 mV dec−1 and a small overpotential of 151 mV at a current density of 10 mA cm−2 was achieved for the high-content 1T-phase MoS2 thin film, even outperforming MoS2 powder. Oxygen reduction reaction electrocatalysts The oxygen reduction reaction (ORR) is a crucial step in metal-air batteries and fuel cells.120,121 ORR activity is commonly evaluated by the rotating disk electrode (RDE) technique. Due to the square shape and insulating nature of some substrates, the electrocatalytic activity of thin films for the ORR is assessed by sampled current voltammetry. In addition, a self-supported electrode can be attached to the glassy carbon insert of an RDE using silver paste.122 Similar to the OER, the ORR is also kinetically sluggish. Commercial Pt/C has been regarded as the benchmark electrocatalyst for the ORR. Decreasing the consumption of noble metals while simultaneously improving the ORR activity and stability is the main task when designing Pt-based electrocatalysts. Mohamedi et al.123 synthesized Mn2O3/Pt/carbon nanotubes (CNTs) by a two-step process. Pt was initially deposited onto CNTs with an ultralow loading of Pt and further coated with a porous Mn2O3 layer, achieving a layer-on-layer architecture. Electrochemical testing revealed that the Mn2O3/Pt/CNTs exhibited ORR activity identical to that of Pt in alkaline media. Moreover, the bilayer catalyst showed high tolerance to the methanol oxidation reaction, an outcome attributed to the synergistic effect of the two active sites. Ag-