One-dimensional anodic TiO2nanotubes coated by atomic layer deposition:Towards advanced applications

Applied Materials Today 14 (2019) 1–20 Contents lists available at ScienceDirect Applied Materials Today j ourna l h o mepage: www.elsevier.com/locate/apmt One-dimensional anodic TiO2nanotubes coated by atomic layer deposition: Towards advanced applications Filip Dvoraka, Raul Zazpea,b, Milos Krbala, Hanna Sophaa,b, Jan Prikryla, Siowwoon Ngb, Ludek Hromadkoa, Filip Buresc, Jan M. Macaka,b,∗ aCenter of Materials and Nanotechnologies, Faculty of Chemical Technology, University of Pardubice, Nam. Cs. Legii 565, 53002 Pardubice, Czech Republic bCentral European Institute of Technology, Brno University of Technology, Purkynova 123, 612 00 Brno, Czech Republic cInstitute of Organic Chemistry and Technology, Faculty of Chemical Technology, University of Pardubice, Studentska 573, 53210 Pardubice, Czech Republic a r t i c l e i n f o Article history: Received 21 September 2018 Received in revised form 11 October 2018 Accepted 2 November 2018 Keywords: Atomic layer deposition TiO2nanotube Coatings Functionalization Aspect ratio a b s t r a c t Atomic layer deposition (ALD) represents a unique deposition technique that allows to coat uniformly various high aspect ratio (HAR) porous nanostructures, in addition to its traditional role to coat flat substrates (e.g. Si wafers). Self-organized anodic TiO2nanotube (TNT) layers belong among the most investigated inorganic nanostructures. They possess highly functional materials with promising application potential across many technological fields. Herein, we review the utilization of ALD for the functionalization of anodic TNT layers by secondary materials to advance their physicochemical and photoelectrochemical properties. First, the application of ALD for functionalization of porous aluminium oxide, which represent fundamental HAR nanostructure, is briefly introduced. Then the main experimental parameters governing the uniformity and the conformality of ALD coating within HAR nanostructures are discussed. Finally, the review focuses on the use of ALD to deposit secondary materials into TNT layers for various purposes — the introduction of pioneering studies is followed by particular examples of ALD based functionalizations of coated TNT layers for optimized visible-light absorption, charge separation and passivation, (photo)catalysis, stability, gas sensing, and energy storage. © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. ALD into porous aluminium oxide — overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Aspects of ALD into high aspect ratio nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 4. ALD into TiO2nanotube layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 4.1. Pioneering efforts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4.2. Light absorption and photoelectrochemical conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4.3. Charge separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4.4. Photocatalysis and electrocatalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 4.5. Stability and improved physical properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13 4.6. Gas sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 4.7. Energy storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Abbreviations: AAO, anodic aluminium oxide; ALD, atomic layer deposition; CBD, chemical bath deposition; DSSC, dye-sensitized solar cell; EDX, energy-dispersive X-ray spectroscopy; EOR, ethanol oxidation reaction; FTIR, Fourier transform infrared spectroscopy; HAADF, high-angle annular dark-field; (H)AR, (high) aspect ratio; IPCE, incident photon to electron conversion efficiency; MB, methylene blue; MC, Monte Carlo; PEC, photoelectrochemical; QCM, quartz crystal microbalance; QD, quantum dots; SEM, scanning electron microscope; SILAR, successive ionic layer adsorption and reaction; (S)TEM, (scanning) transmission electron microscope; TMA, trimethylaluminium; TNT, TiO2nanotube; UV, ultraviolet spectral range; vis, visible spectral range; XRR, X-ray reflectivity. ∗Corresponding author. E-mail address: [email protected] (J.M. Macak). https://doi.org/10.1016/j.apmt.2018.11.005 2352-9407/© 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4. 0/). 2 F. Dvorak et al. / Applied Materials Today 14 (2019) 1–20 1. Introduction Titanium dioxide (TiO2) represents a fascinating and highly functional material. Until now, TiO2has found its main commercial applications in pigments, food industry and cosmetics. Pioneering observation of photocatalytic decomposition of water at TiO2 electrode in 1970s [1] launched the tremendous interest in the development of applications of TiO2in solar energy conversion, energy storage, and photo-assisted bioremediation, which represent essential components for the concept of renewable energy sources and of sustainable environment [2]. These applications also take the advantages of TiO2to be a low cost material with an extremely low photocorrosion rate. Additionally, thanks to inherent biocompatibility, low toxicity, and high chemical stability of TiO2, emergence of biomedical applications of TiO2, including photodynamic therapy for cancer treatment, drug delivery systems, cell imaging, biosensors, and genetic engineering has also taken place [3]. TiO2is a wide band gap (3.0–3.2 eV) n-type semiconductor and thus it absorbs light only in the UV spectral range. Generally, the material properties can be tuned by nanostructuring, realized by a gradual decrease of the characteristic size of applied materials down to the nanometer scale [4]. Nanostructuring of materials results in an extremely increased surface to volume ratio with significant impact on material physicochemical properties, including light absorption. Since many applications of TiO2are to great extent influenced by the surface of TiO2[5], nanostructuring of TiO2represent an important step for its efficient and flexible utilization in various applications. Probably the most prominent utilization of nanostructured TiO2 is represented by the utilization of TiO2nanoparticles in the dyesensitized solar cell (DSSC), introduced by O’Regan and Grätzel [6] and by the recent utilization in perovskite solar cells [7,8]. Another class of nanostructured TiO2is presented by 1-D nanostructured materials such as nanowires, nanorods, nanobelts, and nanotubes [9]. 1-D nanomaterials share with nanoparticles (0-D materials) the high surface area and promotional size quantum confinement effects, but have one dimensions outside the nanoscale, typically pore depth or length (applies for tube, rods, wires, fibres). 1-D nanomaterials offer better unidirectional charge/reactant/ion transport properties and better mechanical integrity. TiO2nanotube (TNT) layers prepared by anodization of Ti represent a very popular type of 1-D material with the key advantages of facile growth, selfordering, well-controlled geometry, scalability, high surface area, direct contact to metal-support, and suitability for coating by secondary materials [10–12]. The important characteristic parameter of TNT layers is their aspect ratio (AR), which is the ratio between length and diameter of TNT. In general, the higher AR offers the larger surface area which enhances the benefits of the nanotube structures. Another important aspect of TNT layers is their structural variability – they are amorphous in the as-anodized state, but can be converted, depending on the temperature, into anatase, rutile or their mixtures [11] with large flexibility towards applications. Last, but not least, they possess very good adhesion to the underlying Ti substrates, however, upon specific treatments, they can be detached into free-standing membranes [13,14] or nanotube powders [15]. To address complex functionalities of TNT layers in applications, it is beneficial to combine them with secondary materials. In order to keep advantages of the high surface nanotubular geometry, it is necessary to coat the entire surface of nanotubes. The most often used approaches to fill/coat the nanotube layers, such as electrodeposition [16–19], chemical bath deposition [20–22], spincoating [23,24], or sputtering [25–27], are able to uniformly coat TNTs layers with specific dimensions (predominantly with a low AR). The atomic layer deposition (ALD) represents the only deposition Fig. 1. Illustration of utilization of ALD for advanced functionalization of TNT layers. technique able to coat uniformly highly porous nanostructures of virtually any size [28]. ALD is an established deposition technique based on sequential self-limited adsorption of vapours of compatible precursors on surfaces leading to the formation of required coatings. The selflimited adsorption is the key aspect of ALD making it different from other deposition techniques. Under optimized conditions (temperature, pressure, and exposition time), the precursor molecules are adsorbed all over the surface in just one monolayer – further adsorption of precursor molecules on the already occupied surface sites is not allowed. No matter of the geometry of the substrate, the coatings prepared by ALD have unprecedented conformity all over the surface and their thickness is controllable on the atomic level [29]. As illustrated in Fig. 1, ALD coatings can help to tune/enhance many properties of TNT layers, such as light absorption, separation of photogenerated electron-hole pairs, photocatalytic and electrocatalytic properties, chemical, thermal, and mechanical stability, gas sensing, and energy storage. In particular, ALD is suitable for very thin coatings while it is only to a limited extent (from the point of both high precursor costs and high time demands) usable for the growth of thick coatings (>tens of nanometers). The restricted temperature window for self-limited regime of ALD deposition and high investment costs of the tools/precursors represent main drawback of nowadays ALDs, but these are overcome step by step by continuous advancements in the ALD technology and also market competition. Recently, the usability of ALD for functional coatings of nanostructures towards energy/light-driven applications has been thoroughly reviewed [29–40]. In this review, we address the utilization of ALD to tailor the functionality of anodic TNT layers for improved performance to realize new types of devices for various purposes. First, we briefly summarize the existing literature on ALD deposition into model 1-D material – porous anodic aluminium oxide (AAO) – and introduce specific theoretical features of ALD coating of HAR (HAR) nanostructures; next we focus on the utilization of ALD for functionalization of anodic TNT layers. We introduce pioneering studies of ALD into anodic TNT layers and finally we follow by the description of the utilization of ALD for improved light absorption, charge separation, (photo)catalytic properties, stability (mechanical, thermal, and chemical), gas sensing, and energy storage of ALD modified TNT layers. F. Dvorak et al. / Applied Materials Today 14 (2019) 1–20 3 2. ALD into porous aluminium oxide — overview Pioneering studies of utilization of ALD for functional coating of 1-D HAR nanostructures by secondary materials were performed on porous anodic aluminium oxide (AAO) – a well-established self-ordered 1-D material [41,42]. Porous AAO is characterized by its mechanical, chemical, and thermal stability, nanoscale pore diameters, high surface-area ratio and low cost fabrication. These properties attracted a considerable interest for a wide number of applications in catalysis, molecular separation, electronics and photonics, (bio)sensors, energy storage and conversion, drug delivery, and templated synthesis among others reviewed by Md Jani et al. [43]. Porous AAO can be fabricated via electrochemical anodization of aluminium. The application of appropriated processing parameters during the anodization (e.g. applied voltage, electrolyte concentration, etc.) allows a very good control of the pore size, the inter-pore distance, the thickness of the porous layer and the overall AR of the pores. The inherent high surface area and AR makes porous AAO a particularly appealing nanoscale template for investigations of the kinetics and mechanisms of the surface chemical reactions that the atomic layer deposition (ALD) method is based on. In particular, Fourier transform infrared spectroscopy (FTIR) was employed to study the binary chemical reactions to deposit SiO2and Al2O3using porous AAO as a support [44–47]. ALD properties as sub-nanometer thickness control and uniformity of the deposited material into the pores were exploited in turn, to shrink the pore diameter of porous AAO. The reduction of the pore diameter to molecular dimensions by the deposition of Al2O3, TiO2and SiO2dramatically improved the gas separation properties of the porous AAO [48,49]. Elam et al. used ultrahigh AR (AR ≈ 5000) porous AAO for a comprehensive study on uniformity of ALD coating of HAR structures [50], which will be discussed in detail in the next section. ALD enables to deposit a whole range of materials into AAO as demonstrated extensively in literature. Johansson et al. first performed and demonstrated ALD of a metallic compound via the deposition of copper nanoparticles into porous AAO. Therein, homogeneous particle size and particle distribution along the pores were demonstrated [51]. Following works reported on the deposition of different metals into porous AAO using ALD, such as Pd [52], Ni and Co [53,54], Ru [55], Pt/Ir [56], Ag [57], and Pt [58]. Also, the deposition of sulfides [59], nitrides [60], a wide number of different oxides as Al2O3[61–63], TiO2 [57,62–68], Ta2O5[66], Fe2O3[53,54,63,69], SiO2[63,70–73], ZnO [63,74–78], SnO2[79], Co3O4[80], together with heterojunctions TiO2/Sb2S3/CuSCN(not ALD) [81] and NiO/Sb2S3[82], multilayered structures as SiO2/Fe2O3/SiO2[83,84], Fe3O4/ZrO2/Fe3O4[85], and Au−CoxFe3−xO4core–shell nanowires [86] have been reported. The successful utilization of ALD for build-up of functional nanostructures on AAO templates for tunable magnetism [69] and photovoltaic devices [81] is illustrated in Fig. 2. Porous AAO has served as an excellent substrate to evaluate the ability of novel precursors to coat/infiltrate HAR substrates [59,70,79,80]. The ALD sub-nanometer thickness control allowed fine-tuning of pore diameters [61–63,71,72] and evaluate the impact of reduced pore diameter on the single-molecule sensing properties [61], the membrane separation properties [63,71], and the ionic transport through the nanoporous membranes [72]. The deposition of secondary materials enabled the functionalization of porous AAO as different sensors [52,56,58,68], anode for Li ion batteries [67] and photoanode for dye-sensitized solar cells [74,81,82]. Except these applications, taking the unique ALD properties of conformal deposition and the wide range of materials that can be deposited, porous AAO has been intensively exploited as a template for the synthesis of different nanostructures. Nanotubular structures of different nature were reported, such as TiN nanotubes Fig. 2. Illustration of functional nanostructures based on templates of porous AAO utilizing ALD. (a)–(c) Micrographs (SEM and TEM) of ALD deposited iron oxide nanotubes for tunable magnetic properties, adopted from [69]. (d) ALD deposited TiO2 (green) and Sb2S3(red) with (not ALD) CuSCN (yellow) in AAO for integration into photovoltaic device, reproduced from [81]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) (exhibiting good ductility [60]), ZnO nanotubes (with superior photoluminescence characteristics [76]), or ferromagnetic nanotubes composed of Ni and Co [53,54] and Fe2O3[53,54,69] shown in Fig. 2a. Single wall TNTs [64,66] and Ag loaded TiO2nanotubes [57], wherein an enhanced photocatalytic activity compared to unloaded TNTs was revealed, and hierarchical multiwall TNTs, fabricated via deposition of alternating TiO2/Al2O3nanolaminates followed by wet etching of sacrificial Al2O3layer [65], have been also reported. The ALD benefits were also applied for the fabrication of multilayered nanotubes, such as TiO2/Sb2S3/CuSCN for application in solid state solar cells [81] illustrated in Fig. 2d, and SiO2/Fe2O3/SiO2 [83,84] or Fe3O4/ZrO2/Fe3O4[85], whose magnetic properties were explored and their potential applications foreseen. Porous AAO also assisted the fabrication of Ru nanowire arrays as a platform for sensor devices [55] and ZnO nanorods [75,76], and ALD coated porous AAO followed by electrodeposition enabled the fabrication of complex nanostructures as TiO2-coated Ni nanowire arrays [64], Ta2O5-coated Ni nanorods [66], and Au–Co ferrite nanowires [86]. Likewise, porous AAO has been proposed as suitable platform for different health and environmental applications – see Ref. [87] and references therein. In particular, ALD ZnO coated porous AAO exhibited encouraging antimicrobial activity with potential dermatological applications [77,78]. The utilization of ALD for coating porous AAO has provided a very strong motivation case for all other high-aspect 1-D nanostructures, including nanopores [88], nanotubes [89,90], nanorods [91], nanowires [92], and branching nanostructures [93,94] of various materials, to be processed in a similar fashion. Among those, TNT layers formed by anodization maintain forefront position and 4 F. Dvorak et al. / Applied Materials Today 14 (2019) 1–20 promise to be treated by ALD, due to facile preparation and high functionality of TiO2compared to other materials. 3. Aspects of ALD into high aspect ratio nanostructures In order to design an ALD process, leading to conformal coatings of HAR nanostructures, it is necessary to understand the kinetics and chemistry of that particular ALD process on the fundamental level. Numerous theoretical studies were devoted to the description of ALD surface chemistry and kinetics. A comprehensive review on theoretical aspects of ALD is beyond the scope of this review. Instead, in this section we want to focus on important theoretical aspects governing ALD in HAR nanostructures. The description of the ALD process in the HAR nanostructures was theoretically addressed by numerous models: a simple kinetic model [95], a kinetic model coupled with Langmuir adsorption [96], Monte Carlo (MC) based simulations in molecular [50,97–102], Knudsen and diffusive [103] gas flow regimes, and numeric or analytic solutions of Boltzmann transport equation coupled with Langmuir adsorption models [104–108]. Although the studies used different theoretical frameworks, the common idea describing the parameters leading to the conformal coating of HAR nanostructures allows to draw general qualitative overview, as follows. One of the key aspects for achieving conformal and uniform coatings is to reach the saturation regime of on ALD deposition. This situation occurs when the precursor molecules fill (chemisorb on) all available adsorption positions of the substrate surface within every ALD cycle (leading to the saturation coverage). The corresponding saturating exposure dose (multiplication of saturation exposure time and pressure of precursor molecules) depends on the surface area (number of available adsorption positions [109]), the surface reaction probability (surface sticking coefficient), the diffusion rate, and the recombination loss probability (in case of plasma ALD processes) of precursor molecules. Fig. 3 illustrates the evolution of step coverage profiles of HAR nanostructure under characteristic deposition regimes of ALD discussed below. The light blue area together with arrows indicate the evolution of step coverage profile of HAR nanostructure (black area). Black dashed lines represent complete coverage profile under saturated regime of ALD. Depending on the surface reaction probability of precursors and the aspect ratio (AR) of nanostructure, the formation of saturated Fig. 3. The illustration of evolution of step coverage profiles (light blue) of HAR nanostructure (black) by ALD under various process regimes. Light blue area and arrows indicate evolution of step coverage with increasing ALD exposure dose. Dashed black lines represent coverage profiles under saturated regime conditions (saturation exposure dose). (a) The reaction-limited, (b) the diffusion-limited, and (c) the recombination-limited regimes of ALD deposition. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) coverage during ALD deposition into HAR structures can be limited either by the diffusion or the reaction ability of precursors [50]. The so-called reaction-limited regime takes place for low surface reaction probability of precursor molecules as compared to the diffusion probability for a given AR. This corresponds to the situation when the exposure time and pressure in combination with the sufficient diffusion rate allow precursor molecules to reach every adsorption position available within the pore/trench and the only parameter governing the coverage is the surface reaction probability. The saturating exposure dose is not affected by the porous nature of substrate to be coated and nearly equals to the saturating dose for planar surfaces with the equivalent surface area [100]. The corresponding step coverage profile of the pore/trench evolves almost uniformly with increasing exposure as it is schematically illustrated in Fig. 3a. Even unsaturated coverage within one cycle of ALD can lead to the growth of uniform coatings of HAR nanostructures. This is similar to chemical vapor deposition where the conformality increases for low reaction probabilities as a result of reduced depletion of precursors along the via [110]. On the other hand, the diffusion-limited regime occurs when the reaction probability (sticking coefficient) of precursor molecules is much larger than their diffusion probability to reach bottom of an HAR nanostructure. The precursor molecules chemisorb on the available surface sites successively from the entrance-opening of the nanostructure and the precursor coverage profile evolves in the front-like manner with the increasing exposure as depicted in Fig. 3b. The saturating exposure dose approximately scales with second power of AR [50,95]. The gradient of the coverage profile within the front is step-like for reaction probability of 100% and it gradually smoothens with decreasing reaction probability [97]. In the case of plasma ALD processes, the saturation exposure dose is strongly affected by the recombination probability of the precursors, e.g. the wall recombination of plasma radicals or ozone [100,101]. Precursor molecules rather tend to undergo recombination then to reach bottom of HAR. The coverage profiles in recombination-limited regimes are complex and not uniform, as it is drawn in Fig. 3c, and the formation of saturation coverage is possible only for the limited AR with significant saturation doses [100,106,111,112]. Repeated ALD cycles (with constant exposure time) lead to the growth of thicker coating and consequently to change of characteristic dimension/radius of opening of HAR nanostructure. As a consequence, when the thickness of coating is comparable to radius of the opening of pore), the resulting thickness profile is affected as the diffusion becomes more limited. For such cases, the exposure time throughout repeated ALD cycles needs to be properly scaled in order to achieve the uniform thickness of the final coating [107]. Generally, the works introduced above show examples of set of experimental parameters governing the uniformity of ALD coating within HAR nanostructures. The determination of the sufficient saturation dose of precursor within one ALD cycle is critical for the successful ALD of uniform coatings of HAR nanostructures. In the reality, the ALD process is influenced by many experimental parameters depending on type of precursor, substrate material, ALD process parameters, and/or ALD reactor design. The results of simulation models of ALD deposition represent qualitative support for experimental determination of parameters leading to saturated regime of ALD depositions, which need to be performed and optimized for each system and coating material individually. 4. ALD into TiO2nanotube layers Anodic TiO2nanotubes, prepared by the anodization of Ti, got into the forefront of 1-D nanostructured inorganic materials F. Dvorak et al. / Applied Materials Today 14 (2019) 1–20 5 since the development of organic based electrolytes (such as ethylene glycol or glycerol), which allowed growth of HAR TNT layers [10,113,114]. The ability to prepare HAR TNT layers represents a key step for successful applications of TNT layers. The next important step in the development of TNT-based applications is their tailoring by secondary materials, which represents a target inherently suitable for ALD utilization. 4.1. Pioneering efforts The first works devoted to coating of anodized TNT layers by ALD have been focused on sensitization of TNT layers by In2S3for solar energy conversion [115] and on the role of Al2O3, Ta2O5, and TiO2ALD coatings on electric properties of coated TNT layers [116]. The work of Sarkar et al. [111] presents an extensive experimental study of ALD growth of In2S3applied practically on the coating of TNT layers with AR of ≈62. The ALD has been performed by utilization of indium(III) acetylacetonate (In(acac)3) and H2S precursors. The authors performed thorough analysis of In2S3ALD process on flat substrates first. Quartz crystal microbalance (QCM), X-ray reflectivity (XRR), and FTIR were used in order to monitor mass gains during each reactant cycles, to measure the growth per cycle and its temperature dependence, to verify the self-limited surface chemistry of utilized precursors, and to identify surface species formed during sequential surface reactions. The verified ALD process parameters with increased exposure times were utilized for sensitization of HAR TNT layers by 5 nm thick In2S3ALD coating (Fig. 4a). Unfortunately, detailed SEM or TEM investigations to prove the uniformity of ALD coating of TNT layer were missing in that paper. The prominent effect of the In2S3coating on the photoelectrochemical (PEC) properties of the TNT layers is illustrated in Fig. 4b. The In2S3coated TNT layers exhibited a reduced optical band gap of 2.07 eV and reached 10% external quantum efficiency for photon energy >2.5 eV while the uncoated TNT layer was generally photo-inactive for photon energy <3.0 eV. The observed quantum efficiency was significantly below the theoretically predicted maximum quantum efficiency (≈70%). The limited quantum efficiency was attributed to recombination losses and charge collection/injections processes. The work of Tupala et al. [116] presents ALD coatings of TNT layers with AR of ≈10 by selected metal oxides: Al2O3, Ta2O5, and TiO2. TNT layers were prepared by anodization of Ti thin layers evaporated on ITO conductive glass. The TNT layers prepared on transparent conductive support are a very suitable material for solar energy applications (e.g. perovskite solar cells [8]) and their functionalization is an important technological step. The ALD was performed with established precursors: trimethylaluminium (TMA, Al(CH3)3), tantalum pentoxide (Ta(OEt)5), and titanium tetraisopropoxide (Ti(OiPr)4) in combination with water. The target thickness of the coating was 5 nm. The example of TiO2coated TNT layers is shown in Fig. 4c. TNT layers coated by ALD were shown to have higher electric conductivities (across the layer) compared to their uncoated counterparts, as shown in Fig. 4d. Despite both these pioneering works clearly demonstrated the functionality of ALD coated TNT layers, they did not provide any direct evidence of uniformity of ALD coatings within the TNT layers (such as SEM and TEM images). The follow up work of Macak et al. [117] presented interestingly improved light trapping properties of plasma ALD In2O3coated TNT layers with AR up to ≈80 together with microscopic evidence of the presence of the In2O3coating. The ALD deposited In2O3was found to successfully coat the entire TNT interiors with gradually decreasing thickness of the coating from 30 to 5 nm from top to the bottom of the TNT layer (with AR ≈ 80, and layer thickness of ≈8 ␮m), respectively. The observed thickness gradient of In2O3resulted from a limitation of plasma assisted ALD Fig. 4. Pioneering ALD coatings of TNT layers. (a) The In2S3coating of TNT layers and (b) corresponding quantum efficiency. (c) The TiO2coated TNT layer and (d) the current density through the TNT layer modified by various metal oxide coatings. (a) and (b) were reproduced from [115] and (c) and (d) were reproduced from [116]. 6 F. Dvorak et al. / Applied Materials Today 14 (2019) 1–20 Fig. 5. ALD Al2O3coating of TNT layers with aspect ratio of 180: (a) Influence of exposure time of TMA precursor during one ALD cycle on uniformity of thickness of Al2O3 coating within TNT layer; (b–e) Illustration of uniform thickness of Al2O3coating for 5 s TMA exposure time as revealed by SEM inspection through four different depth levels – top, near-top, middle, and bottom, respectively. Reproduced from [118]. to coat uniformly HAR nanostructures due to the limited lifetime of plasma-generated radicals and recombination effects. Contrary, utilizing the thermal ALD process, Zazpe et al. [118] reported uniform ALD deposition of Al2O3coatings within TNT layers with a high AR of ≈180. The conformality of ALD Al2O3coatings with various exposure times of TMA precursor was inspected by SEM at different depth levels of the TNT layers (and supported by in situ QCM measurements and STEM investigations). With an increasing exposure time of TMA, the ALD coating was found to be uniform down to the deepest levels of TNT layers, as shown in Fig. 5a. Optimal exposure time led to uniform coatings within the entire TNT layers, as demonstrated by SEM images measured at four different depth levels of the coated TNT layers, shown in Fig. 5b–e. The observed results correspond to diffusion limited regime of ALD growth, as introduced in [50] and discussed in the previous section. These pioneering works present the successful utilization of ALD for coating of TNT layers by secondary materials with advanced functionalities, relevant for solar energy conversion and other optoelectronic applications. Obtained results clearly demonstrate that the ultra-thin ALD coating is able to significantly alter the functionality of TNT layers and prove that ALD is the most suitable and practical technique to tailor properties of HAR nanostructures. Table 1 presents a complete list of reports utilizing ALD for coating HAR anodic TNT layers. In the following text, these reports will be introduced in thematically selected sections, as illustrated in Fig. 1. 4.2. Light absorption and photoelectrochemical conversion While the TiO2efficiently absorbs light in the UV spectral range, the main limitation for solar-light-driven applications of TiO2is its relatively large intrinsic band gap (3.2 eV for anatase and 3.0 eV for rutile) which does not allow light absorption within the visible-light spectral range. Besides a dye sensitization [119] or a metal/non-metal doping [120–124], an alternative way to improve visible-light photoelectrochemical (PEC) performance of TiO2is the modification of TiO2by a suitable narrow band gap semiconductor. The photoexcitation connected with the effective charge separation leading to the electron injection to the TiO2conduction band can significantly enhance the visible-light photoresponse of TiO2based heterostructures, such as nanoparticulate and nanotubular layers. In order to absorb most of the incident light, the range of thicknesses of the compact absorber layers should be on the microscale. Thus, the ALD does not qualify itself as most suitable deposition technique at first glance for this purpose, since it is technique suitable for production of coatings with thickness on the nanometer scale. However, the situation is diametrically different in the case when HAR nanostructures are used as substrates. Only a few tens of nm thick absorber layer (or even less) are effective enough for efficient solar energy harvesting, as a result of tremendously increased specific surface area of the HAR nanostructure, allowing to reach the effective absorption mass of sensitizer with comparably thinner coatings than for planar substrates, and the multiple light scattering events, prolonging significantly optical path in HAR nanomaterials [125–127]. Moreover, a low thickness of absorber layer is also critical for the reduction of charge recombination losses during charge transfer through the material. The length-scale of effective charge separation is characterized by the size of the depletion layer. On the nanometer scale, the ALD serves as an excellent tool for deposition of uniform coatings with minimum amount of defects, which represents fundamental building block for highly efficient TiO2 nanomaterials to be used for solar energy conversion applications. The first work utilizing ALD to coat TNT layers by secondary visible-light absorbing material was carried out by Sarkar et al. [115], as discussed in the previous section. The following work was performed by Huang et al. [128], who deposited Co3O4into TNT layers with AR over 100. The ALD was performed utilizing bis(cyclopentadienyl)cobalt(II) (Co(Cp)2) and O3as ALD precursors. The thickness of the coating was ranging up to 20 nm and it was verified by SEM and by the calibration on reference Si wafer. ALD Co3O4coated TNT layers were explored for the visible-light (>420 nm) induced photocurrent generation. The best photocurrent response was observed for TNT layers coated by Co3O4with a thickness of 4 nm. The corresponding PEC performances of the 4 nm ALD Co3O4/TNT layer, a reference Co3O4/TNT layer coated by impregnation method, and a reference blank TNT layer are shown in Fig. 6a and b. The enhanced PEC performance of ALD Co3O4/TNT layers was attributed to the favourable band alignment of Co3O4/TNT layers as revealed by XPS, shown in Fig. 6c, and to the ALD technique used, providing uniform Co3O4coating of controlled thickness and with minimal amount of defects, which maintained the large open surface area of TNT layers and minimized charge recombination losses. However, it is worth to note that the reported absolute values of the photoconversion efficiency (Fig. 6b) were extremely low. A significantly pronounced positive effect on the visible-light photoresponse can be expected, when CdS is utilized as a light F. Dvorak et al. / Applied Materials Today 14 (2019) 1–20 7 Table 1 Summary of ALD coatings on TNT layers together with the information about geometry of TNT layers (* difference between inner and outer tube diameter taken into account). Material Year L (␮m) D (nm) AR Reference ALD uniformity Al2O32016 20 110 182 [118] Light absorption In2S32010 4 65 62 [115] Co3O42015 10 85 118 [128] CdS 2017 0.6 95–56–35 6; 11; 17 [134] CdS 2018 1–130 80–230 13–897 [125] MoSeOx2017 5 230 22 [135] Charge separation Al2O32014 23 100 230 [139] Al2O32014 x 97 x [140] Al2O3+ QD 2016 1.7 120 14 [141] TiO2, Al2O3, ZnO + QD 2017 1.7 110 15 [142] ZnO 2014 4.5 120 38 (*60) [143] ZnO 2014 0.5 60 8 [144,145] ZnO + QD 2016 2.4 97 25 [146] ZnO 2016 0.5 60 8 [147] ZnO + P3HT 2017 0.5 60 8 [148] TiO22017 5 230 22 [149] (Photo)catalysis Ti, Al, Zn, Sn, Cu, W 2014 7.5 95 79 [175] Pd 2015 1 70 14 [172] SnO2+ Pd 2017 1 70 14 [179] Pt 2018 7 100 70 [173] Pt 2018 1–5–20 80–230–110 13–22–181 [174] Stability and physical properties Al2O3, TiO2, Ta2O52012 1 100 10 [116] In2O32015 0.5–2–8 80–80–100 6–25–80 [117] Al2O32017 20 110 182 [181] Gas sensing ZnO 2017 5 230 22 [185] Energy storage ZnO Al2O32013 2 100 20 [200] Al2O32017 5 230 22 [201] L = tube length; D = inner tube diameter. absorber with TiO2[129]. CdS is predominantly deposited in the form of quantum dots (QD) by conventional techniques such as chemical bath deposition [20,130], successive ionic layer adsorption (SILAR) [21,131], or electrodeposition [132], which can generally provide decoration of HAR nanostructures with much more limited uniformity than ALD. The expected efficiency of CdS sensitized TNT layers scales with the total surface area of TNT layer as long as the TNT layer can be uniformly coated by sensitizer (and diffusion length of charge carriers is long enough to reach collecting electrodes [133]). A direct way to increase surface area (AR) of TNT layers is (i) to decrease the diameter of nanotubes (increase their areal density) or (ii) to increase the overall thickness of TNT layers. The next two works well describe cases (i) and (ii). Krbal et al. [134] reported downscaling the tube diameter of ALD CdS sensitized TNT layers for efficient PEC energy conversion. TNT layers with thickness of 600 nm and with different average tube diameters of 35, 56, and 96 nm – featuring various surface areas (52, 29, and 23 cm2/cm2, respectively) – were coated by 6 nm thick CdS layers by ALD (using dimethylcadmium and H2S as precursors). The corresponding morphology of CdS coated TNT layers with various tube diameters, captured by SEM, is shown in Fig. 7a–c. The obtained incident photon to conversion efficiencies (IPCE) are shown in Fig. 7d, respectively. The CdS coating of TNT layers led to the strong enhancement of their photoresponse in the visible-light spectral range. The photoresponse rose with the decreasing TNT layer tube diameter (increasing surface area). The composite ALD CdS/TNT layer with the smallest tube diameter ≈ 35 nm (largest surface area ≈ 52 cm2/cm2) exhibited IPCE above 50% up to 470 nm with the photoresponse onset around 520 nm (corresponding with the band gap of CdS ≈ 2.4 eV). In a follow up study, Zazpe et al. [125] presented on the upscaling of the thickness of ALD CdS coated TNT layers. Ultra HAR TNT layers with thicknesses between 1 ␮m and 130 ␮m (AR up to 900) were coated by ALD CdS with thicknesses between 2 nm and 10 nm. The evidence of 10 nm CdS coating of a fraction of TNT layer featured by HAADF STEM image and STEM/EDX elemental mapping with corresponding line profiles is depicted in Fig. 7e–g, respectively. The utilization of CdS coated ultra HAR TNT layers resulted first of all into a superior PEC performance, as demonstrated by high IPCE values of around 70%. In addition, it resulted in an apparent shift of the onset of visible-light photoresponse of CdS-TNT layers up to 675 nm (≈1.8 eV, significantly below the band gap of CdS) while the IPCE around 70% was kept in the range from 300 to 600 nm, as it is shown in Fig. 7h. The unexpectedly significant shift of the onset of photoresponse was attributed to the advantageous geometry of the ultra HAR TNT layers giving rise to multiple light-scattering effects, which extremely prolong the optical path of light within nanostructure as illustrated in Fig. 7i and therefore enhances the probability of capturing light by sub-band gap transitions in conformal and uniform ALD CdS coatings. The obtained results clearly demonstrate the profitability of ALD for light harvesting applications to effectively utilize the deposited material in form of a uniform ultrathin coating, perfectly adopting a large surface/interface area of the support nanostructure, using TNT layer as a model example. The improved photoresponse of ALD coated TNT layers was also presented by Ng et al. [135], who deposited molybdenum oxyselenide (MoSexOy) coating utilizing Mo(CO)6and (CH3Si)2Se ALD precursors. The optimal thickness of the MoSexOycoating for maximal photocurrent generation and photocatalytic degradation of model dye (methylene blue (MB)) was found to be within the range of 2–5 nm. The study represents a fundamental step for future tailored deposition of transition metal dichalcogenides, such as MoSe2 [136], into TNT layers by ALD. 4.3. Charge separation Generally, one of the key limiting factors of the performance of solar cell devices is the recombination of photogenerated charge carriers. Main charge recombination centres are represented by defects, trap states and charge impurities at surfaces and interfaces, 8 F. Dvorak et al. / Applied Materials Today 14 (2019) 1–20 Fig. 6. PEC performance of 4 nm thick Co3O4coating of TNT layers by ALD. (a) The photocurrent density and (b) photoconversion efficiency of Co3O4/TNT layer heterostructure upon visible-light illumination (>420 nm, 100 mW/cm2). (c) Illustration of band alignment within the heterostructure as determined by XPS. Reproduced from [128]. e.g. Ti3+ interstitials giving rise to Ti 3d states below conduction band edge in TiO2[137,138]. For HAR nanostructures (which are also high surface area materials), the prevention of recombination at surfaces/interfaces is of utmost importance. One way to suppress the recombination on surface defects is to prepare surfaces with lowest possible concentration of surface defects. This can be realized either by optimization of the growth conditions of that particular material itself or by deposition of an overlayer of a suitable defect-depleted material – passivation layer – which passivates the surface defects of an underlying material. However, such an overlayer should not alter the properties of the underlying material and should be very thin not to limit the charge transfer through the layer itself. The charge recombination at the interfaces can be suppressed by a combination of compatible materials, forming a high quality interface with low amount of defects and with an inherent energy barrier for the back transfer of electrons or holes. ALD, being inherently ideal for the deposition of conformal and uniform coatings with minimal amount of defects and thickness controlled on the nanometer scale, represents very promising technique for the growth of blocking/passivating layers. The credit of ALD is reflected by a high number of recent studies utilizing ALD for deposition of blocking layers. For the case of TNT layers, the ALD was utilized for the deposition of Al2O3[139–142], ZnO [142–148], and TiO2[142,149] thin coatings as blocking layers in order to improve the PEC performance of TNT layers. In addition, the same materials were also utilized as blocking layers in mesoporous TiO2layers used in DSSC [150–154]. Wide band gap and almost ideal ALD growth make Al2O3a prototypical material for a blocking layer deposited by ALD. ALD growth of Al2O3is usually realized using TMA and water as precursors. Gui et al. [140] coated TNT layers by Al2O3by 10–200 ALD cycles at 100–400 ◦C temperature range in order to enhance the PEC water splitting by the surface passivation of TNT layers. The best PEC performance was observed for Al2O3coating by 25 cycles (2.6 nm) at 200 ◦C yielding 1.8 times higher photocurrent density than bare TNT layers. Kim et al. [139] coated TNT layers with AR of ≈230 by ultrathin Al2O3coatings with thickness of 0.1–0.6 nm (deposited by 1–6 ALD cycles) at 200 ◦C and subsequently sensitized them with N-719 ruthenium based dye. The study showed that already after one ALD cycle, the Al2O3coating led to an increase of open circuit voltage and prolonged the electron life times. In contrast, Al2O3coatings deposited by more than two ALD cycles was found to reduce the photocurrent density as a result of an increase of the energy barrier for the injection of electrons from the dye to the TiO2conduction band. Zeng et al. [141] examined the role of ALD Al2O3overlayers with various thicknesses on PEC performance of coated CdS and PbS QD sensitized TNT layers. The Al2O3overlayers were deposited by 3–100 ALD cycles. The optimal thickness leading to improved charge collection efficiency was found to be 1.5 nm (achieved by 30 ALD cycles) yielding 1.6 times higher photocurrent density than for bare QD sensitized TNT layers. For the illustration of properties of Al2O3ALD coating of TNT layers, the structure of 30 ALD cycles Al2O3coated QD-TNT layers and PEC efficiency with model of beneficial interface band alignment of ALD Al2O3coated QD-TNT layers are shown in Fig. 8a–c. Fundamental mechanisms behind the passivation effect of Al2O3overlayers can be learned from the field of silicon solar cells [155]. Al2O3coatings help to reduce the surface/interface recombination rate by (i) passivation of surface/interface defects, so-called chemical passivation which is realized by hydrogen incorporated naturally in bulk Al2O3[156] and (ii) by significant reduction of the concentration of one type of charge carrier at the surface/interface by an electric field denoted as field effect passivation, that stems from the inherent accumulation of negative charge in Al2O3near the interface, connected to defects in form of Al vacancies and O interstitials [157]. The higher is the temperature of ALD process, the lower amount of hydrogen is found in Al2O3, resulting in less effective chemical passivation [158]. The optimal temperature range for Al2O3ALD deposition is found to be between 150 and 250 ◦C. The post-deposition annealing of Al2O3improves the passivation effect with onset above 300 ◦C as it promotes diffusion of H towards the interface and also increases the negative charge accumulated in Al2O3. The thickness also affects the passivation capability of Al2O3. The chemical passivation starts to be limited for coatings thinner than 5 nm, while the field effect passivation remains down to 2 nm. On the other hand, increased thickness of the passivation layer limits its charge transfer properties. The trade-off between F. Dvorak et al. / Applied Materials Today 14 (2019) 1–20 9 Fig. 7. ALD coating of TNT layers by CdS light-absorber layer for improved visible-light photoresponse. (a–c) SEM images of 6 nm CdS coated TNT layers with average tube diameters 35 nm, 56 nm, and 95 nm, respectively, scale bar 100 nm. (d) IPCE of TNT layers with various nanotube diameter sensitized by 6 nm thick ALD CdS coating upon illumination with a monochromatized light source (e) High angle annular dark field (HAADF) STEM image and (f) STEM/EDX elemental map of fraction of nanotube coated by 10 nm thick CdS overlayer, the green arrows mark the corresponding elemental line profiles shown in (g). (h) IPCE of TNT layers with various thicknesses sensitized by 5 nm thick ALD CdS coating. (i) Illustration of multiple light scattering effects prolonging the optical path of light in TNT layer and facilitating efficient light harvesting. (a–d) Reproduced from [134] and (e–i) reproduced from [125]. the passivation and charge transfer properties yields optimal Al2O3 thicknesses of around 2 nm employed as capping passivation layer [155]. The above discussed results clearly demonstrate the uniqueness of ALD to tailor the coatings of HAR nanostructures on atomic level and ultrahigh precision, non-feasible for alternative deposition techniques. In an analogous way to Al2O3, several works addressed to which extent the PEC performance of TNT layers based photoanodes can benefit from the secondary coating by thin ZnO overlayer, deposited by ALD. ZnO represents a direct band gap semiconductor with the band gap value (3.4 eV) close to TiO2, which is known to promote efficiency of DSSC [159]. Jeong et al. [143] coated TNT layers by 10 nm thick ZnO (using 50 ALD cycles) and sensitized it by N719 dye. The ZnO coating was found to form a particulate-like decoration of TNT layers increasing the surface area of the photoanode. The short circuit photocurrent density improved from 3.98 to 4.31 mA/cm2. Correspondingly, the efficiency improved from 1.23 to 1.42% by ZnO coating. The positive effect of the ZnO coating was attributed to the increased surface area of ZnO coated TNT layers and to the high isoelectric point of ZnO forming an inherently positive charge at the ZnO/electrolyte interface, facilitating an efficient dye adsorption. Moreover, the difference of TiO2and ZnO isoelectric points caused a negative shift of the TiO2conduction band, resulting in an increased energy level difference with the liquid electrolyte leading to an increased open circuit voltage. In several studies, Cai et al. [144,145,147,148] inspected the effect of 2, 5, and 10 nm thick ZnO coating (deposited by 10, 25, and 50 ALD cycles) on the PEC performance of ZnO/TNT layers and of functionalized poly(3-hexylthiophene)/ZnO/TNT layers. The best PEC performance was revealed for 2 nm thick ZnO film (10 ALD cycles). The photocurrent density rose by a factor of 1.6 and the results of impedance spectroscopy pointed to the improved charge separation featured by lowered charge-transfer resistance, negative shift of flat band potential, and longer electron lifetimes. The additionally poly(3-hexylthiophene) functionalized ZnO/TNT layers showed increased light absorption and promoted photoluminescence quenching as a mark of supressed radiative recombination of photogenerated charge carriers [148]. Zeng et al. [146] deposited a thin ZnO layer by ALD as an interlayer for PbS and CdS QD sensitized TNT layers. The authors utilized TNT layers with various thicknesses (1.5–2.8 ␮m) and coated them by ALD ZnO with thickness between 0.7 and 10.5 nm (5, 10, 30, and 70 ZnO ALD cycles). The ALD ZnO/TNT layers were afterwards loaded by CdS/PbS QD using SILAR technique. 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