Conformal TiO₂ Aerogel-Like Films by Plasma Deposition: from Omniphobic Antireflective Coatings to Perovskite Solar Cell Photoelectrodes

Conformal TiO2Aerogel-Like Films by Plasma Deposition: from Omniphobic Antireflective Coatings to Perovskite Solar Cell Photoelectrodes Jose M. Obrero, Lidia Contreras-Bernal, Francisco J. Aparicio Rebollo, Teresa C. Rojas, Francisco J. Ferrer, Noe Orozco, Zineb Saghi, Triana Czermak, Jose M. Pedrosa, Carmen López-Santos, Kostya Ken Ostrikov, Ana Borras, Juan Ramón Sánchez-Valencia,*and Angel Barranco* Cite This: ACS Appl. Mater. Interfaces 2024, 16, 39745−39760 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: The ability to control the porosity of thin oxide films is a key factor determining their properties. Despite the abundance of dry processes for synthesizing oxide porous layers, a high porosity range is typically achieved by spin-coating-based wet chemical methods. Besides, special techniques such as supercritical drying are required to replace the pore liquid with air while maintaining the porous network. In this study, we propose a new method for the fabrication of ultraporous titanium dioxide thin films at room or mild temperatures (T≤120 °C) by a sequential process involving plasma deposition and etching. These films are conformal to the substrate topography even for high-aspect-ratio substrates and show percolated porosity values above 85% that are comparable to those of advanced aerogels. The films deposited at room temperature are amorphous. However, they become partly crystalline at slightly higher temperatures, presenting a distribution of anatase clusters embedded in the sponge-like open porous structure. Surprisingly, the porous structure remains after annealing the films at 450 °C in air, which increases the fraction of embedded anatase nanocrystals. The films are antireflective, omniphobic, and photoactive, becoming superhydrophilic when subjected to ultraviolet light irradiation. The supported, percolated, and nanoporous structure can be used as an electron-conducting electrode in perovskite solar cells. The properties of the cells depend on the aerogel-like film thickness, which reaches efficiencies close to those of commercial mesoporous anatase electrodes. This generic solvent-free synthesis is scalable and applicable to ultrahigh porous conformal oxides of different compositions, with potential applications in photonics, optoelectronics, energy storage, and controlled wetting. KEYWORDS: aerogel-like coatings, plasma deposition, omniphobic films, antireflective films, perovskite solar cells, titanium dioxide 1. INTRODUCTION Porous nanostructured metals and metal-oxide films have attracted recent solid attention owing to their diverse emerging applications in catalysis, photonics, photovoltaics, electrodes for batteries, intelligent materials, energy harvesting, sensing, nanomembranes, and biointerfaces. 1−4 To meet the requirements for the most recent applications, it is essential to develop synthetic procedures for customized porous structures directly compatible with their fabrication on processable substrates such as transparent conductive electrodes, flexible polymers, and on-device architectures such as interdigitated electrodes. 5−9 Tunable deposition of porous metal and metal oxide nanosystems, including solution methods, electrochemical, electrospinning, and physical and chemical vapor deposition processes, has recently emerged as a highly topical area of research. 10−15 Vacuum-based deposition methods are especially suited for developing devices integrating functional coatings with advantages such as precise control over thickness and composition, uniformity, purity, versatility, and scalability to large-scale production, as demonstrated by many examples from optoelectronics and microelectronics. 16−18 Properly designed, the vacuum-based processes can be energy efficient and produce a very low or negligible environmental impact. Porous structures have been developed by vacuum-based methods such as oblique angle deposition (OAD) or glancing Received: February 13, 2024 Revised: July 8, 2024 Accepted: July 10, 2024 Published: July 20, 2024 Research Articlewww.acsami.org © 2024 The Authors. Published by American Chemical Society 39745 https://doi.org/10.1021/acsami.4c00555 ACS Appl. Mater. Interfaces 2024, 16, 39745−39760 This article is licensed under CC-BY 4.0 Downloaded via UNIV DE SEVILLA on February 19, 2025 at 12:31:25 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. angle deposition (GLAD) in many different technological areas. 14,19 By tuning the deposition parameters, it is possible to obtain high-surface-area structures with anisotropic pore structure distribution and enhanced diffusion properties. For example, porosity control in TiO2films by OAD/GLAD 20 has been applied for the development of gas sensors, 21,22 photonic structures, 19,23 microfluidic devices, 24 and solar cell electrodes by liquid infiltration. 25 Other vacuum-based methods, such as plasma-enhanced chemical vapor deposition and magnetron sputtering, which typically produce compact and dense films, have also been applied for the development of porous oxide thin films for diverse applications, albeit to a lesser extent. 26−29 Despite the many reports on the preparation of porous oxides by vacuum-based methods, a very high porosity range is typically achievable by wet chemistry methods. Aerogels are a class of nanostructured materials with ultrahigh porosity (80− 98%), extremely low density, and a high surface area. 30 Aerogels are used in structural applications as catalysts and catalytic supports, adsorption materials, and thermal insulators. 31,32 Aerogels are usually prepared in powder form by sol−gel methods combined with special drying methods such as supercritical drying to remove the solvent and fill the network with air, avoiding the collapse of the original gel structure due to surface tension. Different wet methods, such as spin-coating, dip-coating, and spray-coating, have been proposed to obtain supported aerogel thin films. 33,34 Titanium oxide films have been the subject of extensive research and application due to their remarkable properties. They have demonstrated utility as photocatalysts, electron transport layers in solar cells, optical materials due to their high refractive index and effective absorption of UVA radiation, gas sensors, components in biomaterial development for medical applications, surfaces with bactericidal properties, and dielectric protective layers. 35−38 The properties of titanium dioxide films, such as band gap position, crystallinity, and porosity, are highly dependent on their preparation conditions. 39,40 TiO2aerogels were first prepared by Teichner in 1976. 39 Since then, they are usually prepared from Ti alkoxides and inorganic salts (e.g., TiCl4), mostly for photocatalytic and thermal applications. 32 In this work, we present a new approach for the synthesis of ultraporous conformal TiO2dioxide films reaching the levels of high porosities and low densities characteristic of aerogel materials. 41 The deposition process combines remote plasma polymerization of a Ti phthalocyanine precursor to produce conformal homogeneous Ti-containing plasma polymers and plasma etching to remove the organic component of the polymer, yielding highly porous TiO2films. The combined plasma polymer deposition and plasma etching process can be repeated successively to increase the thickness of the films, increasing their overall porosity. Due to their structure, the TiO2aerogel-like films developed are antireflective and omniphobic and can be further thermally annealed to increase their crystallinity without collapsing the porous structure. Interestingly, despite the high porosity levels achieved and partial crystallinity, the films are photoactive. An initial feasibility study about the use of TiO2aerogel-like films as electron transport layers in perovskite cells shows results comparable to standard commercial mesoporous titania coatings fabricated by sol−gel methods, indicating the potentiality of this type of film for their integration in devices after proper optimization. This technique is generic and can be applied to other metal precursors to synthesize aerogel-like conformal functional oxide nanocoatings. In addition, the plasma and vacuum processes employed are industrially scalable. 2. EXPERIMENTAL SECTION 2.1. Aerogel-Like Thin Film Synthesis. The main precursor molecule in this study, titanium(IV) phthalocyanine dichloride (TiPc), and additional precursors such as titanium phthalocyanine, titanium acetylacetonate, silicon(IV) phthalocyanine dichloride, and iron(II) phthalocyanine were purchased from Sigma-Aldrich and used as received. Conformal metal-containing plasma polymer films were deposited by remote plasma-assisted vacuum deposition (RPAVD). Full details of the experimental setup can be found elsewhere. 42,43 In summary, the precursor molecules were sublimated in the downstream region of an electron cyclotron resonance microwave Ar plasma (150 W, 2.45 GHz) by using a Knudsen cell placed about 10 cm from the plasma. The substrates were at room temperature (RT) and placed facing the Knudsen cell and away from the plasma discharge. The growth of the polymer film was monitored in situ with a quartz crystal microbalance (QCM) placed next to the sample holder. Argon pressure was dosed by a calibrated mass flow controller to reach a deposition pressure of 2 ×10−2mbar. The base pressure of the reactor was <10−6mbar. Silicon (100) and fused silica were used as substrates. Special substrates, such as supported nanowires on Si(100), for core@shell deposition and perovskite solar cell (PSC) electrodes, were prepared as specified below. 2.2. Plasma Etching Treatments by ECR-MW. The posttreatment of the plasma polymer films by plasma etching was carried out with the samples facing an Ar/O2(1:1 mass flow) plasma discharge (2 ×10−2mbar, 350 W). The distance between the substrates and the glow discharge region was 8 cm. Substrate temperatures range from RT to 120 °C, and treatment durations range from several minutes up to 30 min, depending on the sample characteristics. Cycles of plasma polymer deposition and plasma etching were conducted in the same reactor by rotating the sample holder and using the described experimental conditions. 2.3. Synthesis of Supported Organic Nanowires (ONWs) by Physical Vapor Deposition (PVD). H2−Phthalocyanine (H2Pc) from Sigma-Aldrich was used as received. The base pressure in the deposition system was 1 ×10−6mbar. The sublimation of the molecule was carried out using a Knudsen cell, placed at 8 cm from the substrates, under 10−2mbar of Ar flow, which was dosed by a calibrated mass flow controller. The growth rate and equivalent thickness of H2Pc NWs were monitored using a QCM, and the growth rate was adjusted to 0.3−0.4 Å/s setting a density in the QCM electronics of 0.5 g/cm3. The substrate temperature was imposed at 170 °C in a heatable sample holder connected to an electric current source to induce the formation of the ONWs. Additional details about the synthesis of ONWs can be found elsewhere. 44,45 2.4. Preparation of Perovskite Solar Cells. Fluorine-doped tin oxide glass TEC 15 (FTO, XOP Glass, resistance 12−14 Ω/□) were used as substrates for the cells. The substrates were cleaned using an ultrasonic bath and following the sequence of solvents: Hellmanex solution (2:98% V soap/water), deionized water, 2-propanol, and acetone (15 min for each solvent). After the cleaning sequence, the substrates were dried by using a nitrogen gas flow. Then, an ultraviolet (UV)/O3treatment was applied for 15 min. Thin, compact layers (20 nm) of TiO2(c-TiO2) were deposited on top of the FTO substrates by spray pyrolysis. The precursor of the compact layers consisted of 1 mL of titanium di-isopropoxide bis(acetylacetonate) solution (75% in 2-propanol, Sigma-Aldrich) in 14 mL of absolute ethanol. The precursor solution was sprayed over annealed FTO (400 °C) using oxygen as the carrier gas. The FTOs were kept at 450 °C for 30 min to achieve an anatase phase. Once the substrates were cooled down, the ultraporous TiO2layer was deposited, as indicated above. For the preparation of reference cells, a mesoporous solution was prepared by adding 1 mL of absolute ethanol to 150 mg of a commercial TiO2paste (Sigma-Aldrich, 18NRT). The mesoporous ACS Applied Materials & Interfaces www.acsami.org Research Article https://doi.org/10.1021/acsami.4c00555 ACS Appl. Mater. Interfaces 2024, 16, 39745−39760 39746 dispersion was left to stir overnight. After that, 100 μL were deposited on c-TiO2samples by spin-coating at 4000 rpm for 10 s. After being spun, the samples were immediately placed on a hot plate at 100 °C for 10 min. The mesoporous TiO2layer (m-TiO2) was then sintered following a programmed temperature variation according to the previous report. 46 Perovskite layers were grown on the ultraporous electrode following the method reported by Saliba et al. 46 That is, a perovskite precursor solution consisting of ((FAPbI3)83(MAPbBr3)17 + 5% CsI) + 5% RbI was prepared by the addition of 1 M formamidinium lead triiodide (FAPbI3) and 1 M methylammonium lead tribromide (MAPbBr3) solutions (5/1%V, respectively) both in 1:4% V DMSO/ DMF (dimethyl sulfoxide and N,N-dimethylformamide, respectively). A 5% V of 1.7 M of CsI solution in DMSO was added to this solution, and then 5% V of RbI solution in 1:4% V DMSO/DMF (0.2:99.8% mol, respectively). The perovskite film (RbCsMAFA) was deposited by two-step spin-coating: (1) 1000 rpm, 10 s; (2) 6000 rpm, 20 s. 15 s after the beginning of the second step, 200 μL of chlorobenzene was added as an antisolvent. Immediately, the substrate was annealed at 100 °C for 45 min. Once the substrate was cooled down, a doped solution of Spiro-OMeTAD was deposited by spin-coating on top of the perovskite layer. This solution consisted of 70 mM 2,2,7,7- tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9-spirobifluorene (Sigma-Aldrich) in chlorobenzene and the dopants Lithium bis- (trifluoromethanesulfonyl)imide (LiTFSI, 520 mg/mL in acetonitrile), tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine) cobalt(III)tris- (bis(trifluoromethylsulfonyl)imide), and 4-tert-butylpyridine in a molar ratio of 0.5, 0.03 and 3.3, respectively. The perovskite and Spiro-OMeTAD layers were deposited in a glovebox under very low humidity and oxygen conditions (<0.1 ppm of O2and <0.1 ppm of H2O). Finally, gold contacts (thickness ∼80 nm) were deposited on the devices by evaporation in high vacuum (<10−6mbar). 2.5. Experimental Characterization Methods. High-resolution scanning electron microscopy (SEM) images of the samples deposited on silicon wafers were obtained in a Hitachi S4800 microscope at an acceleration voltage of 2 kV. Cross-sectional views were obtained by cleaving the Si(100) substrates. Focused ion beam 3D analysis (FIB- 3D) of selected samples was performed on a Zeiss crossbeam 550 FIB-SEM. Scanning TEM (STEM), high-resolution transmission electron microscopy (HRTEM), and high-angle annular dark field (HAADF)-STEM images were acquired in a TALOS F200S microscope, working at 200 kV, and in a Tecnai G2F30 S-Twin STEM, working at 300 kV, both with a HAADF detector. DigitalMicrograh software has been used to analyze the HRTEM images and to obtain the digital diffraction patterns (DDP). X-ray photoemission spectroscopy (XPS) experiments were performed in a Phoibos 100 DLD X-ray spectrometer from SPECS. The spectra were collected in the pass energy constant mode at a value of 50 eV by using magnesium and aluminum sources. The C 1s signal at 284.8 eV was utilized for the calibration of the binding energy in the spectra. The assignment of the BE to the different elements in the spectra corresponds to the data in XPS reference databases. 47 UV−vis transmission spectra of the samples deposited on fused silica slides were recorded by using a PerkinElmer spectrophotometer in the range of 190 to 2500 nm. Reflection spectra were recorded on a Cary 5000 from Agilent using a universal measurement accessory in the range of 190−2500 nm. Variable angle spectroscopic ellipsometry was carried out on a Woollam VASE ellipsometer. Rutherford backscattering spectroscopy (RBS) and nuclear reaction analysis (NRA) were carried out in the 3MV Tandem Accelerator at the Centro Nacional de Aceleradores (US-CSIC, Sevilla, Spain). The RBS spectra were obtained using 2.0 MeV alpha particles and collecting the backscattered particles with a PIPS (passivated implanted planar silicon) particle detector at 165°. NRA was used to determine C, N, and O elements in the film from the 12C(d,p)13C, 14N(d,α1)12C y 16O(d,p1)17O nuclear reactions using deuterons of 1.0, 1.4 y 0.9 MeV, respectively. The spectra were obtained by using a particle detector at a 150°collection angle in combination with a 13 μm thick Mylar filter to stop the retrodispersed particles. The NRA and RBS spectra were simulated using the SIMNRA 6.0 code. 48 The film densities were determined from the combined RBS and NRA analyses and thickness values obtained from cross-sectional SEM micrographs of the films. 28 Adsorption/desorption isotherms were measured with a QCM using quartz crystal sensors with deposited porous oxide layers following the procedure described in previous refs 28 and 49 For this analysis, a set of samples were grown directly on quartz crystals in the plasma reactor. The isotherms were obtained by sequentially introducing varying amounts of water vapor into a closed chamber containing the QCM. Before the adsorption experiment, the samples were heated under a vacuum at approximately 120 °C to eliminate any condensed or adsorbed water in the films during their exposure to air. The total pore volume was estimated under the assumption that all pores were filled with water at saturation pressure. Atomic force microscopy (AFM) images were obtained using the tapping mode of a Nanotec microscope with Dulcinea electronics and then analyzed with the WSxM software. 50 The wetting behavior was studied by contact angle measurements of liquid droplets with Milli-Q water (2 μL) and CH2I2diiodo- methane (1 μL) by the static sessile droplet method on an OCA20 from DataPhysics Instruments GmbH. Contact angle values are presented as an average of 5 measurements. Fluorine-based chemical derivatization was carried out by exposing a previously plasmaactivated surface to 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFOTES) vapor maintained in a thermal bath at 80 °C for 3 h after a prior vacuum pump. These conditions favor the reaction of the surface −OH groups with a perfluorinated silane. 51 Photoactivation by UV irradiation in air was performed with a 175 W ASB-XE-175 xenon light source lamp. Steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements were carried out using an Edinburgh Instruments FLS1000-DD-STM fluorescence spectrophotometer. 3. RESULTS AND DISCUSSION 3.1. Development of Ultraporous Aerogel-Like TiO2 Films by Plasma Deposition and Etching. In a previous work, we demonstrated that plasma oxidation of porphyrin and phthalocyanine sublimated films produced conformal nanostructured oxide surfaces. 9,42 Plasma oxidation effectively removes organic components from such films, producing oxide surface aggregates whose properties (i.e., oxidation state, degree of aggregation, and surface percolation) depend strongly on the nature of the metal cation and the thickness of the initial sublimated film. Therefore, porous nanostructured Zn, Co, Cu, oxide, and Pt metal surfaces were prepared by direct plasma oxidation of sublimated Zn(II), Co(II), Cu(II), and Pt(II) porphyrins or phthalocyanines. 44,52 Herein, we present the evolved synthetic approach by exploiting the use of sacrificial polymeric films obtained by RPAVD of Ti(IV) phthalocyanine dichloride (hereafter, TiPc plasma polymer). The RPAVD method gives rise to plasma polymer films by adjusting the plasma interactions with a sublimated functional precursor. The details of the methods have been discussed in previous works. 42,43,53 RPAVD films are continuous, conformal, homogeneous, insoluble, and crosslinked plasma polymers containing some fraction of unreacted precursor molecules, depending on the synthetic conditions used. In this work, the RPAVD TiPc films are used as conformal sacrificial layers to be oxidized by the interaction with oxygen plasma at low temperature or RT. The polymeric Ti-containing plasma polymer films act here as the precursor layers for Ti atoms that will eventually form TiO2films by reacting with the oxygen plasma species. In addition, the plasma etching, or plasma oxidation, steps remove C and the ACS Applied Materials & Interfaces www.acsami.org Research Article https://doi.org/10.1021/acsami.4c00555 ACS Appl. Mater. Interfaces 2024, 16, 39745−39760 39747 remaining heteroatoms by forming volatile species that are pumped out of the chamber. The TiO2films are thus deposited by the iterative process shown in Figure 1a. In each of the synthetic cycles, a Ti-containing plasma polymer is deposited and then subjected to an oxygen plasma treatment to form a titanium oxide film. The plasma polymerization and oxidation cycles are repeated several times to increase the final layer thickness. As indicated in the schematic, each cycle comprises conformal TiPc plasma polymerization (step 1) and plasma etching (step 2) steps. These cycles are required to fully oxidize the layers after oxygen plasma treatment. As will be shown below, thin layers of several tens of nanometers are fully oxidized at mild temperatures and plasma powers. However, the plasma polymer is not completely oxidized for thicker films under the same experimental conditions. Thus, the repeated cycling procedure is an essential requirement for the controlled growth of thicker and porous layers of TiO2. Figure 1b shows the planar and cross-sectional films of a TiPc polymeric film deposited on Si(100) by RPAVD. The micrographs show that this film is continuous and homogeneous, with a very smooth surface. AFM characterization of a set of polymeric films shows RMS surface roughness is <0.5 nm for films with thicknesses in the range of 50−500 nm (Figure S1a); this is a common feature of polymer films synthesized by RPAVD. 42,43,53 These films exhibit intense absorption bands ascribed to embedded Ti phthalocyanine molecules (Figure S1b). Apart from these localized molecular absorption features, the films are transparent in the visible (VIS) and near-infrared (NIR) range. When the TiPc plasma polymer films are exposed to oxygen plasma, oxygenated species from the plasma are incorporated to form nanostructured inorganic TiO2films. The C, N, and Cl elements from the initial polymeric films were removed as volatile species. The surface elemental and chemical analysis was conducted by X-ray photoelectron spectroscopy (XPS) on a sublimated TiPc film, a titanium-containing plasma polymer deposited using RPAVD, and three titanium dioxide (TiO2) oxide films deposited through one, three, and five deposition and etching cycles (referred to as C1, C3, and C5, respectively) in Figure S2. The RPAVD process generated plasma polymers with a composition closely resembling the stoichiometry of the precursor molecule, exhibiting only a minimal oxygen enrichment of less than 10 atomic %. It is worth noting that such oxygen enrichment is characteristic of any plasma deposition process due to postdeposition reactions of the samples in air or by direct incorporation of residual oxygen species in the plasma. 42 After the plasma oxidation, the resulting TiO2films were found to be slightly overstoichiometric (i.e., surface oxygen enrichment). Importantly, the films were analyzed in their as-deposited state without undergoing any surface cleaning procedures. A minor carbon content (∼17%) was detected in the oxide films, particularly in C3 and C5. This carbon content and the excess oxygen could be attributed to surface contamination (adsorbed hydrocarbon and water molecules and surface hydroxyl groups). Note that a small percentage of oxygen is also present on the surface of the precursor powder reference. However, a small percentage of carbon content from incomplete oxidation of the sacrificial polymer precursor cannot be avoided. Notably, no traces of nitrogen (N) or chlorine (Cl) from the precursor molecules were detected on the surfaces of the oxide films. Figure 2a shows the FESEM micrographs of the film shown in Figure 1b after the plasma etching process. The figure shows how the initially homogeneous polymeric films were converted into a porous structure, presenting a distribution of nanometersized holes that are spread from the film surface to the Si substrate. These deposition and etching steps are repeated successively to increase the thickness of the resulting porous film (cycles Cn in Figure 1a). Figures 2a−e shows that the film thickness increases with each cycle. The microstructure evolves from an open hole interconnected oxide to a highly open, three-dimensional percolated porous structure. It can be noted that after every cycle, in the resulting TiO2films (Figure 2b), the pores of the TiO2film increased in size with respect to those produced in the previous deposition−oxidation cycle. From the third cycle onward, the film evolves into a more columnar-like structure, which is mainly formed by interconnected voids forming an open network where the empty space is predominant. This interconnected structure is preserved in cycles C4 and C5. The deposition and plasma treatment cycles shown are very effective in increasing the thickness of the films while maintaining the porous structure of the previous layers. This is possible because, in every first deposition step of each cycle, the TiPc plasma polymer conformally coats the open porous surface obtained in the previous cycle (as shown in Figure 1c) before plasma oxidation. After plasma oxidation of the plasma polymer, the previous pore structure is preserved, and the new modified pore structure remains interconnected. Therefore, the Figure 1. Sequential deposition procedure of porous oxide thin films by plasma polymerization and remote etching. (a) Schematic of sequential plasma deposition (RPAVD) and plasma etching processes to obtain ultraporous TiO2thin films. The thickness of the films increases as a function of the number of deposition (step 1) plus etching (step 2) cycles denoted as C1, C2,...,Cn. (b) FESEM cross and planar micrographs of an RPAVD Ti-containing polymeric film on Si(100). This polymeric film deposition is the starting step for the synthesis of porous TiO2films. (c) RPAVD polymeric film deposited on top of a porous TiO2film grown on two previous cycles of plasma polymerization and plasma etching. ACS Applied Materials & Interfaces www.acsami.org Research Article https://doi.org/10.1021/acsami.4c00555 ACS Appl. Mater. Interfaces 2024, 16, 39745−39760 39748 conformal polymer coating also has the role of preventing the closure of the previously formed pores. This process is most likely due to the deposition of conformal polymeric films without diffusion inside the micropores. Microscopy measurements confirm that all pores of the film structure are hollow after the etching process, as will be seen below. An in-depth evaluation of the porous structure of these films can be obtained from FIB-SEM analysis of the films. Figure 3a shows a FIB-3D micrograph of the TiO2porous film deposited in 5 cycles. It is possible to distinguish in the micrograph the different interfaces corresponding to the first three deposition cycles, reaching a thickness of about 500 nm. Apart from these three interfaces, the film is very porous and continuous. From cycle C4 and above, less interconnected porous columnar structures develop, and the clear interface between cycles disappears. These columnar structures can also be recognized in the cross-sectional FESEM views of Figure 2d,e. The set of xz slices obtained by FIB etching at successive y positions shown in Figure 3b reveals the completely interconnected three-dimensional (3D) porosity and the lateral connection of the oxide nanostructures in the films [see the complete video sequence of this FIB-SEM in S3 (Video S3)]. The image sequence allows one to observe that porous structures are continuous and connect the film surface with the substrate. Apart from the interconnected porosity, a comparison between the FESEM and FIB-SEM micrographs reveals an extremely low-density 3D TiO2interconnected network in films. Figure 4a shows the evolution of the film density (black dots) as a function of the layer thickness corresponding to a set of films prepared by 1 to 5 deposition/plasma oxidation cycles. The figure also shows the corresponding porosity values calculated using the relationship Γ= 100(1 −ρ/ρr) taking the density of anatase as ρr= 3.9 g/cm3. 54 The first deposition/ plasma etching cycle yielded films with a density of less than 1 g/cm3and a porosity of 74.8%. As the number of cycles increases, the density decreases dramatically, reaching a value of 0.47 g/cm3at 440 nm of thickness (C5), which corresponds to a film porosity as high as 87.8%. These values increase slightly to 88.5% when the thickness increases to 660 nm with another deposition cycle, reaching a minimum density of 0.45 g/cm3. The shape of the density and porosity versus the layer thickness curves indicates that both magnitudes are not constant over the thickness. If we calculate the density/ porosity profile (i.e., the density/porosity of each section of the film) as a function of film thickness (Figure 4b), it can be noted how the film density/porosity decreases/increases rapidly up to a thickness of about 220 nm, where it reaches a value of 0.35 g/cm3/91.1% and then decreases/increases more slowly until it reaches a minimum density of 0.29 g/cm3 at 440 nm, which corresponds to the porosity of 92.5%. In the last studied cycle at 660 nm, the density/porosity changes and increases/decreases up to 0.40 g/cm3/89.9%. Note that the porosity values in Figure 4a,b are referenced to the reported value of anatase. Using other density reference values, like those of amorphous TiO2(3.8−3.9 g/cm3) 55,56 or rutile (4.23 g/cm3), 54 we can obtain slightly lower or higher porosity values. Thus, for thickness up to 200 nm, the density and porosity values are in the range of those reported for aerogel films obtained by supercritical drying methods 57 and are even higher Figure 2. Characterization of the film structure by SEM. Top-view (left) and cross-sectional (right) FESEM images of films deposited on Si(100) were obtained by using deposition-etching cycles. (a−e) Synthetic cycles C1 to C5, respectively. Figure 3. Determination of the pore structure by electron microscopy (a) FIB-SEM cross-sectional micrograph of a C5 porous TiO2sample. (b) Sequence of FIB images from the previous sample showing the connectivity between the porous structures in the film. ACS Applied Materials & Interfaces www.acsami.org Research Article https://doi.org/10.1021/acsami.4c00555 ACS Appl. Mater. Interfaces 2024, 16, 39745−39760 39749 than typical values reported for TiO2aerogel films. 31,33,34,58 For this reason, we will hereafter refer to them as aerogel-like TiO2films. However, their characteristic structure differs from those reported for conventional aerogels, as seen in the following sections. The porosity of the samples was evaluated in detail by measuring water adsorption−desorption isotherms using a quartz crystal monitor. Figure 4c shows normalized isotherms corresponding to two samples of 310 and 440 nm (C3 and C4 cycles). The two isotherms have similarly shaped adsorption− desorption curves. As expected, the total water adsorption capacity increases with the film thickness. The isotherms belong to group IV of the IUPAC classification, indicating that most of the porosity is due to mesopores. 59 This type of curve presents an initial region that changes rapidly with adsorbed water volume and corresponds to the adsorption of micropores (P/P0< 0.05) and a second region that changes with a lower slope and corresponds to the adsorption of mesopores. The large absorption−desorption cycle hysteresis in both samples indicates a complex porous structure. However, unlike water, adsorption isotherms reported for columnar porous TiO2thin films prepared by PECVD 28 and GLAD 49 determined by a similar methodology, the absorption−desorption cycles are fully reversible at RT even in the micropore region very likely due to the very high accessibility of the open, porous structure of the films. Pore size distribution curves of the two studied films are shown in Figure 4b. The curves indicate that the two films have a relatively broad pore distribution in the range of 2 to 10 nm, with maxima in the region near 2 nm. The highest pore concentration corresponds to values between 2 and 6 nm. In that region, the curves derived from the adsorption and desorption branches are similar. 3.2. Synthesis of Aerogel-Like Films at Room Temperature. As indicated in the Experimental Section, all the aerogel-like films shown in the figures above have been synthesized using deposition cycles at RT and plasma oxidation treatments at 120 °C. Oxidation at this mild temperature allows the oxidation of polymeric films up to a ∼200 nm thick range. Oxidation at RT, under the plasma conditions used in this work, is limited to thicknesses of less than ∼50 nm. Above this thickness, only the surface of the polymeric layer is oxidized, forming an oxide layer that protects the deeper polymeric layer from further oxidation (Figure S4). However, such a limitation can be easily overcome by working with polymer sacrificial layers of reduced thicknesses (i.e., below 50 nm), making it possible to obtain fully oxidized porous structures at RT. Note that this implies a higher number of cycles to get a given thickness in comparison with the previous examples (Figures 2−4). The synthesis at RT enhances the compatibility of the proposed method with temperature-sensitive substrates such as plastics and polymers. Figure 5a shows a TiO2aerogel-like film fully synthesized (deposition plus etching) at RT in 4 cycles. In a cross-sectional view (Figure 2a, bottom), it is possible to observe a nanocolumnar structure delimited by gaps from the surface to the substrate. However, the very thin initial layer, in contact with the substrate, is denser with the development of the columnar structure some nm over the substrate. The corresponding HRTEM image (Figure 5b) shows that the film solid structures present a very low density, where it is possible to identify continuous thin TiO2structures of no more Figure 4. Characterization of the thin film density and porosity. (a) Thin film overall density and porosity versus thickness. (b) Evolution of the film density and porosity as a function of thickness. (c) Normalized absorption−desorption water isotherms determined for films of 310 and 440 nm. (d) Pore size distribution calculated from the adsorption isotherms in (c), as indicated. ACS Applied Materials & Interfaces www.acsami.org Research Article https://doi.org/10.1021/acsami.4c00555 ACS Appl. Mater. Interfaces 2024, 16, 39745−39760 39750 than 4 nm thickness. These tenuous TiO2structures forming the solid part of the films are also interconnected. In summary, the structure of the RT-deposited films resembles that of the films prepared at a mild temperature (Figure 2), with some differences in the scale of the observed features. Besides, the films are fully amorphous, as shown in the digital electron diffraction pattern in Figure 5b. 3.3. Conformal Deposition. One critical advantage of the RPAVD technique is that plasma-assisted deposition gives rise to conformal coatings even on high-aspect-ratio supported nanostructures. 43 To demonstrate the extension of this critical advantage to the fabrication of aerogel-like materials, we have applied the combination of polymerization/oxidation cycles on supported single-crystalline nanowires (ONWs). These nanowires are formed by the self-assembly of conjugated small molecules by π-staking (see ref 45 for further experimental details). Figure 5c showcases characteristic SEM images of the different steps and cycles of the formation of the TiO2aerogel on a previously deposited array of ONWs. Due to the strong conformality of the RPAVD process, the plasma polymer (i.e., C1-step 1) forms a uniform shell around the ONW with high homogeneity in thickness from the tip to the base of each nanowire (see ref 43). After the etching step, the shell is converted, forming the aerogel-like structure that prevents the collapse of the 1D nanostructure but forms even smaller features in comparison with the thin film counterpart, as demonstrated in the two bottom panels in Figure 5c. This result exemplifies the potential of the methodology for being applied to the coating of supported delicate nanostructures of different natures. 3.4. Thin Film Crystallinity. The morphology and microstructure of the samples deposited by five cycles of polymerization/plasma oxidation can be observed in the STEM cross-sectional micrographs in Figure 6. The film presents an ultraporous structure (Figure 6a) that is more clearly observed in the HAADF-STEM image (Figure 6b). The very thin walls surrounding the pores can be neatly observed in the TEM images obtained at higher magnification (Figure 6c). This structure presents similarities to those observed in the RT sample (Figure 5b). In both cases, the samples are of very low density, showing a similar sponge-like structure of percolated oxide walls of ∼4 nm. Although both types of samples have the same internal structure despite the difference in thickness, there is a very noticeable difference between them that can be attributed to the difference in the plasma etching experimental conditions. While the films synthesized by plasma polymerization−oxidation cycles at RT are essentially amorphous (Figure 5b), the samples subjected to plasma oxidation at 120 °C show crystalline domains of anatase nanoparticles distributed along their otherwise amorphous-like structure (see the HRTEM images, the DDP of Figures 6d and S5). As will be discussed in the next section, one of the applications of the TiO2ultraporous films is selective contact in PSCs. For this type of application, a higher crystalline film is necessary, which is usually obtained by a thermal treatment at 450 °C during the cell manufacturing process. Figure 7 shows the TEM analysis of the film in Figure 6 after thermal annealing at 450 °C in air following the same protocol described for the preparation of TiO2electrodes in the Experimental Section. It is interesting to observe that the ultraporous structure remains stable after this treatment without significant modifications in the percolated structures, which still maintain the same thickness and shape as in the original film (Figure 7 a−c). However, the sample annealed at 450 °C presents a more crystalline structure, with a higher density of anatase domains in the range of 4−10 nm. These domains are larger than those observed in the 120 °C sample (compare the HRTEM images and DDP in Figures 6d and 7d). Thus, a thermally activated diffusion process is responsible for the growth and fusion of the anatase nanocrystals within the amorphous matrix. It is worth highlighting that the development of these anatase nanocrystals occurs without densification, cracking, or the collapse of the porous structure (Figure 7b), which is often a characteristic of thermally induced processes. 60 In our case, for the range of temperatures studied it was not necessary to add a percentage of SiO2or to perform rapid heat treatments to preserve the porous structure as previously proposed to avoid thermal collapse in mesoporous TiO2films for high-temperature applications. 32,61 3.5. Optical Properties. The optical properties of aerogellike TiO2films were studied on samples deposited in fused silica and stored in air with a relative humidity between 45 and 50% for at least two months. Measurements were carried out at RT without any prior heat treatment. Figure 8a shows the transmittance spectra of a set of samples from the first to the fifth cycle (C1−C5) deposited on fused silica as well as an uncoated fused silica substrate. All of the samples are transparent in the UV−vis−NIR region without any significant absorption corresponding to the sacrificial plasma polymer film that is intensely absorbing in the visible, as shown in Figure S1. The enlarged transmission region in Figure 6b shows that all Figure 5. Examples of conformal coating of supported singlecrystalline ONWs and deposition at RT. (a) SEM normal and crosssectional micrographs of a TiO2aerogel-like film fully synthesized (deposition plus etching) at RT in four deposition and oxidation cycles (C4). (b) HRTEM micrograph of the film presented in (a), with an inset showing the corresponding digital diffraction pattern. (c) SEM micrographs showing examples of conformal deposition of RPAVD TiPc plasma polymer and TiO2aerogel-like films on single crystalline supported nanowires. RT stands for room temperature in panels (a,b) Key experimental steps are indicated in panel (c). ACS Applied Materials & Interfaces www.acsami.org Research Article https://doi.org/10.1021/acsami.4c00555 ACS Appl. Mater. Interfaces 2024, 16, 39745−39760 39751 the films are antireflective, with transmittances higher than those of the fused silica substrate. The overall transmittance of the films increases with the film thickness reaching values higher than 96% in the NIR region. Similarly, the reflectance of the films (Figure 8b) decreases with the layer thicknesses, reaching values in the range of 3.5−3.2% in the NIR. As expected, the overall light transmission of the films increases as the density of the films decreases with thickness. Figure 8c shows the evolution of the refractive index as a function of the thickness determined by variable-angle spectroscopic ellipsometry. All the calculated values are lower than the fused silica refractive index, i.e., n= 1.47, in the visible-NIR regions (i.e., n= 1.47−1.43 in the spectral region 400−2500 nm), which explains the high light transmission values observed. The lowest refractive index value of 1.17 is achieved with the thickest sample. To complete the optical characterization of the films, Section S6 includes a study of the relationship between film porosity and refractive index using an effective medium approximation (EMA). The optical characterization demonstrates that the plasmaassisted deposition and etching methodology of aerogel-like films can find potential uses for the design of optical elements in antireflecting systems. The normal transmission and reflection values are lower than those reported for hierarchical mesoporous TiO2(SiO2) films 61 and close to those reported for more complex optical coatings as moth-eye structures obtained by relatively elaborated template methodologies. 30,62,63 The optical properties of the annealed films are similar to those of the aerogel films, presenting only a minor increase in the diffuse transmittance at shorter wavelengths, as shown in Section S7. 3.6. Wetting Properties. Figure 9a,b shows in blue the evolution of the contact angle of water and diiodomethane droplets on the surface of the TiO2aerogel-like films, respectively. In the case of water, the aerogel-like film surfaces are hydrophobic over the whole range of thicknesses, increasing from about 107°for the 50 nm samples to 128° for the 150 nm thick sample. At higher thicknesses, the values are stable, increasing slightly up to ∼131°for the thickest sample (i.e., 700 nm). Although some works have reported the hydrophobicity values slightly higher than those reported here for rutile aerogel powder (c.a, 145°), 64 most of the authors commonly reported hydrophilicity of highly porous TiO2and TiO2aerogel surfaces. 58,61,65 Superhydrophobic water contact angle values have been reported mainly for TiO2aerogels chemically derivatized with nonhydrolyzable surface organic groups and silane derivatives. 64,66,67 In the case of nonpolar liquids such as diiodomethane, the evolution is similar: the thinner samples show a contact angle of ∼80°, increasing up to ∼106°for a thickness of 150 nm to remain stable in the range of 106−107°up to a thickness of 700 nm. Thus, the deposited pristine TiO2aerogel-like surfaces are omniphobic for thicknesses higher than 150 nm. These surfaces can be permanently chemically derivatized by attaching fluorinated chains to the surface hydroxyl groups by gas-phase silane chemistry (see the Experimental Section). Figure 6. Determination of the pore structure and embedded anatase nanocrystals by high-resolution electron microscopy. STEM micrographs of an aerogel-like TiO2thin film prepared by five cycles of polymerization-plasma oxidation at 120 °C: (a) cross-sectional image, (b) HAADF-STEM cross-sectional micrograph, (c) high magnification TEM micrograph, and (d) HRTEM image of the square area marked in (c) and the corresponding electron diffraction pattern. ACS Applied Materials & Interfaces www.acsami.org Research Article https://doi.org/10.1021/acsami.4c00555 ACS Appl. Mater. Interfaces 2024, 16, 39745−39760 39752 In this case, the contact angles of both water and diiodomethane increase over the entire range of thicknesses studied (blue lines in Figure 9a,b). The resulting derivatized films develop a more noticeable omniphobic behavior throughout the thickness range studied. For water, superhydrophobicity values (i.e., θ> 150°) are reached at thicknesses above 150 nm, reaching a contact angle above 160°for the thickest layers. The values corresponding to iodomethane also increase in parallel for all thicknesses, reaching remarkably high values above 130° in the range of 170−700 nm. The omniphobic behavior obtained before and after derivatization can be explained by the high roughness and porosity characteristic of the studied surfaces that allow reaching the so-called Cassie−Baxter state even with low surface tension liquids such as diiodomethane. 66 This state would be reached thanks to the reentrant texture of the surface (see, for example, Figure 3) in which the liquid drop would not be able to wet the surface texture. Note that all the reported contact angle values correspond to porous layer surfaces deposited on flat substrates. Thus, the values shown could be optimized by introducing additional surface structuring levels, a common strategy for the design of liquid-repellent hierarchical surfaces. Although these synthetic options will not be explored here, it is interesting to note the compatibility of the technique with conformal deposition on nanostructured substrates (Figure 5c). The stability of fluorinated TiO2 aerogel surfaces has been tested for up to three months after the derivatization process. Although further studies are needed to determine stability over time under different environmental conditions, similar treatments have proven to be extremely robust. 51 The hydrophobic and superhydrophobic aerogel-like films can become superhydrophilic by exposure to UV light for several minutes, reaching a water contact angle of 0°, as shown in Figure 9c. A similar result is observed for the annealed samples (Figure S7c). These results demonstrate that the intrinsic photoactive nature of TiO2is preserved in the aerogellike films. The effective absorption of water observed can be attributed to the presence of an interconnected porous network that allows water absorption by capillarity. 23,68,69 Figure 7. Embedded anatase nanocrystals and porous structure after annealing. STEM micrographs of the aerogel-like film shown in Figure 6 after annealing at 450 °C in an Ar atmosphere. (a−c) Cross-sectional images at different magnifications. (b) HAADF-STEM micrograph. (d) HRTEM image and the corresponding digital diffraction pattern (inset). ACS Applied Materials & Interfaces www.acsami.org Research Article https://doi.org/10.1021/acsami.4c00555 ACS Appl. Mater. Interfaces 2024, 16, 39745−39760 39753 (67) Pan, F.; Zhang, B.; Cai, W. The Effect of Hydrophilicity/ Hydrophobicity of TiO2-SiO2 Composite Aerogels in the Epoxidation Reaction. Catal. Commun. 2017,98, 121−125. (68) Kota, A. K.; Kwon, G.; Tuteja, A. 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