Anodic TiO2 nanotube walls reconstructed: Inner wall replaced by ALD TiO2 coating

Applied Surface Science 549 (2021) 149306 Available online 16 February 2021 0169-4332/© 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Full Length Article Anodic TiO 2 nanotube walls reconstructed: Inner wall replaced by ALD TiO 2 coating Martin Motola a , 1 , Raul Zazpe a , b , Ludek Hromadko a , b , Jan Prikryl a , Veronika Cicmancova a , Jhonatan Rodriguez-Pereira a , Hanna Sopha a , b , Jan M. Macak a , b , * a Center of Materials and Nanotechnologies, Faculty of Chemical Technology, University of Pardubice, Nam. Cs. Legii 565, 530 02 Pardubice, Czech Republic b Central European Institute of Technology, Brno University of Technology, Purkynova 123, 612 00 Brno, Czech Republic ARTICLE INFO Keywords: TiO 2 nanotube layers Single-wall Atomic layer deposition Photoelectrochemistry Photocatalysis ABSTRACT A reconstruction process of TiO 2 nanotube (TNT) layers towards their superior photoelectrochemical performance and photocatalytic activity is presented. At first, TNT layers (~5 μ m thick, ~250 nm in diameter) were prepared via electrochemical anodization to obtain double-wall (DW) TNT layers. Second, a selective chemical treatment was conducted to etch the inner wall, yielding single-wall (SW) TNT layers. Third, TNT layers were coated by an additional approx. 5.5 nm, 11 nm, and 16 nm thick TiO 2 coatings, respectively, using Atomic Layer Deposition (ALD). A pronounced increase in the incident photon-to-electron conversion efficiency (reaching ~85% at λ =350 nm) was achieved on SW TNT layers coated with 11 nm thick ALD coatings compared to SW without coating (~35% at λ =350 nm). This is due to the optimal thickness of the ALD TiO 2 coating that passivates surface states and improves the separation of the photogenerated charge carriers. Photocatalytic performance of SW TNT layers with 11 nm thick ALD coatings (rate constant; k =0.1156 min −1 ) was increased by approx. 10-times compared to that of the nowadays most reported blank DW TNT layers (rate constant; k = 0.0119 min −1 ). 1. Introduction The pioneering effort of Fujishima and Honda on the photoelectrochemical water splitting using TiO 2 [1] opened new perspectives in materials research. Ever since, hydrogen evolution [2], photodegradation of pollutants [3], and dye-sensitized solar cells (DSSCs) [4,5] are few of the many possible photoelectrochemical applications of metal oxide semiconductors that have attracted a great scientific attention. One of the most crucial factors of a semiconductor for its optimal employment in photoelectrochemical applications is its efficient incident light absorption and utilization. From the variety of TiO 2 structures, one-dimensional anodic TiO 2 nanotube (TNT) layers [6–9], represent an exceptional material with unique geometry and physicochemical properties [10–14]. TNT layers can be grown [15,16] with controllable dimensions (e.g., inner tube diameter and layer thickness) [17] directly from Ti by optimized anodization in suitable electrolytes containing fluoride ions [15,16]. Throughout the past years, Ti foil [9,18,19], Ti wire [20], Ti mesh/grid [21–23], Ti spheres [24], Ti alloys (e.g., Ti-Au [25], Ti-6Al-7Nb [26], Ti6Al-4V [26], Ti-7.5Mo [27]), magnetron sputtered Ti on Si [22,28,29], ITO [30,31], or FTO [32–34], and electrodeposited Ti on Ni [35] have been used as a starting substrates for TNTs synthesis. Since their introduction in 1984 [6] and after pioneering works [16,18,36] regarding their advanced synthesis and promising application prospects (e.g., photoelectrochemistry, batteries, and as biomedical material) in 2005–2007, numerous electrolytes (aqueous and/or organic based) containing HF or fluoride salts were employed to obtain both lowand high-aspect ratio TNT layers [15,16,18,37,38]. Among all the electrolytes, ethylene glycol-based ones are the most popular electrolytes for preparation of such TNT layers. Nevertheless, there is an important fact about TNT layers prepared in some organic-based electrolytes (including ethylene glycol), which is often overlooked, yet it is a fundamental aspect of nanotube growth that significantly affects their morphology and intrinsic properties [39–42]. In these electrolytes, a double-wall (DW) structure is obtained during the anodization process and the resulting nanotubes possess an outer and an inner wall [40,43]. * Corresponding author. E-mail address: [email protected] (J.M. Macak). 1 Present address: Department of Inorganic Chemistry, Faculty of Natural Sciences, Comenius University in Bratislava, Ilkovicova 6, 842 15, Bratislava, Slovakia. Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc https://doi.org/10.1016/j.apsusc.2021.149306 Received 4 December 2020; Received in revised form 9 February 2021; Accepted 10 February 2021 Applied Surface Science 549 (2021) 149306 2 This phenomenon can be explained either by the plastic flow model [44–46] or by the bubble mold model [39,47,48]. As reported, by removing the inner wall to obtain a single-wall (SW) structure, the efficiency of DSSCs is increased [49,50], the photoelectrochemical [41,42,51,52] and photocatalytic [51,53] performances are enhanced, and the conductivity [41,42] of such SW TNT layers is improved compared to that of DW ones. Even though a large part of the originally photoactive TiO 2 material is removed in the SW compared to DW TNT layers, the effect of higher TiO 2 purity prevails and the charge carrier transport in such SW layers is more efficient [42,52]. However, it has never been investigated so far, what would an addition of a high purity TiO 2 mass back to the remaining walls with high purity TiO 2 do with the photoelectrochemical and photocatalytic response of such reconstructed nanotube walls. From all the possible techniques available for such reconstruction, Atomic Layer Deposition (ALD) is considered to be the most suitable technique [54,55]. It is especially suited for coating of highly-ordered TNT layers, as it allows homogeneous coating throughout the whole nanotube volume [56]. By introducing a secondary material by ALD within TNT layers (e.g., MoS 2 [57,58], MoSe 2 [59], Pt [60,61], ZnO [62], Al 2 O 3 [63,64], or TiO 2 [65]), their photoelectrochemical and photocatalytic performance can be significantly enhanced due to a stronger ability to produce charge carriers, an improved incident light absorption, or by annihilation of the TiO 2 surface states. Besides the beneficial effect on the TNT layers’ photoelectrochemical and photocatalytic performance, ALD coatings (e.g., Al 2 O 3 [66,67], MoS 2 [57], or TiO 2 [68]) improve also their thermal, chemical, and mechanical stability [67], their biocompatibility [69], prevents the nanotubes from unwanted morphological and structural changes [68], and increases the efficiency of Li-ion based batteries [57,66]. Overall, the combination of TNT layers and a secondary material coated by ALD is phenomenal and more than promising towards the TNTs advanced application [56] and the ever increasing number of reports confirm this bold statement. Nevertheless, all reports [52,53,55–63,65], show ALD coatings on DW TNT layers (i.e., containing the inner wall). The use of such ALD coated DW TNT layers for photoelectrochemical or photocatalytic applications has led to improvement in all these cases. However, the improvement could be even stronger, if a suitable ALD coating would be placed directly and exclusively on a high purity TiO 2 outer wall instead of also the inner wall, as it was conducted in all previous works. Based on this knowledge gap, in this work, we present a reconstruction procedure of self-organized DW TNT layers (~5 μ m thick, ~250 nm in diameter). A selective chemical etching treatment was conducted to remove the inner wall of the DW TNT layers. Subsequently, the TNT layers were coated with an additional TiO 2 coating (nominal thicknesses ~5.5 nm, ~11 nm, and ~16 nm) by an optimized ALD process. The obtained TNT layers were explored for their photoelectrochemical performance and photocatalytic activity and compared with DW and SW TNT layers without any ALD coating. 2. Experimental 2.1. Synthesis and etching of TNT layers According to our previous work [70], ~5 µm thick DW TNT layers with approx. 250 nm inner nanotube diameter were prepared via electrochemical anodization of Ti foils. All anodizations were conducted using PGU-200V high-voltage potentiostat (Elektroniklabor, GmbH) at 100 V for 4 h at room temperature in ethylene glycol-based electrolyte containing 10% water and 0.15 M NH 4 F. The inner wall of the DW TNT layers was removed based on our previous reports on 1 cm 2 [42,52,53] and 2.25 cm 2 [51] area to obtain SW TNT layers on such areas. For 1 cm 2 area TNT layers, first, the asprepared DW TNT layers were pre-annealed at 135 ◦C for 1 h in air using a heating rate of 15 ◦C/min. Second, the pre-annealed layers were immersed in piranha solution (H 2 SO 4 :H 2 O 2 =3:1) for 10 min at 70 ◦C. For 2.25 cm 2 area TNT layers, the as-prepared DW TNT layers were preannealed at 150 ◦C for 1 h in air (heating rate 15 ◦C/min). After preannealing, the layers were immersed in piranha solution for 5 min at 55 ◦C and additional 8 min at 65 ◦C. After etching, all layers (1 cm 2 and 2.25 cm 2 ) were immersed in H 2 O and EtOH for 5 min, respectively, and dried with a nitrogen jet. All TNT layers were annealed (400 ◦C, 1 h, air atmosphere) to obtain photocatalytically active anatase TiO 2 [42]. 2.2. ALD process Preliminary ALD processes (TFS200, Beneq) were carried out with different Ti precursors to determine the most suitable Ti precursor that could be used for main ALD TiO 2 coatings of the TNT layers. The results are shown in Electronic Supplementary Information (ESI, Fig. S1). Based on these preliminary results, titanium (IV) isopropoxide (TTIP, min. 98.8% purity) and O 3 (ozone generator, BMT Messtechnik, Stahnsdorf, Germany; 8 g/h output) were selected as titanium and oxygen precursors, respectively, for coating of TNT layers (both DW and SW) employed in this work. Different number of ALD cycles (namely 266, 532 and 798) were deposited to obtain nominal thick 5.5 nm, 11 nm, and 16 nm coatings, respectively (growth rate of 0.20 Å/cycle). The nominal thicknesses of the three different ALD TiO 2 coatings were confirmed by analyses of SEM images. A heating temperature of 60 ◦C was applied to TTIP (in order to get enough vapor pressure) and the deposition temperature was 250 ◦C. One ALD cycle was compromised of the next steps: TTIP (750 ms) – N 2 purge (5 s) – O 3 pulse (8 s) – N 2 purge (15 s). 2.3. Characterization A field-emission scanning electron microscope (FE-SEM, JEOL JSM 7500F), X-ray diffractometry (XRD, PANalytical Empyrean Cu K α radiation, λ =1.5418 Å), X-ray photoelectron spectroscopy (XPS, ESCA2SR, Scienta-Omicron) using a monochromatic Al K α (1 484.7 eV) X-ray source, and diffuse reflectance UV–VIS spectra (DRS, Shimadzu UV36000Plus Series UV–VIS spectrophotometer) were used for characterization of TNT layers. 2.4. Photoelectrochemical and photocatalytic performance measurements Photoelectrochemical performance measurements were carried out in a three-electrode cell using Ag/AgCl as a reference electrode, TNT layers as a working electrode and Pt wire as a counter electrode at 0.4 V vs Ag/AgCl in an aqueous 0.1 M Na 2 SO 4 solution in the wavelength range from 325 nm to 450 nm according to our previous work [53]. Electrochemical impedance spectra (EIS) were recorded at bias potential +0.4 V vs Ag/AgCl in the frequency range from 0.1 Hz to 100 kHz (voltage perturbation amplitude of 10 mV) in the dark and under UV light irradiation (λ =350 nm), respectively. Mott-Schottky (MS) plots were recorded at frequency of 1 kHz in the range −0.4 V vs Ag/AgCl to +1.0 V vs Ag/AgCl in the dark. Photocatalytic activity was evaluated according to our previous report [58]. Photocatalytic activity measurements were repeated three-times for each sample and the differences in the photocatalytic performance did not exceed ±5% (based on the rate constant). This confirms a good stability of all TNT layers as a UV-light photocatalyst. 3. Results and discussion 3.1. Materials characterization SEM images of the as-prepared TNT layers are shown in Fig. 1A and B (top and cross-sectional view, respectively). Fig. 1C reveals the inner wall in DW TNT layers in the bottom parts of nanotubes with a typical porous structure. Etching in piranha solution removed the inner wall M. Motola et al. Applied Surface Science 549 (2021) 149306 3 and the depicted SEM image (Fig. 1D) shows no presence of the porous structure in SW TNT layers. A representative SEM image (Fig. 2) shows blank and ALD coated SW TNT layers. After ALD, the nanotube wall thicknesses gradually increased with higher number of ALD TiO 2 cycles. Indeed, the nominal thicknesses of the additional ALD TiO 2 coatings were ~5.5 nm, ~11 nm, and ~16 nm after 266, 532, and 758 ALD TiO 2 cycles, respectively. As a result, the inner tube diameter decreased in ALD TiO 2 coated TNT layers (Fig. 2B-D) compared to that of blank TNT layers (Fig. 2A). The inner tube diameters of ~250 nm, ~239 nm, ~228 nm, and ~218 nm were obtained for blank, +5.5 nm TiO 2 , +11 nm TiO 2 , and +16 nm TiO 2 coated TNT layers, respectively. As previously reported [65], uniformity and homogeneity of the additional ALD coatings was achieved throughout the whole TNT layers. To further confirm the uniformity of the ALD TiO 2 coatings in this work, a high-resolution SEM image of the bottom part of ALD TiO 2 coated SW TNT layer +11 nm TiO 2 is shown in Fig. S2. The crystalline structure and the surface chemical composition of blank and ALD TiO 2 coated DW and SW TNT layers was characterized by XRD and XPS and the results are shown in ESI (Fig. S3 and Table S1), respectively. 3.2. Photoelectrochemical performance Fig. 3 shows photocurrent densities (maximum at ~350 nm) and photon-to-electron conversion efficiencies (IPCE) obtained for blank and ALD TiO 2 coated DW and SW TNT layers (λ =325–450 nm). The recorded photocurrent transients are shown in ESI (Fig. S4). A pronounced increase in photocurrent density (Fig. 3A) and IPCE (Fig. 3B) was recorded for the blank SW layers compared to that of the blank DW ones. More specifically, the IPCE values increased by ~5% for SW layers in the wavelength range from 325 to 375 nm. For example, at ~350 nm, the IPCE reached ~30% and ~35% for DW and SW TNT layers, respectively. This pronounced increase is due to the inner wall removal, which resulted in an improved charge carrier transport along the nanotube walls in SW TNT layers [42,52]. Even though such increase among DW and SW samples is encouraging, much more pronounced increases were obtained for samples with the additional ALD TiO 2 coatings. More specifically, at wavelength of ~350 nm, 2–3 times higher photocurrent densities (Fig. 3A) and up to 85% IPCE values (Fig. 3B) were obtained for ALD coated samples. The maxima were achieved for TNT layers +11 nm TiO 2 . Indeed, the surface states are passivated and the utilization of the charge carriers is more efficient due to the additional ALD TiO 2 coating, as we discussed in our previous work on ALD TiO 2 coating of DW TNT layers [65]. The thickness of the additional ALD TiO 2 coating is a decisive parameter regarding the efficient charge carrier transport and decreased recombination rate of charge carriers along the ALD TiO 2 coating [71,72]. This can be ascribed to the presence of surface and bulk trap states in the ALD TiO 2 coating. In a recent report [72], a detailed investigation of the optimal ratio of surface/bulk traps in ALD TiO 2 coatings with different thicknesses towards photoelectrochemical water splitting was evaluated. More specifically, different thicknesses of ALD TiO 2 coatings were coated on planar ITO glass and series of experimental techniques were conducted to determine the optimal thickness of the coating for photoelectrochemical water splitting. Based on the reported results [72], the differences in the photoelectrochemical performances in our blank and ALD TiO 2 coated TNT layers can be ascribed to the following. In thinner ALD TiO 2 coatings (~5.5 nm after 266 ALD TiO 2 cycles), surface traps dominate the overall charge transfer rate, thus the recombination rate of the charge carriers is substantial compared to that in thicker layers. With increased thickness of the ALD TiO 2 coating (~11 nm after 532 ALD TiO 2 cycles), the recombination of charge carriers is decreased due to the increased amount of the bulk trap states and as a result of an interaction between the surface and bulk trap states. Moreover, the bulk traps improve the charge carrier transfer along the ALD TiO 2 coating. Nevertheless, thicker ALD TiO 2 coatings (~16 nm after 758 ALD TiO 2 cycles) are detrimental for the efficient incident light utilization. Although the charge carrier recombination is similar to that in ~11 nm thick ALD TiO 2 coating, the charge carrier transfer along the TiO 2 coating is not efficient due to exceeding the critical thickness of the coating. In general, the surface traps are responsible for the efficient photogeneration of the charge carriers and the bulk traps are responsible for the efficient charge carrier transport along TiO 2 [73,74]. To provide an additional photoelectrochemical insight, cyclic voltammograms (CV) were recorded and are discussed in more detail in ESI (Fig. S5). The electronic structure of blank and ALD TiO 2 coated SW and DW TNT layers was investigated by electrochemical impedance spectra (EIS) and is represented as Nyquist plots along with the equivalent circuit in the dark (Fig. S6A) and under UV light irradiation (λ =350 nm; Fig. S6B). Overall, the presence of diffusive effects was associated to the nanotubular layer whereas the underlying Ti substrate was associated with behavior close to the ideal capacitor in all SW and DW (blank and ALD TiO 2 coated) TNT layers. Regarding the electronic structure of TNT layers, lower resistance value was obtained for blank SW compared to that of DW ones as the result of the inner wall removal. After ALD TiO 2 coatings, the resistance of both SW and DW TNT layers increased towards higher values. Indeed, by adding an additional TiO 2 as an n-type Fig. 1. SEM images of blank TiO 2 nanotube layers annealed at 400 ◦C (A) top view, (B) cross-sectional view, (C) double-wall structure, and (D) single-wall. Fig. 2. Top view SEM images of blank and ALD TiO 2 coated single-wall TiO 2 nanotube layers (A) blank, (B) +5.5 nm TiO 2 , (C) +11 nm TiO 2 , and (D) +16 nm TiO 2 . The “Xnm” specifies the thickness of the additional TiO 2 ALD coating. M. Motola et al. Applied Surface Science 549 (2021) 149306 4 semiconductor on TNT layers, such increase is expected, and the resistance values increased in parallel with thicker coating due to additional TiO 2 material. Although the higher resistances result in lower electronic conductivity in general in thicker ALD TiO 2 coatings [72], the differences between here reported blank and ALD TiO 2 coated TNT layers are measurable, but not very significant (approx. 30 Ωcm 2 between blank and +16 nm TiO 2 ). Nevertheless, further coating with thicker ALD TiO 2 coatings (>16 nm) would be probably accompanied by a substantial increase in resistance (in orders of magnitude), as recently reported for planar coatings [72]. To obtain additional insights regarding the material from the semiconductor point of view, Mott-Schottky (MS) analyses (Fig. 4) were Fig. 3. (A) Photocurrent density vs wavelength and (B) corresponding IPCE vs wavelength obtained for blank and ALD TiO 2 coated doubleand single-wall TiO 2 nanotube layers. Guiding lines at λ =350 nm are intentionally drawn to show the IPCE values at maximum photocurrents (shown in (A)). The “Xnm” specifies the thickness of the additional TiO 2 ALD coating. All data were recorded in an aqueous 0.1 M Na 2 SO 4 at +0.4 V vs Ag/AgCl . Fig. 4. MS plots with obtained V fb and calculated N D for blank and ALD TiO 2 coated doubleand single-wall TiO 2 nanotube layers. The “Xnm” specifies the thickness of the additional TiO 2 ALD coating. All data were recorded in an aqueous 0.1 M Na 2 SO 4 at 1 kHz and +0.4 V vs Ag/AgCl . M. Motola et al. Applied Surface Science 549 (2021) 149306 5 recorded and the donor density N D and the flat band potential V fb of blank and ALD TiO 2 coated DW and SW TNT layers were evaluated by following the equation: 1 C2=2 ε r ε 0qND [(Vappl −Vfb) − kT q](1) where C is the capacitance, ε r the relative dielectric constant of TiO 2 ( ε r =41.4 at 1 kHz), ε 0 the vacuum dielectric constant, q the electron charge, V appl the applied potential (+0.4 V vs Ag/AgCl ), k the Boltzmann constant, and T the absolute temperature. The intercept point with the xaxis of the plot in Fig. 4 represents V fb whereas N D was calculated using Eq. (1). The frequency of 1 kHz was applied during the measurements, which is generally considered sufficient for the MS analysis of porous structures, such as TNT layers [75–77]. No significant differences in the donor density were observed for blank and ALD TiO 2 coated DW and SW TNT layers and the calculated N D values ranged from approx. 5.8 ×10 19 cm −3 to 2.28 ×10 20 cm −3 for all TNT layers. Exact numbers are shown in Fig. 4. Notably, there was an increasing trend in N D values with added TiO 2 , reaching their maxima at 1.25 ×10 20 for DW with 5.5 nm and at 2.28 ×10 20 cm −3 SW with 11 nm ALD coated TNT layers, respectively. However, the N D value of DW TNT layers at 11 nm (1.24 ×10 20 ) was also very close to maximum (1.25 × 10 20 ). These trends are in a very good match with photocurrent and IPCE values shown in Fig. 3. Overall, the obtained N D values are also in accord with previous reports [75–78] on anodic oxides and TNT layers, where the donor densities were typically within the range of 10 19 – 10 20 cm −3 . The calculated N D values suggest that all TNT layers contain a similar amount of oxygen vacancies and defects in general and the additional ALD TiO 2 coating is of comparable quality (in terms of purity) as the TiO 2 prepared by anodization of Ti. This is in contrast with our previous report [65], where the ALD TiO 2 coated TNT layers possessed N D value of ~10 18 cm −3 , which was lower than for blank TNT layer. However, in that work, different precursors were used for the ALD process (TiCl 4 as Ti and H 2 O as O precursor) that resulted in coatings with high purity TiO 2 (i.e., reduced number of oxygen vacancies compared to that of blank TNT layers). Moreover, in that work the photoelectrochemical and photocatalytic performances were not as good as in this work, because the TiCl 4 derived ALD coatings were almost too pure. In other words, some optimal amount of defects is needed in TiO 2 in general for its optimal photoelectrochemical and photocatalytic performance. The more efficient transfer of the photogenerated charge carriers is due to the additional ALD TiO 2 coatings described here (by using TTIP and O 3 as Ti and O precursors, respectively). Indeed, the optimal number of oxygen vacancies and recombination sites in general (e.g., surface states) in TiO 2 enhances the charge carrier transport and decreases the charge carrier recombination [10,79]. Differences were observed in flat band potentials V fb for different TNT layers used in this work (i.e., SW vs DW), as shown in Fig. 4. A pronounced shift of V fb was observed for blank SW TNT layers (+0.155 V) compared to that of DW ones (-0.025 V). However, the shape of the MS curve was similar in both cases. This suggest that the V fb potential change is related to the Fermi level shift in TiO 2 [78] due to i) an increased conductivity of TiO 2 or ii) an alteration of the potential drop at the material/electrolyte interface due to change in the surface chemistry. Indeed, previous reports [41,42] show increased conductivity in SW TNT layers. After ALD, the trends in V fb were different among SW and DW coated TNT layers. For DW and SW layers the highest V fb were reached for 16 nm and 11 nm of ALD TiO 2 coatings, respectively. The shift to positive values for ALD coated samples, compared to uncoated ones, can be explained by a modification of the TNT surface by TiO 2 of different nature and composition. In particular, it is believed that thickening of the available TiO 2 and changing the mass ratio between anodic and ALD-based TiO 2 causes this shift towards positive values. However, this interplay is more complex and as the compositional differences are not traceable (e.g., by XPS), we cannot explicitly be sure about the most dominant effects that influence this trend. 3.3. Photocatalytic activity Photodegradation of MB solution was explored under UV light irradiation (λ =365 ±5 nm) on blank and ALD TiO 2 coated DW and SW TNT layers. It followed the first-order reaction typical for the TiO 2 photocatalyst and an organic dye [8]. The corresponding degradation rates are shown in Fig. 5A (repeated photocatalytic runs are shown in ESI, Fig. S7 and Table S3). The obtained kinetic rate constants along with the degradation rates are shown in Fig. 5C. The obtained photoactivities of blank and ALD TiO 2 coated SW and DW TNT layers are in line with the recorded photoelectrochemical performances (Fig. 3 and Fig. S4-S5). The highest decomposition rate was obtained using SW TNT layers +11 nm TiO 2 (k =0.1156 min −1 ), which is ~9.7 times higher compared to that of blank DW TNT layers (k =0.0119 min −1 ), i.e., the most commonly used TNT layers nowadays. In general, the SW TNT layers, both blank and ALD TiO 2 coated, possess enhanced photocatalytic activity compared to the DW TNT layers. Indeed, the inner wall in DW TNT layers (Fig. 1C) contributes negatively to the charge carrier recombination [43,53]. Regarding the three different thicknesses of the additional ALD TiO 2 coating, the photoactivity followed the trend of TNT layers +11 nm TiO 2 >+16 nm TiO 2 >+5.5 nm TiO 2 . This trend is due to the optimal thickness of the ALD coating along with the most efficient incident light utilization for TNT layers +11 nm TiO 2 , discussed earlier in this work. A substantial photoactivity improvement was obtained also for TNT layers +16 nm TiO 2 . This is in correlation with the obtained photocurrent density (Fig. 3A) and IPCE (Fig. 3B) data. The incident light utilization in TNT layers +16 nm TiO 2 is enhanced due to the additional ALD TiO 2 coating, i.e., diminished charge carrier recombination and improved the transfer along TiO 2 . Nevertheless, both photoelectrochemical (Fig. 3) and photocatalytic (Fig. 5) performances of TNT layers +16 nm TiO 2 were decreased compared to that of TNT layers +11 nm TiO 2 . Indeed, thickening of the ALD TiO 2 coating is responsible for such decreased performances. In thicker coatings, the charge carrier’s recombination occurred in the bulk of the ALD TiO 2 coating thus decreased the materials performances. Moreover, as the photocatalytic degradation of organic pollutants proceeds on the interface (i.e., TNT layers/dye), the available surface area influences the photocatalytic activity of a material. Indeed, in TNT layers +16 nm TiO 2 , the total available surface area is decreased due to the additional ALD TiO 2 coating (situated on both outer and inner tube wall) and the inner tube diameter is decreased (diameter of ~218 nm; Fig. 2D) compared to that of blank TNT layers (diameter of ~250 nm; Fig. 2A). At last, DRS were recorded to determine the reflectance and the indirect optical band gap energy of blank and ALD TiO 2 coated SW and DW TNT layers was calculated from Kubelka-Munk theory (Fig. 5B). The reflectance edge (at λ =~400 nm) is ascribed to anatase TiO 2 (E BG = 3.0–3.2 eV) [42,80]. The optical band gap (Fig. 5C) gradually decreased with the increased thickness of the additional ALD TiO 2 coating from 3.1 eV (blank) to 2.99 eV (+16 nm TiO 2 ). This is due to the penetration depth (derived from the DRS spectra) that is higher than 100 nm. Thus, it is far deeper than the combined thickness of the nanotube wall (10–60 nm) and the additional ALD TiO 2 coating (~5.5 nm, ~11 nm, and ~16 nm after 266, 532, and 758 ALD TiO 2 cycles, respectively). Therefore, the amount of photons absorbed by the material is increasing with the thicker ALD TiO 2 coatings. As a result, the optical band gap shifts towards lower values (from 3.1 eV to 2.99 eV). 4. Conclusions Two-step morphology reconstruction process of anodic DW TNT layers towards superior photoelectrochemical and photocatalytic performance was conducted. The inner wall was removed via a selective M. Motola et al. Applied Surface Science 549 (2021) 149306 6 chemical etching treatment. A more efficient charge carrier transport was achieved in such SW TNT layers. This led to 5–10% increase in photoelectrochemical and photocatalytic performances of SW TNT layers. To further enhance the incident light utilization of the SW TNT layers, coating by an additional ALD TiO 2 (thickness of ~5.5 nm, ~11 nm, and ~16 nm, respectively) was conducted. The additional TiO 2 coating improved utilization of the photogenerated charge carriers and passivated the surface states on TiO 2 . The ~11 nm thick additional TiO 2 coating was optimal for increased performances of TNT layers due to optimal ratio of surface and bulk traps. Indeed, the IPCE values increased by approx. 2.5-times and reached ~85% at the wavelength of 350 nm for SW TNT layers +11 nm TiO 2 compared to that of blank DW TNT layers (IPCE value of ~30% at λ =350 nm). Moreover, the photocatalytic activity increased by approx. 10-times for SW TNT layers + 11 nm TiO 2 with a rate constant of 0.1156 min −1 compared to that of blank DW TNT layers (k =0.0119 min −1 ). Overall, the here presented morphology reconstruction process significantly improved the TNT layers’ performance in photoelectrochemical applications. Authors contributions MM and JMM designed experiments; MM, HS – synthesis of nanotubes; LH – XRD and SEM characterizations; RZ and JP – atomic layer deposition; JP – UV/VIS diffuse reflectance; JRP – XPS; MM – (photo) electrochemical performance measurements; MM – photocatalytic activity measurements; MM, HS and JMM wrote the manuscript and carried out revisions; all others reviewed and edited the manuscript; JMM supervised the team and provided support. All authors have read and agreed the published version of the manuscript. Declaration of Competing Interest There are no conflicts to declare. 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