Effect of Pt Deposits on TiO2 Electrocatalytic Activity Highlighted by

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Surfaces, Interfaces, and Applications

Effect of Pt deposit on TiO2 electrocatalytic activity highlighted by an electron tomography Mariusz Andrzejczuk, Agata Roguska, Marcin Pisarek, Piotr Kedzierzawski, and Malgorzata Lewandowska ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03932 • Publication Date (Web): 23 Apr 2019 Downloaded from http://pubs.acs.org on April 23, 2019

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ACS Applied Materials & Interfaces

Effect of Pt Deposit on TiO2 Electrocatalytic Activity Highlighted by an Electron Tomography Mariusz Andrzejczuk*, Agata Roguska+, Marcin Pisarek+, Piotr Kędzierzawski+, Małgorzata Lewandowska* *Faculty

of Materials Science and Engineering, Warsaw University of Technology, Woloska 141, 02–507 Warsaw, Poland

+Institute

of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01–224 Warsaw, Poland

KEYWORDS Electron tomography, nanotubes, TiO2, Pt nanoparticles, thin films, STEM (scanning transmission electron microscopy)

ABSTRACT

Characterizing materials at small scales presents major challenges in the engineering of nanocomposite materials having a high specific surface area. Here, we show the application of electron tomography to describe the three-dimensional structure of highly-ordered TiO2 nanotube

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arrays modified by Pt nanoparticles. The titanium oxide nanotubes were prepared by the electrochemical anodization of a Ti substrate, after which Pt was deposited by magnetron sputtering. Such a composite shows high electrochemical activity that depends on the amount of the metal and the morphological parameters of the microstructure. However, a TiO2 structure modified by metallic nanoparticles has never been visualized in 3D, making it very difficult to understand the relationship between electrocatalytic activity and morphology. In this paper, TiO2 nanotubes of different sizes and different amounts of Pt were analyzed using the electron microscopy technique. Electrocatalytic activity was studied using the cyclic voltammetry (CV) method. For selected samples, electron tomography 3D structure reconstruction was performed to describe their fine microstructure. The highest activity was detected in the sample having bigger nanotubes (25 V) where the porosity of the structure was high and the Pt content was 0.1 mg cm2.

3D imaging using electron tomography opens up new possibilities in the design of

electrocatalytic materials.

INTRODUCTION

Titanium oxide (TiO2) is an attractive material in various fields of application due to its unique properties, such as chemical inertness, biocompatibility, inexpensiveness, high photocatalytic and electrochemical activity.1,2 The activity and applicability of this material depends substantially on its morphology. TiO2 has been developed in many different forms, including thin layers, nanoparticles, nanotubes, nanofibers and nanosheets.3-5 Nanotubular TiO2 is one of the most promising carrier materials due to its large specific surface area and well-ordered structure,


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providing a direct pathway for charge transport and thereby decreasing carrier path length. However, its surface activity and morphology vary depending on the conditions under which it is fabricated. It is well known that in electrochemical anodization processes the diameter of nanotubes increases linearly along with anodization voltage.6 Using highly viscous electrolytes consisting of glycerol or ethylene glycol, smooth-walled nanotubes are produced. The formation of ripples on the side-walls of nanotubes is notable for aqueous electrolyte applications.7 The semiconductive nature of TiO2 can be modified by the addition of small quantities of noble metals such as platinum. The metal particles enhance electron transfer and suppress electron-hole recombination.8 The highly organized structures of TiO2 nanotubes with Pt nanoparticles make such nanotubes useful for applications involving photocatalysis,9 electrocatalysis,10 and sensors.11 A titanium oxide support loaded with a Pt catalyst displays high electrocatalytic activity and good CO tolerance of the catalyst. Such materials can be used for direct methanol fuel cell (DMFC) applications.12 The small Pt nanoparticles deposited on 3D TiO2 structures create a large Pt surface, resulting in a large electrochemically active surface area. The influence of the amount and particle size of Pt on electrocatalytic activity towards methanol oxidation reaction was investigated by Ting et al.13 It was found that there is an optimum size of Pt particles that provides the best electrocatalytic performance in terms of CO tolerance and electrochemical stability. It should be noted that the electrocatalytic activity of catalysts is increases along with an increase in the Pt loading on the TiO2 support. In order to deposit metallic nanoparticles on nanotubes, various methods of deposition have been used, including electrodeposition,14 photochemical reduction15 and magnetron deposition.16 The filling of narrow channels of nanotubes with metal nanoparticles is challenging technologically, and so the choice of a suitable method is crucial for proper structural optimization. Then,


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evaluating the homogeneity and morphology of a metal deposit must be performed using advanced techniques of electron microscopy, such as electron tomography. Electron tomography is a very powerful technique for characterizing nanoscale materials in three dimensions (3D), with a resolution of a few nanometers.17,18 Even with atomic-level resolution, similar results can be obtained using the atom probe tomography technique.19 However, this technique is more suitable for the 3D characterization of solid materials, especially in terms of local chemical composition. For porous materials with nano-sized features, the more suitable method for analysis seems to be electron tomography.20 This technique is very often used also to visualize the real structure of nanosized core-shell materials.21 Such multiphase materials consist of phases of different mean atomic number, providing very good contrast on high-angle annular dark field STEM projection images and fully satisfying the requirements of electron tomography.22 Bright field STEM imaging, where diffraction contrast is present, can be also used for electron tomography reconstruction, but only with poorly crystallized, single phase materials.23 Apart from the 3D characterization of particles and homogenous porous materials, electron tomography can be also used for bulk nanomaterials having an ordered structure.24-26 Such materials need advanced techniques of sample preparation, such as FIB milling and special holders for image acquisition, to minimize the missing wedge effect.27 A special sample preparation technique for full-range tomography, were proposed also for samples in form of particles.28 In this study, we describe the use of electron tomography to characterize the 3D structure of titania nanotubes modified by Pt nanoparticles. The electron tomography technique provides makes it possible to obtain precise information on the structure of complex nanoscale materials. Combining the data on 3D structure with the electrocatalytic activity results, we attempted to select


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the optimal morphology. It was assumed that the distribution of Pt inside the TiO2 nanotubes would be crucial to catalytic efficiency.

EXPERIMENTAL DETAILS In this work, titania nanotubes were obtained by the electrochemical oxidation of pure titanium at voltages of 10 and 25 V, resulting in the formation TiO2 nanotubes ~ 40 nm and ~ 110 nm in diameter, respectively.6 A Ti foil of 0.25 mm thickness (99.5% purity, Alfa Aesar) was used as the substrate. The titanium oxide nanotube layers were fabricated by the electrochemical anodization of the Ti samples in an optimized mixture of DI water + glycerol (volume ratio 50:50) with 0.27 M NH4F, for 40 min. After anodization, heat treatment was performed at 450oC for 1h to change the amorphous structure of the TiO2 nanotubes into a crystalline anatase structure. The deposition of Pt nanoparticles was performed using the DC magnetron sputtering technique, using a Leica EM MED020 apparatus with amounts of from 0.05 up to 0.2 mg/cm2. The average amount of metal deposited per cm2 was strictly controlled by quartz microbalance in situ measurements. Certainly, both the true average amount and local amount of the metal deposits may vary substantially due to the highly-developed specific surface area of the nanotube arrays and the resulting non-uniform distribution of the metal deposits. The configuration of the setup was perpendicular to the surface of the sample. The deposition parameters of the Pt nanoparticles are listed in Table 1.


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Table 1. Deposition parameters of Pt nanoparticles using magnetron sputtering Pt, mg/cm2

PVD methods

magnetron sputtering

0.05 Pressure, Pa

2

Current, A

25 x 10-3

Time, s

~170

0.1

0.2

~370

~760

The chemical composition of the oxide layers after the platinum deposition process was examined using AES and XPS spectroscopy, with a Microlab 350 (Thermo Electron). The AES was used to determine the composition profiles of TiO2 NTs on Ti substrates with Pt deposits. For this purpose an Ar+ ion gun (EX05) was used, where, discontinuous sputtering, applied in 120 s and 360 s etching steps to gradually remove the oxide layers functionalized by Pt nanoparticles. The sputtering parameters were: ion energy 3 keV, beam current 1.3 μA, and crater size 4 mm2. The Auger spectra were excited at a primary energy of E = 10 keV and recorded after each sputtering period. The appropriate sensitivity factors from the Thermo VG Scientific database for the elemental components (Ti, O, Pt) were used to convert the Auger signals into atomic percentages (at.%). Separate tests showed that the sputtering rate was 0.14 nm/s, which was determined by ion etching of a silicon sample covered by a SiO2 layer of known thickness (15 nm).The chemical state of the surface species was identified using the high-energy resolution spherical sector analyzer of the XPS spectrometer (the maximum energy resolution was 0.83 eV for the XPS method). The XPS spectra were excited using AlKα (hν = 1486.6 eV) radiation as a source. High-resolution spectra were recorded using a pass energy of 40 eV. Avantage-based data system software (Version 4.16) was used for the data processing.


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Surface observations of the TiO2 nanotubes modified by Pt nanoparticles were performed using a Hitachi SU8000 scanning electron microscope. Transmission electron microscopy examinations were carried out with a scanning transmission electron microscope (Hitachi HD2700) operated at 200 kV. Two modes - BF (bright field) and HAADF (high angle annular dark field) - STEM were used for microstructure imaging. For the transmission microscopy investigations, cross-sectional TEM lamellae were fabricated using a Hitachi NB5000 focused ion beam system with a gallium ion beam. The electron tomographic studies were performed on selected samples: nanotubes prepared at 10 and 25 V with a Pt deposition of 0.2 mg/cm2, and at 25 V with a Pt deposition of 0.1 mg/cm2. 2D projection images were acquired using a Hitachi HD-2700 STEM microscope operated at 200 kV with an HAADF detector. Images were collected over a range of 0-180o at increments of 3o using a Hitachi holder with a 360o tilt range, at a magnification of 130,000 - 250,000 times, with a pixel size of 1-2 nm, a frame time of 10 s and 512x512 pixels image resolution. Next, the HAADF-STEM tilt series was aligned using a cross correlation with the help of IMOD software.29 The reconstruction, with 20 iterations, was performed using the simultaneous iterative reconstruction technique (SIRT) algorithm implemented in TomoJ software.30 The 3D visualization of the reconstructed volume, segmentation and quantification were performed with commercial Avizo software. The samples for electron tomography were prepared as needle-shaped specimens using a Hitachi NB5000 focused ion beam system. The diameter of the specimens was in a range of 200-400 nm. On the top of each sample, a thin layer of carbon was deposited to protect platinum against gallium ion beam sputtering. Electrocatalytic activity was studied using the cyclic voltammetry (CV) method. CV plots were recorded at room temperature (22 ± 1 oC) using an EP-20 potentiostat with an EG-20 function


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generator from Elpan, in a three-compartment electrochemical cell with a Pt gauge as counter electrode and Ag/AgCl/1 M KCl as reference electrode.

RESULTS AND DISCUSSION Figure 1 f - j show an SEM top view image of titania nanotubes after Pt deposition. As can be seen, an increase in Pt loading leads to a subsequent decrease in the nanotube window. In the case of low anodization voltage (10 V) when small nanotubes are fabricated, 0.2 mg/cm2 of Pt is enough to completely close the nanotubes. The surface looks rough but compact. Partially closed nanotubes are observed for the sample of small nanotubes with a Pt deposit of 0.1 mg/cm2. For the lowest content of deposited Pt, 0.05 mg/cm2, the rounded shape of the nanotubes is clearly visible, with particles of platinum adorning their edges. In the case of large nanotubes, even at the highest amount of Pt deposit, 0.2 mg/cm2, the pores remain open. Platinum is located on the edges, and probably on the walls, of the nanotubes. These results suggest that, using the same amount of deposited metal, large nanotubes will be modified more deeply than small nanotubes. Large nanotubes should be more prone to surface modification because they provide easier access to the wall surface. Therefore, after surface modification, the electrochemical activity of nanotubes of different sizes should be different. To determine the relationship between the depth of nanotube surface modification and diameter, TEM observations of cross-sectional samples were performed. As can be seen in the BF-STEM images (Figure 1 a - e), the TiO2 nanotubes are covered by a platinum layer visible as a black layer; it has a thickness of 50 - 100 nm, depending on the amount of metal deposited. The average diameter of the nanotubes is about 40 nm at 10 V and 110 nm at 25 V. There are no significant differences in the thickness of the Pt layer for the different sizes of nanotubes. However,


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determining the Pt deposit thickness accurately is very challenging due to the highly irregular form of the nanotubes. Analyzing the maximum depth of nanotube modification, a depth of about 250 nm was noted for the larger nanotubes, and about 140 nm for the smaller ones.


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Figure 1. Microstructures of TiO2 nanotubes obtained at 10 and 25V with different amounts of Pt deposit (a-e) cross-sectional BF-STEM (f-j) surface SEM images. The platinum deposit is in the form of a bulk layer in the upper part of the nanotubes, and changes to separated clusters and fine particles as the distance from the surface inside the nanotubes increases. The size of the Pt clusters varies, from tens of nanometers in the region close to the bulk Pt layer to only a few nanometers farther away. The high-resolution observations confirmed the nano-sized and fully crystalline structure of the Pt clusters (Figure 2). As can be seen in the high resolution and HAADF-STEM images, the Pt nanoparticles are a few nanometers in diameter, and they join into larger clusters covering the titania nanotube surface.

Figure 2. (a) HR-STEM and (b) HAADF-STEM images showing Pt nanoparticles deposited on TiO2 nanotube. The chemical composition of the Pt loaded TiO2 NTs was examined by XPS technique. The XPS survey spectra (data not shown) shows characteristic peaks originating from the metals (Pt, Ti),


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oxygen and carbon (surface contamination). The high resolution XPS spectra revealed that for all investigated samples with various amount of platinum, Pt exists in metallic state: Pt4f7/2 ~ 71.0 eV (Pt0) and Ti is in oxidized sate: Ti2p3/2 ~ 458.8 eV (Ti4+), see Figure 3 a,b. The XPS results suggests that there are no interactions (SMSI effect) between the nanoporous oxides and the Pt metal deposit, 31 no characteristic shifts of the maximum peaks position of Pt and Ti were detected. However, it is worth to mention that, the thickness of the Pt deposit may be too high to reveal this effect using XPS technique.

Figure 3. High resolution XPS spectra for (a) Pt4f and (b) Ti2p regions: 10V NTs + 0.05 mg∙cm-2 Pt, 10V NTs + 0.1 mg∙cm-2 Pt, 25V NTs + 0.1 mg∙cm-2 Pt.


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The AES method was applied in order to determine precisely the depth of metal penetration into the tubes and the chemical state of the elements. The AES depth profile in Figure 4 a,b,c shows that the platinum is mainly located on the tops of the nanotubes - Pt top layer. The thickness of this layer is increasing with the total amount of Pt deposit and reaches 50 nm for 0.05 mg/cm2 and 85 nm for 0.1 mg/cm2. It was found also that the depth of penetration by Pt nanoparticles follows the same trend: 185 nm for 0.05 mg/cm2 of Pt and 285 nm for 0.1 mg/cm2 of Pt, which agrees with our previous microscopic observations (see Figure 1). Some differences in the estimates of the depth of Pt distribution result from the research methods applied and from possible local variations in chemical composition.


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Figure 4. AES chemical depth profile (a) 10V nanotubes with 0.05 mg/cm2 Pt (b) 10V nanotubes with 0.1 mg/cm2 Pt, (c) 25V with 0.1 mg/cm2 Pt. Catalysts based on TiO2 nanotubes (25V) with 0.05 mg/cm2, 0.1 mg/cm2 and 0.2 mg/cm2 Pt deposits were employed in the electro-oxidation of formic acid. Steady-state cyclic voltammograms were recorded in 0.5 M H2SO4 electrolyte, and the results of the electrochemical measurements are presented in Figure 5 a-c. The CV curves display all the characteristic regions


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for a catalytically active Pt metal: hydrogen adsorption (cathodic scan) - desorption (anodic scan) between –0.3 V and 0 V; the formation of a metal oxide monolayer between 0.5 V and 1.2 V (PtO) (anodic scan); and a metal oxide reduction peak (cathodic scan) at ~0.4 V for 0.1 and 0.2 mg/cm2 Pt. The electrochemical formic acid oxidation behavior of the same catalysts was investigated by cyclic voltammetry in 0.5 M HCOOH + 0.5 M H2SO4 electrolyte (Figure 5 d-f). During the positive scan, the reaction on Pt started at 0.05 V (0.05 mg/cm2 Pt) and 0.45 V (0.1 and 0.2 mg/cm2 Pt), respectively. Such a difference in the electrochemical behavior on Pt (a shift of more than 0.4 V in the reaction start) is probably a consequence of the various amounts of Pt and the distribution of Pt nanoparticles in the TiO2 layers. In a potential range of from -0.05 V to 0.45 V, the Pt surface (0.1 and 0.2 mg/cm2 Pt) was entirely covered with COad, rendering it completely inactive towards formic acid electrooxidation. The catalyst regained activity only at potentials above 0.45 V, where Pt-OH surface groups are formed (see the current increase in voltammograms 5 e, f) which removed COad from the surface, oxidizing it to CO2. It is well known that formic acid oxidation at Pt occurs mostly through the indirect pathway, during which a rapid dehydration step is followed by the oxidative removal of the adsorbed CO: HCOOH  COad + H2O  CO2 + 2H+ + 2e- 32,33 The maximum of the specific peak current ~ 250 mA mg-1 Pt was measured for the titania nanotubes fabricated at 25 V and modified with 0.1 mg/cm2 of Pt. A lower current of about 180 mA mg-1 Pt was measured for the nanotubes with a deposit of 0.2 mg/cm2 of Pt. For the surface adornment with the lowest content of Pt 0.05 mg/cm2, the effect of electro-oxidation of FA was very weak, due to an insufficient amount of Pt on the surface of the TiO2 NT. For all the samples measured, after reaching the maxima, the currents of formic acid electrooxidation decreased gradually. The current decrease during the back scan took place not only because of the decrease


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in potential, but also because of anion re-adsorption, the build-up of a COad layer on the Pt nanoparticles, and surface reconstruction. Similar electrochemical behavior was observed for catalysts based on 10 V nanotubes (data not shown). For these types of samples also, a maximum specific peak current of electrooxidation of formic acid was obtained for the sample with a deposit of 0.1 mg/cm2 of Pt (180 mA mg-1 Pt).

Figure 5. Cylic voltametric curves TiO2 nanotube (25 V) electrodes with (a,d) 0.05 mg/cm2 (b,e) 0.1 mg/cm2 (c,f) 0.2 mg/cm2 Pt in 0.5 M H2SO4 and 0.5 M H2SO4 + 0.5 M HCOOH. In order to determine the relationship between the microstructure and the electrochemical activity of the nanotubes, an electron tomography analysis was performed on selected samples. Two different sizes of nanotubes, fabricated at 10 and 25 V and with a deposit of 0.2 mg/cm2 were analyzed to gather information on the depth of platinum penetration inside the nanotubes. As we know from the SEM results, this amount of platinum is enough to close the smaller nanotubes from the top and to significantly densify the larger ones. This means that this amount of metal ensures


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maximum modification of the interior of the nanotubes. Electron tomography was used to evaluate the scale of surface modification for different sizes of nanotubes. According to the STEM observations, a greater modification was anticipated for the larger nanotubes because the deposited metal particles had easier access to the TiO2 surface (Figure 1). As can be seen in Figure 6, the TiO2 nanotubes are covered by a Pt deposit, visible as a dark area in the BF-STEM image and as a bright area in the HAADF-STEM image (Figure 6 a,b). A reconstructed image is presented in Figure 6c. Very careful manual segmentation revealed the localization of Pt and TiO2 phases in three-dimensional space (Figure 6d).

Figure 6. 3D imaging of 25V TiO2 nanotubes with 0.2 mg/cm2 Pt, (a) BF-STEM, and (b) HAADFSTEM images of the TiO2/Pt sample analyzed, (c) reconstructed image, (d) phase segmented image An analysis of the metal depth penetration into the TiO2 porous structure was performed on a cropped 3D volume comprising only a few nanotubes (Figure 7). In the case of the large-diameter nanotubes, platinum particles were observed at a depth of up to 250 nm from the top of the nanotubes, whereas in the case of the small nanotubes the maximum depth was 140 nm for the same amount of Pt, 0.2 mg/cm2 (Figure 7b, a). These results confirm previous observations of


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different depths of penetration performed on STEM cross-section images, and are different from the results of AES chemical profiles performed for 0.1 mg/cm2 Pt, where the depth of modification is similar for both sizes of nanotubes. The lower content of Pt (0.1 mg/cm2) should not be the crucial to the differences observed. The differences in the measured depth of modification could be result of the limited resolution of electron tomography and the small size of the Pt nanoparticles. The size of the nanotubes can affect the size of the Pt clusters created due to different access to the surface of the nanotubes. The small-diameter nanotubes were quickly closed by the platinum during the magnetron deposition process. The process of surface adornment was shorter in the case of the smaller nanotubes due to the limited access to the interior of the nanotubes. The influence of the deposited metal content on the depth of penetration of the large nanotubes was analyzed on 25V TiO2 nanotubes with 0.1 and 0.2 mg/cm2 Pt (Figure 7 b, c). Based on the 3D results, we found out that the depth of platinum penetration did not depend on Pt content. The only difference visible for both structures was that there was much less platinum deposited on the tops of the nanotubes in the case of the lower Pt content. The better electrochemical activity of the sample with 0.1 mg/cm2 Pt was probably related to the easier access of the electrolyte to the TiO2/Pt surface. Another important issue is the size of the Pt nanoparticles. It is known that smaller Pt nanoparticles have a larger specific surface area and therefore display better catalytic properties.13 However, we have to assume that the size of the nanoparticles was similar for all amounts of the deposit, and only the number of large agglomerates could increase with an increased amount of Pt deposited. The presence of such Pt agglomerates may affect electrochemical activity.


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Figure 7. Surface rendering of segmented reconstruction of Pt/TiO2 nanotubes, (a) TiO2 10V with 0.2 mg/cm2 Pt, (b) TiO2 25V with 0.2 mg/cm2 Pt, (c) TiO2 25V with 0.1 mg/cm2 Pt A quantitative analysis of the segmented reconstruction images made it possible to obtain morphological parameters such as volume, porosity and surface area that accurately describe the fabricated nanostructures (Table 2). All parameters were calculated for a volume limited to 140 nm from the top of the nanotubes. This was the part of the TiO2 material most exposed to platinum deposition. The differences measured in the volumes and areas of the Pt and TiO2 phase samples derived mostly from the size of the segmented volume. Therefore, the ratio parameters were calculated to better describe the morphology of the structure. As can be seen, the ratio of the volume of Pt to TiO2 is similar for the larger nanotubes - about 40 % - and lower for smaller nanotubes - 30 %. The relatively lower Pt content inside the smaller nanotubes might be related to the diameter of the nanotubes and limited access for the deposited metal. In this case, Pt is located more on the top of the nanotube layer. Such a conclusion can be also drawn from the ratio of the


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area of Pt to TiO2 parameter, which is very similar for both the small and large nanotubes. As can be seen, the ratio of the area of Pt to TiO2 is much higher in the case of the larger nanotubes, which might suggest that more of the surface of the large nanotubes is covered by the Pt deposit. The porosity is much higher for the larger nanotubes, which is in agreement with previous calculations.25 The greater the amount of Pt deposited, the more the porosity of the nanotubule layer is reduced. It must be mentioned that porosity may be much higher close to the top of the TiO2 nanotube layer than in its lower part. This is related to the nature of nanotube growth and to the changing dimensions of the nanotubes with growth direction.25 Therefore, the values of porosity measured should only be used for comparison in this specific investigation. The correlation between the morphology parameters and the voltametric curves analysis made it possible to better understand the electrocatalytical activity in relation to material structure. The large nanotubes with high porosity provided good access of the electrolyte to the TiO2/Pt surface and thus enhanced electrocatalytic activity. The small nanotubes, despite their large surface area, have a lower maximum current of volumetric curves. This is due to the small diameter of those nanotubes and the difficulty with which the electrolyte enters them. There is an optimum content of Pt that ensures the greatest activity. Too high a Pt content leads to the formation of a thick layer on the tops of the nanotubes and closes the open nanotubular structure. It must be mentioned that electrocatalytic activity will be changing with electrochemical cycles due to Pt morphology changes. Such effect was observed and analyzed using electron tomography technique.34 The nanostructure compaction and active surface area reduction will be especially visible for thin deposit of Pt with initially high surface area.


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Table 2. Morphological parameters, Volume, Area, Porosity determined by 3D image analysis, together with calculated porosity and surface area.

TiO2 10V_0.2 TiO2 25V_0.2 TiO2 25V_0.1

VPt,

VTiO2,

AreaPt/ AreaTiO2, %

Porosity,

nm3

VPt/ VTiO2, Area Pt, Area TiO2, % nm2 nm2

nm3

6.6*10^5

2.2*10^6

29.9

5.4*10^5

1.7*10^6

31.3

33.8

1.1*10^6

2.9*10^6

37.4

2.2*10^5

3.7*10^5

59.8

56.4

4.7*10^5

1.1*10^6

43.9

7.7*10^4

1.3*10^5

59.7

73.6

%

CONCLUSIONS This paper dealt with a morphological analysis using the results of electron tomography to examine the electrocatalytic activity of a TiO2/Pt structure. Different diameters of TiO2 nanotubes as the support and different amounts of Pt were investigated. The highest electrocatalytic activity was noted for large nanotubes fabricated at 25V with a 0.1 mg/cm2 of Pt deposit. This content of metal seems to be optimal for Pt active surface fabrication and for maintaining access by the electrolyte to the interior of the nanotubular structure. A larger amount of Pt deposit created a thick layer on the top of the nanotubes, closing them, which could reduce electrocatalytic activity. Also, smaller nanotubes displayed worse properties because they tended to become closed faster by the deposited metal. The homogenous distribution of Pt deposit on the TiO2 nanotube surface was confirmed by electron tomography investigations of selected samples. Generally speaking,


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electron tomography is a very powerful technique that makes it possible to characterize 3D nanostructured materials that are difficult to characterize using other techniques.

ASSOCIATED CONTENT (Word Style “TE_Supporting_Information”). Supporting Information. A listing of the contents of each file supplied as Supporting Information should be included. For instructions on what should be included in the Supporting Information as well as how to prepare this material for publications, refer to the journal’s Instructions for Authors. The following files are available free of charge. movie of TiO2 nanotubes obtained at 25V with 0.1 mg of Pt (MPG) movie of TiO2 nanotubes obtained at 25V with 0.2 mg of Pt (MPG) AUTHOR INFORMATION Corresponding Author [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources National Science Centre through research grant UMO-2014/13/D/ST8/03224 Notes


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The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Science Centre through research grant UMO2014/13/D/ST8/03224.

REFERENCES (1) Hashimoto, K.; Irie, H.; Fujishima, A. TiO2 Photocatalysis: A Historical Overview and Future Prospects. Japanese Journal of Applied Physics 2005, 44 (12), 8269–8285. https://doi.org/10.1143/JJAP.44.8269. (2) Roy, P.; Berger, S.; Schmuki, P. TiO2 Nanotubes : Synthesis and Applications Angewandte. 2011, 2904–2939. https://doi.org/10.1002/anie.201001374. (3) Luka, G.; Witkowski, B. S.; Wachnicki, L.; Andrzejczuk, M.; Lewandowska, M.; Godlewski, M. Kinetics of Anatase Phase Formation in TiO2 Films during Atomic Layer Deposition

and

Post-Deposition

Annealing.

CrystEngComm

2013,

15

(46),

9949.

https://doi.org/10.1039/c3ce40893k. (4)

Han, X.; Kuang, Q.; Jin, M.; Xie, Z.; Zheng, L. Synthesis of Titania Nanosheets with a

High Percentage of Exposed (001) Facets and Related Photocatalytic Properties Synthesis of Titania Nanosheets with a High Percentage of Exposed (001). Society 2009, No. 1, 1–3. https://doi.org/10.1021/ja8092373.


22

Page 23 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(5)

Nakata, K.; Fujishima, A. TiO2photocatalysis: Design and Applications. Journal of

Photochemistry and Photobiology C: Photochemistry Reviews 2012, 13 (3), 169–189. https://doi.org/10.1016/j.jphotochemrev.2012.06.001. (6)

Roguska, A.; Pisarek, M.; Belcarz, A.; Marcon, L.; Holdynski, M.; Andrzejczuk, M.; Janik-

Czachor, M. Improvement of the Bio-Functional Properties of TiO2 Nanotubes. Applied Surface Science 2016, 388, 775–785. https://doi.org/10.1016/j.apsusc.2016.03.128. (7)

Macak, J. M.; Schmuki, P. Anodic Growth of Self-Organized Anodic TiO2 Nanotubes in

Viscous

Electrolytes.

Electrochimica

Acta

2006,

52,

1258–1264.

https://doi.org/10.1016/j.electacta.2006.07.021. (8)

Linsebigler, A. L.; Lu, G.; Yates, J. T. Photocatalysis on TiO2 Surfaces: Principles,

Mechanisms,

and

Selected

Results.

Chemical

Reviews

1995,

95

(3),

735–758.

https://doi.org/10.1021/cr00035a013. (9)

Ching, L.; Hon, K.; Saravanan, P.; Ibrahim, S. Applied Surface Science Rapid Thermal

Reduced Graphene Oxide / Pt – TiO2 Nanotube Arrays for Enhanced Visible-Light-Driven Photocatalytic

Reduction

of

CO2.

Applied

Surface

Science

2015,

358,

122–129.

https://doi.org/10.1016/j.apsusc.2015.08.065. (10) Cao, H.; Huang, K.; Wu, L.; Hou, G.; Tang, Y.; Zheng, G. Applied Surface Science Enhanced Catalytic Performance of Pt/TNTs Composite Electrode by Reductive Doping of TNTs. Applied Surface Science 2016, 364, 257–263. https://doi.org/10.1016/j.apsusc.2015.12.128. (11) Bai, J.; Zhou, B. Titanium Dioxide Nanomaterials for Sensor Applications. Chemical Reviews 2014, 114 (19), 10131–10176. https://doi.org/10.1021/cr400625j.


23


Page 24 of 28

(12) Chen, C.; Pan, F. Applied Catalysis B : Environmental Electrocatalytic Activity of Pt Nanoparticles Deposited on Porous TiO2 Supports toward Methanol Oxidation. 2009, 91, 663– 669. https://doi.org/10.1016/j.apcatb.2009.07.008. (13) Ting, C. C.; Liu, C. H.; Tai, C. Y.; Hsu, S. C.; Chao, C. S.; Pan, F. M. The Size Effect of Titania-Supported Pt Nanoparticles on the Electrocatalytic Activity towards Methanol Oxidation Reaction Primarily via the Bifunctional Mechanism. Journal of Power Sources 2015, 280, 166– 172. https://doi.org/10.1016/j.jpowsour.2015.01.081. (14) Olgun, U.; Gülfen, M.; Üstel, F.; Arslan, H. Electro-Optics and Band Gap Energies of Nanosilver-Coated TiO2 Nanotubes on Titanium Metal. Acta Metallurgica Sinica (English Letters) 2018, 31 (2), 153–163. https://doi.org/10.1007/s40195-017-0664-6. (15) Lv, J.; Gao, H.; Wang, H.; Lu, X.; Xu, G.; Wang, D.; Chen, Z.; Zhang, X.; Zheng, Z.; Wu, Y. Applied Surface Science Controlled Deposition and Enhanced Visible Light Photocatalytic Performance of Pt-Modified TiO2 Nanotube Arrays. Applied Surface Science 2015, 351, 225–231. https://doi.org/10.1016/j.apsusc.2015.05.139. (16) Maria Dalcin Fornari, A.; de Araujo, M. B.; Bergamin Duarte, C.; Machado, G.; Teixeira, S. R.; Weibel, D. E. Photocatalytic Reforming of Aqueous Formaldehyde with Hydrogen Generation over TiO2nanotubes Loaded with Pt or Au Nanoparticles. International Journal of Hydrogen Energy 2016, 41 (27), 11599–11607. https://doi.org/10.1016/j.ijhydene.2016.02.055. (17) Andrzejczuk, M.; Roguska, A.; Michalska, M.; Lipinska, L.; Czerwinski, A.; Cantoni, M.; Krawczynska, A. T.; Lewandowska, M. STEM Study of Li4Ti5O12 Anode Material Modified with

Ag

Nanoparticles.

Journal

of

Microscopy

2016,

264

(1),

41–47.

https://doi.org/10.1111/jmi.12414.


24

Page 25 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(18) Song, H.; Yang, Y.; Geng, J.; Gu, Z.; Zou, J.; Yu, C. Electron Tomography: A Unique Tool Solving Intricate Hollow Nanostructures. Advanced Materials 2018, 1801564, 1801564. https://doi.org/10.1002/adma.201801564. (19) Herbig, M.; Choi, P.; Raabe, D. Combining Structural and Chemical Information at the Nanometer Scale by Correlative Transmission Electron Microscopy and Atom Probe Tomography. Ultramicroscopy 2015, 153, 32–39. https://doi.org/10.1016/j.ultramic.2015.02.003. (20) Shukla, A. K.; Ercius, P.; Gautam, A. R. S.; Cabana, J.; Dahmen, U. Electron Tomography Analysis of Reaction Path during Formation of Nanoporous NiO by Solid State Decomposition. 2014, 2459 (2). https://doi.org/10.1021/cg5001649.. (21) Hernández-Garrido, J. C.; Moreno, M. S.; Ducati, C.; Pérez, L. a; Midgley, P. a; Coronado, E. a. Exploring the Benefits of Electron Tomography to Characterize the Precise Morphology of Core-Shell Au@Ag Nanoparticles and Its Implications on Their Plasmonic Properties. Nanoscale 2014, 6, 12696–12702. https://doi.org/10.1039/c4nr03017f. (22) Weyland, M.; Midgley, P. a. Electron Tomography. Materials Today 2004, 7 (12), 32–40. https://doi.org/10.1016/S1369-7021(04)00569-3. (23) Van Doren, E. A.; De Temmerman, P.-J. R.; Francisco, M. A. D.; Mast, J. Determination of the Volume-Specific Surface Area by Using Transmission Electron Tomography for Characterization and Definition of Nanomaterials. Journal of nanobiotechnology 2011, 9 (1), 17. https://doi.org/10.1186/1477-3155-9-17. (24) Fakron, O. M.; Field, D. P. 3D Image Reconstruction of Fiber Systems Using Electron Tomography. Ultramicroscopy 2014, 149, 21–25. https://doi.org/10.1016/j.ultramic.2014.10.012.


25


Page 26 of 28

(25) Andrzejczuk, M.; Roguska, A.; Pisarek, M.; Hołdy ń ski, M.; Lewandowska, M.; Kurzydłowski, K. J. Morphology of TiO2 Nanotubes Revealed through Electron Tomography. Micron 2017, 95, 35−41. (26) Roguska, A.; Belcarz, A.; Zalewska, J.; Hołdyński, M.; Andrzejczuk, M.; Pisarek, M.; Ginalska, G. Metal TiO2 Nanotube Layers for the Treatment of Dental Implant Infections. ACS Applied

Materials

&

Interfaces

2018,

10

(20),

17089–17099.

https://doi.org/10.1021/acsami.8b04045. (27) Yaguchi, T.; Konno, M.; Kamino, T.; Watanabe, M. Observation of Three-Dimensional Elemental Distributions of a Si Device Using a 360??-Tilt FIB and the Cold Field-Emission STEM System. Ultramicroscopy 2008, 108, 1603–1615. https://doi.org/10.1016/j.ultramic.2008.06.003. (28) Padgett, E.; Hovden, R.; Dasilva, J. C.; Levin, B. D. A.; Grazul, J. L.; Hanrath, T.; Muller, D. A. A Simple Preparation Method for Full-Range Electron Tomography of Nanoparticles and Fine

Powders.

Microsc.

Microanal.

23,

1150–1158,

2017,

https://doi.org/10.1017/S1431927617012764. (29) Kremer, J., Mastronarde, D.N., McIntosh, J.R. Computer visualization of three-dimensional image data using IMOD. Journal of structural biology 116 1 (1996): 71-76 (30) Messaoudii, C.; Boudier, T.; Sanchez Sorzano, C. O.; Marco, S. TomoJ: Tomography Software for Three-Dimensional Reconstruction in Transmission Electron Microscopy. BMC bioinformatics 2007, 8, 288. https://doi.org/10.1186/1471-2105-8-288. (31) Manikandan, M.; Vedarajan, R.; Materials, N.; Kodiyath, R.; Ueda, S. Pt Decorated FreeStanding TiO 2 Nanotube Arrays : Highly Active and Durable Electrocatalyst for Oxygen


26

Page 27 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Reduction

and

Methanol

Oxidation

Reactions.

2016,

No.

April

2017.

https://doi.org/10.1166/jnn.2016.11772. (32) Rice, C.; Ha, S.; Masel, R. I.; Wieckowski, A. Catalysts for Direct Formic Acid Fuel Cells. 2003, 115, 229–235. https://doi.org/10.1016/S0378-7753(03)00026-0. (33) Jiang, K.; Zhang, H.-X.; Zou, S.; Cai, W.-B. Electrocatalysis of Formic Acid on Palladium and Platinum Surfaces: From Fundamental Mechanisms to Fuel Cell Applications. Phys. Chem. Chem. Phys. 2014, 16 (38), 20360–20376. https://doi.org/10.1039/C4CP03151B. (34) Ustarroz, J.; Geboes, B.; Vanrompay, H.; Sentosun, K.; Bals, S.; Breugelmans, T.; Hubin, A. Electrodeposition of Highly Porous Pt Nanoparticles Studied by Quantitative 3D Electron Tomography: Influence of Growth Mechanisms and Potential Cycling on the Active Surface Area. ACS Applied Materials & Interfaces 2017 9 (19), 16168-16177; DOI: 10.1021/acsami.7b01619.


27


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Effect of Pt Deposits on TiO2 Electrocatalytic Activity Highlighted by

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