Photocatalytic H2 Generation Using Dewetted Pt-Decorated TiO2

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Article 2

Photocatalytic H Generation Using Dewetted PtDecorated TiO Nanotubes – Optimized Dewetting and Oxide Crystallization by a Multiple Annealing Process 2

JeongEun Yoo, Marco Altomare, Mohamed Mokhtar, Abdulmohsen A. Alshehri , Shaeel A. Al-Thabaiti, Anca Mazare, and Patrik Schmuki J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b12050 • Publication Date (Web): 29 Mar 2016 Downloaded from http://pubs.acs.org on March 29, 2016

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The Journal of Physical Chemistry

Photocatalytic H2 Generation Using Dewetted PtDecorated TiO2 Nanotubes – Optimized Dewetting and Oxide Crystallization by a Multiple Annealing Process

JeongEun Yoo,1‡ Marco Altomare,1‡ Mohamed Mokhtar,2 Abdelmohsen Alshehri,2 Shaeel A. Al-Thabaiti,2 Anca Mazare,1 and Patrik Schmuki1,2*

1

Department of Materials Science, Institute for Surface Science and Corrosion WW4-LKO,

Friedrich-Alexander University, Martensstraße 7, D-91058 Erlangen, Germany. 2

Chemistry Department, Faculty of Sciences, King Abdulaziz University, 80203 Jeddah, Saudi

Arabia Kingdom. * Corresponding Author. E-mail: [email protected]; Tel.: +49-9131-852-7575; Fax: +49-9131-852-7582. ‡ These authors contributed equally.

1 Environment ACS Paragon Plus


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Abstract In the present work we use TiO2 nanotube arrays, deposit onto them a thin layer of Pt, dewet the Pt to nanoparticles and use the structures as photocatalyst to generate H2. In order to achieve a maximum H2-generation efficiency, on the one hand an optimal thermal dewetting of the Pt layer is needed (this requires an oxygen free heat treatment), and on the other hand an optimal crystallization of the TiO2 nanotubes into a suitable crystalline oxide is required (achieved by annealing in an O2 containing environment). To combine the two requirements, we provide an adequate multiple annealing treatment under reducing and oxidizing conditions, and obtain Pt nanoparticle-decorated TiO2 nanotubes composed of anatase-rutile mixed phase that show a significantly enhanced photocatalytic H2 generation ability compared to tube layers that underwent any single step annealing process.


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Introduction Since the groundbreaking work of Fujishima and Honda,1 photocatalytic water splitting on semiconductors has been widely investigated, as in principle it provides a most promising way for a sustainable generation of the energy carrier H2. A photocatalytic process is generally based on light absorption by a semiconductor that leads to charge carrier generation, separation and reaction. In sequence, light of energy higher than the bandgap energy promotes electrons from the valence band to the conduction band, while holes are left in the valence band. Electrons and holes on the respective bands may then migrate to the semiconductor surface and induce red-ox reactions with species in the adjacent phase.2,3 Among the different photocatalytic materials that have been developed in the last decades, TiO2 still represents one of the most promising semiconductor metal oxides, being cheap, nontoxic, (photo-)chemically stable and, most importantly, having suitable band edge positions for triggering red-ox reactions with H2O. For anatase TiO2, the conduction band edge lies at approximately – 0.25 V (vs. SHE, pH 1)4,5 – photo-generated conduction band electrons are thus thermodynamically able to reduce water into H2. Nevertheless this reaction is not only affected by conventional charge carrier recombination and trapping phenomena, but also by the sluggish kinetics of electron transfer from TiO2 to the aqueous electrolyte. Among different successful strategies, charge transfer issues can be tackled by i) nanostructuring the semiconductor,6,7 and by ii) using suitable co-catalysts (mostly Pt, Pd and Au).8–11 When nanostructures of TiO2 are used as photocatalyst, the charge carriers have to migrate only over relatively short distances (nm-scale) to reach the surface and react with the environment (e.g., the adjacent liquid phase), and this reduces the probability of charge carrier



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recombination and trapping.12 One of the most facile techniques to fabricate defined nanostructures of TiO2 is the electrochemical anodization of metallic Ti under self-organizing conditions that provide highly-ordered and vertically aligned TiO2 nanotubes (NTs) on the metal substrate.13,14 A key advantage of this approach is that the geometry of the nanotubes (wall thickness, tube diameter and length, etc.) can be easily tailored simply by the electrochemical conditions (e.g., adjusting applied voltage, water and fluoride contents).15,16 The second typical measure to increase the efficiency of TiO2 in photocatalysis is the deposition of noble metal particles, such as Au, Pd or Pt, onto the TiO2 surface. These cocatalysts not only act as “electron sink” (i.e., by trapping conduction band electrons),17,18 but, as in the case of Pt, may promote the recombination of H0 surface species to H2. This is the reason why Pt is frequently found (under the same deposition conditions) to be more active than other noble metals. Recently we reported on a most straightforward way to decorate TiO2 nanotube layers with co-catalyst particles that is to first coat the tubes with a thin sputter-deposited film of the noble metal (Au, Pt) and then to thermally dewet this thin film to particles.19–21 Thermal dewetting of thin Au films into small particles on TiO2 can be achieved by a simple annealing process in air at 450°C. These annealing conditions are ideal as not only they lead to the dewetting of the thin Au films but they also lead to crystallization of the TiO2 nanotubes (that after anodic formation are amorphous) into a photocatalytically active mixture of anatase and rutile. In other words, a single step annealing can at the same time be used to crystallize the metal oxide semiconductor and induce Au dewetting.19 In contrast to Au an air treatment at 450°C is not suitable for Pt due to its much higher melting point.21,22 To achieve dewetting of Pt thin films, temperatures > 500°C are normally


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required. Moreover, the process must be carried out in Ar or N2 atmosphere to avoid the formation of Pt-oxide at these elevated temperatures.22–25 On the other hand, annealing in a reductive atmosphere leads to a partial reduction of TiO2 and to a significant photoactivity decay due to an enhanced charge carrier trapping and recombination.26,27 In the present work we explore a combination of reducing and oxidizing annealing treatments to address both these issues. We show that an optimized annealing sequence can lead to drastically increased photocatalytic efficiency in terms of a maximized H2 evolution rate.

Experimental Ti foils (0.125 mm thick, 99.7 % purity, Goodfellow, England) were degreased by sonicating in acetone, ethanol, deionized water, and then dried in a N2 stream. The Ti foils were then anodized to fabricate the highly ordered TiO2 nanotubes arrays in an electrolyte based on o-H3PO4 with 3 M HF (Sigma-Aldrich).19 For the anodic growth, a two-electrode configuration was used, where the Ti foil (15 mm x 15 mm) and a Pt gauze were the working and counter electrodes, respectively. The anodization experiments were carried out by applying a potential of 15 V (for 2 h) provided by a Volcraft VLP 24 Pro DC power supply. After the anodization, the nanotube films were rinsed with ethanol and dried in a N2 stream. The tube oxide layers, either as-formed (i.e., amorphous) or thermally treated, were coated by thin Pt films with a nominal thickness in the range of 1-25 nm, using a high vacuum sputter coater (Leica – EM SCD500) and a 99.99 % pure Pt target, Hauner Metallische Werkstoffe. The pressure of the sputtering chamber was reduced to 10-4 mbar, and then set at 10-2 mbar of Ar. The applied current was 15 mA. The amount of sputtered Pt, i.e., the nominal thickness of the Pt film, was in situ determined by an automated quartz crystal film-thickness sensor.



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The tubes (either pristine or Pt-coated) were exposed to different thermal treatments. Annealing in ambient air was carried out at different temperatures (350-600°C) for 1 h, using a Rapid Thermal Annealer (Jipelec Jetfirst 100 RTA), with a heating/cooling rate of 30°C min-1. Annealing in pure O2 (99.95 %, Linde) was carried out in a VMK80S Linn High Therm tubular furnace at 450°C for 30 min (or 5 h, in the case of the samples characterized by XPS in view of their Pt(II) and Pt(IV) oxide content). The O2 flux was set to 100 mL min-1. Annealing in pure N2 (99.999 %, Linde) was carried out at 600°C for 1 h, in a ZEW 1041-5 Heraeus tubular furnace. The N2 flux was set to 20 mL min-1 (noteworthy, the gas flow rate in the furnace was found to affect the dewetting process due to an altered heat transfer). For morphological characterization, a field-emission scanning electron microscope (Hitachi FE-SEM S4800) was used. X-ray diffraction analysis (XRD, X’pert Philips MPD with a Panalytical X’celerator detector) using graphite monochromized Cu Kα radiation (wavelength 1.54056 Å) was used for determining the crystallographic composition of the samples. The crystallographic phase composition of the anodized films was determined by Rietveld refinement performed using the GSAS software.28 Structural models for anatase TiO2, rutile TiO2 and metallic Ti phases were taken from refs.29,30 The background was subtracted using the shifted Chebyshev polynomials and the diffraction peak profiles were fitted with a modified pseudoVoigt function. The composition and the chemical state of the films were characterized using X-ray photoelectron spectroscopy (XPS, PHI 5600, US), and the spectra were shifted in relation to the C1s signal at 284.8 eV (Pt4f peaks were fitted with Multipak software). For photocatalytic H2 generation experiments, the Pt-decorated TiO2 nanotube layers on the Ti foils were welded to a Ti wire. Then, these samples were immersed into a 20 vol.%


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ethanol-water solution within a quartz tube that was used as photocatalytic reactor (ethanol was used as hole-scavenger).8,31,32 The ethanol-water solution (volume = 7 mL, kept under static conditions during the runs) and the cell head-space (volume = 8 mL) were purged with N2 gas for 30 min prior to photocatalysis. Before sealing the quartz tube with a gas-tight cap, the Ti wire was stuck into the bottom side of the cap, in order to hold and immobilize the tube layers within the photocatalytic reactor at the same position. The light source used for the photocatalytic experiments was a HeCd laser with emission λ = 325 nm, and an emission intensity of 18.5 mW cm−2 (Kimmon, Japan). The laser beam was expanded to a circle-shaped light spot of 1 cm in diameter, thus illuminating a sample surface of ~ 0.78 cm2. In order to determine the amount of H2 generated under irradiation, the headspace of the quartz reactor was analyzed by gas chromatography (using a GCMS-QO2010SE chromatograph, Shimadzu). The GC was equipped with a thermal conductivity detector (TCD), a Restek micropacked Shin Carbon ST column (2 m x 0.53 mm) and a Zebron capillary column ZB05 MS (30 m x 0.25 mm). GC measurements were carried out at a temperature of the oven of 45°C (isothermal conditions), with the temperature of the injector set to 280°C and the TCD fixed to 260°C. The flow rate of the carrier gas, i.e., argon, was 14.3 mL min-1. All the experiments lasted 9 hours and the amount of evolved H2 was measured at the end of the experiments.

Results and Discussion Fig. 1 shows the morphological features of the anodic TiO2 nanotube layers used in this work. These short aspect ratio nanotubes were selected to achieve conformal sputter-coating and also since they provide almost ideal hexagonal packing. They have an average diameter and length of



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the tubes of 70-80 nm and 150-170 nm, respectively.19 Fig. 1(b,c) show the morphology of the tube substrates after sputter-coating the tubes with a layer of Pt with a nominal thickness of 5 nm. The profile of the noble metal deposition is particularly clear in Fig. 1(c), where one can see that the Pt film preferentially coats the tube top, and the thickness decreases gradually toward the bottom of the cavities. The thin Pt metal films present on the tube substrates can split up into nano-sized Pt particles by a thermal treatment at 600°C in N2 atmosphere for 1 h,33 leading to the Pt/TiO2 structures shown in Fig. 1(d,e). One can see that in the case of 5 nm-thick sputter-coated Pt films, the particles formed by dewetting are globular and have diameters of ca. 5-25 nm, with smaller particles at the bottom and bigger aggregates at the top of the tubes. Such a distribution of the nanoparticles (NPs) is in line with the different initial thickness of the Pt film on the tube walls, that is, the size of the dewetted particles strictly depends on the thickness of the metal film and thinner films generally split into small particles, while thicker coatings form large metal islands.21 Noteworthy, only a thermal treatment at 600°C (or higher) in N2 (or Ar) was found to lead to defined dewetted Pt clusters without affecting the TiO2 tubular structures, while lower temperatures did not affect the morphology of the Pt films even after extended annealing times. If oxidizing conditions are used, e.g., an O2-containing atmosphere at 600°C, a complete collapse of the tube structures is observed (see Fig. 1(f)). Moreover, in an O2-containing atmosphere, extensive growth of thermal oxide from the Ti metal substrate underneath the tubes is observed (this aspect is discussed below in more detail).34,35 A first round of photocatalytic experiments (the results are compiled in Table 1) showed that Pt dewetting is highly beneficial for the photocatalytic H2 generation. Dewetted films


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showed a H2 production of ~ 2.3 ℎ in comparison to non-dewetted layers that produced ~ 1 ℎ , that is, a 100% enhancement can be achieved through the dewetting step.19,36 This enhancement of the activity can be ascribed to the fact that the as-sputtered noble metal film coats the TiO2 tubes in a conformal fashion, and thus shades uniformly the underneath TiO2 structure. Dewetting forms Pt nanoparticles homogeneously distributed on the oxide surface – thus not only free TiO2 surface is generated but also a higher specific area of exposed Pt is provided. The results is improved light absorption by the oxide and an improved electron transfer to the environment and, as a consequence, an enhanced H2 generation efficiency is attained.37 Nevertheless, Pt-coated tubes annealed in air at 450°C show an even higher H2 production of ~ 6.6 ℎ (see Table 1), although this thermal treatment does not induce Pt dewetting. This means that an enhanced H2 evolution must be related also to a higher quality oxide in the tubes (lower defect density oxide formed by crystallizing in non-reductive conditions).38 Thus, two factors, namely the occurrence of Pt dewetting (high temperature treatment in N2) and a suitable crystallization of the oxide (air annealing) need to be optimized to achieve a maximum photocatalytic activity. In order to adequately combine the two factors we explored several annealing conditions of the Pt-coated tubes, at various temperatures and in different atmospheres, and investigated compositional and structural aspects as well as their photocatalytic performance. Fig. 2, Table 2 and Table 3 summarize the results of XPS and XRD characterization of these samples that were prepared in every case by depositing on the nanotube layers 5 nm-thick Pt films, and that then were exposed to different thermal treatments. The XPS results in Fig. 2(a) show the films to be generally composed of TiO2 and Pt metal, with a small content of fluoride, due to F-ion uptake by the oxide during the electrochemical



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growth in the HF electrolyte. Previous work indicated such minor bulk fluoride remnants not to have significant effect on the photocatalytic properties of the anodic TiO2 nanotubes.5,13 From the XPS data in Table 2 the occurrence of Pt dewetting is also traceable: the surface Ti amount for non-dewetted films (NT + 5 nm Pt + 450°C air) is relatively low (2.2 at%) while Pt is accordingly high (42.8 at%), confirming that the oxide surface is well coated by the sputtered Pt film. After treating the films in N2 (NT + 5 nm Pt + 600°C N2) the Ti surface content increased up to 16 at%, indicating that Pt agglomerated into particles and thus a more open titanium oxide surface becomes detectable. Fig. 2(b) shows the XRD patterns of Pt-decorated TiO2 NT layers that underwent different thermal treatments, and the refined XRD results in term of crystallographic phase composition are compiled in Table 3. The layers show the characteristic reflections of crystalline TiO2 (anatase and rutile phases) and Ti metal (owing to the porosity and to the relative short thickness of the tube oxide layers, the Ti metal substrate is still visible)39. Particularly, from the data in Table 3 it is clear that the annealing treatments largely affects the crystallographic features of the tube layers, both in terms of degree of crystallinity (intended as total amount of crystalline anatase and rutile TiO2 that is present in the anodic layer) and relative amount of formed anatase with respect to the total amount of crystalline TiO2. More precisely, we find that the single annealing step in air at 450°C leads on the one hand to high relative anatase content (~ 44% of the total amount of crystalline TiO2) but on the other hand to a low degree of crystallinity of the tube layer (the content of crystalline TiO2 in the sample is only ~ 11 wt%). On the contrary, the multiple annealing “600ºC N2 + 450ºC air” leads to both high relative anatase content (~ 30%) and higher degree of crystallinity (~ 29 wt%) – in other words, the multiple annealing forms larger amounts of crystalline TiO2 in the anodic films compared to


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the single annealing step, as determined from the refinement of the anatase and rutile XRD signals (Table 3). Also, from the XRD pattern of the sample “NT + 5 nm Pt + 600ºC N2 + 450ºC air” one can see the characteristic reflections of Pt, with the main peak at ~ 46.3°.40 The appearance of the Pt reflections for this sample may be ascribed to the fact that the thermal treatment in N2 at 600ºC leads to Pt agglomeration and hence to an increase of the crystallographic coherence of the Pt particles. Conversely, no crystalline Pt was detected when the noble metal was as-deposited on the tube layers (in relatively small amounts, e.g. ≤ 5 nm) or after a thermal treatment in air at 450ºC. The photocatalytic results in Fig. 2(c) (summarized in Table 1) showed that the different thermal treatments (different temperatures and atmospheres) have dramatic effects not only on the morphology and crystallographic structure of the Pt-decorated nanotube layers, but also on their H2 generation performance. The results show that the two key concepts, optimized dewetting and TiO2 crystallization, can be beneficially combined through a multiple annealing approach. In contrast to the single step processes (that yield 2.3 and 6.6 ℎ ), the sample “NT + 5 nm Pt + 600ºC N2 + 450ºC air” lead to ~ 19.3 ℎ, i.e., clearly the highest H2 generation efficiency among the differently annealed Pt/TiO2 photocatalysts. Interestingly, if the sequence is inverted, i.e., Pt is dewetted (600°C in N2) after crystallizing the tubes in air, the H2 evolution drops to ~ 0.2 ℎ (sample “NT + 450ºC air + 5 nm Pt + 600ºC N2”). This is most likely due to the generation of oxygen vacancies during the second annealing in reductive conditions (such a defect generation under reductive conditions is widely reported in the literature26,27).



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Noteworthy, a comparison in terms of photocatalytic H2 production rate with earlier works on Au dewetting shows that, by using a similar sputtering-dewetting strategy, Au-modified TiO2 nanotubes layers20,36 lead to a maximized photocatalytic activity of ~ 6-7 ℎ, while the Ptmodified nanotubes fabricated in the present work lead to a ~ 3 times higher H2 evolution rate, i.e., 19.3 ℎ. The air-crystallization step was further explored by exposing dewetted layers to airannealing at different temperatures in the 350-550°C range. The photocatalytic results summarized in Fig. 3(a) show that compared to an air-treatment at 450°C (~ 19.3 ℎ ), annealing at 350 and 550°C leads to significantly lower H2 generation efficiencies (~ 2.8 and 1.2 ℎ , respectively). The reasons for this become evident from the XRD data (Fig. 3(b) and Table 3), where one can see that the treatment in air at 350°C does not induce oxide crystallization into anatase TiO2 – only rutile is formed (~ 26 wt%). Air-annealing at 550°C leads to anatase formation along with a high degree of crystallinity of the oxide (the total content of crystalline TiO2 is 64.5 wt%), but it causes at the same time the formation of a large amount of rutile (~ 52 wt%).41 This remarkable rutile formation at 550°C can be ascribed to a thermal oxidation of the Ti metal substrate. In line with other works, the growth of rutile occurs firstly by rutile formation at the metal/TiO2 interface and then proceeds (with higher annealing temperatures or longer thermal treatments) up the tube walls and towards the tube tops.34,39,42,43This rutile layer under the tubes is clearly visible from the cross-sectional SEM images in Fig. 3(c-f) where one can see that rutile forms (from the Ti metal substrate) as a layer of some hundreds of nanometers underneath the tubular oxide. An air-treatment at 550°C leads to a ca. 500 nm-thick rutile film, that is ~ 3 times the thickness of that formed at 450°C (i.e., ~ 150 nm). The thickening of the


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rutile under-layer with increasing the temperature of the air-annealing is also apparent from the XRD data in Table 3. Not only the rutile content significantly increases from 26.4 to 52.5 wt%, but also the contribution of the Ti XRD reflections ascribed to the metal substrate become weaker. Additionally, the coarsening of the tube walls for the tube layers annealed in air at 550°C is in line with the large content of rutile and its formation not only at the TiO2/Ti interface but also in the tube walls (please compare Fig. 3(d) and (f)). Therefore, the absence of anatase phase in the layers air-treated at 350°C, and the predominant rutile content in the oxides crystallized at 550°C, are plausible reasons for the low H2 generation yield observed under these conditions. Another noteworthy finding is that annealing in O2-containing atmosphere can, if not properly adjusted, affect the Pt oxidation state23 and therefore the co-catalyst efficiency.44 Pt/TiO2 nanotubes annealed after dewetting in pure O2 at 450°C (5 h) produced ~ 4.5 ℎ . Please note that films treated at the same temperature in air lead instead to ~ 19.3 ℎ (see Fig. 3(a) and Table 1). From the XPS data in Fig. 4 and in Table 4 it can be seen that the noble metal for airtreated samples at 450°C is present as metallic Pt (i.e., Pt0),23 and the fitting of the HR spectrum in the Pt4f region shows besides the Pt0 signals only a small contribution (small shoulder) of adsorbed oxygen (i.e., PtOads). On the contrary, for 5 nm and 25 nm Pt films annealed in pure O2 an inversion of the relative intensity of the Pt4f signals is evident (the 4f7/2 peak at ~ 72.2 eV has a lower intensity than the 4f5/2 peak at ~ 75.8 eV), and even more importantly a broad shoulder (at ~ 76-80 eV) appears that can be attributed to the formation of PtO (PtII) and PtO2 (PtIV).23 The more intense Pt signals in Fig.4(a) are in line with the larger amount of sputter-coated Pt (i.e., 25 nm as compared to 5 nm). However, more interesting is that for this layer, when



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exposed to an identical multiple annealing (600°C in N2 followed by 450°C in O2 for 5 h), the relative HR XPS spectrum (Fig. 4(a)) shows an even more pronounced shoulder. Fitting of these data (Fig. 4(b)) matches well with the reference Pt4f signals of PtII and PtIV oxides.45,46Moreover, considering the XPS data summarized in Table 4, we observe that for the 5 nm Pt layers there is a significant difference between annealing in air or in O2, as annealing in air leads to more adsorbed oxygen (PtOads) while annealing in O2 forms more platinum oxides (PtO and PtO2) and leads to a lower amount of PtOads. The low H2 generation efficiency of tube layers annealed in pure O2 is thus ascribed to the formation of Pt oxides which deteriorates the efficiency of Pt as a co-catalyst.24,44–46 After optimizing the Pt dewetting and oxide crystallization, we also evaluated the effect of the amount of co-catalyst on the H2 evolution efficiency. The photocatalytic results in Fig. 5(a) show a clear enhancement of the H2 generation when the amount of co-catalyst is increased to 5 nm, while larger amount of co-catalyst, e.g., 7-15 nm-thick Pt films, led to significantly lower photocatalytic activity. From the SEM images in Fig. 5(b-d) (and in Fig. 1(d,e)) it is evident that an increase of the Pt film thickness leads to an increase of the average Pt nanoparticle size after dewetting. For Pt film of 2, 7 and 10 nm, the average Pt NPs size ranges are 5-20, 30-70 and 80-100 nm, respectively. The smaller nanoparticles (e.g., Fig. 5(b)) are round in shape, show a relatively narrow size distribution and are ordered at the tube tops in hexagonal arrangement. From thicker metal films, Pt islands are formed that are several tens of nanometers, as shown for example in Fig. 5(d).21 The reason for the drop of photocatalytic activity observed when increasing the co-catalyst amount is therefore that thick noble metal films dewet into large Pt islands that shade the


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underneath oxide and limit light absorption. Additionally, thinner metal films form Pt nanoparticles with a higher specific surface area than the large Pt islands and this enhances the area for electron transfer to the environment and therefore the H2 evolution efficiency. Lower Pt amounts lead also to a limited coverage of the oxide and therefore photogenerated holes can efficiently diffused at the oxide/environment interface, thus reducing the probability of carrier recombination.

Conclusions In this work we demonstrate for TiO2 nanotubes that the quality of the oxide, in terms of crystallinity and defect density, and the morphology and chemical state of the co-catalyst (here Pt) play a crucial role in determining the photocatalytic efficiency of noble metal/TiO2 systems. We show that a sputter-dewetting approach is a suitable technique for decorating anodic nanotube TiO2 surfaces with fine Pt co-catalyst nanoparticles. More importantly, we introduce a multiple annealing strategy by combining adequate reducing and oxidizing conditions that leads to Pt dewetting and at the same time to the tube crystallization into a photocatalytically active anatase-rutile mixture. As a consequence, we achieve a maximized H2 generation from such Pt/TiO2 photocatalysts. This concept can likely be extended to other functional metal/oxide systems where the metal is even more susceptible to the annealing conditions.

Acknowledgment This project was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University. The authors would also like to acknowledge the ERC, the DFG, and the DFG



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“Engineering of Advanced Materials” cluster of excellence for financial support. Dr. M. Allieta is acknowledged for the XRD data refinement and for related valuable discussion.

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Figure Captions

Figure 1 SEM images of TiO2 nanotube arrays a) as-formed; b) and c) sputter-coated with a 5 nm-thick Pt film; d) and e) after thermal dewetting of the Pt film by heat treatment in N2 at 600ºC for 1 h; f) that underwent structural collapse by a thermal treatment in air at 600ºC for 1 h.

Figure 2 a) XPS survey, b) XRD patterns, and c) photocatalytic H2 generation results of Pt/TiO2 nanotube layers exposed to different thermal treatments.

Figure 3 a) Photocatalytic H2 generation data and b) XRD patterns of samples exposed to different thermal treatment in air and O2. The SEM images (c-f) show the rutile under-layer formed by annealing at 450ºC (c,d) and at 550ºC (e,f) in air.

Figure 4 a) High-resolution Pt4f XPS spectra of Pt (reference) and of TiO2 nanotube layers decorated with sputter-coated Pt films (nominally 5 and 25 nm-thick) and exposed to different thermal treatments in air and O2 (the arrows indicate the shoulder ascribed to Pt(II) and Pt(IV) oxides formation); b) Pt4f high-resolution XPS spectrum (experimental data) of the sample “NT + 25 nm Pt + 600°C N2 + 450°C O2” along with the fitting curve and the deconvoluted doublets accounting for Pt0, PtOads, Pt(II) and Pt(IV) oxides.


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Figure 5 a) Photocatalytic H2 generation results and (b-d) SEM images of TiO2 nanotube layers decorated by sputter-coating different Pt amounts: b) 2 nm, c) 7 nm, and d) 10 nm. The samples were annealed in N2 at 600°C and then in air at 450°C.



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Tables

Table 1 Photocatalytic H2 generation results of Pt/TiO2 nanotube layers sputter-coated with different amount of Pt and exposed to different single and multiple thermal treatments. The labels of the samples illustrate the sequence and provide details of the different treatments that the layers underwent (i.e., amount of sputter-coated Pt, and annealing temperature and atmosphere).

Table 2 Summary of the XPS results of TiO2 nanotube layers sputter-coated with 5 nm-thick Pt layers that were subsequently exposed to single and multiple thermal treatments in air and N2.

Table 3 Summary of the XRD results of TiO2 nanotube layers, pristine and Pt-decorated, that were exposed to different thermal treatments.

Table 4 Surface elemental composition and content (at%) obtained from the XPS data (Fig. 4) for Pt reference sample and TiO2 nanotube layers decorated with sputter-coated Pt films (5 and 25 nmthick) and exposed to different multiple thermal treatments in N2 (600°C, 1 h), air (450°C, 1 h) and O2 (450°C, 5 h).


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Figure 1



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Figure 2


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Page 27 of 34

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Figure 3



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Figure 4


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Figure 5



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Table 1

H2 generation rate ( )

Sample NT + 600ºC N2 + 5 nm Pt

1.0

NT + 5 nm Pt + 600ºC N2

2.3

NT + 5 nm Pt + 450ºC air

6.6

NT + 5 nm Pt + 600ºC N2 + 450ºC air

19.3

NT + 450ºC air + 5 nm Pt + 600ºC N2

0.2


2.8


1.2

NT + 5 nm Pt + 600ºC N2 + 450ºC O2

4.5


4.5


10.0


8.9

NT + 10 nm Pt + 600ºC N2 + 450ºC air

9.2

NT + 15 nm Pt + 600ºC N2 + 450ºC air

3.4


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Table 2

Atomic concentration (at%)

Sample

Ti

O

Pt

C

F

N

NT + 5 nm Pt + 600ºC N2

16.1

51.9

14.0

12.4

0.7

4.9


2.2

18.1

42.8

29.5

0.6

6.9


17.4

56.4

12.6

7.2

0.4

6.0



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Table 3

Crystalline phase content (wt%) Ti metal

Anatase (A)

Rutile (R)

+

× % +


88.9

4.9

6.2

11.1

44.1

NT + 5 nm Pt + 600ºC N2

68.7

7.2

24.1

31.3

23.0


71.0

8.8

20.2

29.0

30.3


73.6

-

26.4

26.4

0.0


62.9

13.9

23.2

37.1

37.5


35.5

12.0

52.5

64.5

18.6

Sample


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Table 4

Atomic concentration (at%) Sample

Ti

O

-


Pt

C

F

1.9

26.9

-

3.2

1.5

12.4

0.5

5.1

7.7

1.9

9.1

-

15.2

8.1

3.2

8.9

-

Pt

PtOads

PtO

PtO2

21.8

39.2

7.2

3.0

13.8

47.9

9.3

11.3

NT + 5 nm Pt + 450ºC O2

10.3

53.4

12.6

NT + 25 nm Pt + 450ºC O2

4.9

40.9

18.8

Pt reference



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Table of Contents (TOC) Image


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Photocatalytic H2 Generation Using Dewetted Pt-Decorated TiO2

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