Effect of Thermal Treatment on the Photocatalytic Activity for Hydrogen

Article pubs.acs.org/JPCC

Ta2O5 Nanotubes Obtained by Anodization: Effect of Thermal Treatment on the Photocatalytic Activity for Hydrogen Production Renato V. Gonçalves,† Pedro Migowski,‡ Heberton Wender,§ Dario Eberhardt,† Daniel E. Weibel,∥ Flávia C. Sonaglio,† Maximiliano J. M. Zapata,† Jairton Dupont,‡ Adriano F. Feil,*,† and Sergio R. Teixeira*,† †

Laboratório de Filmes Finos e Fabricaçaõ de Nanoestruturas (L3Fnano), UFRGS, Instituto de Física, Av. Bento Gonçalves 9500, P.O. Box 15051, 91501-970, Porto Alegre, RS, Brazil ‡ Laboratório de Catálise Molecular (LMC), UFRGS, Instituto de Química, Av. Bento Gonçalves 9500, P.O. Box 15003, 91501-970, Porto Alegre, RS, Brazil § Laboratório Nacional de Luz Síncrotron (LNLS), Rua Giuseppe Máximo Scolfaro, 10.000 P.O. Box 6192, 13083-970, Campinas, SP, Brazil ∥ Laboratório de Fotoquímica e Superfícies (LAFOS), UFRGS, Instituto de Química, Av. Bento Gonçalves 9500, P.O. Box 15003, 91501-970, Porto Alegre, RS, Brazil S Supporting Information *

ABSTRACT: Freestanding tantalum oxide nanotubes (Ta2O5 NTs) were easily fabricated by controlling only the electrolyte temperature during anodization in a sulfuric acid solution. When the electrolyte temperature decreased, the adherence of NTs to the Ta substrate increased. High electrolyte temperatures facilitated formation of freestanding NTs. Thermal treatment of the freestanding Ta2O5 NTs below 750 °C resulted in an amorphous structure. The orthorhombic crystalline phase appeared only at temperatures higher than 750 °C. The effect of thermal treatment on the crystalline structure and morphology of Ta2O5 NTs showed that the NTs retained their tubular shape up to 800 °C. In addition, it was shown that the crystallinity of the NTs was enhanced from 11% to 34% by increasing the treatment time for the NTs at 800 °C from 0.5 to 1 h. High crystallinity and low surface contamination increased the photocatalytic activity of the freestanding NTs for hydrogen production by water splitting using a water/ethanol solution under UV radiation. The sample annealed at 800 °C for 1 h showed the highest photocatalytic activity for hydrogen generation. Additionally, changes to the physicochemical properties of the surface and bulk of the photocatalyst showed decreased selectivity for minor products (C2H4 and C2H6).

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INTRODUCTION Modern life requires abundant and economical energy, and the sun is the best source of energy on earth. Research into materials and techniques which enable the transformation of energy from the sun has increased in recent decades due to the environmental problems associated with the use of fossil fuels. In this context, after the pioneering work of Honda and Fujishima using semiconductor photoanodes for water splitting,1 photocatalytic hydrogen production has emerged as a low-cost alternative process for producing clean and renewable fuels. In this process, photon energy is converted into chemical energy by overcoming the positive uphill change in free energy.1−7 When a semiconductor is irradiated with UV © 2012 American Chemical Society

light, electrons and holes are generated. The photogenerated electrons reduce water to form H2, while the holes oxidize it to form O2. Two important factors that influence the photocatalytic water splitting process are the crystalline structure of the semiconductor and the number of exposed catalytic sites on its surface. Thus, the main goal in the search for new and more active photocatalysts is to fine tune all of the main factors that control the global photoredox reactions. Received: April 5, 2012 Revised: June 4, 2012 Published: June 4, 2012 14022

dx.doi.org/10.1021/jp303273q | J. Phys. Chem. C 2012, 116, 14022−14030

The Journal of Physical Chemistry C

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was H2 production by UV irradiation of NT semiconductors dispersed in ethanol/water solutions. First, the attainment of freestanding NTs was optimized by lowering the adherence of the oxide layer on the Ta substrate by changing the anodization temperature. The physicochemical and morphological changes to the NTs due to thermal treatments were characterized by Xray powder diffraction, X-ray photoelectron spectroscopy, specific surface area (BET), and electron microscopy. Furthermore, the effects of the thermal treatments on Ta2O5 NTs crystallinity and surface contaminants on the photochemical properties of the nanotubular semiconductors for hydrogen production by water splitting were investigated. Additionally, changes to the physicochemical properties of the surface and bulk of the photocatalyst showed decreased selectivity for minor products (C2H4 and C2H6).

Despite its wide band gap (3.8 to 4.0 eV) and thus poor solar light-harvesting ability, Ta2O5 has been shown to be one of the most active photocatalysts for water splitting under UV irradiation reported to date.8 Moreover, Ta2O5 has also received attention because it is a precursor material in the synthesis of NaTaO3,9 KTaO3, and LiTaO3,10 which are promising materials for use as photocatalysts. The high photocatalytic activity of Ta2O5 probably relies on its surface structure and bulk electron diffusion lengths that together generate more surface-active catalytic sites than other semiconductors.11 One way to enhance the number of surface-active sites and reduce the bulk recombination of electron−hole pairs is to control the semiconductor’s size and shape. By choosing the proper synthesis methodology, it is possible to control the shape of the particles, thus favoring the exposure of specific catalytic sites as well as increasing the overall surface area of the catalysts. The anodization technique is a powerful tool that allows the manufacture of highly ordered and self-organized nanoporous or nanotubular oxide structures with a wide variety of metals, such as Al,12−15 Ti,16−23 W,24,25 Zr,26,27 Nb,28−30 Fe,31−33 and Ta.34−36 These nanomaterials have attracted much attention due to a wide range of possible applications, such as in sensors,16,37 photoelectrodes,37−40 photocatalysts,18,41 solar cells,42−44 decomposition of organic compounds,11,21,27,45,46 and biomedical applications.47,48 Anodization of Ta foils in fluoride electrolytes dissolved in H2SO4, with or without organic additives such as ethylene glycol (EG) and dimethyl sulfoxide (DMSO), enables fabrication of high aspect ratio Ta2O5 nanotube (NT) arrays. On the other hand, anodization of Ta metal with high fluoride concentrations in an H2SO4 solution has resulted in formation of freestanding Ta2O5 NTs.49 Ta2O5 NTs prepared by anodization have also been used as a template for the synthesis of TaS2,50 TaON,51 and Ta3N5 NTs.40 It has been shown that the annealing conditions affect the crystal structure and photoresponse of anodic oxide semiconductor NTs.52,53 Furthermore, one strategy to induce changes in the morphological properties of these nanomaterials is control of the annealing temperature. This alters the physicalchemical properties of the NTs and, as a consequence, the electron−hole dynamics and photochemical properties for hydrogen production by water splitting reactions.47,54 The temperature at which the as-prepared Ta2O5 crystallizes is still a topic of discussion. Some studies have reported that the NT arrays are crystallized at 550 °C,40 while others have shown that crystallization occurs at 750 °C in Ar36 or at 300 °C in an oxygen atmosphere.34 Generally, the as-anodized samples are amorphous and further annealing is required to crystallize the oxide NTs. For different oxide semiconductors, the annealing process transforms the as-anodized amorphous oxide phase into a polycrystalline oxide phase.16,26,34,40,52,53,55 Actually, two mechanisms describe the crystalline phase transition in oxide NTs: (i) crystallization of NTs during the annealing process is induced by the metal substrate52 and (ii) dopants or impurities from the electrolyte occupy the vacant sites in the NT structure, changing the transition thermodynamics of the amorphous−crystalline phase.53 Thus, a specific study in order to evaluate and better understand the phenomena involved in the crystallization of Ta2O5 is still needed. This work reports on the photocatalytic properties of freestanding Ta2O5 NTs after thermal treatments at different temperatures for variable periods of time. The reaction studied

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EXPERIMENTAL SECTION General Considerations. Sulfuric acid (H2SO4, 98%), hydrofluoric acid (HF, 40%), ethanol (EtOH, 95%), and tantalum discs (Ta, 0.25 mm thick, 99.99%) were purchased from Synth, Nuclear, Fmaia, and Goodfellow, respectively. Solvents and reagents were of analytical grade and used as received. Ta2O5 NTs photocatalytic activity was compared with a standard powder of Ta2O5 (99.98%) purchased from Sigma Aldrich, denoted as SP. Preparation of Ta2O5 NTs. Anodization Process. Prior to anodization, Ta discs (10 mm diameter) were cleaned ultrasonically in acetone and isopropanol baths for 20 min, rinsed with distilled water, and further dried under nitrogen flux. Ta2O5 NTs were prepared by anodization in a standard two-electrode electrochemical cell configuration18,21 with platinum foil as the counter electrode under constant applied voltage (50 V for 20 min) at different temperatures (0, 10, 20, 30, 40, and 50 °C) using a Thermo Neslab-RTE7 (Electron Corp.) bath to control the temperature. The distance between the electrodes was maintained at 1 cm in all experiments. The electrolyte consisted of H2SO4 + 1 vol % HF + 4 vol % deionized water, ultrasonicated during anodization. Immediately after anodization, Ta2O5 NTs were carefully immersed in distilled water to remove the excess HF and H2SO4. The NTs prepared at 50 °C could be easily removed from the Ta substrate by rinsing with distilled water. The freestanding NTs were collected in a receptacle with water and decanted. After removal of the water, the obtained white powder was dried in air. Thermal Treatment. Samples were annealed in a furnace in atmospheric air at different temperatures with a heating rate of 5 °C·min−1. The adherence of NTs was qualitatively estimated using adhesive tape by attaching it to the sample surface and slowly removing it. NTs with lower adherence to the Ta substrate remained stuck to the tape. Ta2O5 NT Characterization. SEM and TEM Analysis. The morphology of Ta2O5 NTs was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) using a JEOL 6060 and a JEOL JEM1200 EXII operating at 80 kV, respectively. For TEM analysis, the samples were prepared by dispersing a few milligrams of freestanding NTs in acetone at room temperature followed by ultrasonication. One or two drops were further deposited on a 400 mesh carbon-coated Cu grid. Chemical analyses by line scans and mapping were performed with an electron probe energy-dispersive X-ray spectrometer (EDX) in the SEM microscope. 14023



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Effects of Electrolyte Temperature on Ta2O5 NTs Formation. Ta2O5 NTs were obtained by anodization of Ta metal discs in an H2SO4 + 1 vol % HF + 4 vol % H2O electrolyte at 50 V (∼25 °C), as reported in a recent work.36 SEM images show that anodization of Ta metal for 20 min resulted in vertically aligned and cracked Ta2O5 NTs with lengths of ∼2.5 μm (see Supporting Information, Figure S1). These results are in accordance with previously reported data.36 Furthermore, in some areas, the NTs spontaneously detached from the Ta substrate, leaving printed dimples on the Ta metal. Thus, anodization experiments were carried out under different electrolyte temperatures in order to study the adherence of Ta2O5 NTs to the substrate. Figure 1a shows the current−time behavior during the potentiostatic anodization at different electrolyte temperatures.

Crystalline Structure. X-ray diffraction (XRD) of the freestanding Ta2O5 NTs was recorded using a Philips X’PERT diffractometer with Cu Kα radiation (λ = 1.54 Å) at a 2θ range from 5° to 100° with a 0.02° step size and measuring time of 5 s per step and complementary glancing angle X-ray diffraction (GAXRD) were recorded at the Brazilian Synchrotron Light Laboratory (LNLS) using the XRD2 beamline with λ = 1.50 Å. Crystal structure analyses of Ta2O5 NTs and standard, after thermal treatment, was performed by Rietveld profile refinement using Fullprof software.56 The degree of crystallinity (XC) of the NTs annealed at different temperatures was determined from the ratio of the integral intensity of the crystalline contribution to the total intensity.57 Chemical Composition. X-ray photoelectron spectroscopy (XPS) spectra were obtained using conventional equipment equipped with a high-performance hemispheric SPECSLAB II (Phoibos-Hs 3500 150 analyzer, SPECS, 9 channels) energy analyzer and a nonmonochromatic Al Kα (hν = 1486.6 eV) radiation as the excitation source at the LNLS. The operating pressure in the ultrahigh vacuum chamber (UHV) during analysis was 10−7 Pa. Energy steps of 50 and 20 eV were used for the survey and single-element spectra, respectively. The position of the C 1s signal corresponding to C−C/C−H was used for energy calibration by setting the energy value at 285.0 eV. The envelopes were analyzed and peak fitted after subtraction of the Shirley background using Gaussian− Lorenzian peak shapes obtained from the CasaXPS software package. Specific Surface Area. Measurements of the specific surface area (SBET) were performed according to the Brunauer− Emmett−Teller method (BET) using nitrogen absorption isotherms with a Micromeritics TriStar II 3020 apparatus. Samples were degassed at 150 °C overnight on a vacuum line prior to nitrogen adsorption measurements. Photocatalytic Activity Measurements. Photogeneration of hydrogen by water splitting was carried out in a doublewall quartz photochemical reactor using in all cases 8 mg of the photocatalyst suspended by magnetic stirring in 8 mL of ethanol/water solution (23.8%). The temperature of the reaction system was maintained constant at 25 °C by water circulation through the photochemical reactor using a thermostatic bath. Prior to irradiation, the system was deaerated by bubbling Ar/vacuum for 30 min to remove any other gases. A 240 W Hg−Xe lamp (Cermax) was used as the excitation source. The gaseous products of the photocatalytic reaction were quantified by gas chromatography at room temperature on an Agilent 6820 GC chromatograph. Gases H2 and CO, CO2, CH4, and C2H4 were analyzed simultaneously with a thermal conductivity detector (TCD) and a flame ionization detector (FID), respectively. Argon was used as the carrier gas. Using a gastight syringe with a maximum volume of 100 μL, the amount of gases produced was measured in 0.5 h intervals.

Figure 1. (a) Current density curves for anodization of Ta discs at 50 V at different electrolyte temperatures for 1200 s, and (b) effect of electrolyte temperature on the outer diameter and length of the NTs.

Initially, at all temperatures the current density curves show a sharp decrease due to formation of a compact Ta oxide barrier film, similar to that already observed in Ti anodization in fluoride media.22 After 250 s of anodization at 0, 10, 20, and 30 °C, the current density curves remained constant at ∼1, 3, 5, and 7 mA·cm−2, respectively. Additionally, at electrolyte temperatures of 40 and 50 °C, the current density curves increased with the increase in temperature and remained constant at higher current densities (30 and 50 mA·cm−2 respectively). Figure 1b shows the effect of electrolyte temperature on the Ta2O5 NT dimensions. The NT lengths gradually increased from 1.3 to 4.6 μm when the electrolyte temperature increased from 0 to 50 °C (see Supporting Information, Figure S2). However, an increase in the electrolyte temperature from 0 to 50 °C resulted in a decrease in the NT diameter from 143 to 90 nm; on the other hand, the wall thickness under all anodization conditions was 30 nm. Anodization at higher electrolyte temperatures also reduced the adherence of the Ta2O5 NT arrays on the Ta substrate.

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RESULTS The strategy used in this work to evaluate the effects of the crystallinity of Ta2O5 in H2 photoproduction was to crystallize freestanding NTs at different temperatures and times. Moreover, to obtain the powdered form of the nanotubes, it was first necessary to optimize the anodization conditions to easily detach the anodic films adhered onto the Ta metal. The photocatalytic activity of the obtained NTs was compared with commercial powdered Ta2O5 nanoparticles provided by Sigma Aldrich. 14024



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Figure 2. (First line) Optical images of Ta2O5 NTs prepared by anodization at 50 V showing the dependence of the adherence of the NTs to the substrate on the electrolyte temperature. (Second line) Adhesive tape test on Ta2O5 NTs.

of thermal treatment in air at different temperatures and times was performed. It was observed that all freestanding Ta2O5 NTs annealed below 750 °C were amorphous (Figure 4) and started to crystallize above 750 °C in an orthorhombic phase (JPCDS file 25-0922). Actually, three peaks began to appear at 22.94°, 28.48°, and 36.45° after annealing at 750 °C for 30 min, indicating that crystallization of the orthorhombic phase started at this temperature. Annealing of NT samples above 800 °C, i.e., 900 °C or more, destroyed the nanotubular shape, and for this reason, these samples were not considered in the present study. Figure 4 also shows the Rietveld refinements of the XRD patterns of the as-prepared NTs, the NTs annealed for 0.5 h at 550, 750, and 800 °C, NTs annealed for 1 h at 800 °C, and an SP purchased from Sigma-Aldrich. Rietveld refinements showed that the grain size of the sample treated for 30 min at 800 °C was 15.5 nm, while the samples annealed for 1 h were 15.4 nm, indicating that the medium grain size of the Ta2O5 NTs did not change the crystallite size by the thermal treatment periods used here. Rietveld refinements used a preferred orientation (Ph) using the Modified March’s function (eq 1)58,59

Figure 2 shows optical microscope images of samples anodized at different bath temperatures. At 0, 10, and 20 °C, the NTs surfaces were smooth and well adhered to the Ta substrate. When the electrolyte temperature increased to 30 °C, the NTs started to detach from the Ta substrate; see Figure 2 (red arrows on top images). Detachment of Ta2O5 NTs was clearly observed at 40 and 50 °C; see Figure 2 (first line). The second line of Figure 2 shows the NTs that were totally free from the Ta substrate and could be easily removed with the use of adhesive tape. These effects were not observed in the samples anodized at 0, 10, and 20 °C (see the green circles, bottom line of Figure 2). Thus, by systematically controlling the anodization conditions, the Ta2O5 NTs could be fully released from the Ta substrate, resulting in a white powder composed of freestanding Ta2O5 NTs with the same morphological characteristics as those observed when they were attached to the Ta substrate (Figure 3a and 3b). The freestanding NTs production rate was

−3/2 ⎛ sin 2 αh ⎞ ⎟ Ph = G2 + (1 − G2)⎜(G1 cos αh)2 + G1 ⎠ ⎝

(1)

where G1 corresponds to the Bragg−Brentano geometry, G2 represents the fraction of the sample without orientation, and αh is the acute angle between the scattering vector and the normal to the crystallites. In addition, the ratio of the crystalline/amorphous phases was determined by the area ratio method.57 The crystallinity was greatly increased from 11% to 34% by increasing the crystallization time from 0.5 to 1 h. SEM and TEM Microscopied. Figure 5 shows SEM and TEM images of the freestanding Ta2O5 NTs annealed at different temperatures. SEM and TEM analyses show that the samples annealed at 800 °C maintained the initial NT morphology without suffering any kind of collapse or structural deformation. Chemical Surface. XPS spectroscopy was used to analyze the surface chemistry of the prepared freestanding Ta2O5 NTs

Figure 3. (a) Optical and (b) SEM images of freestanding Ta2O5 NTs prepared by anodization at 50 V with the anodization electrolyte at 50 °C.

15.8 mg·h−1·cm−2 at an electrolyte anodization temperature of 50 °C. Under these conditions, the lower the electrolyte temperature, the better the adherence of Ta2O5 NTs to the Ta substrate. In this way, it is possible to achieve both freestanding and well-adhered Ta2O5 NTs just by choosing the appropriate electrolyte temperature during the anodization process. Structural and Chemical Surface Analysis. Crystalline Structure. In order to evaluate the crystalline behavior, freestanding Ta2O5 NTs were prepared by anodization at 50 V at an electrolyte temperature of 50 °C. To understand the crystallization process of the freestanding Ta2O5 NTs, a study 14025



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Figure 4. XRD diffractograms of the as-anodized and annealed freestanding Ta2O5 NTs samples. NTs were prepared at 50 V and an electrolyte temperature of 50 °C and then annealed at temperatures from 550 to 800 °C for different periods, except the top diffractogram that shows the SP sample.

as well as standard Ta2O5 NPs (SP). Survey XPS data are shown in Figure 6, and quantitative results are summarized in Table 1. Sputtering of the samples was not carried out, and in consequence, the C1s signal originating from carbon-containing contamination (adventitious carbon) was always present in the survey XPS spectra. It has already been shown that morphological and chemical evolution of the surface during low-energy ion sputtering is rather a complex issue for Ta2O5.60 It can be observed that, depending on the annealing temperature, the O/Ta ratio departed from the theoretical value of 2.5 for Ta2O5 by ±16%. At temperatures lower than 750 °C, the contribution of highly oxidized S2p mainly increased the intensity signal of O1s. When the temperature increased, the amount of S2p decreased together with an increase in the O/Ta ratio approaching the 2.5 theoretical ratio. At temperatures above 800 °C, a defect in the oxygen concentration at the surface was observed. Photocatalytic Activity. Table 1 summarizes the photocatalytic rate for hydrogen production by the prepared NTs and of an SP sample in UV-irradiated aqueous ethanol solutions. As can be seen in entries 2 and 8, use of ethanol as a sacrificial oxidant enhanced the hydrogen evolution rate by 12.5 times, justifying its use. Additionally, the photodecomposition of ethanol under UV light exposure evolved just 5.4 μmol·h−1 of H2, at a much lower production rate (entry 1), when compared with all experiments using Ta2O5 as a photocatalyst (entries 3−

Figure 5. SEM and TEM images of the freestanding Ta2O5 NTs prepared at a potential of 50 V with the anodization electrolyte at 50 °C. All samples were annealed at different temperatures for 0.5 h, except the bottom image that shows Ta2O5 annealed for 1 h at 800 °C.

8). Furthermore, all Ta2O5 NTs photocatalysts were more active than the crystalline SP Ta2O5 (entries 3−8), even the amorphous as-anodized sample (entry 4). Table 1 also depicts a clear trend in H2 photoproduction using nanotubular Ta2O5. An increase in photocatalytic activity as a function of the thermal treatment conditions could be observed. The H2 photoproduction rate gradually increased from 2600 μmol·h−1·g−1 with the amorphous NTs sample (entry 4) to 4900 μmol·h−1·g−1 with the NTs sample annealed at 800 °C for 1 h (entry 8). Interestingly, during the experiments, several other products were observed on the obtained chromatograms, allowing quantification of the carbon-containing gaseous products of ethanol photoreforming. In the performed experiments, it was possible to observe carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), ethylene (C2H4), and ethane (C2H6). The evolution rates of these compounds were also observed during photolysis of ethanol/water mixtures (entries 3−8). Analyzing Table 1, it is possible to observe that these products 14026



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8). Actually, the evolution rate of the minor products seemed to decay with an increase in annealing temperature and then increased again for the most active sample in this study (entry 8). The SP sample also produced all five side products (entry 3) but at much lower rates than all the nanotubular photocatalysts. However, this was easily understandable since it was shown to have the lowest hydrogen evolution activity of all the tested Ta2O5 samples.

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DISCUSSION

Optimization of NTs released from tantalum foils revealed interesting features of the dynamics of the anodization process. An increase in electrolyte temperature considerably increased the anodization current densities and length of the Ta2O5 nanotubes. This effect was also previously observed in anodization performed under isothermic conditions with a relatively higher HF concentration at the electrolyte.49 The gradual increase in the current density with the increase in temperature may be due to the higher ionic conductivity of the electrolytes and pitting of oxide by fluoride ions. 61 Furthermore, higher electrolyte temperatures also decreased the adherence of the formed NTs films over Ta metal. This effect may be related to the high stress forces generated between the anodic film and the metal substrate during more rapid growth of the anodic film at higher temperatures. An increase in electrolyte temperature also decreased the external diameter of the nanotubes. Recently, the effect of electrolyte temperature on the diameter of anodic nanotubes has been studied for formation of Fe2O332 and TiO262 NTs in organic and aqueous electrolytes. It was shown that, in aqueous electrolytes, lowering the temperature leads to a decrease in the inner diameter of TiO2 NTs while maintaining the same outer diameter.63 For anodization in organic electrolytes, e.g., EG, DMSO, and glycerol, it has been shown for Fe2O332 and TiO262 that there is a direct relationship between the electrolyte temperature and the NT diameter. In both cases, the NT diameter increased with increasing electrolyte temperature and retained a wall thickness of 30 nm, independent of the electrolyte temperature. It seems that the predominant effect is related to the mobility of fluorine ions which can be governed by the characteristics of the electrolyte solvent.63 However, the results found herein for Ta2O5 follow the opposite trend to what was observed in the previous studies. One possible explanation for the Ta2O5 NT diameter decrease and length

Figure 6. Survey XPS data of the freestanding Ta2O5 NTs prepared at a potential of 50 V with the anodization electrolyte at 50 °C. All samples were annealed at different temperatures for 0.5 h, except the top spectrum that shows Ta2O5 annealed for 1 h at 800 °C.

were formed at much lower velocities, ∼100 less, than hydrogen. It was not possible to observe any kind of trend on the evolution rates of CO2, CO, and CH4 in the photolysis experiments performed with the different samples. However, the photoproduction rates of C2H4 and C2H6 seemed to be dependent on the photocatalyst used. In the blank reactions and without ethanol (entries 1 and 2), the CO, CO2, CH4, C2H4, and C2H6 residual gases were only detected in trace amounts. Comparison of the evolution velocities of C2H4 and C2H6 of each photocatalyst is a hard task, since it would be expected that a more active catalyst for hydrogen evolution would produce these compounds at higher rates. It could be observed that the less active nanotubular photocatalyst, i.e., the as-anodized sample (entry 4), produced both ethane and ethylene at rates higher than all other NTs samples (entries 5−

Table 1. Ethanol Photoreform Product Generation Rates by UV Irradiation in a 23.8% (v/v) Ethanol/Water Solution at 25 °Ca gas production rate (μmol·h−1·g−1) entry

catalytic system

1 2 3 4 5 6 7 8

no photocatalystb no ethanol SP Ta2O5 NTs

annealing temperature (°C) 800c as-anodized 550 750 800 800d

H2 5.4 390 830 2600 2900 3200 3100 4900

± ± ± ± ± ± ± ±

C2H6 0.8 125 15 120 66 260 230 320

2.5 53 61 31 28 51

± ± ± ± ± ±

0.1 5.1 15 11 2.7 2.7

C2H4

0.8 23 10 1.6 2.9 6.3

± ± ± ± ± ±

0.3 2.2 4.0 0.2 2.1 3.3

CH4

6.5 26 28 27 26 42

± ± ± ± ± ±

0.1 2.2 1.9 2.2 2.2 0.9

CO

3.1 12 4.2 8.8 7.8 9.5

± ± ± ± ± ±

0.6 2.1 1.1 1.3 1.3 1.3

CO2

3.8 9.7 6.7 7.3 6.8 11

± ± ± ± ± ±

0.9 2.3 0.5 1.3 1.1 1.2

The freestanding Ta2O5 NTs were prepared at an anodization electrolyte temperature of 50 °C and annealed at different temperatures. bBlank reaction, only water and ethanol solution. cThe photocatalytic reaction for hydrogen production was performed in pure water solution without ethanol as the sacrificial reagent using a photocatalyst that was annealed for 1 h at 800 °C in an air atmosphere. dThe photocatalyst was annealed for 1 h at 800 °C in an air atmosphere. a

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Table 2. Chemical-Physical Characterization of Freestanding Ta2O5 NTs Synthesized under Different Thermal Treatment Conditionsa chemical surface composition (%) samples SP Ta2O5 NTs

annealing temperature (°C) as anodizedb 550 750 800 800c

XC (%)

CS (nm)

SBET (m2·g−1)

PD (nm)

PV (cm3·g−1)

Ta4f

O1s

C1s

S2p

O/ Ta

37

49.5

4.7 16.2 15.0 13.4 13.2 18.2

164 221 223 333 248 314

0.0162 0.089 0.082 0.094 0.080 0.142

22 16 16 17 19 21

44 47 45 43 43 46

34 26 35 37 38 33

6 4 3 >0.5

2.0 2.9 2.8 2.5 2.3 2.2

2 11 34

15.5 15.4

a

A comparison with the SP sample is also included. XC = crystalline proportion, CS = crystal size, SBET = specific surface area, PD = pore diameter, and PV = pore volume. bThe as-anodized sample has an amount of 5% of F1s on the Ta2O5 surface, while the annealed photocatalyst does not have F on the surface. cThe photocatalyst was annealed for 1 h at 800 °C in an air atmosphere.

NT structure may explain the differences in the photocatalytic activities shown in Table 1. Crystallization and removal of weakly bonded species at the NT surface seemed to have a positive influence on the photocatalytic activity for hydrogen production. As can be seen in Table 1, the increase in hydrogen photoproduction followed the same trends observed for the increase in crystallinity and loss of surface sulfur shown in Table 2. An increase in the surface area due to thermal treatment could be claimed to explain the observed results. However, what the results show does not seem likely. Actually, annealing the samples for 0.5 h at different temperatures showed that an increase in temperature can decrease the surface area of the tubes (Table 2). The only sample for which the surface area was increased by thermal treatment was the one that was submitted to 800 °C for 1 h; however, this did not seem to have a predominant effect on the increased rate of H2 evolution. The surface area and photocatalytic activity of hydrogen production of this sample were 1.4 and 1.6 times higher, respectively, than the values observed for the NTs treated at 800 °C for 0.5 h. The influence of the crystalline structure and/or surface contamination also has an effect on formation of side products of the reaction, especially C2H4 and C2H6. It should be pointed out, to the best of our knowledge, that this was the first time that these side products have been found for ethanol photoreforming using Ta2O5 semiconductors. Evolution of C2H4 and C2H6 is somewhat related to the crystallinity and/or surface sulfur content. As can be seen in Figure 7a and 7b, the

increase with higher anodization electrolyte temperatures may be related to the kinetics (oxidation/dissolution) of the initial tantalum oxide formation.61 In the case presented herein, the oxide formation velocity increased with electrolyte temperature. In the initial stages, highly dense pits were formed at the metal surface. As the electric field action in the pits is preferred, it will increase the pit diameter, leading to NTs. With increasing anodization time, the NT size increases and the size is limited by adjacent NT walls, similar to what was observed for alumina nanopores obtained by Al anodization.64 Additionally, as the electrolyte solvent plays a crucial role on the anodization kinetics, the highly acidic nature of H2SO4 may change the oxidation/dissolution behavior of the anodization process performed in this study. Anodization of Ta foils at 50 °C was beneficial for obtaining freestanding Ta2O5 NTs. This produced greater amounts of tubes due to the higher current densities and also facilitated formation of anodic films from the Ta metal. This powdered form of Ta 2 O5 NTs is amorphous; to increase their photocatalytic activity, samples should be crystallized. The thermal treatments to the samples were carefully performed to increase the crystallinity of the tubes without collapsing the form, thus preserving the nanotubular shape of the catalysts. It was observed that NTs annealed at temperatures around 900 °C destroyed the tube walls (results not shown). Therefore, the samples were annealed up to 800 °C. The thermal treatments carried out to increase NT crystallinity completely changed the physicochemical properties of both the bulk and the surface of the Ta2O5 NTs. The as-anodized NT samples were amorphous and had the highest sulfur surface content (6%) of all NT materials (Table 2). As the thermal treatment temperature increased, the S content of the surface started to decease until it was completely removed by heating the NTs at 800 °C for 1 h (Table 2). Energy-dispersive X-ray spectroscopy (EDX) also supports these results (see Supporting Information; Table S1), showing that the sulfur concentration decreased on the surface and in the bulk of the NTs when the annealing temperature increased. The NTs annealed below 750 °C for 0.5 h were completely amorphous (∼2% crystalline) at this threshold temperature. A further increase in temperature to 800 °C yielded NTs with crystallite sizes around 15 nm, and extending the annealing time to 1 h enhanced the crystallinity of the NTs from 11% to 34%. Moreover, the physicochemical changes to Ta2O5 had dramatic influences on the photocatalytic activities of these semiconductors. The effect of the thermal treatments on the

Figure 7. Final percentages of the gaseous-phase composition of the photoreformation of an ethanol/water mixture using different photocatalysts catalyzed by (a) H2 and (b) CH4, C2H4, C2H6, CO, and CO2. 14028



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final percentage composition of the gaseous phase of the reaction catalyzed by the amorphous catalyst with a high surface sulfur composition had relatively higher amounts of C2H6 and C2H4 than the crystalline and clean catalysts. This behavior can also be extended to the SP reaction. The decrease in formation of these compounds was followed by an increase in the final hydrogen concentration (Figure 7a). Thus, controlling the annealing temperature allows production of crystalline and less contaminated semiconductors that are more active and selective for formation of gaseous hydrogen under UV illumination. One last aspect of the results of this study is that the higher apparent photocatalytic activity of the 800 °C, 1 h Ta2O5 NT catalysts compared to SP in terms of hydrogen production should be emphasized. Both materials have the same crystallinity and no sulfur on the surface and differ only by the crystal size, shape, and surface area. Thus, the activity can be related to the surface area, crystal size, and shape as shown by Kudo et al.65 The nanotubes produced H2 5.9 times faster than SP, and the surface area of the nanotubes was only 3.9 times higher than that of SP, also indicating that the surface area is not the only factor that enhances photocatalytic activity. These results led us to think that the nanotubular shape of the semiconductor is a key factor for the high activity of the photocatalysts produced in this work. Studies focused on photocatalyst shapes and their effects on photocatalytic activity are in progress and will be published soon.

Article

ASSOCIATED CONTENT

S Supporting Information *

Details of SEM images of Ta2O5 NTs prepared by anodization at 50 V with different electrolyte temperatures and sulfur concentration percent measured by energy-dispersive X-ray spectroscopy (EDX) to compare crystallization of the freestanding Ta2O5 NTs after different annealing temperatures in air atmosphere for 30 min. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: aff[email protected] (A.F.F.) and [email protected] (S.R.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially sponsored by CNPq (no. 471220/ 2010-8), FAPERGS (no. 11/2000-4), CAPES (Brazilian funding agencies), P&D ANEEL-CEEE GT (no. 9945481). Thanks also to the “Centro de Microscopia Eletrônica (CME) of the Universidade Federal do Rio Grande do Sul” and the National Synchrotron Light Laboratory (LNLS) for the use of their XRD2 beamline under proposal no. 10835 and conventional XPS equipment. The authors also thank the “Laboratório de Altas Pressões do IF-UFRGS” and Mr. Otelo J. Machado for XRD powder analysis.

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CONCLUSIONS Freestanding Ta2O5 NTs synthesis was optimized by adjusting the electrolyte temperature during anodization in an H2SO4, H2O, and HF solution at 50 V for 20 min. The results show that when the electrolyte temperature is 50 °C, the adherence of NTs is low enough to easily remove the NTs from the Ta substrate. This detachment is probably related to the high anodic film growth rate observed at 50 °C, which generated high interface stress forces between the Ta metal and Ta2O5. Furthermore, the NT diameters gradually decreased from 145 to 90 nm by increasing the electrolyte temperature from 0 to 50 °C. The effect of thermal treatment on the crystalline structure and morphology of Ta2O5 NTs showed that the NTs retained their tubular shape up to 800 °C. Similarly, annealing of the freestanding Ta2O5 NTs below 750 °C resulted in a mainly amorphous structure. The orthorhombic crystalline phase appeared only in Ta2O5 NTs annealed at temperatures higher than 750 °C. In addition, it was shown that the crystallinity of the NTs was enhanced from 11% to 34% by increasing the treatment time of the NTs at 800 °C from 0.5 to 1 h. The surface sulfur content also decreased with increasing annealing temperature and time. High crystallinity and low surface contamination increased the photocatalytic activity of the freestanding NTs for hydrogen production by water splitting. The sample annealed at 800 °C at 1 h was the most active of all the photocatalysts tested. Moreover, changes to the surface and bulk NT physicochemical properties modified the photoreforming side product selectivity and simultaneously increased the selectivity for hydrogen photogeneration. The amorphous and sulfur-contaminated samples produced more C2H4 and C2H6 than the crystalline and sulfur-free catalysts. To the best of our knowledge, this is the first time that these side products have been assessed in ethanol photoreforming using Ta2O5 semiconductors.

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Effect of Thermal Treatment on the Photocatalytic Activity for Hydrogen

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