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Effect of Oxygen Content on the Photoelectrochemical Activity of Crystallographically Preferred Oriented Porous Ta3N5 Nanotubes Sherdil Khan,† Maximiliano J. M. Zapata,† Daniel L. Baptista,† Renato V. Gonçalves,‡ Jesum A. Fernandes,∥ Jairton Dupont,⊥ Marcos J. L. Santos,§ and Sérgio R. Teixeira*,† †

Instituto de Física, UFRGS, Av Bento Goncalves 9500 PO Box -15051 91501-970, POA-RS, Brazil Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, São Carlos 13560−970, SP Brazil § Instituto de Quimica, UFRGS, Av Bento Goncalves 9500 PO Box -15051 91501-970, POA-RS, Brazil ∥ Department of Chemistry, University of Victoria, Victoria, BC V8W 3 V6, Canada ⊥ School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, U.K. ‡

S Supporting Information *

ABSTRACT: Crystallographically preferred oriented porous Ta3N5 nanotubes (NTs) were synthesized by thermal nitridation of vertically oriented, thick-walled Ta2O5 NTs, strongly adhered to the substrate. The adherence on the substrate and the wall thickness of the Ta2O5 NTs were finetuned by anodization, thereby helping to preserve their tubular morphology for nitridation at higher temperatures. Samples were studied by scanning electron microscopy, high-resolution electron microscopy, X-ray diffraction, Rietveld refinements, ultraviolet−visible spectrophotometry, X-ray photoelectron spectroscopy, photoluminescence spectra, and electrochemical techniques. Oxygen content in the structure of porous Ta3N5 NTs strongly influenced their photoelectrochemical activity. Structural analyses revealed that the nitridation temperature has crystallographically controlled the preferential orientation along the (110) direction, reduced the oxygen content in the crystalline structure and the tubular matrix, and increased the grain size. The preferred oriented porous Ta3N5 NTs optimized by the nitridation temperature presented an enhanced photocurrent of 7.4 mA cm−2 at 1.23 V vs RHE under AM 1.5 (1 Sun) illumination. Hydrogen production was evaluated by gas chromatography, resulting in 32.8 μmol of H2 in 1 h from the pristine porous Ta3N5 NTs. Electrochemical impedance spectroscopy has shown an effect of nitridation temperature on the interfacial charge transport resistance at the semiconductor−liquid interface; however, the flat band of Ta3N5 NTs remained unchanged. composed of a single crystal or preferentially oriented grains.18 In the case of Ta3N5, there have been few reports on the syntheses of NTs applied in photoelectrochemical water splitting.2,10 Thus, the synthesis of single-crystal-like or preferentially oriented pristine Ta3N5 NTs is highly desirable. Previously, we have shown that the tubular morphology of Ta2O5 NTs annealed under air atmosphere remained preserved until 800 °C, and the NTs remained amorphous.19 For higher temperatures and longer thermal treatments, the NTs collapsed.20 Therefore, a challenge remains to synthesize Ta3N5 NTs preserving the tubular morphology at higher temperatures and presenting improved crystallinity. Recently, structural and PEC properties of Ta3N5 thin films have been studied as a function of nitridation temperature for a range of 600−800 °C.21 However, because of the limitation of morphological preservation at higher temperature, such study has never been attempted for Ta3N5 NTs once single

1. INTRODUCTION Over the past several decades, the generation of renewable energy has been a hot research topic of growing interest, with solar energy being one of the main renewable resources available on earth. Solar photoelectrochemical (PEC) water splitting has been regarded as one of the most promising directions.1,2,3 Great efforts have been made in the development of new materials for efficient photocatalysts.4−6 Recently, tantalum nitride (Ta3N5) has received increasing attention because of its near optimal band structure.7−9 To enhance photocatalytic performance, several Ta3N5 nanostructures such as nanotubes (NTs),2,10 nanoparticles,11 and nanorods12,13 have been employed for water-splitting reactions. In the context of improved charge transportation properties, nanostructures, particularly the highly oriented one-dimensional nanotubular architectures of several semiconductors, e.g., TiO2, have already proven to be promising candidates for catalysis, photovoltaic, and PEC devices.14,15 Nevertheless, in some cases the grain boundaries in the NTs hindered efficient charge transportation, resulting in poor solar driven energy conversion.16,17 Therefore, for efficient charge transportation, the NTs should be © 2015 American Chemical Society

Received: June 8, 2015 Revised: July 30, 2015 Published: July 31, 2015 19906

DOI: 10.1021/acs.jpcc.5b05475 J. Phys. Chem. C 2015, 119, 19906−19914

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The Journal of Physical Chemistry C

Figure 1. (a) SEM and (b) TEM images of as-anodized Ta2O5 NTs.

air followed by nitridation at 900 °C for 3 h to convert it into a Ta3N5 film. 2.2. Characterization. The samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), Rietveld refinements, X-ray photoelectron spectroscopy (XPS), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), Mott−Schottky curves, and photoluminescence (PL) spectra. More details on experimental methods, characterizations, and computation are available in Section S1 of Supporting Information. 2.3. Photoelectrochemical Studies. The photoelectrochemical measurements were performed in a three-electrode configuration employing Ta3N5 NTs as the working electrode, Pt wobbling electrode as the counter electrode, and Ag/AgCl as a reference electrode. For all of the PEC measurements, aqueous solution of 0.1 M K4[Fe(CN)6 (Merck) and 0.1 mM K3[Fe(CN)6] (Merck) with a pH of 7.5 was used. An AM 1.5 filter was used to simulate the solar spectrum. The light intensity was calibrated using a silicon photodiode with a known responsivity of 100 mW cm−2 (1 Sun). For Nyquist plots, a fixed potential of 0.2 V vs Ag/AgCl (0.84 V vs RHE) was applied and scanned over the frequency range of 100 kHz to 100 mHz with an amplitude of 5 mV. Moreover, Mott− Schottky plots were obtained under a frequency of 500 Hz at 5 mV of amplitude for a range of the applied potentials. The measured potential versus the Ag/AgCl was converted to the RHE scale according to the following relation:7

temperatures have been applied for the syntheses.2,10 Thus, synthesizing Ta3N5 NTs at higher nitridation temperature preserving the tubular morphology and studying the effect of nitridation temperature on structural, surface, and PEC properties are highly warranted. In the crystalline structure of Ta3N5, oxygen has been found to be a natural impurity that cannot be eliminated completely. It helps improve the mechanical and atomic cohesion properties of Ta3N5.22 However, excessive oxygen content may act as recombination centers for photogenerated carriers.23 Therefore, for an enhanced PEC water-splitting activity, the oxygen content in the Ta3N5 NTs should be controlled. In this work, we describe the synthesis of porous Ta3N5 NTs by preserving tubular morphology at high temperatures, presenting a preferred orientation along the (110) direction and possessing a lower oxygen content. The nitridation was carried out for a range of temperatures (800−1000 °C). The porous NTs were found to demonstrate enhanced PEC watersplitting activity compared to the previously studied nanostructured Ta3N5.24 Detailed morphological, structural, surface, and electrochemical characterizations are presented to clarify the mechanism of the enhanced PEC activity of the photoanode.

2. EXPERIMENTAL SECTION 2.1. Preparation of Ta3N5 NTs. The Ta3N5 NTs were synthesized starting from Ta2O5 NTs that were prepared by anodization. The anodization was carried out using a previously developed method,19 employing a two-electrode configuration (Ta foil as anode and Cu disk as cathode) in an electrolyte consisting of a mixed solution of H2SO4 (Lab-Synth Products Laboratory LTD, 98.0%) and 1 vol % of HF and 4 vol % of distilled water. A DC voltage of 50 V was applied for 20 min by an initial ramping of 10 V/s at an electrolyte temperature of 10 °C. To synthesize Ta3N5 NTs by thermal nitridation, the Ta2O5 NTs were placed on an alumina boat and then inserted into a horizontal quartz tube furnace. The as-anodized Ta2O5 NTs were nitrided at 800, 850, 900, and 1000 °C under 100 mL/min of gas flux consisting of a special mixture of ammonia:argon (1:9 vol/vol) for 3 h and labeled as NTs-800, NTs-850, NTs900, and NTs-1000, respectively. The heating and cooling rate of the furnace were maintained at 5 °C/min. For comparison, a Ta3N5 film was also prepared via a twostep process: first, Ta2O5 was grown by heating a similar Ta foil (without electrochemical anodization) at 500 °C for 20 min in

VRHE = VAg/AgCl + (0.059) × pH + 0.197 V

For gas chromatography, an Agilent 6820 GC instrument was used. H2 was analyzed with a thermal conductivity detector (TCD), and argon was used as the carrier gas. For the GC experiment, an airtight three-electrode cell was used. Before measurements, the PEC cell was pumped to a small pressure (due to the low vapor pressure of water) and then backfilled with Ar. Using a gastight syringe at the Pt counter electrode compartment, a maximum volume of 50 μL of the gas was taken. The amount of H2 produced was measured for 1 h under AM 1.5 (1 Sun) illumination.

3. RESULTS AND DISCUSSION 3.1. Morphology. Figure 1 displays the morphology of the as-anodized Ta2O5 NTs presenting the vertically oriented and 19907


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Figure 2. SEM and TEM images of the Ta3N5 NTs: (a) NTs-800, (b) NTs 850, and (c) NTs-900.

smooth-walled features. The nanotube grew to a length of approximately 1.60 ± 0.03 μm with an average outer diameter and wall thickness of 130 ± 4 nm and 34 ± 2 nm, respectively. According to our previous results, the adherence of the NTs on the substrate obtained from anodization conducted in H2SO4/HF electrolyte was controlled by the temperature of the electrolyte.19 Decreasing the temperature increases the adherence of the Ta2O5 NTs onto the Ta substrate; it also increases the wall thickness of the NTs. From previous studies on Ta3N5 nanorods and TiO2 NTs, the adherence on the substrate and the wall thickness have been found to be key factors for sustaining the morphology at high temperatures and improving PEC activity.12,25 Considering these results, one might expect that well-adhered and thick-walled Ta2O5 NTs that are anodized at lower temperature are more likely to preserve the tubular morphology during transformation to Ta3N5 NTs, at high nitridation temperatures, as well as to improve PEC activity. Figure 2 displays the SEM and TEM images of the samples after thermal nitridation. An interesting result is that even at high temperatures the tubular morphology is clearly preserved, although the walls of the nanotubes became rough and porous. In addition, after nitridation, the dimensions of the samples were reduced. The NTs were shortened from 1.6 ± 0.03 μm (as-anodized) to approximately 1.4 ± 0.05, 1.25 ± 0.06, 1.1 ± 0.07 μm for NTs-800, NTs-850 and NTs-900, respectively. For NTs-1000, the morphology was not completely tubular once agglomeration was observed on the top and NTs appeared as a mosaic of interconnected rings (Figure S1). Therefore, we have excluded NTs-1000 from further characterizations. However, for the sake of comparison, we have used NTs-1000 for PEC measurements. The details of the geometrical parameters for tubular samples are presented in Table S1. The nitridation process replaces N3− for O2− by keeping the Ta5+ in valence state and two N3− substitute three O2− anions, resulting in a natural change in density of the compounds. The density of Ta3N5 has been reported as 9.85 g/ cm,26 whereas that for Ta2O5 as 8.18 g/cm;27 therefore, a volume shrinkage of 22.2% could possibly occur simultaneously during the nitridation process.11 Unlike commercial precursor powders which are crystalline in nature and difficult to nitride,28 we have used amorphous NTs which are more likely to shrink during crystallization.29 Therefore, we consider that porosity and shrinkage of the Ta3N5 NTs at higher temperatures is simultaneously related to the density difference of Ta3N5 and the Ta2O5 with the amorphous nature of the precursor NTs. 3.2. Crystalline Structure of Ta3N5 NTs. The crystallographic structures of the Ta3N5 NTs were determined by X-ray diffraction employing Rietveld refinement (Tables 1 and S2). Figure 3 shows that all of the peaks match with the

Table 1. Grain Size, Lattice Strain, and Stoichiometry Obtained from the Rietveld Refinements sample

size (nm) along (110)

strain (× 10−4)

stoichiometry

NTs-800 NTs-850 NTs-900

21.24 ± 2.4 26.4 ± 2.0 48.7 ± 2.7

13.85 ± 0.05 32.43 ± 0.02 36.90 ± 0.03

Ta2.89N4.18O0.84 Ta2.98N4.29O0.48 Ta2.99N4.61O0.22

Figure 3. Rietveld refinement profile for porous Ta3N5 NTs. •, observed Yobs; red solid lines, calculated Ycalc; blue solid lines, Yobs − Ycalc difference of phase.

orthorhombic phase of Ta3N5 (JCPDS card no.: 79-1533). An important feature of the XRD patterns is the increment in the relative peak intensity at 2θ degree of 24.4°, corresponding 19908


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narrowing of the bandgap. The optical bandgaps of the porous Ta3N5 NTs were found to range between 2.1−2.2 eV, which is consistent with the literature.2,11,31,32 The valence and conduction bands of Ta3N5 have been attributed to the N 2p and Ta 5d orbitals, respectively.8,33 Nevertheless, the occupied valence band and the vacant conduction band determine the compound color, which is red for the samples synthesized in the current study; this agrees with previous reports.34,35 Figure 4 shows three transitions in the absorption spectra of the porous Ta3N5 NTs. One broad absorption from 800 nm to ca. 600 nm (transition A), the main transition from 600 nm to ca. 540 nm, and a shoulder above 540 nm (transition B). The presence of the broad absorption (transition A) has been described as a formation of surface defects.36−38 Transition B was earlier observed in the literature; however, its origin was not explained.11,36,39,40 An interesting feature in the absorption spectra (Figure 4) is the dependence of the intensity of transition B with temperature. As observed in the analyses of Rietveld refinements (Table 1), the oxygen content in porous Ta3N5 NTs decreases with the nitridation temperature. In addition, further analyses by spatial-localized EDX and PL spectroscopy (discussed further below) have confirmed the same dependency of oxygen content with the nitridation temperature. Therefore, these results strongly suggest that transition B is related to the content of oxygen in the porous Ta3N5 NTs. 3.4. XPS Spectra of Ta3N5 NTs. The XPS spectra of Ta2O5 and Ta3N5 NTs are presented in Figure S2. At the surface of asanodized Ta2O5 NTs, in addition to O, Ta and C, the presence of F and S impurities can be observed. The incorporation of F and S at the surface is related to the anodization process itself once the Ta disk was anodized in H2SO4 and HF-containing solution. After thermal nitridation, nitrogen was incorporated to form Ta3N5 NTs, thereby decreasing the oxygen content when compared to the as-anodized Ta2O5 NTs and eliminating the surface impurities of F and S (survey spectra of Figure S2). Table 2 compares the surface atomic ratios N/Ta and O/Ta of

to (110) crystallographic plane with increasing temperature. The Rietveld refinement presented the same tendency of preferential orientation and anisotropic increase of the grain size along the (110) direction (Table 1). The results clearly show a preferential oriented crystallization induced in the amorphous tubular matrix during nitridation. On the right side of Figure 3 the scheme represents the dependence of grain size and crystallographically preferred orientation (the yellow arrows) with temperature. At low temperature, the grains are smaller and grow randomly; as the temperature increases, the grains become larger and crystallographically preferred oriented. Another striking feature observed by the Rietveld refinement is the isotropic microstrain associated with the boundary of the grains which increases with temperature. We may expect that the increase in microstrain is related to the defects in the crystalline structure; in addition, it compensates to preserve the tubular morphology. By the values of the occupation factor in the Rietveld refinement we have calculated the percentage of defects in the crystalline structure of porous Ta3N5 NTs (Table S2). Schottky defects were found at Ta1(4c), N1(8f); on the other hand, improved refinements were obtained by incorporating oxygen substitutional defects at the 3-coordinated site N2(4f) (bottom of Figure 3). Based on the refinements, the nominal stoichiometry was calculated from the percentage values of site occupancy and vacancy obtained from the occupation factor (Table S2) and is presented in Table 1. It can be seen that by increasing the temperature the oxygen content decreases and more nitrogen is incorporated into the unit cell. In addition, for NTs-900 there are fewer Ta defects (recombination centers) and the stoichiometry is closer to Ta3N5. From the above discussion, we may deduce that oxygen is present in the whole crystalline structure of porous Ta3N5 NTs. Thermal nitridation at higher temperatures decreases the oxygen content; however, oxygen cannot be removed completely.30 Nevertheless, the presence of oxygen in the Ta3N5 structure was marked as being essential to sustaining the mechanical cohesion of the compound.22 3.3. Ultraviolet−Visible Diffuse Reflectance Spectra. Figure 4 displays the absorption spectra of the Ta2O5 NTs and porous Ta3N5 NTs. The absorption edges of the porous Ta3N5 NTs are red-shifted from the Ta2O5 edge, indicating the

Table 2. Atomic Ratios of N/Ta and O/Ta on the Surface of Ta3N5 NTs and the Area Percentages of Doublets in Ta 4f Regions Obtained by XPS Analyses sample

N/Ta

O/Ta

doublet area

NTs-800 NTs-850 NTs-900

1.29 1.00 0.97

0.27 0.28 0.29

5% 6% 6%

the deconvoluted Ta 4f, N 1s, and O 1s XPS spectra of the porous Ta3N5 NTs (Figure S2). In the precursor (Ta2O5 NTs), the Ta 4f7/2 and Ta 4f5/2 appeared at 28.4 and 26.5 eV, respectively, corresponding to Ta5+ in Ta2O5.41,42 The binding energies are reproducible to ±0.3 eV. For porous Ta3N5 NTs, these energy peaks are shifted to the lower-energy values indicating the transformation of oxide to nitride due to the lower electronegativity of nitrogen compared to the oxygen. The values of the binding energies for nitride samples “Ta 4f5/2 at 26.7 eV” and “Ta 4f7/2 at 24.8 eV” match the values presented in previous reports (Ta5+ of Ta3N5).8,31,43 Deconvoluted Ta 4f spectra (Figure S2) of the nitride samples are divided into two doublet peaks; one doublet, which is higher in intensity and shifted to a lower binding energy (compared to as-anodized Ta2O5 NTs), belongs to Ta3N5, and another with lower intensity but shifted to higher binding

Figure 4. Ultraviolet−visible diffuse reflectance spectra of Ta2O5 NTs and porous Ta3N5 NTs. 19909


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The Journal of Physical Chemistry C energy is positioned in between Ta3N5 and Ta2O5.8 From the binding energy position of the lower-intensity doublets, we might suggest that these doublets correspond to the (oxy)nitride species of tantalum.8,44 The percentages of these doublets are presented in Table 2. While finding the N/Ta and O/Ta ratios, we have subtracted the area of this doublet from Ta 4f to remove this contribution from the Ta3N5. One can observe that N/Ta decreases indicating that nitrogen is released from the surface of the Ta3N5 NTs by increasing the temperature. The N/Ta ratio found here was smaller than the theoretical value (1.67); therefore, it is quite reasonable to suggest the presence of other tantalum species on the surface of NTs.32 For the as-anodized Ta2O5 NTs, the O 1s peak (Figure S2) is observed at 530.7 eV whereas for all Ta3N5 NTs samples, it appeared at ca. 529 eV corresponding to lattice oxygen in Ta3N5.45 The other peaks at higher binding energies are associated with the hydroxide species at the surface.32,43 Moreover, the slight increase in the O/Ta ratio (Table 2) with increasing temperature corroborates the suggestion of forming the (oxy)nitride species of tantalum. The above discussion indicates that the amorphous Ta2O5 NTs that are used as a precursor for the synthesis of Ta3N5 NTs are more likely to lose a large amount of nitrogen from the surface at higher nitridation temperature; they are also more likely to form oxide-rich phases of tantalum nitride. Therefore, proper cleaning of the surface of Ta3N5 NTs is highly desirable for their application in PEC experiments. 3.5. PEC Performance of Porous Ta3N5 NTs. Prior to each measurement, the samples were cleaned by immersion in HF:H2O (1:9) for 30 s and rinsed with distilled water. Figure 5 shows the chopped linear sweep voltammetry curves of all the samples. The electrochemical parameters obtained by PEC analyses are presented in Table. S3. The sample prepared at 900 °C (NTs-900) has presented enhanced photocurrent of 7.4 mA cm−2 at 1.23 V vs RHE, when compared to the literature where a photocurrent of ca. 4.8 mA cm−2 was obtained only after doping the Ta3N5 with alkaline metal ions that has produced 15.9 μmol of hydrogen for 1 h.24 Compared to the undoped sample described in that report, the pristine NTs-900 presented a 25-fold improvement at 1.23 V vs RHE.24 It is important to note that in order to obtain highly crystalline Ta3N5 NTs, with lower defect formation and morphological preservation, the annealing temperature should be high enough and the annealing time should be short enough to avail all of the advantageous features of the tubular morphology so they can be used in solar-energy driven devices. Hereafter, the origin of the enhanced PEC activity of NTs-900 will be discussed in detail on the basis of a thorough characterization of the samples. To verify the importance of the effect that tubular morphology has on the PEC activity, Ta2O5 films have been thermally grown on the Ta substrate at 550 °C for 20 min. The film was nitrided under the same conditions as NTs-900. The thickness of the film is almost the same as the average length of the porous Ta3N5 NTs (Figure S3). In addition, NTs-1000 was employed for the same measurements (Figure S4). The Ta3N5 film and NTs-1000 have presented photocurrent of 2.2 mA cm−2 and 2.4 mA cm−2, respectively, at 1.23 V vs RHE. Interestingly, the nanotubes synthesized at the lower temperature of 850 °C (NTs-850) presented PEC activity higher than that of the Ta3N5 films and NTs-1000. Therefore, the vertically oriented nature of the NTs provided a straight pathway for the electrons and reduced the hole diffusion path, which resulted in

Figure 5. (a) Linear sweep voltammetry curves of porous Ta3N5 NTs recorded under simulated 1 Sun (AM 1.5) illumination. (b) Gas chromatography of the H2 evolved from the counter electrode compartment (inset: cathode compartment). The red dotted line indicates the expected amount of H2 with unity H2 evolution efficiency, and the blue spheres present the amount of H2 evolved.

3.4 times enhancement in the photocurrent as compared to the Ta3N5 films prepared in the current study. The sample NTs-900 was chosen for hydrogen gas measurement at the same experimental conditions reported previously.24 The gas evolved from the Pt electrode compartment was quantified by gas chromatography. The amount of electrons passing through the outer circuit was 96.2 μmol cm−2 for 1 h. The amount of hydrogen evolved from the counter electrode is measured as 32.8 μmol (Figure 5b). The efficiency of H2 evolution was 67%, and the rest can be considered as the reduction of hole scavenger. To study that the observed photocurrent by porous Ta3N5 NTs is not just an artifact of anodic photo-oxidation of sacrificial agent, we have measured the LSV curves in the inert 0.1 M phosphate buffer electrolyte adjusted to pH 12 (Figure S5). It can be seen that the NTs-900 has shown improved PEC activity compared to other samples. Considering the importance of the crystalline structure as discussed earlier, the highly crystalline structure of the NTs900, its preferential crystallization orientation along (110), its larger grain size, and its stoichiometrically ideal nature are the main reasons why it has demonstrated the best PEC performance. To further understand the origin of the enhanced PEC activity of NTs-900 on a nanoscopic scale, we have performed high-angle annular dark field (HAADF)-STEM and HRTEM. These analyses were conducted with a specific focus on the edge and the middle of the NTs. Panels a and b of Figure 6 display the HAADF-STEM images of NTs-800 and 19910


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Figure 6. HAADF-STEM images of (a) NTS-800 and (b) NTs-900; (c) spatial-localized EDX spectra of NTS-800 recorded in the middle and on the edge of the nanotube; and (d) HRTEM image and simulated diffraction patterns (inset) of NTs-900.

NTs-900, respectively, viewed along the [1-10] zone axis. The d-spacing of 0.36 Å is in a good agreement with the (110) plane of the orthorhombic phase already confirmed by XRD. Comparing panels a and b of Figure 6, one can observe that the periodic lattice fringes for NTs-800 disappear at the borders, thereby demonstrating amorphism on the edges of the walls. To further study the chemical composition of NTs-800, spatial-localized EDX spectra were acquired at two positions: the middle and the edge of the nanotube (Figure 6c). The EDX spectra clearly indicate a higher amount of oxygen at the borders of NTs-800, suggesting the presence of some amorphous oxide/oxynitride of tantalum (shown as the blue layer in the inset of Figure 6a). From these results it is intuitive that the amorphous regions around the boundaries of the NTs increase the resistance for the flow of the photogenerated charge carriers, thereby explaining why NTs-800 could not present a better PEC response. We might expect that during the nitridation, the oxygen segregates to the borders of the NTs. Therefore, to decrease oxygen content and the amorphism to improve the PEC performance, Ta3N5 NTs should be synthesized at a higher temperature. Figure 6d shows the HRTEM image of NTs-900. The crystallite domains are extended to the entire diameter, showing single crystal-like nature. Similar results can be confirmed from selected area electron diffraction patterns (Figure S6). The inset of Figure 6d presents the simulation of electron diffraction patterns viewed along the [1-10] zone axis, which is in agreement with the experimental distances, and the (110) can be observed clearly;

the red-colored line shows the direction of the nanotube along its length. These results indicate that, by increasing the temperature, the grains stack up along the preferential crystallization orientation of (110) forming a mosaic of larger grains, thereby corroborating the results obtained by the Rietveld refinement. When NTs-800 is compared with NTs900, it is clear that the highly crystalline structure of NTs-900 with a larger grain and a preferential crystallization direction along (110) and lower oxygen contents around the walls make it a better candidate for improving the PEC water-splitting activity. Based on the crystalline structure analyzed by HRTEM and the Rietveld refinements, a scheme of the grain growth for porous Ta3N5 NTs as a function of temperature is shown in Figure 7. In the context of understanding the reasons behind the formation of porous Ta3N5 NTs with preferred orientation along (110), we suggest that the presence of the oxygen content in the precursor matrix might be a key factor. HRTEM and XRD analyses have shown that the oxygen content decreases as the temperature increases. During the nitridation process, oxygen is replaced by nitrogen, and if the nitridation temperature is high enough to remove the oxygen, the amorphous tubular matrix will be transformed into highly crystalline larger grains. On the other hand, if the nitridation temperature is lower, a large amount of oxygen cannot be removed. The presence of this oxygen across the grain boundaries may hinder crystallization into highly oriented 19911


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which is characteristic of a single time constant system.46 The EIS data have been fitted to an equivalent circuit (EC) consisting of a series resistance, Rs, and CPE (constant phase element) parallel with the charge-transfer resistor, Rct (inset of Figure 8a).47 By fitting the impedance spectroscopy results into the proposed equivalent circuit, we have found the values of the EC components. As seen in the inset of Figure 8a, NTs-900 exhibited the lowest interfacial charge-transfer resistance; this indicates improved charge transportation dynamics, which clearly corroborates the excellent PEC performance of the synthesized NTs. The facile interface charge transfer in NTs900 is further verified by photoluminescence (PL) spectra (Figure 8b). A strong emission peak at about 510 nm is observed for all the samples, and the smallest intensity of this peak from NTs-900 indicates the low electron−hole pair recombination. This result shows good agreement with the EIS analysis and the enhanced PEC activity of NTs-900. To separate the emission peaks of oxide and nitride, PL of the asanodized Ta2O5 and the Ta3N5 NTs were measured (Figure S7). The PL spectra were obtained from the powders that are scratched off from the substrates. A similar emission peak at 510 nm is observed in the Ta2O5 NTs; however, compared to Ta2O5 NTs, it is almost absent for the porous Ta3N5 NTs. HRTEM and XRD analyses have already revealed that the oxygen is present all the way to the bottom of the NTs. Therefore, we may expect that the peak at 510 nm is related to the oxygen in Ta3N5 NTs which acts as recombination centers for the photogenerated carriers.23 From the Nyquist plots and the PL spectra, it is clear that the oxygen content should be controlled for better PEC performance of Ta3N5 NTs. The Mott−Schottky plots (Figure S8) present the n-type semiconductor transition for porous Ta3N5 NTs. The flat band potential (Efb) was obtained by the extrapolation of the linear region which matches the previously reported flat band for Ta3N5 NTs by Bard’s group.2 Interestingly, the flat band of all the samples is the same. These results show that the enhanced PEC activity of NTs-900 is not related to the flat band. In fact, it is a trade-off between the favored crystalline structure and the electrolyte for improved charge transportation of the photogenerated carriers. The approximated values of the Efb were −0.05 ± 0.03 V vs RHE for all the samples. Nevertheless, the slopes of the curves vary, which is related to the variation in the density of states that is obtained at different nitridation conditions.

Figure 7. Schematic diagram of the fabrication process: (a) asanodized smooth-walled Ta2O5 NTs and (b) nitrided porous Ta3N5 NTs (c) growth of grains to crystallographically preferred orientation by increasing nitridation temperature; the yellow arrows show preferential orientation, and the blue layer shows the presence of amorphous oxide/oxynitride of tantalum on the borders of the NTs.

larger grains. However, the mechanism described above is not completely understood and further investigations are required. To elucidate the mechanism of charge transfer across the interface of porous Ta3N5 NTs−electrolyte, we performed EIS. Figure 8a compares the Nyquist plots of the porous Ta3N5 NTs. For all the samples, single semicircles can be observed,

4. CONCLUSIONS In summary, we have synthesized porous Ta3N5 NTs by thermal nitridation of the precursor Ta2O5 NTs. The morphological, structural, surface, and charge transportation properties of the porous Ta3N5 NTs were investigated. Anodization in a lower electrolyte temperature helped to preserve the tubular morphology for nitridation at higher temperatures. XRD and HRTEM analyses demonstrated that thermal nitridation controlled the preferential orientation along the (110) plane of orthorhombic Ta3N5. The oxygen content is one limiting source that influences the PEC activity of the photoanode. Increasing the nitridation temperature increased the grain size, decreased the oxygen substitution defects and Schottky defects in the crystalline structure, and removed the amorphous layer on the walls of the NTs. Crystallographically oriented along (110), highly crystalline, larger grain size, and lower oxygen containing NTs-900 demonstrated enhanced photocurrent, which was 3.4-times and 25-fold higher than the

Figure 8. (a) Nyquist plots obtained under dark conditions and (b) PL spectra of Ta3N5 NTs. 19912


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The Journal of Physical Chemistry C Ta3N5 film prepared in this study and the literature reported values, respectively.24 In addition, NTs-900 exhibited the lowest charge-transfer resistance across the semiconductor−electrolyte interface. Applying NTs-900 for gas chromatography, 32.8 μmol cm−2 of H2 was evolved in 1 h. Therefore, for enhanced PEC performance of porous Ta3N5 NTs, nitridation temperature should be high enough and time should be short enough to control the oxygen content and to avail all advantageous features of the tubular morphology. These results show that porous Ta3N5 NTs with crystallographically preferential orientation along (110) and low oxygen content can improve PEC activity, so it is a promising candidate for photoelectrochemical devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b05475. Details of experimental methods and characterizations; SEM images, XPS spectra, LSV curves, SAED patterns, PL spectra, and Mott−Schottky plots; and tables displaying data on the geometrical dimensions of the NTs, Rietveld refinement, and PEC parameters. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +55-51-33086498. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for financial support from the following Brazilian agencies: CNPq (Process 477804/2011-0, 490221/2012-2, and 472243/2013-6) and FAPERGS − PqG 2011 Proc. 11/0837-0. The authors acknowledge Dr. Fabiano Bernardi for helpful discussion on XPS analyses, Dr. Pedro Migowski for useful discussion on water splitting, Dr. Paulo Franzen for measuring PL spectra, MSc. Luana de Lucca de Costa for help on the graphical design, and Mr. Otelo José Machado for performing XRD measurements.

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REFERENCES

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