Synthesis and Visible-Light-Driven Photocatalytic Activity of Ta4+ Self

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Synthesis and Visible-Light-Driven Photocatalytic Activity of Ta4+ Self-Doped Gray Ta2O5 Nanoparticles Luiz E. Gomes,† Marcio F. da Silva,† Renato V. Gonçalves,‡ Giovanna Machado,§ Glaucia B. Alcantara,∥ Anderson R. L. Caires,† and Heberton Wender*,† †

Instituto de Física, Universidade Federal de Mato Grosso do Sul, Av. Costa e Silva S/N, Bairro Universitário, CEP 79070-900, Campo Grande, MS, Brazil ‡ Instituto de Física de São Carlos, Universidade de São Paulo, PO Box 369, São Carlos, SP 13560-970, Brazil § Centro de Tecnologias Estratégicas do Nordeste − CETENE, Recife, PE, Brazil ∥ Instituto de Química, Universidade Federal de Mato Grosso do Sul, Av. Costa e Silva S/N, Bairro Universitário, CEP 79070-900, Campo Grande, MS, Brazil S Supporting Information *

ABSTRACT: C-impregnated/Ta4+ self-doped ultrafine Ta2O5 nanoparticles (NPs) were prepared by a one-step solvothermal method through reaction of Ta(V) chloride with benzyl alcohol. These NPs showed a large specific surface area of 253.4 m2 g−1, mean diameter of 2−3 nm, and superior photocatalytic activity in the photodegradation of Rhodamine B dye (RhB), compared to TiO2 (P25) and Ta2O5 commercial NPs. The obtained materials were annealed at different temperatures and their structure, morphology and optical and photocatalytic properties were investigated in detail. The C-impregnated Ta4+ self-doped Ta 2O 5 NPs presented enhanced and extended absorption in the visible range of the solar spectrum, increasing significantly their photocatalytic activity. The best photocatalyst could completely remove RhB after only 12 min of UV irradiation and yielded 68% and 43% of RhB photodegradation after 120 min of simulated sunlight and visible irradiation, respectively, with an apparent quantum efficiency of 3.6% at 447 nm. This enhanced photocatalytic performance is ascribed to the combined effects of better light harvesting properties, high surface area and longer electron lifetimes in the excited sub-band, due to the presence both of oxygen vacancies neighboring Ta4+ and Ta−O−C on the Ta2O5 surface. The photocatalyst systems presented good stability, confirming their promise as candidates for photocatalytic applications.

1. INTRODUCTION In the past few decades, photocatalysts have been used extensively to deal with energy and environmental problems, such as solar-to-chemical energy conversion by means of artificial photosynthesis, photodegradation of dyes or photocatalytic chemical synthesis.1−3 Among all photocatalysts studied to date, tantalum oxide (Ta2O5) emerges as one of the most attractive transition metal oxide semiconductors for both photocatalytic water splitting and photodegradation of organic pollutants, due to its unique physical and chemical properties.2,4−7 High to outstanding dielectric constant, and high refractive index, photoelectric properties and chemical stability, Ta2O5 has been widely used not only in photocatalysis but also as a material in dynamic random access memory,8 insulators,9 atomic switches,10 antireflective coating layers,11 CO oxidation,12 and gas sensors.13 Lu et al. have described Ta2O5 nanowire photocatalysts with low-dimensional structures, high surface area and an impressive hydrogen generation rate under Xe lamp irradiation with no cocatalyst.14 Vertically stacked Ta2O5 films were recently © XXXX American Chemical Society

reported as photocatalysts for photodegradation of Rhodamine B (RhB) and hydrogen production by water splitting, showing attractive stability and recycling under UV irradiation.4 Tao et al. reported well-dispersed mesoporous Ta2O5 submicrospheres with significantly enhanced photocatalytic activity for photodegradation of organic pollutants such as methylene blue and RhB.15 However, due to its wide band gap (about 4 eV) their light-absorption property is limited to a very narrow part of the solar spectrum, and generally, strong UV light is required for performing photocatalysis with Ta2O5. Actually, different and complicated processes are usually involved in a semiconductor-based photocatalytic reaction. First of all, photons have to be absorbed by the semiconductor material, to promote valence-band electrons to the conduction band and consequently generate electron−hole pairs, which individually and separately need to migrate to the photocatalyst Received: November 30, 2017 Revised: February 2, 2018 Published: March 5, 2018 A

DOI: 10.1021/acs.jpcc.7b11822 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Ta2O5 NPs are spherical with 3.3 nm mean diameter, resulting in a high surface area of 253.4 m2 g−1. Carbon impregnation and Ta4+ self-doping resulted in an improved and high visiblelight absorption with an apparent quantum efficiency of 3.6% at 447 nm. In addition, the reusability tests showed that the material, proposed herein for the first time, is a promising candidate for photocatalytic applications.

surface and then take part in reduction and oxidation reactions to complete the energy conversion process.2,16−18 During all these processes a good photocatalyst candidate should be able to avoid electron−hole recombination. In case of Ta2O5-based materials, different strategies have being widely explored in order to increase their photocatalytic activity. Among them can be highlighted: (i) obtaining materials with large surface areas to increase reactive sites and improve light absorption;19 (ii) integrating other visiblelight-responsive semiconductors with Ta2O5 in the form of hybrid heterojunction materials to extend the limits of photon absorption wavelength range;20,21 (iii) tuning morphology such as nanowires and nanotubes2 favoring carrier transport, so avoiding charge recombination; and (iv) doping external ions into the Ta2O5 structure.22 Doping a foreign ion or combining a second element with a wide band gap oxide semiconductor is a well-known approach to narrow the band gap or to create an intermediate energy band to act as a donor or acceptor level between, e.g., Ta2O5 conduction and valence bands.23,24 This can result in a shift of light absorption to the visible range, which is totally desirable in order to lower the total cost of practical applications. It is worth pointing out that the search for visible-light absorbing photocatalysts is in response to the fact that visible light comprises almost 55% of solar light, which is a renewable, abundant and a natural resource. Hashimoto et al. reported visible-light-sensitive Ta2O5‑xNx powder photocatalysts produced by nitrogen doping at the oxygen sites, keeping the Ta2O5 mother structure. Nitrogendoped Ta2O5 could decompose gaseous 2-propanol under both visible and ultraviolet light irradiation.25 The authors reported photocatalytic activities to be dependent on dopant concentration and wavelength of light irradiation, showing that this process is efficient for engineering the light absorption in wide band gap semiconductors, extending their absorption to the visible range. Zhu et al. have, in turn, reported oxygen-deficient gray Ta2O5 nanowires through a hydrothermal reaction and subsequent aluminum reduction at different temperatures to improve photoactivity for photoelectrochemical water splitting.26 In this case, oxygen vacancies and disordered shells were induced into this gray Ta2O5 nanowire, enhancing solar absorption and photocatalytic performance. Very recently, Xin Yu et al. reported the synthesis and photocatalytic activity for hydrogen evolution of Ta4+ self-doped Ta2O5 nanorods27 and quantum dots.28 These materials presented enhanced visible light absorption from 400 to 800 nm and visible light photocatalytic activity compared to commercial Ta2O5, mainly because of the formation of Ta4+ species. Ta4+-self-doping was also reported to increase significantly the visible-light photocatalytic activity of NaTaO3 nanostructured catalysts due to reduced band gaps at ∼2 eV.29,30 Similar visible-light photocatalytic activity and lightabsorption enhancements were also observed for Ti3+-selfdoped TiO2 nanoparticles.19,31 The presence of Ta4+ selfdoping (or oxygen vacancy defects) in Ta2O5 photocatalysts is considered to form new defect energy levels between the Ta 5d and O 2p, stated to be responsible for conduction band minimum and valence band maximum of Ta2O5, respectively, giving rise to an intriguing visible-light response. Herein, we report for the first time a one-step synthesis of Cimpregnated/ultrafine Ta4+ self-doped Ta2O5NPs photocatalysts with remarkable visible-light absorption and improved photocatalytic activity for RhB dye removal. SAXS and TEM revealed that the as-obtained C-impregnated Ta4+ self-doped

2. EXPERIMENTAL SECTION 2.1. Chemicals. Tantalum(V) chloride (99.99%) and benzyl alcohol (99+% anhydrous) were purchased from Sigma-Aldrich Chemical Co, Brazil. Rhodamine B (RhB, 99.8%) was purchased from Vetec. Commercial TiO2 P25 NPs (D-60287) and Ta2O5 NPs (99.9%) were purchased from Degussa and Puratronic, respectively. All solvents and reagents were of analytical grade and used as received. Tantalum chloride was manipulated inside of a fume hood and immediately closed and sealed with Parafilm M. Deionized water was used in all experiments. 2.2. Synthesis of C-Impregnated Ta4+ Self-Doped Ta2O5 Nanoparticles. The synthesis of C-impregnated Ta4+ self-doped Ta2O5 NPs was performed by adapting previously reported methods 32,33 as follows: 0.4888 g of tantalum chloride (TaCl5) was added to 48.9 mL of benzyl alcohol. The solution was first kept under ultrasound (UltraCleaner 1400A 40 kHz) for 20 min and then magnetically stirred for 30 min. After complete solubilization, the reaction mixture was transferred into a 110 mL Teflon-lined stainless steel autoclave and carefully sealed. The solvothermal reaction was performed inside an oven at 220 °C for 72 h with a heating rate of 2 °C min−1. After spontaneous cooling, the final product was centrifuged at 4500 rpm for 15 min and then washed twice with acetone and once with ethyl alcohol to remove organic impurities. The obtained powder was dried at 80 °C for 4 h in air and stored in Eppendorf tubes. After that, the powders were submitted to different thermal treatments at 600, 700, and 800 °C for 4 h, reaching the desired temperature at a heating rate of 10 °C min−1. 2.3. Photocatalyst Characterization. Transmission electron microscopy (TEM) was used to investigate the morphology and size of the particles in a FEI-Tecnai G2 200 KV microscope. For each analysis, approximately 4 mg of sample was dispersed in acetone and homogenized by ultrasound for 15 min. An aliquot of the solution was dropped onto a 300-mesh Cu grid covered with an ultrathin film of amorphous carbon. The deposited samples were dried in vacuum for 3 h before being inserted into the microscope for analysis. Size histograms were obtained after counting around 150 particles using ImageJ software. Small angle X-ray scattering (SAXS) experiments were carried out at the SAXS1 beamline of Brazilian Synchrotron Light Source LNLS, located at Campinas, SP, Brazil. The beamline was configured to a X-ray wavelength of λ = 1.488 Å and scattering vector q measuring regions from 0.16 to 3.5 nm−1, where q = 4π sin(2θ)/λ and θ is the scattering angle. The solids-containing nanoparticles were placed in the hole of a steel washer, sealed with Kapton tape and placed in the sample holder. The fitting of the SAXS curves was performed using SASfit software, developed by Kohlbrecher of the Paul-Scherrer Institut (PSI), following the procedures described elsewhere.34 The crystalline structure of the obtained powders was analyzed by X-ray diffraction (XRD) in an Ultima IV Rigaku diffractometer using Cu Kα radiation measuring the range 20 B


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Figure 1. Representative TEM images of as-obtained Ta2O5 NPs (A) and Ta2O5 NPs annealed at 800 °C (B). Histograms of size distribution obtained by SAXS (C) and TEM (D) for the different temperatures of annealing studied. Xc and σ represent the mean and standard deviation respectively, as obtained by a Gaussian fit in the data.

spectra were acquired in the 4000−500 cm−1 range with 2 cm−1 resolution. Steady-state and time-resolved photoluminescence studies were performed using an FS-2 fluorescence spectrometer (Scinco, Korea) and a FluoTime 100 fluorescence lifetime spectrometer (PicoQuant, Germany), respectively. The photoluminescence (PL) spectra were obtained using a 150W Xe-arc lamp as excitation source, Czerny−Turner monochromators for selecting excitation and emission wavelengths, and a R928 photomultiplier for collecting the photoluminescence signal. The samples were excited at 280 and 370 nm and the PL emission spectra collected. Photoluminescence lifetime measurements were performed using a picosecond pulsed diode laser at 283 nm as excitation source. The excitation LED provided 820 ps pulses with a repetition rate of 10 MHz, and the time-resolved intensity decays were collected by a timecorrelated single-photon counting (TCSPC) method. PL lifetimes were obtained by fitting the PL intensity decay curves, using the equation I(t) = Aet/τ where I, A, t, and τ represent the PL intensity, background correction constant, decay time, and PL lifetime, respectively. The PL measurements were carried out in solutions containing 5 mg of powder sample diluted in 2 mL of ethyl alcohol. All PL data were collected at room

≤ 2θ ≤ 80, with an angular step of 0.02° counting 5 s in each step. X-ray photoelectron spectroscopy (XPS) analyses were performed in order to identify and quantify chemical species on the surface of the photocatalysts. The equipment used was a spectrometer from Thermo Scientific, model K-Alpha equipped with a 128-channel detector spherical analyzer. The powder samples were supported on double-sided carbon tape. Al Kα monochromatic radiation was used (hν = 1486.6 eV) as the excitation source. The spectra were recorded with a passing energy of 200 eV for the survey and 50 eV for the highresolution spectra, measuring 30 scans for Ta (dwell = 100) and 20 scans for C and O (dwell = 50). In all spectra, the peak positions were corrected using C 1s adventitious carbon set at 284.8 eV. The CasaXPS software package was used for the treatment of the acquired spectra. UV−vis Diffuse Reflectance spectra of the powders were obtained using a LAMBDA 650 UV−vis spectrometer (PerkinElmer) equipped with an integrating sphere. Fourier transform infrared spectroscopy (FTIR) measurements were conducted using a JASCO 4100 spectrophotometer, along with an attenuated total reflectance unit at room temperature. The C


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irradiation setup used for calculating EQF, with RhB and Ta2O5 concentrations of 40 ppm and 3 mg/mL, respectively. The maximum absorption of RhB at 554 nm was monitored and converted to concentration in mols m−2 s−1 after performing a standard calibration curve with different concentrations of RhB and taking into account the incident light focused area of π × 10−4 m2 measured at the liquid surface position. Here, φap depends on the incident photons instead of the photons absorbed by the catalyst, so the quantum efficiency may be higher than the value measured in this paper.

temperature using right-angle geometry (90° excitation/ emission geometry) and a quartz cuvette with four polished faces and 10 mm optical path length. Brunauer−Emmett−Teller (BET) specific surface area measurements were carried out in equipment from Micromeritics, model ASAP 2020. For each measurement, 300 mg of sample was used. Samples were treated in a vacuum line (∼10−3 mbar) for 24 h before starting the measurements. Nuclear magnetic resonance (NMR) spectroscopy and ζ-potential experimental conditions are detailed in the Supporting Information. 2.4. Photocatalytic Reactions. The photocatalytic activity of Ta2O5 NPs was evaluated by following the degradation rate of RhB dye, as a model organic pollutant, under UV (highpressure 300 W Hg vapor lamp) as well as under simulated solar irradiation. The UV system was adjusted to irradiate a power of 73.5 mW cm−2 using a Newport power meter, model 1918-C with 918D-SL-OD3 silicon photodetector calibrated at λ = 405 nm. The distance between the UV lamp and the RhB liquid surface was 8.7 cm for all the experiments. For simulated solar irradiation, an Abet Tech solar simulator model 10500, equipped with a 150 W xenon lamp and an AM1.5G filter was used for ensuring reproducibility of the solar spectrum. This system was calibrated for irradiating at approximately 238 mW cm−2 by using an Abet Tech calibration cell, model 15151. Photons directly illuminated the liquid surface in the top of a borosilicate beaker of 50 mL capacity. Reactions were performed by filling the remarkable visible-light with 25 mL of 40 mg L−1 RhB solution, under continuous magnetic stirring for homogenization, followed by addition or not of a certain amount of photocatalyst. The system was maintained under dark conditions and stirring for 30 min to ensure dye adsorption. Immediately after stirring in the dark, the beaker was exposed to the irradiation source and 300 μL aliquots were taken at different time intervals and analyzed using a UV−vis spectrometer (Ocean Optics USB 4000), by following the absorption intensity at 554 nm (the maximum absorption wavelength of RhB). For those runs performed under visible light, a 400 nm cutoff filter was inserted between the lamp and the reactor. The best photocatalyst was finally tested for reusability. All experiments were conducted in triplicate and the photocatalysts washed, centrifuged, dried and reused immediately after the end of each cycle. Accordingly to IUPAC recommendations,35 two optical filters (BG37 and FSQ-GC420 cutting-on 420 nm, both from Newport) were used in order to almost monochromatic incident light at 447 nm with fwhm = 52 nm for calculating the apparent quantum efficiency (φap). The irradiance (E) measured under these conditions was 1.38 W m−2 and was converted to photon flux (EQF) by using the following relationship: EQF = E × λ × 0.836 × 10−2 = 5.1569 μmol photons m−2 s−1, where E and λ are given in W m−2 and nm, respectively. For this conversion it was considered the photon energy (Ep = hc/λ), the number of photons (Np = E/ Ep) and finally the EQF = Np/NA, where NA is the Avogadro number. The apparent quantum efficiency was calculated by using eq 1.35−37 φap (%) =

3. RESULTS AND DISCUSSION 3.1. Photocatalyst Preparation and Characterization. Parts A and B of Figure 1 show representative TEM images of the as-obtained Ta2O5 NPs and Ta2O5 NPs annealed at 800 °C, respectively. The size histograms obtained by SAXS and TEM are presented in parts C and D of Figure 1, respectively. As can be seen, as-obtained Ta2O5 are ultrasmall sized particles with spherical mean diameters of 3.3 ± 0.8 and 2.1 ± 0.5 nm as determined by SAXS and TEM histograms, respectively. All SAXS curves and fittings are presented in Figure S1 of the Supporting Information. The results show that by increasing the annealing temperature from ambient to 600, 700, and 800 °C, the NP size and size distribution increased significantly, in which a good agreement was observed between SAXS and TEM results. Moreover, at 800 °C the NP’s seem to be anisotropic with ellipsoid-like shapes. Figure 2 shows the XRD pattern results of as-obtained and Ta2O5 NPs annealed at 600, 700, and 800 °C. As can be seen,

Figure 2. X-ray diffraction data of Ta2O5 NPs before and after annealing at different temperatures.

the as-obtained sample is amorphous. The diffraction peaks of the samples treated at 600 °C were indexed in a hexagonal structure δ-Ta2O5, (ICSD 19-1299, SG, P6/mmm, a = b = 7.24 and c = 11.61). As the annealing temperature increased to 700 °C, the hexagonal structure was converted to the orthorhombic phase of β-Ta2O5 (ICSD 25-0922, SG, P21212, a = 6.198, b = 40.29, and c = 3.888). No other additional crystalline peaks could be observed. Noticeably, the crystallinity of the sample treated at 700 °C is superior to that treated at 600 °C. Another important factor is the unexpected peak broadening observed with increasing annealing temperature. Theoretically, the reverse would be expected; i.e., an increase in annealing temperature should induce the formation of bigger crystallites and, consequently, smaller peak widths. However, the orthorhombic phase of

n° of photodegraded molecules [mol m−2 s−1] EQF[mol m−2 s−1] × 100

(1)

The numerator of eq 1 was obtained by performing the photocatalytic degradation of RhB for 180 min under the same D


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from 318 to 750 nm. The area under the curve was obtained by integrating over these established limits taking the zero point as a baseline; the results are presented in Table 1. The area corresponding to the former segment (A1) does not change significantly when comparing the samples to each other. However, the latter segment (A2) reduced drastically to approximately 11% of the initial value after annealing at 600 °C. For higher annealing temperatures (700 and 800 °C) this percentage increased again, reaching approximately 17% of the initial value. In order to determine the band gaps of the samples, a straight line was adjusted in the linear portion of the Tauc plots ((F(R)hν)n vs photon energy hν) where R is reflectance, F(R) = (1 − R)2/2R is the Kubelka−Munk function and n is determined by the type of transition.39 Since F(R) is proportional to the absorption coefficient,40 the exponent n was determined by the best fit to a straight line near the absorption edge and was found for n = 1/2, indicating that Ta2O5 NP samples present an indirect allowed transition band gap. The Tauc plots are shown in Figure S2 of the Supporting Information, and the obtained band gap energy values are presented in Table 1. All samples showed a band gap of around 3.9 eV, which matches experimental values previously reported in the literature for Ta2O5.15 However, a second band gap could also be identified and was revealed to be dependent on the annealing temperature, with values ranging from 2.7 eV for the as-obtained Ta2O5 sample to 2.3 eV for Ta2O5-800 °C. Similar additional band gaps were observed for carbon-doped TiO2,41,42 F- and N-doped-Ta2O5,43,44 and Ta4+-doped Ta2O5.27,28 Therefore, the presence of this second absorption peak in the visible range can be understood as a consequence of the Ta2O5 NP doping. The chemical composition of as-prepared Ta2O5 NPs was explored by XPS measurements. It is well-known that XPS is a powerful tool for determining the composition and oxidation state of chemical components present on catalyst surfaces. The survey spectra (Figure S3, see Supporting Information) of asprepared and annealed Ta2O5 NPs were used for quantification of the surface elements, and the results are presented in Table 1. No contaminant peaks could be identified in the spectra, indicating the observed product to be highly pure. Sputtering of the samples was not carried out, resulting in a strong presence of C 1s signal in the XPS spectra originating from both adventitious carbon and residual carbon from hydrothermal synthesis. It has already been shown that morphological and chemical evolution of the surface during low-energy ion sputtering is a rather complex issue for Ta2O5, which is why the process was not carried out.2 High-resolution C 1s spectra of Ta2O5 NPs presented a main peak, which was attributed to adventitious graphitic carbon (C−C sp2), and used to calibrate

Ta2O5 has many peaks with very short spacing due to fusion of multiple diffraction orders, especially those near 2θ = 29 and 37°, which caused the observed broadening.38 At 800 °C, the sample retained the orthorhombic beta phase with a visible improvement in crystallinity. UV−vis diffuse reflectance spectra were measured in order to study the optical properties of the Ta2O5 NPs, and the results are presented in Figure 3. As can be seen, all Ta2O5 samples

Figure 3. UV−vis diffuse reflectance spectra of Ta2O5 NPs before and after annealing at different temperatures.

presented a strong and characteristic absorption band in the UV range and a second absorption band in the visible range. This visible absorption presented a remarkable intensity for asobtained Ta2O5 NPs, which reduced after annealing at 600 °C and slightly increased again after annealing at 700 and 800 °C. Taking the wavelength at 550 nm as a reference, the asobtained Ta2O5 NPs presented over 39.4% of light absorption, compared to 2.4, 4.6, and 5.2% for samples annealed at 600, 700, and 800 °C, respectively. These results are in agreement with the color alterations observed in the powders (pictures in the top of Figure 3) that changed from gray for as-synthesized Ta2O5 to almost white by increasing the annealing temperature to 800 °C. To obtain a quantitative comparison of this second contribution to the absorption spectra, the absorption curves were divided into two segments; one from 250 to 318 nm, corresponding mostly to the Ta2O5 gap contribution, and other

Table 1. Chemical−Physical Characterization of Ta2O5 Nanoparticles after Annealing at Different Temperaturesa XPS chemical surface composition (%) Ta2O5 NPs

BG1 (eV)

BG1 (eV)

A1

A2

C 1s

O 1s

Ta 4f

O/Ta

Ta4+

SBET (m2 g−1)

as-obtained 600 °C 700 °C 800 °C

3.94 3.98 3.94 3.88

2.70 2.45 2.40 2.30

60.8 54.2 55.2 56.1

201.6 22.4 34.9 35.1

48.34 31.88 31.24 30.50

37.63 48.69 48.34 48.28

14.03 19.43 20.43 21.22

2.68 2.51 2.37 2.28

3.19 3.84 3.56 3.85

253.4 121.2 46.1 30.0

a

BG1 = band gap in the UV region; BG2 = band gap in the visible region; A1 = integrated area under the absorption curve limited to the region 250− 318 nm (i.e., corresponding to BG1 contribution); A2 = integrated area under the absorption curve limited to the region 318−750 nm (i.e., corresponding to BG2 contribution); SBET = specific surface area. E


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Figure 4. Ta 4f high resolution XPS spectra of as-obtained Ta2O5 NPs (A) and Ta2O5 NPs annealed at 600 (B), 700 (C), and 800 °C (D). The insets indicated by black arrows present details of the Ta4+ suboxide contribution. See Table 1 for quantification results.

and 28.26 eV for the as-obtained sample, corresponding to Ta 4f7/2 and 4f5/2, respectively. Exact values for all temperatures are presented in Table S1. As expected, they correspond to Ta5+ with an energy separation of 1.91 eV and area ratio of 3/4. In addition, to completely fit the Ta 4f spectra it was necessary to include a second contribution with a small doublet at 22.43 and 24.34 eVan energy separation of 1.91 eVthat formed a tail at the lower-energy side of the spectrum (regions indicated by black arrows in Figure 4). This second contribution refers to Ta4+ suboxide.49 This peak, around 22 eV, could also be hypothetically attributed to Ta2C formation, as referred to in the literature,50 due mainly to the high carbon content observed in the quantification results, but this was discarded since the corresponding C 1s peak at ∼282 eV could not be detected. Therefore, the as-obtained Ta2O5 NPs sample was self-doped with Ta4+ and impregnated with C-based species, hereinafter called C-impregnated/Ta4+ self-doped Ta2O5 NPs. Specific surface area (SBET) was performed according to the Brunauer− Emmett−Teller (BET) method using nitrogen adsorption isotherms. As can be seen in Table 1, C-impregnated/Ta4+ selfdoped Ta2O5 NPs presented the largest surface area, of 253.4 m2 g−1, compared to the annealed samples. The higher the annealing temperature, the smaller was the surface area of resulting Ta2O5. This reduction in surface area can be directly related to increased average size of the nanoparticles, as demonstrated by TEM and SAXS results. C-impregnated/Ta4+ self-doped Ta2O5 NPs presented a high carbon content, as shown by the XPS analysis. To investigate the origin of this carbon content the organic residue arising from the solvothermal reaction between tantalum pentachloride

the energy at 284.8 eV (Figure S4, Supporting Information). Besides the main peak, two well-defined C 1s peaks at slight higher binding energies were also observed and taken into account. For as-obtained Ta2O5 NPs, these two peaks were centered at 286.3 and 289.3 eV and were attributed to C−O−H (or C−O−C) and O−CO, respectively.45,46 The total carbon content obtained by the quantitative surface analysis was 48.34% for the as-obtained sample, diminishing to 30.5% after annealing at 800 °C, as presented in Table 1. The incorporation of carbon products into the structure and/or surface of amorphous Ta2O5 NPs arises from benzyl alcohol decomposition during solvothermal synthesis, and subsequent reaction during annealing helped the elimination of volatile compounds (see NMR discussion in further detail). It is important to note that the dark gray color obtained for the powders also indicates that the samples were impregnated with C. The O 1s spectra could be decomposed into two components (Figure S5, Supporting Information). The main one, at around 530.5 eV, is attributed to oxygen in tantalum oxide, and the other component, at the higher binding energy of ∼531.6 eV, can be attributed to lattice oxygen atoms; contamination by C−O−H (or C−O−C) and/or oxygen vacancy neighbors of Ta4+ species.28,47,48 This latter component peak did not show a Supporting Information). This result is in accordance with the reduction in the total amount of carbon with annealing. Moreover, FTIR analysis was performed and corroborated the presence of suboxides and carbon impregnation; see Figure S6 of the Supporting Information. Figure 4 shows Ta 4f high-resolution spectra. Two main peaks can easily be identified in all samples, centered at 26.35 F


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Figure 5. RhB photodegradation under UV irradiation for different concentrations of Ta4+ self-doped Ta2O5-800 °C photocatalysts (A). Comparison of RhB photodegradation under UV irradiation using [3 mg mL−1] of C-impregnated Ta4+ self-doped Ta2O5 NPs photocatalysts annealed at different temperatures (B). Photocatalytic activity of the as-obtained Ta4+ self-doped Ta2O5 versus Ta2O5 and TiO2 commercial NPs for RhB photodegradation under UV irradiation (C). Absorbance curve of RhB for the best photocatalyst showing complete removal of the 554 nm absorption band after 12 min of UV irradiation (D).

visible-light absorption ability. In contrast, Ta4+ content did not change significantly with annealing, as shown in Table 1. 3.2. Photocatalytic Activity of Ta2O5 NPs. For the photocatalytic activity study, all experiments were conducted with a 40 mg L−1 RhB solution as standard, since this concentration does not interfere too much with the penetration of incident photons, while the self-degradation effect was lessened, especially for UV light tests. The optimum amount of photocatalyst relative to the dye concentration was studied using UV irradiation and Ta2O5-800 °C photocatalyst, as can be seen in Figure 5A. With increasing mass concentration of the photocatalyst in solution, the photodegradation rate presented two behaviors. There was a considerable increase up to a concentration of 3 mg mL−1, where 100% degradation was achieved within 60 min of UV irradiation, while the photoactivity decreased for higher amounts of photocatalyst, this being associated with the blocking of light passing through the solution due to scattering of photons by the excess of NPs present in the reaction environment.2,16,54 Therefore, the optimum concentration of photocatalyst in RhB solution was found to be 3 mg mL−1. In all cases studied, the photocatalytic degradation of RhB using Ta2O5 NPs as photocatalysts was noticeably higher than direct photolysis (without photocatalyst). This result shows that the Ta2O5 NPs photocatalysts are effective in accelerating the degradation of the dye by converting light energy into chemically active species, attacking and breaking the chemical bonds of the organic pollutants. As different Ta2O5 NPs with distinct physical-chemical properties were obtained after annealing, a study was

and benzyl alcohol was analyzed by NMR spectroscopy. Results revealed that benzyl alcohol was completely converted to several organic byproducts after the solvothermal reaction; see Supporting Information for more details. Among these, two major compounds were identified through one- and twodimension NMR experiments, showing typical CH2 hydrogen neighboring the amine nitrogen at 3.86 and 3.87 ppm, as presented in Table S2 and Figure S7. Benzaldehyde (9.99 and 192.8 ppm), toluene (2.22 and 20.5 ppm), and dibenzyl ether (4.40 and 71.8 ppm) were identified from typical 1H and 13C chemical shifts (Figure S8, Supporting Information). Other several byproducts with signals very close to benzaldehyde, toluene and dibenzyl ether suggested the presence of chlorosubstituted compounds. Indeed, reactions including the disproportionation of benzyl alcohol to form toluene, benzaldehyde and water, and the dehydration of benzyl alcohol to form dibenzyl ether have been reported and corroborate our results.51,52 Although benzoic acid could not be detected, the presence of benzyl benzoate validates the complete oxidation of the benzyl alcohol during the solvothermal synthesis. Therefore, Ta2O5 NPs could probably help the decomposition of benzyl alcohol, acting as a heterogeneous catalyst by creating active sites for the selective alcohol oxidation.53 The formation of the Ta2O5 NPs in the presence of these organic byproducts resulted in the impregnation with C species and at the same time in the creation of oxygen vacancies (Ov). After annealing the samples, the amount of C was reduced, as presented in Table 1, and the sample color changed from gray to almost white, followed by a significant reduction in the G


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The Journal of Physical Chemistry C performed to identify which samples showed better photocatalytic performance under UV irradiation for the photodegradation of RhB. These results are shown in Figure 5B and revealed a noticeable difference in photocatalytic performance of catalysts annealed at different temperatures. Surprisingly, the efficiency of as-obtained C-impregnated/Ta4+ self-doped Ta2O5 NPs was higher than all the other studied samples, resulting in 100% of RhB photodegradation after only 12 min of UV irradiation, whereas using photocatalysts with thermal treatment at 600, 700, and 800 °C yielded, respectively, 34, 48 and 11% photodegradation during the same period. Therefore, photocatalytic activity of C-impregnated/Ta4+ self-doped Ta2O5 NPs decreased after annealing. The phodegradation of RhB under the experimental conditions applied herein notably agreed with the zero-order kinetics. The photodegradation behavior of RhB may be represented by linear decay as shown in eq 2, where [RhB]0 and [RhB]t represents the initial concentration of RhB dye and the concentration at a time interval t, respectively. K is the pseudozero order constant usually expressed in units of concentration divided by time.55,56 [RhB]t = [RhB]0 − kt

(2)

Figure S9 and Table S3 show the linear fits and k values obtained for the photocatalysts studied herein. The highest k value was 4.56 ppm min−1 when using the C-impregnated/Ta4+ self-doped Ta2O5 NPs (as-obtained photocatalyst). It can also be noted that reaction initiated in a pseudozero order and changed to a pseudo-first order kinetic for this photocatalyst after about 7.5 min of UV light irradiation, as observed elsewhere.56 Figure 5C shows the comparison of the photocatalytic activity of as-obtained and commercial Ta2O5 NPs. The results show that the photocatalyst proposed herein possesses superior photocatalytic activity for RhB degradation compared to Degussa TiO2 P25 and Puratronic Ta2O5 NPs. In Figure 5D the complete removal of the RhB after 12 min of UV irradiation using the as-obtained photocatalyst can be visualized by the suppression of the absorption band centered at 554 nm, characteristic of RhB dye, without wavelength shift of the maximum absorption signals. The photoactivities of the photocatalysts under solar irradiation were evaluated using a solar simulator equipped with an AM1.5G filter to ensure reproducibility of the solar spectrum. Figure 6A presents the results of RhB photodegradation where the as-obtained C-impregnated/Ta4+ selfdoped Ta2O5 NPs photocatalyst showed higher efficiency than the annealed samples, as was also observed under UV irradiation. The reactions followed a pseudozero order kinetic under simulated solar irradiation and the linear fits and k values are presented in Figure S10 and Table S3. After 120 min of irradiation, 68% of RhB photodegradation was achieved for the as-obtained photocatalyst, whereas for those annealed at 600, 700, and 800 °C it was 28, 12, and 45%, respectively. It is worthy to note that photodegradation of RhB was faster when using UV irradiation compared to simulated solar light because of the higher absorption ability of the photocatalyst to harvest UV photons. However, as measured by UV−vis diffuse reflectance, the C-impregnated/Ta4+ self-doped Ta2O5 NPs presented absorption extending to wavelengths in the visible range, which also contributed to the total photocatalytic activity. The quantum apparent efficiency estimated for the as-obtained C-impregnated/Ta4+ self-doped Ta2O5 NPs photocatalyst is 3.6% at λ = 447 nm, revealing an unprecedented

Figure 6. RhB photodegradation under simulated solar irradiation using Ta4+ self-doped Ta2O5 NPs photocatalysts [3 mg mL−1] (A). Final RhB degradation after 120 min of simulated solar irradiation with and without using the 400 nm cutoff filter (B). Reusability of Cimpregnated Ta4+ self-doped Ta2O5 NPs photocatalyst (as-obtained) under solar irradiation for photodegradation of RhB (C).

result regarding photodegradation efficiency of organic pollutants with tantalum oxide photocatalysts under visible irradiation. In order to evaluate the visible light activity of the asobtained photocatalyst, RhB photodegradation experiments were performed by using a 400 nm cutoff filter, and the results compared with the sample annealed at 800 °C, Figure 6B. Both photocatalysts (before and after annealing) were active for RhB photodegradation under visible light irradiation. As-obtained and 800 °C-annealed C-impregnated/Ta4+ self-doped Ta2O5 NPs induced almost 43% and 38% photodegradation of RhB, H


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Figure 7. Steady-state PL emission spectra of the as-obtained C-impregnated/Ta4+ self-doped Ta2O5 NPs, commercial Ta2O5 and Ta2O5-800 °C photocatalysts for light excitation wavelengths of 280 nm (A) and 370 nm (B). Time-resolved PL spectra of the as-obtained C/Ta4+ - Ta2O5 NPs, commercial Ta2O5 and Ta2O5-800 °C photocatalysts (C). Scheme of the photocatalytic activity of the C-impregnated Ta4+ self-doped Ta2O5 NP photocatalysts (D).

neutral pH, while the 800 °C annealed sample have a negatively charged surface with ζ-potential values of −25.9 ± 0.6 mV. This result indicates that the surface charge is substantially influenced by the annealing process. As the as-obtained Ta2O5 NPs is positively charged and showed the best photocatalytic activity, we discard that electrostatic attraction was the main factor favoring the adsorption of cationic RhB dye molecules. 3.3. Photocatalytic Mechanism. The high photocatalytic activity observed for C-impregnated/Ta4+ self-doped Ta2O5 may be a combined result of elevated surface area, a capability to harvest efficiently UV and visible light, reduced electron− hole recombination, and longer electron lifetimes in the excited sub-band (as will be discussed in the present section). The first property is due to formation of ultrafine NPs and the others may arise from a synergism of C-impregnation and Ta4+ selfdoping, creating a unique and novel electronic structure with an additional band gap in the visible range. In order to demonstrate the existence of the additional energy band between valence (VB) and conduction bands (CB), hereafter called the Ov/Ta4+ sub-band, and the correlation between the Ov/Ta4+ sub-band and the visiblelight photocatalytic activity observed, a close photoluminescence analysis was performed by steady-state and time-resolved fluorescence measurements. Parts A and B of Figures 7 present the PL spectra of as-obtained and 800 °C-annealed samples, as well as the PL spectrum of a commercial Ta2O5 sample for comparison, using excitation wavelengths of 280 and 370 nm.

respectively, during the first 120 min under visible light irradiation (λ > 400 nm). The C-impregnated/Ta4+ self-doped Ta2O5 NPs photocatalyst was finally tested for reusability under solar irradiation, as presented in Figure 6C. The results show that after the first, second, and third run, the photodegradations (mean value ± standard deviation) were 73.9 ± 4.9%, 69.2 ± 4.4%, and 67.6 ± 7.3%, respectively. Therefore, the photocatalyst preserved almost constant photocatalytic properties after three runs, with very low tendency of photoactivity decrease, showing that these photocatalyst are promising candidates for application in wastewater treatment. Actually, only few works could be found in the literature discussing the use of pure (or doped) Ta2O5 as photocatalyst for photodegradation of RhB dye.15,28,57−64 Table S4 presents a summary of these works. As it can be seen, several authors do not mention the radiometric or photometric characteristic (W m−2, lm m−2, etc.) of the used light sources when reporting their photocatalytic activities, which make difficult to perform a suitable comparison. Even so, to the best of our knowledge, the photocatalytic activities described here are the largest reported to date for photodegradation of RhB using powder Ta2O5 photocatalysts. As-obtained and 800 °C annealed samples were submitted to ζ potential analysis in order to determine the surface charge of the photocatalysts and a possible electrostatic interaction with RhB. The as-obtained Ta2O5 NPs show a positively charged surface with the averaged ζ-potential value of 7.5 ± 0.3 mV at I


Article

The Journal of Physical Chemistry C The results show that the PL spectrum of the 800 °C-annealed sample is similar to that observed for the commercial sample, presenting an emission band in the UV range (290−400 nm) with a maximum at 320 nm (∼3.9 eV) for excitation at 270 nm, which is a result of radiative transition of the electrons from the CB to VB of Ta2O5. In contrast, the as-obtained sample has almost zero PL emission originating from CB to VB of Ta2O5, emitting mainly in the visible range (between 400 and 525 nm) with a maximum at 460 nm (∼2.7 eV), in agreement with the energy of BG2 observed in Table 1. Therefore, our results show that after promotion of the electrons to the Ta2O5 CB, they migrate to the Ov/Ta4+ sub-band energy level by nonradiative transitions, and then return to the VB emitting in the blue region. In addition, time-resolved PL results (Figure 7C) revealed that the electrons have a longer lifetime in the excited state when migrating to the Ov/Ta4+ sub-band, as observed for the as-obtained sample. The PL lifetime of the electrons in the Ov/Ta4+ sub-band of as-obtained C-impregnated/Ta4+ selfdoped Ta2O5 NPs was almost three times higher (7.2 ns) than the PL lifetime of the CB electrons in the 800 °C-annealed (2.8 ns) and commercial Ta2O5 (2.6 ns). These PL results confirm that the as-obtained sample has a greater capacity for absorbing light in the visible range due to the existence of the Ov/Ta4+ sub-band. The Ta4+ species neighboring Ov defects were generated during the solvothermal reaction using benzyl alcohol as medium, forming a new defect sub-band energy level between the VB maximum and CB minimum of C-impregnated/T4+ self-doped Ta2O5 NPs and, consequently, making visible light absorption possible. Additionally to the capacity of absorbing light in visible range, the PL results also demonstrated that the longer lifetime of the electrons in the excited state of the Ov/Ta4+ sub-band contributes significantly to enhancing the photocatalytic activity. However, it is important to point out that the observed longer lifetime is probably due to a synergistic effect of Cimpregnation and Ta4+ self-doping, as no longer electron lifetime was detected for the 800 °C-annealed sample, as presented in Figure 7C (i.e., no longer electron lifetime was obtained after reducing the C content in the annealed samples). In fact, it is well established that longer electron lifetimes can avoid charge recombination by facilitating photoinduced electron−hole separation in the semiconductor. Similar results were recently reported in the literature,28 indicating that Ta− O−C chemical bonds play a key role in the separation of photoinduced electrons and hole pairs, facilitating charge transfer. Figure 7D presents a scheme for the photocatalytic activity of the C-impregnated/Ta4+ self-doped Ta2O5 NPs photocatalysts. As the best photocatalyst proposed herein, the as-obtained Cimpregnated/Ta4+ self-doped Ta2O5 NPs absorb UV and visible light simultaneously, promoting electrons from Ta2O5 VB to Ta2O5 CB and then to the [Ov•Ta4+]+ sub-band, leaving holes in the Ta2O5 VB. After photoexcitation and electron− hole separation, the electrons migrate to the sub-band by nonradiative processes, having enough time to diffuse to the NP surface to react with O2 and form •O2− anionic radicals which, in turn, form •OOH and •OH radicals through a sequence of reactions. As consequence, the electron−hole pair recombination is suppressed, allowing the holes also to migrate to the photocatalyst surface to oxidize water to •OH radicals or to oxidize RhB molecules directly.

4. CONCLUSIONS C-impregnated/Ta4+ self-doped Ta2O5 NPs were prepared successfully for the first time by a one-step solvothermal reaction using benzyl alcohol as solvent. The as-obtained Ta2O5 NPs were spherical in shape, with 3.3 nm mean diameter, and presenting high surface area (253.4 m2 g−1), amorphous structure, self-doping with Ta4+ and high carbon content. They resulted in gray-colored powders with two distinct band gaps; one in the UV range (∼3.9 eV), as expected for Ta2O5, and another in the visible range (2.7−2.3 eV). This additional electronic band in the visible region for the C-impregnated/ Ta4+ self-doped Ta2O5, in association with its longer electron lifetime and high surface area, was responsible for superior photocatalytic activity under UV irradiation compared to commercial TiO2 and Ta2O5 NPs. In addition, the Cimpregnated/Ta4+ self-doped Ta2O5 photocatalysts were also active under visible light irradiation, reaching almost 43% RhB photodegradation during the first 120 min of reaction with an unprecedented apparent quantum efficiency of 3.6% at 447 nm. The best photocatalyst was reused and was revealed to be stable after three runs, showing that the material herein proposed is a promising candidate for photocatalytic applications. Annealing was detrimental to the photocatalytic activity of the samples under UV, visible or simulated solar irradiation. The results revealed that the size of the NPs increased with annealing, strongly decreasing their surface area. In addition, the material structure underwent a transformation from amorphous to crystalline hexagonal system at 600 °C, and orthorhombic at 700 and 800 °C. XPS results showed that the Ta2O5 NPs were self-doped with approximately 4% of Ta4+, which did not change with annealing. In contrast, the amount of carbon seen on the photocatalyst surface decreased from 48.3% for the asobtained sample to 30.5% after annealing at 800 °C. The PL results confirmed the existence of an additional energy band, i.e., a sub-band, between the valence and conduction bands of Ta2O5, due to the presence of Ta4+ species neighboring oxygen vacancy defects. Furthermore, the time-resolved PL also demonstrated that the electron lifetime in the Ov/Ta4+ subband of the as-obtained C-impregnated/Ta4+ self-doped Ta2O5 NPs was almost three times higher than in Ta2O5-800 °C or commercial Ta2O5, suppressing the electron−hole pair recombination.

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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.7b11822. NMR experimental conditions and analysis, ζ potential experimental conditions, SAXS fittings, Tauc plots used for calculating indirect band gaps, survey of C 1s and O 1s XPS spectra, FTIR spectra, and photocatalytic kinetic evaluations (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(H.W.) E-mail: [email protected]; hbtwender@ gmail.com. ORCID

Renato V. Gonçalves: 0000-0002-3372-6647 Glaucia B. Alcantara: 0000-0003-2549-3000 Heberton Wender: 0000-0002-1417-3581 J


Article

The Journal of Physical Chemistry C Author Contributions

Ta2O5 Nanotubes in the Catalytic Oxidation of CO. Phys. Chem. Chem. Phys. 2014, 16 (12), 5755. (13) Baruwati, B.; Varma, R. S. Synthesis of Monodispersed tantalum(V) Oxide Nanospheres by an Ethylene Glycol Mediated Route. Cryst. Growth Des. 2010, 10 (8), 3424−3428. (14) Lü, X.; Ding, S.; Lin, T.; Mou, X.; Hong, Z.; Huang, F. Ta2O5 Nanowires: A Novel Synthetic Method and Their Solar Energy Utilization. Dalton Trans. 2012, 41 (2), 622−627. (15) Tao, C.; Xu, L.; Guan, J. Well-Dispersed Mesoporous Ta2O5 Submicrospheres: Enhanced Photocatalytic Activity by Tuning Heating Rate at Calcination. Chem. Eng. J. 2013, 229, 371−377. (16) Wender, H.; Gonçalves, R. V.; Dias, C. S. B.; Zapata, M. J. M.; Zagonel, L. F.; Mendonça, E. C.; Teixeira, S. R.; Garcia, F. Photocatalytic Hydrogen Production of Co(OH)2 NanoparticleCoated α-Fe2O3 Nanorings. Nanoscale 2013, 5 (19), 9310. (17) Wender, H.; Feil, A. F.; Diaz, L. B.; Ribeiro, C. S.; Machado, G. J.; Migowski, P.; Weibel, D. E.; Dupont, J.; Teixeira, S. R. S. R. SelfOrganized TiO2 Nanotube Arrays: Synthesis by Anodization in an Ionic Liquid and Assessment of Photocatalytic Properties. ACS Appl. Mater. Interfaces 2011, 3 (4), 1359−1365. (18) Cavalcante, R. P.; Dantas, R. F.; Wender, H.; Bayarri, B.; González, O.; Giménez, J.; Esplugas, S.; Machulek, A. Photocatalytic Treatment of Metoprolol with B-Doped TiO2: Effect of Water Matrix, Toxicological Evaluation and Identification of Intermediates. Appl. Catal., B 2015, 176−177, 173−182. (19) Zhou, Y.; Chen, C.; Wang, N.; Li, Y.; Ding, H. Stable Ti3+ SelfDoped Anatase-Rutile Mixed TiO2 with Enhanced Visible Light Utilization and Durability. J. Phys. Chem. C 2016, 120 (11), 6116− 6124. (20) Gonçalves, R. V.; Wender, H.; Migowski, P.; Feil, A. F.; Eberhardt, D.; Boita, J.; Khan, S.; Machado, G.; Dupont, J.; Teixeira, S. R. Photochemical Hydrogen Production of Ta2O5 Nanotubes Decorated with NiO Nanoparticles by Modified Sputtering Deposition. J. Phys. Chem. C 2017, 121 (11), 5855−5863. (21) Reza Gholipour, M.; Dinh, C.-T.; Béland, F.; Do, T.-O. Nanocomposite Heterojunctions as Sunlight-Driven Photocatalysts for Hydrogen Production from Water Splitting. Nanoscale 2015, 7 (18), 8187−8208. (22) Grewe, T.; Tüysüz, H. Alkali Metals Incorporated Ordered Mesoporous Tantalum Oxide with Enhanced Photocatalytic Activity for Water Splitting. J. Mater. Chem. A 2016, 4 (8), 3007−3017. (23) Yan, H.; Wang, X.; Yao, M.; Yao, X. Band Structure Design of Semiconductors for Enhanced Photocatalytic Activity: The Case of TiO2. Prog. Nat. Sci. 2013, 23 (4), 402−407. (24) Li, J.; Wu, N. Semiconductor-Based Photocatalysts and Photoelectrochemical Cells for Solar Fuel Generation: A Review. Catal. Sci. Technol. 2015, 5 (3), 1360−1384. (25) Murase, T.; Irie, H.; Hashimoto, K. Visible Light Sensitive Photocatalysts, Nitrogen-Doped Ta2O5 Powders. J. Phys. Chem. B 2004, 108 (40), 15803−15807. (26) Zhu, G.; Lin, T.; Cui, H.; Zhao, W.; Zhang, H.; Huang, F. Gray Ta2O5 Nanowires with Greatly Enhanced Photocatalytic Performance. ACS Appl. Mater. Interfaces 2016, 8 (1), 122−127. (27) Yu, X.; Li, W.; Li, Z.; Liu, J.; Hu, P. Defect Engineered Ta2O5 Nanorod: One-Pot Synthesis, Visible-Light Driven Hydrogen Generation and Mechanism. Appl. Catal., B 2017, 217, 48−56. (28) Yu, X.; Li, Z.; Liu, J.; Hu, P. TaOC Chemical Bond Enhancing Charge Separation between Ta4+ Doped Ta2O5 Quantum Dots and Cotton-like g-C3N4. Appl. Catal., B 2017, 205, 271−280. (29) Zhou, Y.; Wang, Y.; Wen, T.; Chang, B.; Guo, Y.; Lin, Z.; Yang, B. Enhanced Visible-Light-Driven Photocatalytic Activity in Yellow and Black Orthorhombic NaTaO3 Nanocubes by Surface Modification and Simultaneous N/Ta4+ Co-Doping. J. Colloid Interface Sci. 2016, 461, 185−194. (30) Wang, J.; Su, S.; Liu, B.; Cao, M.; Hu, C. One-Pot, LowTemperature Synthesis of Self-Doped NaTaO3 Nanoclusters for Visible-Light-Driven Photocatalysis. Chem. Commun. 2013, 49 (71), 7830−7832.

The manuscript was written with contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would greatly thank CAPES (L.E.G. M.Sc. funding), CNPq (Project Nos. 486342/2013-1, 427835/20160, 311798/2014-4 and 310066/2017-4) and FUNDECT (Process No. 23/200.247/2014). Our thanks are extended to ́ the “Laboratório Nacional de Luz Sincrotron (LNLS)” for SAXS beamline (SAXS1-proposal IDs: 20160302; 17986); to the “Laboratório Nacional de Nanotecnologia (LNNano)” for XPS measurements (Proposals XPS-20373 and XPS-18144). The authors also acknowledge Prof. Dr. Gleison A. Casagrande and Prof. Dr. Marco A. U. Martines for UV−vis diffuse reflectance and ζ potential measurements, respectively; and Prof. Dr. Samuel L. de Oliveira for providing access to UV−vis and FTIR equipment.

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REFERENCES

(1) Zhang, P.; Zhang, J.; Gong, J. Tantalum-Based Semiconductors for Solar Water Splitting. Chem. Soc. Rev. 2014, 43 (13), 4395. (2) Gonçalves, R. V.; Migowski, P.; Wender, H.; Eberhardt, D.; Weibel, D. E.; Sonaglio, F. C.; Zapata, M. J. M.; Dupont, J.; Feil, A. F.; Teixeira, S. R. Ta2O5 Nanotubes Obtained by Anodization: Effect of Thermal Treatment on the Photocatalytic Activity for Hydrogen Production. J. Phys. Chem. C 2012, 116 (26), 14022−14030. (3) Zhang, P.; Wang, T.; Chang, X.; Gong, J. Effective Charge Carrier Utilization in Photocatalytic Conversions. Acc. Chem. Res. 2016, 49 (5), 911−921. (4) Li, J.; Dai, W.; Wu, G.; Guan, N.; Li, L. Fabrication of Ta2O5 Films on Tantalum Substrate for Efficient Photocatalysis. Catal. Commun. 2015, 65, 24−29. (5) Jiang, Y.; Liu, P.; Liu, Y.; Liu, X.; Li, F.; Ni, L.; Yan, Y.; Huo, P. Construction of Amorphous Ta2O5/g-C3N4 Nanosheet Hybrids with Superior Visible-Light Photoactivities for Organic Dye Degradation and Mechanism Insight. Sep. Purif. Technol. 2016, 170, 10−21. (6) Gömpel, D.; Tahir, M. N.; Panthöfer, M.; Mugnaioli, E.; Brandscheid, R.; Kolb, U.; Tremel, W. Facile Hydrothermal Synthesis of Crystalline Ta2O5 Nanorods, MTaO3 (M = H, Na, K, Rb) Nanoparticles, and Their Photocatalytic Behaviour. J. Mater. Chem. A 2014, 2 (21), 8033. (7) Sreethawong, T.; Ngamsinlapasathian, S.; Yoshikawa, S. Facile Surfactant-Aided Sol−gel Synthesis of Mesoporous-Assembled Ta2O5 Nanoparticles with Enhanced Photocatalytic H2 Production. J. Mol. Catal. A: Chem. 2013, 374−375, 94−101. (8) Chu, K.; Chang, J. P.; Steigerwald, M. L.; Fleming, R. M.; Opila, R. L.; Lang, D. V.; Van Dover, R. B.; Jones, C. D. W. Material and Electrical Characterization of Carbon-Doped Ta2O5 Films for Embedded Dynamic Random Access Memory Applications. J. Appl. Phys. 2002, 91 (1), 308. (9) Alimardani, N.; King, S. W.; French, B. L.; Tan, C.; Lampert, B. P.; Conley, J. F. Investigation of the Impact of Insulator Material on the Performance of Dissimilar Electrode Metal-Insulator-Metal Diodes. J. Appl. Phys. 2014, 116 (2), 024508. (10) Gu, T.; Tada, T.; Watanabe, S. Conductive Path Formation in the Ta2O5 Atomic Switch: First-Principles Analyses. ACS Nano 2010, 4 (11), 6477−6482. (11) Koc, K.; Tepehan, F. Z.; Tepehan, G. Antireflecting Coating from Ta2O5 and SiO2 Multilayer Films. J. Mater. Sci. 2005, 40, 1363− 1366. (12) Gonçalves, R. V.; Wojcieszak, R.; Uberman, P. M.; Teixeira, S. R.; Rossi, L. M. Insights into the Active Surface Species Formed on K


Article

The Journal of Physical Chemistry C (31) Xing, M.; Fang, W.; Nasir, M.; Ma, Y.; Zhang, J.; Anpo, M. SelfDoped Ti3+-Enhanced TiO2 Nanoparticles with a High-Performance Photocatalysis. J. Catal. 2013, 297, 236−243. (32) Buha, J.; Arčon, D.; Niederberger, M.; Djerdj, I. Solvothermal and Surfactant-Free Synthesis of Crystalline Nb2O5, Ta2O5, HfO2, and Co-Doped HfO2 Nanoparticles. Phys. Chem. Chem. Phys. 2010, 12 (47), 15537. (33) Adhikari, S. P.; Hood, Z. D.; More, K. L.; Ivanov, I.; Zhang, L.; Gross, M.; Lachgar, A. Visible Light Assisted Photocatalytic Hydrogen Generation by Ta 2 O5 /Bi 2 O 3 , TaON/Bi 2 O 3 , and Ta 3 N 5 /Bi 2 O 3 Composites. RSC Adv. 2015, 5 (68), 54998−55005. (34) Wender, H.; de Oliveira, L. F.; Feil, A. F.; Lissner, E.; Migowski, P.; Meneghetti, M. R.; Teixeira, S. R.; Dupont, J. Synthesis of Gold Nanoparticles in a Biocompatible Fluid from Sputtering Deposition onto Castor Oil. Chem. Commun. 2010, 46 (37), 7019. (35) Braslavsky, S. E.; Braun, A. M.; Cassano, A. E.; Emeline, A. V.; Litter, M. I.; Palmisano, L.; Parmon, V. N.; Serpone, N. Glossary of Terms Used in Photocatalysis and Radiation Catalysis (IUPAC Recommendations 2011). Pure Appl. Chem. 2011, 83 (4), 931. (36) Serpone, N. Relative Photonic Efficiencies and Quantum Yields in Heterogeneous Photocatalysis. J. Photochem. Photobiol., A 1997, 104 (1−3), 1−12. (37) Li, Q.; Guo, B.; Yu, J.; Ran, J.; Zhang, B.; Yan, H.; Gong, J. R. Highly Efficient Visible-Light-Driven Photocatalytic Hydrogen Production of CdS-Cluster-Decorated Graphene Nanosheets. J. Am. Chem. Soc. 2011, 133 (28), 10878−10884. (38) Bright, T. J.; Watjen, J. I.; Zhang, Z. M.; Muratore, C.; Voevodin, A. A.; Koukis, D. I.; Tanner, D. B.; Arenas, D. J. Infrared Optical Properties of Amorphous and Nanocrystalline Ta2O5 Thin Films. J. Appl. Phys. 2013, 114 (8), 083515. (39) López, R.; Gómez, R. Band-Gap Energy Estimation from Diffuse Reflectance Measurements on Sol−gel and Commercial TiO2: A Comparative Study. J. Sol-Gel Sci. Technol. 2012, 61 (1), 1−7. (40) Köferstein, R.; Jäger, L.; Ebbinghaus, S. G. Magnetic and Optical Investigations on LaFeO3 Powders with Different Particle Sizes and Corresponding Ceramics. Solid State Ionics 2013, 249−250, 1−5. (41) Park, J. H.; Kim, S.; Bard, A. J. Novel Carbon-Doped TiO2 Nanotube Arrays with High Aspect Ratios for Efficient Solar Water Splitting. Nano Lett. 2006, 6 (1), 24−28. (42) Dong, F.; Guo, S.; Wang, H.; Li, X.; Wu, Z. Enhancement of the Visible Light Photocatalytic Activity of C-Doped TiO2 Nanomaterials Prepared by a Green Synthetic Approach. J. Phys. Chem. C 2011, 115 (27), 13285−13292. (43) Dabirian, A.; Van De Krol, R. High-Temperature Ammonolysis of Thin Film Ta2O5 Photoanodes: Evolution of Structural, Optical, and Photoelectrochemical Properties. Chem. Mater. 2015, 27 (3), 708−715. (44) Yu, H.; Sun, D.; Liu, J.; Fang, Y.; Li, C. C. Monodisperse Mesoporous Ta2O5 Colloidal Spheres as a Highly Effective Photocatalyst for Hydrogen Production. Int. J. Hydrogen Energy 2016, 41 (39), 17225−17232. (45) Eshkenazi, V.; Peled, E.; Burstein, L.; Golodnitsky, D. XPS Analysis of the SEI Formed on Carbonaceous Materials. Solid State Ionics 2004, 170 (1−2), 83−91. (46) Ni, M.; Ratner, B. D. Differentiating Calcium Carbonate Polymorphs by Surface Analysis Techniques - An XPS and TOF-SIMS Study. Surf. Interface Anal. 2008, 40 (10), 1356−1361. (47) Atanassova, E.; Spassov, D. X-Ray Photoelectron Spectroscopy of Thermal Thin Ta2O5 Films on Si. Appl. Surf. Sci. 1998, 135 (1−4), 71−82. (48) Tsuchiya, T.; Imai, H.; Miyoshi, S.; Glans, P.-A.; Guo, J.; Yamaguchi, S. X-Ray Absorption, Photoemission Spectroscopy, and Raman Scattering Analysis of Amorphous Tantalum Oxide with a Large Extent of Oxygen Nonstoichiometry. Phys. Chem. Chem. Phys. 2011, 13 (38), 17013. (49) Muto, A.; Yano, F.; Sugawara, Y.; Iijima, S. The Study of Ultrathin Tantalum Oxide Films before and after Annealing with XRay Photoelectron Spectroscopy. Jpn. J. Appl. Phys. 1994, 33 (5R), 2699.

(50) Stern, K. H. Theme and Variations on Tantalum-Carbonate Reactions in Molten Fluorides. J. Electrochem. Soc. 1989, 136 (12), 3760. (51) Sankar, M.; Nowicka, E.; Tiruvalam, R.; He, Q.; Taylor, S. H.; Kiely, C. J.; Bethell, D.; Knight, D. W.; Hutchings, G. J. Controlling the Duality of the Mechanism in Liquid-Phase Oxidation of Benzyl Alcohol Catalysed by Supported Au-Pd Nanoparticles. Chem. - Eur. J. 2011, 17 (23), 6524−6532. (52) Albonetti, S.; Mazzoni, R.; Cavani, F. CHAPTER 1. Homogeneous, Heterogeneous and Nanocatalysis. Transition Metal Catalysis in Aerobic Alcohol Oxidation 2014, 1−39. (53) Alhumaimess, M.; Lin, Z.; Weng, W.; Dimitratos, N.; Dummer, N. F.; Taylor, S. H.; Bartley, J. K.; Kiely, C. J.; Hutchings, G. J. Oxidation of Benzyl Alcohol by Using Gold Nanoparticles Supported on Ceria Foam. ChemSusChem 2012, 5 (1), 125−131. (54) Guettaï, N.; Ait Amar, H. Photocatalytic Oxidation of Methyl Orange in Presence of Titanium Dioxide in Aqueous Suspension. Part I: Parametric Study. Desalination 2005, 185 (1−3), 427−437. (55) Gaya, U. I.; Abdullah, A. H.; Zainal, Z.; Hussein, M. Z. Photocatalytic Treatment of 4-Chlorophenol in Aqueous ZnO Suspensions: Intermediates, Influence of Dosage and Inorganic Anions. J. Hazard. Mater. 2009, 168 (1), 57−63. (56) Ding, Y.; Yang, F.; Zhu, L.; Wang, N.; Tang, H. Bi3+ Self Doped NaBiO3 Nanosheets: Facile Controlled Synthesis and Enhanced Visible Light Photocatalytic Activity. Appl. Catal., B 2015, 164, 151− 158. (57) Jiang, S.; Guo, Y.; Wang, C.; Qu, X.; Li, L. One-Step Sol−gel Preparation and Enhanced Photocatalytic Activity of Porous Polyoxometalate−tantalum Pentoxide Nanocomposites. J. Colloid Interface Sci. 2007, 308 (1), 208−215. (58) Yang, X.; Xu, L.; Yu, X.; Li, W.; Li, K.; Huo, M.; Guo, Y. Enhanced Photocatalytic Activity of Eu2O3/Ta2O5 Mixed Oxides on Degradation of Rhodamine B and 4-Nitrophenol. Colloids Surf., A 2008, 320 (1−3), 61−67. (59) Guo, G.; Huang, J. Preparation of Mesoporous Tantalum Oxide and Its Enhanced Photocatalytic Activity. Mater. Lett. 2011, 65 (1), 64−66. (60) Ambreen, S.; Pandey, N. D.; Mayer, P.; Pandey, A. Characterization and Photocatalytic Study of Tantalum Oxide Nanoparticles Prepared by the Hydrolysis of Tantalum Oxo-Ethoxide Ta8(μ3-O)2(μ-O)8(μ-OEt)6(OEt)14. Beilstein J. Nanotechnol. 2014, 5 (1), 1082−1090. (61) Zhou, C.; Shang, L.; Yu, H.; Bian, T.; Wu, L. Z.; Tung, C. H.; Zhang, T. Mesoporous Plasmonic Au-Loaded Ta2O5 Nanocomposites for Efficient Visible Light Photocatalysis. Catal. Today 2014, 225, 158−163. (62) Li, J.; Dai, W.; Yan, J.; Wu, G.; Li, L.; Guan, N. Hydrothermal Synthesis and Photocatalytic Properties of Tantalum Pentoxide Nanorods. Chin. J. Catal. 2015, 36 (3), 432−438. (63) Ismail, A. A.; Faisal, M.; Harraz, F. A.; Al-Hajry, A.; Al-Sehemi, A. G. Synthesis of Mesoporous Sulfur-Doped Ta2O5 Nanocomposites and Their Photocatalytic Activities. J. Colloid Interface Sci. 2016, 471, 145−154. (64) Ambreen, S.; Danish, M.; Pandey, N. D.; Pandey, A. Investigation of the Photocatalytic Efficiency of Tantalum Alkoxy Carboxylate-Derived Ta2O5 Nanoparticles in Rhodamine B Removal. Beilstein J. Nanotechnol. 2017, 8, 604−613.

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