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NaTaO3 heterojunction interface for enhanced photocatalytic and photoelectrochemical H2 generation
Journal of Photochemistry & Photobiology A: Chemistry 391 (2020) 112363
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Tailoring charge transport in BaBiO3/NaTaO3 heterojunction interface for enhanced photocatalytic and photoelectrochemical H2 generation
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J.M. Mora-Hernandeza, Ali M. Huerta-Floresb, Leticia M. Torres-Martínezb,* a
CONACYT - Universidad Autónoma de Nuevo León, UANL, Facultad de Ingeniería Civil, Departamento de Ecomateriales y Energía, Av. Universidad S/N Ciudad Universitaria, San Nicolás de los Garza, Nuevo León, C.P. 66455, Mexico b Universidad Autónoma de Nuevo León, UANL, Facultad de Ingeniería Civil, Departamento de Ecomateriales y Energía, Av. Universidad S/N Ciudad Universitaria, San Nicolás de los Garza, Nuevo León, C.P. 66455, Mexico
A R T I C LE I N FO
A B S T R A C T
Keywords: Hydrogen production Heterogeneous photocatalysis Photoelectrochemistry
Photoactive perovskites NaTaO3 and BaBiO3 were synthesized by the solid-state method. The resulting compounds were mixed by wet impregnation, adding BaBiO3 (BBO) onto the base material NaTaO3 (NTO) to obtain the compounds X-BBO/NTO (X = 5–30 wt.%). The sample 20-BBO/NTO presented the largest crystallites (135 nm). Since BaBiO3 presents narrower Eg value (≈ 2 eV) than NaTaO3 (≈ 4 eV), the addition of BBO to NTO promoted a better visible light harvesting. This feature can be explained by the formation of a heterojunction, which improves the photocatalytic activity by an abatement of the charge carrier recombination. Photoelectrochemical tests showed an improvement in the charge transfer resistance and enhanced photoelectrocatalytic activity for the 20-BBO/NTO sample, over the other BBO concentrations and bare photocatalysts. An optimal amount of BaBiO3 (20 wt.%) increased the synergy of the n-type heterojunction by an enhancement in the charge transfer processes to perform the photoelectrocatalytic water splitting. 20-BBO/NTO sample showed the highest hydrogen production of 54 μmol g−1, which outstripped 1.4 times the photocatalytic hydrogen generation of the bare NaTaO3 semiconductor.
1. Introduction Since the report of hydrogen evolution from water splitting in 1972 using TiO2 photocatalyst [1], numerous materials have been developed for achieving competitive efficiencies in this and other related applications, such as organic dye degradation [2] and carbon dioxide reduction [3]. Titanates, niobates, and tantalates have exhibited promising efficiencies [4], but their activity is limited to UV light, using only around 4% of the solar spectrum [5]. NaTaO3 (NTO) is a perovskite structured photocatalyst with a bandgap of around 4 eV that has shown higher efficiencies than TiO2 for water splitting due to the negative character of the conduction band (CB) composed by Ta 5d orbitals [6]. To extend the light absorption capacity of NaTaO3 into the visible range, different strategies have been employed, including synthesis methods [7–10], cation [11] and anion doping [12], the use of co-catalysts [13], the formation of solid solutions [14], and the coupling with other materials such as amorphous carbon [15] g-C3N4 [16], Ag2O [17], RuO2 [8], NiO [18], and SrTiO3 [19], among others. On the other hand, BaBiO3 (BBO) perovskite is a semiconductor that has gained considerable attention in the field of photocatalysis due to
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the low bandgap (≈ 2 eV) and high capacity to harvest visible light [20], originated from the hybridized O 2p and Bi 6s orbitals in the valence band (VB) which narrow the bandgap [21]. Also, the charges exhibit high mobility in BaBiO3 due to the high dispersion of the s orbital in this material [22]. BaBiO3 powders have been successfully synthesized by solid-state and some wet-chemistry methods [23,24]. This semiconductor has been applied as powders in the removal of methylene blue and acetaldehyde [22], the degradation of rhodamine B, showing an excellent stability and attractive yields (80% of RhB degradation), the production of hydrogen (61 μmol g−1 h−1) [24], as well as thin films for photoelectrochemical water splitting and photocatalytic H2 evolution and CO2 reduction [25]. It has also been studied in composites with n-type TiO2 for the photocatalytic degradation of gaseous benzene [26], enhancing the activity of pure TiO2. These results indicate that BaBiO3 can act efficiently as a sensitizer of a large bandgap semiconductor, such as NaTaO3, and extend its activity in the visible-light range. Moreover, it is expected that the electrons injection from BaBiO3 to NaTaO3 promotes an improvement in the charge transport properties, thus enhancing the photocatalytic activity of the heterojunction to produce H2.
Corresponding author. E-mail address: [email protected] (L.M. Torres-Martínez).
https://doi.org/10.1016/j.jphotochem.2020.112363 Received 31 October 2019; Received in revised form 7 December 2019; Accepted 7 January 2020 Available online 07 January 2020 1010-6030/ © 2020 Elsevier B.V. All rights reserved.
Journal of Photochemistry & Photobiology A: Chemistry 391 (2020) 112363
J.M. Mora-Hernandez, et al.
In this work, we report for the first time the production through a fast impregnation method of a BaBiO3-NaTaO3 heterojunction (BBO/ NTO) with high photocatalytic activity for the hydrogen evolution. The appropriate semiconductor coupling allowed an efficient charge separation and migration in the interface of the heterojunction as well as an improved photocatalytic performance. A complete study of the physicochemical properties of the bare and heterostructured photocatalysts is included, as well as the effect of the variation of BaBiO3NaTaO3 proportions on the photocatalytic performance and the mechanism of charge transference involved.
2.4. Photocatalytic hydrogen generation To perform the hydrogen generation tests, 200 mg of the photocatalysts powders were dispersed by vigorous stirring in 200 mL of deionized water into a 250 mL Pyrex reactor at room temperature without the addition of sacrificial agents. Then, nitrogen gas was bubbled through the solution for 30 min to remove the oxygen dissolved in water. A UV pen-ray lamp with an emission peak centered at 254 nm and a nominal power of 4400 μW cm−2 was used as a lighting source; the hydrogen evolution was monitored by a gas chromatograph (Thermo Scientific, TCD detector and fused silica column, 30 mm X 0.53 mm), using N2 as the gas carrier. The reaction time under lighting conditions and continuous stirring was 3 h; samples were taken at intervals of 30 min.
2. Experimental section 2.1. Synthesis of the NaTaO3 via solid-state method In the synthesis of NaTaO3, Ta2O5 and Na2CO3 were used as precursors, both reagents were dried and stoichiometrically weighed (5 wt. % excess of Na2CO3 to avoid sodium evaporation). Then, the reagents were mixed and ground into an agate mortar in the presence of acetone. After the acetone evaporation, the mixed powder was placed into a platinum crucible and heat threated at 600 °C for 12 h to degas CO2. The sample was ground one more time using acetone; then, the powder sample was annealed at 800 °C for 12 h in air atmosphere until the formation of NaTaO3 powder.
3. Results and discussion 3.1. X-ray diffraction X-ray diffraction was used to determine crystalline properties, cristallite size calculation and the progressive addition of BaBiO3 to the NaTaO3 matrix. Fig. 1a depicts the XRD patterns for the bare samples NTO and BBO. In this figure, the positions of the main diffraction peaks for NTO and BBO are in agreement with the joint committee for powder diffraction standard cards (JCPDS) 01-074-2478 and 01-078-059, respectively, which indicates an adequate synthesis process to obtain the pure phases. Fig. 1b shows the X-ray diffraction patterns for NTO modified by the incorporation of BBO (X-BBO/NTO, X = 5 to 30 wt.%). It is possible to observe short diffraction peaks at 2θ values of 29°, 41.5° and 51.5° corresponding to the three-main signals of BBO, and these peaks increase their intensity as the BBO amount increases. The Scherrer equation [27] was employed to calculate the crystallite sizes from the full-width at the half maximum (FWHM) of the NTO main peak (2θ = 23°); L = kλ/βcos(θ), where L is the crystallite size, k is the Scherrer constant (0.9), λ is the wavelength of the X-ray radiation (0.15418 nm for Cu Kα), β is the full width at half maximum (FWHM) of the diffraction peak at 2θ, and θ is the preferential angle diffraction peak. From Fig. 1c, it is possible to observe that bare materials present similar crystallite size (116 and 117 nm for NTO and BBO, respectively); however, after the BBO and NTO mixing NTO, excepting for 15BBO/NTO, all the mixed samples increased their crystallite size, 20BBO/NTO presents the largest crystallite size, 135 nm. It is important to mention that materials with large crystallite sizes present fewer grain limits, which is an important parameter to explain the decrease of recombination processes (e− and h+) inside the material structure [28], and therefore a higher photocatalytic activity.
2.2. Synthesis of the BaBiO3 via solid-state method BaBiO3 was synthesized by a solid-state method, using BaCO3 (99%) and Bi2O3 (99%). The precursors were dried in an oven at 120 °C. Then, equivalent amounts were weighed and mixed in an agate mortar, using acetone for homogenizing the mixture. The powder was placed in a platinum crucible and a thermal treatment of 800 °C for 24 °C under air atmosphere to obtain the BaBiO3 crystalline phase. 2.3. Materials characterization Physicochemical, optical and electrochemical characterizations were carried out as follows described: X-ray diffraction (XRD) technique was used to determine the crystalline structure, ensure the obtaining of the expected phases, and to calculate the crystallite size of the samples, these measurements were performed employing a Bruker D8 Advance diffractometer operating at 40 kV and 40 mA with Cu Kα radiation (λ = 1.5406 A) in the 2θ range of 15-70°. The textural characterization was carried out by the specific surface area (SBET) determination using the Bel Japan Inc. Belsorp mini II instrument. Before the analysis, the samples were degassed at 300 °C for 1 h under high vacuum. A scanning electron microscope (SEM) JEOL-6490LV was employed to observe the morphological characteristics of the samples. The absorbance spectra were used to calculate the Tauc plots which were employed in the determination of the bandgap energy (Eg); the samples were excited in an energy range from 1500 to 200 nm using a UV–vis NIR (Cary 5000) spectrophotometer coupled with an integrating sphere for diffuse reflectance measurements. The charge carrier recombination rate was estimated by photoluminescence spectra (PL) using a fluorescence spectrometer (Agilent Cary Eclipse) with an excitation wavelength of 254 nm. A potentiostat/galvanostat AUTOLAB PGSTAT302 N, a standard three-electrode quartz cell, and a 0.5 M Na2SO4 aqueous solution were employed in the electrochemical measurements. A platinum plate and an Ag/AgCl 3 M KCl electrode were used as counter and reference electrodes, respectively. Working electrodes were made mixing the photocatalysts powders with water and ethanol to form a slurry. The photocatalytic ink was deposited by spin coating onto FTO glass. The charge transfer resistance measurements (EIS) and the photocurrent tests (chronoamperometry) were performed under UV–vis illumination (Newport 66884, QTH lamp, 250 W output).
3.2. Specific surface area (SBET), and pore size (BHJ) It is well known that a high surface area is an indication of a better catalytic activity as result of a higher number of actives sites where chemical species can be adsorbed and subsequently react [29]. In addition, the presence of small pores is characteristic of high surface materials. In this way, Fig. 2a shows the adsorption-desorption isotherms to determine the specific surface area (SBET), the samples present type-ll isotherms, which are related with the existence of micropores [30]. From this plot, samples 20-BBO/NTO and 25-BBO/NTO show the highest adsorption volume than the other BBO wt.% concentrations. Fig. 2b depicts the relation between the surface area and the pore sizes distribution. Even though all the X-BBO/NTO photocatalysts present low surface area values (less than 4 m2 g−1), it is clear that 20 and 25 wt.% BBO/NTO samples show the highest difference between surface area and pore size, which could suggest an improvement to perform catalytic processes for these two samples [31]. 2
Journal of Photochemistry & Photobiology A: Chemistry 391 (2020) 112363
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Fig. 1. X-ray diffraction measurements for the BBO/NTO and pure materials (a) Pure photocatalysts, (b) X-BBO/NTO samples (X = 5–30 wt.%) and (c) Crystallite sizes determined by Scherrer equation.
3.3. Scanning Electron microscopy
respectively. A summary of the mass composition for each sample is also reported in Table S1, the elemental composition confirms the increase of Ba and Bi atoms according to the addition of BBO to the NTO photocatalysts. Fig. 3(a–b) shows the micrographs of bare semiconductors; while it is true that bare samples present shapeless particles, the main difference lies in the agglomerated small particles
The scanning electron microscopy was used to observe and analyze the morphology of pure samples, and subsequent changes suffered after the mixing process. Energy-dispersive X-Ray Spectroscopy (EDS) measurements and elemental mapping are shown in Figures S1 and S2,
Fig. 2. (a) Adsorption-desorption isotherm and (b) Surface area and pore size diameters for the X-BBO/NTO samples (X = 5–30 wt.%). 3
Journal of Photochemistry & Photobiology A: Chemistry 391 (2020) 112363
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Fig. 3. Micrographs at 10,000 X of the (a–b) bare materials, (c–h) X-BBO/NTO samples (X = 5–30 wt.%).
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valence band and Bi 6 s orbitals in the conduction band, NaTaO3 shows a direct transition typical of UV light absorption, characteristic of the band to band transition O 2p (valence band) to Ta 5d (conduction band) [15]. While it is true, after the BBO incorporation, the bandgap value remains practically the initial NaTaO3 value (4.05 eV), the inset of Figs. 4c depicts an expected effect, a redshift in the absorption edge for the BBO/NTO samples. Compared to the pure NaTaO3, the BBO/NTO samples show an enhanced light absorption in the visible range. X-BBO/ NTO show a progressive tuning from 5 to 20 wt.% BBO, being the 20BBO/NTO the curve most red-shifted (Fig. 4c), above 20 wt.% BBO/ NTO samples present a regression trend. To further understand the optical behavior observed by UV–vis DRS, photoluminescence measurements (PL) were carried out to the BBO/NTO samples. The emission spectra are depicted in Fig. 4d; all the samples present a peak at 420 nm, which corresponds to the band gap transition of NaTaO3. From these plots, it is clear that the highest emission peak corresponds to the pure NaTaO3 sample, which is directly related to a higher recombination rate of the photogenerated charge carriers. As observed during the UV–vis DRS measurement, the addition of BBO into the NTO semiconductor enhance the visible light harvesting. For the X-BBO/NTO samples, the 15, 20 and 25 wt.% BBO seems to be the materials with lower recombination rate; however, based on the UV–vis DRS measurements, we can predict that 20-BBO/NTO it is the most suitable compound to perform the HER. It is well known that a high crystallinity of the sample enhances the photocatalytic activity of a semiconductor due to a lower presence of crystalline defects which decrease the recombination of charge carriers (h+ and e−) [36,37]. In this way; up to this point, it is suggested that the improvement in the photocatalytic activity for the BBO/NTO systems lies in the formation of particles with
showed by NaTaO3 compared to BaBiO3 which present much bigger shapeless particles. After the BBO/NTO mixing process, it is clear that all the materials decrease their particle sizes; however, slight differences can be observed in these samples. For the X-BBO/NTO batch (Fig. 3(c – h)), the addition of 5 wt. % BBO causes that the sample remains similar to the NTO pure sample, but the agglomeration degree is reduced. For the samples containing 10, 15 and 20 wt. % BBO, it is possible to observe two main characteristics; first, an increase in the particle size, and second, the formation of pseudo-cubic particles; these last aspects cannot be found in the samples containing 25 and 30 wt. % SS-BBO, which again reduces their particle sizes and present a re-agglomeration effect. Is it important to mention the sample 20-BBO/NTO shows the best particles dispersion. These slight morphological differences, such as particle size, shape, and particle agglomeration, can play a fundamental role in the photogeneration of charge carriers, which are the main responsible for an appropriate photocatalytic activity [32,33].
3.4. UV–vis diffuse reflectance spectroscopy and photoluminescence The measurement of the light absorption rate was carried out by the UV–Vis Diffuse Reflectance Spectroscopy (UV–vis DRS). From this data and considering a direct transition [34], the bandgap of the samples was calculated through the Tauc plots presented in Fig. 4. Tauc plots for bare semiconductors are presented in Fig. 4a and b; BBO presents a lower bandgap than NTO (1.97 and 4.08 eV, respectively). It is important to mention that BaBiO3 presented a very similar band gap value associated with a direct transition, as reported in literature 1.94 eV [24] and 2.20 eV [35]. While BaBiO3 present light absorption in the visible region due to a bands arrangement composed by O 2p orbitals in the
Fig. 4. Tauc plots for (a) NaTaO3, (b) BaBiO3, and (c) X-BBO/NTO, (d) Photoluminescence spectra for X-BBO/NTO samples (X = 5–30 wt.%). 5
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photocatalyst and the bare materials. All semiconductors present a cathodic peak around ≈ -0.5 V vs. Ag/AgCl 3 M, corresponding to the metal oxide (Bi, Ba, Ta) reduction, as well as the metal re-oxidation at 1.1 V vs. Ag/AgCl 3 M followed by the oxygen evolution reaction. It is interesting to observe that in dark conditions, BBO present the lowest overpotential to perform both, the H2 (framework 1) and O2 evolution (framework 2) reaction (-1.2 V and 1.3 V vs. Ag/AgCl 3 M, respectively); although, it is necessary to perform the transient photocurrent tests to determine the photoelectrochemical activity at different potentials. Fig. 6b demonstrates a remarkable feature, the photocurrent transient in the negative potential scan direction does not show any difference between the dark and the light conditions; thus, any sample present photocatalytic activity under a negative BIAS. On the contrary, under a positive BIAS (oxidation potential), all samples present a welldefined photocatalytic current (Fig. 6c). 20-BBO-NTO presents the highest photocurrent at potential values above 1.3 V vs. Ag/AgCl 3 M, which suggest these materials work as photoanodes to split the water molecule by an effective activity to perform the oxygen evolution reaction. Finally, It is well-known light irradiation promotes the charge carrier separation and transportation; thus, for efficient semiconductor materials, the charge transference resistance has to decrease under light conditions. Fig. 6d depicts the Nyquist - electrochemical impedance spectroscopy (EIS) plots in dark and light conditions at a BIAS potential of 1.45 V vs. Ag/AgCl 3 M. All the photocatalysts present a low charge transference resistance under light irradiation vs. dark conditions. The best photocatalytic charge carrier separation can be positioned as 20BBO/NTO < BBO < NTO.
intermedium sizes compared to the pure materials; but more important, less crystal defects by an increasing in the crystallite size as we observed by XRD [38,39]. Also, the red shift induced by the BBO addition which is a compound capable of absorbing light in the visible spectra; and finally, an improved charge carrier transference induced by the formation of an efficient heterojunction BBO/NTO. 3.5. Photoelectrochemical characterization Electrochemical measurements were carried out to investigate further the charge transfer effects involved at the photocatalyst surface – electrolyte interface. The electrochemical impedance spectroscopy (EIS) technique was employed to obtain the Nyquist plots in dark conditions; a 0.5 M Na2SO4 solution was used as an electrolyte, and the frequency range was varied from 100 kHz to 1 Hz with the signal amplitude being ± 10 mV from a bias potential of -0.1 V vs. Ag/AgCl 3 M KCl. The charge transfer resistance (Rct) can be understood as a measure of the exchange capacity of electrons between a solid electrolyte and the chemical species contained in an electrolyte. Since the Nyquist plot represents on the complex plane, the real part of a frequency response function (FRF) against its imaginary part as an implicit variable [40], a smaller diameter size of the semicircles on this plots reveals less Rct, an ease for the charges transfer and hence, an enhanced photocatalytic activity [41]. Fig. 5a depicts the Nyquist plots of the X-BBO/NTO samples (X = 5 to 30 wt.%), from this figure we can observe these samples present a decreasing trend in the Rct as the BBO is added until the optimal value of 20-BBO/NTO; at higher BBO concentrations (25 and 30 wt.%), the Rct present a backward in the charge transfer ease. In addition to the enhanced optical properties determined by UV–vis DRS and photoluminescence, the EIS tests, allows predicting an enhanced photocatalytic activity for the 20-BBO/NTO sample. The transient photocurrent tests evaluated the photocatalytic activity and partial stability for the BBO/NTO materials. Fig. 5b shows the photocurrent transients for the X-BBO/NTO samples (X = 5 to 30 wt.%) at a BIAS potential of -0.62 V vs. Ag/AgCl 3 M (H+/H2 equilibrium redox potential). It is clear that the photocurrent generated under illumination increases from 5 until 20 wt.% BBO added, above 20 wt.%, BBO/NTO samples do not present good stability and show a mixed current decreasing its photocatalytic response. 20-BBO/NTO presented the highest photocurrent value (≈2 μA). Besides, 20-BBO/NTO shows an efficient photocatalytic charge carrier generation since it can generate and eliminate the photocurrent signal in a short time. Since UV–vis DRS, photoluminescence, and electrochemical characterizations suggest the 20-BBO/NTO sample presents the best photocatalytic performance, Fig. 6a depicts the cyclic voltammetry for this
3.6. Photocatalytic hydrogen production The photocatalytic activity of the BBO/NTO heterojunctions and pure semiconductors to perform the water splitting from pure water under UV irradiation is shown in Fig. 7. It is worth to mention that even when BaBiO3 is supposed to present photocatalytic activity because its narrow band-gap; however, the UV-light irradiation used in this tests can produce an excess of charge carriers which promotes the inefficient extraction of the photo-generated charges in perovskites, thus suppressing the BaBiO3 photocatalytic activity [42]. On the other hand, several reports have demonstrated that pure NaTaO3 is one of the best materials to perform the hydrogen generation [7–9,43]. In agreement with this statement, the NaTaO3 sample presented in this work presented the highest H2 production among the bare materials. Fig. 7a shows the BBO/NTO hydrogen generation profile, from this figure, we can point out how with a minimum addition of BBO (5 wt.%), the BBO/ NTO presents almost the same H2 generation rate than bare BaBiO3. As a result of a higher BBO addition, the H2 photogeneration increase until
Fig. 5. (a) Nyquist plots and, (b) photocurrent response for X-BBO/NTO samples (X = 5–30 wt.%) samples at equilibrium potential under UV–vis illumination (Newport 66884, QTH lamp, 250 W output). 6
Journal of Photochemistry & Photobiology A: Chemistry 391 (2020) 112363
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Fig. 6. Photoelectrochemical characterization for X-BBO/NTO samples (X = 5–30 wt.%) samples, (a) Cyclic voltammetry, (b) photocurrent transients (negative scan) (c) photocurrent transients (positive scan) and (d) Nyquist Plots in dark and light conditions.
a maximum value of 54 μmol g−1 for 20-BBO/NTO; above 20 wt.% BBO, the production decline unto minimum values. In a general way, the activity of BBO was improved in the composites, exhibiting a volcano-type characteristic trend (Table S3). This trend could be described as follows: when small amounts of the BBO (below 20%) are mixed with NTO, the charge separation and transference is improved, until it reaches an optimal amount (20% of BBO), where the overpotential for the water-splitting reaction is reduced, and the photocatalytic activity is superior in the composite compared to the bare materials. Above this optimal value (loadings higher to 20%), the activity decreases due to the covering of the active sites and the surface of the material, which
reduces the light absorption and the generation of charges [44]. It is feasible to suggest that the high conversion rate achieved by the 20BBO/NTO photocatalyst (1.4 times hydrogen than pure NaTaO3 and 1.5 times higher than TiO2 P25 (Figure S3)) showed in Fig. 7b, is a result of parameters combination such as i) A higher crystallite size which theoretically presents a lower number of grain boundaries responsible for a higher charge carrier resistance recombination degree [45,46], ii) the presence of an optimal amount of BaBiO3 which acts as a secondary phase with the ability to form a heterojunction BBO/NTO capable of enhance the visible light-harvesting, inducing a rapid charge separation and an inhibition of the electron-hole pairs recombination [47,48].
Fig. 7. (a) Photocatalytic hydrogen production of X-BBO/NTO samples (X = 5–30 wt.%) and (b) Hydrogen generation rate for bare materials and X-BBO/NTO. 7
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semiconductor to be translated to the counter electrode (platinum rod) to perform the reduction reactions [56,57]. Similarly, photogenerated holes (h+) on the NTO surface are transferred to the BBO VB, where they react with the surrounding chemical species (water) to perform the oxidation processes (generation of ∙OH radicals). In this way, the heterojunction BBO/NTO presents an enhancement in the charge carrier transference due to a continuous BBO/NTO charge flow, which acts to prevent recombination issues and increase the lifetime of the photogenerated charge carriers. Since the 20-BBO/NTO heterojunction presents an n-type semiconductor behavior (Figure S3) [15], this material possesses a higher number of electrons than holes; in this way, majority carriers (e−) flow from the semiconductor (working electrode) to the platinum counter electrode. The electrochemical system using 20-BBO/ NTO performs the water-splitting process by the initial ∙OH radicals generation and subsequent water oxidation reaction onto the semiconductor surface [58], as depicted in the electrochemical evaluations of Fig. 6c.
3.7. Photocatalytic and photoelectrochemical mechanisms The suggested photocatalytic transfer mechanism was obtained by the calculation of the theoretical band positions for the BBO/NTO heterojunction following the equations (1) and (2), respectively [49]: Evb = X - Ee + 0.5Eg
(1)
ECB = EVB - Eg
(2)
where Evb is the valence band (VB) edge potentials; X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms (XBa =2.4 eV, XBi =4.69 eV, XNa =2.85 eV, XTa =4.11 eV, and XO =7.54 eV); Ee is the energy of free electrons on the hydrogen scale (4.5 eV) [49], Eg is the bandgap energy of semiconductor, and ECB is the conduction band (CB) edge potentials. According to the atomic ratio of each constituent element, the X values for BaBiO3 and NaTaO3 are 5.45 and 5.49, respectively. In this way, the position for Evb and ECB were calculated to be 1.75 eV (VB) and 0.15 eV (CB) for BaBiO3, and 3.025 eV (VB) and -1.045 eV (CB) for NaTaO3. These previous calculations allow the theoretical proposal of a photocatalytic mechanism as the one illustrated in Fig. 8a. It is possible to observe the band diagrams for pure semiconductors before contact. Because of their extrinsic nature (p-type for BaBiO3 and n-type for NaTaO3), before contact, the fermi levels (EF) for each compound present a different position [50]. The EF is located slightly above the VB in BaBiO3 (EFp); meanwhile, this value can be found nearly under the CB in NaTaO3 (EFn) [51,52]. Once the semiconductors are in contact, electrons flow from the NaTaO3 CB to the BaBiO3 CB until the junction reaches an equalization of the Fermi Level; this event induces a new bands positions in the new heterojunction [53]. After contact, because of the Fermi Level equalization, BaBiO3 bands shift in the negative direction. In this way, the BaBiO3 CB presents now a more negative potential position than NaTaO3 CB to form a type-ll heterojunction [54,55]. Hence, when the new heterostructure is irradiated, BaBiO3 is the main responsible for the light absorption and charge carriers generation due to its narrow bandgap; therefore, the photogenerated electrons migrate from the BaBiO3 CB to the NaTaO3 CB due to a less of the barrier which acts preventing recombination issues. Meanwhile, electrons accumulated on the NaTaO3 CB reacts with the surrounding medium to perform the protons (H+) reduction coming from the photocatalyst anodic site, the accumulated holes on the BaBiO3 VB are responsible for the water oxidation reaction. The lifetime of the photogenerated charge carriers is prolonged, thus improves the photocatalytic efficiency of the BBO/NTO heterojunction [56,57]. On the other hand, Fig. 8b depicts the proposed photoelectrochemical mechanism. Since the conduction CB and VB of BBO and NTO are in different energetic levels, the photogenerated electrons migrate from the BBO CB to the NTO CB where are extracted from the BBO/NTO
4. Conclusions The photocatalytic activity of semiconductors NaTaO3 and BaBiO3 synthesized by the solid-state method was studied to perform the hydrogen production process without the use of sacrificial agents. This study demonstrated that BBO is a suitable material to modify the physicochemical NTO properties. In general, X-BBO/NTO (X = 5–30 wt.%) presented lower particle size than bare BaBiO3, being the 20-BBO/NTO the sample that presented the lowest particles agglomeration degree and the largest crystallite size. The addition of BBO to the NTO improved the light harvest by a red-shift. The formation of a heterojunction BBO/NTO enhanced the photocatalytic activity to perform the photocatalytic water splitting process by an improved initial water oxidation step. It is suggested this photocatalytic improvement was caused by: i) an increase of the X-BBO/NTO crystallite size compared to bare materials, which indicates a lower number of grain boundaries defects, so reducing the recombination rate. ii) an optimal addition of BaBiO3 (20 wt.%) that increase the synergy of the BBO/ NTO; thus, decreasing the charge transference resistance, and iii) a continuous charge carrier transference caused by the overlapped bands position (heterojunction) which prevent the recombination issues, thus enhancing the charge transfer processes to perform the photocatalytic hydrogen generation. Author Statement Leticia M. Torres-Martinez and J. Manuel Mora-Hernandez conceived of the present idea. Ali M. Huerta-Flores verified the analytical methods and supervised the findings of this work, all authors discussed
Fig. 8. The proposed photo(electro)catalytic reaction mechanisms for the water-splitting process onto the 20-BBO/NTO heterojunction. 8
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the results and contributed to the final manuscript. J. Manuel Mora-Hernandez and Ali M. Huerta-Flores carried out the experiment. J. Manuel Mora-Hernandez wrote the manuscript with support from Ali M. Huerta-Flores. J. Manuel Mora-Hernandez and Ali M. Huerta-Flores conceived and planned the experiments. J. Manuel Mora-Hernandez and Ali M. Huerta-Flores carried out the experiments. J. Manuel Mora-Hernandez and Ali M. Huerta-Flores contributed to sample preparation. J. Manuel Mora-Hernandez and Ali M. Huerta-Flores contributed to the interpretation of the results. J. Manuel Mora-Hernandez took the lead in writing the manuscript. All authors provided critical feedback and helped shape the research, analysis and manuscript.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors would like to thank Cátedras CONACyT – ID7708, and thank CONACYT for the financial support through the following projects: CB-2014-237049 and FC-2016-01-1725, also thank UANL: PAIFIC 2018-10, PAIFIC 2018-5, PAICYT 2019 IT1053-19, PAICYT IT1021-19, FIC-UANL.SEP-PROFIDES 511-6/18-11852. Additionally, the authors thank Jesus Meza-Bustillos, Williams González-Suárez, and J. David Fernandez-Reyes by their technical support. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jphotochem.2020. 112363. References [1] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (5358) (1972) 37–38. [2] A.R. Khataee, M.B. Kasiri, Photocatalytic degradation of organic dyes in the presence of nanostructured titanium dioxide: influence of the chemical structure of dyes, J. Mol. Catal. A Chem. 328 (1) (2010) 8–26. [3] A.D. Tjandra, J. Huang, Photocatalytic carbon dioxide reduction by photocatalyst innovation, Chinese Chem. Lett. 29 (6) (2018) 734–746. [4] P. Kanhere, Z. Chen, A Review on Visible Light Active Perovskite-Based Photocatalysts Vol. 19 (2014), pp. 19995–20022. [5] S.R. Thakare, et al., Development of new, highly efficient and stable visible light active photocatalyst Ag2ZrO3 for methylene blue degradation, Catal. Commun. 62 (2015) 39–43. [6] F.-F. Li, et al., Improved Visible-Light Photocatalytic Activity of NaTaO3 With Perovskite-Like Structure via sulfur Anion Doping Vol. 166-167 (2014). [7] C. Gómez-Solís, et al., Facile solvo-combustion synthesis of crystalline NaTaO3 and its photocatalytic performance for hydrogen production, Fuel 130 (2014) 221–227. [8] C. Gómez-Solís, et al., RuO2–NaTaO3 heterostructure for its application in photoelectrochemical water splitting under simulated sunlight illumination, Fuel 166 (2016) 36–41. [9] A.M. Huerta-Flores, et al., Laser assisted chemical vapor deposition of nanostructured NaTaO3 and SrTiO3 thin films for efficient photocatalytic hydrogen evolution, Fuel 197 (2017) 174–185. [10] J. Rodríguez-Torres, et al., Synthesis and characterization of Au-Pd/NaTaO3 multilayer films for photocatalytic hydrogen production, J. Photochem. Photobiol. A: Chem. 332 (2017) 208–214. [11] L. An, H. Onishi, Electron–Hole Recombination Controlled by Metal Doping Sites in NaTaO 3, Photocatalysts 5 (2015) 3196–3206. [12] B. Wang, et al., Anion-doped NaTaO3 for visible light photocatalysis, J. Phys. Chem. C 117 (44) (2013) 22518–22524. [13] A. Iwase, H. Kato, A. Kudo, The Effect of Au Cocatalyst Loaded on La-Doped NaTaO3 on Photocatalytic Water Splitting and O2 Photoreduction Vol. 136–137 (2013), pp. 89–93. [14] P. Kanhere, et al., Electronic Structure, Optical Properties, and Photocatalytic Activities of LaFeO3–NaTaO3 Solid Solution Vol. 116 (2012), pp. 22767–22773. [15] J.M. Mora-Hernandez, A.M. Huerta-Flores, L.M. Torres-Martínez, Photoelectrocatalytic characterization of carbon-doped NaTaO3 applied in the photoreduction of CO2 towards the formaldehyde production, J. Co2 Util. 27
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degradation of pollutants, Adv. Sustain. Syst. 1 (7) (2017) 1700006. [55] J. Low, et al., Heterojunction photocatalysts, Adv. Mater. 29 (20) (2017) 1601694. [56] Y. Chen, et al., Extended carrier lifetimes and diffusion in hybrid perovskites revealed by Hall effect and photoconductivity measurements, Nat. Commun. 7 (2016) 12253. [57] D. Kiermasch, et al., Improved charge carrier lifetime in planar perovskite solar cells by bromine doping, Sci. Rep. 6 (2016) 39333. [58] H. Zhuang, et al., Construction of one dimensional ZnWO4@SnWO4 core-shell heterostructure for boosted photocatalytic performance, J. Mater. Sci. Technol. 35 (10) (2019) 2312–2318.
[50] J. Liu, Origin of high photocatalytic efficiency in monolayer g-C3N4/CdS heterostructure: a hybrid DFT study, J. Phys. Chem. C 119 (51) (2015) 28417–28423. [51] W.A. Smith, et al., Interfacial band-edge energetics for solar fuels production, Energy Environ. Sci. 8 (10) (2015) 2851–2862. [52] N. Sedaghati, et al., Boosted visible-light photocatalytic performance of TiO2-x decorated by BiOI and AgBr nanoparticles, J. Photochem. Photobiol. A: Chem. 384 (2019) 112066. [53] Y. Sun, et al., A Bi/BiOI/(BiO)2CO3 heterostructure for enhanced photocatalytic NO removal under visible light, Chinese J. Catal. 40 (3) (2019) 362–370. [54] F. Opoku, et al., Recent progress in the development of semiconductor-based photocatalyst materials for applications in photocatalytic water splitting and
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Contents lists available at ScienceDirect
Journal of Photochemistry & Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Tailoring charge transport in BaBiO3/NaTaO3 heterojunction interface for enhanced photocatalytic and photoelectrochemical H2 generation
T
J.M. Mora-Hernandeza, Ali M. Huerta-Floresb, Leticia M. Torres-Martínezb,* a
CONACYT - Universidad Autónoma de Nuevo León, UANL, Facultad de Ingeniería Civil, Departamento de Ecomateriales y Energía, Av. Universidad S/N Ciudad Universitaria, San Nicolás de los Garza, Nuevo León, C.P. 66455, Mexico b Universidad Autónoma de Nuevo León, UANL, Facultad de Ingeniería Civil, Departamento de Ecomateriales y Energía, Av. Universidad S/N Ciudad Universitaria, San Nicolás de los Garza, Nuevo León, C.P. 66455, Mexico
A R T I C LE I N FO
A B S T R A C T
Keywords: Hydrogen production Heterogeneous photocatalysis Photoelectrochemistry
Photoactive perovskites NaTaO3 and BaBiO3 were synthesized by the solid-state method. The resulting compounds were mixed by wet impregnation, adding BaBiO3 (BBO) onto the base material NaTaO3 (NTO) to obtain the compounds X-BBO/NTO (X = 5–30 wt.%). The sample 20-BBO/NTO presented the largest crystallites (135 nm). Since BaBiO3 presents narrower Eg value (≈ 2 eV) than NaTaO3 (≈ 4 eV), the addition of BBO to NTO promoted a better visible light harvesting. This feature can be explained by the formation of a heterojunction, which improves the photocatalytic activity by an abatement of the charge carrier recombination. Photoelectrochemical tests showed an improvement in the charge transfer resistance and enhanced photoelectrocatalytic activity for the 20-BBO/NTO sample, over the other BBO concentrations and bare photocatalysts. An optimal amount of BaBiO3 (20 wt.%) increased the synergy of the n-type heterojunction by an enhancement in the charge transfer processes to perform the photoelectrocatalytic water splitting. 20-BBO/NTO sample showed the highest hydrogen production of 54 μmol g−1, which outstripped 1.4 times the photocatalytic hydrogen generation of the bare NaTaO3 semiconductor.
1. Introduction Since the report of hydrogen evolution from water splitting in 1972 using TiO2 photocatalyst [1], numerous materials have been developed for achieving competitive efficiencies in this and other related applications, such as organic dye degradation [2] and carbon dioxide reduction [3]. Titanates, niobates, and tantalates have exhibited promising efficiencies [4], but their activity is limited to UV light, using only around 4% of the solar spectrum [5]. NaTaO3 (NTO) is a perovskite structured photocatalyst with a bandgap of around 4 eV that has shown higher efficiencies than TiO2 for water splitting due to the negative character of the conduction band (CB) composed by Ta 5d orbitals [6]. To extend the light absorption capacity of NaTaO3 into the visible range, different strategies have been employed, including synthesis methods [7–10], cation [11] and anion doping [12], the use of co-catalysts [13], the formation of solid solutions [14], and the coupling with other materials such as amorphous carbon [15] g-C3N4 [16], Ag2O [17], RuO2 [8], NiO [18], and SrTiO3 [19], among others. On the other hand, BaBiO3 (BBO) perovskite is a semiconductor that has gained considerable attention in the field of photocatalysis due to
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the low bandgap (≈ 2 eV) and high capacity to harvest visible light [20], originated from the hybridized O 2p and Bi 6s orbitals in the valence band (VB) which narrow the bandgap [21]. Also, the charges exhibit high mobility in BaBiO3 due to the high dispersion of the s orbital in this material [22]. BaBiO3 powders have been successfully synthesized by solid-state and some wet-chemistry methods [23,24]. This semiconductor has been applied as powders in the removal of methylene blue and acetaldehyde [22], the degradation of rhodamine B, showing an excellent stability and attractive yields (80% of RhB degradation), the production of hydrogen (61 μmol g−1 h−1) [24], as well as thin films for photoelectrochemical water splitting and photocatalytic H2 evolution and CO2 reduction [25]. It has also been studied in composites with n-type TiO2 for the photocatalytic degradation of gaseous benzene [26], enhancing the activity of pure TiO2. These results indicate that BaBiO3 can act efficiently as a sensitizer of a large bandgap semiconductor, such as NaTaO3, and extend its activity in the visible-light range. Moreover, it is expected that the electrons injection from BaBiO3 to NaTaO3 promotes an improvement in the charge transport properties, thus enhancing the photocatalytic activity of the heterojunction to produce H2.
Corresponding author. E-mail address: [email protected] (L.M. Torres-Martínez).
https://doi.org/10.1016/j.jphotochem.2020.112363 Received 31 October 2019; Received in revised form 7 December 2019; Accepted 7 January 2020 Available online 07 January 2020 1010-6030/ © 2020 Elsevier B.V. All rights reserved.
Journal of Photochemistry & Photobiology A: Chemistry 391 (2020) 112363
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In this work, we report for the first time the production through a fast impregnation method of a BaBiO3-NaTaO3 heterojunction (BBO/ NTO) with high photocatalytic activity for the hydrogen evolution. The appropriate semiconductor coupling allowed an efficient charge separation and migration in the interface of the heterojunction as well as an improved photocatalytic performance. A complete study of the physicochemical properties of the bare and heterostructured photocatalysts is included, as well as the effect of the variation of BaBiO3NaTaO3 proportions on the photocatalytic performance and the mechanism of charge transference involved.
2.4. Photocatalytic hydrogen generation To perform the hydrogen generation tests, 200 mg of the photocatalysts powders were dispersed by vigorous stirring in 200 mL of deionized water into a 250 mL Pyrex reactor at room temperature without the addition of sacrificial agents. Then, nitrogen gas was bubbled through the solution for 30 min to remove the oxygen dissolved in water. A UV pen-ray lamp with an emission peak centered at 254 nm and a nominal power of 4400 μW cm−2 was used as a lighting source; the hydrogen evolution was monitored by a gas chromatograph (Thermo Scientific, TCD detector and fused silica column, 30 mm X 0.53 mm), using N2 as the gas carrier. The reaction time under lighting conditions and continuous stirring was 3 h; samples were taken at intervals of 30 min.
2. Experimental section 2.1. Synthesis of the NaTaO3 via solid-state method In the synthesis of NaTaO3, Ta2O5 and Na2CO3 were used as precursors, both reagents were dried and stoichiometrically weighed (5 wt. % excess of Na2CO3 to avoid sodium evaporation). Then, the reagents were mixed and ground into an agate mortar in the presence of acetone. After the acetone evaporation, the mixed powder was placed into a platinum crucible and heat threated at 600 °C for 12 h to degas CO2. The sample was ground one more time using acetone; then, the powder sample was annealed at 800 °C for 12 h in air atmosphere until the formation of NaTaO3 powder.
3. Results and discussion 3.1. X-ray diffraction X-ray diffraction was used to determine crystalline properties, cristallite size calculation and the progressive addition of BaBiO3 to the NaTaO3 matrix. Fig. 1a depicts the XRD patterns for the bare samples NTO and BBO. In this figure, the positions of the main diffraction peaks for NTO and BBO are in agreement with the joint committee for powder diffraction standard cards (JCPDS) 01-074-2478 and 01-078-059, respectively, which indicates an adequate synthesis process to obtain the pure phases. Fig. 1b shows the X-ray diffraction patterns for NTO modified by the incorporation of BBO (X-BBO/NTO, X = 5 to 30 wt.%). It is possible to observe short diffraction peaks at 2θ values of 29°, 41.5° and 51.5° corresponding to the three-main signals of BBO, and these peaks increase their intensity as the BBO amount increases. The Scherrer equation [27] was employed to calculate the crystallite sizes from the full-width at the half maximum (FWHM) of the NTO main peak (2θ = 23°); L = kλ/βcos(θ), where L is the crystallite size, k is the Scherrer constant (0.9), λ is the wavelength of the X-ray radiation (0.15418 nm for Cu Kα), β is the full width at half maximum (FWHM) of the diffraction peak at 2θ, and θ is the preferential angle diffraction peak. From Fig. 1c, it is possible to observe that bare materials present similar crystallite size (116 and 117 nm for NTO and BBO, respectively); however, after the BBO and NTO mixing NTO, excepting for 15BBO/NTO, all the mixed samples increased their crystallite size, 20BBO/NTO presents the largest crystallite size, 135 nm. It is important to mention that materials with large crystallite sizes present fewer grain limits, which is an important parameter to explain the decrease of recombination processes (e− and h+) inside the material structure [28], and therefore a higher photocatalytic activity.
2.2. Synthesis of the BaBiO3 via solid-state method BaBiO3 was synthesized by a solid-state method, using BaCO3 (99%) and Bi2O3 (99%). The precursors were dried in an oven at 120 °C. Then, equivalent amounts were weighed and mixed in an agate mortar, using acetone for homogenizing the mixture. The powder was placed in a platinum crucible and a thermal treatment of 800 °C for 24 °C under air atmosphere to obtain the BaBiO3 crystalline phase. 2.3. Materials characterization Physicochemical, optical and electrochemical characterizations were carried out as follows described: X-ray diffraction (XRD) technique was used to determine the crystalline structure, ensure the obtaining of the expected phases, and to calculate the crystallite size of the samples, these measurements were performed employing a Bruker D8 Advance diffractometer operating at 40 kV and 40 mA with Cu Kα radiation (λ = 1.5406 A) in the 2θ range of 15-70°. The textural characterization was carried out by the specific surface area (SBET) determination using the Bel Japan Inc. Belsorp mini II instrument. Before the analysis, the samples were degassed at 300 °C for 1 h under high vacuum. A scanning electron microscope (SEM) JEOL-6490LV was employed to observe the morphological characteristics of the samples. The absorbance spectra were used to calculate the Tauc plots which were employed in the determination of the bandgap energy (Eg); the samples were excited in an energy range from 1500 to 200 nm using a UV–vis NIR (Cary 5000) spectrophotometer coupled with an integrating sphere for diffuse reflectance measurements. The charge carrier recombination rate was estimated by photoluminescence spectra (PL) using a fluorescence spectrometer (Agilent Cary Eclipse) with an excitation wavelength of 254 nm. A potentiostat/galvanostat AUTOLAB PGSTAT302 N, a standard three-electrode quartz cell, and a 0.5 M Na2SO4 aqueous solution were employed in the electrochemical measurements. A platinum plate and an Ag/AgCl 3 M KCl electrode were used as counter and reference electrodes, respectively. Working electrodes were made mixing the photocatalysts powders with water and ethanol to form a slurry. The photocatalytic ink was deposited by spin coating onto FTO glass. The charge transfer resistance measurements (EIS) and the photocurrent tests (chronoamperometry) were performed under UV–vis illumination (Newport 66884, QTH lamp, 250 W output).
3.2. Specific surface area (SBET), and pore size (BHJ) It is well known that a high surface area is an indication of a better catalytic activity as result of a higher number of actives sites where chemical species can be adsorbed and subsequently react [29]. In addition, the presence of small pores is characteristic of high surface materials. In this way, Fig. 2a shows the adsorption-desorption isotherms to determine the specific surface area (SBET), the samples present type-ll isotherms, which are related with the existence of micropores [30]. From this plot, samples 20-BBO/NTO and 25-BBO/NTO show the highest adsorption volume than the other BBO wt.% concentrations. Fig. 2b depicts the relation between the surface area and the pore sizes distribution. Even though all the X-BBO/NTO photocatalysts present low surface area values (less than 4 m2 g−1), it is clear that 20 and 25 wt.% BBO/NTO samples show the highest difference between surface area and pore size, which could suggest an improvement to perform catalytic processes for these two samples [31]. 2
Journal of Photochemistry & Photobiology A: Chemistry 391 (2020) 112363
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Fig. 1. X-ray diffraction measurements for the BBO/NTO and pure materials (a) Pure photocatalysts, (b) X-BBO/NTO samples (X = 5–30 wt.%) and (c) Crystallite sizes determined by Scherrer equation.
3.3. Scanning Electron microscopy
respectively. A summary of the mass composition for each sample is also reported in Table S1, the elemental composition confirms the increase of Ba and Bi atoms according to the addition of BBO to the NTO photocatalysts. Fig. 3(a–b) shows the micrographs of bare semiconductors; while it is true that bare samples present shapeless particles, the main difference lies in the agglomerated small particles
The scanning electron microscopy was used to observe and analyze the morphology of pure samples, and subsequent changes suffered after the mixing process. Energy-dispersive X-Ray Spectroscopy (EDS) measurements and elemental mapping are shown in Figures S1 and S2,
Fig. 2. (a) Adsorption-desorption isotherm and (b) Surface area and pore size diameters for the X-BBO/NTO samples (X = 5–30 wt.%). 3
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Fig. 3. Micrographs at 10,000 X of the (a–b) bare materials, (c–h) X-BBO/NTO samples (X = 5–30 wt.%).
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valence band and Bi 6 s orbitals in the conduction band, NaTaO3 shows a direct transition typical of UV light absorption, characteristic of the band to band transition O 2p (valence band) to Ta 5d (conduction band) [15]. While it is true, after the BBO incorporation, the bandgap value remains practically the initial NaTaO3 value (4.05 eV), the inset of Figs. 4c depicts an expected effect, a redshift in the absorption edge for the BBO/NTO samples. Compared to the pure NaTaO3, the BBO/NTO samples show an enhanced light absorption in the visible range. X-BBO/ NTO show a progressive tuning from 5 to 20 wt.% BBO, being the 20BBO/NTO the curve most red-shifted (Fig. 4c), above 20 wt.% BBO/ NTO samples present a regression trend. To further understand the optical behavior observed by UV–vis DRS, photoluminescence measurements (PL) were carried out to the BBO/NTO samples. The emission spectra are depicted in Fig. 4d; all the samples present a peak at 420 nm, which corresponds to the band gap transition of NaTaO3. From these plots, it is clear that the highest emission peak corresponds to the pure NaTaO3 sample, which is directly related to a higher recombination rate of the photogenerated charge carriers. As observed during the UV–vis DRS measurement, the addition of BBO into the NTO semiconductor enhance the visible light harvesting. For the X-BBO/NTO samples, the 15, 20 and 25 wt.% BBO seems to be the materials with lower recombination rate; however, based on the UV–vis DRS measurements, we can predict that 20-BBO/NTO it is the most suitable compound to perform the HER. It is well known that a high crystallinity of the sample enhances the photocatalytic activity of a semiconductor due to a lower presence of crystalline defects which decrease the recombination of charge carriers (h+ and e−) [36,37]. In this way; up to this point, it is suggested that the improvement in the photocatalytic activity for the BBO/NTO systems lies in the formation of particles with
showed by NaTaO3 compared to BaBiO3 which present much bigger shapeless particles. After the BBO/NTO mixing process, it is clear that all the materials decrease their particle sizes; however, slight differences can be observed in these samples. For the X-BBO/NTO batch (Fig. 3(c – h)), the addition of 5 wt. % BBO causes that the sample remains similar to the NTO pure sample, but the agglomeration degree is reduced. For the samples containing 10, 15 and 20 wt. % BBO, it is possible to observe two main characteristics; first, an increase in the particle size, and second, the formation of pseudo-cubic particles; these last aspects cannot be found in the samples containing 25 and 30 wt. % SS-BBO, which again reduces their particle sizes and present a re-agglomeration effect. Is it important to mention the sample 20-BBO/NTO shows the best particles dispersion. These slight morphological differences, such as particle size, shape, and particle agglomeration, can play a fundamental role in the photogeneration of charge carriers, which are the main responsible for an appropriate photocatalytic activity [32,33].
3.4. UV–vis diffuse reflectance spectroscopy and photoluminescence The measurement of the light absorption rate was carried out by the UV–Vis Diffuse Reflectance Spectroscopy (UV–vis DRS). From this data and considering a direct transition [34], the bandgap of the samples was calculated through the Tauc plots presented in Fig. 4. Tauc plots for bare semiconductors are presented in Fig. 4a and b; BBO presents a lower bandgap than NTO (1.97 and 4.08 eV, respectively). It is important to mention that BaBiO3 presented a very similar band gap value associated with a direct transition, as reported in literature 1.94 eV [24] and 2.20 eV [35]. While BaBiO3 present light absorption in the visible region due to a bands arrangement composed by O 2p orbitals in the
Fig. 4. Tauc plots for (a) NaTaO3, (b) BaBiO3, and (c) X-BBO/NTO, (d) Photoluminescence spectra for X-BBO/NTO samples (X = 5–30 wt.%). 5
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photocatalyst and the bare materials. All semiconductors present a cathodic peak around ≈ -0.5 V vs. Ag/AgCl 3 M, corresponding to the metal oxide (Bi, Ba, Ta) reduction, as well as the metal re-oxidation at 1.1 V vs. Ag/AgCl 3 M followed by the oxygen evolution reaction. It is interesting to observe that in dark conditions, BBO present the lowest overpotential to perform both, the H2 (framework 1) and O2 evolution (framework 2) reaction (-1.2 V and 1.3 V vs. Ag/AgCl 3 M, respectively); although, it is necessary to perform the transient photocurrent tests to determine the photoelectrochemical activity at different potentials. Fig. 6b demonstrates a remarkable feature, the photocurrent transient in the negative potential scan direction does not show any difference between the dark and the light conditions; thus, any sample present photocatalytic activity under a negative BIAS. On the contrary, under a positive BIAS (oxidation potential), all samples present a welldefined photocatalytic current (Fig. 6c). 20-BBO-NTO presents the highest photocurrent at potential values above 1.3 V vs. Ag/AgCl 3 M, which suggest these materials work as photoanodes to split the water molecule by an effective activity to perform the oxygen evolution reaction. Finally, It is well-known light irradiation promotes the charge carrier separation and transportation; thus, for efficient semiconductor materials, the charge transference resistance has to decrease under light conditions. Fig. 6d depicts the Nyquist - electrochemical impedance spectroscopy (EIS) plots in dark and light conditions at a BIAS potential of 1.45 V vs. Ag/AgCl 3 M. All the photocatalysts present a low charge transference resistance under light irradiation vs. dark conditions. The best photocatalytic charge carrier separation can be positioned as 20BBO/NTO < BBO < NTO.
intermedium sizes compared to the pure materials; but more important, less crystal defects by an increasing in the crystallite size as we observed by XRD [38,39]. Also, the red shift induced by the BBO addition which is a compound capable of absorbing light in the visible spectra; and finally, an improved charge carrier transference induced by the formation of an efficient heterojunction BBO/NTO. 3.5. Photoelectrochemical characterization Electrochemical measurements were carried out to investigate further the charge transfer effects involved at the photocatalyst surface – electrolyte interface. The electrochemical impedance spectroscopy (EIS) technique was employed to obtain the Nyquist plots in dark conditions; a 0.5 M Na2SO4 solution was used as an electrolyte, and the frequency range was varied from 100 kHz to 1 Hz with the signal amplitude being ± 10 mV from a bias potential of -0.1 V vs. Ag/AgCl 3 M KCl. The charge transfer resistance (Rct) can be understood as a measure of the exchange capacity of electrons between a solid electrolyte and the chemical species contained in an electrolyte. Since the Nyquist plot represents on the complex plane, the real part of a frequency response function (FRF) against its imaginary part as an implicit variable [40], a smaller diameter size of the semicircles on this plots reveals less Rct, an ease for the charges transfer and hence, an enhanced photocatalytic activity [41]. Fig. 5a depicts the Nyquist plots of the X-BBO/NTO samples (X = 5 to 30 wt.%), from this figure we can observe these samples present a decreasing trend in the Rct as the BBO is added until the optimal value of 20-BBO/NTO; at higher BBO concentrations (25 and 30 wt.%), the Rct present a backward in the charge transfer ease. In addition to the enhanced optical properties determined by UV–vis DRS and photoluminescence, the EIS tests, allows predicting an enhanced photocatalytic activity for the 20-BBO/NTO sample. The transient photocurrent tests evaluated the photocatalytic activity and partial stability for the BBO/NTO materials. Fig. 5b shows the photocurrent transients for the X-BBO/NTO samples (X = 5 to 30 wt.%) at a BIAS potential of -0.62 V vs. Ag/AgCl 3 M (H+/H2 equilibrium redox potential). It is clear that the photocurrent generated under illumination increases from 5 until 20 wt.% BBO added, above 20 wt.%, BBO/NTO samples do not present good stability and show a mixed current decreasing its photocatalytic response. 20-BBO/NTO presented the highest photocurrent value (≈2 μA). Besides, 20-BBO/NTO shows an efficient photocatalytic charge carrier generation since it can generate and eliminate the photocurrent signal in a short time. Since UV–vis DRS, photoluminescence, and electrochemical characterizations suggest the 20-BBO/NTO sample presents the best photocatalytic performance, Fig. 6a depicts the cyclic voltammetry for this
3.6. Photocatalytic hydrogen production The photocatalytic activity of the BBO/NTO heterojunctions and pure semiconductors to perform the water splitting from pure water under UV irradiation is shown in Fig. 7. It is worth to mention that even when BaBiO3 is supposed to present photocatalytic activity because its narrow band-gap; however, the UV-light irradiation used in this tests can produce an excess of charge carriers which promotes the inefficient extraction of the photo-generated charges in perovskites, thus suppressing the BaBiO3 photocatalytic activity [42]. On the other hand, several reports have demonstrated that pure NaTaO3 is one of the best materials to perform the hydrogen generation [7–9,43]. In agreement with this statement, the NaTaO3 sample presented in this work presented the highest H2 production among the bare materials. Fig. 7a shows the BBO/NTO hydrogen generation profile, from this figure, we can point out how with a minimum addition of BBO (5 wt.%), the BBO/ NTO presents almost the same H2 generation rate than bare BaBiO3. As a result of a higher BBO addition, the H2 photogeneration increase until
Fig. 5. (a) Nyquist plots and, (b) photocurrent response for X-BBO/NTO samples (X = 5–30 wt.%) samples at equilibrium potential under UV–vis illumination (Newport 66884, QTH lamp, 250 W output). 6
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Fig. 6. Photoelectrochemical characterization for X-BBO/NTO samples (X = 5–30 wt.%) samples, (a) Cyclic voltammetry, (b) photocurrent transients (negative scan) (c) photocurrent transients (positive scan) and (d) Nyquist Plots in dark and light conditions.
a maximum value of 54 μmol g−1 for 20-BBO/NTO; above 20 wt.% BBO, the production decline unto minimum values. In a general way, the activity of BBO was improved in the composites, exhibiting a volcano-type characteristic trend (Table S3). This trend could be described as follows: when small amounts of the BBO (below 20%) are mixed with NTO, the charge separation and transference is improved, until it reaches an optimal amount (20% of BBO), where the overpotential for the water-splitting reaction is reduced, and the photocatalytic activity is superior in the composite compared to the bare materials. Above this optimal value (loadings higher to 20%), the activity decreases due to the covering of the active sites and the surface of the material, which
reduces the light absorption and the generation of charges [44]. It is feasible to suggest that the high conversion rate achieved by the 20BBO/NTO photocatalyst (1.4 times hydrogen than pure NaTaO3 and 1.5 times higher than TiO2 P25 (Figure S3)) showed in Fig. 7b, is a result of parameters combination such as i) A higher crystallite size which theoretically presents a lower number of grain boundaries responsible for a higher charge carrier resistance recombination degree [45,46], ii) the presence of an optimal amount of BaBiO3 which acts as a secondary phase with the ability to form a heterojunction BBO/NTO capable of enhance the visible light-harvesting, inducing a rapid charge separation and an inhibition of the electron-hole pairs recombination [47,48].
Fig. 7. (a) Photocatalytic hydrogen production of X-BBO/NTO samples (X = 5–30 wt.%) and (b) Hydrogen generation rate for bare materials and X-BBO/NTO. 7
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semiconductor to be translated to the counter electrode (platinum rod) to perform the reduction reactions [56,57]. Similarly, photogenerated holes (h+) on the NTO surface are transferred to the BBO VB, where they react with the surrounding chemical species (water) to perform the oxidation processes (generation of ∙OH radicals). In this way, the heterojunction BBO/NTO presents an enhancement in the charge carrier transference due to a continuous BBO/NTO charge flow, which acts to prevent recombination issues and increase the lifetime of the photogenerated charge carriers. Since the 20-BBO/NTO heterojunction presents an n-type semiconductor behavior (Figure S3) [15], this material possesses a higher number of electrons than holes; in this way, majority carriers (e−) flow from the semiconductor (working electrode) to the platinum counter electrode. The electrochemical system using 20-BBO/ NTO performs the water-splitting process by the initial ∙OH radicals generation and subsequent water oxidation reaction onto the semiconductor surface [58], as depicted in the electrochemical evaluations of Fig. 6c.
3.7. Photocatalytic and photoelectrochemical mechanisms The suggested photocatalytic transfer mechanism was obtained by the calculation of the theoretical band positions for the BBO/NTO heterojunction following the equations (1) and (2), respectively [49]: Evb = X - Ee + 0.5Eg
(1)
ECB = EVB - Eg
(2)
where Evb is the valence band (VB) edge potentials; X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms (XBa =2.4 eV, XBi =4.69 eV, XNa =2.85 eV, XTa =4.11 eV, and XO =7.54 eV); Ee is the energy of free electrons on the hydrogen scale (4.5 eV) [49], Eg is the bandgap energy of semiconductor, and ECB is the conduction band (CB) edge potentials. According to the atomic ratio of each constituent element, the X values for BaBiO3 and NaTaO3 are 5.45 and 5.49, respectively. In this way, the position for Evb and ECB were calculated to be 1.75 eV (VB) and 0.15 eV (CB) for BaBiO3, and 3.025 eV (VB) and -1.045 eV (CB) for NaTaO3. These previous calculations allow the theoretical proposal of a photocatalytic mechanism as the one illustrated in Fig. 8a. It is possible to observe the band diagrams for pure semiconductors before contact. Because of their extrinsic nature (p-type for BaBiO3 and n-type for NaTaO3), before contact, the fermi levels (EF) for each compound present a different position [50]. The EF is located slightly above the VB in BaBiO3 (EFp); meanwhile, this value can be found nearly under the CB in NaTaO3 (EFn) [51,52]. Once the semiconductors are in contact, electrons flow from the NaTaO3 CB to the BaBiO3 CB until the junction reaches an equalization of the Fermi Level; this event induces a new bands positions in the new heterojunction [53]. After contact, because of the Fermi Level equalization, BaBiO3 bands shift in the negative direction. In this way, the BaBiO3 CB presents now a more negative potential position than NaTaO3 CB to form a type-ll heterojunction [54,55]. Hence, when the new heterostructure is irradiated, BaBiO3 is the main responsible for the light absorption and charge carriers generation due to its narrow bandgap; therefore, the photogenerated electrons migrate from the BaBiO3 CB to the NaTaO3 CB due to a less of the barrier which acts preventing recombination issues. Meanwhile, electrons accumulated on the NaTaO3 CB reacts with the surrounding medium to perform the protons (H+) reduction coming from the photocatalyst anodic site, the accumulated holes on the BaBiO3 VB are responsible for the water oxidation reaction. The lifetime of the photogenerated charge carriers is prolonged, thus improves the photocatalytic efficiency of the BBO/NTO heterojunction [56,57]. On the other hand, Fig. 8b depicts the proposed photoelectrochemical mechanism. Since the conduction CB and VB of BBO and NTO are in different energetic levels, the photogenerated electrons migrate from the BBO CB to the NTO CB where are extracted from the BBO/NTO
4. Conclusions The photocatalytic activity of semiconductors NaTaO3 and BaBiO3 synthesized by the solid-state method was studied to perform the hydrogen production process without the use of sacrificial agents. This study demonstrated that BBO is a suitable material to modify the physicochemical NTO properties. In general, X-BBO/NTO (X = 5–30 wt.%) presented lower particle size than bare BaBiO3, being the 20-BBO/NTO the sample that presented the lowest particles agglomeration degree and the largest crystallite size. The addition of BBO to the NTO improved the light harvest by a red-shift. The formation of a heterojunction BBO/NTO enhanced the photocatalytic activity to perform the photocatalytic water splitting process by an improved initial water oxidation step. It is suggested this photocatalytic improvement was caused by: i) an increase of the X-BBO/NTO crystallite size compared to bare materials, which indicates a lower number of grain boundaries defects, so reducing the recombination rate. ii) an optimal addition of BaBiO3 (20 wt.%) that increase the synergy of the BBO/ NTO; thus, decreasing the charge transference resistance, and iii) a continuous charge carrier transference caused by the overlapped bands position (heterojunction) which prevent the recombination issues, thus enhancing the charge transfer processes to perform the photocatalytic hydrogen generation. Author Statement Leticia M. Torres-Martinez and J. Manuel Mora-Hernandez conceived of the present idea. Ali M. Huerta-Flores verified the analytical methods and supervised the findings of this work, all authors discussed
Fig. 8. The proposed photo(electro)catalytic reaction mechanisms for the water-splitting process onto the 20-BBO/NTO heterojunction. 8
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the results and contributed to the final manuscript. J. Manuel Mora-Hernandez and Ali M. Huerta-Flores carried out the experiment. J. Manuel Mora-Hernandez wrote the manuscript with support from Ali M. Huerta-Flores. J. Manuel Mora-Hernandez and Ali M. Huerta-Flores conceived and planned the experiments. J. Manuel Mora-Hernandez and Ali M. Huerta-Flores carried out the experiments. J. Manuel Mora-Hernandez and Ali M. Huerta-Flores contributed to sample preparation. J. Manuel Mora-Hernandez and Ali M. Huerta-Flores contributed to the interpretation of the results. J. Manuel Mora-Hernandez took the lead in writing the manuscript. All authors provided critical feedback and helped shape the research, analysis and manuscript.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors would like to thank Cátedras CONACyT – ID7708, and thank CONACYT for the financial support through the following projects: CB-2014-237049 and FC-2016-01-1725, also thank UANL: PAIFIC 2018-10, PAIFIC 2018-5, PAICYT 2019 IT1053-19, PAICYT IT1021-19, FIC-UANL.SEP-PROFIDES 511-6/18-11852. Additionally, the authors thank Jesus Meza-Bustillos, Williams González-Suárez, and J. David Fernandez-Reyes by their technical support. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jphotochem.2020. 112363. References [1] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (5358) (1972) 37–38. [2] A.R. Khataee, M.B. Kasiri, Photocatalytic degradation of organic dyes in the presence of nanostructured titanium dioxide: influence of the chemical structure of dyes, J. Mol. Catal. A Chem. 328 (1) (2010) 8–26. [3] A.D. Tjandra, J. Huang, Photocatalytic carbon dioxide reduction by photocatalyst innovation, Chinese Chem. Lett. 29 (6) (2018) 734–746. [4] P. Kanhere, Z. Chen, A Review on Visible Light Active Perovskite-Based Photocatalysts Vol. 19 (2014), pp. 19995–20022. [5] S.R. Thakare, et al., Development of new, highly efficient and stable visible light active photocatalyst Ag2ZrO3 for methylene blue degradation, Catal. Commun. 62 (2015) 39–43. [6] F.-F. Li, et al., Improved Visible-Light Photocatalytic Activity of NaTaO3 With Perovskite-Like Structure via sulfur Anion Doping Vol. 166-167 (2014). [7] C. Gómez-Solís, et al., Facile solvo-combustion synthesis of crystalline NaTaO3 and its photocatalytic performance for hydrogen production, Fuel 130 (2014) 221–227. [8] C. Gómez-Solís, et al., RuO2–NaTaO3 heterostructure for its application in photoelectrochemical water splitting under simulated sunlight illumination, Fuel 166 (2016) 36–41. [9] A.M. Huerta-Flores, et al., Laser assisted chemical vapor deposition of nanostructured NaTaO3 and SrTiO3 thin films for efficient photocatalytic hydrogen evolution, Fuel 197 (2017) 174–185. [10] J. Rodríguez-Torres, et al., Synthesis and characterization of Au-Pd/NaTaO3 multilayer films for photocatalytic hydrogen production, J. Photochem. Photobiol. A: Chem. 332 (2017) 208–214. [11] L. An, H. Onishi, Electron–Hole Recombination Controlled by Metal Doping Sites in NaTaO 3, Photocatalysts 5 (2015) 3196–3206. [12] B. Wang, et al., Anion-doped NaTaO3 for visible light photocatalysis, J. Phys. Chem. C 117 (44) (2013) 22518–22524. [13] A. Iwase, H. Kato, A. Kudo, The Effect of Au Cocatalyst Loaded on La-Doped NaTaO3 on Photocatalytic Water Splitting and O2 Photoreduction Vol. 136–137 (2013), pp. 89–93. [14] P. Kanhere, et al., Electronic Structure, Optical Properties, and Photocatalytic Activities of LaFeO3–NaTaO3 Solid Solution Vol. 116 (2012), pp. 22767–22773. [15] J.M. Mora-Hernandez, A.M. Huerta-Flores, L.M. Torres-Martínez, Photoelectrocatalytic characterization of carbon-doped NaTaO3 applied in the photoreduction of CO2 towards the formaldehyde production, J. Co2 Util. 27
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