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RuO2–NaTaO3 heterostructure for its application in photoelectrochemical water splitting under simulated sunlight illumination
Fuel 166 (2016) 36–41
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RuO2–NaTaO3 heterostructure for its application in photoelectrochemical water splitting under simulated sunlight illumination C. Gómez-Solís, J.C. Ballesteros, L.M. Torres-Martínez ⇑, I. Juárez-Ramírez Universidad Autónoma de Nuevo León (UANL), Facultad de Ingeniería Civil, Departamento de Ecomateriales y Energía, Ciudad Universitaria, San Nicolás de los Garza, Nuevo León C.P. 66455, Mexico
a r t i c l e
i n f o
Article history: Received 17 June 2015 Received in revised form 26 October 2015 Accepted 26 October 2015 Available online 31 October 2015 Keywords: NaTaO3 RuO2 Photoelectrochemical Band gap Hydrogen production
a b s t r a c t In this paper, we report the synthesis of RuO2–NaTaO3 films for their application as photocathodes in photoelectrochemical (PEC) cells for hydrogen generation from water splitting. RuO2 was electrodeposited on NaTaO3 films at constant current varying the deposition time with the finality to obtain 1–3 wt.% RuO2–NaTaO3. The characterization of the photoelectrodes RuO2–NaTaO3 was carried out by linear voltammetry (VL), chronoamperometry (CA), scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM/EDS), X-ray diffraction (XRD), ultraviolet–visible absorption spectroscopy (UV–vis) and X-ray photoelectron spectroscopy (XPS). The hydrogen production was conducted in a photoelectrochemical reactor coupled with a gas chromatograph. One of the main contributions of this work is the incorporation of RuO2 on NaTaO3 films in order to enhance its activity in the visible region, achieving a maximum photoelectrochemical hydrogen production of 15.7 mmol h1 g1 for the film with 2 wt.% RuO2 with a solar-to-hydrogen (STH) conversion efficiency of 4.29%. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction The combination of solar energy with water is one of the promising alternatives to solve the energy need that society will demand in future years. This strategy is an option to obtain hydrogen, from water-splitting process, as a clean and sustainable energy source [1,2]. Water splitting can be carried out by heterogeneous photocatalysis (HPC) and photoelectrochemical process (PEC) [3–5]. In the literature [6,7], it is reported that the photoelectrochemical water splitting started in Japan in the late 1960s with the study of Fujishima and Honda, in which TiO2 was used as semiconductor material. Since then, new materials have been prepared with finality to surmount the high-energy barriers in the watersplitting reaction. Among these materials, NaTaO3 has been widely studied due to its high activity for this reaction under UV-light [8]. Kudo et al., using the HPC system [9–14], have extensively reported the study of tantalates, for its application in the hydrogen production. Recently, our group has reported a photocatalytic hydrogen production of 10 mmol h1 g1 using sodium tantalate [15,16]. Furthermore, other studies about the use of tantalates
⇑ Corresponding author. E-mail address: [email protected] (L.M. Torres-Martínez). http://dx.doi.org/10.1016/j.fuel.2015.10.104 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.
for hydrogen production can be found in literature [17–27]. Most of those reports are focused on the relation of sodium tantalate performance with hydrogen production rate Several strategies have been conducted in order to enhance the photocatalytic activity of the tantalates, such as synthesis route, doping of metal cations and incorporation of co-catalysts; also it is reported that different morphologies can increase the amount of hydrogen production. Specifically, the synthesis methods used in these studies are solid state, hydrothermal, sol–gel, moltensalt synthesis, hydrothermal, polymerizable complex, deposition and/or doping of metal cations such as: Zn, La, Sm, Gd, Si, Zr, In, Nb, Ce, and Al [8]. Also the use of co-catalysts like Ni, NiO, Au nanoparticles, RuO2 and [Mo3S4]4+ have been applied as a strategy to enhance activity [10,13,15,16]. Despite the fact that there is a huge amount of papers related to hydrogen production using perovskite-type tantalate compounds, only few studies are related to the use of PEC systems [28–31]. These studies have been focused to get information about the electrochemical behavior to understand the mechanism of photocatalysis for water splitting reaction and O2 photoreverse reaction. Among the metallic materials that can be combined with NaTaO3 to enhance the photocatalytic activity of this semiconductor in visible region, the RuO2 can be a good candidate due to its high chemical stability, high conductivity and excellent diffusion barrier
C. Gómez-Solís et al. / Fuel 166 (2016) 36–41
properties. In particular, RuO2 has been applied as a catalyst in the reaction of water splitting and several organic compounds [32–35] through photoinduced processes. For example, Bloh et al. [33,34] have showed that zinc oxide containing ruthenium oxide is highly active in the visible light spectrum. In our previous studies, the hydrogen production, using HPC system, was enhanced due to the incorporation of RuO2 as co-catalyst in NaTaO3 [15,16]; however, the heterointerface of RuO2–NaTaO3 remains uninvestigated. In this work, we focus by first time on the photoelectrochemical hydrogen production using RuO2 as co-catalyst on NaTaO3; it is used as photocathode under simulated solar irradiation.
2. Experimental 2.1. Synthesis of NaTaO3 NaTaO3 was synthesized by the solvo-combustion method in concordance with the procedure reported by our group [16], where NaTaO3 was formed by a reaction between tantalum (V) ethoxide (99.98% Sigma Aldrich) and sodium acetate (98% DEQ) in stoichiometric amounts. The reaction mixture was kept at 70 °C under stirred and refluxed for a time of 30 min. Then, 1 mL of nitric acid was added and the flask immediately placed onto a hot plate at 180 °C during 5 min. The obtained product was annealed at 600 °C for 2 h. The obtained NaTaO3 powder was applied by screen printing on a copper substrate area (1 1 cm2) to form the NaTaO3 film. Later, this sample was used as working electrode for electrodeposition of RuO2 and subsequently, in the experiments of photoelectrochemical hydrogen production. The amount of powder applied on the copper substrate was 1.2 ± 0.1 mg. 2.2. Synthesis of RuO2 on NaTaO3 RuO2 was incorporated on each NaTaO3 film by chronopotentiometry technique from an electrolytic solution with a chemical composition of 1.0 M HNO3 + 0.01 M RuCl3 (both from Sigma Aldrich), which was kept at 40 °C. Before the experiments, the solution was bubbled with O2 for 1 h. The electrochemical experiments were carried out in a conventional three-electrode cell; platinum wire and saturated Ag/AgCl electrode were used as counter and reference electrodes, respectively. The electrochemical tests were conducted using a potentiostat/galvanostat AUTOLAB PGSTAT302N equipment coupled to a computer with the NOVA 1.10 software for control of the experiments and data acquisition. The applied current for electrodeposition of RuO2 on the NaTaO3 surface was 0.5 mA cm2 with different duration time in order to obtain three electrodes containing 1, 2 and 3 wt.%. of RuO2, respectively. 2.3. Structural, morphological and optical characterization The crystal structure of the powder was analyzed by X-ray diffraction (XRD) using Cu Ka radiation (1.5406 Å) in a Bruker D8 Advance X-ray diffractometer. The surface morphology and elemental distribution were analyzed using a scanning electron microscope (SEM) in a JEOL 6490 LV equipment coupled with an energy dispersive X-ray spectroscopy detector (EDS). X-ray photoelectron spectra of photocatalysts were recorded using a PerkinElmer Phi 560 XPS/Auger system. In order to determinate the band gap energy (Eg), Tauc’s plot was employed from the acquired data of absorbance spectra using the transformed diffuse reflectance technique according to the Kubelka–Munk theory. UV–Vis spectrophotometer (Lambda 35 Perkin Elmer Corporation) equipped with an integrating sphere attachment was used for optical absorption measurements.
37
2.4. Photoelectrochemical study The electrochemical experiments were carried out in a conventional three-electrode photoelectrochemical quartz cell in a 0.5 M Na2SO4 aqueous solution at room temperature. The photoelectrodes fabricated were used as working electrodes; Platinum wire and saturated Ag/AgCl were used as counter and reference electrodes, respectively. In this work all potentials are reported with respect to Ag/AgCl electrode. The photocurrents were recorded under simulated solar light (AM1.5G, 100 mW cm2) with a 450 W Xe-lamp. The experiments were performed under inert atmosphere and at room temperature. The activity of RuO2– NaTaO3 for the hydrogen evolution reaction (HER) was studied by linear voltammetry and chronoamperometry techniques. The PEC cell was connected to a Shimadzu’s versatile GC-2014 (TCD) gas chromatograph equipment for hydrogen quantification. Before measurements, the chromatograph was calibrated for hydrogen and oxygen with the next reference gases containing 99.998% H2 and 99.999% O2, respectively. 3. Results and discussion 3.1. Synthesis, preparation and characterization of RuO2–NaTaO3 electrode The RuO2–NaTaO3 photoelectrode was prepared as follow: NaTaO3 nanocubes were firstly synthesized by the solvocombustion process to obtain a powder, which was coated in the form of a film on ITO substrate by screen printing. The RuO2 coating on the NaTaO3 films was carried out by electrodeposition technique with a constant applied current from a HNO3/RuCl3 aqueous solution. Kim et al. [36] have reported the electrodeposition method used in this work to obtain RuO2. Fig. 1a shows the XRD patterns registered for the RuO2–NaTaO3 electrodes. In all cases, the orthorhombic NaTaO3 phase was clearly identifiable in the X-ray diffraction patterns accordingly with the JCPDS 89-8061. Additionally, an enlargement (Fig. 1b) in the 2h range 27–37° shows the presence of small peaks that have been associated to RuO2 (JCPDS 70-2662). Fig. 2 shows: (a) SEM image and its corresponding (b) EDS spectrum of the RuO2–NaTaO3 film on the copper substrate. Fig. 2a reveals that two different morphologies of RuO2–NaTaO3 electrode are present. The analysis EDS indicated that cube-like agglomerated structures correspond to NaTaO3, while the small spherical particles are associated to the RuO2 compound. The chemical analysis by EDS (Fig. 2b) showed that the film contains only Cu, Na, Ta, Ru and O. The measured weight percentage of RuO2 (2.1 wt.%) agrees with the expected one, i.e., 2.0 wt.%. In order to determine the optical properties and calculate the energy band gap (Eg) of the RuO2–NaTaO3 films, additional experiments of UV–vis spectroscopy were conducted. Fig. 3a shows the absorbance spectra of the RuO2–NaTaO3 films. It is clear to observe that the light absorption in visible region increases with increasing RuO2 content. This behavior is attributed to the resonant oscillation of conduction electrons at the interface between RuO2 and NaTaO3 as it has been mentioned in the literature [35]. This phenomenon is stimulated by incident light provoking an enhancement ultraviolet light emission and increase of visible light absorption. Tauc’s plot (Fig. 3b) was constructed from the absorbance spectrum. According to the Kubelka–Munk equation [37,38], absorption coefficient (a) can be calculated using reflectance data:
FðRÞ ¼
ð1 RÞ2 a ¼ 2R s
ð1Þ
38
C. Gómez-Solís et al. / Fuel 166 (2016) 36–41
(a)
*
NaTaO3
(a)
Intensity / a. u.
# Na2Ta4 O11 o Cu RuO -NaTaO (3 wt.%) 2 3
o
RuO -NaTaO (2 wt.%) 2 3
*
* #
#
*
*
*
o
*
*
RuO -NaTaO (1 wt.%) 2 3
10
20
30
40
50
60
70
2 θ, degree
(b)
RuO2
RuO2
# (b)
#
30
[F(R)hν]
2
20
28
32
36
10
3.0 wt. RuO2
2θ, degree
2.0 wt. RuO
Fig. 1. XRD patterns of (1) RuO2–NaTaO3 (1 wt.%), (2) RuO2–NaTaO3 (2 wt.%) and (3) RuO2–NaTaO3 (3 wt.%) films.
2
1.0 wt. RuO 2
0
where R is the percentage of reflected light, s is the scattering coefficient. Assuming that s is wavelength independent, the incident photon energy (hm) and the optical band gap energy (Eg) can be related to the transformed Kubelka–Munk function:
ðFðRÞhmÞ ¼ Aðhm Eg Þ n
3.50
3.75
4.00
4.25
4.50
hν /eV Fig. 3. (a) UV–visible absorption spectra and (b) Tauc plot for (1) RuO2–NaTaO3 (1 wt.%), (2) RuO2–NaTaO3 (2 wt.%) and (3) RuO2–NaTaO3 (3 wt.%).
ð2Þ
where A, is the constant depending on transition probability and n is the power index that is related to the optical absorption process; n is equal to ½ or 2 for indirect or direct allowed transition, respectively [37,38]. The obtained Eg values were 4.04, 3.98 and 3.90 eV for RuO2–NaTaO3 (1 wt.%), RuO2–NaTaO3 (2 wt.%) and RuO2–
NaTaO3 (3 wt.%), respectively. These results indicate that as the RuO2 content is increased on NaTaO3, the Eg value diminished due to improvement in visible light absorption. XPS was used to determine the surface condition of RuO2–NaTaO3 films. Fig. 4a shows the survey spectra acquired from the RuO2–NaTaO3 (3 wt.%) film, where it is possible to
Fig. 2. (a) SEM image and (b) EDS spectrum of RuO2–NaTaO3 (2 wt.%) film.
C. Gómez-Solís et al. / Fuel 166 (2016) 36–41
39
(a)
Fig. 5. Photocurrent–time curves of (a) RuO2–NaTaO3 (1 wt.%), (b) RuO2–NaTaO3 (2 wt.%) and (c) RuO2–NaTaO3 (3 wt.%) under simulated sunlight on/off from 0.5 M Na2SO4 aqueous solution.
4+ Ru
Intensity / a. u.
(b)
280.7
285.4
4+ Ru
290
288
286
284
282
280
278
Binding Energy / eV Fig. 4. (a) XPS survey spectrum of RuO2–NaTaO3 (3 wt.%) and (b) high-resolution Ru 3d spectrum of NaTaO3.
observe the presence of tantalum, ruthenium, sodium and oxygen. Fig. 4b shows two emission lines at 280.7 and 285.4 eV assigned to Ru 3d5/2 and Ru 3d3/2, which are characteristic of the Ru4+ oxidation state of Ru [35]. This result indicates that the electrodeposition method allows the deposit of RuO2 on NaTaO3 by a simple an economic method; the presence of RuO2 was also corroborated by XRD and EDS analyses.
Fig. 6. Linear voltammetry obtained for (a) RuO2–NaTaO3 (1 wt.%), (b) RuO2– NaTaO3 (2 wt.%) and (c) RuO2–NaTaO3 (3 wt.%) from 0.5 M Na2SO4 aqueous solution at a scan rate of 10 mV s1.
the RuO2–NaTaO3 films under this test, which can be interpreted as a high chemical stability of these materials. The steady photocurrents determined from these transients were used to calculate the solar-to-hydrogen (STH) conversion efficiency using the next equation [38]
g¼I 3.2. Photoelectrochemical water splitting The following experiments were carried out to examine the photoelectrochemical behavior of RuO2–NaTaO3 films under simulated sunlight. Fig. 5 shows the recorded chronoamperogram at constant potential of 0.3 V for 1 h under cycles of dark (300 s) and light (300 s). In general, it is possible to note that when the light was switched on, the cathodic photocurrent of the three electrodes was decreased. The cathodic photocurrent value decreased to zero immediately after the sunlight was switched off. However, the cathodic photocurrent is dependent of RuO2 content, being the optimum RuO2 amount of approximately 2 wt.%. This suggests an efficient separation of the photogenerated electron/hole pair. Additionally, Fig. 5 shows a good and reproducible photoresponse on all
ð1:23 EÞ Plight
where I is the photocurrent density at the measurement applied bias, V is the applied bias (versus RHE), and Plight is the incident light intensity of 100 mW/cm2. The measured potentials versus Ag/AgCl can be converted to the RHE scale according to the Nernst equation [37,38]: ERHE ¼ EAg=AgCl þ 0:059pH þ EoAg=AgCl . Where ERHE is the converted potential versus RHE, EoAg=AgCl = 0.197 V at 25 °C and EAg=AgCl is the experimentally measured potential against Ag/AgCl. Accordingly, the STH efficiency of the RuO2–NaTaO3 films is calculated to be 0.79%, 4.29% and 2.24% for RuO2–NaTaO3 (1 wt.%), RuO2–NaTaO3 (2 wt.%) and RuO2–NaTaO3 (3 wt.%) at a bias potential of 0.3 V. These results confirm that the RuO2–NaTaO3 (2 wt.%) is the film with highest conversion of absorbed light into photocurrent and with the lowest recombination rate of hole–electron pair.
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Acknowledgements Authors want to thank the financial support for this research to CONACYT – México through projects: FON.INST./75/2012 ‘‘Fotosíntesis Artificial”, CNPq México-Brasil 2012-174247, CB2011-168730, CB-2014-237049, RET-2014-239391 and PAICYTUANL. Also thanks to Juan Luis Peña Chapa and CINVESTAV-IPN unidad Mérida for the facilities to carried out the XPS analysis.
References
Fig. 7. GC-TCD spectra of the gas product obtained on RuO2–NaTaO3 (2 wt.%) at different times: (a) 0.5 h and (b) 1 h.
Fig. 6 shows a set of linear sweep voltammograms recorded on the RuO2–NaTaO3 films in the dark and with simulated sunlight irradiation. All scan linear sweep voltammograms were conducted from 0.08 to 0.55 V at scan rate of 10 mV s1. Upon illumination of the RuO2–NaTaO3 films, a pronounced cathodic photocurrent starting at ca. 0.2 V was observed, which continues its increment until final potential at 0.55 V. The highest cathodic value 1.9 mA/cm2 at 0.55 V, was achieved for the NaTaO3–RuO2 (2 wt.%) which is 2.1 and 1.2 times greater than films in curves (a) and (c), respectively. The curve (d) corresponds to the voltammetric response obtained under dark conditions, where a negligible photocurrent was registered. 3.3. Photoelectrochemical hydrogen production coupled to GC With the finality to evaluate the amount of photoelectrochemical hydrogen production, the chronoamperometry (at 0.3 V for 1 h) coupled with GC-TCD technique under simulated sunlight irradiation were used. Fig. 7 represents the chromatographic separation of the evolved H2 and O2 during electrochemical operation at applied constant potential, where tH2 (0.76 min) and tO2 (0.86 min) are the respective retention times. These results indicate that cathodic photocurrent recorded during the electrochemical process is due to H2 and O2 production, i.e., the photoelectrochemical water splitting is occurring. Hydrogen amount was calculated from calibration data and the results were: 4.5, 15.7 and 8.3 mmol g1 cm2 for RuO2–NaTaO3 (1 wt.%), RuO2– NaTaO3 (2 wt.%) and RuO2–NaTaO3 (3 wt.%), respectively. These results indicate that RuO2–NaTaO3 is a good material for the hydrogen production under simulated sunlight and controlled-applied potential. 4. Conclusions Photoelectrochemical hydrogen production from water splitting was carried out using RuO2–NaTaO3 films as photocathodes. The effect of RuO2 concentration on NaTaO3 was studied in the range 1–3 wt.% RuO2 and it was found that the optical properties of RuO2–NaTaO3 films were enhanced for the visible region. The XRD, EDS and XPS analysis revealed the presence of RuO2 on the NaTaO3 surface. PEC-GC results shown that a maximum photoelectrochemical hydrogen production of 15.7 mmol h1 g1 was achieved for the film with 2 wt.% RuO2 with solar-to-hydrogen (STH) conversion efficiency of 4.29%.
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Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
RuO2–NaTaO3 heterostructure for its application in photoelectrochemical water splitting under simulated sunlight illumination C. Gómez-Solís, J.C. Ballesteros, L.M. Torres-Martínez ⇑, I. Juárez-Ramírez Universidad Autónoma de Nuevo León (UANL), Facultad de Ingeniería Civil, Departamento de Ecomateriales y Energía, Ciudad Universitaria, San Nicolás de los Garza, Nuevo León C.P. 66455, Mexico
a r t i c l e
i n f o
Article history: Received 17 June 2015 Received in revised form 26 October 2015 Accepted 26 October 2015 Available online 31 October 2015 Keywords: NaTaO3 RuO2 Photoelectrochemical Band gap Hydrogen production
a b s t r a c t In this paper, we report the synthesis of RuO2–NaTaO3 films for their application as photocathodes in photoelectrochemical (PEC) cells for hydrogen generation from water splitting. RuO2 was electrodeposited on NaTaO3 films at constant current varying the deposition time with the finality to obtain 1–3 wt.% RuO2–NaTaO3. The characterization of the photoelectrodes RuO2–NaTaO3 was carried out by linear voltammetry (VL), chronoamperometry (CA), scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM/EDS), X-ray diffraction (XRD), ultraviolet–visible absorption spectroscopy (UV–vis) and X-ray photoelectron spectroscopy (XPS). The hydrogen production was conducted in a photoelectrochemical reactor coupled with a gas chromatograph. One of the main contributions of this work is the incorporation of RuO2 on NaTaO3 films in order to enhance its activity in the visible region, achieving a maximum photoelectrochemical hydrogen production of 15.7 mmol h1 g1 for the film with 2 wt.% RuO2 with a solar-to-hydrogen (STH) conversion efficiency of 4.29%. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction The combination of solar energy with water is one of the promising alternatives to solve the energy need that society will demand in future years. This strategy is an option to obtain hydrogen, from water-splitting process, as a clean and sustainable energy source [1,2]. Water splitting can be carried out by heterogeneous photocatalysis (HPC) and photoelectrochemical process (PEC) [3–5]. In the literature [6,7], it is reported that the photoelectrochemical water splitting started in Japan in the late 1960s with the study of Fujishima and Honda, in which TiO2 was used as semiconductor material. Since then, new materials have been prepared with finality to surmount the high-energy barriers in the watersplitting reaction. Among these materials, NaTaO3 has been widely studied due to its high activity for this reaction under UV-light [8]. Kudo et al., using the HPC system [9–14], have extensively reported the study of tantalates, for its application in the hydrogen production. Recently, our group has reported a photocatalytic hydrogen production of 10 mmol h1 g1 using sodium tantalate [15,16]. Furthermore, other studies about the use of tantalates
⇑ Corresponding author. E-mail address: [email protected] (L.M. Torres-Martínez). http://dx.doi.org/10.1016/j.fuel.2015.10.104 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.
for hydrogen production can be found in literature [17–27]. Most of those reports are focused on the relation of sodium tantalate performance with hydrogen production rate Several strategies have been conducted in order to enhance the photocatalytic activity of the tantalates, such as synthesis route, doping of metal cations and incorporation of co-catalysts; also it is reported that different morphologies can increase the amount of hydrogen production. Specifically, the synthesis methods used in these studies are solid state, hydrothermal, sol–gel, moltensalt synthesis, hydrothermal, polymerizable complex, deposition and/or doping of metal cations such as: Zn, La, Sm, Gd, Si, Zr, In, Nb, Ce, and Al [8]. Also the use of co-catalysts like Ni, NiO, Au nanoparticles, RuO2 and [Mo3S4]4+ have been applied as a strategy to enhance activity [10,13,15,16]. Despite the fact that there is a huge amount of papers related to hydrogen production using perovskite-type tantalate compounds, only few studies are related to the use of PEC systems [28–31]. These studies have been focused to get information about the electrochemical behavior to understand the mechanism of photocatalysis for water splitting reaction and O2 photoreverse reaction. Among the metallic materials that can be combined with NaTaO3 to enhance the photocatalytic activity of this semiconductor in visible region, the RuO2 can be a good candidate due to its high chemical stability, high conductivity and excellent diffusion barrier
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properties. In particular, RuO2 has been applied as a catalyst in the reaction of water splitting and several organic compounds [32–35] through photoinduced processes. For example, Bloh et al. [33,34] have showed that zinc oxide containing ruthenium oxide is highly active in the visible light spectrum. In our previous studies, the hydrogen production, using HPC system, was enhanced due to the incorporation of RuO2 as co-catalyst in NaTaO3 [15,16]; however, the heterointerface of RuO2–NaTaO3 remains uninvestigated. In this work, we focus by first time on the photoelectrochemical hydrogen production using RuO2 as co-catalyst on NaTaO3; it is used as photocathode under simulated solar irradiation.
2. Experimental 2.1. Synthesis of NaTaO3 NaTaO3 was synthesized by the solvo-combustion method in concordance with the procedure reported by our group [16], where NaTaO3 was formed by a reaction between tantalum (V) ethoxide (99.98% Sigma Aldrich) and sodium acetate (98% DEQ) in stoichiometric amounts. The reaction mixture was kept at 70 °C under stirred and refluxed for a time of 30 min. Then, 1 mL of nitric acid was added and the flask immediately placed onto a hot plate at 180 °C during 5 min. The obtained product was annealed at 600 °C for 2 h. The obtained NaTaO3 powder was applied by screen printing on a copper substrate area (1 1 cm2) to form the NaTaO3 film. Later, this sample was used as working electrode for electrodeposition of RuO2 and subsequently, in the experiments of photoelectrochemical hydrogen production. The amount of powder applied on the copper substrate was 1.2 ± 0.1 mg. 2.2. Synthesis of RuO2 on NaTaO3 RuO2 was incorporated on each NaTaO3 film by chronopotentiometry technique from an electrolytic solution with a chemical composition of 1.0 M HNO3 + 0.01 M RuCl3 (both from Sigma Aldrich), which was kept at 40 °C. Before the experiments, the solution was bubbled with O2 for 1 h. The electrochemical experiments were carried out in a conventional three-electrode cell; platinum wire and saturated Ag/AgCl electrode were used as counter and reference electrodes, respectively. The electrochemical tests were conducted using a potentiostat/galvanostat AUTOLAB PGSTAT302N equipment coupled to a computer with the NOVA 1.10 software for control of the experiments and data acquisition. The applied current for electrodeposition of RuO2 on the NaTaO3 surface was 0.5 mA cm2 with different duration time in order to obtain three electrodes containing 1, 2 and 3 wt.%. of RuO2, respectively. 2.3. Structural, morphological and optical characterization The crystal structure of the powder was analyzed by X-ray diffraction (XRD) using Cu Ka radiation (1.5406 Å) in a Bruker D8 Advance X-ray diffractometer. The surface morphology and elemental distribution were analyzed using a scanning electron microscope (SEM) in a JEOL 6490 LV equipment coupled with an energy dispersive X-ray spectroscopy detector (EDS). X-ray photoelectron spectra of photocatalysts were recorded using a PerkinElmer Phi 560 XPS/Auger system. In order to determinate the band gap energy (Eg), Tauc’s plot was employed from the acquired data of absorbance spectra using the transformed diffuse reflectance technique according to the Kubelka–Munk theory. UV–Vis spectrophotometer (Lambda 35 Perkin Elmer Corporation) equipped with an integrating sphere attachment was used for optical absorption measurements.
37
2.4. Photoelectrochemical study The electrochemical experiments were carried out in a conventional three-electrode photoelectrochemical quartz cell in a 0.5 M Na2SO4 aqueous solution at room temperature. The photoelectrodes fabricated were used as working electrodes; Platinum wire and saturated Ag/AgCl were used as counter and reference electrodes, respectively. In this work all potentials are reported with respect to Ag/AgCl electrode. The photocurrents were recorded under simulated solar light (AM1.5G, 100 mW cm2) with a 450 W Xe-lamp. The experiments were performed under inert atmosphere and at room temperature. The activity of RuO2– NaTaO3 for the hydrogen evolution reaction (HER) was studied by linear voltammetry and chronoamperometry techniques. The PEC cell was connected to a Shimadzu’s versatile GC-2014 (TCD) gas chromatograph equipment for hydrogen quantification. Before measurements, the chromatograph was calibrated for hydrogen and oxygen with the next reference gases containing 99.998% H2 and 99.999% O2, respectively. 3. Results and discussion 3.1. Synthesis, preparation and characterization of RuO2–NaTaO3 electrode The RuO2–NaTaO3 photoelectrode was prepared as follow: NaTaO3 nanocubes were firstly synthesized by the solvocombustion process to obtain a powder, which was coated in the form of a film on ITO substrate by screen printing. The RuO2 coating on the NaTaO3 films was carried out by electrodeposition technique with a constant applied current from a HNO3/RuCl3 aqueous solution. Kim et al. [36] have reported the electrodeposition method used in this work to obtain RuO2. Fig. 1a shows the XRD patterns registered for the RuO2–NaTaO3 electrodes. In all cases, the orthorhombic NaTaO3 phase was clearly identifiable in the X-ray diffraction patterns accordingly with the JCPDS 89-8061. Additionally, an enlargement (Fig. 1b) in the 2h range 27–37° shows the presence of small peaks that have been associated to RuO2 (JCPDS 70-2662). Fig. 2 shows: (a) SEM image and its corresponding (b) EDS spectrum of the RuO2–NaTaO3 film on the copper substrate. Fig. 2a reveals that two different morphologies of RuO2–NaTaO3 electrode are present. The analysis EDS indicated that cube-like agglomerated structures correspond to NaTaO3, while the small spherical particles are associated to the RuO2 compound. The chemical analysis by EDS (Fig. 2b) showed that the film contains only Cu, Na, Ta, Ru and O. The measured weight percentage of RuO2 (2.1 wt.%) agrees with the expected one, i.e., 2.0 wt.%. In order to determine the optical properties and calculate the energy band gap (Eg) of the RuO2–NaTaO3 films, additional experiments of UV–vis spectroscopy were conducted. Fig. 3a shows the absorbance spectra of the RuO2–NaTaO3 films. It is clear to observe that the light absorption in visible region increases with increasing RuO2 content. This behavior is attributed to the resonant oscillation of conduction electrons at the interface between RuO2 and NaTaO3 as it has been mentioned in the literature [35]. This phenomenon is stimulated by incident light provoking an enhancement ultraviolet light emission and increase of visible light absorption. Tauc’s plot (Fig. 3b) was constructed from the absorbance spectrum. According to the Kubelka–Munk equation [37,38], absorption coefficient (a) can be calculated using reflectance data:
FðRÞ ¼
ð1 RÞ2 a ¼ 2R s
ð1Þ
38
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(a)
*
NaTaO3
(a)
Intensity / a. u.
# Na2Ta4 O11 o Cu RuO -NaTaO (3 wt.%) 2 3
o
RuO -NaTaO (2 wt.%) 2 3
*
* #
#
*
*
*
o
*
*
RuO -NaTaO (1 wt.%) 2 3
10
20
30
40
50
60
70
2 θ, degree
(b)
RuO2
RuO2
# (b)
#
30
[F(R)hν]
2
20
28
32
36
10
3.0 wt. RuO2
2θ, degree
2.0 wt. RuO
Fig. 1. XRD patterns of (1) RuO2–NaTaO3 (1 wt.%), (2) RuO2–NaTaO3 (2 wt.%) and (3) RuO2–NaTaO3 (3 wt.%) films.
2
1.0 wt. RuO 2
0
where R is the percentage of reflected light, s is the scattering coefficient. Assuming that s is wavelength independent, the incident photon energy (hm) and the optical band gap energy (Eg) can be related to the transformed Kubelka–Munk function:
ðFðRÞhmÞ ¼ Aðhm Eg Þ n
3.50
3.75
4.00
4.25
4.50
hν /eV Fig. 3. (a) UV–visible absorption spectra and (b) Tauc plot for (1) RuO2–NaTaO3 (1 wt.%), (2) RuO2–NaTaO3 (2 wt.%) and (3) RuO2–NaTaO3 (3 wt.%).
ð2Þ
where A, is the constant depending on transition probability and n is the power index that is related to the optical absorption process; n is equal to ½ or 2 for indirect or direct allowed transition, respectively [37,38]. The obtained Eg values were 4.04, 3.98 and 3.90 eV for RuO2–NaTaO3 (1 wt.%), RuO2–NaTaO3 (2 wt.%) and RuO2–
NaTaO3 (3 wt.%), respectively. These results indicate that as the RuO2 content is increased on NaTaO3, the Eg value diminished due to improvement in visible light absorption. XPS was used to determine the surface condition of RuO2–NaTaO3 films. Fig. 4a shows the survey spectra acquired from the RuO2–NaTaO3 (3 wt.%) film, where it is possible to
Fig. 2. (a) SEM image and (b) EDS spectrum of RuO2–NaTaO3 (2 wt.%) film.
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39
(a)
Fig. 5. Photocurrent–time curves of (a) RuO2–NaTaO3 (1 wt.%), (b) RuO2–NaTaO3 (2 wt.%) and (c) RuO2–NaTaO3 (3 wt.%) under simulated sunlight on/off from 0.5 M Na2SO4 aqueous solution.
4+ Ru
Intensity / a. u.
(b)
280.7
285.4
4+ Ru
290
288
286
284
282
280
278
Binding Energy / eV Fig. 4. (a) XPS survey spectrum of RuO2–NaTaO3 (3 wt.%) and (b) high-resolution Ru 3d spectrum of NaTaO3.
observe the presence of tantalum, ruthenium, sodium and oxygen. Fig. 4b shows two emission lines at 280.7 and 285.4 eV assigned to Ru 3d5/2 and Ru 3d3/2, which are characteristic of the Ru4+ oxidation state of Ru [35]. This result indicates that the electrodeposition method allows the deposit of RuO2 on NaTaO3 by a simple an economic method; the presence of RuO2 was also corroborated by XRD and EDS analyses.
Fig. 6. Linear voltammetry obtained for (a) RuO2–NaTaO3 (1 wt.%), (b) RuO2– NaTaO3 (2 wt.%) and (c) RuO2–NaTaO3 (3 wt.%) from 0.5 M Na2SO4 aqueous solution at a scan rate of 10 mV s1.
the RuO2–NaTaO3 films under this test, which can be interpreted as a high chemical stability of these materials. The steady photocurrents determined from these transients were used to calculate the solar-to-hydrogen (STH) conversion efficiency using the next equation [38]
g¼I 3.2. Photoelectrochemical water splitting The following experiments were carried out to examine the photoelectrochemical behavior of RuO2–NaTaO3 films under simulated sunlight. Fig. 5 shows the recorded chronoamperogram at constant potential of 0.3 V for 1 h under cycles of dark (300 s) and light (300 s). In general, it is possible to note that when the light was switched on, the cathodic photocurrent of the three electrodes was decreased. The cathodic photocurrent value decreased to zero immediately after the sunlight was switched off. However, the cathodic photocurrent is dependent of RuO2 content, being the optimum RuO2 amount of approximately 2 wt.%. This suggests an efficient separation of the photogenerated electron/hole pair. Additionally, Fig. 5 shows a good and reproducible photoresponse on all
ð1:23 EÞ Plight
where I is the photocurrent density at the measurement applied bias, V is the applied bias (versus RHE), and Plight is the incident light intensity of 100 mW/cm2. The measured potentials versus Ag/AgCl can be converted to the RHE scale according to the Nernst equation [37,38]: ERHE ¼ EAg=AgCl þ 0:059pH þ EoAg=AgCl . Where ERHE is the converted potential versus RHE, EoAg=AgCl = 0.197 V at 25 °C and EAg=AgCl is the experimentally measured potential against Ag/AgCl. Accordingly, the STH efficiency of the RuO2–NaTaO3 films is calculated to be 0.79%, 4.29% and 2.24% for RuO2–NaTaO3 (1 wt.%), RuO2–NaTaO3 (2 wt.%) and RuO2–NaTaO3 (3 wt.%) at a bias potential of 0.3 V. These results confirm that the RuO2–NaTaO3 (2 wt.%) is the film with highest conversion of absorbed light into photocurrent and with the lowest recombination rate of hole–electron pair.
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Acknowledgements Authors want to thank the financial support for this research to CONACYT – México through projects: FON.INST./75/2012 ‘‘Fotosíntesis Artificial”, CNPq México-Brasil 2012-174247, CB2011-168730, CB-2014-237049, RET-2014-239391 and PAICYTUANL. Also thanks to Juan Luis Peña Chapa and CINVESTAV-IPN unidad Mérida for the facilities to carried out the XPS analysis.
References
Fig. 7. GC-TCD spectra of the gas product obtained on RuO2–NaTaO3 (2 wt.%) at different times: (a) 0.5 h and (b) 1 h.
Fig. 6 shows a set of linear sweep voltammograms recorded on the RuO2–NaTaO3 films in the dark and with simulated sunlight irradiation. All scan linear sweep voltammograms were conducted from 0.08 to 0.55 V at scan rate of 10 mV s1. Upon illumination of the RuO2–NaTaO3 films, a pronounced cathodic photocurrent starting at ca. 0.2 V was observed, which continues its increment until final potential at 0.55 V. The highest cathodic value 1.9 mA/cm2 at 0.55 V, was achieved for the NaTaO3–RuO2 (2 wt.%) which is 2.1 and 1.2 times greater than films in curves (a) and (c), respectively. The curve (d) corresponds to the voltammetric response obtained under dark conditions, where a negligible photocurrent was registered. 3.3. Photoelectrochemical hydrogen production coupled to GC With the finality to evaluate the amount of photoelectrochemical hydrogen production, the chronoamperometry (at 0.3 V for 1 h) coupled with GC-TCD technique under simulated sunlight irradiation were used. Fig. 7 represents the chromatographic separation of the evolved H2 and O2 during electrochemical operation at applied constant potential, where tH2 (0.76 min) and tO2 (0.86 min) are the respective retention times. These results indicate that cathodic photocurrent recorded during the electrochemical process is due to H2 and O2 production, i.e., the photoelectrochemical water splitting is occurring. Hydrogen amount was calculated from calibration data and the results were: 4.5, 15.7 and 8.3 mmol g1 cm2 for RuO2–NaTaO3 (1 wt.%), RuO2– NaTaO3 (2 wt.%) and RuO2–NaTaO3 (3 wt.%), respectively. These results indicate that RuO2–NaTaO3 is a good material for the hydrogen production under simulated sunlight and controlled-applied potential. 4. Conclusions Photoelectrochemical hydrogen production from water splitting was carried out using RuO2–NaTaO3 films as photocathodes. The effect of RuO2 concentration on NaTaO3 was studied in the range 1–3 wt.% RuO2 and it was found that the optical properties of RuO2–NaTaO3 films were enhanced for the visible region. The XRD, EDS and XPS analysis revealed the presence of RuO2 on the NaTaO3 surface. PEC-GC results shown that a maximum photoelectrochemical hydrogen production of 15.7 mmol h1 g1 was achieved for the film with 2 wt.% RuO2 with solar-to-hydrogen (STH) conversion efficiency of 4.29%.
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