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Improved photoelectrochemical water oxidation under visible light with mesoporous CoWO4
Materials Letters 183 (2016) 281–284
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Improved photoelectrochemical water oxidation under visible light with mesoporous CoWO4 M.I. Ahmed a, A. Adam b, A. Khan a, A.u. Rehman c, M. Qamaruddin a, M.N. Siddiqui b, M. Qamar a,n a
Center of Excellence in Nanotechnology (CENT), King Fahd University of Petroleum and Minerals, Dhahran 31261, Kingdom of Saudi Arabia Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran 31261, Kingdom of Saudi Arabia c Center for Engineering Research (CER), King Fahd University of Petroleum and Minerals, Dhahran 31261, Kingdom of Saudi Arabia b
art ic l e i nf o
a b s t r a c t
Article history: Received 26 June 2016 Received in revised form 20 July 2016 Accepted 27 July 2016 Available online 28 July 2016
Herein, a hydrothermal synthesis of nanocrystalline cobalt tungstate (m-CoWO4) with mesoporous texture in the presence of amphiphilic organosilane surfactant is described. As-prepared nanocrystals are spherical in shape with size between 8 and 10 nm consisting of monoclinic (wolframite structure) phase. In addition, these nanocrystals are endowed with high surface area (50 m2 g 1), textured mesoporous surface and absorption threshold into visible region. Furthermore, these textured and fine nanocrystals exhibit improved photoelectrochemical activity (PEC), as compared to non-mesoporous CoWO4, under visible light (λ 4420 nm) for water oxidation in a photoelectrochemical cell. & 2016 Published by Elsevier B.V.
Keywords: Mesoporous CoWO4 Photoelectrochemical oxidation Hydrothermal
1. Introduction Utilization of photoelectrochemical (PEC) process to produce energy-rich product, such as H2, through water oxidation is of great significance as it involves in-expensive semiconductor photocatalyst and is driven by renewable solar energy [1]. The quest for energy via PEC in the visible regime of the solar spectrum is advancing at a fervent pace for which new photocatalysts are being developed and explored [2]. Photocatalysts which are active under visible light are of particular significance because visible light constitutes 45% of the solar spectrum. Recently, among binary metal oxides, transition metal tungstates of MIIWO4 type (M ¼ Mn, Co, Ni, or Cu) have been studied extensively. They have been used as environmental benign material for various applications. In particular, CoWO4 has been investigated for different applications owing to its versatile properties as compared to other catalysts of this group [3–6]. Since the photoelectrochemical reactions are predominantly a surface-dictated phenomenon, engineering of photocatalyst's surface is crucial. Among surface engineering strategies, creating roughness or porosity on surface seems promising in particular because it is likely to provide sufficient surface area for the adsorption of reactants and faster migration or diffusion of parent as n
Corresponding author. E-mail address: [email protected] (M. Qamar).
http://dx.doi.org/10.1016/j.matlet.2016.07.137 0167-577X/& 2016 Published by Elsevier B.V.
well as intermediate products thereby enhancing the overall efficiency of the process [7]. Different synthesis approaches have been tried for CoWO4 [8,9]. Here, we report a novel strategy for the synthesis of mesoporous CoWO4 nanoparticles via amphiphilic-organosilane assisted hydrothermal templating technique. Furthermore, visiblelight-mediated photoelectrochemical behavior of m-CoWO4 was compared with non-porous CoWO4 for water oxidation.
2. Materials and methods 2.1. Reagents and chemicals Analytical grade Cobalt acetate, Sodium tungstate, Dimethyl octadecylamine and Chloropropyl trimethoxysilane and absolute methanol were used. All the solutions were prepared with deionized water ( 418.2 MΩ-cm). 2.2. Surfactant synthesis The surfactant [3-(trimethoxysilyl) propyl] hexadecyl dimethyl ammonium chloride (TPHAC) was synthesized by mixing dimethyl octadecylamine with chloropropyl trimethoxysilane in methanol in the presence of potassium iodide. The mixture was refluxed for 7 days to obtain TPHAC solution (60% by weight in methanol), which was utilized as a surfactant.
282
M.I. Ahmed et al. / Materials Letters 183 (2016) 281–284
2.3. Synthesis of nanocrystalline m-CoWO4 Solution A was prepared by dissolving Co(CH3COO)2 (2 mM) along with 0.5 mM TPHAC surfactant in 60 mL H2O at room temperature. Solution B was prepared by dissolving Na2WO6 (2 mM) in 4 mL H2O. Solution B was added dropwise to solution A and the mixture was stirred at room temperature for 3 h and then transferred to 125 mL acid digestion vessel for hydrothermal treatment at 150 °C for 24 h. After reaction, product was washed several times with deionized water, ethanol and dried in vacuum oven at 100 °C. The dried sample was calcined at 450 °C for 2 h at a ramp rate of 2 °C/min. Non-mesoporous nanocrystalline CoWO4 was also prepared in a similar way without using TPHAC template. 2.4. Structural and morphological characterization X-Ray Diffraction (XRD) was used to examine the phase structure of the samples. The morphologies of the samples were examined by High-Resolution Transmission Electron MicroscopySelected Area Electron Diffraction (HRTEM-SAED). Diffuse Reflectance Spectroscopy (DRS) was employed to study the optical properties and Brunauer–Emmett–Teller (BET) was used for surface area measurement. 2.5. Photoelectrochemical water oxidation The photoelectrocatalytic (PEC) activities of the synthesized CoWO4 and m-CoWO4 nanoparticles were evaluated in a 3-electrode cell assembly connected to a potentiostat, using sodium sulfate solution (0.5 M) as the electrolyte. Saturated calomel electrode and coiled platinum wires were used as the reference and counter electrodes, respectively. The working electrode was prepared by the deposition of a homogeneous suspension of catalyst in Nafions and ethanol on an ITO substrate. A 300 W Xenon lamp with a cut-off filter was used to obtain radiation of λ 4 420 nm.
3. Results and discussion
Intensity (a.u.)
Fig. 1 shows the representative XRD patterns of samples prepared with and without surfactant. The characteristic peaks appeared at 2θ values 19.2°, 23.9°, 24.8°, 30.8°, 36.4°, 38.8°, 41.6°, 54.3° and 65.4° may be attributed to 〈001〉, 〈110〉, 〈011〉, 〈111〉, 〈200〉, 〈002〉, 〈121〉, 〈 202〉 and 〈 113〉 crystal planes of monoclinic crystal with wolframite structure for CoWO4 and m-CoWO4 [JCPDS-15-0867]. Furthermore, m-CoWO4 was somewhat less crystalline as compared to that of CoWO4 due to the condition of
10
m-CoWO4 (200) (-202)
(111) (110) (001) (011)
(-113) (121) (002)
CoWO4
20
30
40
50
60
2θ (deg.) Fig. 1. Comparative XRD patterns of CoWO4 and m-CoWO4.
70
high saturation with respect to the surfactant solution, under which rapid nucleation of CoWO4 particles is likely to take place but, due to the longer induction time required for the crystal formation, less crystalline particles were formed. Mesoporosity in the prepared m-CoWO4 sample was confirmed by N2 sorption analysis. The BET isotherm of m-CoWO4 sample (Fig. 2B) was in conformity to the classical type IV behavior. Absence of hysteresis loop in CoWO4 sample (Fig. 2A) prepared without TPHAC confirmed that surfactant played an active role in introducing mesoporosity. Pore size distribution of both the samples is presented as inset figures in Fig. 2A and B. As evident, a wide pore size distribution was observed in case of CoWO4. However, in m-CoWO4 a narrow pore size distribution was observed. In addition, approximately 2.2 fold increase in surface area of m-CoWO4 (50 m2 g 1) was observed as compared to CoWO4 (22 m2 g 1). This improvement in the surface area could presumably be attributed to the presence of mesoporous surface as well as smaller particle size. The band gaps of the CoWO4 and m-CoWO4 (Fig. 2C and D) were determined by measuring the optical properties by DRS and were found to be approximately 2.4 and 2.3 eV, respectively [10]. Both the samples are capable of absorbing visible light radiation. Further structural and morphological details of CoWO4 and mCoWO4 were investigated by TEM, HR-TEM and SAED (Fig. 3). TEM micrograph (Fig. 3A) shows the CoWO4 nanocrystals that are in the range of 30–40 nm in size (near spherical shape). SAED pattern and the HR-TEM image (Fig. 3B and C) of CoWO4 show high degree of crystallinity and the interplanar distance was calculated to be 0.287 nm, which is in close agreement with 0.29 nm estimated by the XRD data for 〈111〉 plane of monoclinic crystal. Fig. 3E shows the formation of fine (between 5 and 10 nm) spherical particles of m-CoWO4, which is also indicated by XRD peak broadening. A homogeneous distribution of particle size observed that could be attributed to the presence of surfactant which controls the nucleation and growth of particles. SAED and the HR-TEM images of m-CoWO4 (Fig. 3F and G) show polycrystalline nature and the interplanar distance was calculated to be 0.243 nm, which could be due to 〈200〉 plane of monoclinic crystal as indicated by XRD d-spacing. In addition to BET, mesoporous textured surface in m-CoWO4 was substantiated by SAED and TEM studies, as shown in Fig. 3F and G. SAED pattern presented somewhat diffused rings, which are characteristics of porous nano-architecture. A thorough scanning of m-CoWO4 through TEM study suggested the presence of worm-like discontinued surface as shown in Fig. 3H. However, the surface of CoWO4 was smooth and such surface texture was absent (Fig. 3D). The photoelectrochemical water oxidation was carried out under visible light irradiation (λ 4420 nm) in a three-electrode photoelectrochemical cell. Variation in photocurrent with the applied voltage was obtained, and the photoelectrochemical response of m-CoWO4 was better as compared to CoWO4 (Fig. 4A). The improved photocurrent for m-CoWO4 could be attributed to its mesoporous texture, which could ameliorate the kinetics of electron transfer process necessary for catalysis [7]. Since the specimens may show a certain degree of current drift over a time scales of 5–10 min, photocurrent was measured for CoWO4 and mCoWO4 by turning the light on and off every 50 s for more than 10 min, keeping the applied voltage constant at of 0.4 V (Fig. 4B). Upon illumination, the photocurrent increased instantaneously and reached a steady state quickly; no current was generated in dark, even up to 0.4 V applied voltage for both samples. However, for m-CoWO4 there is small increase in current at the start of every cycle, this may be due to the enhancement of number of charge carriers or good charge separation because of mesoporous textured surface.
M.I. Ahmed et al. / Materials Letters 183 (2016) 281–284
283
Fig. 2. BET isotherms of CoWO4 [A] and m-CoWO4 [B] with their pore size distributions [inset A and B], DRS spectra of CoWO4 (C) and m-CoWO4 (D).
100 nm
5 1/nm
0.243 nm
0.287 nm
2 nm
5 1/nm
100 nm
5 nm
2 nm
2 nm
Fig. 3. TEM, SAED, HR-TEM, of CoWO4 (A–D) and m-CoWO4 (E–H).
4. Conclusions In summary, an amphiphilic-organosilane assisted hydrothermal technique is demonstrated for the synthesis of nanostructured CoWO4 endowed with mesoporous surface. TEM, HRTEM and N2 sorption analysis confirmed the presence of
mesopores. The particle size was decreased and the surface area of mesoporous sample was improved by 2.2 times. Furthermore, visible-light-driven photoelectrochemical water oxidation was better with mesoporous CoWO4 as compared to nonporous CoWO4, and such activity improvement was attributed to its modified surface and optical attributes.
M.I. Ahmed et al. / Materials Letters 183 (2016) 281–284
A
2.0 1.5
m-CoWO4
1.0 CoWO4
0.5 0.0 -0.1
0.0
0.1
0.2
0.3
0.4
Time (seconds)
m-CoWO4
B
2
2.5
Current density (µA/cm )
2
Current density (µA/cm )
284
4
CoWO4
light off
light on
3
2
1 0
100
200
300
400
500
600
700
Time (seconds)
Fig. 4. Comparative potentiodynamic (I V) response of CoWO4 and m-CoWO4 (A), consistent generation of photocurrent under intermittent visible light illumination of CoWO4 and m-CoWO4 (B).
Acknowledgments This project was funded by National Plan for Science, Technology and Innovation (MAARIFAH), King Abdulaziz City for Science and Technology (KACST) through the Science & Technology Unit at King Fahd University of Petroleum & Minerals (KFUPM), Kingdom of Saudi Arabia, award number (10-NAN1387-04).
References [1] E.L. Miller, Photoelectrochemical water splitting, Energy Environ. Sci. 8 (2015) 2809–2810. [2] P. Kalisman, Y. Nakibli, L. Amirav, Perfect photon-to-hydrogen conversion efficiency, Nano Lett. 16 (2016) 1776–1781. [3] S.M. Montemayor, A.F. Fuentes, Electrochemical characteristics of lithium insertion in several 3D metal tungstates (MWO4, M¼ Mn, Co, Ni and Cu) prepared by aqueous reactions, Ceram. Int. 30 (2004) 393–400.
[4] U.M. Garcia-Perez, A. Martinez-De La Cruz, J. Peral, Transition metal tungstates synthesized by co-precipitation method: basic photocatalytic properties, Electrochim. Acta 81 (2012) 227–232. [5] X. Xu, J. Gao, G. Huang, H. Qiu, Z. Wang, J. Wu, Z. Pan, F. Xing, Fabrication of CoWO4@NiWO4 nanocomposites with good supercapacitve performances, Electrochim. Acta 174 (2015) 837–845. [6] C. Ling, L.Q. Zhou, H. Jia, First-principles study of crystalline CoWO4 as oxygen evolution reaction catalyst, RSC Adv. 4 (2014) 24692–24697. [7] T. Grewe, X. Deng, C. Weidenthaler, F. Schuth, H. Tuysuz, Design of ordered mesoporous composite materials and their electrocatalytic activities for water oxidation, Chem. Mater. 25 (2013) 4926–4935. [8] J. Deng, L. Chang, P. Wang, E. Zhang, J. Ma, T. Wang, Preparation and magnetic properties of CoWO4 nanocrystals, Cryst. Res. Technol. 1007 (2012) 1004–1007. [9] S. Rajagopal, D. Nataraj, O.Y. Khyzhun, Y. Djaoued, J. Robichaud, D. Mangalaraj, Hydrothermal synthesis and electronic properties of FeWO4 and CoWO4 nanostructures, J. Alloy. Compd. 493 (2010) 340–345. [10] R. Lacomba-Perales, J. Ruiz-Fuertes, D. Errandonea, D. Martínez-García, A. Segura, Optical absorption of divalent metal tungstates: correlation between the band-gap energy and the cation ionic radius, Europhys. Lett. Assoc. 83 (2008), 37002 pp. 1–5..
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Improved photoelectrochemical water oxidation under visible light with mesoporous CoWO4 M.I. Ahmed a, A. Adam b, A. Khan a, A.u. Rehman c, M. Qamaruddin a, M.N. Siddiqui b, M. Qamar a,n a
Center of Excellence in Nanotechnology (CENT), King Fahd University of Petroleum and Minerals, Dhahran 31261, Kingdom of Saudi Arabia Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran 31261, Kingdom of Saudi Arabia c Center for Engineering Research (CER), King Fahd University of Petroleum and Minerals, Dhahran 31261, Kingdom of Saudi Arabia b
art ic l e i nf o
a b s t r a c t
Article history: Received 26 June 2016 Received in revised form 20 July 2016 Accepted 27 July 2016 Available online 28 July 2016
Herein, a hydrothermal synthesis of nanocrystalline cobalt tungstate (m-CoWO4) with mesoporous texture in the presence of amphiphilic organosilane surfactant is described. As-prepared nanocrystals are spherical in shape with size between 8 and 10 nm consisting of monoclinic (wolframite structure) phase. In addition, these nanocrystals are endowed with high surface area (50 m2 g 1), textured mesoporous surface and absorption threshold into visible region. Furthermore, these textured and fine nanocrystals exhibit improved photoelectrochemical activity (PEC), as compared to non-mesoporous CoWO4, under visible light (λ 4420 nm) for water oxidation in a photoelectrochemical cell. & 2016 Published by Elsevier B.V.
Keywords: Mesoporous CoWO4 Photoelectrochemical oxidation Hydrothermal
1. Introduction Utilization of photoelectrochemical (PEC) process to produce energy-rich product, such as H2, through water oxidation is of great significance as it involves in-expensive semiconductor photocatalyst and is driven by renewable solar energy [1]. The quest for energy via PEC in the visible regime of the solar spectrum is advancing at a fervent pace for which new photocatalysts are being developed and explored [2]. Photocatalysts which are active under visible light are of particular significance because visible light constitutes 45% of the solar spectrum. Recently, among binary metal oxides, transition metal tungstates of MIIWO4 type (M ¼ Mn, Co, Ni, or Cu) have been studied extensively. They have been used as environmental benign material for various applications. In particular, CoWO4 has been investigated for different applications owing to its versatile properties as compared to other catalysts of this group [3–6]. Since the photoelectrochemical reactions are predominantly a surface-dictated phenomenon, engineering of photocatalyst's surface is crucial. Among surface engineering strategies, creating roughness or porosity on surface seems promising in particular because it is likely to provide sufficient surface area for the adsorption of reactants and faster migration or diffusion of parent as n
Corresponding author. E-mail address: [email protected] (M. Qamar).
http://dx.doi.org/10.1016/j.matlet.2016.07.137 0167-577X/& 2016 Published by Elsevier B.V.
well as intermediate products thereby enhancing the overall efficiency of the process [7]. Different synthesis approaches have been tried for CoWO4 [8,9]. Here, we report a novel strategy for the synthesis of mesoporous CoWO4 nanoparticles via amphiphilic-organosilane assisted hydrothermal templating technique. Furthermore, visiblelight-mediated photoelectrochemical behavior of m-CoWO4 was compared with non-porous CoWO4 for water oxidation.
2. Materials and methods 2.1. Reagents and chemicals Analytical grade Cobalt acetate, Sodium tungstate, Dimethyl octadecylamine and Chloropropyl trimethoxysilane and absolute methanol were used. All the solutions were prepared with deionized water ( 418.2 MΩ-cm). 2.2. Surfactant synthesis The surfactant [3-(trimethoxysilyl) propyl] hexadecyl dimethyl ammonium chloride (TPHAC) was synthesized by mixing dimethyl octadecylamine with chloropropyl trimethoxysilane in methanol in the presence of potassium iodide. The mixture was refluxed for 7 days to obtain TPHAC solution (60% by weight in methanol), which was utilized as a surfactant.
282
M.I. Ahmed et al. / Materials Letters 183 (2016) 281–284
2.3. Synthesis of nanocrystalline m-CoWO4 Solution A was prepared by dissolving Co(CH3COO)2 (2 mM) along with 0.5 mM TPHAC surfactant in 60 mL H2O at room temperature. Solution B was prepared by dissolving Na2WO6 (2 mM) in 4 mL H2O. Solution B was added dropwise to solution A and the mixture was stirred at room temperature for 3 h and then transferred to 125 mL acid digestion vessel for hydrothermal treatment at 150 °C for 24 h. After reaction, product was washed several times with deionized water, ethanol and dried in vacuum oven at 100 °C. The dried sample was calcined at 450 °C for 2 h at a ramp rate of 2 °C/min. Non-mesoporous nanocrystalline CoWO4 was also prepared in a similar way without using TPHAC template. 2.4. Structural and morphological characterization X-Ray Diffraction (XRD) was used to examine the phase structure of the samples. The morphologies of the samples were examined by High-Resolution Transmission Electron MicroscopySelected Area Electron Diffraction (HRTEM-SAED). Diffuse Reflectance Spectroscopy (DRS) was employed to study the optical properties and Brunauer–Emmett–Teller (BET) was used for surface area measurement. 2.5. Photoelectrochemical water oxidation The photoelectrocatalytic (PEC) activities of the synthesized CoWO4 and m-CoWO4 nanoparticles were evaluated in a 3-electrode cell assembly connected to a potentiostat, using sodium sulfate solution (0.5 M) as the electrolyte. Saturated calomel electrode and coiled platinum wires were used as the reference and counter electrodes, respectively. The working electrode was prepared by the deposition of a homogeneous suspension of catalyst in Nafions and ethanol on an ITO substrate. A 300 W Xenon lamp with a cut-off filter was used to obtain radiation of λ 4 420 nm.
3. Results and discussion
Intensity (a.u.)
Fig. 1 shows the representative XRD patterns of samples prepared with and without surfactant. The characteristic peaks appeared at 2θ values 19.2°, 23.9°, 24.8°, 30.8°, 36.4°, 38.8°, 41.6°, 54.3° and 65.4° may be attributed to 〈001〉, 〈110〉, 〈011〉, 〈111〉, 〈200〉, 〈002〉, 〈121〉, 〈 202〉 and 〈 113〉 crystal planes of monoclinic crystal with wolframite structure for CoWO4 and m-CoWO4 [JCPDS-15-0867]. Furthermore, m-CoWO4 was somewhat less crystalline as compared to that of CoWO4 due to the condition of
10
m-CoWO4 (200) (-202)
(111) (110) (001) (011)
(-113) (121) (002)
CoWO4
20
30
40
50
60
2θ (deg.) Fig. 1. Comparative XRD patterns of CoWO4 and m-CoWO4.
70
high saturation with respect to the surfactant solution, under which rapid nucleation of CoWO4 particles is likely to take place but, due to the longer induction time required for the crystal formation, less crystalline particles were formed. Mesoporosity in the prepared m-CoWO4 sample was confirmed by N2 sorption analysis. The BET isotherm of m-CoWO4 sample (Fig. 2B) was in conformity to the classical type IV behavior. Absence of hysteresis loop in CoWO4 sample (Fig. 2A) prepared without TPHAC confirmed that surfactant played an active role in introducing mesoporosity. Pore size distribution of both the samples is presented as inset figures in Fig. 2A and B. As evident, a wide pore size distribution was observed in case of CoWO4. However, in m-CoWO4 a narrow pore size distribution was observed. In addition, approximately 2.2 fold increase in surface area of m-CoWO4 (50 m2 g 1) was observed as compared to CoWO4 (22 m2 g 1). This improvement in the surface area could presumably be attributed to the presence of mesoporous surface as well as smaller particle size. The band gaps of the CoWO4 and m-CoWO4 (Fig. 2C and D) were determined by measuring the optical properties by DRS and were found to be approximately 2.4 and 2.3 eV, respectively [10]. Both the samples are capable of absorbing visible light radiation. Further structural and morphological details of CoWO4 and mCoWO4 were investigated by TEM, HR-TEM and SAED (Fig. 3). TEM micrograph (Fig. 3A) shows the CoWO4 nanocrystals that are in the range of 30–40 nm in size (near spherical shape). SAED pattern and the HR-TEM image (Fig. 3B and C) of CoWO4 show high degree of crystallinity and the interplanar distance was calculated to be 0.287 nm, which is in close agreement with 0.29 nm estimated by the XRD data for 〈111〉 plane of monoclinic crystal. Fig. 3E shows the formation of fine (between 5 and 10 nm) spherical particles of m-CoWO4, which is also indicated by XRD peak broadening. A homogeneous distribution of particle size observed that could be attributed to the presence of surfactant which controls the nucleation and growth of particles. SAED and the HR-TEM images of m-CoWO4 (Fig. 3F and G) show polycrystalline nature and the interplanar distance was calculated to be 0.243 nm, which could be due to 〈200〉 plane of monoclinic crystal as indicated by XRD d-spacing. In addition to BET, mesoporous textured surface in m-CoWO4 was substantiated by SAED and TEM studies, as shown in Fig. 3F and G. SAED pattern presented somewhat diffused rings, which are characteristics of porous nano-architecture. A thorough scanning of m-CoWO4 through TEM study suggested the presence of worm-like discontinued surface as shown in Fig. 3H. However, the surface of CoWO4 was smooth and such surface texture was absent (Fig. 3D). The photoelectrochemical water oxidation was carried out under visible light irradiation (λ 4420 nm) in a three-electrode photoelectrochemical cell. Variation in photocurrent with the applied voltage was obtained, and the photoelectrochemical response of m-CoWO4 was better as compared to CoWO4 (Fig. 4A). The improved photocurrent for m-CoWO4 could be attributed to its mesoporous texture, which could ameliorate the kinetics of electron transfer process necessary for catalysis [7]. Since the specimens may show a certain degree of current drift over a time scales of 5–10 min, photocurrent was measured for CoWO4 and mCoWO4 by turning the light on and off every 50 s for more than 10 min, keeping the applied voltage constant at of 0.4 V (Fig. 4B). Upon illumination, the photocurrent increased instantaneously and reached a steady state quickly; no current was generated in dark, even up to 0.4 V applied voltage for both samples. However, for m-CoWO4 there is small increase in current at the start of every cycle, this may be due to the enhancement of number of charge carriers or good charge separation because of mesoporous textured surface.
M.I. Ahmed et al. / Materials Letters 183 (2016) 281–284
283
Fig. 2. BET isotherms of CoWO4 [A] and m-CoWO4 [B] with their pore size distributions [inset A and B], DRS spectra of CoWO4 (C) and m-CoWO4 (D).
100 nm
5 1/nm
0.243 nm
0.287 nm
2 nm
5 1/nm
100 nm
5 nm
2 nm
2 nm
Fig. 3. TEM, SAED, HR-TEM, of CoWO4 (A–D) and m-CoWO4 (E–H).
4. Conclusions In summary, an amphiphilic-organosilane assisted hydrothermal technique is demonstrated for the synthesis of nanostructured CoWO4 endowed with mesoporous surface. TEM, HRTEM and N2 sorption analysis confirmed the presence of
mesopores. The particle size was decreased and the surface area of mesoporous sample was improved by 2.2 times. Furthermore, visible-light-driven photoelectrochemical water oxidation was better with mesoporous CoWO4 as compared to nonporous CoWO4, and such activity improvement was attributed to its modified surface and optical attributes.
M.I. Ahmed et al. / Materials Letters 183 (2016) 281–284
A
2.0 1.5
m-CoWO4
1.0 CoWO4
0.5 0.0 -0.1
0.0
0.1
0.2
0.3
0.4
Time (seconds)
m-CoWO4
B
2
2.5
Current density (µA/cm )
2
Current density (µA/cm )
284
4
CoWO4
light off
light on
3
2
1 0
100
200
300
400
500
600
700
Time (seconds)
Fig. 4. Comparative potentiodynamic (I V) response of CoWO4 and m-CoWO4 (A), consistent generation of photocurrent under intermittent visible light illumination of CoWO4 and m-CoWO4 (B).
Acknowledgments This project was funded by National Plan for Science, Technology and Innovation (MAARIFAH), King Abdulaziz City for Science and Technology (KACST) through the Science & Technology Unit at King Fahd University of Petroleum & Minerals (KFUPM), Kingdom of Saudi Arabia, award number (10-NAN1387-04).
References [1] E.L. Miller, Photoelectrochemical water splitting, Energy Environ. Sci. 8 (2015) 2809–2810. [2] P. Kalisman, Y. Nakibli, L. Amirav, Perfect photon-to-hydrogen conversion efficiency, Nano Lett. 16 (2016) 1776–1781. [3] S.M. Montemayor, A.F. Fuentes, Electrochemical characteristics of lithium insertion in several 3D metal tungstates (MWO4, M¼ Mn, Co, Ni and Cu) prepared by aqueous reactions, Ceram. Int. 30 (2004) 393–400.
[4] U.M. Garcia-Perez, A. Martinez-De La Cruz, J. Peral, Transition metal tungstates synthesized by co-precipitation method: basic photocatalytic properties, Electrochim. Acta 81 (2012) 227–232. [5] X. Xu, J. Gao, G. Huang, H. Qiu, Z. Wang, J. Wu, Z. Pan, F. Xing, Fabrication of CoWO4@NiWO4 nanocomposites with good supercapacitve performances, Electrochim. Acta 174 (2015) 837–845. [6] C. Ling, L.Q. Zhou, H. Jia, First-principles study of crystalline CoWO4 as oxygen evolution reaction catalyst, RSC Adv. 4 (2014) 24692–24697. [7] T. Grewe, X. Deng, C. Weidenthaler, F. Schuth, H. Tuysuz, Design of ordered mesoporous composite materials and their electrocatalytic activities for water oxidation, Chem. Mater. 25 (2013) 4926–4935. [8] J. Deng, L. Chang, P. Wang, E. Zhang, J. Ma, T. Wang, Preparation and magnetic properties of CoWO4 nanocrystals, Cryst. Res. Technol. 1007 (2012) 1004–1007. [9] S. Rajagopal, D. Nataraj, O.Y. Khyzhun, Y. Djaoued, J. Robichaud, D. Mangalaraj, Hydrothermal synthesis and electronic properties of FeWO4 and CoWO4 nanostructures, J. Alloy. Compd. 493 (2010) 340–345. [10] R. Lacomba-Perales, J. Ruiz-Fuertes, D. Errandonea, D. Martínez-García, A. Segura, Optical absorption of divalent metal tungstates: correlation between the band-gap energy and the cation ionic radius, Europhys. Lett. Assoc. 83 (2008), 37002 pp. 1–5..