oxygen evolution

Materials Research Bulletin 64 (2015) 283–287

Contents lists available at ScienceDirect

Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu

A facile one step synthesis of Cu/Cu2O nanocomposites: Enhanced hydrogen/oxygen evolution Bharat Kumar a , Soumen Saha a , Kasinath Ojha a , Ashok K. Ganguli a,b, * a b

Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India Institute of Nano Science and Technology, Mohali, Punjab 160062, India

A R T I C L E I N F O

A B S T R A C T

Article history: Received 7 May 2014 Received in revised form 23 September 2014 Accepted 17 October 2014 Available online 3 January 2015

The exploration of metal/metal oxide nanocomposites as catalysts for hydrogen and oxygen evolution is highly desirable for renewable and clean energy applications. Cu/Cu2O nanocomposites have obtained one step process by thermal decomposition of aligned copper oxalate nanorods and CuO in 2:1 molar ratio in argon atmosphere at 350
C. Hydrogen and oxygen evolution reaction (HER and OER) were carried out using these catalyst on glassy carbon as working electrodes in KOH electrolyte solution. Cu/Cu2O nanocomposites produce 8 times higher current density than Cu and 4.5 times higher than Cu2O during HER whereas in case of OER study, it is 36 times higher than Cu and 2.9 times higher than Cu2O. The electrocatalyst is highly stable over 50 cycles. Our studies show an improvement in electrocatalytic activity by properly choosing the composite, and thus, it may be expanded to other electrocatalysts for obtaining increased activity. ã 2015 Elsevier Ltd. All rights reserved.

Keywords: Nanocomposites Hydrogen evolution reaction Oxygen evolution reaction Current density

1. Introduction Due to the dwindling natural resources like coal and oil, production of renewable and clean energy is a major challenge in this decade for serving the community. Hydrogen is an important form of renewable and clean energy (byproduct is H2O) due to their high energy content and has a great potential to provide sustainable energy through fuel cells and battery devices. Hydrogen can be obtained by splitting of water by photo or electrocatalysis (hydrogen evolution reaction, HER) or by hydrolysis of ammonia borane or by solar hydrogen generation whereas O2 can be obtained by oxygen evolution reaction (OER) using electro and photocatalysis. The availability of cheap and highly efficient electrocatalysts is thus very important area of research. Nanostructured catalysts are being pursued due to the higher surface area to volume ratio which enhanced the catalytic ability, and they have different geometric and electronic properties from bulk materials. Cuprous oxide (Cu2O) is a p-type semiconductor having a direct band gap of 1.9–2.2 eV with widespread applications in several fields whereas Cu metal nanoparticles are of special interest

* Corresponding author at: Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India. Tel.: +91 11 26591511; fax: +91 11 26854715. E-mail address: [email protected] (A.K. Ganguli). http://dx.doi.org/10.1016/j.materresbull.2014.10.076 0025-5408/ ã 2015 Elsevier Ltd. All rights reserved.

compared to silver, gold and platinum nanoparticles due to their lower cost and wide availability. Fabrication of metal/semiconductor [1] (Cu/Cu2O) nanocomposites are of interest for energy storage [2,3], optical [4], photocatalysis [5–7], electrocatalysis [8,9] and organic catalysis [10]. Different types of heterostructures of Cu/Cu2O such as nanocomposites, core–shell, multilayer thin film, and different morphologies such as flowers, spheres, cubes, hexapods, wires, and hollow structures have been synthesized by solvothermal [6], wet chemical [3,8–10], hydrothermal [11–13], electrochemical deposition [4,14–16], ball milling [5,7,17] and microemulsion method [18]. To the best of our knowledge, HER and OER studies have not been investigated using Cu/Cu2O nanocomposites as electrocatalysts whereas there are several reports on these studies (HER and OER) by nanosized Co [19] & Cu [20,21] metal, Cu2O [22] metal oxide and alloys like Co–Ni [19], Fe– Co [23] and Cu–Co [24]. In our previous study, we had successful synthesized Cu [20] and Cu2O [22] nanoparticles and used them as electrocatalysts for H2 and O2 evolution. The aim of this work is to see the effect of metal/semiconductor nanocomposites or heterostructures (Cu and Cu2O) on electrocatalysis. We have synthesized Cu/Cu2O nanocomposites by solid state route and characterized by powder X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, surface area analyzer and diffuse reflectance spectroscopy. The electrocatalytic properties for hydrogen and oxygen evolution reaction (HER and OER) were investigated and compared with those of Cu [20] and Cu2O [22]

[(Schem_1)TD$FIG]

284

B. Kumar et al. / Materials Research Bulletin 64 (2015) 283–287

CuC2O4.H2O

CuC2O4.H2O + CuO

2CuC2O4.H2O + CuO

Cu

350 C/6h, Ar

Cu2O

350 C/6h, Ar

Cu/Cu2O (48% & 52 %)

350 C/6h, Ar

of 0.5 M KOH. For each experiment fresh KOH solution was used. Cyclic voltammetry was carried out at a scan rate of 0.025 V/s in the potential range of 1.5 to 0 V in HER studies whereas 0 to 1 V potential was applied to the electrodes for the OER study in case of glassy carbon and 0 to 0.9 V in case of Pt as working electrode. The electroactive surface area of the catalyst on the working electrodes was measured by CV in 0.5 M KOH. 3. Results and discussion

nanoparticles. A systematic scheme for the reduction of Cu2+ is shown in Scheme 1.

Fig. 1 shows the powder X-ray diffraction pattern of Cu/Cu2O nanocomposites synthesized at 350
C in inert (Ar) atmosphere by mixing copper oxalate nanorods and CuO in 2:1 molar ratio. From our previous study, it was observed that heating copper oxalate nanorods and CuO in 1:1 molar ratio gives pure Cu2O [22] while copper oxalate nanorods will only give Cu [20] metal nanoparticles at 350
C in inert (Ar) atmosphere. Heating the CuO at 350
C in inert (Ar) atmosphere, only reduces it to 7% of Cu2O whereas at 850
C, it is completely reduced to Cu2O. Heating of CuO and oxalic acid in 1:1 ratio at 350
C in inert (Ar) atmosphere will give a mixture of Cu, Cu2O and CuO in the ratio of 39%, 43% and 18%, respectively. Heating of copper oxalate nanorods or CuO in air always results in CuO. All the reactions methodology is given in Scheme 1. The reflection pattern (Cu/Cu2O nanocomposites, Fig. 1) shows the presence of both Cu (JCPDS: 892838, Fm3m) as well as Cu2O (JCPDS: 782076, Pn3m) without any other impurity phases. The mass fraction of Cu and Cu2O in Cu/Cu2Onanocomposites is calculated by using the given formulae:

2. Synthesis and characterization

Cuð%Þ ¼

Cu/Cu2O nanocomposites were synthesized by mixing copper oxalate nanorods [20] and CuO (Aldrich, 99.99%) in 2:1 molar ratios followed by heating at 350
C for 3 h in Ar atmosphere. Powder Xray diffraction studies (PXRD) were carried out using Ni filtered CuKa radiation where scans were recorded with a step size of 0.02
and step time of 1 s. Transmission electron microscopic (TEM) studies were carried out using a Tecnai G2 20 electron microscope operated at 200 kV. TEM specimens were prepared by dispersing the Cu/Cu2O nanocomposites in ethanol by ultrasonic treatment, dropping onto a porous carbon film supported on a copper grid, and then drying in air. Field emission scanning electron microscopy (FESEM) of the compounds was carried out on FEI quanta 3D FEG–FESEM by coating the powder samples with gold. Nitrogen adsorption–desorption isotherms were recorded at liquid nitrogen temperature (77 K) using a Nova 2000e (Quantachrome Corp.) equipment, and the specific area was determined by the Brunauer–Emmett–Teller (BET) method. The samples were degassed at 150
C for 4 h prior to the surface area measurements. Diffuse–reflectance spectra (DR) spectra were recorded on Shimadzu UV-2450 spectrophotometer where the baseline was fixed using a barium sulfate reference. Cyclic voltammetry (CV) was carried out with a computer controlled electrochemical workstation (Autolab PGSTAT 302 N). Hydrogen evolution and oxygen evolution reactions were studied by using Ag/AgCl as reference electrode while Pt was used as counter electrode whereas glassy carbon (0.02 cm2) was used as working electrodes. Working electrode was polished using (0.05 mm) alumina paste, ultrasonicated in distilled water and then in ethanol. 2 mg of Cu/Cu2O nanocomposites was sonicated in 20 mL of isopropanol, and then 10 mL of Nafion was added. One drop (nearly 10 mL) of this paste was placed on the working electrode and dried for half an hour. Nafion acts as a proton conducting binder for nanoparticles which forms a membrane over the surface of the electrode (membrane electrode assembly, MEA). All the three electrodes were placed in a freshly prepared solution

where ICu(111) and ICu2O(111) maximum counts of characteristics reflection of Cu(111) and Cu2O(111). Based on above calculation, the amount of Cu and Cu2O was found to be 48% and 52%, respectively. Fig. 2 shows the field emission scanning electron micrograph (FESEM) of Cu/Cu2O nanocomposites. Cu2Oparticles exhibited cube shaped morphology whereas Cu particles are spherical in nature (Fig. 2a). These cube and spherical shape particles are agglomerated and fused with each other. On higher magnification (Fig. 2b) it was observed that these Cu/Cu2O nanocomposites are very dense in nature where spherical nanoparticles of Cu are embedded within Cu2O. Fig. 3 shows the transmission electron micrograph (TEM) of Cu/ Cu2O nanocomposites obtained from the thermal decomposition of copper oxalate and copper oxide. These Cu2O and Cu particles were agglomerated to each other in nanocomposites and are

ICuð111Þ ICuð111Þ þ ICu2 Oð111Þ

[(Fig._1)TD$IG]

18000 15000

Cu Cu2O

12000

6000 3000

(311) (220) (222)

9000 (220)

Scheme 1.

(200)

CuO

(211)

350 C/6h, Air

Cu/Cu2O/CuO (39 %, 43% & 18%)

(111)

350 C/6h, Ar

(200)

CuC2O4.H2O

Cu2O

850 C/6h, Ar

(111)

CuO + H2C2O4

350 C/6h, Ar

(110)

CuO

Cu2O/CuO (7% & 93 %)

Intensity

CuO

0 10

20

30

40 50 2 Scale

60

70

80

Fig. 1. Powder X-ray diffraction pattern of Cu/Cu2O nanocomposites.

B. Kumar et al. / Materials Research Bulletin 64 (2015) 283–287

[(Fig._2)TD$IG]

[(Fig._3)TD$IG]

285

Fig. 2. FESEM micrograph of Cu/Cu2O nanocomposites (a) low (b) high magnification.

Fig. 3. TEM micrograph of Cu/Cu2O nanocomposites (a) low (b) high magnification.

M + H2O + e ! MHads + OH

MHads + H2O + e ! H2 + OH + M

[(Fig._4)TD$IG] 0.20

0.16

Cu-Cu2O

0.12 Abs.

spherical in nature. The average diameter of Cu2O nanoparticles was found to be 25–30 nm in which spherical copper nanoparticles of size 15–20 nm are embedded and form Cu/Cu2O nanocomposites (Fig. 3a). On higher magnification, it was observed that these Cu2O particles are in cubic morphology where as the Cu nanoparticles are spherical in nature (Fig. 3b). There are reports of synthesis of Cu/Cu2O nanocomposites, but most of the earlier studies of Cu/Cu2O did not report any investigation of the electrochemical properties (HER and OER) using the Cu/Cu2O nanocomposites as electrocatalyst. Surface area of the synthesized Cu/Cu2O nanocomposites was found to be 15.8 m2/g which is 3.5 times higher than the reported Cu@Cu2O [10] core–shell nanostructure. A characteristic band of 619 nm (Fig. 4) was observed for Cu/Cu2O nanocomposites by diffuse reflectance spectroscopy which is in the visible region of the solar spectrum. The band gap was found to be 2.01 eV which is in the visible region of the solar spectrum. Electrochemical properties of Cu/Cu2O nanocomposites were carried out by using the cyclic voltammetry. In negative potential range of 1.5 to 0 V, the hydrogen evolution reaction is carried out for these nanocomposites where glassy carbon was used (0.02 cm2) as working electrode in 0.5 M KOH solution at scan rate of 0.025 V/ s. The following reactions occur during the HER at surface of electrode:

0.08

0.04 2.01 eV

0.00 400

500

600 700 Wavelength (nm)

800

Fig. 4. Diffuse reflectance spectra of Cu/Cu2O nanocomposites.

286

[(Fig._5)TD$IG]

B. Kumar et al. / Materials Research Bulletin 64 (2015) 283–287

30

1

(b)

(a) Current density (mA/cm2)

0

Current (mA)

0

-1

-30

-60

-2

-3 -1.5

-90

-1.2

-0.9

-0.6

-0.3

0.0

-120 -1.5

-1.2

-0.9

-0.6

-0.3

0.0

Potential Applied (V)

Potential Applied (V)

Fig. 5. HER of Cu/Cu2O nanocomposites (a) current (b) current density.

where M is the electrocatalyst. Overall reaction, 2H2O + 2e ! H2 + 2OH Fig. 5 shows the cyclic voltammograms of HER of Cu/Cu2O nanocomposites. The two redox peak observed at –0.62 V and 0.36 V during the HER of Cu/Cu2O nanocomposites is due oxidation of Cu to Cu+ and to Cu++. The overpotential for the Cu/Cu2O nanocomposites was found to be 1.0 V which is lower than Cu (1.2 V) as well as Cu2O (1.2 V). The maximum current at 1.5 V was found to be 2.75 mA. It is 4.8 times higher than Cu [20] and 5.3 times higher than Cu2O [22]. The electroactive surface area was determined by extrapolating the curve obtained from CV [20]. It was found to be 0.0276 cm2/mg. Current density was calculated by using the electroactive surface area, and it was found to be 99.6 mA/cm2 which is 8.3 times higher than Cu [20] and 4.5 times higher than Cu2O [22] (Fig. S1). The higher current and current density of Cu/ Cu2O nanocomposites is due to the low overpotential [25,26]

(onset potential) than pure Cu and pure Cu2O during HER study. This may be due to the fast electron transfer rate at the interfaces in nanocomposites. The copper/cuprous oxide nanocomposites obtained by us show an excellent stability. Oxygen evolution reaction was carried out by using Cu/Cu2O nanocomposites as a catalyst on GC electrode in 0.5 M KOH solution at a scan rate of 0.025 V/s in potential range of 0–1.0 V at room temperature. Cyclic voltammograms of OER study of Cu/ Cu2O nanocomposites is shown in Fig. 6. There is an increase in oxidation current at 0.52 V due to the amount of oxygen generated during electrolysis. The reaction at the electrode surface during OER is as follows. 4OH ! 2H2O + O2 + 4e The overpotential during the OER for Cu/Cu2O nanocomposites was found to be 0.52 V which is higher than Cu (0.41 V) and Cu2O (0.19 V). The maximum current is found to be 1.6 mA (at 1.0 V) which is 18 times higher than reported for Cu [20] nanoparticles and 3 times

[(Fig._6)TD$IG] 80

1.8 1.5

60 Current density (mA/cm2)

(a) Current (mA)

1.2 0.9 0.6 0.3

(b)

40

20

0

0.0 -0.3 0.0

0.2

0.4

0.6

Potential Applied (V)

0.8

1.0

-20 0.0

0.2

0.4

0.6

Potential Applied (V)

Fig. 6. OER of Cu/Cu2O nanocomposites (a) current (b) current density.

0.8

1.0

B. Kumar et al. / Materials Research Bulletin 64 (2015) 283–287

higher for Cu2O [22]. The current density was found to be 58 mA/ cm2 which was 36 times higher than Cu [20] and 2.9 times higher than Cu2O [22]. The higher current and current density of Cu/Cu2O nanocomposites is due to the high overpotential [27] (onset potential) than pure Cu and pure Cu2O during OER study (Fig. S2). There is no other study on HER and OER using Cu/Cu2O nanocomposites as electrocatalyst. This electrocatalyst is reusable and very stable. To check the stability of the Cu/Cu2O nanocomposites for the electrocatalytic activity, we performed the 50 cycles experiment (continuously). It was found that this electrocatalyst showed excellent stability during both the electrocatalytic processes, i.e., hydrogen and oxygen evolution reaction (HER and OER). Since our studies show improvement in activity and stability of electrocatalysts of Cu/Cu2O nanocomposites, therefore, this methodology may be expanded to obtain other electrocatalysts with increased activity. 4. Conclusions We have synthesized Cu/Cu2O nanocomposites by single step solid state reaction methodology. The Cu/Cu2O nanocomposites are 8 times more efficient as electrocatalyst (higher current density) than Cu and 4.5 times more efficient than Cu2O for hydrogen evolution reaction (HER). In case of OER, the current density was 36 times higher than Cu and 2.9 times higher than Cu2O. The Cu/Cu2O electrocatalyst is very stable over 50 cycles. Acknowledgments Financial assistance from DST (nano mission), DeitY and IIT Delhi, Government of India is gratefully acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. materresbull.2014.10.076.

287

References [1] J.A. Switzer, C.J. Hung, E.W. Bohannan, M.G. Shumsky, T.D. Golden, D.C. Van Aken, Adv. Mater. 9 (1997) 334–338. [2] X.P. Gao, H.X. Yang, Energy Environ. Sci. 3 (2010) 174–189. [3] Y. Bian, M. Yeng, X. Zhao, L. Ma, C. Jin, Y. Ding, X. Shen, Adv. Mater. Res. 509 (2012) 28–35. [4] E.D. Mishina, K. Nagai, S. Nakabayashi, Nano Lett. 1 (2001) 401–404. [5] T. Kou, C. Jin, C. Zhang, J. Sun, Z. Zhang, RSC Adv. 2 (2012) 12636–12643. [6] B. Zhou, H. Wang, Z. Liu, Y. Yang, X. Huang, Z. Lü, Y. Sui, W. Su, Mater. Chem. Phys. 126 (2011) 847–852. [7] T. Kou, Y. Wang, C. Zhang, J. Sun, Z. Zhang, Chem. Eng. J. 223 (2013) 76–83. [8] M. Yang, X. Zhao, L. Ma, Y. Yao, Y. Ding, X. Shen, Electrochim. Acta 56 (2011) 5783–5787. [9] K.S. Park, S.D. Seo, Y.H. Jin, S.H. Lee, H.W. Shim, D.H. Lee, D.W. Kim, Dalton Trans. 40 (2011) 9498–9503. [10] J. Kou, A. Saha, C.B. Stamperb, R.S. Varma, Chem. Commun. 48 (2012) 5862– 5864. [11] S. Dehghanpour, A. Mahmoudi, M.M. Ghazi, N. Bazvand, S. Shadpour, A. Nemati, Powder Tech. 246 (2013) 148–156. [12] B. Zhou, Z. Liu, H. Wang, Y. Yang, W. Su, Catal. Lett. 132 (2009) 75–80. [13] Z. Ai, L. Zhang, S. Lee, W. Ho, J. Phys. Chem. C 113 (2009) 20896–20902. [14] X. Wang, C. Li, G. Chen, L. He, H. Cao, B. Zhang, Solid State Sci. 13 (2011) 280– 284. [15] Y.H. Lee, I.C. Leu, M.T. Wu, J.H. Yen, K.Z. Fung, J. Alloys Compd. 427 (2007) 213– 218. [16] E.W. Bohannan, L.Y. Huang, F.S. Miller, M.G. Shumsky, J.A. Switzer, Langmuir 15 (1999) 813–818. [17] M.J. Nine, B. Munkhbayar, M.S. Rahman, H. Chung, H. Jeong, Mater. Chem. Phys. 141 (2013) 636–642. [18] C.Y. Wang, Y. Zhou, Z.Y. Chen, B. Cheng, H.J. Liu, X. Mo, J. Colloid Interface Sci. 220 (1999) 468–470. [19] J. Ahmed, S. Sharma, K.V. Ramanujachary, S.E. Lofland, A.K. Ganguli, J. Colloid Interface Sci. 336 (2009) 814–819. [20] B. Kumar, S. Saha, M. Basu, A.K. Ganguli, J. Mater. Chem. A 1 (2013) 4728–4735. [21] J. Ahmed, P. Trinh, A.M. Mugweru, A.K. Ganguli, Solid State Sci. 13 (2011) 855– 861. [22] B. Kumar, S. Saha, A. Ganguly, A.K. Ganguli, RSC Adv. 4 (2014) 12043–12049. [23] J. Ahmed, B. Kumar, A.M. Mugweru, P. Trinh, K.V. Ramanujachary, S.E. Lofland, Govind, A.K. Ganguli, J. Phys. Chem. C 114 (2010) 18779–18784. [24] J. Ahmed, A. Ganguly, S. Saha, G. Gupta, P. Trinh, A.M. Mugweru, S.E. Lofland, K. V. Ramanujachary, A.K. Ganguli, J. Phys. Chem. C 115 (2011) 14526–14533. [25] L. Feng, H. Vrubel, M. Bensimon, X. Hu, Phys. Chem. Chem. Phys. 16 (2014) 5917–5921. [26] J.M. McEnaney, J.C. Crompton, J.F. Callejas, E.J. Popczun, C.G. Read, N.S. Lewis, R. E. Schaak, Chem. Commun. 50 (2014) 11026–11028. [27] N.A. Al Abass, G. Denuault, D. Pletcher, Phys. Chem. Chem. Phys. 16 (2014) 4892–4899.