Overall Water Splitting under Visible Light Irradiation Using ...

Available online at www.sciencedirect.com

ScienceDirect Energy Procedia 54 (2014) 221 – 227

4th International Conference on Advances in Energy Research 2013, ICAER 2013

Overall Water Splitting under Visible Light Irradiation Using Nanoparticulate RuO2 Loaded Cu2O Powder as Photocatalyst Tatsat Banerjeea, Alakananda Mukherjeea * a

Department of Chemical Engineering, Jadavpur University, Kolkata – 700032, West Bengal, India

Abstract Overall decomposition of water into hydrogen and oxygen in presence of a heterogeneous photocatalyst has received prodigious attention due to its potential for the production of clean and recyclable hydrogen energy. However, most of the efficient photocatalysts developed till date, works primarily on ultra-violate range of light. To develop photocatalysts that can decompose water under more abundant visible range of light, efforts have already been made by researchers who basically tried to synthesize materials which have such a narrow band gap that they can utilize less energetic photons in visible range. To do so, the catalysts that they have prepared to exhibit high stability and to give decent reaction rate and quantum efficiency are of extremely complex structure. Moreover, cumbersome synthesis route involving doping of different materials, complicated coreshell nanostructure preparation, etc is necessary in most of the cases. Here, we report a facile and efficient approach to facilitate photocatalytic water-splitting under visible light, in single step. We have modified Cu2O, a well-known p-type semiconductor having a band-gap ~2.1 eV, with RuO2 nanoparticles and used it as photocatalyst. We have observed that it has a possibility of near-stoichiometric overall water decomposition under visible light with appreciable quantum efficiency. © 2014 2014Alakananda The Authors. Published by Elsevier Ltd. Ltd. This is an open access article under the CC BY-NC-ND license © Mukherjee. Published by Elsevier Selection and peer-review under responsibility of Organizing Committee of ICAER 2013. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of Organizing Committee of ICAER 2013 Keywords: Solar Hydrogen; Photocatalytic Water Splitting; Cu2O; RuO2; Heterogeneous Catalysis

1. Introduction Overall photocatalytic decomposition of water into hydrogen and oxygen in presence of a heterogeneous catalyst provides an excellent way of producing hydrogen energy which can supersede conventional energy sources

* Corresponding author. Tel.: +91 9330859933 E-mail address: [email protected]

1876-6102 © 2014 Alakananda Mukherjee. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of Organizing Committee of ICAER 2013 doi:10.1016/j.egypro.2014.07.265

222

Tatsat Banerjee and Alakananda Mukherjee / Energy Procedia 54 (2014) 221 – 227

by its capacity of zero carbon dioxide emission and its ability of utilization of solar energy, an effectively inexhaustible source of recyclable energy [1][2]. As this route does not involve any external electrical power supply, high temperature, and carbon dioxide generation, it is considered as potentially more sustainable than all other existing ways (like steam reforming of fossil fuel, electrolysis of water, thermal power concentration, biomass conversion, etc.) [3] those lead to “Hydrogen Economy”, arguably the most-sought solution of global energy crisis, which advocates the transition from the utilization of energy sources (like fossil fuels) to the use of energy vectors (i.e. hydrogen) in various sectors including motive power, automobiles, portable electronics, etc [4]. Many efficient photocatalysts which can act under ultraviolet spectrum has already been developed [3]. However, unfortunately, only 4% of the incoming solar energy consists of spectrum in the UV range, whereas 43% of it consists of visible light [5]. So, a facile and versatile recipe to synthesize a stable photocatalyst that is capable of harvesting less energetic but more abundant visible photons is a matter of extensive research, till now[3][6]. In attempt to develop such a catalyst, the systems those were synthesized exhibiting high stability and giving decent reaction rate with good quantum efficiency are of extremely complex structure. Moreover, cumbersome synthesis route involving doping of different materials, complicated core-shell nanostructure preparation, etc is necessary in most of the cases [6]. Here, in this paper, we describe a simple and efficient approach to facilitate photocatalytic water-splitting under visible range of light, via single-stage-route (i.e. not mimicking Z-Scheme). We are here modifying Copper (I) Oxide, a well-known p-type semiconductor [7][8], with Ruthenium (IV) oxide nanoparticles to use as a photocatalyst under visible light range, for overall water splitting. 2. Choice of photocatalyst The overall photocatalysis scheme consists of three intermediate steps: a) absorption of photon (of visible light range) and electron-hole pair generation, b) charge separation and migration to surface and c) surface reaction. For successful decomposition, there exist some stringent requirements those should be satisfied by the catalyst material.

Figure 1. Three steps of photocatalytic water-splitting. Step 1: Absorption of photon (of visible light range) and electron-hole pair generation, Step 2: Charge separation and migration to surface. Step 3: Surface reaction (oxidation and reduction).


In the first step, photocatalyst molecule absorbs photon from incident natural sunlight or any other artificial light. After the energy has been absorbed, the reaction may or may not go to the second step depending upon the generation of electron-hole pair, feasibility of which is determined by the band gap of catalyst and also on the energy of excitation. Most of the photocatalysts, used in this reaction till date, are basically semiconductors. In that case, the bottom level of the conduction band has to be negative with respect to the redox potential of H+/H2 (0V vs. NHE), while the top level of the valence band has to be positive with respect to the redox potential of O2/H2O (1.23 V vs. NHE). Therefore, the theoretical minimum band gap for water splitting is 1.23 eV. On the other hand, if the band gap becomes greater than 3 eV, the semiconductor photocatalyst can no longer use visible photons to generate electron-hole pair. So ideally a material should be found (or have to be prepared via band-gap modification techniques like doping) whose band gap is greater than 1.23 eV and less than 3 eV, for overall water splitting under visible light range of light. It was seen that metal oxides with d0 and d10 metal cations are promising candidates [9][10]. In that case also, the difference between the bottom of conduction band (consisting of empty d or sp orbitals) and top of the valance band (consisting of O2P orbitals) is usually greater than 3 eV [9] and that is why doping is necessary for almost all materials. However, copper (I) oxide, a well-known p-type semiconductor with metal cation configuration of d10, has a band gap of ~ 2.1 eV [7][8][11] and seems to be an ideal choice for water decomposition reaction under visible light range [8][11][12]. We took it as our basic catalyst. After generation of charged pair, second step starts where they get separated and move to the surface of photocatalyst molecule. If there are crystal defects, then there is probability of recombination of electron and hole. Again if there is no active site on the surface, reaction cannot take place and electron-hole pair will be lost. To prevent this, we modified Cu2O with RuO2 nanoparticle which is well-known for providing reaction site [5][6][9][10][13] for photocatalytic water decomposition. In the final step, surface reaction takes place where H+ get reduced to hydrogen gas by accepting electron and water get oxidized to oxygen gas by accepting a hole. It is believed that loaded co-catalyst (RuO2 in our case) helps here too by decreasing the activation energy for hydrogen gas evolution [6]. 3. Experimental 3.1. Materials Copper (I) Oxide [Cu2O] of 99.99% purity was bought from Sigma-Aldrich. Triruthenium dodecacarbonyl [Ru3(CO)12] of 99% purity was also from Sigma Aldrich. Both of them were used without any pre-treatment. Tetrahydrofuran (THF) was purchased from Merck Millipore. All other chemicals were local products of analytical grade. Distilled water was used throughout the whole experiment. 3.2. Preparation of photocatalyst For the photocatalyst preparation, we have used modified route of RuO2 loading reported by Sato et al. [10]. Basically, triruthenium dodecacarbonyl [Ru3(CO)12] was impregnated up to incipient wetness into Cu2O. Then the ruthenium carbonyl complex was oxidized to RuO2. First the pore volume of copper(I) oxide was determined, which came out to be 0.45 cc/g. Some amount of Cu2O was taken in a beaker. THF was taken (equal to the pore volume of Cu2O) in a separate test tube and some amount Ru3(CO)12 was added into it and a solution was prepared in such a way that final catalyst contains 1 wt% RuO2. This solution was added into the beaker containing Cu2O. Now THF was removed from the mixture by heating it at 85°C for one hour. Finally Ru3(CO)12 was oxidized in air to produce RuO2 by heating at 350 °C for 5 hours. The prepared catalyst was later characterized by Field emission scanning electron microscope (FE-SEM). The process can be shown schematically as:

223

224


Figure 2. Scheme for photocatalysis synthesis

3.3. Water splitting reaction Photodecomposition of water was done in a Pyrex cell, taking 0.5g of as-prepared catalyst in 200 mL distilled water. As Cu2O is insoluble in water, the water-catalyst mixture was vigorously stirred magnetically to ensure that catalyst remained suspended. A 300 W Xe lamp was used as the light source. We focused the light on the Pyrex cell and then used a longpass filter so that YLVLEOH VSHFWUXP ZLWK ZDYHOHQJWK Ȝ!50 nm can pass through it. We used distilled water with pH nearly equal to 7 (i.e. the neutral water). The gases produced were determined by a gas chromatograph which was attached to the Pyrex cell in such a way that produced gases can directly enter it after evolution, without any contamination. We took total four runs, each lasting for hundred hours. The reaction system was emptied after each run. 4. Results and discussion From the FE-SEM images below it is quite clear that RuO2 nanoparticles of around 10 nm created a layer over Cu2O particles of submicrometer range. These nanoparticles acted as co-catalyst throughout the reaction and acted as hydrogen evolution site.

Figure 3. a) FESEM image of Bare Cu2O b) SEM image of Cu2O Loaded with RuO2 Nanoparticle


Quite interestingly, we got some nanowire morphology in some catalyst particles. That is quite evident from the FE-SEM image below.

Figure 4. FE-SEM images of CuO nanowires loaded with RuO2 nanoparticles

These were probably CuO nanowires, produced due to oxidation of Cu2O, in high temperature. We need to investigate further to determine the exact composition of photocatalyst (the ratio of CuO and Cu2O). However, we can expect that this CuO nanowires will help in the overall photocatalysis process instead of hindering as it has a band gap of 1.2-1.3 eV and it is known for exhibiting catalyst property in water-splitting under ultraviolet as well as visible light range [11]. The graph below indicates that water is splitting to produce hydrogen and oxygen simultaneously and nearly stoichiometrically. The weight of the catalyst remained virtually unchanged, even after 400 hours of reaction.

Figure 5. Rate of production of Hydrogen and Oxygen with Time

225

226


Initially rate of production of oxygen seems to be more than that of hydrogen. This queer phenomenon can be explained from the fact that Cu2O absorbs huge amount of oxygen in bulk as well as surface as O- or O2- form naturally [14]. So it may be possible that absorbed oxygen got released at the initial stage of reaction. We have observed that the reaction does not take place if it is done in dark. That confirms that the reaction is indeed dependent upon incident photons. Again, using different cut-off filter we have seen that the rate of water VSOLWWLQJGHFUHDVHVFRQVLGHUDEO\DIWHUȜ!550 nm and there is practically no splitting when light wavelength exceeds 600nm. The quantum efficiency was measured using a silica photodiode and we found it is nearly 0.45% at 500nm wavelength of light. 5. Conclusion In conclusion, here we present a facile protocol of making a photocatalyst system for water splitting under visible range of light. Cu2O was chosen as catalyst in the first place as it excellently satisfies the stringent band gap requirement, mentioned earlier. RuO2 nanoparticle loaded Cu2O powder was successfully developed to work as photocatalyst, without employing any cumbersome synthesis route like doping of one material into another, coreshell nanostructure preparation, photodeposition, etc. Using the as-prepared photocatalyst, near-stoichiometric overall decomposition of water under visible light range with decent quantum efficiency was achieved. RuO2 nanoparticles, impregnated up to incipient wetness, worked as a co-catalyst to help hydrogen generation, as expected. The troublesome via-mediator-electron-transfer mechanism of Z-scheme was avoided here intentionally. Interestingly, the weight of the photocatalyst material remains practically unchanged even after four hundred hours of reaction and this indicates the stability of our prepared catalyst system. Again, the moles of hydrogen and oxygen evolved are clearly comparable with the moles of catalyst taken. This shows the decent rate of reaction and also gives good indication that gases are getting produced from water itself. 6. Future work As our future work, we have planned to immobilize the catalyst system onto a glass substrate to increase its quantum efficiency. We are particularly interested in low-cost and energy-efficient immobilization via “Click” chemistry, i.e. Copper(I)-catalyzed-azide-alkyne cycloaddi tion. While well-established protocols for alkynefunctionalization and azide-functionalization of glass substrate are already there [15], we are currently exploring options to functionalize a suitable photocatalyst material with either azide or alkyne, to attach it to glass via “click” reaction and to modify the catalyst via co-catalysts so that entire system can decompose water effectively. Also, we have noted that there is a huge potential of increasing oxygen evolution rate by loading suitable nanoparticulate co-catalyst that can act as hole capture site to enhance water oxidation reaction [16]. This approach enables us to bypass the use of problematic shuttle redox mediators in traditional Z-Scheme water splitting. That is why we are right now searching for a convenient nanoscale material to load into our catalyst system that can efficiently work as oxygen-evolution co-catalyst, in harmony with original system. Acknowledgements We would like to take this opportunity to thank Chemical Engineering Department of Jadavpur University at first, where we have carried out most of our experiments. Then we thank Dr. Abhishek Dey and his group at the Inorganic Chemistry Department of IACS (Indian Association for the Cultivation of Science) for technical assistance and valuable suggestions. We thank Central Scientific Services Department of IACS for helping us in taking FESEM images. Finally, we acknowledge financial support from Jadavpur University. References [1] Brillet J, Yum JH, Cornuz M, Hisatomi T, Solarska R, Augustynski J et al. Highly efficient water splitting by a dual-absorber tandem cell. Nat Photonics 2012; 6(12): 824-828.


[2] Blankenship RE, Tiede DM, Barber J, Brudvig GW, Fleming G, Ghirardi M et al. Comparing photosynthetic and photovoltaic efficiencies and recognizing the potential for improvement. Science 2011; 332(6031): 805-809. [3] Kudo A, Miseki Y. Heterogeneous photocatalyst materials for water splitting. Chem Soc Rev 2009; 38(1): 253-278. [4] Marbán G, Valdés-Solís T. Towards the hydrogen economy?. Int J Hydrogen Energ 2007; 32(12): 1625-1637. [5] Zou Z, Ye J, Sayama K, Arakawa H. Direct splitting of water under visible light irradiation with an oxide semiconductor photocatalyst. Nature 2001; 414(6864): 625-627. [6] Maeda K, Domen K. Photocatalytic water splitting: recent progress and future challenges. J Phys Chem Lett 2010; 1(18): 2655-2661. [7] Nakano Y, Saeki S, Morikawa T. Optical bandgap widening of p-type Cu2O films by nitrogen doping. Appl Phys Lett 2009; 94(2): 022111022111. [8] Hara M, Kondo T, Komoda M, Ikeda S, Kondo JN, Domen K et al. Cu2O as a photocatalyst for overall water splitting under visible light irradiation. Chem Commun 1998; 3: 357-358. [9] Maeda K, Takata T, Hara M, Saito N, Inoue Y, Kobayashi H et al. GaN: ZnO solid solution as a photocatalyst for visible-light-driven overall water splitting. J. Am. Chem. Soc. 2005; 127(23): 8286-8287. [10] Sato J, Saito N, Nishiyama H, Inoue Y. New photocatalyst group for water decomposition of RuO2 -loaded p-block metal (In, Sn, and Sb) oxides with d10 configuration. J Phys Chem B 2001; 105(26): 6061-6063. [11] Barreca D, Fornasiero P, Gasparotto A, Gombac V, Maccato C, Montini T et al.The potential of supported Cu2O and CuO nanosystems in photocatalytic H2 production. ChemSusChem 2009; 2(3): 230-233. [12] Hu CC, Nian JN, Teng H. Electrodeposited p-type Cu2O as photocatalyst for H2 evolution from water reduction in the presence of WO3. Solar Energy Materials and Solar Cells 2008; 92(9): 1071-1076. [13] Maeda K, Teramura K, Lu D, Takata T, Saito N, Inoue Y et al. Photocatalyst releasing hydrogen from water. Nature 2006; 440(7082): 295295. [14] Wood BJ, Wise H, Yolles RS. Selectivity and stoichiometry of copper oxide in propylene oxidation. J Catal 1969; 15(4): 355-362. [15] Upadhyay AP, Behara DK, Sharma GP, Bajpai A, Sharac N, Ragan R et al. Generic Process for Highly Stable Metallic NanoparticleSemiconductor Heterostructures via Click Chemistry for Electro/Photocatalytic Applications. ACS Appl Mater Interfaces 2013; 5(19): 9554-9562. [16] Ma SSK., Maeda K, Hisatomi T, Tabata M, Kudo A, Domen K. A Redox-Mediator-Free Solar-Driven Z-Scheme Water-Splitting System Consisting of Modified Ta3N5 as an Oxygen-Evolution Photocatalyst. Chem Eur J 2013; 19(23): 7480-7486.

227