CC BY
43
KiMYA PROBLEML9RÎ № 4 2016
343
UDC 621
PHOTOELECTROCHEMICAL SYSTEM BASED ON SEMICONDUCTOR NANOMATERIALS AND GRAPHENE STRUCTURES FOR SOLAR HYDROGEN PRODUCTION
G.Ya.Kolbasov, I.A.Rusetskii, I.A.Slobodyanyuk, M.O.Danilov
V.I. Vernadski Institute of General and Inorganic Chemistry of the Ukrainian NAS, Palladina Ave.,.32-34, 03680 Kyiv 142, Ukraine, e-mail: [email protected]
The photo-electrochemical processes on epitaxial films of n-GaAs and polycrystalline films CdSe after modifying their surface by nano-sizedparticles of CdS, Zn, Pt, graphene oxide and Nb2O5 have been studied. It revealed that the modification of the electrode causes increase in their photosensitivity, which comes from reduced rates of surface recombination and photocathode reactions that take place with the participation of surface electronic states. As cathodes for the hydrogen production there were used the nano-composites based on multiwalled carbon nanotubes and reduced graphene oxide. Materials analyzed are rather promising to be used as electrodes for electrochemical cells in solar hydrogen production.
Keywords: GaAs, CdSe, nanostructures, graphene oxide, reduced graphene oxide, solar hydrogen production.
Development of renewable energy sources is one of the most challenging scientific and technical problems of our time. In this respect, an emphasis is laid on the use of solar energy by means of semiconductor solar cell converters. Currently developed are photo-electrochemical solar energy converters to accumulate hydrogen without the gas generation phase [1, 2]. Success in using these devices is dependent on the technology of producing low-cost and effective semiconductor and electro-catalytic materials. Principal factor in the production of these is the development of methods of their synthesis, which ensure high durability and reliability of solar energy electrochemical converters. An important aspect of these systems is to increase the efficiency of photo-conversion. One of the ways to attain this goal is the
modification and nano-structuring of semiconductors to prevent the recombination of charge carriers and thus enhance their catalytic properties.
With the advent of the new nanocarbon material - graphene, a lot of papers have been published to identify their use as a base material for anode and cathode in the photo-electrochemical cells [3-5].
The aim of the work is to examine photo-electrochemical properties of GaAs and CdSe composite films with the nanostructured surface that contain graphene oxide (GO) and make it possible to determine the applicability of these films and cathode materials based on graphene structures in the photo-electrochemical system for the hydrogen production.
Experimental results and discussion
We are engaged in studying epitaxial films of n-GaAs, ~ 1 microns thick, which are chemically deposited on a single crystal GaAs-substrate, as well as electrodes on the basis of polycrystalline CdSe. Composite photo-electrodes based on photosensitive CdSe film
were obtained by applying an aqueous dispersion of mechanical CdSe powder in the polyethylene glycol of CdCl2 solution (2 mass % of CdCl2) and 0.19-2.42 mass % of graphene oxide (by weight of CdSe) on the Ti substrate with preformed layer of TiO2
nanotubes. Note that TiO2 nanotubes were obtained through the use of anodic potentiostatic polarization of the titanium foil in F- ions-containing [6, 7] electrolyte. Photo-electrodes were annealed in air at 530 ° C for 1 hour to prevent the possible destruction of the material [6, 8]. The surface of the electrodes was studied by means of Auger spectrometer 09-I-OC-10-005, mass spectrometer IMS4f and electron microscopes JSM 6700F and JEM-100 CXII.
To examine the current-voltage characteristics, potentiostat-galvanostat P-8S (Elins, Russia) has been used. The electrode potential has been measured with respect to silver chloride reference electrode. Spectral dependences of photocurrent have been measured on a quartz cell that included an MDR-2 mono-chromator, and as a light source there was used a xenon lamp (DKSSh-500) with stabilized discharge current. The xenon lamp with power incident light P = (16 ^ 50) mW / cm2 was used to illuminate photo-electrode effect. Values of P have been established with help of photodiode head PD300 - UV and the display NOVA II (Ophir-Spiricon). Pulsed nitrogen laser LGI-21 (X = 0.337 micron, Pi = 1300 W / cm2, Ti = 15 ns) has been used to study the relaxation kinetics of the photo-EMF. The time resolution of the measurement setup made up 50 ns. As electrolyte, we used an aqueous solution of KOH and Na2S containing 1 mol / L of each component.
The electrodes have been modified by
2+
treatment in the solutions containing Zn ions. The presence of Zn on the surface was determined through the use of mass -spectrometry and by means of Auger spectroscopy; Zn concentration on the surface constituted about ~1013 atoms / cm2. Results of electron-microscopic analysis showed that at the resolution of ~ 2 nm which took place during the experiment, the Zn metal nanoparticles were not observed; this may indicate that the nanoparticles have sizes, at least, 2 nm. It should be noted that similar conclusions were made by authors of [9], who carried out studies into the effect of modifying TiO2 electrode by platinum on photo-catalysis; in this case the nanoparticles were not
observed due to their extremely small size. We performed deposition of CdS nanoparticles on GaAs during the formation of CdS colloidal particles in aqueous solutions. Note that an average particle diameter changed from 2 nm to 5 nm depending on the initial concentration of reagents [10]. For the deposition of Pt nanoparticles on the surface of the GaAs, there was used a method of Pt electro-deposition for chlor-platinic acid solutions under strong light on the semiconductor in the area of fundamental absorption light. Average diameters of Pt particles ~ 10 nm on the surface coverage were 30-70%.
Nb2O5 particles were deposited on the surface of CdSe by means of photo-electrochemical method from the solution of aqueous electrolyte based on poly peroxy niobium acids. Electrolyte content constituted Nb2O5-xH2O - 0.01 mol / L (in terms of pure Nb2O5); 20 ml / L the solution of 35 mass % of H2O2. The solution was adjusted to pH = 3 by sulfuric acid. The electrochemical dissolution powder of Nb2O5 carried out in a pocket_ like a titanium cathode in the solution of 40 mass % of H2SO4 and 5 mass % of H2O2 at a current density of 0.5 ^ 1 A / cm [11].
Graphene oxide (GO) and reduced graphene oxide (RGO) were obtained by chemical synthesis from multiwalled carbon nanotubes (MCNT) [12-14]. An outer diameter of carbon nanotubes was about 10-30 nm; a specific surface area - 130 m2 / g; a bulk density - 25-30 g / dm3; an amount of wall from 8 to 15. Fig. 1 shows graphene oxide prepared out of multiwalled carbon nanotubes.
Mb? - '
. 20 nm
Figure 1. Micrograph of graphene oxide.
When using semiconductor electrodes for solar energy conversion, in particular, for production of hydrogen, it is necessary to estimate the value of photo-voltage E < -1 V (with respect to silver chlorine reference electrode) at which hydrogen can be efficiently reduced on cathodes [2, 15]. The main difficulty in the use of relatively narrow-gap semiconductors to produce hydrogen lies in small negative value of photo-voltage (Eph) and recombination processes occurring on the surface, and as a result, low voltage of photo-electrochemical cell. Nano-structuring Ti substrate by TiO2 nanotubes contributes to the gathering of major carriers in this hetero-structure and leads to an increase of Eph [6] thus improving the adhesion of photoactive coating with a substrate.
Studies into the effect of modifying the surface of a semiconductor on its surface charge, which is determined from the photovoltage values under illumination [16] and measurements of voltage-capacitance characteristics of the Schottky layer [15] made it possible to establish that the modified TiO2 nanotubes / CdSe / GO - electrode result in raising potentials of flat bands Efb in the cathode area by 60 - 100 mV, which, in turn, leads to rise in photopotential AEpf. A similar increase in the negative area of Epf after modifying the surface by Zn was observed by us in the matter of GaAs electrodes. We found that the modification of the semiconductor electrode by Nb2O5 raises load parameters of TiO2 nanotubes / CdSe- electrode by 10 - 20% and reduces relaxation processes arising from adsorption processes and slowing recombination processes, as well as traps carriers charge on the electrode surface (Fig. 2).
A reason for improved performance of the TiO2 nanotubes / CdSe / GO - electrode composite structure is to improve the separation of electron-hole pairs in the space charge area (SCR) modified CdSe, which contacts with the electrolyte, since graphene structures are electron acceptors [17, 18] and contribute to an increase in negative charge of the surface of semiconductor in contact with CdSe / GO and, as a consequence, leads to an increase of electric field in the SCR.
Figure 2. Photocurrent density j dependence on potential E for modified Nb2O5 (1), and for initial TiO2 nanotubes / CdSe- electrode (2). 1M Na2S + 1M NaOH solution. The light output P=16 mW/cm2.
Changes in the value of flat-band potential can also be related to an increase in negative surface charge as a result of the restructuring of the "slow" surface electron states, interaction of quantum dots of metals and semiconductors with semiconductor surfaces, in particular, GaAs, and cause changes in the content of the surface oxide layer.
It was found that modification of the surface semiconductors led to an increase in quantum yield of photo-electrochemical current if, in a wide spectral range (Fig. 3).
0.8
Hi
0.6 -
0.4
0.2 -
200
400
600
800 X, nm
Figure 3. Spectral dependence of the quantum yield of photo-electrochemical current, ni, in the solution of 1 mol/L KCl for a GaAs electrode at the potential E = 0.4 V. 1 -initial electrode, and modified with Pt (2); (3); CdS (4).
Maximum value of ni for the modified GO-CdSe electrode was n =0.65 at a potential of 0.4 V. A decrease in n in the short-wavelength area of the spectrum is characterized by the start of GaAs electrode, on whose surface there were no nanoparticles (Fig. 3, curve 1) [16]. The largest increase in n was observed in the case of CdS (Fig. 3, curve 4) and the least in Pt (Fig. 3, curve 2). The deposition of Pt nanoparticles was accompanied by the increase in n in the visible area and the decrease in the ultraviolet one (Fig. 3, curve 2).
To analyze the obtained results, we applied to the theory of photo-generated charge carrier transfer across the semiconductor/electrolyte interface [15, 16] bearing in mind that the total photocurrent across the interface consists of minority-carrier current (hole photocurrent) and majority-carrier current (electronic photocurrent), while in the course of interfacial charge transfer through the interface there were involved surface electronic states. Note that the rise in the quantum yield of photocurrent, n, in the visible area after the modification of Pt, Zn, CdS and GO (Fig. 3)
surfaces may be accounted for by intensification of photo-catalytic activity of nanostructured surface and, hence, the increase in anodic photocurrent together with the decrease in the surface recombination velocity of the holes and, consequently, rise in the hole photocurrent [16]. At the same time, to explain the decrease in n after deposition of Pt in the ultraviolet area it is essential to assume that the cathodic photocurrent contributes to the rise in the cathodic reaction rate in total photocurrent. Analysis of experimental data shows that the modification of the GaAs surface by Pt gives rise to the cathodic reaction rate (in this case, the reaction of hydrogen photo evolution). This is proven by a significant decrease in the overvoltage of hydrogen evolution on GaAs after the deposition of Pt nanoparticles on its surface [19].
It should be noted that the loss of photogenerated carrier on the recombination after modification of semiconductors also manifested itself in the rise of characteristic relaxation time and the increase in photovoltage amplitude, which leads to the rise in photosensitivity electrode.
Figure 4. Comparative summarized current-voltage characteristics of different photo-anodes in the solution of 1M KOH + 1M Na2S and cathodes in the solution of 30 % of KOH: CdSe -photo-anode (1); Pt - cathode (2); cathode from MCNTs with deposited Pt (3); cathode from RGO with deposited Pt (4); composite photo-anode from CdSe with 1 % of GO. P=16 mW / cm2. Potential sweep rate of 20 mV/s.
Fig. 4 shows the current-voltage anodes obtained provide a hydrogen evolution
characteristics of photo-anodes and different on platinum and carbon nano-composites.
cathode materials for the production of Composite photo-anodes from TiO2 nanotubes
hydrogen. As is seen from Fig. 4, photo- / CdSe with GO have better load
characteristics than non-modified films of CdSe. As a cathode material, we applied nano-composites based on carbon nanotubes and RGO with deposited Pt. Note that modified Pt
carbon materials can effectively generate the hydrogen gas in the system, as well as on the cathodes based on RGO with essentially improved electro-catalytic parameters.
Conclusions
It revealed that modification of the surface of CdSe and GaAs electrodes with nanoparticles of CdS, Zn, Pt, graphene oxide and Nb2O5 leads to an increase in the negative surface charge, reduces the surface recombination and as a consequence raises the photo-conversion efficiency. The formation of TiO2 nanotubes on titanium substrate increases the adhesion of CdSe polycrystalline, and reduces the amount of the cathode dark current. It found that base materials for the cathode in
photo-electrochemical cell for hydrogen production might be identified as materials with low hydrogen evolution over-potential and new nano-composites based on multi-walled carbon nanotubes and reduced graphene oxide, which are apparent in the reaction of hydrogen evolution close to the platinum metal. Results obtained testify that the analyzed photo-electrochemical system is highly promising for solar hydrogen production.
References
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ФОТОЭЛЕКТРОХИМИЧЕСКАЯ СИСТЕМА, ОСНОВАННАЯ НА ПОЛУПРОВОДНИКОВЫХНАНОМАТЕРИАЛАХ ИГРАФЕНОВЫЕ СТРУКТУРЫ ДЛЯ ПРОИЗВОДСТВА "СОЛНЕЧНОГО " ВОДОРОДА
Г.Я.Колбасов, Л.А.Русецкий, И.А.Слободьянюк, М.О.Данилов
Институт общей и неорганической химии им. В.И.Вернадского Национальной Академии Наук Украины, пр. Палладина 32-34, 03680 Киев 142, Украина; e-mail: [email protected]
Изучены фотоэлектрохимические процессы на эпитаксиальных пленках n-GaAs и поликристаллических пленках CdSe после модифицирования их поверхности наноразмерными частицами CdS, Zn, Pt, оксидом графена и Nb2O5. Показано, что модифицирование электродов приводит к увеличению их фоточувствительности, что связано с уменьшением скорости поверхностной рекомбинации и фотокатодной реакции с участием поверхностных электронных состояний. Показано, что базовыми материалами катода в фотоэлектрохимической ячейке для получения водорода могут быть как известные материалы с низким перенапряжением выделения водорода, так и нанокомпозиты на основе углеродных нанотрубок и восстановленного оксида графена, характеристики которых в реакции выделения водорода близки к значениям на платиновых металлах. Исследованные материалы перспективны для применения в качестве электродов электрохимических преобразователей солнечной энергии для получения водорода. Ключевые слова: GaAs, CdSe, наноструктуры, оксид графена, восстановленный оксид графена, производство "солнечного " водорода.
Поступила в редакцию 30.10.2016.
