Mass spectrometry quantification of hydrogen produced under illumination of p-CuInSe2 and modified surfaces

Int. J. Hydrogen Energy, Vol. 22, No. 6. pp. 581-584, 1997 ‘c’: 1997 International

Pergamon PII: SO360-3199(96)00196-h

Association

for Hydrogen Energy Elsevier Science Ltd

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MASS SPECTROMETRY QUANTIFICATION OF HYDROGEN PRODUCED UNDER ILLUMINATION OF p-CuInSe, AND MODIFIED SURFACES S. FERNANDEZ-VALVERDE,*t E. ORDOREZ-REGIL,? R. VALENCIA-ALVARADO,? R. RIVERA-NORIEGAS and 0. SOLORZA-FERIAf t Gerencia de Ciencias Basicas, Instituto National de Investigaciones Nucleares, A. Postal 1X-1027. 11870 Mexico D. F. : Depto de Quimica, CINVESTAV-IPN. A. Postal 14-470,070OO Mexico D. F.

(Receiwdfor

publication 25 July 1996)

Abstract---Hydrogen gas obtained from illuminated p-CuInSez (CIS) on 0.5 M sulfuric acid solution was collected in a glass balloon using argon as carrier gas, and quantified with a mass spectrometer. p-CuInSe, thin films were electrodeposited on SnO,-coated glass plates. according to references already reported. Electrochemical deposition of Se and chemical modifications on the semiconducting surfaces with adsorbed ruthenium were also tested for the hydrogen evolution reaction. The E, and E, of the modified p-CuInSe, were determined. Atomic absorption spectrometry was used to quantify the chemical compositions of the semiconductors before and after the illumination. The modified semiconductors had a composition of CIS/Se,, and CIS/Se,/(RuO,), ,Z,under illumination at 0.8 V in

0.5 M H,SO, for 4 min, the volumes of evolved hydrogen were 1 cm’ and 2.4 cm3,respectively. c: 1997International Association for Hydrogen Energy

INTRODUCTION Thin film solar cells from compound semiconductors are still considered as important options of low cost photovoltaics. The CuInSe, (CIS) has been studied for non linear optics, photovoltaic and photoelectrochemcial applications, although considerable attention has been focused on single crystals and thin films [l-5]. One of the most important limitations on photoelectrochemcial cells is the durability caused by degradation by gradual recombinations and reactions with the electrolyte over the specimen surface [6]. In this connection a great effort has focused on modifying the semiconducting surface with metal deposition and chemical compound adsorption [791. The effect of redox solutions on CIS thin films modified surface with metal aggregates has also been investigated [lo]. It has been shown that Pt, Pd and Ru on the surface catalyse the hydrogen evolution reaction allowing the transfer of the generated photocurrent, otherwise it is very difficult on non-treated surfaces. In this way, the aim of the paper is to present an experimental system to quantify, by mass spectrometry,

*Author

to whom correspondence should be addressed

the hydrogen generated photoelectrochemically on ptype CIS thin films, prepared by electrodeposition on SnO,-electric conducting glasses, and by surface modifications of the semiconductor with electrodeposited selenium and chemically adsorbed ruthenium, and to distinguish between hydrogen evolution and the surface degradation processes.

EXPERIMENTAL A standard electrochemical interface (Schlumberger model 1286) combined with a frequency response analyzer (Schlumberger model 1250) set-up were used for the current potential and impedance measurements. The electrodeposition of p-type CIS thin films was performed as reported in Refs [ 1, 111.It involves preparing an aqueous oxygen free solution containing 5.0 mM Cu,S04, 20 mM In,(SO,),, 10 mM SeO,, 60 mM K,SO, and 0.06 M sodium citrate, adjusting the pH to 1.7 with dilute sulfuric acid and setting the temperature to 310 K to avoid the citrate decomposition. The electrode was dipped 1 cm into the solution and supported with a metallic holder. The electrodepositions were carried out for I5 min in potentiostatic conditions in a stirred solution. The reference electrode was a saturated sulfate electrode (SSE). 581

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S. FERNANDEZ-VALVERDE

A platinum grid was used as counterelectrode. Under these conditions, CIS thin films were electrodeposited at -0.97 V. The photoelectrochemical experiments were carried out in a Pyrex typical electrochemical cell with oxygen free 0.5 M HzS04 solution. The light source was a 1000 watts xenon lamp, with an illumination intensity of 45 mWcm-*. The low energy IR radiation was eliminated by passing the light through a 10 cm glass tube water filter before arriving to the glass Pyrex cell. A thin film of selenium was obtained by scanning the potential three times from -0.30 V to -0.90 V vs (SSE), in a 0.1 M SeO,+0.5 M H2S04 solution, under the same light intensity. The chemical modification of the surface electrode with ruthenium was obtained by dipping the working electrode in 0.1 M HNO, f0.01 M RuCI, solution for 20 min. The electrodes were illuminated in a sealed electrochemical cell, using argon as carrier gas and polarizing the working electrodes at - 0.80 VjSSE, in a direct light beam for approximately 4 min. The gases were collected in a 250 ml glass balloon by displacing water and quantified in a mass spectrophotometer (Cuadruvac Q 200). Prior to the experiments the glass balloon was lyophilized. For the hydrogen quantification, the equipment was heated and stabilized for 4 h before analysis. The calibration was performed with hydrogen-argon gas mixtures with concentration of 0.1, 0.5 and 1% in volume. These gases were introduced using the same 250 ml glass balloon. The E, of the semiconductors were determined from transmittance spectra obtained from a Cary 17D spectrophotometer, taking a thin film of dioxide conducting glass as a reference. The flatband potential was determined from the impedance measurement in the dark in 0.5 M H2S04 solution. The composition of the thin films were determined by atomic absorption spectrometry (Perkin-Elmer 2380). The thin films were dissolved in hot concentrated nitric acid solution and standardized to 10 ml with distilled water. Standard solutions of indium, copper and selenium were used for calibration. Hollow cathode lamps and air acetylene flames have been used in the determination. The l,,, are 196, 304, 325 and 350 nm for Se, In, Cu and Ru, respectively. RESULTS

AND DISCUSSION

Photoelectrochemical responses of the semiconductors measured under illumination are shown in Fig. 1. Curve (a) represents the behavior of the CIS, curve (b) CIS/Se, and curve (c) CIS/Se,/Ru, in 0.5 M sulfuric acid solution. With CIS the onset photocurrent starts around -0.40 V, and at -0.70 V vs SSE the photocurrent is around 80 mA cm-*. When the CIS was immersed in an acidified solution of 0.1 M SeO,, and scanned under illumination, a thin film of selenium was electrodeposited on the semiconductor surface, increasing the photocurrent response with time and stabilized after six cycles. The mechanism for the deposition of selenium is pH dependent [ 121and can be described by either of the following reactions:

et al.

-0.9

-0.8

-0.7 potential

-0.6

-0.5

(SE)

-0.4

-0.3

/ V

Fig. 1. Current potential behavior of CIS and their modified structure under illumination in 0.5 M H, SO,. HSeO:+3H++4em

+Se+2H,O

HSe0,+5H++5em

+Se+3H,O

(2)

with the two species, HSeO: and HSeO,, equilibrium in aqueous media:

coexisting in

(1)

HSeO:+H,OoHSeO,+2H+. (3) The behavior of the CIS/SE, in the 0.5 M sulfuric acid solution, is shown in curve 1b, the overpotential for proton reduction starts at -0.40 V, and at -0.70 V vs SSE the photocurrent was around 550 mA cm-*. CIS/SE, dipped in a solution containing Ru+~ gives RuO, as observed by X-ray diffraction (2). The presence of this oxide may be explained by the formation of a complex hydrated species as reported recently by Tyrlic et a/. [13]. These species in air result in the formation of the ruthenium dioxide as is well known for the exposure to air of the species formed during the reduction of commercial RuCl, in acidic medium by metallic mercury [ 141. The dioxide was chemisorbed on the surface: CIS/ Se,/(RuO,),. The photo response obtained under the same experimental conditions, is shown in Fig. l(c). Here the photocurrent rises compared with those described above. The effect of ruthenium could be interpreted as a reaction with a surface chemical entity associated with a surface state near the conduction band, similar to the process observed on GaAs [15], where the increase in photoresponse was associated with the change of the composition of the surface state, either by electrostatic interaction with the surface state or by forming a bond in which electrons are shared. The band gap, E,, obtained from transmission spectra of each semiconductor before illumination is reported in Table 1. The E, increases with the deposition of selenium Table 1. Energy gap, flat-band potential and hydrogen quantification (%) after illumination at -0.80 VjSSE for 4 min Semiconductor CuInSe, CuInSe,/Se,, CuInSe,/Se,/(RuO,),

E&V) 1.04 1.21 ,* 1.33

E,(V/SSE)

% hydrogen

-0.10 -0.10 -0.03

0 2.5 6.3

H, PRODUCED

UNDER

ILLUMINATION

0.05 0.04

0.03

0.02

0.01

ot...“‘.“..‘.‘.‘..‘;u.e.l - 1 -0.6 -0.6

-2 -0.4

-0.2

0

0.2

potential (SSE) / V Fig. 2. Capacitance potential behavior of CIS and their modified surface in 0.5 M H,SO,, in dark and 20 KHz.

and adsorption of ruthenium. These changes in the energy gap are associated with the modification of the films. Capacitance-voltage measurement of the CIS and their modified surface were carried out to determine the flatband potential, E,, and the carrier concentration in the same aqueous solution, assuming a dielectric constant value, e, as 10 [16]. Figure 2 shows the behavior for each of these semiconductors tested under dark conditions. Table 1 summarizes the values of the flatband potentials obtained by extrapolation to C5;’ = 0. The electrodeposition of selenium did not modify the E,; however, a shift to a more cathodic value was observed with the adsorption of ruthenium although the carrier concentration is maintained in the range of lOI cmm3, meaning that the bulk of the CIS was not modified in the dark. The results of the hydrogen percentages, determined by the partial pressure of the gases present in the balloons are given in Table 1, and the chemical compositions of the semiconductor, determined by atomic absorption spectrometry after illumination at 0.8 V for 4 and 20 min, are reported in Table 2. In p-CuInSe,, no hydrogen was detected, in both dark conditions and after illumination for 4 min. The photocurrent observed in this case was associated with the degradation process of the surface. This fact was confirmed by the atomic absorption spectrometry results. When the semiconductor was illuminated for 4 min the amount of selenium increases on the surface, implying a dissolution of copper and indium to form a material with a composition of CuInSe,,. After 20 min illumination Table 2. Composition of the semiconductor before and after illumination for 4 and 20 min at 0.80 VjSSE Before illumination CuInSe, CuInSe,/Se, CuInSe#e&RuO&

After illumination 4 min CuInSe, 4 CuInSe, I2 CulnSes/RuO 05

After illumination 20 min Cub de, b CuIno de15 Cub ,,Se, 4

OF p-CuInSe,

583

the majority of the indium leaves the CIS forming the compound CuIn, ,,Se,,,. The electrodeposited selenium on CIS gave a composition of CIS/Se,,. After illumination at 0.8 V in 0.5 M sulfuric acid solution for 4 min, a dissolution of the superficial selenium was detected. The final composition was CIS/Se, and the hydrogen evolved corresponded to 1 cm3. When the illumination was 20 min the semiconductor was modified to CuIn, ,,Se,,. The modified CIS/Se,, with ruthenium had a composition CIS/Se,/(RuO,), ,2. Under the same experimental conditions the evolved hydrogen was 2.4 cm3. The composition of the semiconductor was also modified to CIS/Se,/(RuO,),,,, due to the photocorrosion after 4 min of illumination. After 20 min illumination, changes in the composition were determined, and resulted in the formation of CuIn, ,,Se, + CONCLUSIONS The observed photocurrent on CIS corresponds to a photodegradation process. No hydrogen was quantified. The observed increased photocurrent with electrodeposited selenium was attributed to the hydrogen evolution reaction and also to a photodegradation process. A similar result was obtained with ruthenium adsorbed as oxide on the surface of CIS/Se,,. The photodegradation process could be associated with the porosity of the electrodeposited and chemically modified semiconductors. In addition, the utilization of mass spectrometry for the hydrogen quantification and atomic absorption spectrometry of the chemical composition determination, allowed us to distinguish between the hydrogen evolution reaction and the semiconductor photodegradation.

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