Indian Journal of ChemistryVol. 31A, January 1992, pp. 6-11

Effect of photodeposition of nickel on the photocatalytic,photoelectrochemical and surface properties of CdS

I Bernard Rufus, B Viswanathan", V Ramakrishnan & J C KuriacoseDepartment of Chemistry, Indian Institute of Technoiogy, Madras 600 036

Received 22 August 1991; accepted 10 October 1991

In situ deposition of ferromagnetic metals (Fe, Co and Ni) increases photocatalytic activity and alsoavoids the aerial oxidation of the metals. The deposition of sulphides of Fe, Co and Ni also enhances thephotocatalytic activity of CdS. Effect of photodeposition of nickel on the photocatalytic activity of CdSfor decomposition of aqueous sulphide has also been studied and compared with that of in situ photode-position of Fe, Co, Rh and Pd. The following order of activity is observed: Rh > Pd > > Co > Ni - Fe.Apart from Ni'', the various Ni - 0 species which are formed by the aerial oxidation of Nio have beenidentified by x,ps studies of the NilCdS surface. Phctodeposition of Ni increases the photocatalytic ac-tivity and the photostability of CdS. A mechanism for the photocatalytic decomposition of aqueous sul-phide has been proposed. Photodeposition of Ni decreases the photocorrosion of CdS, photovoltage andphotocurrent of the PEC cell and also shifts the photocurrent onset potential to a more positive value.

Deposition of noble metals on CdS increases itsphotocatalytic activity and photostabilityl-4. How-ever, the use of noble metals- is not cost effectiveeither in the conversion and storage of solar energyusing metallized CdS as photoanodes in photoelec-trochemical (PEe) cells or as photocatalysts forphotocatalytic reactions.

The effect of deposition of ferromagnetic metalssuch as Fe, Co and Ni on the photocatalytic activityof CdS has not been studied in detail. The photoca-talytic decomposition of aqueous sulphide! - 10 usingCdS as a photocatalyst is an important reactionfrom the point of view of conversion and storage ofsolar energy. Further, it has other added advantageslike (i) production of hydrogen-a clean fuel and astarting material for many industries, (ii) destructionof a pollutant, viz. H2S, (ill) production of sul-phur-a useful by-product and (iv) recycling the hy-drogen used in hydrosulphurization (HDS)-a pro-cess widely used in petroleum industries. In thepresent investigation photocatalytic decompositionof hydrogen sulphide in the presence of metallisedCdS has been used to follow the effect of in situ dep-osition of Fe, Co and Ni on the photocatalytic activ-ity of CdS. Nickel has been selected as one of therepresentative of this group of metals and detailedX-ray photoelectron spectroscopic (XPS) andphotoelectrochemical studies have been carried outon NilCdS in order to have a better understandingof the photocatalytic properties of NilCdS. Further,no detailed reports are available in literature on thephotoelectrochemical and surface properties of Nil

CdS.

Materials and MethodsIron(m) chloride (FeCI3• 6H20; AR, S D Fine

Chemicals Pvt Ltd), nickel(II) chloride(NiCI2' 6H20; AR, BDH), cobalt(II) chloride(CoCI2' 6H20; AR, Glaxo), cadmium sulphide(CdS, 99.999%; Fluka), sodium sulphide(NazS' 7H20; GR, Merck), potassium chloride (AR,Glaxo), glacial acetic acid (100% aldehyde-free; GR,Merck), sodium hydroxide (AR S D Fine Chemi-cals Pvt Ltd) were used as such. Triply distilled wa-ter was used in all the experiments.

Preparation of NilCdS for XPS analysisNilCdS was prepared by illuminating a sintered

(4 hr at 873 Kin N2) CdS pellet (8 mm diam) su-spended in 2.28 x W-3 MNiClz (20 ml) (acetic acidmedium, pH =4.5, sodium hydroxide solution wasused to adjust the pH) for 20 min, deaerating thesystem and maintaining a nitrogen atmosphere. Af-ter washing the CdS pellet with triply distilled water,it was dried at room temperature in air. The illumi-nated face of NilCdS was analyzed by XPS using anESCA Lab Mk II (VG Scientific Co., UK) at roomtemperature and at a pressure of 5 x W-9 mbar.The XP spectra were taken before and after sputter-ing NilCdS with argon ions at 150 I1Afor 5 min. TheX-ray source used was MgKa with an energy of1253.6 eY. The carbon Is XPS peak at 285.0 eV wasused as the internal standard.

RUFUS et al. EFFECT OF PHOTODEPOSITION OF NICKEL ON PROPERTIES OF CdS 7

Photocatalytic studiesPhotocatalytic decomposition of aqueous sul-

phide was carried out under deaerated condition(deaeration was done by freeze-pump-thaw proce-dure) in a double walled pyrex glass reactor with wa-ter circulation in the outer jacket to prevent heatingduring irradiation. The gas evolved during illumina-tion was analyzed using a Toshniwal gas chromato-graph using thermal conductivity detector molecu-lar sieve-SA column and nitrogen as the carrier gas.The gas evolved was identified as hydrogen. Forsubsequent studies the pressure of hydrogen formedwas monitored using an in-built manometer. In allthe experiments the photocatalyst (100 mg) and0.25 M aqueous solution (20 mI) of Na2S (pH 13) asthe reactant were used. The amount of sulphide wasestimated iodimetrically!'. The dead volume of thereactor was determined and using the pressure ofthe gas inside the reactor, the volume of hydrogen atSTP was calculated. The reaction was carried out atroom temperature (303 K). The reaction mixturewas stirred during irradiation by a magnetic stirrer.Before starting the illumination, the reaction mix-ture was stirred for 1 hr in the dark for equilibration.The metal ions (Fe3 ", Co2+ and Nil +) were addedto the reaction mixture (S2- + CdS) for in situ metal-lization.

Photoelectrochemical studiesThe photoelectrochemical studies were per-

formed in a single compartment PEC cell with thestandard three-electrode configuration (photo-anode: CdS or NilCdS; counter electrode: Pt-foil;reference electrode: saturated calomel electrodeSCE and electrolyte: 0.1 MKCI). The PEC cell wasprovided with a flat quartz window for illuminatingthe photoanode. The photovoltage and photocur-rent of the PEC cell were measured before and afterdeposition of Ni on CdS. All the photoelectrochem-ical and electrochemical studies were made using aWenking POS 73 potentioscan coupled with anOminigraphic 2000 X - Y - t recorder. The currentversus the applied potential (I-V) curves were re-corded before and after deposition of Ni on CdSboth in the dark and under illumination.

Photocorrosion studiesThe photoanodes (CdS and NilCdS) were illumi-

nated for 30 min at a bias of + 1V (vs SCE) in 0.5 MKCI (ref. 12). After irradiation, the amount of Cd2+present in the electrolyte was determined by cyclicvoltammetry .

Light sourceA Philips (India) 1000W tungsten-halogen lamp

was used UJ the light source for both photocatalyticand photoelectrochemical studies.

Results and DiscussionXPS studies on NilCdS

The Ni/CdS was analyzed by XPS to know theextent and oxidation state of photodeposited Ni.The feeble Ni 2pJ/2 XPS peak shows that the amountof photodeposited Ni is very small. The several XPSpeaks in the Ni 2pV2 region and in the 0 1s regionsuggest that in addition to Ni", various types ofNi - °species are also present on the surface of NilCdS. The different oxides of Ni identified are givenin Table 1. Sputtering with argon ions considerablydecreases the intensity of the °is XPS peaks, imply-ing the removal of oxide layers. However, even afterargon ion sputtering, small amount of differenttypes of Ni - 0 species remain on tne surface of NilCdS. Thus the XPS studies on NilCdS surfaceshows that very small quantity of Ni is photodepo-sited on CdS. Further, several kinds of nickel spe-cies like Nio, NiO, NiOads, NiOx (x < 1), Ni203 andNi(OH)2 are present on the surface of NilCdS(Table 1). These oxides could have been formed bythe aerial oxidation of Ni during exposure of NilCdS to atmosphere. On n-type semiconductors likeCdS the reduction reactions occur on the darkside!". But the photodeposition of Ni on the illumi-nated face of CdS indicates that the photoreductionof Ni2+ is mediated by surface states.

Effect of in situ deposition of Fe, Co and Ni on thephotocatalytic activity of CdS

The photocatalytic decomposition of aqueous

Table I-Various species present on Ni/CdS surface

Ni 2p.1l2 XPS peak binding energy in eV Speciesassigned

Before After 5 rnin Reported I]

Ar ' (Ar + sputteringsputtering at 150 mA)

852.4 852.6 852.6,852.4 Ni(ref. 14)

859.0 858.7 858.8P Ni853.5 853.5 853.5 NiOladsl854.9 854.3 854.5 (ref. 4-16) NiO

856.5 856.34 NiO861.4 861.7 861.7' NiO855.7 855.4 855.8 Ni203

86l.2 861.7 861.4' Ni20J

857.0 856.5 856.6 Ni(OHh862.4' Ni(OHh

p-energy loss peak, q-rnultiplet splitting and r-electronshake-up satellite peak.

8 INDIAN J CHEM, SEe. A, JANUARY 1992

sulphide' - In has been used as a test reaction to fol-low the effect of in situ deposition of Fe, Co and Nion the photocatalytic activity of CdS. The reductionpotentials of Fe3+IFe (- 0.278 V), C02+ ICo(-0.519 V) and Ni2+INi (-0.492 V) are negativeindicating that the reduction of these metal ions isthermodynamically unfavourable. As these valueslie below the conduction band level of CdS(- 0.952 V vs SCE), the conduction band electronsof CdS can reduce these metal ions. It is evidentfrom the XPS studies on NilCdS that the amount ofphotodeposited Ni is very small. Even though the re-duction potential of Ni2+INi couple lies below theconduction band edge of CdS, as the difference be-tween them is very small and as this reaction is ther-modynamically unfavourable, the extent of reduc-tion is also small, resulting in the deposition of onlya small quantity of Ni. For the same reasons theamounts of photodeposited Fe and Co are also ex-pected to be low. So in the case of in situ depositionof Fe, Co and Ni on CdS, the enhancement in thephotocatalytic activity of CdS (Table 2) could bemainly due to the deposition of sulphides of the Fe,Co and Ni on CdS, which are formed during the ad-dition of these metal ions to the reaction mixture(S2- + CdS). The sulphides of Fe, Co and Ni whichare known to be active for HDS'9 could serve as thesite for the oxidation of sulphide ions. The nakedCdS and the metal sites (very small amount) couldact as reduction centers, as these are better sinks forthe photoelectrons than the sulphides of Fe, Co andNi.

In the case of NilCdS, the rate of hydrogen evolu-tion observed is almost the same as that in the case

Table 2-Effect of in situ deposition of Fe, Co, Ni, Pel and Rh onthe photocatalytic activity of CdS

[Reactant: 0.25 MNa~S aqueous solution; wt of thephotocatalyst = 100 mg; temp. = 303 K]

Photocatalyst Initial H2 evolutionrate (111at STP )/hr

31

5330o

5158o

52940

255025304150

SINo.

CdS2 CdS + 1.37 wt% Ni ' as NiCI2

CdS+CI- ([Cnsameas that in 2)Ni ' 1.37 wt% as NiCl2 (blank)NiS/CdS (Ni 1.37 wt%)CdS + 1.37 wt% Co'; as CoCl,Co' + 1.37 wt% as CoCI2 (blank)CdS + 1.37 wt% Fe"" as FeCI,Pd/CdS (Pd 1.37 wt%) [3]CdS+ 1.37wt%Pd2> as PdCl,[3]Rh/CdS (Rh 1.37 wt°j.,)[41CdS + 1.37 wt% Rh'" as RhCl,14j

34

567

89

10

II12

of in situ deposition of Ni (Table 2). This shows thatthe enhanced activity observed is largely due to dep-osition of NiS. This further supports the fact that theincreased photocatalytic activity observed in thecase of in situ deposition of Fe, Co and Ni is mainlydue to the deposition of the sulphides of Fe, Co andNi. The sulphides of these metals by themselves donot act as photocatalysts (Table 2). The followingorder of activity is observed in the case of in situ me-tallization of CdS: Co> Ni = Fe. In the case of noblemetals like Pd and Rh3.4 the conduction band elec-trons can be easily transferred to the noble metalsand further the hydrogen evolution overvoltages ofnoble metals are lower than those of the ferromag-netic metals considered here. So noble metals act asbetter reduction centers than the ferromagnetic me-tals like Fe, Co and Ni. This is obvious from thehigher hydrogen evolution rates observed in thecase of Pd/CdS and Rh/CdS (Table 2).

Mechanism of photocatalytic decomposition ofaqueous sulphide

Based on the experimental findings the mechan-ism shown in Scheme 1 is proposed for the photoca-talytic decomposition of aqueous sulphide:S2- +2H+ ?SH- +H+ ... (1)

hv _ ( )MS-M-CdS~ecB MS-M-CdSCdS

+h;B(MS-M-CdS) ... (2)2e~B(MS-M-CdS)+2H+ -H2

+2MS-M-CdS ... (3)

2h;B (MS - M - CdS) + SH - - S + H ++2MS-M-CdS ... (4)

overall reaction,

... (5)

In alkaline medium:S+S2- - S~-In the presence of sulphite:S2- +S02- - S 02- + S2-2 3 2 3

Scheme 1

... (6)

... (7)

The band gap illumination of the photocatalyst(MS - M - CdS, M = Fe, Co, Ni; MS, the corre-sponding metal sulphide). results in the excitation ofelectron from the valence band to the conductionband leading to the formation of valence band holes(Eq. 2). The electrons migrate to the photocatalystl

RUFUS et al. EFFECT OF PHOTODEPOSITION OF NICKEL ON PROPERTIES OF Cd!S 9

aqueous sulphide interface, where the reduction ofprotons takes place (Eq. 3) and the holes react withthe SH - ions (pH 13) leading to the formation ofsulphur (Eq. 4). In alkaline medium, the sulphurformed reacts with the sulphide ions to give disul-phide ions (Eq. 6), which in turn reacts with sulphurto give polysulphide ions (S2-) and they being yel-low in colour reduce the light available for absorp-tion by CdS. Hence the reaction slows down afterprolonged irradiation. The colourless sulphide solu-tion turns into yellow after the reaction, indicatingthe formation of polysulphide ions. The formationof yellow disulphide and hence that of polysulphidecan be avoided by the addition of sulphite ions tothe reaction mixture, which react with the disul-phide ions to give colourless thiosulphate ions(Eq. 7) (see ref. 1). As the in situ metallization andphotocatalytic reaction are carried out in vacuo,therefore is no possibility of.formation of metal ox-ides on the surface of the metallized CdS.

Photoelectrochemical studies on NilCdSillumination of CdS photoanode in NiCl2 solution

for longer periods of time brings about significantchanges in the photoelectrochemical characteristicsof CdS (Table 3 and Fig. 1). Photodeposition of Nion CdS photoanode decreases both the photo vol-tage and the photocurrent of the PEC cell (Table 3)and shifts the photocurrent on-set potential towardsa more positive value. The surface coverage?", E>Ni'increases with the time of deposition (Table 3). Con-ductivity of Ni (metal) is more than that of CdS-asemiconductor. Thus the deposition of Ni over CdSphotoanode decreases the cell resistance Rcell(dark)'

The ~ell(dark) decreases with the increase in theamount of Ni deposited (Table 3). In the presentcase, only the photo anode is beingcoated with nick-el and all other elements of the PEC cell like, theelectrolyte, the counter electrode and the referenceelectrode remain unchanged. Hence the change inRcell(dark) is mainly due to change in the photoanode.

0·85r------------------------- ...•

()'51

-0.851 0·17

t-0.55

-()'51

Appllfll Voltag., V va. SCE

Fig. l-e-Current-voltage curves of the PEC cell with (a) CdS photoanode in the dark, (b) Ni/CdS(eNi = O.10) and (c) CdS during illumination

Table 3- Effect of photodeposition of Ni on CdS photoanode on its photoelectrochemical properties

Time of photo Dark ~cll(darkl Net Photo ~cll(pholo! el'i=des position of =V/I =V/I 1"-1/1"

Nilmin Yoc Isc (k ohm) Yoc Isc (kohm)

1,(mY) (mA) (mY) (mA)

0 140 0.001 140 370 0.154 2.4 0.00I 125 0.001 125 365 0.154 2.4 0.003 92 0.001 92 358 0.149 2.4 0.03t 15 74 0.001 74 338 0.144 L.4 0.07

\ 16" 44 0.001 44 346 0.138 2.5 0.10

Yoc-open circuit voltage; Isc-short circuit current; 8Ni-fraction of the CdS surface covered by Ni [20]; 1(1 and I-photocurrentsobserved before and after deposition of Ni on CdSI respectively; DIICIla kl resistance of PEC cell In the dark and D under iIIu-. . . .. "''ce r, "''cell(phoiolrnmanon; ~-after photodeposition of NI from a 2.75 x 10-3 M solution of NiCI2 for 15 min; Ni was photodeposited from a2.75 x 10-- Msolution.

10 INDIAN J CHEM, SEe. A, JANUARY 1992

With a decrease in Rcell(dark)' normally an increase inthe photocurrent is expected. But the photocurrentdecreases (Table 3). Even though the deposition ofNi increases progressively, the Rcell(photo) remainsconstant. When there is a decrease in the intensity oflight, the number of charge carriers produced alsodecrease and hence both photovoltage and photo-current of the PEC cell decrease. In the present casealso, the decrease in the photocurrent and thephotovoltage is attributed to the geometrical block-ing of the CdS surface by Ni. This is further con-firmed by the constancy of Rcell(photoi (Table 3). If theconduction band electrons of CdS were to be trans-ferred to Ni, the Rcell(photo) would increase with in-crease in eNi (ref. 21). Thus the constancy ofRcell(photo) indicates that not many of the conductionband electrons of CdS are transferred to Ni. So inNi/CdS, Ni does not function as a good reductioncenter due to non-availability of electrons on Ni..During illumination of Ni/CdS, the charge carriers(e - and h + ) produced aid the effective transport ofthe current. Therefore, Rcell(photo) is less thanRcell(dark)'

The positive shift in the photocurrent on-set pot-ential (Fig. 1) indicates that a neutral or a positively

-~OL--L~ __~~~ __~~-1·0 -0.8 -0·8

Voltag., V \/S. seEFig. 2-Cyclic voItammograms taken after photocorrosion (il-lumination at a bias of + 1V vs SCE for 30 min) of (a) CdS and(b) Ni/CdS (0Ni=O.16) [Electrolyte=0.5 M KCI; scanning

rate=50mV/sj

't -200-"c:•~ -300u

-400

-500

-800

100so

o

-100

charged species is being deposited on CdS photo-anode. The XPS studies indicate that Ni? is deposit-ed on CdS. Hence this positive shift in the photocur-rent on-set potential could be attributed to the dep-osition of Ni", The other Ni - 0 species detected byXPS are formed from Ni" only. So initially only Ni?is deposited on CdS and when Ni/CdS is exposed tothe atmosphere, different types of Ni - 0 speciesare formed.

The photocorrosion of CdS (Eq. 8) is one of thedisadvantages of using CdS as a photocatalyst andas a photoanode for the conversion and storage ofsolar energy. As the photo corrosion leads to the for-mation of Cd2 + ,

CdS -- Cd2+ +S ... (8)the extent of Cd2+ dissolved in the electrolyte is adirect measure of the photocorrosion. The concen-tration of Cd2 + present in the electrolyte has beenmonitored by cyclic voltammetry.lt can be seen thatthe amount of Cd2 + present in the electrolyte afterthe photocorrosion of Ni/CdS is 23% less than thatpresent in the case of CdS (Fig. 2). This shows thatthe photodeposition of Ni decreases the photocor-rosion of CdS. The photocurrent at + 1V (vs SCE)(Fig. 1) is mainly due to photocorrosion of CdS (refs20, 22). Hence a decrease in the photocurrent at+ 1V (vs SCE) in the case of Ni/CdS also implies areduction in photocorrosion of CdS. Solvation ofCd2+ ions formed (during the photocorrosion) ben-eath the Ni layer is difficult. Thus decrease in photo-corrosion is largely attributed to geometrical block-ing of CdS surface by Ni. Even though, photodepo-sition of Ni on CdS decreases the photovoltage andphotocurrent of PEC cell, it has other advantageslike stabilization of CdS against photocorrosion.

ReferencesI Buhler N, Meier K & Rebber J-F, J phys Chem. 88 ( 1(84)

3261.2 Reber J-F & Rusek M, J phys Chern, 90 (1986) 824.3 Rufus I B, Ramakrishnan V, Viswanathan B & Kuriacose J

C,lndianJTech,27(I989) 171.4 Rufus I B, Ramakrishnan V, Viswanathan B & Kuriacose J

C, Langmuir, 6 (1990) 565.5 Borgarello E, Kalyanasundaram K, Gratzel M & Pelizzetti

E, Helv ChimActa, 65 (1982) 243.6 Mau A H W, Huang C-B, Kakuta N, Bard A J, Fox M A,

WhiteJ M& WebberSE,J Am chem Soc, 106 (1984)6537.7 Serpone N, Borgarello E, Barbeni M & Pelizzetti E. lnorg

ChimActa, 90 (1984) 191.8 Furlog D N, Grieser F, Hayes D, Hayes R, Sasse W & Wells

D, J phys Chern, 90 (1986) 2388.9 Trocot Y-M & Fendler J H, JAm chem Soc. 106 (1984)

2475.10 Green M & Elofson R M, J chem Soc Chem Commull,

(1985) 830.

RUFUS et al. EFFECT OF PHOTODEPOSITION OF NICKEL ON PROPERTIES OF CdS

11 Vogel A I, A text book of quantitative inorganic analysis(ELBS and Longman, London) 1962, p. 370.

12 Rajeshwar K, Kaneko M, Yamada A & Noufi R N, J physChern, 89 (1985) 806.

13 Kim KS& Winogard N, SuifSci.43 (1974)625.14 Brundle C R & Carley A F, Chem phys Letts, 31 (1975) 423.15 Barr T L,J phys Chern, 82 (1978) 1801.16 Kim K S, Baitinger W E, Amy J W & Winogard N, J electron

Spectrosc Relat Phenom; 5 (1974) 351.17 Bard AJ,J Photochem; 10 (1979) 59.

11

18 Latimer W M, The oxidation states of the elements and theirpotentials in aqueous solutions (Prentice-Hall, New Jersey)1964.

19 Harris S & Chianelli R R, J Catal, 86 (1984) 400.20 Elmorsi M A& Juttner K, Electrochim Acta; 31 (1986) 211.21 Rufus I B, Ramakrishnan V, Viswanathan B & Kuriacose J

C, ProcJndian A cad Sci Chem Sci; 101 (1989) 487.22 Curran J S, Domenech N, Renault N J & Philippe R, J phys

Chem; 89 (1985) 957.