Photoelectrolytic reactivity of Ru pairs in semiconducting RuP3 electrodes

J Electn,anal. Chem, 198 (1986) 269-281

269

Elsevier Sequoia S .A ., Lausanne - Printed in The Netherlands

PHOTOELECTROLYTIC REACTIVITY OF Ru PAIRS IN SEMICONDUCTING RUP, ELECTRODES

It . TRIBUTSCH

Hahn-Mealner-lnsutul fur Xernforsehung Berlin . Bereach Strahlenehernte, D-1600 Berlin 39 /FLAG ) W . HONLE

Max-Planck -Institur fur Fertkmperforschung, Hersenhergsrr . l, D-7(X10 Stuttgart 80 (RR .G.) (Received 22nd January 1985 ; in revised form 29th July 1985)

ABSTRACT

From a theoretical point of view, RuP, should he a promising semiconductor electrode for light-induced oxygen evolution from water . Owing to the presence of RuP 6 octahedra in the crystal lattice and as a result of Ru-Ru bonds, the valence band of RuP, (aE Q =1,67 eV) is derived from Ru d-states . In addition, the existence of Ru pairs (Ru-Ru = 286 .9 and 279 .5 pm) indicates the possibility of cooperative oxidation of water molecules on two adjacent reaction sites . Experimental evidence confirms the photoelectrochemical reactivity of RuP 3 with water but equally establishes the generation of oxidation products of Ru (RuO 2 , RuO4 ) and P (POq - ) which lead to photocorrosion and to a gradual deterioration of the electrode surface . Interestingly, the addition of POa - to the electrolyte enhances photoevolution of oxygen strongly . Possibly because of its less electronegative character and owing to its stronger bonds to oxygen, phosphorus is apparently a less favourable chemical ligand than sulphur in a semiconducting system capable of evolving oxygen under illumination (e.g. RuS 2 ) . The investigation shows that the ligand environment of transition metal centres, including the double layer, in photoelectrochemical catalysis can control the reaction pathway . Some general photoelectrochemical properties of RuP, are described-

INTRODUCTION

Attempts to identify semiconducting transition-metal compounds with valence bands derived from transition metal d-states have proved to be a valuable step towards light-induced evolution of oxygen from water using low energy photons [1-7], The photogeneration of holes in such d-bands leads to coordination bonding of OH - ions or water to the electrode surface which facilitates successive electrontransfer steps without the generation of radical intermediates with unfavourable positive redox potentials . d-Band semiconductors such as MoS 2 . MoSe2 , WS2 and WSe2 undergo a photoreaction with water, but are by themselves oxidized to sulphate and selenate, respectively [1]. PtS2 and RuS2 are sufficiently catalytic to O(i22-0728/86/$03 .50

m 1986 Elsevier Sequoia S.A,

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evolve oxygen from water under illumination while remaining reasonably stable against corrosion [2-7] . Comparative photoelectrochemical and XPS measurements of RuSz , RuSe2 and RuTe2 have clearly shown that catalytic activity for photoevolution of oxygen from water and corrosion stability increase with the purity of the d-valence band [6,7] . The necessary potential bias for oxygen evolution at a high efficiency is more than 0 .5 V higher in the case of PtS2 (AE, =0 .95 cV) in comparison with RuS2 (AF_ 0 =1 .2 eV) . Interestingly, FeS, (pyrite) with an energy gap of AEI, = 0 .9 V does not photooxidize water to oxygen but corrodes to sulphate, although its crystal and electronic structures are practically identical to those of RuS, [8-10] . This result indicates that the chemical nature of the transition metal is of crucial importance for the photocatalytic properties of the transition metal compounds . In the case of ruthenium, the favourable photocatalytic behaviour is apparently partially due to the high oxidation state it can reach . Such a high oxidation state is necessary when water is oxidized to molecular oxygen via individual transition-metal centres . This is the case for RuS 2 , the pyrite structure of which does not provide Ru pairs . The occurrence of ruthenium in a high oxidation state during the process of oxygen evolution is also indicated by the liberation of small amounts of volatile Ru04 from an electrochemically polarized RuS, electrode under illumination [4] . Iron in FeS, cannot attain such a high oxidation state and, as ESCA measurements indicate [8], it passes oxygen on to sulphur which is oxidized to SO„ a process which is not evident in the case of RuSz . The best presently available transition metal compound which is able to utilize low energy photons for the oxidation of water to molecular oxygen under energy gain is RuS, . which is sensitive to IR light (AE0 = 1 .2 eV) . Crystals with high IR sensitivity can be grown from a tellurium melt in the presence of small quantities of iron [4] or from a bismuth melt [11,12] (crystals with lower IR sensitivity, which are also available, have a smaller solar energy conversion efficiency) . Photoinduced evolution starts to become detectable at I V (vs . SHE), clearly below the threshold determined by the thermodynamic potential (E0 =1 .23 V) and the inevitable additional losses caused by the overpotential (0.2-0 .3 V) . The photoelectrochemical reactivity of the new semiconducting ruthenium compound RuP, with water, discussed in this paper, is of interest with respect to two particular points . First, the compound has a valence band derived from ruthenium d-states and is chemically nearly as stable as RuS, (like this compound, it is not attacked by aqua regia) but Ru has P and not S as ligands. Secondly, the compound has Ru-Ru bonds which provide short distances between catalytic transition-metal centres and thus maybe new catalytic pathways for light-induced oxygen evolution . As in the case of n-RuS z , the energy gap of RuP, is too narrow to allow thermodynamically the photoelectrolysis of water without a supporting electrode potential . This new semiconducting material may nevertheless provide a helpful step towards a better understanding and the control of energy-efficient, photoinduced multi-electron transfer reactions .

27 1 MATERIAL PREPARATION AND CRYSTAL STRUCTURE

Preparation Plate-like single crystals of RuP3 with faces up to 10 turn2 in size were prepared by the method of Jolibois [13], using tin as a flux metal . A mixture of Ru, Sri and P in the molar ratio of 1 : 8 :12 was sealed into a quartz ampoule and heated up to 1270 K for 4 days . After cooling down to room temperature, the entire reaction product was treated with dilute HCl at 370 K . RuP3 and the by-product RuP4 (microcrystalline powder) were not attacked by the acid . Chemical analysis revealed that RuP, was free of tin .

Crystal structure The crystal structure was solved by conventional single-crystal methods . RuP, crystallizes triclinically into space group P1 (No . 2) with a = 592.3(3) pm, b = 821 .3(6) pm, c = 586 .6(3) pm, a =112 .35(4)°, /3 =107 .41(4)° , y = 98 .19(5) ° and Z=4 formula units in the unit cell . Each Ru atom is surrounded by a distorted octahedron of P atoms . Two of these octahedra share a common edge, thus leading to formal Ru 2 P70 units . These units are linked into chains via common corners and additional P-P bonds . The asymmetric unit contains three-fold, two-fold and one-fold bonded P atoms with respect to P-P bonds, two of each kind . The P network together with the interpretation as formally zero, 1- and 2-charge of the P atoms implies a formal Ru 3+ cation, thus forming a Ru-Ru single bond .

Fig . 1 . Main structural building units of RuP, . The open circles are P atoms : the dotted ones are Ru atoms . The metal-metal bonds are indicated by bold lines, the Ru-P bonds by thin lines . The numbers within the circles refer to the crystallographic labelling. The estimated standard deviations of the distances are ±0 .3 pm . Further bonds, not included in the figure, are: Pl-P4, Pl-P5, PI-P6 = 219 .4 .. 218 .6, 220 .1 pm ; P3-P3, P3-P4 = 220.8, 218 .0 pm ; P4-PI, P4-P3 = 219 .4, 218 .0 pm ; P5-Pt, P5-P5 - 218 .6, 223 .3 Put .



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This interpretation is supported by the observed diamagnetism and the short metal-metal bonds (cf . Fig . 1) in the compound . The observed Rul-P (228-240 pm) and Ru2-P (234-240 pm) bond distances are quite normal and correspond to the lengths reported previously in RuP 2 [14] and RuP4 [15] . This is also true for d(P-P) . A more detailed description of the structure together with thermal and magnetic properties is given elsewhere [l6] . ELECTRONIC PROPERTIES

Detailed band-structure calculations do not yet exist for this new ruthenium compound but by a combination of several pieces of information and arguments a qualitative energy scheme can be deduced . The diamagnetic properties as well as the results presented in this paper clearly classify RuP P as a semiconductor. Ruthenium exists as Ru 3i in a distorted octahedral environment . This implies three lowered ruthenium orbitals of non-identical energy occupied by five electrons . Semiconducting instead of metallic behaviour is the result of the Ru-Ru bonds, which introduce additional splitting of the d_7, states . The electrons involved in the Ru-Ru metal bonds occupy electronic levels belonging to the highest occupied electronic states in RuP3 . Consequently they contribute to the formation of the upper edge of the valence band of this semiconductor. They mix with the remaining occupied Ru d-states, which also form the lower parts of the RuP, valence band. The electronic states accounting for the electrons transferred from Ru to P are situated still deeper

Ru P 3

P/ P

P /'u-P P P

I

\/P

\

d,r

)-4-

desmona .a

c

a,

,

P /Rlu-P P P

U n

Fig . 2 . Qualitative electronic scheme of RuP 3 indicating the forbidden energy gap, the structure of the valence band and the energetic position of holes supplied for photoelectrochemtcal reactions .

27 3

in the electronic scheme, which is qualitatively depicted in Fig . 2 . An interesting consequence of this energy scheme is that the holes generated by light in the valence band of Rap, would be available for reactions on the electrode surface of Ru states . EXPERIMENTAL

n-Type RuP, crystals with surface dimensions between 1 and 2 mm- were electrically contacted with silver amalgam which had been covered with a conductive silver epoxy layer . The crystals were mounted onto Teflon cylinders which had a copper core and were electrically insulated with an epoxy glue so that only the RuP, crystal had electrical contact with the electrolyte into which the electrode was immersed . A standard photoelectrochemical set-up with potentiostatic control and a three-electrode electrochemical cell were used . The reference was a mercury sulphate electrode. A tungsten halogen lamp was used as the light source . The RuP, electrodes did not undergo chemical treatment before the photoelectrochemical experiments . They are not attacked by aqua regia, and other etching treatments which improve photo-effects have not yet been found . RESULTS

When an electrode potential is applied to a RuP3 electrode in contact with I M H 2 SO4 and its magnitude is varied between negative and positive voltages, only anodic photocurrents are observed (Fig . 3) . This characterizes the electrode as an n-type material with a pronounced limiting photocurrent behaviour at positive electrode potentials . In the potential region of the largest change in photocurrent, a clear hysteresis is observed between the positive and negative sweep directions . Surprisingly, a small anodic photocurrent is maintained until very negative electrode potentials (larger than - 1 .5 V) and it even increases slightly towards the negative direction . The onset of a significant cathodic dark current is not observed in the potential region under study, which indicates that the electrochemical conditions for efficient hydrogen evolution are not reached . This fact together with the occurrence of still significant anodic photocurrents (i ph = 10 µA cm -22 at 130 MW cm - '-) indicates that a small positive space-charge layer is maintained until very negative electrode potentials . This means that the major portion of the applied negative electrode potential must drop in the Helmholtz layer, causing the energy bands of RuP, to rise gradually in the energy scheme relative to the redox levels in the electrolyte . Such behaviour is also reflected in the capacitive behaviour of this ruthenium compound . A Mott-Schottky type dependence is found for a surprisingly large range of negative electrode potentials which permits extrapolation to a flatband potential close to -2 .85 V against a mercury/mercury sulphate electrode, corresponding to a value of -2 .20 V against the SHE (Fig . 4) . Because of the unpinning of energy bands, the practical significance of this Mott-Schottky extrapolation remains questionable. Attempts to determine the carrier concentration by Hall effect measurements failed due to excessive noise of the samples .



274 3 .0

25

0

-1 .0

-0 .5

0

0 .5

1 .0

1 .5

2 .0

electrode potential / V Fig . 3 . Photocurrent-voltage behaviour of RuP 3 measured with the lock-in technique (above) and by do detection while interrupting the incident light at a low frequency (below) . Linear sweep : 10 mV s -1 ; electrolyte : t M H,SO 4 .

Extrapolation of the main photocurrent slope in the spectral region between 600 and 800 nm (Fig . 5) yields a band gap of E 6 = 1 .67 eV. Attempts have been made to determine the character of the optical transition involved . Neither the formula for a direct transition (q)=quantum efficiency -(hv-Er;)1 /2 ) nor that for an indirect

0 .2

15

E U- 1 .0

-2

0

electrode potential/ V Fig . 4. C-V and C-2-V plots for a RuP, electrode in contact with I M H 2 SO 4 . SHE) .

V, .-

-2 .20 V (vs.



275

RuP3

10 RuP3

1



345678910111213 102 (hv-E9~11_ eV) 3/2 by

Fig. 5 . Absorption edge of RuP, (inset) and plot verifying a forbidden direct electron transition .

transition (,p-(hv-Er;)2) could be made to fit the experimental data . However, they fit the formula [17] (1) ¢-(he-Eu)3 .2hy t which is valid for a forbidden direct transition (Fig . 5) . In such a case, quantum selection rules forbid transitions at k = 0, but allow them at k # 0 . The transition probability therefore increases with k 2 . The hysteresis of the photocurrents of RuP3 between -0.5 and +I V indicates reductive and oxidative changes of the electrode surface . Figure 6 (top) shows cyclic photocurrent voltage sweeps in which the negative polarization is increased gradually or a waiting period (2, 4 min) is introduced at -1 .3 V . It is apparent that anodic reoxidation occurs following cathodic reduction . After a prolonged cathodic current flow (the electrode potential was swept between -1 and - 3 V) anodic reoxidation starts at -0 .75 V and the oxidation curve appears to be considerably deformed . These observations indicate that oxidative and reductive processes cannot be neglected at a RuP3 /1 M H,SO4 interface. The dependence of photocurrent on light intensity was investigated on a cathodically pretreated RuP3 electrode (Fig . 7) . A linear relationship was found in the



276

0 -1.5

-1.0

-0 .5 0 .0.5 electrode potential / V

.1.0

.1.5 .

Fig. 6 . Cyclic photocurrent voltage sweeps involving different boundary conditions . Top : 50 mV s - ' . waiting periods of 4, 2 and 0 min at -1 .3 V and 0 min at -0 .3 V. Bottom : 10 mV s -1 after prolonged negative polarization (1 h) while sweeping the electrode potential between -1 and -3 V .

saturation region of the photocurrent (1 .7 V) . At 0 V, where oxidative changes in the electrode surface take place, the efficiency of photocurrent generation clearly decreases above 10 mW cm of incident light energy . Several long-term experiments were carried out to test the photoelectrochemical reaction behaviour of RuP 3 with water . The photoelectrochemical oxidation of water is the principal process of photocurrent generation at a RuP3 /water interface ; however, light-induced oxygen liberation is only one of several reaction paths for

L 10-5

10

102

light intensity / mW cm -2 Fig. 7 . Dependence of photocurrent density on light intensity for a cathodically pretreated Rap, electrode.



277

Ru P3 O d 7 a O 00 d

1 1 1 5 10 15 time / h

Fig. 8. Time dependence of the photocurrent density for RuP,' (1) 1 M H, SO, (0 .7 V) . (2) saturated %ff,PO,-H20 (1 V) . which experimental evidence is available . The photoreactivity of RuP, depends on the composition of the electrolyte . In the presence of 1 M H,SO„ only very little oxygen evolution is detected visually . The RuP, electrode deteriorates gradually with time (Fig. 8, curve 1) . In the electrode surface some intermediate products accumulate which, however, do not include amorphous phosphorus in sufficient quantities to be detected . RuO, and volatile RuO4 are definitely reaction products . They decompose leaving a black deposit of the hydrated dioxide RuO, • H,O as a kind of convection pattern above the illuminated and anodically polarized RuP, electrode . Also crystal bound phosphorus is eventually oxidized to phosphate . The electrode properties for the photoevolution of oxygen do not apparently improve in the presence of 1 M NaOH . However, strong photoevolution of oxygen is observed when 1 M H,PO, is present . This increased oxygen evolution is

E 40 c 20 0 0

1 2 3 electrode potential / V Fig . 9. Current-voltage behaviour of RuP, in contact with a saturated NaH2 PO, solution . Inset : 10-fold magnification of photocurrent onset .

'''78

Fig . 10 (a) Scanning electron micrograph : typical aspect of RuP, single crystals with cylindrical patterns caused by the So melt (ca . 42 x ) . (b) Photograph of the black Ru0 2 H,O convection pattern above the RaP, electrode after passage of approximately 1 W C em - ' (experiment 2 of Fig . 8) .

sustained when RuP, is placed in contact with saturated NaH,PO 3 • H20 (85 g/100 ml) . An I-V dependence of this system is shown in Fig . 9 . However, in this case, side reactions also lead to a deterioration of the electrode surface . Curve 2 of Fig . 8 shows how the photocurrents gradually decrease . After the passage of 100 C cm - ', the photocurrents reduced to approximately 50% . Above the illuminated electrode a dark pattern of Ru0 2 . H,O also appeared in this case (Fig . 10, Ru was identified on the epoxy by X-ray fluorescence analysis) . The presence of H,PO, in the electrolyte gradually alters the originally favourable semiconductor properties of RuP, . The anodic dark currents increase, suggesting the generation of a high concentration of surface states . When negative electrode potentials are applied, large cathodic dark currents are observed . These indicate that the energy bands are less easily unpinned and shifted towards negative potentials . As observed with other d-band semiconductors [1,5,6], the photocurrents of RuP, shift with the redox potential of the electrolyte . The addition of Cl - ions to an aqueous electrolyte increases the photocurrents . Chloride photooxidation to chlorine is kinetically more favourable than photoevolution of oxygen even though its redox potential is more positive . DISCUSSION

RuP1 which provides photogenerated holes in a valence band derived from transition metal d-states and simultaneously (owing to the existence of Ru pairs) on



2,9

reaction sites which are only approximately 280 pm (2 .8 A) apart (Figs . 1 and 2), is an interesting model system for visible light photoelectrolysis of water . It combines two basic photoelectrochemical and catalytic elements which are also present in the manganese reaction centres for oxygen evolution in photosynthesis (holes provided on Mn d-states, Mn-Mn distance of 250 pm (2 .5 A) [18,19]) . In the case of photooxidation of water on two adjacent transition-metal sites with subsequent combination of bound atomic oxygen to molecular oxygen, the transition-metal oxidation state reached would be much lower than that attained in a reaction involving complete oxidation on individual transition-metal sites (one manganese atom would not be able to perform such a complete oxidation) . However, the experimental results obtained with RuP3 indicate that the Ru-Ru pairs in this electrode do not behave ideally, as expected from this simple concept . The liberation of RuO, or RuO, from the anodically polarized photoelectrode shows that high oxidation states of Ru are reached nevertheless, In addition, the marked indications for oxidation and reduction of the RuP, surface show that profound changes occur before liberation of oxygen becomes possible . This implies that the crystalline neighbourhood of Ru in RuP, undergoes a modification which might affect its catalytic properties negatively . The formation of H 3 PO, and RuO, from RuP, is thermodynamically much more favoured than the formation of H,SO, and RuO, from RuS 2 if one compares the ,G values : 4 RuP, + 23 02 (g) + 18 H 2 O -+ 4 RuO, (g) + 12 H 3 PO,

(2)

4(-55 .9) +23(-14 .61)+18(-73 .3) . 4(-64 .71)+12(-313 .5) AG° = -535 .5 kcal/mol RuR,

(-22405 kJ/mol ')

RuS 2 + 5 0 2 (g)+2 H 2 0-+ Ru04 (g)+2 H, S04

(3)

-53 .1 + 5(-14 .6)+2(-73 .3)-+ -64 .71 + 2(-205 .±5) AG° = -203 .5 kcal/mol RuS2

(-851 .4 kJ mol - ' )

On the other hand, RuS2 would also easily corrode if this were not kinetically inhibited . The problem in the case of RuP, is that the kinetic inhibition for the corrosion reaction (as reflected in activation barriers and rate constants) is not sufficient for this strongly exothermic reaction . It is well known from metallic ruthenium-based electrodes (RuO 2 ) that both the electrolyte composition and the preparation procedure for the materials can strongly influence the catalytic properties and the stability against corrosion [20] . A similar opportunity of improving the oxygen photoevolution properties of RuP, by influencing the electrochemical double layer and the electrode surface existed and was verified by adding H,PO, to the electrolyte. The action of PO,- is not yet clear . We do not believe that PO can affect the equilibrium of reaction (2) significantly . On the other hand, PO4 is known to be an efficient complexing agent for transition metals and is used for anti-corrosion treatment of iron and zinc, We consider it likely that the interaction of PO with RuP, affects the molecular and

290

energetic structures of the double layer . In this way the kinetic inhibition of corrosion by way of hole reactions from the d-valence band of RuP 3 is increased . At present it is impossible to give molecular details on the configuration of ligands around Ru in a RuP,/electrolyte interface . We cannot even confirm whether ruthenium pairs still exist in the interface . However, we have shown that suitable chemical conditions can channel photoanodic reactions into oxygen evolution . This opens up new ways for research into the visible light photoelectrolysis of water . The determined energy gap of AEr; = 1 .67 is too small to allow photooxidation of water to oxygen without the assistance of a supporting electrical potential (theoretical minimum : AE,, =2 .2 eV) . There is some ambiguity concerning the determination of the energy gap of a semiconductor by photoelectrochemical measurements but the identification of a "forbidden" direct transition in RuP, makes photochemical sense since d-d transitions in molecular species are equally forbidden . A puzzling property of the new material is its ability to support, in certain electrolytes, a small anodic photocurrent (- 10 ItA cm') up to high negative potentials while suppressing higher cathodic dark currents (Fig . 3) . Capacity measurements indicate an apparent flatband-potential situation only at -2 .20 V (vs . SHE) which means that upon applying an electrode potential the energy bands of the material can be shifted up and down in a remarkable way with respect to the redox levels in the electrolyte . This behaviour is similar to that of p- and n-RuS 2 . For this electrode material, capacity measurements [21,22] and electroreflection studies [23] have shown that the flatband potential shifts in a systematic way with the redox potential of the electrolyte . As long as the charging of surface states, which are present in high concentration, is not limited and is controlled by an ongoing redox reaction, the applied electrode potential drops mainly in the Helmholtz layer . When the electrode potential, at which electron transfer from the redox system occurs, is reached, the applied electrode potential starts to bend the energy bands while it increases the potential drop in the space-charge layer . While all the d-band semiconductors studied up to now are susceptible to some shifting of the energy bands, there must be a special reason for the extreme unpinning of energy bands in RuP3 . We believe that a high overpotential for hydrogen evolution is involved . These results show that there is still much to be learned about the photoelectrochemistry of transition metal compounds in general and about RuP3 in particular . We feel that these experiments are in line with already existing information [1-7] on other photoelectrodes based on d-band semiconductors, which equally provide evidence for interfacial coordination photoelectrochemistry which gives access to kinetically demanding mechanisms (e .g. photoeleetrolysis of water) . REFERENCES 1 2 3 4

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