- Email: [email protected]
Photoelectrochemical study of electrochemically formed semiconducting yttrium hydride (YH3−x)
Electrochimica Acta 44 (1999) 4051±4059
Photoelectrochemical study of electrochemically formed semiconducting yttrium hydride (YH3ÿx) F. Di Quarto*, M.C. Romano, S. Piazza, C. Sunseri Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, UniversitaÁ di Palermo, Viale delle Scienze, 90128 Palermo, Italy Received 4 January 1999; received in revised form 30 March 1999
Abstract The ®rst photoelectrochemical study of semiconducting YH3ÿx ®lms formed by etching bulk Y metal in 0.5 M H2SO4 solution is reported. The formation of semiconducting hydride having an indirect optical band gap, Eopt g , of about 2.35 eV is con®rmed by in situ photocurrent spectroscopy. The photoelectrochemical behaviour of such a phase was investigated both in alkaline and in acidic solutions. The ¯at band potential was estimated to be Ufb= ÿ 1.25 V/NHE, independent of pH. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction In recent papers the possible technological use of yttrium hydrides as electrochromic materials was suggested by dierent authors [1±4]. It was shown that pronounced changes in the optical properties of YHn occur as a function of the hydrogen content on going from YH2 to YH2.86. From a more fundamental point of view the optical changes were associated with a metal-to-insulator transition, which has been reported for other trivalent rare earth elements forming hydrides [5]. In Refs. [1,4] it was suggested that under high H2 pressures (several GPa) a stoichiometric YH3 phase having a band gap around 2 eV (1.8 < Eg < 2.3 eV) is formed, at variance with recent band structure calculations, suggesting that YH3 has semimetallic behaviour [6] or it is an excitonic insulator [7] having a low band gap (Eg=0.35 eV). In Refs. [1±4] all the experiments were performed with evaporated yttrium ®lms covered by palladium caps of dierent thickness (5±50 nm) in order to avoid any metal oxidation by the environment and to pro-
vide (electro)-catalytic activity for atomic hydrogen insertion [3]. In this work the preliminary results of a more extensive study of the electrochemical behaviour of yttrium metal will be reported; the investigation has been addressed mainly to the photoelectrochemical characterization of thin layers of YH3ÿx formed on the bulk metal during chemical etching in 0.5 M sulphuric acid solution. Some information on the nature of the surface layers formed on Y metal in dierent conditions will be presented in order to support the identi®cation of the hydride phase by means of photocurrent spectroscopy (PCS). The in situ determination of the optical band gap as well as the ¯at band potential provides the energetics of the YH3ÿx/aqueous electrolyte junction. The photoelectrochemical study of the yttrium hydrides, YH3ÿx, with a high hydrogen content (x R 0.14) con®rms the semiconducting nature of the defective trihydride and gives some indications on the stability of such a compound in dierent solutions. 2. Experimental
* Corresponding author. Fax: +39-91-656-7280. E-mail address: [email protected] (F. Di Quarto)
An yttrium rod of purity 99.9% (Goodfellow Metals, Cambridge), having an external diameter of
0013-4686/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 9 9 ) 0 0 1 6 6 - 8
4052
F. Di Quarto et al. / Electrochimica Acta 44 (1999) 4051±4059
6.35 mm, was sealed into a te¯on cylinder using a two component epoxy resin (Torr-seal, Varian Associates). The surface was polished mechanically with abrasive papers, rinsed twice (5 min each time) ultrasonically in cold distilled water and then etched chemically in 0.5 M H2SO4 solution for dierent times. In some cases, after the mechanical treatment, the electrode was scraped by a blade and then immersed directly into the electrolytic solution with or without etching. Unless otherwise stated, the results reported below pertain to electrodes usually etched in 0.5 M H2SO4 for 30 s. After etching, the electrodes were rinsed with distilled water and immersed in solutions with dierent pH values (2 < pH < 15), where the photoelectrochemical characterization was performed. In some experiments, after etching the electrodes for 30 s in 0.5 M H2SO4 electrolyte, known amounts of NaOH solution were added to the etching solution in order to shift its pH to the ®nal value at which the photoelectrochemical experiments were carried out. The experimental setup used for the photoelectrochemical study is that reported in our previous paper [8]: it consists of a 150 W UV-VIS xenon lamp (Spectral Energy) coupled with a monochromator (Bausch&Lomb), which allows monochromatic irradiation of the specimen surface through the electrochemical cell quartz windows. A two-phase lock-in ampli®er (EG&G) was used in connection with a mechanical chopper (frequency: 10 Hz) in order to separate the photocurrent from the total current circulating in the cell due to the potentiostatic control. Photocurrent spectra were corrected for the photon ¯ux emitted at each wavelength by the light source (detected by a calibrated thermopile) and normalized to the number of photons measured at 400 nm (about 1016 photons cmÿ2 sÿ1), thus giving a relative quantum yield in photocurrent units. Before normalization, raw spectra were smoothed according a standard computer procedure, where necessary (high dark- to photocurrent ratio or high noise levels). Electrode potentials were measured with respect to mercurous sulphate or Hg/HgO reference electrodes, but in the following they will be referred to the normal hydrogen reference electrode (NHE). 3. Experimental results During the etching process of yttrium in 0.5 M H2SO4 a strong hydrogen evolution was observed over the entire metal surface, while the open circuit potential, Uoc, was ranging between ÿ1.35 and ÿ1.25 V(NHE). The result obtained in 0.005 M H2SO4 is shown in Fig.1 (curve a), where we report Uoc vs. time curve for an unetched yttrium metal electrode polished mechanically. A rapid increase in the corrosion poten-
Fig. 1. Open circuit potential recorded during the free immersion in aerated 0.005 M H2SO4 of unetched yttrium metal (curve a) and in 0.1 M NaOH of a previously etched Y sample (curve b).
tial, starting from negative values (ÿ1.44 V), was observed followed by a levelling to a quasi-steady state value of about ÿ1.20 V. Electrodes etched in 0.5 M sulphuric acid solution for 30 s and then immersed in dierent solutions (0.005 M H2SO4, borate buer at pH=9.2 or 0.01±1 M NaOH solutions) displayed an initial Uoc value of about ÿ1.2 V slowly levelling o at a value around ÿ1.0 2 0.1 V after about 240 s, regardless of the solution pH (Fig. 1, curve b). In Fig. 2 we report the total current vs. time curves under potentiostatic control, in dark conditions and under illumination with light of dierent photon energies, for a Y electrode etched 30 s in 0.5 M H2SO4 and then immersed in 1 M NaOH solution. A cathodic dark current of about 2.2 mA was circulating during the experiment. The ®gure shows clearly the presence of a quite large anodic photocurrent owing to the presence of a n-type semiconducting (SC) layer formed by etching the Y metal in 0.5 M H2SO4. Similar results were obtained in 0.005 M H2SO4 acid solution, but larger and more stable photocurrents were usually measured in alkaline solutions, probably due to the higher stability of the semiconducting layer (see below). In Fig. 3 we report the photocurrent spectrum (corrected for the photon emission of the monochromatorlamp system, see Experimental section) recorded at ÿ1.0 V(NHE) in 0.005 M sulphuric acid on an Y electrode, previously etched in 0.5 M H2SO4 solution. As evidenced in Fig. 3 the photocurrent onset is at around 500 nm; the photocurrent intensity reaches a maximum
F. Di Quarto et al. / Electrochimica Acta 44 (1999) 4051±4059
4053
Fig. 2. Total current in the dark and under illumination with light of dierent wavelengths recorded upon manual chopping on an etched Y sample polarized at ÿ1.02 V(NHE) in 1 M NaOH electrolyte.
near 400 nm and then falls to negligible values at high photon energies (l 1 300 nm in acidic solutions), probably due to a surface recombination process of the photocarriers generated under strongly absorbed light [9]. In fact a light absorption coecient in the order of 105 cmÿ1 was reported for defective trihydride ®lms at
a photon energy of 2.8 eV [1]. However, in principle, it cannot be ruled out that a change of re¯ectivity, R(l ), at the electrode/electrolyte interface, with changing wavelength could account for the drop in the quantum yield at the shortest wavelengths. The determination of the optical band gap of the
Fig. 3. . Photocurrent spectrum recorded on an Y sample, etched in 0.5 M H2SO4 for 30 s, and then polarized at ÿ1.0 V(NHE) in 0.005 M H2SO4 solution. Inset a: determination of the optical band gap of the surface layer according to Eq. (1). Inset b: determination of the optical band gap of a surface layer on an Y sample, etched in 0.5 M H2SO4 for 30 s, and then polarized at ÿ1.0 V(NHE) in Na2SO4 solution with pH adjusted to 12.3 (see text).
4054
F. Di Quarto et al. / Electrochimica Acta 44 (1999) 4051±4059
Fig. 4. Photocurrent vs. electrode potential curves recorded on etched Y samples in dierent solutions upon irradiation with a He±Cd laser beam (l=442 nm). (a) 0.005 M H2SO4; (b) 0.1 M NaOH.
®lms is reported in the inset a of Fig.3. The optical band gap, Eopt g , was obtained from the photocurrent action spectrum by using the following relationship [8] Iph hnn=2 const Eopt g ÿ hn,
1
where hn is the photon energy, while n=1 for indirect and n=4 for direct optical transitions. A good ®tting was obtained in the hypothesis of indirect (nondirect for amorphous materials) optical transitions and the value derived for Eopt was equal to g about 2.3520.05 eV. Similar spectra were recorded for Y electrodes etched in 0.5 M H2SO4 solution and then immersed in the other solutions exploited. The value of optical gap obtained in Fig. 3 for the SC ®lm agrees very well with that reported by Huiberts et al. [4] for the hydride phase g-YH2.86 and it is the experimental support to our hypothesis that a g-YH3ÿx hydride phase is formed on the surface of Y metal after prolonged etching in 0.5 M H2SO4 solution. A partial support to the possible copresence of b-YH2 came from XRD data analysis performed on thin (0.1 mm thick) Y foils etched in the same way. In this case the presence of bYH2 re¯ections at 2y=298 was evidenced in X-ray spectra, whilst no evidence of g-YH3ÿx was obtained from such experiments [2], probably due to the small amount of this phase. We have to mention that the etching of Y metal in 0.005 M H2SO4 for a longer time (600 s) produced a photoactive phase having spectra similiar to that reported in Fig. 3, but with photocurrent intensities, at equal electrode potential, much lower than those observed after etching in 0.5 M H2SO4. This ®nding
suggests a more ecient formation process of the SChydride in more acidic solutions. In the inset b of Fig. 3 we report the determination of the optical gap for an Y electrode polarized at an electrode potential, Ue= ÿ 1.0 V(NHE) in a sodium sulphate solution having pH adjusted to 12.3. The electrode, initially etched in 0.5 M H2SO4 solution for 30 s, has been kept in solution during the pH change performed by adding to the initial 0.5 M H2SO4 etching solution a known amount of 1 M NaOH solution sucient to reach the ®nal pH value. Although the shape of the spectrum and the value of optical gap are both very similar to that recorded in acidic solution, larger and more stable photocurrent intensities were measured at corresponding band bending in alkaline solutions. Moreover, a higher quantum yield in the short wavelength region (l < 300 nm) of the photocurrent spectrum was sometime observed in these last solutions. Although the response at high photon energy and the photocurrent intensity depend to some extent on the pH of the electrolytic solution, nevertheless the collection eciency of the injected photocarriers is always decreasing in the region of the highest photon energies. It is worth noting that both the optical band gap value and the shape of the spectra did not change (apart from the occasional tailing in the shortest wavelength region discussed above) if, after the acidic etching, the electrodes were immersed in alkaline solutions (pH>9), provided that the measurements were performed at suciently negative potentials, close to the initial Uoc values reported in Fig. 1. Moreover, we mention that in 0.1 M NaOH solution no appreciable changes were observed in the shape of photocurrent
F. Di Quarto et al. / Electrochimica Acta 44 (1999) 4051±4059
Fig. 5. Total current (a) and photocurrent recorded at l=300 nm (b) vs. potential curves during anodization of a scraped Y sample in 1 M NaOH solution. Scan rate: 5 mV/s.
4055
spectrum by leaving the hydride electrode at the open circuit potential for more than twelve hours, when open circuit potentials as high as ÿ0.4 V(NHE) were sometimes recorded. We have to say that in this case, although the Eopt value was nearly coincident with g that obtained initially, much lower photocurrents were measured at this potential. We like to stress this ®nding because in alkaline solutions the formation of yttrium hydroxide starting from metallic yttrium is thermodynamically favoured even at very negative potentials [10] (see also below). We took advantage of the relatively good stability of the YH3ÿx layers, in alkaline (9 < pH < 15) as well as in acidic (pH=2.3) solutions, for investigating in a large range of pH values the dependence, if any, of the onset photocurrent potential on the solution pH. Fig. 4(a and b) show the Iph vs. Ue plots (photocharacteristics) at constant wavelength (l=442 nm) in 0.005 M H2SO4 and 0.1 M NaOH solutions, respectively. The photocurrent intensity increases on going towards less negative electrode potentials. A negligible hysteresis was observed in such plots provided that the range of electrode potentials spanned was quite small around the onset photocurrent potential, V . Large hysteresis was observed in the photocharacteristics when a wider potential range was exploited (see Fig. 4(a)). From these curves a quite constant onset photo-
Fig. 6. Photocurrent spectrum recorded for the sample of Fig. 5 at the ®nal scan potential (+0.6 V/NHE). Inset: determination of the optical band gap of the surface layer according to Eq. (1).
4056
F. Di Quarto et al. / Electrochimica Acta 44 (1999) 4051±4059
Fig. 7. Total current vs. potential curve during anodization in 1 M NaOH solution of an Y sample, previously etched in 0.5 M H2SO4 for 30 s. Scan rate: 1 mV/s.
current potential value was derived (V= ÿ 1.25 2 0.1 V/NHE) independent of the solution pH. This ®nding rules out the presence of an acid-base equilibrium at the surface of the n-type SC-hydride. Such an equilibrium is responsible for the ¯at band potential dependence on the solution pH usually observed with semiconducting oxides [11]. In order to show the possible in¯uence of the hydride phase on the nature of the ®lm grown upon anodization of Y metal, we performed two experiments on etched and unetched (freshly scraped surface) yttrium specimens in 1 M NaOH solution under analogous conditions. Fig. 5(a) displays the anodization curve of a scraped Y sample, obtained potentiodynamically at low scan rate (5 mV/s) starting from the open circuit potential (about ÿ0.7 V/NHE) up to a maximum potential equal to +1 V and then back to +0.6 V. It is interesting to note that since the very initial instants of the anodization process an anodic photocurrent was recorded (see Fig. 5(b)) under illumination with monochromatic light (l=300 nm). The photocurrent spectrum, reported in Fig. 6, was recorded at the ®nal scan potential (+0.6 V); it is completely dierent with respect to the previous spectra: in the hypothesis of indirect optical transitions an Eopt g value equal to 3.0 2 0.05 eV was derived (see inset of Fig. 6). Another possible extrapolation at higher energy (around 4 eV) will be discussed in a future paper. The Eopt value obtained is much lower than g that reported for Y2O3 (5.8 eV) [12], but quite in agreement with the band gap expected for a hydroxide or an oxy-hydroxide, according to our recent suggestions [13]. In a separate experiment we recorded the photocharacteristic at constant thickness for the same ®lm of Fig. 6, showing an onset photocurrent potential, V , of about ÿ0.65 V(NHE). In Fig. 7 we report the anodization curve (1 mV/s scan rate) of an Y metal electrode, initially etched for 30 s in 0.5 M H2SO4 and then immersed in 1 M NaOH, obtained starting from ÿ1.0 V(NHE) up to
Fig. 8. Determination of the optical band gap for the surface layer of Fig. 7 after anodic polarization at +1.5 V/NHE.
+0.4 V. The photocurrent spectrum recorded at Ue=+0.4 V shows the presence of a very long tail up to wavelengths longer than 700 nm, at variance with the previous result. From this spectrum, it appears evident the presence of a second optical threshold at much lower photon energies (around 1.1 eV), partially masking the optical gap at higher photon energies (about 2.4 eV), which is still present. We have to mention that in these experiments, if the electrode potential was shifted potentiodynamically starting from Ue=+0.4 V towards ÿ1 V, the photocurrent spectrum looses the absorption threshold at low energy and shows only the usual optical gap around 2.35 eV; however, the measured photocurrent intensities are lower than the previous cases. On the other hand by polarizing the etched electrode at much more anodic potentials (Ue r+1.5 V) a second optical threshold appears at around 3.0 eV, together with the initial threshold at around 2.35 eV (see Fig. 8). This last threshold disappears completely after very long polarization times (more than 12 hours) by leaving a photocurrent spectrum similar to that of Fig. 6 with the only threshold at around 3.0 eV. 4. Discussion The Pourbaix diagram of Y [10] reports a very negative equilibrium potential for the reaction of yttrium metal dissolution Y Y3 3eÿ ;
E8=V ÿ2:372 0:0197logY3
2
which, in absence of formation of any passivating oxide at low pH values, accounts for the strong hydro-
F. Di Quarto et al. / Electrochimica Acta 44 (1999) 4051±4059
gen evolution observed during the etching process in H2SO4 solution. However, although the Pourbaix diagram reports in acidic solutions only the Y/Y3+ equilibrium, the simultaneous formation of a hydride phase must be taken into account due to the large negative enthalpy of formation of YH2 (DHf = ÿ 228 kJ/mol H2 [14,15]). The possible formation of yttrium trihydride is also favoured thermodynamically due to the negative enthalpy of the transformation reaction b ÿ YH2 1=2H2 g ÿ YH3
3
with DHb 4g (®lms)=ÿ 38 kJ/mol [1]. The formation of a hydride phase was argued by the rapid change of colour of the metallic surface, from grey to a deep blue/black colour, which is typical of YH2 and YH3 phases, observed at the end of the etching process, before beginning the photoelectrochemical investigation of the electrodes. The presence of YH2 on Y foils etched in 0.5 M H2SO4 is supported also by the presence of the (111) re¯ections in the X-ray patterns, corresponding to the main peak of the b-YH2 phase [2]. The photocurrent spectrum recorded in acidic solution (Fig. 3), where the formation of any oxy-hydroxide phase is excluded according to the Pourbaix diagram, and the value of optical gap, analogous to that reported for g-YH2.86 by Huiberts et al. [4], strongly support the formation of a g-YH3ÿx (0 R x R 0.14) hydride phase on the surface of Y metal during the 30 s etching in 0.5 M H2SO4. As for the electronic structure of the n-type semiconducting YH3ÿx phase our results suggest the existence of indirect optical transitions at photon energies around 2.35 eV. In order to rationalize this experimental ®nding, we suggest the formation, during the etching process, of an yttrium hydride (having a variable hydrogen content) simultaneous to the electrochemical corrosion of Y occurring via local cells. According to this mechanism we assume that, apart from the hydrogen evolution reaction coupled to the anodic reaction (2), the following electrochemical reaction occurs in the cathodic areas of the corroding metal Y 3 ÿ xH 3 ÿ xeÿ YH3ÿx
4
Yttrium dihydride (x=1) as well as defective yttrium trihydride can be formed spontaneously owing to the large value of enthalpy of formation of YH2 and by taking into account the existence of a positive thermodynamic driving force for the transformation of dihydride to defective trihydride in presence of the large hydrogen evolution observed during the etching process. By assuming, according to the literature [3,14], a value of DGf = ÿ 189 kJ/mol for YH2, the equilibrium potentials for the possible reactions represented by Eq. (4) have been estimated to be
E eq 0:98 ÿ 0:059pH
4057
for x 1
5a
for x 0
5b
and E eq 0:72 ÿ 0:059pH
The equilibrium potential reported in Eq. (5b) has been estimated by assuming an enthalpy of formation DHf (YH3)= ÿ 266 kJ/mol and, as a ®rst approximation, an entropy of formation DSf (YH3)= ÿ 196 J/ K mol. This last value was obtained by using the value of entropy of formation, DSf = ÿ 130.8 J/K mol H2 previously used for YH2 [3] and neglecting the change of con®gurational contribution to DSf . According to the literature data, a metallic behaviour is reported for YH2 [1] so that no photocurrent response can be expected if the formation of yttrium dihydride would be the ®nal step of the 30 s long chemical etching process of yttrium metal in 0.5 M H2SO4 solution. The experimental results show that both in sulphuric acid as well as in NaOH solutions a semiconducting phase having the same optical band gap (2.35 2 0.05 eV) is always present on the yttrium surface soon after immersion of etched Y electrodes. According to this experimental ®nding and in agreement with Eq. (4), in the followings we will assume that semiconducting YH3ÿx (0 < x < 0.14) is formed on the etched Y metal. Under open circuit conditions the presence of this SC-hydride phase was observed in acidic solution (0.005 M H2SO4) for electrode potentials values more negative than ÿ0.8 V(NHE), whilst in alkaline solutions it was still observed at more anodic potentials (up to ÿ0.4 V in 0.1 M NaOH solution). A rationale for this ®nding could be traced out to the dierent stability of YH3ÿx in acidic or alkaline solutions, as discussed below. In fact in acidic solutions the dissolution of YH3ÿx should occur according to the following reaction:
YH3ÿx Y 3 3 ÿ xH 6 ÿ x eÿ
6a
Assuming the standard free energy of formation reported in [10] for Y3+ ions, and the DGf (YH3) estimated above, for x=0 the equilibrium potential of reaction (6a) is given by: E eq ÿ0:827 ÿ 0:0295 pH 0:01 logY3
6b
For [Y3+]=10ÿ6, Eq. (6b) implies that reaction (6a) is thermodynamically favoured for potential values more anodic than about ÿ0.9 V at pH=2.3. In alkaline solutions the following reaction of formation of yttrium hydroxide is allowed YH 3ÿx 3H2 O Y OH 3 6 ÿ x H 6 ÿ x eÿ
7a
4058
F. Di Quarto et al. / Electrochimica Acta 44 (1999) 4051±4059
for potential values more anodic than its equilibrium potential, calculated (for x=0) in a similar way as for Eq. (6b) E eq ÿ0:632 ÿ 0:059 pH
7b
The formation of a very thin ®lm of yttrium hydroxide, which is transparent at photons energies around 2.3 eV (where the SC-hydride phase begins to absorb), could explain the higher stability of the YH3ÿx ®lms in alkaline solution and it could account for the higher quantum yield in the short wavelength region sometimes observed in alkaline solutions. The formation of an anhydrous Y2O3 oxide has been excluded by taking into account that a very large band gap value (05.80 eV) is reported on literature for this oxide [12], at variance with the results of Figs. 6 and 8 showing an absorption threshold at around 3 eV for the phase formed by anodic polarization of Y metal. Indeed, an optical band gap lower than 5.8 eV is expected for yttrium hydroxide according to our experimental ®ndings on dierent oxides and hydroxides and on the basis of a correlation between oxide band gap and dierence of electronegativity of the constituents, recently proposed by the present authors [13]. The formation of Y(OH)3 on unetched Y metal in alkaline solution is in agreement with the Pourbaix diagram of yttrium and it is supported by the experimental ®ndings reported in Figs. 5 and 6 showing the presence on the metal surface of a n-type semiconducting or insulating ®lm having a band gap of 3.0 2 0.05 eV, after anodizing up to +1.0 V in 1 M NaOH solution. A more complex picture comes out from the analysis of the experimental data obtained on Y electrodes anodized in alkaline solutions up to more anodic potentials. These results will be discussed in a forthcoming paper. A support in favour of the existence of the electrochemical equilibrium represented by Eq. (7a) comes from the result of Fig. 8, showing that a surface layer having a band gap value around 3.0 eV was also obtained by polarizing at high anodic potential (+1.5 V) an yttrium specimen initially etched for 30 s in 0.5 M H2SO4. Moreover, the very long polarization time necessary for the disappearance of the SC-hydride phase at this potential suggests that the kinetics of oxidation of the YH3ÿx phase to hydroxide or oxyhydroxide is sluggish: this fact can account for the experimental behaviour of hydride ®lms observed under open circuit conditions in alkaline solutions. By taking into account the presence of the new electrochemical equilibria discussed above, the Pourbaix diagram of Y metal should appear dierent than that reported in ref. [10] and much more similar to those relative to alkaline earth metals, like Ca, Sr and Ba. Finally we like to stress that in Fig. 5(b) an anodic
Fig. 9. Energetics at the metal/yttrium hydride/aqueous electrolyte junction at pH=0. For the derivation of the energy levels see text.
photocurrent was recorded, at l=300 nm, since the beginning of the anodization. From the photocurrent vs. potential curves an onset photocurrent potential of about ÿ0.65 V(NHE) was obtained; this value is much more anodic than that derived in Fig. 4(a and b) for the hydride phase, con®rming the dierent nature of the layers grown on etched or anodized Y metal. The location of the energy levels of the SC-hydride/ electrolyte junction with respect to vacuum was obtained according to the relations Ec ÿ eE8NHE ÿ e Ufb DEf ; Ev Ec ÿ Eg ,
8
where Ec and Ev are the conduction band and the valence band edges of the SC, respectively, DEf is the distance in energy between the Fermi level, Ef = ÿ e Ufb, and the conduction band edge, and E8NHE= ÿ 4.45 eV is the energy level of the normal hydrogen electrode with respect to the vacuum [16]. According to the previous relationships and to the experimental ®ndings, the energy band model of the YH3ÿx (with x R 0.14) semiconducting ®lm/electrolyte junction is reported in Fig. 9. The energy levels of the junction were located under the following assumptions: a) the equilibrium Fermi level of the junction has been located by assuming Ufb coincident with the onset photocurrent potential reported above; b) in absence of further information on the donor concentration and the eective density of states at the conduction band edge, Ec, this last level has been located 0.1 eV higher than Ef . The ®rst assumption is supported by the good reproducibility of the onset photocurrent potentials in the dierent solutions investigated, the second assumption is quite compatible with a density of ionized donors around 1019 cmÿ3 [1].
F. Di Quarto et al. / Electrochimica Acta 44 (1999) 4051±4059
It is noteworthy that by assuming in Fig. 9 an electronic equilibrium at the metal/semiconductor interface, in ¯at band conditions it comes out a value of ÿ3.20 eV for the yttrium metal Fermi level, in very good agreement with the work function (3.1 eV) reported in the literature for this element [17]. According to the scheme of Fig. 9 it is clear that by illuminating the SC-hydride/electrolyte junction with photons of energy hn>Eopt the possible photoelectrog chemical reactions consist in the photooxidation processes of the semiconducting hydride by the photoinjected holes YH3ÿx 6 ÿ x h Y3 sol 3 ÿ x H
H2 O Y OH 3 6 ÿ x H
ladium metal caps has been reported at variance with some previous statements [3]. This preliminary photoelectrochemical study is able also to account for the observed higher stability of the semiconducting hydride in alkaline solutions; such a ®nding could be useful for the use of this hydride in liquid switchable electrochromic cells. The results of this study suggest that the Pourbaix diagram of yttrium should be similar to those of the earth metals group once the possible electrochemical equilibria involving the hydride phases are taken into account.
9 References
in acidic solutions and YH3ÿx 6 ÿ x h 3
4059
10
in neutral and alkaline solutions. Eqs. (9) and (10), which are analogous to Eqs. (6a) and (7a), assuming the hole as reagent, represent the photodissolution reactions of the hydride, whose decomposition Fermi energies (coincident with the decomposition redox potentials of Eqs. (6b) and (7b)) lie well above the valence band edge of SC-YH3ÿx, so that under illumination this phase is always unstable at any pH value. This implies an accelerated dissolution of the trihydride phase under illumination in acidic solution, whilst in basic electrolyte the external hydroxide layer protects partially the hydride against photodissolution. 5. Conclusions The results reported above and the comparison with the literature data suggest the formation on the surface of Y metal etched for 30 s in 0.5 M H2SO4 solution of a SC defective yttrium trihydride (YH3ÿx) having a large optical band gap (2.35 eV). The composition of the SC-hydride should be quite close to YH2.86, which is the phase formed on Y metal under 1 atm hydrogen pressure. The energetic picture derived for the YH3ÿx/electrolyte junction agrees both with the photoemission results on the rare earth trihydrides (suggesting the formation of an hydrogen band located 2.35 eV below the conduction band of yttrium metal) and with the work function value of this element. The possibility to investigate Y hydrides in electrolytic solutions without pal-
[1] J.N. Huiberts, R. Griessen, J.H. Rector, R.J. Wijngaarden, J.P. Dekker, D.G. de Groot, N.J. Koeman, Nature 380 (1996) 231. [2] J.N. Huiberts, J.H. Rector, R.J. Wijngaarden, S. Jetten, D. de Groot, B. Dam, N.J. Koeman, R. Griessen, B. Hjorvasson, S. Olafsson, Y.S. Cho, J. Alloys Comp. 239 (1996) 158. [3] P.H.L. Notten, M. Kremers, R. Griessen, J. Electrochem. Soc. 143 (1996) 3348. [4] R. Griessen, J.N. Huiberts, M. Kremers, A.T.M. van Gogh, N.J. Koeman, J.P. Dekker, P.H.L. Notten, J. Alloys Comp. 253-254 (1997) 44. [5] G.G. Libowitz, Ber. Bunsenges. Phys. Chem. 76 (1972) 837. [6] J.P. Dekker, J. van Ek, A. Lodder and, J.H. Huiberts, J. Phys. Condens. Matter 5 (1993) 4805. [7] Y. Wang, M.Y. Chou, Phys. Rev. Lett. 71 (1993) 1226. [8] F. Di Quarto, S. Piazza, C. Sunseri, Mater. Sci. Forum 192±194 (1995) 633. [9] H.J. Hovel, Solar cells, in: R.K. Willardson, A.C. Beer (Eds.), Semiconductors and Semimetals, 11, Academic Press, New York, 1975 (Chap. 2). [10] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon Press, 1966. [11] H. Gerischer, Electrochim. Acta 34 (1989) 1005. [12] W.J. Tropf, M.E. Thomas, in: D. Palik (Ed.), Handbook of Optical Constants of Solids II, Academic Press, New York, 1991, p. 1079. [13] F. Di Quarto, M.C. Romano, S. Piazza, C. Sunseri, J. Phys. Chem. B 101 (1997) 2519. [14] R. Griessen, T. Riesterer, Topics in Applied Physics, in: L. Schlapbach (Ed.), Hydrogen in Intermetallic Compounds, 63, Springer-Verlag, Berlin, 1988, p. 219. [15] G. Sandrock, in: P.D. Bennett, T. Sakai (Eds.), Hydrogen and Metal Hydride Batteries, Proc., 94-27, The Electrochem. Soc. Inc, NJ, 1994, p. 1. [16] H. Reiss, A. Heller, J. Phys. Chem. 89 (1985) 4207. [17] Anon, Handbook of Physics and Chemistry, CRC Press, Boca Raton, 1995.
Photoelectrochemical study of electrochemically formed semiconducting yttrium hydride (YH3ÿx) F. Di Quarto*, M.C. Romano, S. Piazza, C. Sunseri Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, UniversitaÁ di Palermo, Viale delle Scienze, 90128 Palermo, Italy Received 4 January 1999; received in revised form 30 March 1999
Abstract The ®rst photoelectrochemical study of semiconducting YH3ÿx ®lms formed by etching bulk Y metal in 0.5 M H2SO4 solution is reported. The formation of semiconducting hydride having an indirect optical band gap, Eopt g , of about 2.35 eV is con®rmed by in situ photocurrent spectroscopy. The photoelectrochemical behaviour of such a phase was investigated both in alkaline and in acidic solutions. The ¯at band potential was estimated to be Ufb= ÿ 1.25 V/NHE, independent of pH. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction In recent papers the possible technological use of yttrium hydrides as electrochromic materials was suggested by dierent authors [1±4]. It was shown that pronounced changes in the optical properties of YHn occur as a function of the hydrogen content on going from YH2 to YH2.86. From a more fundamental point of view the optical changes were associated with a metal-to-insulator transition, which has been reported for other trivalent rare earth elements forming hydrides [5]. In Refs. [1,4] it was suggested that under high H2 pressures (several GPa) a stoichiometric YH3 phase having a band gap around 2 eV (1.8 < Eg < 2.3 eV) is formed, at variance with recent band structure calculations, suggesting that YH3 has semimetallic behaviour [6] or it is an excitonic insulator [7] having a low band gap (Eg=0.35 eV). In Refs. [1±4] all the experiments were performed with evaporated yttrium ®lms covered by palladium caps of dierent thickness (5±50 nm) in order to avoid any metal oxidation by the environment and to pro-
vide (electro)-catalytic activity for atomic hydrogen insertion [3]. In this work the preliminary results of a more extensive study of the electrochemical behaviour of yttrium metal will be reported; the investigation has been addressed mainly to the photoelectrochemical characterization of thin layers of YH3ÿx formed on the bulk metal during chemical etching in 0.5 M sulphuric acid solution. Some information on the nature of the surface layers formed on Y metal in dierent conditions will be presented in order to support the identi®cation of the hydride phase by means of photocurrent spectroscopy (PCS). The in situ determination of the optical band gap as well as the ¯at band potential provides the energetics of the YH3ÿx/aqueous electrolyte junction. The photoelectrochemical study of the yttrium hydrides, YH3ÿx, with a high hydrogen content (x R 0.14) con®rms the semiconducting nature of the defective trihydride and gives some indications on the stability of such a compound in dierent solutions. 2. Experimental
* Corresponding author. Fax: +39-91-656-7280. E-mail address: [email protected] (F. Di Quarto)
An yttrium rod of purity 99.9% (Goodfellow Metals, Cambridge), having an external diameter of
0013-4686/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 9 9 ) 0 0 1 6 6 - 8
4052
F. Di Quarto et al. / Electrochimica Acta 44 (1999) 4051±4059
6.35 mm, was sealed into a te¯on cylinder using a two component epoxy resin (Torr-seal, Varian Associates). The surface was polished mechanically with abrasive papers, rinsed twice (5 min each time) ultrasonically in cold distilled water and then etched chemically in 0.5 M H2SO4 solution for dierent times. In some cases, after the mechanical treatment, the electrode was scraped by a blade and then immersed directly into the electrolytic solution with or without etching. Unless otherwise stated, the results reported below pertain to electrodes usually etched in 0.5 M H2SO4 for 30 s. After etching, the electrodes were rinsed with distilled water and immersed in solutions with dierent pH values (2 < pH < 15), where the photoelectrochemical characterization was performed. In some experiments, after etching the electrodes for 30 s in 0.5 M H2SO4 electrolyte, known amounts of NaOH solution were added to the etching solution in order to shift its pH to the ®nal value at which the photoelectrochemical experiments were carried out. The experimental setup used for the photoelectrochemical study is that reported in our previous paper [8]: it consists of a 150 W UV-VIS xenon lamp (Spectral Energy) coupled with a monochromator (Bausch&Lomb), which allows monochromatic irradiation of the specimen surface through the electrochemical cell quartz windows. A two-phase lock-in ampli®er (EG&G) was used in connection with a mechanical chopper (frequency: 10 Hz) in order to separate the photocurrent from the total current circulating in the cell due to the potentiostatic control. Photocurrent spectra were corrected for the photon ¯ux emitted at each wavelength by the light source (detected by a calibrated thermopile) and normalized to the number of photons measured at 400 nm (about 1016 photons cmÿ2 sÿ1), thus giving a relative quantum yield in photocurrent units. Before normalization, raw spectra were smoothed according a standard computer procedure, where necessary (high dark- to photocurrent ratio or high noise levels). Electrode potentials were measured with respect to mercurous sulphate or Hg/HgO reference electrodes, but in the following they will be referred to the normal hydrogen reference electrode (NHE). 3. Experimental results During the etching process of yttrium in 0.5 M H2SO4 a strong hydrogen evolution was observed over the entire metal surface, while the open circuit potential, Uoc, was ranging between ÿ1.35 and ÿ1.25 V(NHE). The result obtained in 0.005 M H2SO4 is shown in Fig.1 (curve a), where we report Uoc vs. time curve for an unetched yttrium metal electrode polished mechanically. A rapid increase in the corrosion poten-
Fig. 1. Open circuit potential recorded during the free immersion in aerated 0.005 M H2SO4 of unetched yttrium metal (curve a) and in 0.1 M NaOH of a previously etched Y sample (curve b).
tial, starting from negative values (ÿ1.44 V), was observed followed by a levelling to a quasi-steady state value of about ÿ1.20 V. Electrodes etched in 0.5 M sulphuric acid solution for 30 s and then immersed in dierent solutions (0.005 M H2SO4, borate buer at pH=9.2 or 0.01±1 M NaOH solutions) displayed an initial Uoc value of about ÿ1.2 V slowly levelling o at a value around ÿ1.0 2 0.1 V after about 240 s, regardless of the solution pH (Fig. 1, curve b). In Fig. 2 we report the total current vs. time curves under potentiostatic control, in dark conditions and under illumination with light of dierent photon energies, for a Y electrode etched 30 s in 0.5 M H2SO4 and then immersed in 1 M NaOH solution. A cathodic dark current of about 2.2 mA was circulating during the experiment. The ®gure shows clearly the presence of a quite large anodic photocurrent owing to the presence of a n-type semiconducting (SC) layer formed by etching the Y metal in 0.5 M H2SO4. Similar results were obtained in 0.005 M H2SO4 acid solution, but larger and more stable photocurrents were usually measured in alkaline solutions, probably due to the higher stability of the semiconducting layer (see below). In Fig. 3 we report the photocurrent spectrum (corrected for the photon emission of the monochromatorlamp system, see Experimental section) recorded at ÿ1.0 V(NHE) in 0.005 M sulphuric acid on an Y electrode, previously etched in 0.5 M H2SO4 solution. As evidenced in Fig. 3 the photocurrent onset is at around 500 nm; the photocurrent intensity reaches a maximum
F. Di Quarto et al. / Electrochimica Acta 44 (1999) 4051±4059
4053
Fig. 2. Total current in the dark and under illumination with light of dierent wavelengths recorded upon manual chopping on an etched Y sample polarized at ÿ1.02 V(NHE) in 1 M NaOH electrolyte.
near 400 nm and then falls to negligible values at high photon energies (l 1 300 nm in acidic solutions), probably due to a surface recombination process of the photocarriers generated under strongly absorbed light [9]. In fact a light absorption coecient in the order of 105 cmÿ1 was reported for defective trihydride ®lms at
a photon energy of 2.8 eV [1]. However, in principle, it cannot be ruled out that a change of re¯ectivity, R(l ), at the electrode/electrolyte interface, with changing wavelength could account for the drop in the quantum yield at the shortest wavelengths. The determination of the optical band gap of the
Fig. 3. . Photocurrent spectrum recorded on an Y sample, etched in 0.5 M H2SO4 for 30 s, and then polarized at ÿ1.0 V(NHE) in 0.005 M H2SO4 solution. Inset a: determination of the optical band gap of the surface layer according to Eq. (1). Inset b: determination of the optical band gap of a surface layer on an Y sample, etched in 0.5 M H2SO4 for 30 s, and then polarized at ÿ1.0 V(NHE) in Na2SO4 solution with pH adjusted to 12.3 (see text).
4054
F. Di Quarto et al. / Electrochimica Acta 44 (1999) 4051±4059
Fig. 4. Photocurrent vs. electrode potential curves recorded on etched Y samples in dierent solutions upon irradiation with a He±Cd laser beam (l=442 nm). (a) 0.005 M H2SO4; (b) 0.1 M NaOH.
®lms is reported in the inset a of Fig.3. The optical band gap, Eopt g , was obtained from the photocurrent action spectrum by using the following relationship [8] Iph hnn=2 const Eopt g ÿ hn,
1
where hn is the photon energy, while n=1 for indirect and n=4 for direct optical transitions. A good ®tting was obtained in the hypothesis of indirect (nondirect for amorphous materials) optical transitions and the value derived for Eopt was equal to g about 2.3520.05 eV. Similar spectra were recorded for Y electrodes etched in 0.5 M H2SO4 solution and then immersed in the other solutions exploited. The value of optical gap obtained in Fig. 3 for the SC ®lm agrees very well with that reported by Huiberts et al. [4] for the hydride phase g-YH2.86 and it is the experimental support to our hypothesis that a g-YH3ÿx hydride phase is formed on the surface of Y metal after prolonged etching in 0.5 M H2SO4 solution. A partial support to the possible copresence of b-YH2 came from XRD data analysis performed on thin (0.1 mm thick) Y foils etched in the same way. In this case the presence of bYH2 re¯ections at 2y=298 was evidenced in X-ray spectra, whilst no evidence of g-YH3ÿx was obtained from such experiments [2], probably due to the small amount of this phase. We have to mention that the etching of Y metal in 0.005 M H2SO4 for a longer time (600 s) produced a photoactive phase having spectra similiar to that reported in Fig. 3, but with photocurrent intensities, at equal electrode potential, much lower than those observed after etching in 0.5 M H2SO4. This ®nding
suggests a more ecient formation process of the SChydride in more acidic solutions. In the inset b of Fig. 3 we report the determination of the optical gap for an Y electrode polarized at an electrode potential, Ue= ÿ 1.0 V(NHE) in a sodium sulphate solution having pH adjusted to 12.3. The electrode, initially etched in 0.5 M H2SO4 solution for 30 s, has been kept in solution during the pH change performed by adding to the initial 0.5 M H2SO4 etching solution a known amount of 1 M NaOH solution sucient to reach the ®nal pH value. Although the shape of the spectrum and the value of optical gap are both very similar to that recorded in acidic solution, larger and more stable photocurrent intensities were measured at corresponding band bending in alkaline solutions. Moreover, a higher quantum yield in the short wavelength region (l < 300 nm) of the photocurrent spectrum was sometime observed in these last solutions. Although the response at high photon energy and the photocurrent intensity depend to some extent on the pH of the electrolytic solution, nevertheless the collection eciency of the injected photocarriers is always decreasing in the region of the highest photon energies. It is worth noting that both the optical band gap value and the shape of the spectra did not change (apart from the occasional tailing in the shortest wavelength region discussed above) if, after the acidic etching, the electrodes were immersed in alkaline solutions (pH>9), provided that the measurements were performed at suciently negative potentials, close to the initial Uoc values reported in Fig. 1. Moreover, we mention that in 0.1 M NaOH solution no appreciable changes were observed in the shape of photocurrent
F. Di Quarto et al. / Electrochimica Acta 44 (1999) 4051±4059
Fig. 5. Total current (a) and photocurrent recorded at l=300 nm (b) vs. potential curves during anodization of a scraped Y sample in 1 M NaOH solution. Scan rate: 5 mV/s.
4055
spectrum by leaving the hydride electrode at the open circuit potential for more than twelve hours, when open circuit potentials as high as ÿ0.4 V(NHE) were sometimes recorded. We have to say that in this case, although the Eopt value was nearly coincident with g that obtained initially, much lower photocurrents were measured at this potential. We like to stress this ®nding because in alkaline solutions the formation of yttrium hydroxide starting from metallic yttrium is thermodynamically favoured even at very negative potentials [10] (see also below). We took advantage of the relatively good stability of the YH3ÿx layers, in alkaline (9 < pH < 15) as well as in acidic (pH=2.3) solutions, for investigating in a large range of pH values the dependence, if any, of the onset photocurrent potential on the solution pH. Fig. 4(a and b) show the Iph vs. Ue plots (photocharacteristics) at constant wavelength (l=442 nm) in 0.005 M H2SO4 and 0.1 M NaOH solutions, respectively. The photocurrent intensity increases on going towards less negative electrode potentials. A negligible hysteresis was observed in such plots provided that the range of electrode potentials spanned was quite small around the onset photocurrent potential, V . Large hysteresis was observed in the photocharacteristics when a wider potential range was exploited (see Fig. 4(a)). From these curves a quite constant onset photo-
Fig. 6. Photocurrent spectrum recorded for the sample of Fig. 5 at the ®nal scan potential (+0.6 V/NHE). Inset: determination of the optical band gap of the surface layer according to Eq. (1).
4056
F. Di Quarto et al. / Electrochimica Acta 44 (1999) 4051±4059
Fig. 7. Total current vs. potential curve during anodization in 1 M NaOH solution of an Y sample, previously etched in 0.5 M H2SO4 for 30 s. Scan rate: 1 mV/s.
current potential value was derived (V= ÿ 1.25 2 0.1 V/NHE) independent of the solution pH. This ®nding rules out the presence of an acid-base equilibrium at the surface of the n-type SC-hydride. Such an equilibrium is responsible for the ¯at band potential dependence on the solution pH usually observed with semiconducting oxides [11]. In order to show the possible in¯uence of the hydride phase on the nature of the ®lm grown upon anodization of Y metal, we performed two experiments on etched and unetched (freshly scraped surface) yttrium specimens in 1 M NaOH solution under analogous conditions. Fig. 5(a) displays the anodization curve of a scraped Y sample, obtained potentiodynamically at low scan rate (5 mV/s) starting from the open circuit potential (about ÿ0.7 V/NHE) up to a maximum potential equal to +1 V and then back to +0.6 V. It is interesting to note that since the very initial instants of the anodization process an anodic photocurrent was recorded (see Fig. 5(b)) under illumination with monochromatic light (l=300 nm). The photocurrent spectrum, reported in Fig. 6, was recorded at the ®nal scan potential (+0.6 V); it is completely dierent with respect to the previous spectra: in the hypothesis of indirect optical transitions an Eopt g value equal to 3.0 2 0.05 eV was derived (see inset of Fig. 6). Another possible extrapolation at higher energy (around 4 eV) will be discussed in a future paper. The Eopt value obtained is much lower than g that reported for Y2O3 (5.8 eV) [12], but quite in agreement with the band gap expected for a hydroxide or an oxy-hydroxide, according to our recent suggestions [13]. In a separate experiment we recorded the photocharacteristic at constant thickness for the same ®lm of Fig. 6, showing an onset photocurrent potential, V , of about ÿ0.65 V(NHE). In Fig. 7 we report the anodization curve (1 mV/s scan rate) of an Y metal electrode, initially etched for 30 s in 0.5 M H2SO4 and then immersed in 1 M NaOH, obtained starting from ÿ1.0 V(NHE) up to
Fig. 8. Determination of the optical band gap for the surface layer of Fig. 7 after anodic polarization at +1.5 V/NHE.
+0.4 V. The photocurrent spectrum recorded at Ue=+0.4 V shows the presence of a very long tail up to wavelengths longer than 700 nm, at variance with the previous result. From this spectrum, it appears evident the presence of a second optical threshold at much lower photon energies (around 1.1 eV), partially masking the optical gap at higher photon energies (about 2.4 eV), which is still present. We have to mention that in these experiments, if the electrode potential was shifted potentiodynamically starting from Ue=+0.4 V towards ÿ1 V, the photocurrent spectrum looses the absorption threshold at low energy and shows only the usual optical gap around 2.35 eV; however, the measured photocurrent intensities are lower than the previous cases. On the other hand by polarizing the etched electrode at much more anodic potentials (Ue r+1.5 V) a second optical threshold appears at around 3.0 eV, together with the initial threshold at around 2.35 eV (see Fig. 8). This last threshold disappears completely after very long polarization times (more than 12 hours) by leaving a photocurrent spectrum similar to that of Fig. 6 with the only threshold at around 3.0 eV. 4. Discussion The Pourbaix diagram of Y [10] reports a very negative equilibrium potential for the reaction of yttrium metal dissolution Y Y3 3eÿ ;
E8=V ÿ2:372 0:0197logY3
2
which, in absence of formation of any passivating oxide at low pH values, accounts for the strong hydro-
F. Di Quarto et al. / Electrochimica Acta 44 (1999) 4051±4059
gen evolution observed during the etching process in H2SO4 solution. However, although the Pourbaix diagram reports in acidic solutions only the Y/Y3+ equilibrium, the simultaneous formation of a hydride phase must be taken into account due to the large negative enthalpy of formation of YH2 (DHf = ÿ 228 kJ/mol H2 [14,15]). The possible formation of yttrium trihydride is also favoured thermodynamically due to the negative enthalpy of the transformation reaction b ÿ YH2 1=2H2 g ÿ YH3
3
with DHb 4g (®lms)=ÿ 38 kJ/mol [1]. The formation of a hydride phase was argued by the rapid change of colour of the metallic surface, from grey to a deep blue/black colour, which is typical of YH2 and YH3 phases, observed at the end of the etching process, before beginning the photoelectrochemical investigation of the electrodes. The presence of YH2 on Y foils etched in 0.5 M H2SO4 is supported also by the presence of the (111) re¯ections in the X-ray patterns, corresponding to the main peak of the b-YH2 phase [2]. The photocurrent spectrum recorded in acidic solution (Fig. 3), where the formation of any oxy-hydroxide phase is excluded according to the Pourbaix diagram, and the value of optical gap, analogous to that reported for g-YH2.86 by Huiberts et al. [4], strongly support the formation of a g-YH3ÿx (0 R x R 0.14) hydride phase on the surface of Y metal during the 30 s etching in 0.5 M H2SO4. As for the electronic structure of the n-type semiconducting YH3ÿx phase our results suggest the existence of indirect optical transitions at photon energies around 2.35 eV. In order to rationalize this experimental ®nding, we suggest the formation, during the etching process, of an yttrium hydride (having a variable hydrogen content) simultaneous to the electrochemical corrosion of Y occurring via local cells. According to this mechanism we assume that, apart from the hydrogen evolution reaction coupled to the anodic reaction (2), the following electrochemical reaction occurs in the cathodic areas of the corroding metal Y 3 ÿ xH 3 ÿ xeÿ YH3ÿx
4
Yttrium dihydride (x=1) as well as defective yttrium trihydride can be formed spontaneously owing to the large value of enthalpy of formation of YH2 and by taking into account the existence of a positive thermodynamic driving force for the transformation of dihydride to defective trihydride in presence of the large hydrogen evolution observed during the etching process. By assuming, according to the literature [3,14], a value of DGf = ÿ 189 kJ/mol for YH2, the equilibrium potentials for the possible reactions represented by Eq. (4) have been estimated to be
E eq 0:98 ÿ 0:059pH
4057
for x 1
5a
for x 0
5b
and E eq 0:72 ÿ 0:059pH
The equilibrium potential reported in Eq. (5b) has been estimated by assuming an enthalpy of formation DHf (YH3)= ÿ 266 kJ/mol and, as a ®rst approximation, an entropy of formation DSf (YH3)= ÿ 196 J/ K mol. This last value was obtained by using the value of entropy of formation, DSf = ÿ 130.8 J/K mol H2 previously used for YH2 [3] and neglecting the change of con®gurational contribution to DSf . According to the literature data, a metallic behaviour is reported for YH2 [1] so that no photocurrent response can be expected if the formation of yttrium dihydride would be the ®nal step of the 30 s long chemical etching process of yttrium metal in 0.5 M H2SO4 solution. The experimental results show that both in sulphuric acid as well as in NaOH solutions a semiconducting phase having the same optical band gap (2.35 2 0.05 eV) is always present on the yttrium surface soon after immersion of etched Y electrodes. According to this experimental ®nding and in agreement with Eq. (4), in the followings we will assume that semiconducting YH3ÿx (0 < x < 0.14) is formed on the etched Y metal. Under open circuit conditions the presence of this SC-hydride phase was observed in acidic solution (0.005 M H2SO4) for electrode potentials values more negative than ÿ0.8 V(NHE), whilst in alkaline solutions it was still observed at more anodic potentials (up to ÿ0.4 V in 0.1 M NaOH solution). A rationale for this ®nding could be traced out to the dierent stability of YH3ÿx in acidic or alkaline solutions, as discussed below. In fact in acidic solutions the dissolution of YH3ÿx should occur according to the following reaction:
YH3ÿx Y 3 3 ÿ xH 6 ÿ x eÿ
6a
Assuming the standard free energy of formation reported in [10] for Y3+ ions, and the DGf (YH3) estimated above, for x=0 the equilibrium potential of reaction (6a) is given by: E eq ÿ0:827 ÿ 0:0295 pH 0:01 logY3
6b
For [Y3+]=10ÿ6, Eq. (6b) implies that reaction (6a) is thermodynamically favoured for potential values more anodic than about ÿ0.9 V at pH=2.3. In alkaline solutions the following reaction of formation of yttrium hydroxide is allowed YH 3ÿx 3H2 O Y OH 3 6 ÿ x H 6 ÿ x eÿ
7a
4058
F. Di Quarto et al. / Electrochimica Acta 44 (1999) 4051±4059
for potential values more anodic than its equilibrium potential, calculated (for x=0) in a similar way as for Eq. (6b) E eq ÿ0:632 ÿ 0:059 pH
7b
The formation of a very thin ®lm of yttrium hydroxide, which is transparent at photons energies around 2.3 eV (where the SC-hydride phase begins to absorb), could explain the higher stability of the YH3ÿx ®lms in alkaline solution and it could account for the higher quantum yield in the short wavelength region sometimes observed in alkaline solutions. The formation of an anhydrous Y2O3 oxide has been excluded by taking into account that a very large band gap value (05.80 eV) is reported on literature for this oxide [12], at variance with the results of Figs. 6 and 8 showing an absorption threshold at around 3 eV for the phase formed by anodic polarization of Y metal. Indeed, an optical band gap lower than 5.8 eV is expected for yttrium hydroxide according to our experimental ®ndings on dierent oxides and hydroxides and on the basis of a correlation between oxide band gap and dierence of electronegativity of the constituents, recently proposed by the present authors [13]. The formation of Y(OH)3 on unetched Y metal in alkaline solution is in agreement with the Pourbaix diagram of yttrium and it is supported by the experimental ®ndings reported in Figs. 5 and 6 showing the presence on the metal surface of a n-type semiconducting or insulating ®lm having a band gap of 3.0 2 0.05 eV, after anodizing up to +1.0 V in 1 M NaOH solution. A more complex picture comes out from the analysis of the experimental data obtained on Y electrodes anodized in alkaline solutions up to more anodic potentials. These results will be discussed in a forthcoming paper. A support in favour of the existence of the electrochemical equilibrium represented by Eq. (7a) comes from the result of Fig. 8, showing that a surface layer having a band gap value around 3.0 eV was also obtained by polarizing at high anodic potential (+1.5 V) an yttrium specimen initially etched for 30 s in 0.5 M H2SO4. Moreover, the very long polarization time necessary for the disappearance of the SC-hydride phase at this potential suggests that the kinetics of oxidation of the YH3ÿx phase to hydroxide or oxyhydroxide is sluggish: this fact can account for the experimental behaviour of hydride ®lms observed under open circuit conditions in alkaline solutions. By taking into account the presence of the new electrochemical equilibria discussed above, the Pourbaix diagram of Y metal should appear dierent than that reported in ref. [10] and much more similar to those relative to alkaline earth metals, like Ca, Sr and Ba. Finally we like to stress that in Fig. 5(b) an anodic
Fig. 9. Energetics at the metal/yttrium hydride/aqueous electrolyte junction at pH=0. For the derivation of the energy levels see text.
photocurrent was recorded, at l=300 nm, since the beginning of the anodization. From the photocurrent vs. potential curves an onset photocurrent potential of about ÿ0.65 V(NHE) was obtained; this value is much more anodic than that derived in Fig. 4(a and b) for the hydride phase, con®rming the dierent nature of the layers grown on etched or anodized Y metal. The location of the energy levels of the SC-hydride/ electrolyte junction with respect to vacuum was obtained according to the relations Ec ÿ eE8NHE ÿ e Ufb DEf ; Ev Ec ÿ Eg ,
8
where Ec and Ev are the conduction band and the valence band edges of the SC, respectively, DEf is the distance in energy between the Fermi level, Ef = ÿ e Ufb, and the conduction band edge, and E8NHE= ÿ 4.45 eV is the energy level of the normal hydrogen electrode with respect to the vacuum [16]. According to the previous relationships and to the experimental ®ndings, the energy band model of the YH3ÿx (with x R 0.14) semiconducting ®lm/electrolyte junction is reported in Fig. 9. The energy levels of the junction were located under the following assumptions: a) the equilibrium Fermi level of the junction has been located by assuming Ufb coincident with the onset photocurrent potential reported above; b) in absence of further information on the donor concentration and the eective density of states at the conduction band edge, Ec, this last level has been located 0.1 eV higher than Ef . The ®rst assumption is supported by the good reproducibility of the onset photocurrent potentials in the dierent solutions investigated, the second assumption is quite compatible with a density of ionized donors around 1019 cmÿ3 [1].
F. Di Quarto et al. / Electrochimica Acta 44 (1999) 4051±4059
It is noteworthy that by assuming in Fig. 9 an electronic equilibrium at the metal/semiconductor interface, in ¯at band conditions it comes out a value of ÿ3.20 eV for the yttrium metal Fermi level, in very good agreement with the work function (3.1 eV) reported in the literature for this element [17]. According to the scheme of Fig. 9 it is clear that by illuminating the SC-hydride/electrolyte junction with photons of energy hn>Eopt the possible photoelectrog chemical reactions consist in the photooxidation processes of the semiconducting hydride by the photoinjected holes YH3ÿx 6 ÿ x h Y3 sol 3 ÿ x H
H2 O Y OH 3 6 ÿ x H
ladium metal caps has been reported at variance with some previous statements [3]. This preliminary photoelectrochemical study is able also to account for the observed higher stability of the semiconducting hydride in alkaline solutions; such a ®nding could be useful for the use of this hydride in liquid switchable electrochromic cells. The results of this study suggest that the Pourbaix diagram of yttrium should be similar to those of the earth metals group once the possible electrochemical equilibria involving the hydride phases are taken into account.
9 References
in acidic solutions and YH3ÿx 6 ÿ x h 3
4059
10
in neutral and alkaline solutions. Eqs. (9) and (10), which are analogous to Eqs. (6a) and (7a), assuming the hole as reagent, represent the photodissolution reactions of the hydride, whose decomposition Fermi energies (coincident with the decomposition redox potentials of Eqs. (6b) and (7b)) lie well above the valence band edge of SC-YH3ÿx, so that under illumination this phase is always unstable at any pH value. This implies an accelerated dissolution of the trihydride phase under illumination in acidic solution, whilst in basic electrolyte the external hydroxide layer protects partially the hydride against photodissolution. 5. Conclusions The results reported above and the comparison with the literature data suggest the formation on the surface of Y metal etched for 30 s in 0.5 M H2SO4 solution of a SC defective yttrium trihydride (YH3ÿx) having a large optical band gap (2.35 eV). The composition of the SC-hydride should be quite close to YH2.86, which is the phase formed on Y metal under 1 atm hydrogen pressure. The energetic picture derived for the YH3ÿx/electrolyte junction agrees both with the photoemission results on the rare earth trihydrides (suggesting the formation of an hydrogen band located 2.35 eV below the conduction band of yttrium metal) and with the work function value of this element. The possibility to investigate Y hydrides in electrolytic solutions without pal-
[1] J.N. Huiberts, R. Griessen, J.H. Rector, R.J. Wijngaarden, J.P. Dekker, D.G. de Groot, N.J. Koeman, Nature 380 (1996) 231. [2] J.N. Huiberts, J.H. Rector, R.J. Wijngaarden, S. Jetten, D. de Groot, B. Dam, N.J. Koeman, R. Griessen, B. Hjorvasson, S. Olafsson, Y.S. Cho, J. Alloys Comp. 239 (1996) 158. [3] P.H.L. Notten, M. Kremers, R. Griessen, J. Electrochem. Soc. 143 (1996) 3348. [4] R. Griessen, J.N. Huiberts, M. Kremers, A.T.M. van Gogh, N.J. Koeman, J.P. Dekker, P.H.L. Notten, J. Alloys Comp. 253-254 (1997) 44. [5] G.G. Libowitz, Ber. Bunsenges. Phys. Chem. 76 (1972) 837. [6] J.P. Dekker, J. van Ek, A. Lodder and, J.H. Huiberts, J. Phys. Condens. Matter 5 (1993) 4805. [7] Y. Wang, M.Y. Chou, Phys. Rev. Lett. 71 (1993) 1226. [8] F. Di Quarto, S. Piazza, C. Sunseri, Mater. Sci. Forum 192±194 (1995) 633. [9] H.J. Hovel, Solar cells, in: R.K. Willardson, A.C. Beer (Eds.), Semiconductors and Semimetals, 11, Academic Press, New York, 1975 (Chap. 2). [10] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon Press, 1966. [11] H. Gerischer, Electrochim. Acta 34 (1989) 1005. [12] W.J. Tropf, M.E. Thomas, in: D. Palik (Ed.), Handbook of Optical Constants of Solids II, Academic Press, New York, 1991, p. 1079. [13] F. Di Quarto, M.C. Romano, S. Piazza, C. Sunseri, J. Phys. Chem. B 101 (1997) 2519. [14] R. Griessen, T. Riesterer, Topics in Applied Physics, in: L. Schlapbach (Ed.), Hydrogen in Intermetallic Compounds, 63, Springer-Verlag, Berlin, 1988, p. 219. [15] G. Sandrock, in: P.D. Bennett, T. Sakai (Eds.), Hydrogen and Metal Hydride Batteries, Proc., 94-27, The Electrochem. Soc. Inc, NJ, 1994, p. 1. [16] H. Reiss, A. Heller, J. Phys. Chem. 89 (1985) 4207. [17] Anon, Handbook of Physics and Chemistry, CRC Press, Boca Raton, 1995.