Photoelectrochemical properties of Fe2O3 microcrystallites prepared in nafion

71

J. Electroanal. Chem, 295 (19PO) 71-78 Elsevier Sequoia S.A., Lausanne

Photoelectrochemical properties of Fe,O, microcrystallites prepared in Nafion Hiiokazu Miyoshi, Koji Tanaka, Hiroyuki Uchida and Hiioshi Yoneyama * Department of Applied Chemistry, Fatuity ofEngineering, Osaka Uniwrsiq, Yamada-oka 24, Suita, Osaka 565 ~J~aa~

Hirotaro Mori and Takao Sakata Research Center for UItra-high Voltage Electron Microscopy, Osaka University, Yam&-oka Osaka 565 (Japan)

2-1, Sita,

(Received 5 July 1990)

ABSTRACT

Frio, microcrystaIlites were prepared in Nafion@ by mixing FeC13 aqueous solution with Nafion+ alcoholic solution in molar ratios of l/3 Fe3* to the sulfonate groups of Nafion ranging from 0.1 to 1.0, followed by refluxing at 100 o C at pH 2.5-2.6 for 48 h. The maximum size qua&&ion achieved was 1.2 eV. The energy diagrams of the FezOs microcrystals determined from the action spectra and the onset potential of photocurrents due to oxidation of tartrate at the FezO,/Nafion-coated electrodes showed both positive shifts in the valence bands and negative shifts in the conduction bands with almost equal rate, with an increase in the size quantization.

Studies on phot~he~~~ properties and s~e~qu~t~tion of se~conductor ~cr~~st~~tes @article size less than 10 nm) have been actively conducted [l-4]. Iron oxide is one of the stable oxide semiconductors in aqueous solution. Since Dimitrijevic et al. [5] determined a flat band potential of a-Fe,O, colloid @article size less than 10 nm} by using pulse radiolysis in 1984, phot~he~c~ and photocatalytic properties of F%03 microcrystalline colloids and powders have been widely investigated by Stramel et al. [6], Kiwi et al. [7], Leland et al. [8], and by Faust et al. [9]. However, few studies have been conducted on the photoelectrochemical properties of size-quantized Fe,O, microcrystals. * To whom correspondence should be addressed. 0022-0728/PO,‘$O3.50

0 1990 - Elsevier Sequoia S.A.

72

We reported previously on the photochemical and photocatalytical properties of FqO, microcrystallites prepared in clay interlayers [lo-121 which provide structurally limited space to prevent the agglomeration of the microcrystals. In the present study, Nafion, an ion-exchange polymer, which is soluble in alcohol but structually less limited, has been used as the support for stabilizing FqO, microcrystals, and the photoelectrochemical properties of the prepared Fe@, microcrystallites have been ~vestigat~. EXPERIMENTAL

Fe,O, was prepared by hydrolysis of acidic FeCl, solution [13]. To prepare different sizes of Fq03 microcrystallites in Nafion support, the molar ratio of l/3 Fe3+ to sulfonate groups of Nafion ([l/3 Fe3+]/[SOJ-Nafion), which will be denoted as X in the fo~o~~, was changed. 2.5 cm3 of aqueous HQ solution containing 26.4, 13.2, 5.2, and 2.6 mm01 dmp3 Fe3+ was added to 5 cm3 of 5 wt% Nafion in alcoholic solution (Aldrich, Nafion 117), which contained 37.7 mm01 dmp3 sulfonate groups, to give different X values of 1.0, 0.5, 0.2, and 0.1, respectively. The molar concentration of HCl was six times that of l/3 Fe3+. The solution pH was raised by adding 1 mol dmv3 NaOH solution to pH 2.5-2.6, followed by dilution of 10 cm3 with water to produce Fe,O, (hematite) precipitation from Fe3+ or Fe(OH)2+ [14]; 2 Fe3++ 3 H,O + FsO, Fe3’+

H,O + Fe(OH)2++

Fe(OH)‘+

+ 6 Hf H+

+ H,O -+ Fe,O, + 4 H+

log[Fe3+]

= -0.72

log[Fe(oH)2+1 [Fe3+] log[Fe(OH)‘+]

- 3 pH

(I)

=: -243_tpH * = -3.15

(2) - 2 pH

(3)

The con~ntration of the other complex ions (Fe(OH)l, HFeO;) are negligibly small in the acidic solution. The resulting solution was reflexed at 100 o C in au oil bath for 48 h to hydrolyze FeCl, in the solution. The resulting transparent solutions were colorless for X6 0.15 and brown for X > 0.2. Hereafter, these samples are denoted as FqO,/Nafion. The Fe,O,/Nafion solutions were dialyzed for 2 days with Cellophane Tubing-Seamless (24/32 inch (1 inch = 2.54 cm) for dialysis, Union Carbide Co. Ltd.) having an average pore size of 2.4 nm. The dialysis did not decrease the content of Fe, which was confirmed by atomic absorption spectroscopy before and after the dialysis. As a reference for the Fe~O~/Na~on, bulk Fe,O, powder was prepared using the same hydrolysis method as that described above, but without adding Nafion and NaOH to the FeCl, solutions. In the measurement of X-ray powder diffraction, an X-ray diffractometer (Shimadzu, XD-3A) was used. Absorption spectra were measured using a UV-visible spectrophotometer (Shimadzu, MPS-5000) and a quartz cell with 1 cm path length. The Fe,O, microcrystals of the Fe,O,/Nafion were observed by a high resolution transmission

73

electron microscope (Hitachi H-!9000, 300 kev). A small amount of F%Os/Nafion solution was applied to carbon-evaporated copper (diameter 3 mm), followed by drying. For photoelectrochemical measurements, 80 ~1 cm-’ of FqO,/Nafion in alcoholic solution was coated onto a~@assycarbon electrode (1 X 1 cm), followed by drying. A single compartment cell having a quartz window was used. A platinum plate and SCE served as the counter and the reference electrode, respectively. The electrolyte solution used was 0.1 mol dmm3Na$O, aqueous solution containing 0.1 mol dme3 sodium tartrate as a sacrificial reagent. As a light source, a 500 W super high pressure mercury lamp (Ushio, DSB-501 A) was used. An auto-phase lock-in amplifier (NF-Circuit, LI-574 A) synchronized with a light chopper (NF-Circuit, CH-353) and a band-pass filter (NF-Circuit, FV-604 F) were employed to amplify the photocurrents. For tbe measurement of action spectra., a monochromatic light was obtained by passing ligbt from the 500 W xenon lamp or the 500 W super bigh pressure mercury lamp through interference filters (Nihon Shinku Kogaku) or a mon~~ometer (JASCO, CT 25 N). The number of incident photons was determined by a thermopile (Epply Labo. Model E 6). The action spectrum of the photocurrent was obtained by dividing the photocurrents by the number of photons determined. RESULTS AND DISCUSSION The bulk Fe,O, powder prepared by hydrolysis of FeCl, at pH 2.1 was hematite as determined by X-ray powder diffraction analysis. Refluxing the FeCl, solution containing Nafion at pH 2.5-2.6 resulted in an appreciable change in absorption spectra, indicating the formation of F%03 microcrystals. Considering that the pzcs of Fe(II1) oxides are higher than pH 8 [15,16], the surface of the F%03 microcrystallite must be protonated in acidic solution, being favorable for electrostatic binding at Nafion sulfonate groups. Figure 1 shows a typcial TEM photograph for a Nafion film containing iron(II1) oxide microcrystallites prepared with the [l/3 Fe3’] to [SO;]-Nafion ratio of 0.5. The particle size ranges from 1.8 to 11.8 nm and its average is 5.68 nm. In this figure, regular parallel lines attributable to diffractions of Fe,O, microcrystallites are seen and the average distance between the lines was determined to be 0.367 nm, which corresponds to the lattice spacing of the (012) face (d = 0.368 nm) of hematite. Figure 2 shows absorption spectra obtained in 2-propanol (45 vol W) of Fr;O,/ Nafion prepared with X = 1.0,0.5, and 0.1. The greatest blue-shift in the absorption threshold was observed at the Fe,O,/Nafion prepared at X= 0.1. The absorption threshold of the FqO, prepared with X = 0.2 was almost the same as that of X = 0.5, although not shown in Fig. 4, to avoid complication. Photoelectrochemical properties of Fe,O,/Nafion were examined by using Fe,O,/Nafion-coated glassy carbon electrodes. The electrolyte solution used was 0.1 mol dme3 Na,SO, containing 0.1 mol dmF3 sodium tartrate as a sacrificial reagent (pH 7). The F~O~/N~on-coats electrodes exhibited anodic photocur-

Fig. 1. Transmission electron micrograph of FqO,/Nafion arrows indicate tbe FqOs microcrystallites.

([l/3

Fe3”]/[SO<]-Nafion

= 0.5). The

rents, indicating that the F%03 microcxystals have an n-type ~~duc~~ty. Figure 3 shows phot~u~eut-potenti~ curves measured by using the lock-in technique, This figure shows the result for X = 0.5 and 0.1 only. In the case of X = 0.2 and 1.0, the

200

300

100

500

600

700

Wavelength I nm

Fig. 2. Absorption spectra of Fe&/Nafion of [l/3 Fe3+ ]/@O< ]-Nafion = 0.1 (a), 0.5 (b), and 1.0 (c) in Z-propanol (45 vol Sg), and Fe-$& powder suspended in water (d) ill].

75

E I V vs. SCE Fig. 3. Photocurrent vs. potential curves of FqOJNafion-coated glassy carbon electrodes in 0.1 mol ti3 Na,SO, containing 0.1 mol dme3 sodium tartrate as a sacrificial reagent. [l/3 Fe3+]/ [SO; ]-Nafion = 0.5 (a), 0.1 (b). Scan rate: 1 mV s -’ Light source: 500 W super high pressure mercury lamp.

onset potentials of the photocurrent were comparable to that obtained for X = 0.5, although the magnitude of photocurrents were different. The onset potential of the photocurrent was determined as the crossing point with the dark current. The onset potential which gives roughly the energy level (&a) of the bottom of the conduction band was - 0.52 V vs. SCE for the Fe,O,/Nafion with X = 0.5 and - 0.75 V vs. SCE for X= 0.1, indicating that the greater the blue-shift in the absorption spectra, the greater the negative shift in the onset potential of the anodic photocurrents. It is thought that the amount of incorporated Fe’+ into Nafion determines the energy levels of Fe,O, microcrystallites. Figure 4 shows typical action spectra of photocurrents. If eqn. (4) is applied to the absorption spectrum of the Fe,O, bulk powder shown in Fig. 2 with the use of the indirect band gap of Es = 1.88 eV [17], then and plots of log(ahv) vs. log(hv - Es) gave a straight line with the slope close to n = 4, indicating that the indirect transition is operative in this material [18,19]. Then the band gap of the FqO, microcrystallites prepared with different X were determined by applying eqn. (4) to the action spectra of the photocurrents shown in Fig. 4 with the use of n = 4. It was found that the plots of (cuhv)‘/2 vs. hv gave a straight line without ambiguity, allowing the determination of the band gaps as shown in Table 1. In this table, the band gap values determined by applying eqn. (4) to the absorption spectra shown in Fig. 2 are also included. The absorption spectra

2sI 300

WavekngthI

!500

400 nm

Fig. 4. Action spectra of photocurrents at FqO&kfion-coated electrode at +0.3 V vs. SCE in 0.1 mol dmd3 Na,SO, containing 0.1 mol dme3 sodium tartrate as a sacrificial reagent. [l/3Fe3+ j/[SO; j-Nafion = 0.1 (0),0.5 (A),1.0 (Cl).The photocurrents were normaked for the number of incident photons.

near the absorption threshold were used here. The values determined by the two different methods gave different energy gaps except for the Fe& microcrystals prepared with X- 0.1, as was to be expected from the difference between the onset of the action spectra of the phot~~ents (shown in Fig. 4) and the absorption threshold shown in Fig. 2. The observed difference in the energy gap values may result from the size-distribution of the particles. Highly active fine microcrystalhtes alone contribute to the generation of the photocurrents, and the absorption threshold is determined by the largest Fe,O, particles of the Fe,Os/Nafion. This interpretation seems reasonable considering that the prepared Fe,O, microcrystals had a wide size distribution, as already described and shown in Fig. 1. Figure 5 shows the energy diagrams of the Fe,O,/Nafion together with those of TABLE 1 Energy gap of Fe,?O,/Nafion and a-Fe&$ powder [l/3 Fe3+ ] (SO-JI-Nafion Fe@3/Nafion 0.1

0.2 0.5 1.0 a-Fe,4

Energy pap/eV EWJPta

E &actb

2.92 1.88 1.82 1.77

3.05 2.60 2.57 2.46

powder

1.83 ’ Calculated from absorption spectrum (Fig. 2) using eqn. (4) near the absorption edge. b Calculated from action spectrum (Fig. 5) using eqn. (4).

77

c3.0

--

EvB

+2.?&5V

(a) (b) (d) (c) Fig. 5. &ergy diagram at pH U obtained by assuming that the Fermi level is equal to the bottom of the conduction band. F%O&%fion ([l/3 Fe3+]/[SO~]-N~ion=O.l (a), 0.5 (b)), Fe,O,/clay (c), and a-FqOz powder (d). Fe@s prepared in clay interlayers and the bulk Fe,O,. In the construction of these energy diagrams, the onset potential of photocurrent was approximated to the bottom of the conduction band &a, and the valence band edge was estimated by applying the determined energy gaps to the conduction band edge. Furthermore, it was assumed that the band edge shifted in a negative direction at the rate of 59.1 mV/pH. It seems important to this research that the band gap of the bulk Fe$& obtained in the present study with the use of the absorption spectra shown in Fig. 2 was 1.83 eV, being slightly different from the value usually adopted (2.2 ev). The 2.2 eV is the value for the direct transition, and the value obtained above does not conflict with the indirect gap of FezO, [17-191. It is recognized in Fig. 5 that the increase of the energy gap of the Fe,O, microcrystals resulted in both a positive shift of the valence bands and a negative shift of the conduction bands with almost the same magnitude, irrespective of the kind of solid supports. By comparing the band gap value and the conduction band edge of the Fe,O,/Nafion with those of the Fe,O, microcrystals prepared in 0.66 nm interlayer spacings of clay, which were reported previously [12], it is noticed that the size quantization of the Fe,O, microcrystals prepared in the clay interlayers is small compared to those prepared in Nafion, suggesting that the photoche~cal properties of Fe,O, prepared at the clay supports were mostly determined by Fe,O, microcrystals not in the clay interlayers but on the clay surfaces. ACKNOWLEDGEMENT

This research was supported by Grant-in-aid for Scientific Research on Priority Area No. 02203119, from the Ministry of Education, Science and Culture.

78 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

A. Henglein, Top. Cum Chem., 143 (1985) 115. A. Heqlein, Chem. Rev., 89 (1989) 1861. M.L. Steigetwald and L.E. Brus, Armu. Rev. Mater. Sci., 19 (1989) 471. Y. Wang, H. Herron, W. Mahler and A. Surma, J. Opt. Sot. Am. B, 6 (1989) 808. N.M. Dimitrijevic, D. Savic, 0.1. M&e aad A.J. Nozik, J. Phys. Chem., 88 (1984) 4278. R.D. Stramel and J.K. Thomas, J. Cohoid Interface Sci., 110 (1986) 121. J. Kiwi and M. GrBtzeI, J. Chem. Sot., Faraday Trans. I, 83 (1987) 1101. J.K. Leland and A.J. Bard, J. Phys. Chem., 91 (1987) 5076. B.C. Faust, M.R. Hoffmann and D.W. Bahnemann, J. Phys. Chem., 93 (1989) 6371. S. Yamanaka, T. Doi, S. Sako and M. Hattori, Mater. Res. Bull., 19 (1984) 161. H. Miyoshi and H. Yoneyama, J. Chem. Sot., Faraday Trans. 1, 85 (1989) 1873. H. Miyoshi, H. Mori and H. Yoneyama, Langmuir, to be published. J. Livage, M. Henry and C. Sanchez, Prog, Solid State Chem., 18 (1988) 259. M. Pourbaix, Atlas of Electrochemical Equilibria, Pergamon Press, Oxford, 1960, 308. R.J. Atkinson, A.M. Posner and J.P. Quirk, J. Phys. Chem., 71 (1967) 550. F. Dumont and A. WatiBon, Discuss. Faraday Sot., 52 (1971) 352. G.B. Goodenough and A. Hawmelt, in 0. Madehmg, M. Schultz and H. Weiss, (Eds.), Landolt-Bomstem New Serie Band 17 g, Halbleiter, Springer Verlag, Berlin, 1984, 252. 18 J.H. Kennedy and K.W. Freese, Jr, J. Electrochem, Sot., 125 (1978) 709. 19 A.H.A. Timremans, T.P.M. Koster, D.H.M.W. Tbewissen, A. Mackor and J. Schoonman, Ber. Bunsenges. Phys. Chem., 90 (1986) 383.