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Langmuir 1991, 7, 19-22
Hydrothermal Preparation and Characterization of Optically Transparent Colloidal Chalcopyrite (CuFeS2) Ewen J. Silvester, Thomas W. Healy,* Franz Grieser, and Brett A. Sexton+ School of Chemistry, University of Melbourne, Parkville, Victoria 3052, Australia Received June 27,1990. In Final Form: October 10, 1990 A method is described for the hydrothermal preparation of the mixed metal sulfide chalcopyrite (CuFeSz) a t low temperatures (95-100 “C) and a t atmospheric pressure. The preparation produces optically tronsparent sols with a particle size range of 50-90 A. Optical absorption measurements show that the CuFeSz particles have an absorption maximum at 470 nm ( ~ ~ 7=07.1 X lo4 M-1 cm-1) and a band gap of 0.6 f 0.1 eV.
Introduction Colloidal metal sulfide particles were often used as model “lyophobic colloids” in early research on the stability of dispersions.’ More recently, CdS colloids have featured in photochemical studies where direct spectroscopic observation of bulk a n d surface reactions involved in solar photolysis was facilitated by using sol particles of a size below optical or “visible” scattering wavelengths.2 Such “transparent” sols are intermediate in size between t h e more conventional ca. 0.1 pm colloidal range and t h e ca. 1 nm “cluster” size range. “Cluster colloid” particles in the 5-10nm size range encompass the region where changes in the solid-state properties begin t o become a function of particle size and open up possibilities for t h e design of photoelectrochemically active thin films.2b As p a r t of an ongoing program t o explore the ultrasmall colloidal size regime offered by semiconducting metal sulfide particles, we have concentrated on the mixed metal sulfide chalcopyrite (CuFeS2). This is in part prompted by the self-induced hydrophobicity observed by Trahar et al.394when mineral chalcopyrite in t h e 1-100 pm size range is held at aqueous solution E h values of greater t h a n +200 mV (SHE). It was anticipated t h a t if chalcopyrite could be prepared as a transparent sol, a range of spectroscopic techniques could be employed t o examine redox reactions taking place at the CuFeS2-aqueous interface. Such studies could possibly resolve the debate as t o whether the self-induced hydrophobicity relates to elemental sulfur, polysulfides, or other species at t h e
interface.5 T h e present successful synthesis of pure colloidal chalcopyrite relies on t h e controlled conversion of ultrasmall transparent colloidal amorphous iron(II1) oxide particles, at temperatures in t h e 95-100“C range and at atmospheric pressure. Solution conditions have been adjusted to move the system t o t h e chalcopyrite Eh-pH stability field in t h e Cu-Fe-S systema6
* Author to whom correspondence should be addressed.
t CSIRO Division of Material Science and Technology, Locked Bag 33, Clayton, Victoria 3168,Australia. (1) Overbeek,J. Th. G.In Colloid Science; Kruyt, H. R., Ed.; Elsevier:
Amsterdam, 1952; Vol 1, p 58. (2) (a) For example: Hayes, R.; Freeman,P. A.; Mulvaney, P.; Grieser, F.; Healy, T. W.; Furlong, D. N. Ber. Bunsen-Ges. Phys. Chem. 1987,91, 231. (b)Grltzel, M. In Book of Abstracts; 1989 International Chemical Congress of Pacific Basin Societies; Honolulu, HI, December 17-22,1989;
Paper No. APPL
120. (3) Heyes, G.W.; Trahar, W. J. Int. J. Miner. Process. 1977,4, 317. (4) Trahar, W. J. Int. J. Miner. Process. 1983, 11, 57.
(5) Walker, G. W.; Richardson, P. E.; Buckley, A. N. Int. J. Miner. Process. 1989,25, 153. (6) Garrels, R. M.; Christ, C. L. Solutions, Minerals and Equilibria; Freeman, Cooper and Company: San Francisco, CA 1965.
0743-7463/91/2407-0019$02.50/0
Experimental Section Materials. All chemicals used were of the highest purity commercially available. All solutions and colloidal sols were prepared with Millipore “Milli-Q”water. Preparation of Precursor Iron Oxide Sols. For all chalcopyrite preparations, iron was reacted in the form of an optically transparent iron oxide sol. This sol was prepared by dropwise addition of ferric nitrate solution (50 cm3 of 0.125 mol dm-3 Fe(NO&) to boiling water (1000cm3),as described by Sorum.’ Following ferric nitrate addition, the sol was allowed to cool to room temperature and then dialyzed against water at pH 2.8,as described subsequently. The dialyzed sol was analyzed for total Fe by a standard atomic absorption spectroscopy method. Particles prepared by this procedure were typically amorphous to electronsand 50 log, in diameter. Onset of opticalabsorption in these sols was observed at approximately 600 nm. Preparation of Chalcopyrite Sols. Synthesis. Due to the high temperature (95-100“C) and high pH (ca. pH 12)conditions necessary for chalcopyrite synthesis, all sols were prepared in a Teflon vessel to avoid silica contamination of the particles. Such contamination was observed in sols prepared in glass vessels. Step 1. An Fe(III)-Cu(II) mixture was prepared by the combination of 10 cm3 of the dialyzed iron oxide sol described above with the appropriate amount of 0.25 mol dm-3 CU(NO~)O solution so as to achieve equivalent total molarities of iron(II1) and copper(I1). Step 2. In a closed Teflon vessel, 300 cm3 of water was deaerated with argon and heated to 100 OC. Heating of the Teflon reactor was achieved by enclosingthe vessel in an electrical heating tape. To this boiling solution, 10 cm3of a 0.2 mol dm-3 solution of NaOH was added, the argon bubbling discontinued, and the solution bubbled with H2S vigorously for at least 3 min. While the HzS bubbling was maintained, a 10-cm3aliquotof the Fe(OH)3 sol/Cu(II) mixture was injected rapidly into the boiling solution. Immediatelyafter the addition of the Fe(OH)3sol/Cu(II) mixture, the H2S bubbling was stopped and argon degassing restarted. The pH was then raised to above 12 by the addition of approximately 30 cm3 of 0.2 mol dm-3 NaOH. Aging. CuFeSz sols prepared as described above were aged at 95-100 “C while bubbling with argon gas and the spectral characteristics monitored. It was observed that when the ratio of absorbance at 470 nm to absorbance at 800 nm was in the range 2.4-2.5,the sol was sufficiently aged and the heating was then stopped. These aged sols gave diffraction characteristics and elemental compositionstypical of chalcopyrite (see Results). The duration of the aging process was usually 1-2 h. To avoid delay in cooling, the reaction vessel was placed in an ice-water bath until the sol temperature dropped to 25-30 “C. Dialysis. Metal sulfide sols, because of their susceptibility to oxidation, are difficult to clean via passive dialysis techniques. In the present study, excess sulfide and polysulfides were removed by hollow fiber ultrafiltration (Amicon HlP30-20 cartridge at a flow rate of 1L/min). The supporting solution was replaced by deaerated water adjusted to pH 10 (KOH). By this method,
*
(7) Sorum, C. H. J. Am. Chem. Soc. 1928,50, 1263.
0 1991 American Chemical Society
20 Langmuir, Vol. 7, No. 1, 1991
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10-20 exchanges of the sol volume could be achieved within l / Z h and therefore minimize oxidation of the sol. Iron oxide sols used as the starting material for chalcopyrite synthesis were cleaned in a similar way, with the supporting solution replaced by water adjusted to pH 2.8 (HN03). Characterization. Chalcopyrite sols prepared in the above manner were characterized by transmission electron microscopy (TEM) (Hitachi HS-9 or JEOL JEM-100CX) to determine particle size, crystal structure, and morphology. Scanning tunneling microscopy (STM) was used to image the particles at higher resolution.8 Mineral phases were identified by electron diffraction or X-ray powder diffraction (Philips diffractometer using Cu Ka radiation), with the diffraction spacings compared to known standard^.^ X-ray powder diffraction samples were prepared by deliberate coagulation of the sol by addition of excess salt. Selected samples were analyzed for elemental composition by Energy Dispersed Analytical X-rays (EDX) (Philips 400T analytical electron microscope). UV-visible absorption spectra were recorded between 250 and 900 nm for all sols prepared (Hitachi 150-20 spectrophotometer).
Results Morphology. Photographs of aged sol particles imaged by TEM and STM are shown in parts a and b of Figure 1, respectively. Particle definition and crystallinity, as determined by electron diffraction, both improve with aging. In Figure 2 is shown the X-ray powder diffraction patterns of (a) natural chalcopyrite, (b) transparent colloidal chalcopyrite (this study), and (c) transparent colloidal chalcopyrite after deliberate oxidation by air. As shown in Table I, X-ray and electron diffraction spacings compare well with the accepted values for this mineral.g From TEM photographs, the size of the particles was determined to be between 50 and 90 A. Line broadening of the X-ray diffraction pattern is consistent with a crystal size of ca. 50 A. The STM photograph of a cluster of CuFeS2 particles (Figure lb) shows better resolution of the colloid and supports the size dimensionsobtained from TEM micrographs. Also clearly seen in the STM photograph is the near spherical shape of the particles, which is difficult to determine from TEM images. Spectral Characteristics. All sols prepared by the method described were optically transparent. In Figure 3a is shown the UV-visible absorption spectra of CuFeS2 sols (a) before dialysis, (b) after dialysis, and (c) after deliberate air oxidation. An absorbance maximum at 470 nm is observed in unoxidized sols ( ~ 4 7 0= 7.1 X lo4 M-l cm-l), which does not appear to be affected by the dialysis procedure. Aging of the sol between 95 and 100"C resulted in narrowing of the 470-nm band. The shape of the 470-nm band was found to be particularly dependent on the method of preparation and, in particular, on the initial sulfide to metal ratio. In preparations with low initial sulfide concentrations, the 470-nm band was broad with considerably stronger absorption at longer wavelengths and with additional absorption bands in some cases. Such sols exhibited poorer particle definition and less-well-defined diffraction characteristics. In Figure 3b is shown a comparison of the optical absorption spectrum of the transparent chalcopyrite sol produced in this study with that calculated from reflectivity studies on single crystals of chalcopyrite, taken from the work of Oguchi et al.1° Although the absorption (8) Sexton, B. A.; Cotterill, G. F. J . Vac. Sci. Technol. 1989, A7,2734. (9) Berry, L. G., Ed. Selected Powder DiffractionData for Minerals, 1st ed.;Joint Committee on Powder Diffraction Standards:Philadelphia, PA, 1974. (10) Oguchi, T.; Sato, K.; Teranishi, T. J . Phys. SOC.Jpn. 1980,48, 1 O'Y
lL3.
Figure I. (a) TEM photograph of dialyzed chalcopyrite sol particles. (b) STM photograph of dialyzed chalcopyrite sol particles. Particles were imaged on pyrolytic graphite using a Pt/Ir wire tip (tip bias, 490 mV; tunneling current, -0.4 nA).
calculated from reflectivity is greater than that observed for the sol, both spectra are of the same general form with a low energy absorption band in the visible region and a higher energy band edge. The band gap of chalcopyrite lies well into the nearinfrared and is therefore not easily determined in an aqueous colloidal sol due to strong water absorption in this region. Despite this, analysis of the available low energy absorption edge yields a reasonablefit to an indirect transition,11-13from which an extrapolated band gap value of 0.6 f 0.1 eV is obtained. This compares well with the solid-state value of 0.53 eV.14 (11) Butler, M. A. J . Appl. Phys. 1977,48, 1914. (12) Mills, G.; Zongguan, Li.; Meisel, D. J . Phys. Chem. 1988,92,822. (13) Gratzel, M. HeterogenousPhotochemical Electron Transfer;CRC Press: Boca Raton, FL, 1989.
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Langmuir, Vol. 7, No. 1, 1991 21
1 .o 0.8
0.6 0.4 0.2 0.0
I
I
I
I
I
200
400
600
800
1000
Wavelength (nm) 65
55
-
450
35
25 '
0
15 *
5 e
28
Figure 2. X-ray powder diffraction patterns of (a) natural (mineral)chalcopyrite,(b)transparent colloidalchalcopyrite (this study), and (c) transparent colloidal chalcopyrite deliberately oxidized by air at pH 10 (some evidence of covellite (CuS) and iron(II1) oxide formation).
Table I. Diffraction Data for Natural and Colloidal
-7 b E,
*-O
5
=
9
1.5
z
I
X
I
v
c
c Q)
.-0
I
1.0
-
I
I
I
Chalcopyrite colloidal CuFeS2
4 8,
natural CuFeS2
4A
(>5% intensity)O 3.038 1.857
(1
J
1.5753 Reference 9.
electron diffraction
X-ray diffraction
3.04
3.03
1.867
1.86
1.58
1.59
0.5
U
0
U
0.0 -~ 200
400
600
800
lo00
1200
1400
1800
Wavelength (nm)
Molecular orbital calculations on CuFeSp clusters15 correlate the low-energy band with a charge transfer (CT) transition from the valence band to unoccupied Fe(3d) orbitals. Ligand to metal CT transitions of iron(II1) complexes are known to occur a t low energies when highly reducing ligands are involved,16 so the assignment of the low-energy band to this type of transition would seem reasonable considering that S(3p) orbitals contribute significantly to the valence band.15J7 Extensive delocalization of the unoccupied Fe(3d) state is expected due to strong Fe(3d)-S(3p) orbital mixing. As a consequence, the energy levels corresponding to the unoccupied Fe(3d) states form an effective "conduction band" of CuFeS2. The band to band nature of this transition is supported by the abrupt increase in photoconductivity in the region of the low energy band edge.18 The higher energy absorption edge is attributed to a second band to band transition and appears at the same energy in both the colloidal sol and solid state. This absorption edge is similar in energy to that observed in analogous non-ferrous I-111-VI compoundswith chalcopy~~
(14)Weast, R. C., Astle, M. J., Eds.CRC Handbook of Chemistry and Physics, 63rd ed.; CRC Press: Boca Raton, FL, 1982. (15)Kambara, T. J . Phys. SOC.Jpn. 1974,36, 1625. (16)Balzani, V.; Caraasiti, V. Photochemistry of Coordination Compounds; Academic Press: London, 1970. (17) Tossell, J. A,; Urch, D. S.; Vaughan, D. J.; Wiech, G. J . Chem. Phys. 1982,77,77. (18) Teranishi, T.; Sato, K.; Kondo, K. J . Phys. SOC.Jpn. 1974,36,
1618.
.-0 .-cX w -
Figure3. (a) UV-visible absorptionspectra of (a)an aged chalcopyrite sol before dialysis, (b) an aged chalcopyrite sol after dialysis,and (c) a dialyzed chalcopyrite sol after oxidation by air at pH 10. (b)UV-visible absorption spectra of (a) a transparent chalcopyrite sol and (b) solid chalcopyrite (calculated from reflectivity data ref 10). Sulphur (100%)
Copper (100%)
Iron (100%)
Figure 4. Microprobe EDX analysis of chalcopyrite particles plotted on a three-component diagram (mole percent). The compositionof pure chalcopyrite is indicated by the intersection of the three dashed lines correspondingto 25% Cu, 25% Fe, and 50% S.
rite structure (both CuGaS219 and CuA1S214 have band gaps of 2.5 eV). Doping of these minerals with traces of Fe(II1) yields a low-energy absorption band similar to that observed in bulk c ~ F e S 2 . l ~ (19)Shuey, R. T. Semiconducting Ore Minerals; Eleevier Scientific: Amsterdam, 1975.
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22 Langmuir, Vol. 7,No. 1, 1991
Elemental Composition. Selected samples from prepara tions found to have good morphological and diffraction characteristics were analyzed for elemental composition by EDX. A compositional analysis of CuFeSp sol particles taken from several 0.1 pm2areas of an electron microscope grid is shown in Figure 4. Good correlation with the expected Cu:Fe:S ratio of 1:1:2 is observed. The 25 assays shown in Figure 4 correspond to an elemental composition of Cul.03FeS1.95;no statistical significance can be claimed for such an assignment and is given simply as an example. Discussion
The preparation of a mixed metal sulfide, such a chalcopyrite, is complicated by the possibility of forming several solid products. In the Cu-Fe-S system, the only precedent for such a synthesis is that reported by Roberts20 who successfully synthesized, under mild hydrothermal conditions, both bornite (CuEFeSJ and chalcopyrite of low purity. Other studies into the preparation of mixed metal sulfide colloids have featured dimetal sulfides of zinc and cadmium or lead and cadmium. The simultaneous precipitation of ZnS and CdS has been shown to lead to the formation of a transparent "co-colloid" with optical properties that differ from either ZnS or CdSepl The unique properties of these sols have been attributed to cation substitution at the surface rather than the formation of a new solid phase. The preparation of larger (ca. 1pm) CdS-ZnS and CdSePbS colloids has also been described.22 Again, it appears that only limited cation substitution occurs, with the particles consisting of an amalgamation of pure CdS and ZnS or CdS and PbS particles, respectively. The reaction involved in the preparation of chalcopyrite, as described in this study, can be broadly viewed as (20)Roberts, W. M. B. Econ. Geol. 1963,58,52. (21) Henglein, A. Top. Curr. Chem. 1988,143, 113. (22) Murphy Wilhelmy, D.; Matijevic, E. Colloids Surf. 1985, 16, 1.
-
2Fe(OH),(s) + 2Cu2*(aq)+ 5S2-(aq) + 6H+(aq) 2CuFeS,(s) + So(s) + 6Hp0(1) with excess sulfide solubilizing sulfur to form polysulfide species that are removed by the dialysis procedure. Both magnetic susceptibility studies and Mossbauer s p e c t r o s ~ o p show y~~~ that ~ ~iron and copper exist essentially as Fe(II1) and Cu(1) in chalcopyrite, which is a redox combination that would be untenable if both were reacted in soluble forms. At this point in time, the mechanism of formation is not clear, but it would appear that iron in the form of iron(II1) hydroxide as a reactant is protected from reduction under the highly reducing conditions imposed. Aqueous copper is, however, available for reduction and it is likely that this reduction step precedes the formation of a sulfide sol. Conclusions The successful synthesis of transparent colloidal chalcopyrite enables a number of properties of this mineral to be studied by using direct spectroscopic techniques. In particular, the phenomenon of self-induced or natural flotability can be studied spectroscopically in situ, with the possibility of identifying surface species responsible for the hydrophobicity observed under oxidizing condit i o n ~ The . ~ ~availability ~ of well-characterized CuFeSp sols has also allowed the kinetics of oxidation by oxygen and other oxidants to be studied in detail; a paper dealing with these aspects of colloidal chalcopyrite is in preparation. Acknowledgment. This work was supported in part by the Australian Research Council (ARC). E.S.acknowledges the receipt of a Commonwealth Post-Graduate Research Award (CPRA). The authors thank David Watson and Hans Jaeger of the CSIRO Division of Material Science for electron microscopy work and Dr. Colin Gould of IC1 Advanced Materials Runcorn (U.K.) for EDX analysis. Registry No. CuFeSz, 12015-76-8. (23) Adams, R. L. Diss.Abstr. Int., B 1974, 34, 4338.
