Langmuir 1988,4, 967-976
ar .
From eq 8 we have
967
’[
dRo = ri (X(S;)-xi)x
axi/ao=
-(x?bsd/Mx - X,) sin 6 + (aZrbsd/MZ- 2,) cos 6 = Zi
azi/ae= -(aZ?bsd/MZ- Z,) sin 6 - (Xrbsd/M* - X,) cos 6 = -Xi (A31 Hence
ari/a6 = ( l / r i ) [ ( Z ( s-i )&)Xi- (X(si)- Xi)Zi]
Using the definitions of x f , z f , and p one obtains ax(si) _ax(si) p 2p -- _a -
aR,
(A4)
ap
aRo
- -xf(sJ R,
Similarly, it can be shown that
ari 1 Z?bsd -=--[ ( X ( s i )- Xi) sin 6 + (Z(si)- 2;)cos 61 aa ri M, ari
1
ax,
ri
- = -[(X(si)- X i ) cos 6 - (Z(si)- ZJ sin 61 ari 1 -azo - - -[(X(si) - X i ) sin 0 + (Z(si)- Zi)cos 61 ri
Hence
ari -- -[(X(sJ 1 _ dRo
ri
- XJ(x(sJ + 2Px’(s,))
+ (Z(sJ - ZJ x
(z(si)
+ 2Pzf(si))l (A81
Similarly, it can be shown that
(A5)
Differentiating eq 10 with respect to Royields the following equation:
The values for x(sJ, x f ( s i ) z(si), , and z’(si) may be easily approximated by an appropriate linear interpolation between their values a t the points (Xj,Zj)and (Xj+Jj+J.
Dye Sensitization of van der Waals Surfaces of SnSa Photoanodest B.A. Parkinson E.I. du Pont de Nemours & Co., Central Research and Development Department, Experimental Station E328/216B, Wilmington, Delaware 19898 Received February 15, 1988. In Final Form: April 5, 1988 The sensitization of the van der Waals surface of SnSz (Eg= 2.22 eV) with over 30 different dyes (Arnm
< 2.2 eV) is studied. The van der Waals surface of this material has several advantages for studying
sensitization. It is renewable via cleavage and lacks an oxide layer under ambient conditions. The relevance of the electrochemicalproperties of the dyes to their sensitizationbehavior is discussed. Adsorption isotherms for many of the dyes were measured by relating quantum yield for electron injection to surface coverage. Both J and H aggregates and monomeric dye species sensitize n-SnS2. The photocurrent-voltage behavior of the dye is interpreted by using Spitler’s theory of electron injection into semiconductors. Sensitized photocurrents are also studied as a function of light intensity and supersensitizer concentration to aid the qualitative theoretical analysis. Several unusual effects associated with the layered structure of the semiconductor are observed including dye intercalation, total internal reflection of the incident light, and surface phase changes.
Introduction The sensitization of a silver halide grain via electron injection from a highly absorbing organic dye molecule is the process underlying photography.’P2 Adsorption of dye molecules onto the surface of semiconductor electrodes has for many years been used to model sensitization of silver halides and as a potential method of energy conversion.s13 The semiconductor is sensitized to sub-band-gap light via electron injection into the conduction band of the semiconductor from the excited state of the dye molecule with the overall efficiency of the process directly measurable via the photocurrent. Understanding of dye and dye aggregate sensitization efficiency, of the excited states involved in injection, and of the energetic thresholds for ‘Contribution No. 4685 from E. I. du Pont de Nemours & Co.
. 0743-7463/88/2404-0967$01.50/0
electron injection has mostly resulted from photoelectrochemical and spectroscopic studies of oxide semiconductors (1) The Theory of the Photographic Process, 4th ed.; James, T. H.,
Ed.;MacMillan: New York, 1977.
(2) Berriman, R. W.; Gilman, P. B., Jr. Photog. Sci. Eng. 1973, 17, 235. ( 3 ) Gerischer, H.; Tributsch, H. Ber. Bunsen-Ges. Phys. Chem. 1968, 72, 437. (4) Memming, R.; Tributach, H. J . Phys. Chem. 1971, 75, 562. (5) Memming, R. Photochem. Photobiol. 1972, 16, 325. (6) (a) Gerischer, H.; Willig, F. In Topics in Current Chemistry; Davison, A., Ed.; Springer: New York, 1976; pp 61, 31. (b) Gerischer, H.; Spitler, M. T.; Willig, F. In Electrode Processes 1979; Bruckenstein, S., Ed.; Electrochemical Society: Princeton, NJ, 1980, p 115. (7) Sonntag, L. P.; Spitler, M. T. J . Phys. Chem. 1985, 89, 1453. (8) Natoli, L. M.; Ryan, M. A.; Spitler, M. T. J. Phys. Chem. 1985,89, 1448. (9) Spitler, M. T.; Calvin, M. J. Chem. Phys. 1977, 66, 4294. (10) Spitler, M. T.; Calvin, M. J. Chem. Phys. 1977, 67, 5193.
0 1988 American Chemical Society
968 Langmuir, Vol. 4, No. 4 , 1988
Parkinson
because of their large band gaps and photocorrosion resistant surfaces. The quantum yield, defined as electrons per photon absorbed by the dye (QYAP), on oxide semiconductor single-crystal surfaces was consistently limited to less than 0.5%. The low quantum yield has been attributed to recombination of the photogenerated carriers via back reaction of the injected electron with the dye radical, perhaps through surface states in the forbidden region of the band gap. Surface states are virtually unavoidable on materials that have a three-dimensional structure due to the need to terminate the bonding at every low-index crystal face. Surface quenching of dye excited states, another process which could potentially limit the quantum yield via energy transfer to neighboring dye molecules, was found not to be operative on ZnO singlecrystal surfaces sensitized with rhodamine B or rose ben-
65 66 67 68 69
ga1.14
IR-125
We have recently shown that van der Waals (0001) surfaces of two-dimensional semiconductors such as MoS,, WS,, MoSe,, and WSe, are superior substrates for studying ~ensitizati0n.l~High quantum yields (>4% per incident photon (QYIP) or >BO% per absorbed photon (QYAP)) were measured for the sensitization of the WSe, surface by an infrared absorbing pentathiacyanine dye. The high quantum yields were attributed to the lack of interface states due to the lack of bond termination, characteristic of van der Waals surfaces. UHV studies (LEED and Auger) also demonstrated that the hexagonal closest packed chalcogenide surfaces are highly ordered, free from contamination and oxides even after potential cycling in an electrolyte.16 In this study sensitization of two-dimensional semiconductors is extended to a larger band gap material, SnS2. The larger band gap (2.22 eV) has the significant advantage of making a larger number of dyes available for study as sensitizers. We have used over 30 widely differing dyes to study sensitization of the van der Waals surface of n-SnS,. The results are qualitatively interpreted by using a theory developed by Spitlerl' for electron injection into a semiconductor from an excited state of an adsorbed dye. The theory is based on earlier work by Char16 and Willig'* concerning electron injection from a dye excited state into an insulating crystal.
Experimental Section Dyes were obtained from several sources. The majority of the common dyes was purchased from the Eastman Kodak Co. The long-wavelength unsymmetrical cyanine dyes (nos. 65-69) were kindly provided by S. H. Ma of Du Pont Imaging Systems Department. Evan Laganis of Phillips Du Pont Optical provided other novel long-wavelength absorbing dyes. Chart I shows the structures of all the dyes used in this study and also shows the abbreviations used in the text and figures. The source of the dyes, their redox potentials, and absorbance maxima for the dye monomer are given in Table I. All dyes were of the highest purity available and used as received. The standard test solution for sensitization was 2 M LiCl in methanol with 10 mM hydroquinone (H,Q) added as a supersensitizer. Dye isotherms were measured by small additions of a 1 mM dye solution in methanol to the standard solution and measurement of the photocurrent spectra after each addition. (11)Spitler, M. T.; Lubke, M.; Gerischer, H. Ber. Bunsen-Ges. Phys. Chem. 1979,83, 663. (12) Gerischer,H.; Bressel, B. Ber. Bunsen-Ges. Phys. Chem. 1985,89, 1083. (13) Itoh, K.; Chiyokawa, Y.; Nakao, M.; Honda, K. J . Am. Chem. SOC. 1984,106, 1620. (14) Spitler, M. T. J . Phys. Chem. 1986, 90,2156. (15) Spitler, M. T.; Parkinson, B. A. Langmuir 1986, 2, 549. (16) Stickney, J. S.; Rosasco, S. R.; Solomun, T.; Hubbard, A. T.; Parkinson, B. A. Surf. Sci. 1984, 136, 15. (17) Spitler, M. T. J . Electroanal. Chem. 1987, 228, 69. (18) Charle, K. P.; Willig, F. Chem. Phys. Lett. 1978, 57, 253.
Table I
dyea DMAT HITC DTDC DTTC
ALB AZB CMB
cv
CRY
SQOT EV
MG MER MB NC
NMB NB
ox
PAR PC TBP PYR SQO SQS
SA SRB TAP STIC TP
source Kodak Kodak Kodak Kodak S. H. Ma S. H. Ma S. H. Ma S. H. Ma S. H. Ma Kodak Kodak Kodak Kodak Kodak Laganis Kodak Kodak Kodak Kodak G. F. Smith Kodak Kodak Kodak Kodak Du Pont Kodak Laganis Kodak Merck Laganis Kodak Kodak Laganis Laganis Du Pont
Amax,* nm 618 755 653 773 765 782 780 780 765 665 615 580 601 710 812 590 795 617 540 658 775 608 635 645 520 605 835 545 697 I97 572 554
810 792 590
E,,,' V
E ~ ~v , c
0.83 0.535 0.62 0.475 0.28 0.27 0.285 0.265 0.268 1.16 1.15 0.75 1.35 0.5 0.46 1.15 0.45 1.15 0.79 1.15 0.50 1.15 1.203 1.253 0.75 0.695 0.57 1.285 0.3 0.35 0.7 1.195 0.725 0.30 1.268
-0.7 -0.660 -0.795 -0.665 -0.80 -0.89 -0.83 -0.86 -0.81 -0.28 -0.2 -0.67 -0.34 -0.86 -0.65 -0.757 -0.76 -0.47 -0.44 -0.295 -0.675 -0.2 -0.30 -0.348 -0.52 -1.0 -0.445 -0.655 -0.88 -0.72 -1.01 -0.915 -0.42 -0.83 -0.373
See Chart I for structures. bAbsorbancemaximum measured in methanol. 'First oxidation or reduction potential in volts vs AgfAgCl. SnS2crystals were prepared from a compound synthesized from stoichiometric amounts of high-purityelements (99.9999% sulfur and 99.999% tin) by either chemical vapor transport or slow cooling of a melt. Chemical vapor transport was done in sealed quartz ampules with the addition of iodine as a transport agent and phosphorus as a dopant. (Undoped crystals had too high a resistivity to be useful for sensitization experiments.) The ampules were then placed in a two-zone furnace, and all material was transported to one end of the ampule via a steep temperature gradient. Crystals were grown by reversing the temperature gradient,such that the hot zone was at 800 "C and the cooler zone at 750 "C, and transporting for 24-96 h. The melt-grown crystals were prepared by sealing several grams of compound and 0.5 mol '70 phosphorus into a thick-walled (2 mm) quartz ampule with an i.d. of 7 mm. The ampule was then heated in a shielded furnace to 900 "C and cooled at a rate of 1 "C/h to 830 "C. Due to the high sulfur vapor pressure generated at these temperatures, an external pressure of 20 atm was applied to the reaction vessel. The two growth techniques yielded crystals with different morphologies and doping densities. The CVT method yielded thin hexagonal platelike crystals with doping densities in the low 10'6-cm-3range. The melt growth technique resulted in much ~. thicker crystals but with doping levels around 1015~ m - Attempts to grow crystals with doping levels greater than 10'' cm-3 were unsuccessful. The transported crystals were superior for obtaining higher quantum yields for electron injection but had the major disadvantage, due to their thinness, of not being amenable to multiple cleaves for regeneration of contamination-free surfaces before an experiment. The lower doped crystals were sufficiently thick (2-3 mm) that dozens of experiments could be done with the same sample after cleaving off a thin layer of crystal with tape to expose a virgin surface before each experiment. The surface stability of the crystals was verified by UHV studies. Single crystals of SnS2were cleaved in vacuo and upon examination with LEED showed a hexagonal pattern associated with a hexagonal closest packed (0001) surface. XPS showed no
Dye Sensitization of SnSz Photoanodes evidence of carbon or oxygen contamination. The crystal was then exposed to 1 atm of pure oxygen for 14 h. XPS examination showed only a trace of oxygen on the surface even if the sample was illuminated for 90 min with UV light while being bathed in the oxygen atmosphere. Oxidation of the small percentage of edge sites may account for the trace of oxygen on the surface. A buildup of ubiquitous carbon, at much higher levels than oxygen, was observed over time due to the inability to completely purify such a high pressure of oxygen. Photocurrent spectra and photocurrent voltage curves were obtained with chopped (7-30 Hz) monochromatic light by using a Stanford Research Systems Model SR530 lock-in amplifier and Model SR540 light chopper. A computer-controlled data collection system with a DEC 11/73 processor and various A-D and D-A converters controlled the experiments. A Jarrell Ashe 0.25-m monochromater with a stepping motor under computer control was used to scan the wavelength region of interest. A thermopile detector was used to measure the lamp spectra (Newport 75-W tungsten lamp) needed to correct the photocurrent data. A calibrated silicon photodiode from United Detector Technologies was used to measure the flux at a given wavelength in order to allow calculation of quantum yields. The slit image from the monochromator (=1mm X =2 mm) was focused onto the most nearly perfect region of the crystal after each cleave. The resulting light intensity was
