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Silanization Effects on TiO, Electrodes
The present discussion of the factors which affect the A ~ t B. equilibrium provides a straightforward explanation of several little-understood occasional observations on RB solutions reported in literature. Thus the absorption of RB in ethanolg or water1',12 decreases on heating. Conversely a large hyperchromic shift is shown at low temperatures in EPA or alcohol." In the former solvent the absorption is strongly dependent on con~entration.~ Finally the output power from a flashlamp-pumped RB dye laser is increased as the temperature of the dye solution is reduced, It was also noted that the lasing ability of rhodamine 6 G (whose carboxyl group is blocked by esterification and thus unable to lactonize) was unaffected by co01ing.l~Since only the zwitterion can serve as a lasing medium, this result is at least qualitatively not unexpected in light of our results. Preliminary results obtained with other free carboxyl-rhodamine dyes, e.g., rhodamine 110 and rhodamine 19, indicate a behavior similar to that of rhodamine B. Indeed
we suggest that lactonezwitterion equilibria play a general role in both phlotochemistry and photophysics of other xanthene dyes substituted a t the 9 position with a phenyl-2-carboxylic acid group. References and Notes (1) K. H. Drexhage in "Dye Lasers", E. P. Schaffer, Ed., Springer, Berlin, 1973,Chapter 4. (2) E. Noelting and K. Dziewonski, Chem. Ber., 38, 3516 (1905). (3) D. Deutsch, 2 . fbys. Chem., 138,353 (1928). (4) R. W. Ramette and E. B. Sandell, J. Am. Cbem. SOC.,78, 4872 (1956). (5) H. P. Lundgreri and C. H. Binkley, J. folym. Sci., 14, 139 (1954). (6) U. K. A. Klein and F. W. Hafner, Cbem. fbys. Left., 43, 141 (1976). (7) I. Rosenthal, Opt. Commun., 24, 164 (1978). (8) See, e.g., E. Fischer, Mol. Pbotocbem., 2 , 99 (1970). (9) J. E. Selwvn and J. I. Steinfeld. J . fhvs. Cbem., 76, 762 (1972).
(fOi
R. W. Chimbers, T. Kajiwara, and D. R. Kearns, J . Pbys. &em:, 78, 380 (1974). (11) W. E. Speas, f b y s . Rev., 31, 569 (1928). (12) L. V. Levshin and V. K. Gorshkov. ODt. Spectrosc., 10,401 (1960). (l3j R. B. Huth, G. I . Farmer, and M. R. Kagan, J. Appl. Phys., 40, 5145
(1969).
Chemically Modified Electrodes, 12. Effects of Silanization on Titanium Dioxide Electrodes Harry 0. Finklea and Royce W. Murray* Kenan Laboratories of Cbemistty, University of Nortb Carolina, Chapel Hill, North Carolina 275 14 (Received August 25, 1978) Publication costs assisted by the Office of Naval Research
The binding of a monolayer or less of organosilane to the surface of a TiOzelectrode has little discernable effect on semiconductor properties such as flat-band potential, apparent doping level, hole oxidation of water, and reduction processes via a surface state. The attached silane is stable toward hole oxidation. The results indicate the presence of unreacted Ti-OH groups on the silanized surface.
properties. Kuwana et a1.26and Hawn and A r m ~ t r o n g ~ ~ Titanium dioxide is a wide band-gap semiconductor which has received much recent attention1-13 due to its observed a change in the slope of a Mott-Schottky (MS) plot upon silanization of SnOz, indicating a lower doping stability as a photoelectrode.' Holes generated in the level. In accord with these results, Srinivasan and Lamb2* valence band by illumination are capable of oxidizing noted a decrease in Sn02thin film conductance when the water2 or solutes within the electrolyte3s4without lattice dissolution. Both processes have connotations in energy surface was silanized. However, a later p u b l i ~ a t i o non ~~ conversion technology. More recently, unusual photothe same system revealed no change in the MS slope. The catalyzed oxidations and reductions have been reported highly doped Sn02 electrodes make separation of solution using T i 0 2 p ~ w d e r s . l ~A- further ~~ advantage with TiOz double layer capacitance variations from electrode space lies in the ease of fabrication of polycrystalline electrodes charge capacitance changes difficult. Sensitization phoby chemical vapor d e p ~ s i t i o n . ~ tocurrents have been observed3b32for silane-bound dyes Chemical modification of semiconductor electrode on S n 0 2and Tic&, and one very interesting report exists surfaces has the same attractions of covalent attachment for a derivatized semiconductor (n-Si a silylferrocene) of selected molecular species to the surfaces of electrodes irradiated with bandgap light.33 with metal-like behavior, e.g., the possibility of tailoring We report herein results of a study of effects 02 silanelectrode properties in a predictive manner. Ti02, like ization on such TiOZ semiconductor properties as phometal oxides such as Pt/Pt0,21 Sn02,22and R u O ~ , ~ ~tocurrent generation under bandgap ilhjmination, flatpossesses surface hydroxyl groups24capable of reacting band potential, interfacial capacitance, and electron with an organosilane reagent: transfer from the conduction band to a solution species via a surface state. TiO,--OH + XSiR, Ti0,i-OSiR, + HX
+
+
.X = C1, OMe, OEt
An electroactive site can then be assembled on the immobilized reactive functionalities of the organosilane such as amine or p ~ r i d y l . ~ ~ Little work has been done so far to characterize the effects of surface silanization on semiconductor electrode 0022-3654/79/2083-0353$01 .OO/O
Experimental Section Titanium dioxide single crystal electrodes were fabricated from a boule (N. L. Industries, Inc.) by slicing wafers approximately 1mm thick perpendicular 1.0 the c axis, and cutting disks 6 min in diameter from the slices. The 6-imm diameter allowed insertion of the disk into the E X A
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spectrometer. After polishing smooth the (001) face (6 pm diamond paste), Ga-In eutectic was placed on the rough side, and the disks were doped in hydrogen gas at 450-600 “C for 15 min and mounted on glass tubes with Torr-Seal (Varian). On certain samples, the disk edges were not insulated with Torr-Seal to avoid interference in ESCA spectra. Before each experiment, the TiOz crystal surface was repolished with 1-ym diamond paste and etched for several minutes in 0.3 M HNOBplus 5% HF. Polycrystalline Ti02 disks 6 mm in diameter were prepared by chemical vapor deposition5 on titanium disks or by simply heating the Ti disks in a bunsen burner flame for several minutes. These were also doped in hydrogen a t 600 “C. Electrical contact was made with the Ga-In eutectic after scratching through the oxide layer on one side. After mounting on a glass tube, the disk edges were insulated with Torr-Seal. Prior to silanization, electrodes were polished, etched, and vacuum dried at 50 “C. They were allowed to react at room temperature for 5 min with 1070 silane in anhydrous benzene or toluene, thoroughly rinsed with fresh solvent, and air dried. A final methanol rinse was sometimes used, but ESCA studies indicated that lower silane coverages on TiOg resulted from this procedure. Silanes (Petrarch Systems) were most commonly 3(2-aminoethylamino)propyltrimethoxysilane(en silane), methyltrichlorosilane, and dimethyldichlorosilane. All electrolytes were aqueous deaerated 0.5 M Na2S04 buffered with 0.1 M phosphate. Electrolyte pH values were confined between 3 and 9 to avoid possible hydrolysis of the silane layer. Cyclic voltammograms and photocurrent measurements were performed using a battery-operated potentiostat and current follower mounted on an aluminum Faraday cage. Potentials were referenced to a sodium chloride saturated calomel electrode (SCE). Light from a 150-W xenon arc lamp was focused through a light chopper (13 Hz) and a Bausch and Lomb high-intensity monochromator (Model 33-86-02) before passing through the cage wall and focusing onto the electrode surface. By using chopped light and observing the current on an oscilloscope, photocurrents of 10 nA or less could be measured. Ac measurements were performed by applying a 1 mV rms ac potential (PAR Model 173 potentiostat) and measuring the in-phase and out-of-phase current components (PAR Model 129 a lock-in amplifier). Data analysis assumed a series RC circuit, where R was the uncompensated resistance and C the interfacial capacitance. For Mott-Schottky data a surface roughness factor of 2 was assumed. X-ray photoelectron (ESCA) spectra were obtained with a DuPont Model 650B spectrometer with a Mg anode. Peak positions were referenced to C 1s as 285.0 eV.
Results and Discussion Previous experience34with metal oxide-silane reactions shows that mild, anhydrous silanization reaction conditions as used here produce approximate “monolayer” coverages of silane. ESCA spectra of freshly silanized TiOz are consistent with this description (Figure 1). After the silanization reaction, silicon peaks and, when expected, nitrogen peaks appear, the Ti 2p peak intensity is diminished by approximately 50%, and the 0 1s peak becomes a distinct doublet, with binding energies at 530.0 and 532.0 eV. The former 0 1s peak matches the 0 1s peak on clean Ti02,and the latter has been attributed35 to the -Si-0- linkage. Deliberate or accidental siloxane polymer formation results in a very low Ti 2p signal (
