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The Journal of Physical Chemistry, Vol. 82, No. 11, 1978
D. D. Hawn and N. R. Armstrong
Electrochemical Adsorption and Covalent Attachment of Erythrosin to Modified Tln Dioxide Electrodes and Measurement of the Photocurrent Sensitization to Visible Wavelength Light David D. Hawn and Neal R. Armstrong" Department of Chemistty, Michigan State University, East Lansing, Michigan 48824 (Received September 30, 1977; Revised Manuscript Received January 16, 1978) Publication costs assisted by the Petroleum Research Fund
Erythrosin (2,4,5,7-tetraiodofluorescein) has been attached to SnO, electrode surfaces either through electrochemical adsorption or covalent attachment. SnO, electrodes modified with mercaptopropyltrimethoxysilane, 3-(2-aminoethyl)propyltriethoxysilane,or y-aminopropyltrimethoxysilaneshowed a considerable enhancement of the currents passed during oxidation of erythrosin and a deposition of several monolayers of active chromophore. Erythrosin was covalently attached to the silane-modified Sn02surfaces by means of an amide bond or by the formation of a thiol bond. Small surface concentrations of the adsorbed and covalently attached dye were easily detected by monitoring the I(3d3/2,5/2) x-ray photoelectron transitions. Sensitization of the SnOBcurrent/voltage response to visible wavelength light was observed for both the adsorbed and covalently attached dye electrodes. Enhanced stability and efficiency of the photocurrent response was observed for the covalently attached vs. adsorbed dye molecules.
Introduction Chemical modification of electrode surfaces to alter their catalytic properties has been recently demonstrated for several carbon and metal oxide electrode^.^"^ One promising type of electrode modification involves the covalent attachment or adsorption of photoactive dye molecules to large band gap, n-type, metal oxide, semiconductor electrode^.^,^ Photocurrents at various electrodes have normally been sensitized to visible wavelength light by irradiating the semiconductor in contact with a dilute solution of the dye. Equilibrium adsorption of the dye to the electrode surface accounts for the majority of the photocurrent activitySg-l3The photocurrent or photopotential action spectrum of the electrode can be extended into the visible wavelength region because of the light-induced oxidation of the dye molecule. This photoresponse can be further enhanced with the addition of a reducing agent to solution which can regenerate the reduced form of the dye in an electrocatalytic sequence.%l3 We have been interested in enhancing the photocurrent response of SnOz and TiOz electrodes where the photoactive dye was either permanently adsorbed to the surface or covalently attached, thus removing the necessity of irradiating an electrode/dye solution interface. We report here on (A) our observations of the electrochemical deposition or adsorption of several monolayers of erythrosin (I, 2,4,5,7-tetraiodofluorescein)on various
6'"" I
silane-modified SnOz electrodes, (B) the covalent attachment of erythrosin to these same silane-modified electrodes, (C) the electrochemical and surface analysis characterizations of the above electrodes and, (D) our preliminary data on the comparison of the photoelec0022-365417812082-1288$0 1.OO/O
trochemical response, to visible wavelength, of the SnOz electrodes with adsorbed or covalently attached erythrosin. Erythrosin (I) was selected as the sensitizing agent because of its optimal properties as a photosensitizing dye (A, 527 nm and E 1.3 X lo5 M-l cm-l ), the nature of its adsorption on electrode surfaces, the ability to make covalent surface attachment by more than one type of surface coupling agent, and the ease of monitoring the surface concentrations of the dyes by electron spectroscopic techniques (x-ray photoelectron spectroscopy, XPS). Erythrosin, as well as other xanthene-type dyes, adsorbs from solution to metal oxide electrodes at monolayer and submonolayer concentrations and can sensitize the photocurrent response t o visible wavelength light.11-17 Voltammetric studies of aqueous solutions of erythrosin a t unmodified and modified SnO, electrodes led to the observation of irreversible electrochemical deposition of multilayers of the intact chromophore, coincident with the electrochemical oxidation. We made use of this unusual adsorption process to attach more than one monolayer of the dye to the silane-modified SnO, electrodes. Covalent attachment of erythrosin to SnOz electrodes was accomplished by the formation of an amide linkage to aminosilane-modified electrode surfaces as reported recently for rhodamine B.' A thiol-erythrosin linkage was also made tQattach the dye to the electrode surfaces through mercaptosilane coupling agents.18 Characterization of the modified electrode surfaces was accomplished using XPS analysis of the iodine (3d,/z,s/z) transitions. The high XPS photoionization cross section for the I(3d5,2) transitions (19.33 vs. C(1s) = l.0)19allowed for unambiguous detection of small surface concentrations of covalently bound or electrochemically adsorbed erythrosin. Differential capacitance studies and cyclic voltammetric studies of the ferrocyanide oxidation of both types of dye-modified SnOz electrodes indicated that the electrochemical adsorption of the dye effectively blocked the SnOz electrode mrface toward further electron transfer. The covalent attachment of the silane and dye, however, did not affect the charge transfer behavior of ferrocyanide a t SnO* The photoelectrochemical oxidation of oxalate ion with all types of erythrosin modified SnO, electrodes showed equivalent or
0 1978 American Chemical Society
The Journal of Physical Chemistry, Vol. 82, No. 11, 1978
Study of Erythrosin-SnO, Electrodes
TABLE I: Conditions for Silanization Silane 7-Aminopropyltrimethoxysilane
(pr-silane) 3 4 2-Aminoethyl)propyltriethoxysilane (en-silane) Mercaptoprowltrimethoxssilane (SHIsilanej -
Solution
Reaction conditions
Electrode formed
5% in dry toluene
12 h reflux
pr-SnO,
1% in dry toluene
0.5 h reflux
en-SnO,
1-5% in dry toluene
1h, no reflux
SH-SnO,
greater photocurrents for the electrodes with covalently attached erythrosin compared to those with the electrochemically adsorbed dye. Irradiation for long time periods also indicated considerably better stability for the covalently attached dye.
Experimental Section Electrochemical experiments were carried out in Lucite cells similar to those previously described.l A potentiostat of conventionaldesign was used for all studies. Differential capacitance measurements were made by modulating the electrode potential with a 300-800 Hz, 10-20 mV sine wave and measuring the quadrature component of the potentiostat response with a Princeton Applied Research, 126, lock-in amplifier.'t6 Photoelectrochemical studies were made using a 450- or 1000-W xenon arc lamp, coupled with a 64x1. water filter, Corning IR filter, condensing lens, light chopper (13 Hz), monochromator or optical filters, potentiostat, and lock-in amplifier.'^^^^ All solutions were prepared from doubly deionized water and thoroughly deareated prior to use. pH 4 and 7 buffers were prepared from potassium hydrogen pthalate or potassium dihydrogen pthalate and either nitric acid or sodium hydroxide. Tin oxide electrodes were obtained from Pittsburgh Plate Glass Co. (Pittsburgh, Pa.). We have found that different preparations of the same electrode material from the same commerical source or from other commercial sources caused variations in the electrochemical behavior of the various dyes. Studies reported here were all obtained from the same preparation of tin oxide. Toluene (Drake) was dried in magnesium sulfate and distilled from potassium metal under a nitrogen atmosphere prior to use. Tetrahydrofuran (Mallinkrodt) was dried by the same procedure. 3-(2-Aminoethyl)propyltriethoxysilane, mercaptopropyltrimethoxysilane (Dow Corning), and y-aminopropyltrimethoxysilane(ICN) were not purified prior to use. All silanes were stored in a freezer when not in use. Rhodamine B, erythrosin, eosin, and fluorescein (Eastman Kodak) were used without purification. Erythrosin was dried in a vacuum oven at 35-40 "C for 24 h prior to use in the amide reaction. Dicyclohexylcarbodiimide (DCC) (ICN) was used as obtained from the manufacturer. Chemical modification of the electrodes was carried out using established procedures.1-6 SnOz electrodes were thoroughly cleaned in an ultrasonic cleaner using successive washings of detergent, ethanol, and distilled water. The electrodes were then refluxed for 2 h in 0.1 M HN03 to protonate all of the oxide surface sites. Silanization of the electrode surfaces was carried out in a totally anaerobic environment with the conditions shown in Table I. Following silanization, the electrodes were refluxed in portions of dry toluene to remove excess silane from the surface. Erythrosin was attached to the SH-Sn0, electrodes by soaking the SH-Sn0, electrodes in a pH 7 buffer M erythrosin for 12 h at room temcontaining 1 X perature. Erythrosin was attached to the pr-SnO, and en-Sn02 electrodes by the reaction of the dye and dicyclohexylcarbodiimide (DCC) in dry tetrahydrofuran at
1289
- I
100 mV
I
I
I ov
I
05
/ I 0
Applied Potential (volts vs AgVAgCI)
Figure 1. Linear sweep voltammetry of rhodamine B and erythrosin at SnOp electrodes in pH 7 aqueous media, scan rate 11.5 mV/s: (a) 2.5 X M rohodamine B at unmodified Sn02; (b) 2.5 X M erythrosine at unmodified SnO,; (c-f) 2.5 X M, 5.0 X M, 1.0 X M, 2.5 X M, 5.0 X M erythrosin at pr-Sn02 electrodes: (g) 2.5 X M erythrosin at an en-SnOp electrode.
0 "C for 48 h.' Excess dye was removed from each electrode by sohxlet extraction for several hours in ethanol, acetone, or water. XPS spectra were obtained on a Physical Electronics 548 ESCA/Auger spectrometer, using a magnesium anode operating at 400 W of power. Vacuum was maintained at Torr or less. Spectra were signal averaged with a Data General Nova-800 computer with 42K core memory. Deconvolution of spectra was accomplished by an established procedure.1i20Binding energies of the I(3d3/2,5/~) spectral lines were corrected for charge shift by referencing to the Sn(3dBjz)line (BE = 486.2 eV) from the underlying electrode surfaces.20 Binding energies were compared to standards of erythrosin powder pelletized with SnOz powder or from erythrosin films physically adsorbed to SnOz electrodes.
Results and Discussion Linear Sweep Voltammetry of Erythrosin at SnOz Electrodes. The xanthene-type dyes, such as rhodamine B and erythrosin, undergo irreversible oxidation and adsorption at many electrode ~ u r f a c e s . l ~Linear - ~ ~ sweep voltammetry (LSV) of the solution forms of these dyes was carried out in order to anticipate the electrochemical behavior of these dyes when covalently attached to SnOz electrodes, Figure l a and l b shows the linear sweep voltammograms M solutions of rhodamine B and erythrosin of 2.5 X in pH 7 solutions at an unmodified SnOa electrode. The peak potential for erythrosin oxidation (EP= 0.862 V vs. Ag+lAgCl) was shifted cathodically from that for rhodamine B (EP= 1.055V vs. Ag+lAgCl (saturated KC1)). The faradaic process involves the irreversible adsorption of the dye, considering the current densities obtained at low dye concentrations (1X to 5 X low4M), the symmetrical shape of the faradaic wave, and the apparent visible buildup of product on the electrode surface. The electrodes were allowed to equilibrate with the erythrosin solutions for times up to several minutes prior to initiating the potential scan. There were no observable effects on
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The Journal of Physical Chemistty, Vol. 82, No. 11, 1978
+
+
+ +
I
l
l
1.0 v
1
0.5
0.0
Applied Potential(volts vs. Ag+/AgCI).
Flgure 2. Linear sweep voltammetry of erythrosin at SH-SnO electrodes, scan rate 11.5 mV/s: (a) 5.0 X M; (b) 1.0X 10-I M; (c) 2.5 X lom4M; (d) 5.0 X lo-' M.
the LSV curves brought about by variations in preequilibration times. No cathodic desorption currents were observed when cyclic voltammetric studies of the oxidation reaction were undertaken. At a fixed dye concentration, the amount of oxidative charge transferred decreased with increasing scan rate, indicating that not all of the dye oxidized was adsorbed at potentials cathodic of the onset of the anodic process. In addition, the anodic peak current, or the total anodic charge transferred, was linearly proportional to the bulk dye concentration from 1 X to 1X M. Above 1X lom3M erythrosin, the peak current and charge transferred during oxidation showed nonlinear response to erythrosin concentration, as would be expected if the electrode surface were saturated with adsorbed dye. A cathodic shift in the anodic peak potential of approximately 50-60 mV was observed for the voltammograms obtained in the concentration range 1 X loT5to 1 X M erythrosin. Similar experiments conducted in 0.1 M KC1 or 0.1 M KBr solutions did not show any observable differences in the voltammetric behavior of erythrosin although both C1- and Br- are understood to adsorb to SnOz in the same potential range as er~thr0sin.l~ Linear sweep voltammetry of erythrosin was also conducted on pyrolitic graphite electrodes and showed nearly identical peak potentials and oxidative charge as that observed on SnOzelectrodes. Electrodes subjected to the oxidation of these dyes lost most of the visible evidence of adsorbed dye when rinsed in solvents such as Hz0 or ethanol. XPS data shown below indicated that a small concentration of a tightly bound form of a species resembling unoxidized erythrosin was present on the surface after rinsing of the unmodified Sn02 electrodes. Oxidation of erythrosin on SnOz electrodes modified with the amine-terminal silanes (pr-SnOz and en-SnOz) and mercapto-terminal silanes (SH-SnOZ) was quite different from that on unmodified SnOz electrodes. In summary, larger oxidation currents were observed at every erythrosin concentration, and oxidation peak potentials were shifted cathodically. These effects were most pronounced on SH-Sn02 electrodes and with the tetrahalogenated forms of fluorescein (eosin and erythrosin). Oxidation of erythrosin or eosin on the modified electrode surfaces resulted in a more permanent deposition of dye material than on unmodified electrode surfaces. Linear sweep voltammograms of erythrosin or pr-SnOz, en-Sn02, and SH-SnOZ electrodes are shown in Figure l e g and Figure 2a-d. At low erythrosin concentrations M) on the pr-SnOz and en-SnOz electrodes, the (5 X peak potential was near that observed for the oxidation on unmodified SnOP At higher erythrosin concentrations (up to 5 X low4M) on en- and pr-Sn02 electrodes the overall faradaic peak was broadened and shifted cath-
D. D. Hawn and N. R. Armstrong
odically (Figure If and lg) to potentials 50-60-mV cathodic of the initial peak potential. There was some indication that this may be due to an additional faradaic process, cathodic of the original process by 50-100 mV. The amount of charge transferred during the oxidation at every erythrosin concentration increased 30-50 % over that observed on unmodified SnOz electrodes. At low erythrosin concentrations, the peak potential for the oxidation on SH-SnOZ electrodes was shifted cathodically vs. unmodified SnOz ( E = 0.770 V, Figure 2). The oxidation peak potential diJnot shift significantly with increasing erythrosin concentration on the SH-SnOZ electrodes, however, the amount of oxidative charge transferred for the oxidation on the SH-SnOZ electrodes was nearly twice that observed for unmodified SnOz electrodes. As an example of this effect, a 2.5 X lo-* M solution produced 497 pC/cm2 of oxidative charge on a clean SnOz surface, 710 pC/cm2 of oxidative charge on a pr-SnOz surface, and 1124 pC/cm2 of oxidative charge on an SH-Sn02 surface. These values are precise to &lo% for different experiments because of variations in the active electrode area exposed to solution.lV2 Oxidation of fluorescein (non-halogenated form of erythrosin) on SnOz proceeded as for rhodamine B. However, the peak potential for fluorescein was near the anodic limit of the SnOz electrode (>1.10 V vs. Ag+lAgCl) so it was difficult to ascertain the exact effect of surface modification on its voltammetric oxidation. Transmission spectrophotometric investigation of the SnOzelectrodes used in these LSV experiments indicated that the absorbance band of the surface-boundspecies was identical with that of the solution form of the dye (Arnm 527 nm). The concentration of surface bound dye was estimated from the absorbance at 527 nm. A pr-SnOz electrode with 820 pC/cm2 of erythrosin oxidation charge transferred showed an absorbance at 527 nm of 0.080 following rinsing, correspondingto a surface concentration of approximately 2-5 monolayers (see explanation below). We can conclude that not all of the deposited dye remains on the electrode surface, but certainly more than that observed on an unmodified Sn02 electrode, where no spectrophotometrically detectable dye was retained following oxidation in the same erythrosin solution. Although the surface-bound dye could not be easily removed by rinsing in solvents such as ethanol, H20, or acetone, continuous soxhlet extraction of these electrodes in the above solvents could eventually remove the visible evidence of adsorbed dye. Investigation of a model redox couple on modified electrode surfaces is an important way of determining the extent to which the surface is blocked toward electron transfer to a solution species.2 Following the electrochemical deposition of the dye, each SnOz electrode was rinsed in H 2 0 and then used for cyclic voltammetry in a loT3M (pH 4) ferrocyanide solution. The amount of charge transferred during the linear sweep voltammetricoxidation of erythrosin, QT,was correlated with separations in anodic and cathodic peak potentials, E,, for the ferrocyanide oxidation, determined by cyclic voltammetry, and effective electrode areas, determined chronoamperometrically
Unmodified SnOz electrodes showed only a small change in their behavior toward the ferrocyanide oxidation (AE,, and Aeffdecreased N 10%) following erythrosin oxidation of up to 300 pC/cm2 (erythrosin solution concentrations up to 5 X lo4 M). The dye is not tightly retained to those
The Journal of Physical Chemistry, Vol. 82, No. 11, 1978 1291
Study of Erythrosin-Sn02 Electrodes
TABLE 11: Iodine to Tin Ratios and Binding Energies for Electrochemically and Mechanically Adsorbed and Covalently Attached Dyes Sample Qa&, &/cmZ NIINSna Binding Energy,b eV
1. Dye physically adsorbed on clean SnO, (ca. less than 1 monolayer) 2. Electrochemically adsorbed on pr-silane/SnO, M (dye) = 5.0 x
122.0
3. Electrochemically adsorbed on pr-silane/SnO, (dye) = 1.0 x lou4M 4. Electrochemically adsorbed on pr-silane/SnO, (dye) = 2.5 x M
312.0
5. Electrochemically adsorbed on pr-silane/SnO, (dye) = 5.0 x M
930.0
6. Electrochemically adsorbed on en-silane/SnO, (dye) = 2.5 X lov4M 7. Electrochemically adsorbed on SH-silane/SnO, (dye) = 5.0 X M 8. Erythrosin covalently attached to en-SnO,
496.0
599.0
63.0
* 0.2
0.06
621.5
2.44 1.49 ( 1 ) 0.95 ( 2 )
618.0 i 0.2 (1) 621.2 i: 0.2 (2)
5.08 0.80 (1) 4.28 (2)
618.4 i 0 . 2 (1) 621.0 i. 0.2 ( 2 )
30.4 6.47 (1) 24.0 ( 2 )
618.1i 0.2 (1)
80.8 9.44 ( 1 ) 71.3 ( 2 )
618.6 i: 0.2 ( 1 ) 621.2 + 3.2 ( 2 )
49.3 15.4 (1) 33.9 (2)
618.0 i: 0.2 ( 1 ) 621.6 i 0.2 (2)
9.17
0.5-1.0
621.1 i 0.2 ( 2 )
621.0 + 0.2 (other species unresolved) 621.2 * 0.2
0.5-1.0 621.2 * 0.2 9. Erythrosin covalently attached to SH-SnO, a Corrected for known molecular cross-section difference^,^^ the first number listed is the total N r / N ~ ratio, , the next n u m bers are for each separate iodine species. Referenced to Sn(3d5,,)band at 486.2 eV.20
electrodes during the rinsing procedure. SH-Sn02, pr-Sn02, and en-Sn02 electrodes showed no differences in the electrochemical behavior of the ferrocyanide oxidation vs. unmodified S n 0 2 electrodes. SH-Sn02, pr-Sn02, and en-SnOz electrodes however became passivated toward the ferrocyanide oxidation following electrochemical oxidation of erythrosin. For example, where QT exceeded 200-250 pC/cm2 on the SH-Sn02 electrodes, AE increased about 10% (from ca. 100 to 110 mV) and Aeffcfecreased to ca. 25% of the value obtained for an unmodified Sn02 electrode surface (0.161 vs. 0.636 cm2). Within the erythrosin concentrations explored, measurable currents for the ferrocyanide oxidation could still be obtained on these modified electrodes. The fact that currents for ferrocyanide oxidation were observed at all on the erythrosin/Sn02 electrodes may result from incomplete coverage of the electrode by the adsorbed erythrosin, Le., “islanding” and/or the fact that the multilayer dye assembly exhibits some appreciable conductivity of its own. This hypothesis is supported by photocurrent data discussed later. Surface analysis data shown below further elucidates the state of this electrochemically adsorbed dye. XPS Studies of Electrochemically Adsorbed Erythrosin on SnOz Surfaces. Figure 3 shows a series of XPS, I(3d) spectra for Sn02 electrodes used to oxidize various erythrosin solutions (the same electrodes as in Figure 1). The I(3d) peaks constitute an unambiguous label for the determination of the presence of surface bound dye. Binding energies of the I(3d5 2 ) peaks were determined by referencing to the 486.2-ek, Sn(3d5iz)peak20and are listed in peaks were symmetric with a Table 11. The Sn(3d3/2,5/2) full-width-at-half-maximum within experimental error of previously published values (1.5 eV).20 No change in the chemical state of the tin species was observed as a result of the adsorption reaction. Clean SnOz electrodes with erythrosin physically adsorbed to the surface gave an
1(3d5/2)binding energy of 621.2 f 0.2 eV (Figure 3a). An identical I(3d5,2) binding energy was observed for erythrosin electrochemically adsorbed to a clean SnOz electrode (excess dye removed by solvent washing). I(3d) spectra for erythrosin electrochemically adsorbed to en-Sn02 and pr-Sn02 electrodes indicated the presence of more than one iodine species attached to the surface (Figure 3b-f). Electrodes prepared with low erythrosin concentrations (QT< 150 pC/cm2) showed an I(3d5/2)peak at 618.2 f 0.2 which was nearly twice as intense as the 621.2-eV peak (Figure 5b). As the concentration of erythrosin used to prepare the electrodes was increased, the total I(3d3/2,52) peak intensities increased, the 621.2 eV (3d5/2)peak Aecame the most intense, but the 618.2 eV peak remained in each spectrum. The 618.2 eV binding energy iodine species is consistent with a more oxidized form of iodine than that present in the native erythrosin molecule. The 618.2 eV binding energy is in agreement with that reported for molecular Iz while the 621.2 eV binding energy is in accord for iodo-substituted aromatic molecules.25 The presence of the new iodine surface species is coincident with the enhancement of the faradaic process observed for the erythrosin oxidation/adsorption on the pr-SnOz and en-Sn02 electrodes. The lower binding energy iodine species may be adsorbed at potentials cathodic of the oxidation of clean SnOz,hence the cathodic shift in peak potential and general broadening of the faradaic wave. The pr-SnO2, en-SnOz, and SH-SnOZ surfaces may also initiate or catalyze a new oxidation process for the dye which results in the deposition of an oxidized iodine species. I t is not likely that a covalent attachment is brought about by the oxidation on these surfaces, since solvent extractions can remove the dye. Comparison of the surface coverage of erythrosin with the charge passed on oxidation, QT, is made in Table I1 and Figure 4. I/Sn relative atomic ratios were computed from
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The Journal of Physical Chemishy, Vol. 82, No. 11, 1978
D. D. Hawn and N. R. Armstrong
[L 0
n
b.
N(E)
I\
* cts/sec.
-LkeLkn
I
E
1
640
I
60
I
630 620
I
610
I
600
BINDING ENERGY (eV) Flgure 3. XPS spectra of I(3d3/2,5/2)transitions on (a) several monolayers of erythrosin physically adsorbed to an SnO, electrode; (b-e) electrochemically adsorbed erythrosln on pr-Sn02 electrodes from solutions of 5 X IO-‘ M, 1.0 X I O 4 M, 2 6 X lo4 M, 5.0 X lo4 M, respectively; (f) electrochemically adsorbed erythrosin on an en-Sn02 electrode from a 2.5 X M solution.
the integrated I(3d5j2)and Sn(3dsI2)spectra, normalized to published cross sections.lg The sharp increase of I/Sn ratio with increasing QT is consistent with the exponential relationship between the Sn(3dslz)signal intensity and its distance beneath the dye-film surfacesz6While the total I/Sn ratio increases as a function of electrochemical charge transferred, the I/Sn ratio for the 618.2-eV I(3d5/2)peak alone reached a near constant value vs. increasing surface concentration. This would be expected if the new type of iodine species were deposited first during the voltammetric sweep, while the erythrosin responsible for the 620.2-eV I(3d5/2)peak was deposited on top of the new surface species. The thickness of the adsorbate overlayer can be estimated from the ratio of intensities of the Sn(3d5/J signal for the clean (lo)and modified (I) SnOz electrodes2
Oxidative Charge Transfered.QT
( rCoul/crn2)
Figure 4. Corrected I/Sn ratios vs. estimated surface concentrations of erythrosin on various SnO, electrodes: (a) A, I/Sn ratios computed using the total I(3dSl2 signal intensity on pr-Sn0, electrodes with electrochemically adsorbed erythrosin; (b) V,I/Sn ratios computed Using 618.2-eV I(3dSI2) signal intensity on pr-Sn02 electrodes with electrochemically adsorbed erythrosin; (c) B, as for (a) above on en-SnO, electrode: (d) 0 , as for (a) and (c) above on SH-Sn02 electrode: (e) 4, ca. 1 monolayer of physically adsorbed erythrosin on Sn02 electrode, equivalent to the I/Sn ratio computed for covalently attached erythrosin on an en-Sn, electrode and covalently attached erythrosin on a SH-Sn0, electrode.
where d is the film thickness and X is the escape depth of the Sn(3dslz) photoelectron (ca. 11 A). Estimates of overlayer thicknesses using this technique was plagued by uncertainties in the angular distribution of the ejected electrons, the uncertainty in the true escape depth of the photoelectron through the organic overlayer, and the variation in Io between experiments (f10-20%). Nevertheless, estimates of the adsorbate thickness were made, indicating that an I/Sn ratio of 80.8 corresponded to an approximate thickness of 41.7 A while an I/Sn ratio of 2.44 corresponded to an approximate thickness of 22 A. This thickness includes the silane overlayer thickness (less than 10 A).2 Comparison of these data with the transmission spectrophotometric evidence given above clearly shows that not all of the adsorbed erythrosin on the modified electrode surfaces consists of an active chromophore. The XPS experiments indicate the number of iodine-containing molecules undergoing oxidation on the electrode exceeds the monolayers of active chromophore which are adsorbed. As shown below however, the surface bound molecule can yield a photocurrent response on these electrodes despite the apparent buildup of other organic material on the electrode surface. Further spectroelectrochemicaland dc surface conductance experiments are underway to explore more fully the nature of this reaction. Our preliminary results indicate that the SH-Sn02 electrodes appear to capture more of the active chromophore during the adsorRtion process than pr-SnOZ or en-SnO2 electrodes. It appears possible that the silane layer acts as a “template” to enhance adsorption on the electrode surface. Covalent Attachment of Erythrosin to SnOz. Covalently attached erythrosin that could not be removed from the electrode surface by repeated Soxhlet extractions in solvents such as EtOH, HzO, and acetone for 24 h was obtained for the SH-SnOZ, pr-SnOz, and en-Sn02 electrodes. In the case of the pr-Sn02 and en-SnOz electrodes, covalent attachment was made via amidization to the carboxylate group on erythrosin, The mercapto-terminal function of the SH-Sn02 electrodes acts as an excellent nucleophile to displace at least one of the iodine atoms from erythrosin, resulting in a stable carbon-sulfur bond to hold the dye to the electrode surface. Transmission spectrophotometricassays of the electrode
The Journal of Physical Chemistry, Vol. 82, No. 11, 1978 1293
Study of Erythrosin-SnO, Electrodes
surfaces or of clean glass subjected to the same synthetic conditions indicated that easily observable amounts of the dye could be bonded to the metal oxide or silica surfaces. No difference was observed in the absorbance spectrum of the dye covalently bound to these surfaces vs. the solution spectrum. Assuming the extinction coefficient for the bound dye is the same as in aqueous pH 7 solution (1.3 X lo6 M-l cm-l ) 9 absorbance values of approximately 0.020-0.025 at 527 nm for the thiol or amide attached dye indicated surface coverages of approximately 9 X 1013 molecules/cm2. Assuming 81 A2 per dye molecule (projected flat surface area) a maximum dye coverage of 1.2 X 1014molecules cm2 would be expected. KirkovZ3has estimated 2 X lo1 oxide sites per cm2on the SnOz surface, whereas a lower concentration of ca. 3 X 1014sites per cm2 has been estimated for silica surfaces.24 Linear sweep voltammetry was attempted for the covalently attached erythrosin/Sn02 electrodes using the conditions shown in Figure 1. No detectable oxidation of the dye was observed. This may be a result of the fact that covalently attaching the dye to the electrode surface shifts the oxidation potential beyond the anodic limit of the electrode in this solvent. Since the normal solution oxidation process involves adsorption of the dye molecule, the preattached dye may show significantly different electrochemistry. The photoelectrochemical response of the attached dye is however quite resonable. Changes in the surface structure of the modified SnOz electrodes were qualitatively ascertained by electrode capacitance measurement techniques. Differential capacitance vs. potential measurements were made on unmodified Sn02electrodes and on SH-Sn02, pr-SnOz, and en-SnOz electrodes, before and after covalent attachment of erythrosin. An apparent carrier density, no, was calculated from the slope of the relationship between the electrode potential (E) and the reciprocal of the square of the differential capacitance (1/C2)
i
1/C2= (2/eeoeno)(E- E F B - kT/e)
(3)
where the other terms of this equation have been defined previously.lI6 For the highly doped SnOz electrodes, it has been shown that a significant contribution from the Helmholtz layer capacitance (C,) can cause a shift in the observed flat-band potential (Em,oM) vs. the true flat-band potential (EFB).ls6 E F B = EFB,obsd
- kT/e
f eoeeno/2CH2
(4)
Chemical modification of the Sn02electrode surface may therefore cause a change in both the apparent carrier density, no, and the apparent flat-band potentiaL6 In general, the attachment of the SH-silanes (from dilute silane solutions) to Sn02 electrode surfaces caused only a modest decrease in the apparent carrier density, no, (less than 10%) and little change in the apparent flat-band potential. Further attachment of erythrosin to the SH-SnOZ surfaces caused little further change in either of these parameters. The attachment of pr-silanes or en-silanes and erythrosin to the Sn02electrodes however caused a 50% decrease in the apparent carrier density, no (1.9 vs. 4.7 X 1021/cm3),and a 200-400-mV cathodic shift in the apparent flat-band potential which may be due entirely to a change in the Helmholtz capacitance. These effects appear to be very dependent upon the concentration of silane used in the initial surface modification and are the subject of further study. The change in the electrode surface following attachment of the various silanes and dyes was also monitored
650
640
630
620
BINDING ENERGY (eV)
Figure 5. XPS I(3d3,2,5,2)spectra for (a) erythrosin physically adsorbed to a SnO, electrode at a concentration of ca.1 monolayer; (b) erythrosin covalently attached to a SH-SnO, electrode; (c) erythrosin covalently attached to an en-SnO, electrode.
by cyclic voltammetric and chronoamperometric studies of the ferrocyanide oxidation as discussed above. The AE,, and effective electrode surface areas were relatively unchanged (less than 5% decrease) by attachment of erythrosin to either SH-Sn02, pr-Sn02, or en-Sn02 electrodes. Ferrocyanide apparently remains largely accessible to the electrode surface following chemical modification. This is consistent with a silane + dye surface coverage of an equivalent monolayer or less as shown by the spectrophotometric assay. X P S Studies of Covalently Attached Erythrosin to spectra for en-Sn02 and S n 0 2Surfaces. XPS I(3d3/2,5/2) SH-SnOZ electrodes with covalently attached erythrosin are shown in Figure 5b and 5c and the pertinent binding energies and NI/Nsn ratios listed in Table I1 for various dye modified electrodes. The 1(3d5/2) spectra were symmetric with only one spectral component and had binding energies consistent with the dye which was physically and electrochemically adsorbed on unmodified Sn02 NI/Ns,ratios and approximate thicknesses indicate a much lower surface coverage than for the adsorbed dyes on the modified electrodes. Figure 5a shows the spectra for erythrosin physically adsorbed to a SnOz electrode at a concentration of approximately one monolayer. The similarity in intensities of the spectra in Figure 5a-c confirms our hypothesis that the covalently attached dye is present at roughly monolayer concentrations on the Sn02 electrode surface. It should be pointed out that erythrosin may not be attached to the amide-modified SnOzelectrodes by a strictly covalent bond. Unmodified S n 0 2 electrodes which were subjected to the same amidization reaction conditions as the pr-Sn02 and en-Sn02 electrodes showed an attached form of erythrosin which was stable to solvent extraction. Our surface and spectrophotometric analysis indicated that this erythrosin was present at approximately half the surface concentration of that observed on the en-SnOz electrodes. We have observed similar results for the attachment of rhodamine B to SnOz,which contradicts earlier reports.' Unsilanized Sn02electrodes subjected to the thiol attachment scheme did not show any bound dye, so we are confident that erythrosin found on the SH-Sn02 electrodes is present
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The Journal of Physical Chemistry, Vol. 82, No. 11, 1978 I
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Flgure 6. Photocurrent response (lock-in amplifier response to a 13 Hz, chop ed-light beam, induced photocurrent; bias potential = 1.O V vs. Ag IAgCl) vs. Qads for electrochemicallyadsorbed erythrosin on (0)pr-Sn0, electrodes. Also shown are the responses for erythrosin (0)covalently attached to en-SnO, and SH-SnO, electrodes. (Q) Unmcdified SnO, electrodes and SnO, electrodes modified with en-silane or SH-silane but without erythrosin.
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because of a true covalent attachment. Photocurrent Experiments. Figure 6 shows that comparison of photocurrent response of SnOz electrodes with electrochemically adsorbed and covalently attached erythrosin in the oxidation of oxalate ion. The incident light was from a 450-W xenon arc lamp, filtered to bracket the visible absorbance band of the dye, A,, 527 nm. The output of the lock-in amplifier, used to detect the photocurrent response at an applied bias of +1.0 V vs. Ag+lAgCl, is plotted vs. QT of the electrochemically adsorbed dye (calculated from electrochemical data). The photocurrent response was found to appear at a bias potential of ca. +0.2 V vs. Ag+lAgCl and to be nearly linearly dependent upon applied potential up to +1.0 V, where an apparent plateau value was reached. The response for the photoinduced oxidation of 0.05 M oxalate reached a maximum at 600 < QT < 800 pC/cm2. While the exact surface concentrations of the dye are not known, it is clear that the photocatalytic properties of the adsorbed dye are dependent upon some optimum surface arrangement. Beyond ca. QT = 600-700 pC/cm2 of dye, the photocatalytic response plateaus or decays slightly, probably because of the electrical resistance developed by the thickening adsorbate layerlo and because the surface concentration of active retained dye probably reaches a maximum. I t is important to note that the photocurrent response of SH-SnOZ and en-SnOz electrodes with covalently attached erythrosin is much greater than that for electrodes with adsorbed dyes at higher surface concentrations. Erythrosin attached to en-SnOZ electrodes has a surface concentration of 1-2 monolayers at maximum, yet the photocurrent response is near that observed for several more monolayers of electrochemically adsorbed dye. Murray has hypothesized that covalently attached molecules must necessarily be accessible to the electrode surface via the normal molecular flexing of the coupling ~ i l a n e . ~The - ~ photoinduced oxidation of a supersensitizer such as oxalate (which catalytically regenerates the unoxidized form of the dye) requires the interaction of the dye in its excited state with the electrode surface and, in its oxidized state, with solvated oxalate Clearly this type of interaction is optimized for the covalently attached dye as opposed to the electrochemically adsorbed dye multilayers. Figure 7 shows a typical photocurrent/time response for a covalently attached erythrosin-pr-Sn02 electrode vs. a pr-SnOz electrode with electrochemically adsorbed dye
Flgure 7. Photocurrent (at 13 Hz) vs. time for erythrosin electrochemically adsorbed to pr-SnO, (0)and erythrosin covalently attached to pr-SnO, (+) in a 0.1 M oxalate solution (pH 4).
(the amount of charge transferred during adsorption corresponds to roughly 860 pC/cm2. The data were obtained as the output of the lock-in amplifier, the light was modulated at 13 Hz. The currents shown may therefore include contributions from charge separation in the dye monolayer. Because the light excitation is periodic, regeneration of the ground state dye is also possible. Nevertheless, these long-time aging experiments clearly show the relative stability of the covalently attached dye. The adsorbed dye electrode photocurrent decayed to nearly half its maximum value in approximately 60 min, while the covalently attached dye reached a steady state activity. A turnover number of 8.6 X mol/cm2 s was estimated for the covalently attached dye, which corresponds to one full turnover of every attached dye molecule every 100 s. By increasing the oxalate concentration or by using ascorbic acid (0.1 M) as a reducing agent, the turnover number could be increased to ca. 8.0 X mol/cm2 s or a turnover of every dye molecule in 10-15 s. It will be of interest to see if further experiments can improve on this turnover number, a necessity for future application of these electrode systems. Further experiments with the xanthene-type dyes and phthalocyanine dyes attached by similar covalent attachment schemes are equally enco~raging.~' Acknowledgment. The authors thank Dr. Ted Kuwana, Ohio State University, for the use of the Physical Electronics 548, ESCA/Auger System, and the excellent technical assistance of John Evans and Albert Lin, also Ohio State University. This work was partially supported by the Petroleum Research Fund, administered by the American Chemical Society.
References and Notes (1) N. R. Armstrong, A. W. C. Lin, M. Fujihara, and T. Kuwana, Anal. Chem., 48, 741 (1976). (2) P. R. Moses, L. Wler, and R. W. Murray, Anal. Chem., 47, 1882 (1975). (3) J. C. Lennox and R. W. Murray, J. EWoanal. Chem., 78,395 (1977); D. F. Untereker, J. C. Lennox, L. M. Wier, P. R. Moses, and R. W. Murray, ibid., 81, 309 (1977). (4) P. R. Moses and R. W. Murray, J. Ektroanal. Chem.,81, 393 (1977). (5) J. R. Lenhard and R. W. Murray, J. Necfroanal. Chem., 81, 195 (1977). (6) M. Fujihara, T. Matsue, and T. Osa, Chem. Left., 785 (1976). (7) M. Fujihara, Nature (London), 264, 349 (1976). (8) J. F. Evans, T. Kuwana, M. Henne, and G. R. Royer, J. flectroanal. Chem., 80, 409 (1977). (9) N. R. Armstrong and R. Shepard, manuscript in preparation. (10) H. Gerischer and F. Wiillg, Top. Current Chem., 61, 31 (1976). (11) M. T. Spitler and M. Calvin, J . Chem. Phys., 66, 4294 (1977). (12) T. Watanabe, A. Fujishlma, 0. Tatsuoki, and K. Honda, Bull. Chem. SOC. Jpn., 49, 8 (1976). (13) H. Kim and H. A. Laltlnen, J . Electrochem. Soc., 122, 53 (1975). (14) L. Gouverneur, G. Leroy, and I. Zador, f/ectrochem. Acta, 19,215 (1974). (15) M. Heycorsy, S. Vavricka, and R. Heyrovska, J. Ebcfmnal. Chem.,
The Journal of Physical Chemistry, Vo/. 82, No. 7 1 , 1978 1295
Unusual Behavior of Vaporized Magnesium
(21) H. A. Laitinen, C. A. Vincent, and T. M. Bednarski, J . Nectrochem. SOC.,115, 1024 (1968). (22) K. Hauffe and U. Bode, Discuss. Faraday Soc., 68, 281 (1975). (23) P. Kirkov, Nectrochem. Acta, 17, 519 (1972). (24) V. Ya. Davydov, Trans. Faraday Soc., 61, 2254 (1964). (25) L. Balsenc, H. Berthou, and C. K. Jorgenson, Chimia, 29, 64 (1975). (26) M. P. Seah, Surface Sci., 32, 703 (1972). (27) N. R. Armstrong and R. Shepard, manuscript in preparation.
46, 391 (1973). (16) 1. M. Issa, R. M. Issa, M. Ghonheim, and Y. Temerk, Electrochem. Acta, 18, 265 (1973). (17) N. R. Bannerjee and A. S. Hegi, Electrochem. Acta, 335 (1973). (18) I. M. Chaiken and E. L. Smith, J. Bioi. Chem., 244, 5096 (1969). (19) J. H. Scofieid, J . Nectron Specfrosc., 8 , 129 (1976). (20) A. W. C. Lin, N. R. Armstrong, and T. Kuwana, Anal. Chem., 49, 1228 (1977).
Unusual Behavior of Vaporized Magnesium. 2. Evidence for Gas-Surface Exchange L. B. Knight, Jr.,* K. S. Stewart, and W. T. Beaudry Departmenf of Chemistry, Furman University, Greenville, South Carolina 296 73 (Received June 22, 1977; Revised Manuscript Recelved October 25, 7977) Publication costs assisted by the National Science Foundation
Previous experimental evidence indicated that vaporized magnesium atoms in the presence of trace quantities of certain impurity gases can exhibit a substantially reduced sticking coefficient. Magnesium isotopes have been used to determine if the mechanism responsible for producing the low sticking coefficient could involve exchange between gas phase and surface bound magnesium. Evidence for the occurrence of such exchange is discussed.
Introduction Experimental conditions have been recently reported which can cause vaporized magnesium atoms to exhibit a substantially reduced sticking coefficient.’ The simultaneous impingement of magnesium vapor and trace quantities of certain volatile species containing halogen atoms under high vacuum conditions can condition ambient temperature surfaces so that the sticking coefficient of vaporized magnesium atoms is reduced to such an extremely low value that magnesium behaves as if it were a semipermanent gas. The previous report has presented the detailed experimental evidence used to reach the above conclusion. The evidence included the establishment of the identity of the volatile magnesium species as atomic magnesium, the chemical specificity of the effect, and the determination of an approximate value for the sticking coefficient. The sticking coefficient value of 0.04 previously reported for the isolated glass bulb was calculated incorrectly. The original calculations failed to include the possibility of multiple surface collisions of a single atom. We are grateful to one of the reviewers for suggesting a reexamination of this parameter. The experimental data reported in Figure 6 of ref 1 have been employed to determine a revised sticking coefficient value of 1.3 X 10“. The details of this calculation are presented in Appendix
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1. Efforts have been made to obtain information about the surface mechanism responsible for causing the reduced sticking coefficient. Isotopically labeled magnesium has been employed to determine whether or not exchange between impinging and surface bound magnesium occurs. The experimental results clearly indicate that significant exchange does in fact occur. Furthermore, the evidence seems to indicate that exchange occurs predominantly on glass surfaces of the apparatus. However, this conclusion is only tentative at this stage.
Experimental Section The surface exchange utilized the double cell vaporization assembly shown in Figure 1. Experimental pa0022-385417812082-1295$01.0010
rameters such as the vaporization temperatures, partial pressures of magnesium, and partial pressures of impurity gases are identical with those described in ref 1. This assembly is identical with the single cell vaporization system used previously except it contains two separately controlled tantalum Knudsen cells mounted on watercooled copper electrodes. The double cell flange simply replaces the single cell assembly shown in Figure 1of ref 1. For the surface exchange experiments described in the next section, the 5-L glass sphere was connected between the vaporization chamber and the TOF mass spectrometer via 15-mm teflon stopcocks. Natural magnesium (NatMg = 79% 24Mg,11% 26Mg,10% 25Mg)was loaded into the forward cell while 99.4% 2sMg obtained from the Oak Ridge National Laboratory was placed in the rear vaporization cell. The rear 2sMg cell was surrounded by a water-cooled copper jacket which helped to prevent 24Mg contamination of the 26Mgsource. Since such contamination would invalidate the isotopic exchange results, extensive efforts were expended to prevent the contamination initially and subsequently to prove, by the deliberate design of various experimental procedures, that contamination had not occurred.
Results The basic plan of the exchange studies involved generating volatile magnesium in the “normal” manner until the surfaces in the system were thoroughly conditioned with NatMgfrom the forward cell in Figure 1. The system was taken to be “conditioned” when the Mg+ mass spectrometric signals reached a steady value near maximum. If the system had been conditioned on a given day and no part of the vacuum system had been opened to atmosphere, the time required for subsequent conditioning was about 10 min. Once a NatMgconditioned system was achieved, the forward vaporization cell was switched off and the rear cell containing 2sMgwas switched on. It takes about 10 s for the NatMgcell to cool to the point where no Mg+ ion signal can be detected. Ordinarily, =90 s is the required time for these types of cells to reheat to a point 0 1978 American Chemical Society
