9436
J . Phys. Chem. 1991, 95,9436-9438
Supplementing this central finding, we have determined several of the basic thermal and photochemical properties of phosgene on Pd( 1 1 I): ( I ) phosgene adsorbs molecularly and reversibly on Pd(l11) at 1 IO K, with desorption of the multilayer occurring at 127 K and the monolayer at 144 K; (2) upon UV irradiation, phosgene dissociates to form Cl(a) and CO(g) with no evidence
for the photodesorption of C12C0 (3) the initial photodissociation cross section is (5.3 f 0.8) X cm2.
Acknowledgment. We thank Dr. M. T. Paffett of Los Alamos National Laboratory for loaning us the Pd( 1 11) crystal. This work was supported in part by the US.Army Research Office.
Plasma Oxldatlon versus Photooxidation of Polystyrene A. G. Shard and J. P. S. Badyal* Department of Chemistry, Science Laboratories, University of Durham, Durham, DH1 3LE, U.K. (Received: March 7, 1991)
A comparative study of different ways of oxidizing polystyrene has been conducted. Desired functionalities can be introduced
into the polymer surface by carefully selecting the mode of oxidation (plasma versus vacuum ultraviolet irradiation) and the gaseous environment (02,COz, or N20). Variations in oxidized groups have been monitored by X-ray photoelectron spectroscopy (XPS) and can be correlated to the chemical nature of the surface modification.
Introduction The surface oxidation of polystyrene is an area of considerable scientific interest in terms of improving its wettability/printability and with regards to understanding its oxidative degradati~n.~' Exposure to an oxygen glow discharge is one way of modifying the surface of such a polymer? A pure oxygen plasma contains ions, atoms, ozone, and metastables of atomic and molecular oxygen, as well as electrons and a broad electromagnetic spect r ~ m .However, ~ the interaction of a plasma with a hydrocarbon polymer surface is poorly understood. In this article we report on the relative importance of vacuum ultraviolet radiation in a glow discharge. These modeling studies compare plasma oxidation of polystyrene with photooxidation under oxygen (0,)and carbon dioxide (C02) atmospheres. We consider the extent to which there is incorporation of the chemical character of the parent gas molecule into the eventual surface functionality. Also within this context, the role of atomic oxygen has been explored, by examining the reactivity of polystyrene toward photoexcited nitrous oxide (N20).
'
Experimental Section A 13.56-MHz RF generator was inductively coupled to a cylindrical glass reactor via an externally wound copper coil. This was used for glow discharge investigations. Reactant gas, 0.3 Torr, was introduced into the plasma reactor at a constant flow rate of 1.1 cm3 m i d ; the plasma was run at a power of 10 W for 3 min. This was found to be sufficient, since longer periods resulted in no further changes at the polymer surface (as detected by XPS). An additional glass chamber was attached to the aforementioned plasma reactor for the photooxidation experiments, with the two vessels being connected via a lithium fluoride window (Figure I ) . Both sides were evacuated by two-stage rotary pumps Torr. A nitrogen glow discharge (50 W, to better than 2 X 1. I cm3 min-I) was used to generate vacuum-UV radiation, the lithium fluoride window being transparent to the strongest lines ( I ) Brewis, D. M.; Briggs, D. Polymer 1981, 22, 7. (2) Allen, G., Bevington, J. C., Eds. Comprehensiue Polymer Science; Pergamon: Oxford, 1989; Vol. 6, p 532. ( 3 ) McKellar, J. F.: Allen, N. S. Photochemistry of Man-Made Polymers; Applied Science: London, 1979. (4) Clark, D. T.; Dilks, A. J . Polym. Sci., Polym. Chem. Ed. 1979,17,957. (5) Hollahan, J. R., Bell, A. T., Eds. Techniques and Applications of Plasma Chemistry; Wiley: New York, 1974.
at 174 and 149 nm. A strip of polymer was irradiated under 25 Torr of oxidizing gas. One-hour exposures were found to be sufficient for yielding XPS features that indicated saturation of the surface modification. This was much longer than the time required for the plasma treatments, since the photon flux is much higher within the glow discharge. Research grade quality oxygen, nitrogen, carbon dioxide, and nitrous oxide (BOC)were used. The polystyrene film was carefully washed with isopropyl alcohol and dried in air, this was found to give a contaminant-free surface (as measured by XPS). X-ray photoelectron spectra were acquired on a Kratos E3300 surface analysis instrument. Magnesium KLYradiation was used as the excitation source with electron detection in the fixed retarding ratio (22:l) analyzer mode. The gold 4f, level at 83.8 eV had a full width at half-maximum (fwhm) ofl 1.2 eV. XPS measurements were taken with an electron take-off angle of 30° from the surface normal. No evidence was obtained for radiation damage to the samples during the typical time scale involved in these experiments. Data accumulation and component peak analysis were performed on an IBM PC computer, using linear background subtraction and Gaussian fits with fixed fwhm (except for the T-T* transition). All binding energies are referenced to the hydrocarbon component at 285.0 eV.6 Under the conditions of these experiments, the instrumentally determined sensitivity factors are such that for unit stoichiometry the C(ls):O(ls) intensity ratio is -0.55. Results and Discussion Polystyrene consists of an alkyl chain polymeric backbone, to which phenyl rings are attached. Two peaks are observed in the C( Is) region of the XPS spectrum for the clean starting material: a hydrocarbon component (285.0 eV, 94%of total C(ls) signal) and a distinctive satellite structure at -291.6 eV (6% of total C(1s) signal), which is associated with low-energy 7r T* shake-up transitions that accompany core ionization.' Detailed chemical information about the modified polymer surfaces were obtained by peak fitting the C(1s) XPS spectra to a range of carbon functionalities (see Figure 2 ) : carbon adjacent to a car285.7 eV), carbon singly bonded to boxylate group (C-C02 one oxygen atom (C-0 286.6 eV), carbon singly bound to
-
--
(6) Johansson, G.; Hedman, J.; Berndtsson, A,; Klasson, M.; Nilsson, R. J . Electron Specirosc. Relar. Phenom. 1913, 2, 295. (7) Clark, D. T.; Dilks, A. J . Polym. Sei., Polym. Chem. Ed. 1971, 15, 15.
0022-3654/91/2095-9436%02.50/0 0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 23, 1991 9437
Plasma Oxidation versus Photooxidation TABLE I: Summary of Oxidation Treatments
C-H, treatment untreated O2 plasma
T
-
X ( C - - 0 ) = 100%
T',
% of C(lS) (*O* 1%)
% of C(lS) (io.1%)
O/Z(C--O)
c-0
(*0.02)
(*I%)
94.0 62.4
6.0 3.1
0.00
65.1 63.2 68.1 87.8
0.8
uv C02 plasma co2uv 0 2
N20 UV
Rotary Pump
0.87 1.11 0.97 1.31 0.88
3.1 1.6 4.2
\
Rotary Pump
R F Coils figure I . Schematic of apparatus used for vacuum ultraviolet irradiation.
C-0 (*l%)
38 31 41 37 84
0-C-0 (*1%)
27 27 22 24 16
02(X%;)
02(A?Z[)
-
290
288
286
284
282
BINDING ENERGY (ev) Figure 2. Peak fit of C(ls)XPS spectra for 02/plasma modification of polystyrene.
-
two oxygen atoms are carbon doubly bnded to one oxygen atom (0-C-0 or C=O 287.9 eV), carboxylate groups (0-+=O -289.0 eV), and carbonate carbons (0-0-0 -290.4 eV).* Loss of aromaticity was monitored by the decrease in intensity of the A A* shake-up satellite. Table I summarizes the various treatments (C-C02 can be regarded as not oxidized, since the carbon atom is not directly bonded to oxygen). By taking into account that the O/C- -0ratio has to be consistent with the C- -0 species, it is possible to obtain an approximate idea of what the actual functionalities are (e.g., carboxylic acids versus esters). In the absence of ultraviolet irradiation, polystyrene was found to be unreactive toward the oxidizing gases. 02/Plasma. Surface modification of polystyrene by an oxygen glow discharge yields a broad range of oxygenated functionalities. Oxygen atoms present within the plasma may be responsible for the large number of C-0 groups. Furthermore, a relatively significant proportion of 0 - C 0 - 0 can be taken as an indication of the vigorous nature of the glow discharge treatment. It should be noted that, at this plasma power (IO W), the surface is continually renewed by the evolution of small volatile molecules (e.g., CO, C02, H20, et^.).^ In contrast, at lower powers (0.1 W), the diminished degree of surface ablation results in even more 0CO-0 ~ p e c i e s . ~
-
(8) Pijpers, A. P.;Donners, A. B. J . Polym. Sci., Polym. Chem. Ed. 1985, 23, 453. (9) Mayoux, C.: Antoniou, A.; Ai, 8.;Lascoste, R. Eur. Polym. J . 1973, 9, 1069.
9 9 6
0
0
5
02(A3Z,+)
250-300 nm
O2(B3ZL)
175-200 nm
O('P)
02(X?Z;) O('P)
292
26 37 28 33
02/Vacuum UV. Polystyrene undergoes extensive photochemical oxidation in the presence of ultraviolet radiation.IO The nature of the changes induced at the polymer surface can differ from that in the solid. Oxidation of bulk polymer is highly dependent upon the rate of diffusion of oxygen into the polymer," while the surface is continually exposed to an abundant oxygen supply. Thus, it might be expected that the mechanism of photooxidation and the final products formed are different in the two cases. At wavelengths shorter than 290 nm it has been postulated that photooxidative attack arises from excitation of the benzene ring12 to form either a charge-transfer complex between the phenyl rings and molecular oxygen'' or a peroxy intermediate.I4 In our experiments the alkane backbone must also absorb ( 8) of [Pt(bpy)2]2+(where bpy is 2,2’-bipyridine) in the presence of Ti02-Si02 (or TiOz) colloids or TiOz powder suspensions and an electron donor results in the loss of one bipyridyl ligand and the chemical fixation of Pt(bpy) to the Ti02surface via the formation of Ti-0-Pt bonds. The chemical derivatization of Ti02 with Pt(bpy) is pH dependent and proceeds through a TiQPt(bpy)2 intermediary. X-ray photoelectron spectroscopy analysis indicates that the heterometallic surface complex Ti-O-Pt(bpy) exists in the Pt(0) and Pt(1) oxidation state. The surface complex exhibits a metal-to-ligand charge-transfer absorption band in the visible region of the spectrum. Direct excitation of either the semiconductor or the photosensitizer [Ru(bpy),12+,electrostatically adsorbed to colloidal Si02-Ti02, initiates both the surface derivatization of Ti02 and the subsequent generation of H2. Large turnover numbers (H2 concentration/[Pt(bpy)2]2+ >> 1) are observed, indicating that the Pt(bpy) species on the TiOz surface is catalytic and is therefore regenerated during the photoreduction of water. The rates of H2 evolution with Pt(bpy)-TiO, particles are higher in acidic than basic media. The photocatalytic activity of these particles for H2 production compares favorably with that of other Ti02 systems charged with an equivalent Pt loading.
Photocatalytic reactions in colloidal and suspended semiconductor systems are of much interest from both a fundamental and an applied perspective. The photochemical properties of semiconductor Darticles can easilv be altered bv- Dhvsicallv or chem.
-
‘Present address: IMEC, Kapeldreef 75, B-3001 Leuven. Belgium.
0022-3654/9l/2095-9438$02.50/0
ically modifying their surface or microenvironment. For example, the adsorption of photosensitizing dyes to the surface of a semiconductor affords the possibility of using subbandgap light and the electronic transport properties of the semiconductor to effect charge separation. ‘Both organic dyes and transition-metal complexes have been utilized as photosensitizers of wide-bandgap 0 1991 American Chemical Society
