- Email: [email protected]
Photochemistry at the surface of colloidal cadmium sulfide
21 h1.w 1982
CHGMICAL PHYSICS LLI-KRS
Volume 88, number 5
PHOTOCHEMISTRY AT Ti-IE SURFACE
OF COLLOIDAL
CADMIUM
SULFIDE
J. KUCZYNSKI and J.K. THOMAS Cfmustry Repartmwt,
Urrtuermy of Notre Dame. Notre Dame, brdrarra J6556.
L/S/l
Received 12 February 1982
Pulsed laser studlcs of surhctant stablbzcd CdS coUolds mdlcate a rapld (< 10sg s) c- transfer from CdS to mctbylv~ologcn adsorbed on the collolds A rapId (==10m6 s) subscqucnt back rcnction of someof the reduced methylvtologcn IS observed. PH. solute concentnt~on. etc., affect tbc reactron.
I _Introduction The last decade has shown an Increased interest m photochemistry in unusual ennronments. This ISpartly tnggered by the rapld increase in versatility of photochermcal techmques, and partly by Increased interest in storage of solar energy. The design of photosystems has been influenced by the plant photosynthetlc systems, and hence photochemistry m micelles, microemulsions, vesicles, etc. has received much attention [l-3], with encouragmg results. On the other hand, photovoltalc and photosemiconductor devices, together with photoassisted electrolysn, also provide promlsmg systems for storage of hght energy 14-61. More recently suspensions of semiconductors such a TIO?, and CdS, have shown marked photochemlcal reactivity [7-121. The advent of these latter systems suggests the use of sermconductors in conjunction with surfactants, in the form of colloidal parrlcles much like microemulsIons [ 131. The proJectlon 1s to replace the od of the nucroemulslon center by a senuconductor, and to control the photoinduced chemical events at the surface by use of surfactants and co-surfactants. Earher work with organic microemulsions has indicated that photoinduced charge transfer is readily controlled by the suitable choice of the parUcle surface [ 141. The purpose of the present commumcation ISto indicate our data on the photochemistry of aqueous colloidal surfactant stabilized cadmium sulfide systems, in particular to indicate the features Lnportant
to elIbent
charge separation from tbc parllclc.
2. Experimental Steady-state photolysis was carried out with a 400 W quartz I1 lamp, with sultable heat and wavelength
filters to cut all radiation below 4000 A. Quantum yield measurements were carried out usmg steady hght of 4880 A wavelength from a 2 W Spectra Phyncs argon ion laser, the output of the ldscr was momtored with a Sclcntcch hght meter
Rcduccd
methylvlologen (MV+) yields were mersured at X = 6100 ii where the extuxt~on
cocfkient
1s 1.2 X 104
Pulsed expermients were crrricd out WIIII a Candela dye laser using an LD 490 dye, wh1c11gives pulses of 120 ns fwhm, wavelength 4900 A, and output 50 mJ. Details of the pulsed laser equlpmcnt and monitonng system have been reported previously hi-l
CM’ 1.
[14l.
The colloidal CdS solutions were made via several techniques, and the more photoactive preparations are @ven below (I) H2S was bubbled tnto the surfactant solution followed by gradual addition of CdCI,. (2) A solution of CdClz and EDTA was bubbled wih H,S for five mm. (3) Na,S solution was added slowly to r solutlon of CdC12containing surfactani. All of the above
produced relotwely clear orange
colored colloidti solutions of CdS, with particle sizes rangmg from 300 a in radms upwards. 445
Volu~ncSS, nuntbcr 5
CHChlICAL I’IIYSKS L!ITERS
21 hlay 1982
IncreasmgpH and [MV*+] gives rise to increased ytelds of MV+. In a solutton 8 X lOA M CdS, 0.1 M
3. Data FIN. 1 shows the mcreasc of the reduced methylviologcn concentration [hW], expressed as the optical densrty at 6 100 li, as a function of trme m the photolysrs of a deoxygenated aqueous collordal solutton of CdS stabdized wrth surfactant m the presence of methyhologen, MV?+ . No MV+ ISobserved tn the abscncz of CdS. The reactton giving nse to the blue colored photoproduct is: CdS * (CdS)* + MV”+* (CdS)+ + MV+ The presence of the posrttve hole, (CdS)+, in this system IS mfcrrcd from the photocfremistry of the system. Srgmficant back reaction with MV+ takes place, but posittvc hole scavengers such as ethylene diamine tetraacctate, EDTA, ordy decreased the quantum
yield of the reaction, unlike tts posittve effect u-rsystems such as rutheruum brpyridyl and methylvrologens It is possible th.rt degradation of the posittve hole also takes place: (CdS)+ - Cd-‘)++ S .
No sulfur was observed by vrsud observations m our system, but this may be due to a colloidal form in which tt is formed.
EDTA,pH=
1 IA, [MVz++] = 10-X M, the quantum
yield of MV+ is @(hlV+) = 7 X 10s3, and is independent of whether the exctting wavelength is 4880 or 4336 &The increased #(MV+) with increasing [OH-] IS probably due to modification of the CdS surface by
OH-. The tncreased yield of MV+ with tncreasing [MV?-+] ts due to increasing amounts of MV*+ being adsorbed on the CdS parucle surface. Quantum ytelds are largest wrth negatively charged surfaces, i.e. with sodium lauryl sulphate as surfactant in place of the non-tonic Brij 35, and yields are smallest with posittvely charged surfactants such as cetyl trimethyl ammonium bromide, where MV*+ adsorption at the CdS surface is diminished. Fig. 2 also bears out the above descriptron of the importance of adsorption of MV2+ on the CdS surface. Rg. 2 shows the rate of growth of MV+ in the laser flash photolysis at X = 4900 R, of a deoxygenated CdS/NaLS/MV** solution. The MVt is formed with the laser pulse (fwhm = 120 ns), without delay. Experiments using a 3 ns pulse of 3371 Ir( light from art exirncr laser show that MV+ is formed mprdly compared to the pulse length, the subsequent kinetics being similar to the excitation at 4900 k These data
mdrcatc the tmportance of adsorbmg MV*+ on the particle surface m order for efficient electron transfer to occur. The data m fig. 2 also show that MV+ shows a
6-
. . .
.P--
x_ 6,_ 6-
I
5 t ( minutes
IO
1
Frg. 1. ProductIon of hip M O&j~~, on ordmate. ve’~~stime III the photolyss ofcolloldal CdS systems with MV’ . [CdS] = 8.0 X low5 hl, [surfactant.BnJ] = 2.0 X 10Ms hf. (1): pff= 11 8. (hlV*+] = 4 x lo4 M, (2). pN = 10, [MV*+]=.I x lo+’ hi; (3): pfi = IO, [hlV’+l = 7 X IO’ hl, (4): pH = 10.8, [MV’+j = 4 x 10” hl, (5). PH = 10, [hIV?+I = 4 X 1O-4 hl; (6): pH = 9.2,
[hlV*+] = 4 X IO4 hl, (7): pH = 10, [hlV*+] = 1 X lO-‘hl. 446
I i
z_-J 01 0
’
’
I
’
’
’
2 t (ps)
’
3
’
’
4
’
1
5
Fg. 2. Rate of production of hlV+ a OD at 6 100 A versus time in the pulsed laser photolysrs of colloidal CdS. [CdS] = 2 X 1O-3 ht, [Mv’] = 2 X lOA M, [N&s] = 6 X lo-” hf. h =
4900 A. FuU curve: experimental absorption at 6100 A; black circles: profile of a species produced wth the laser pulse.
Volume 88, number 5
CHEMICAL PHYSICS LlXl-CRS
sharp decrease for some 500 ns following the laser pulse, followed by a yield of stable MV+ that exists for long term observation. These data are taken to be mdlcatlve of a back reaction of MV+ and (CdS)+ m compctltion with escape of MV+ from the surface, or degradation of the positive hole. Vorlous additives, acetone, isopropanol, ferncyanidc, no
EDTA,
21 b1.1~1982
Acknowledgement The authors would hkc to thank the Petroleum Research Fund of the American Chemical Soc~cty for support of this work via Grant No. 126%AC3.
etc. have
effect on the kineric pattern observed tn fig. 2, but
References
tend to decrease the yietd of MV+. Increasing [NaLSJ
from 10-j to IO-2 M decreases the extent of the fast decay of MV+ in fig. 2, whale higher concentrations reduce the MV+ yield. Increasing NaLS leads to the formation of micelles, which tend to extract the MV+ from the CdS surface pnor to back reaction with (CdS)+. A much higher [NaLS] increases [micellc] to an extent where MV2+ itself ISextracted from the CdS surface, and hence decreases the probabdlty of e- transfer from (CdS)* to MV2+.
4. Conclusion Colloidal semlconductors can be arranged as small colloidal particles with a variety of surfaces. The nature of the surface controls the efficiency of etransfer from excited semiconductor to e- acceptor, eg. MV2+. Electron transfer m these systems ts very rapid (<3 ns) and only occurs to species located on the particle surface. Further experiments with morhfied semiconductor surfaces, and other collordd sermconductors are now underway.
[l] N. Two. A. Braun and hl. Gntzel, Ansl. Chem. 19 (1980) 675. [2] T.H. Fendler, I. Phys. Chem. 84 (1980) 1485. [3] J.K.Thomas, Accounts Chcm. Res. lO(1977) 133, Chcm. Rev. 80 (1980) 283. [4] A. Nozrk.Ann. Rev. Phys. Chem. 29 (1978) 189. [S] hl S.Wrighton, AccountsChem. Rcs. I2 (1979) 303. [6] AJ. Bud, 1. Photochcm. 10 (1979) 59 [7] I. Furjshlmaand K. Hands. Nature 238 (1972) 37. (81 J.R. Dwent and GJ. Porter, I. Chem. Sot. Chcm. Commun. (1981) 145 [9] K. Kutyannnsundzun,E. Borgarello. ht Gratzel, Hclv Chcm. Acta 64 (198 I) 362. [lo] W.W. Dunn, Y. Ackown and AJ. Bxd, J. Am Chem. Sot 103 (1981) 6893. [Ill T. fiwu md T. S&kata.Chem. Phys. Letters 72 (1980) 87. 1121 E F. SacvqC.R. OLn md J R. %ir. I. Chem. Soc.Chcm Commun (1980) 401. [ 131 J K. Thomas, Report No
26, Am. Chcm. Sot. Petroleum Researchrund (1981). [ 141 S. At& and J.K. Th0mas.J. Am.Chcm. Sot. 103 (1981) 3550.
447
CHGMICAL PHYSICS LLI-KRS
Volume 88, number 5
PHOTOCHEMISTRY AT Ti-IE SURFACE
OF COLLOIDAL
CADMIUM
SULFIDE
J. KUCZYNSKI and J.K. THOMAS Cfmustry Repartmwt,
Urrtuermy of Notre Dame. Notre Dame, brdrarra J6556.
L/S/l
Received 12 February 1982
Pulsed laser studlcs of surhctant stablbzcd CdS coUolds mdlcate a rapld (< 10sg s) c- transfer from CdS to mctbylv~ologcn adsorbed on the collolds A rapId (==10m6 s) subscqucnt back rcnction of someof the reduced methylvtologcn IS observed. PH. solute concentnt~on. etc., affect tbc reactron.
I _Introduction The last decade has shown an Increased interest m photochemistry in unusual ennronments. This ISpartly tnggered by the rapld increase in versatility of photochermcal techmques, and partly by Increased interest in storage of solar energy. The design of photosystems has been influenced by the plant photosynthetlc systems, and hence photochemistry m micelles, microemulsions, vesicles, etc. has received much attention [l-3], with encouragmg results. On the other hand, photovoltalc and photosemiconductor devices, together with photoassisted electrolysn, also provide promlsmg systems for storage of hght energy 14-61. More recently suspensions of semiconductors such a TIO?, and CdS, have shown marked photochemlcal reactivity [7-121. The advent of these latter systems suggests the use of sermconductors in conjunction with surfactants, in the form of colloidal parrlcles much like microemulsIons [ 131. The proJectlon 1s to replace the od of the nucroemulslon center by a senuconductor, and to control the photoinduced chemical events at the surface by use of surfactants and co-surfactants. Earher work with organic microemulsions has indicated that photoinduced charge transfer is readily controlled by the suitable choice of the parUcle surface [ 141. The purpose of the present commumcation ISto indicate our data on the photochemistry of aqueous colloidal surfactant stabilized cadmium sulfide systems, in particular to indicate the features Lnportant
to elIbent
charge separation from tbc parllclc.
2. Experimental Steady-state photolysis was carried out with a 400 W quartz I1 lamp, with sultable heat and wavelength
filters to cut all radiation below 4000 A. Quantum yield measurements were carried out usmg steady hght of 4880 A wavelength from a 2 W Spectra Phyncs argon ion laser, the output of the ldscr was momtored with a Sclcntcch hght meter
Rcduccd
methylvlologen (MV+) yields were mersured at X = 6100 ii where the extuxt~on
cocfkient
1s 1.2 X 104
Pulsed expermients were crrricd out WIIII a Candela dye laser using an LD 490 dye, wh1c11gives pulses of 120 ns fwhm, wavelength 4900 A, and output 50 mJ. Details of the pulsed laser equlpmcnt and monitonng system have been reported previously hi-l
CM’ 1.
[14l.
The colloidal CdS solutions were made via several techniques, and the more photoactive preparations are @ven below (I) H2S was bubbled tnto the surfactant solution followed by gradual addition of CdCI,. (2) A solution of CdClz and EDTA was bubbled wih H,S for five mm. (3) Na,S solution was added slowly to r solutlon of CdC12containing surfactani. All of the above
produced relotwely clear orange
colored colloidti solutions of CdS, with particle sizes rangmg from 300 a in radms upwards. 445
Volu~ncSS, nuntbcr 5
CHChlICAL I’IIYSKS L!ITERS
21 hlay 1982
IncreasmgpH and [MV*+] gives rise to increased ytelds of MV+. In a solutton 8 X lOA M CdS, 0.1 M
3. Data FIN. 1 shows the mcreasc of the reduced methylviologcn concentration [hW], expressed as the optical densrty at 6 100 li, as a function of trme m the photolysrs of a deoxygenated aqueous collordal solutton of CdS stabdized wrth surfactant m the presence of methyhologen, MV?+ . No MV+ ISobserved tn the abscncz of CdS. The reactton giving nse to the blue colored photoproduct is: CdS * (CdS)* + MV”+* (CdS)+ + MV+ The presence of the posrttve hole, (CdS)+, in this system IS mfcrrcd from the photocfremistry of the system. Srgmficant back reaction with MV+ takes place, but posittvc hole scavengers such as ethylene diamine tetraacctate, EDTA, ordy decreased the quantum
yield of the reaction, unlike tts posittve effect u-rsystems such as rutheruum brpyridyl and methylvrologens It is possible th.rt degradation of the posittve hole also takes place: (CdS)+ - Cd-‘)++ S .
No sulfur was observed by vrsud observations m our system, but this may be due to a colloidal form in which tt is formed.
EDTA,pH=
1 IA, [MVz++] = 10-X M, the quantum
yield of MV+ is @(hlV+) = 7 X 10s3, and is independent of whether the exctting wavelength is 4880 or 4336 &The increased #(MV+) with increasing [OH-] IS probably due to modification of the CdS surface by
OH-. The tncreased yield of MV+ with tncreasing [MV?-+] ts due to increasing amounts of MV*+ being adsorbed on the CdS parucle surface. Quantum ytelds are largest wrth negatively charged surfaces, i.e. with sodium lauryl sulphate as surfactant in place of the non-tonic Brij 35, and yields are smallest with posittvely charged surfactants such as cetyl trimethyl ammonium bromide, where MV*+ adsorption at the CdS surface is diminished. Fig. 2 also bears out the above descriptron of the importance of adsorption of MV2+ on the CdS surface. Rg. 2 shows the rate of growth of MV+ in the laser flash photolysis at X = 4900 R, of a deoxygenated CdS/NaLS/MV** solution. The MVt is formed with the laser pulse (fwhm = 120 ns), without delay. Experiments using a 3 ns pulse of 3371 Ir( light from art exirncr laser show that MV+ is formed mprdly compared to the pulse length, the subsequent kinetics being similar to the excitation at 4900 k These data
mdrcatc the tmportance of adsorbmg MV*+ on the particle surface m order for efficient electron transfer to occur. The data m fig. 2 also show that MV+ shows a
6-
. . .
.P--
x_ 6,_ 6-
I
5 t ( minutes
IO
1
Frg. 1. ProductIon of hip M O&j~~, on ordmate. ve’~~stime III the photolyss ofcolloldal CdS systems with MV’ . [CdS] = 8.0 X low5 hl, [surfactant.BnJ] = 2.0 X 10Ms hf. (1): pff= 11 8. (hlV*+] = 4 x lo4 M, (2). pN = 10, [MV*+]=.I x lo+’ hi; (3): pfi = IO, [hlV’+l = 7 X IO’ hl, (4): pH = 10.8, [MV’+j = 4 x 10” hl, (5). PH = 10, [hIV?+I = 4 X 1O-4 hl; (6): pH = 9.2,
[hlV*+] = 4 X IO4 hl, (7): pH = 10, [hlV*+] = 1 X lO-‘hl. 446
I i
z_-J 01 0
’
’
I
’
’
’
2 t (ps)
’
3
’
’
4
’
1
5
Fg. 2. Rate of production of hlV+ a OD at 6 100 A versus time in the pulsed laser photolysrs of colloidal CdS. [CdS] = 2 X 1O-3 ht, [Mv’] = 2 X lOA M, [N&s] = 6 X lo-” hf. h =
4900 A. FuU curve: experimental absorption at 6100 A; black circles: profile of a species produced wth the laser pulse.
Volume 88, number 5
CHEMICAL PHYSICS LlXl-CRS
sharp decrease for some 500 ns following the laser pulse, followed by a yield of stable MV+ that exists for long term observation. These data are taken to be mdlcatlve of a back reaction of MV+ and (CdS)+ m compctltion with escape of MV+ from the surface, or degradation of the positive hole. Vorlous additives, acetone, isopropanol, ferncyanidc, no
EDTA,
21 b1.1~1982
Acknowledgement The authors would hkc to thank the Petroleum Research Fund of the American Chemical Soc~cty for support of this work via Grant No. 126%AC3.
etc. have
effect on the kineric pattern observed tn fig. 2, but
References
tend to decrease the yietd of MV+. Increasing [NaLSJ
from 10-j to IO-2 M decreases the extent of the fast decay of MV+ in fig. 2, whale higher concentrations reduce the MV+ yield. Increasing NaLS leads to the formation of micelles, which tend to extract the MV+ from the CdS surface pnor to back reaction with (CdS)+. A much higher [NaLS] increases [micellc] to an extent where MV2+ itself ISextracted from the CdS surface, and hence decreases the probabdlty of e- transfer from (CdS)* to MV2+.
4. Conclusion Colloidal semlconductors can be arranged as small colloidal particles with a variety of surfaces. The nature of the surface controls the efficiency of etransfer from excited semiconductor to e- acceptor, eg. MV2+. Electron transfer m these systems ts very rapid (<3 ns) and only occurs to species located on the particle surface. Further experiments with morhfied semiconductor surfaces, and other collordd sermconductors are now underway.
[l] N. Two. A. Braun and hl. Gntzel, Ansl. Chem. 19 (1980) 675. [2] T.H. Fendler, I. Phys. Chem. 84 (1980) 1485. [3] J.K.Thomas, Accounts Chcm. Res. lO(1977) 133, Chcm. Rev. 80 (1980) 283. [4] A. Nozrk.Ann. Rev. Phys. Chem. 29 (1978) 189. [S] hl S.Wrighton, AccountsChem. Rcs. I2 (1979) 303. [6] AJ. Bud, 1. Photochcm. 10 (1979) 59 [7] I. Furjshlmaand K. Hands. Nature 238 (1972) 37. (81 J.R. Dwent and GJ. Porter, I. Chem. Sot. Chcm. Commun. (1981) 145 [9] K. Kutyannnsundzun,E. Borgarello. ht Gratzel, Hclv Chcm. Acta 64 (198 I) 362. [lo] W.W. Dunn, Y. Ackown and AJ. Bxd, J. Am Chem. Sot 103 (1981) 6893. [Ill T. fiwu md T. S&kata.Chem. Phys. Letters 72 (1980) 87. 1121 E F. SacvqC.R. OLn md J R. %ir. I. Chem. Soc.Chcm Commun (1980) 401. [ 131 J K. Thomas, Report No
26, Am. Chcm. Sot. Petroleum Researchrund (1981). [ 141 S. At& and J.K. Th0mas.J. Am.Chcm. Sot. 103 (1981) 3550.
447