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Photochemistry of semiconductor colloids. Size quantization effects in Q-cadmium arsenide
Volume
20
. number
6
CHEMICAL
PHYSICS
25 October
LITTERS
PHOTOCHEMISTRY OF SEMICONDUtIOR COLLOIDS. SIZE QUANTIZATION EFFEflS IN Q-CADMIUM ARSENIDE A FOJTIK
Rrcewed
‘. H WELLER
1985
-‘-
and A HENGLEIN
4 July 1985
were prrpdrsd a collolds ,n aqueous soluuon and m the sohd srate While Extremely small partrcles or Cd,As, mncrocryslallme Cd,As, IS almost a melal absorbmg m the rar-mlrared (band gap = 0 1 eV) Ihe small par~rcles have abhorpuons and fluorescences m the wslble or near-mrrared dependmg on the parwle sne The sohd parllcles can be rcdlssol\ed unhout change m we and are pholoanodlcally decomposed m the Presence ol aw Methyl tlologen is reduced by xlslble hghl m the presence of small (TeLX nm dlamc~er) pnrt~clrs of Cd3As2, lha reducrmn bang enhanced by cxccss arsme Methyl wologen qucnchcs the lluorcxcnce and promow the pholoanodlc dlssoluuon of the parrnzles
L Introduction
Cd(ClO&
It has been reported that cadmium phosphrde, Cd,P?, can be prepared as a colloid in aqueous solution m the form of extremely small particles and that these particles can also be obtamed m the solid state by careful evaporatron of the solvent [I] These small semiconductor particles have unusual optrcal properties, which are caused by the strong perturbations of the electronic energy levels due to tamer confinement [2,3] In fact, the conductron band m these particles degenerates mto a system of rather widely spaced electromc levels; these particles represent a transltlon from scmrconductor to molecular propertres We desrgnate compounds showing such size quantization effects as “Q-maten&” They generally fluoresce strongly and have photocatalytic propertres. After the preparatron and mvestigation of cadmium phosphide, it was a natural step to synthesize the homologous cadrmum arsenide The procedure which was first descrrbed for CdS [3] was apphed Hydrogen arserudc was inJected into a deaerated solution of
acrdrc medra Moreover, the alkalmity determmes the rate of the preciprtation reaction and therefore the size of the colloidal particles formed. The metaphos phate complexes cadmium ions and prevents them from being precipitated as hydroxide in alkahne solution CdaAs, 1s a black matenal in the macrocrystalline state Its energy gap is only 0 1 eV [4], i e. Cd3As2 rs almost a metal It IS shown here that the size quantization effects in thrs matenal are very pronounced as the band gap can be shifted by many electron volts through the whole range of the visible spectrum, i e samples absorbing anywhere - as desrred - rn the visible or rn the infrared can be made by changmg the size of the partrcles. As already found for Cd3P2, the small Cd,Aq particles luminesce wrth the colour of the emitted hght depending on the srze of the particles
’
Pcrmnnux address Cnxhosloxak Academy oC Science. J HeyroL4.y In~~rn~~s or PhJslwl Chemistry and Eleclrochem15~ry Prague. Cixchoslovakm
552
in the presence of metaphosphate The solution has to be alkaline as arsemdes decompose m
2 Results and dlscusson A colorless sample of Cd3As2 was prepared by inJetting half the stoichiometric amount of arsme into a deaerated 2 X lo-4 M Cd(CIO,), solution at pff= 11 5 The solution also contamed 3 X 10m4 M sodium hexametaphosphate The reaction was allowed to pro0 009-2614/85/s
(North-Holland
03.30 0 Elsevier Science Publishers B V Physics Pubhshing Division)
Volume 120, number 6
CHEMICAL
PHYSICS
Fig 1 Absmptton (full lme) and fluorescence (dashed) spcctra of Cd,A% particles of different size (a) Fresh solution; (b) after 1 h;(c) after 4 h; (6) after 1 day.
teed for 10 mm The unreacted arsme was then removed by evacuation with a water pump The amount of Cd3Asl formed was determined iodometncally. It corresponded to 40% conversion of the arsme added origmally The absorption spectrum (fig 1) contains a mzumum at 300 nm which is attnbuted to an cxcrton transItron band The fluorescence spectrum contains a maxlmum at 560 run and a shoulder at 660 nm At pY= 11 5, the absorption spectrum does not change substantially over several hours, 1-e the particles do not grow rapidly The evaporation procedure could therefore be apphed to obtain the particles in the sohd state in the form of a pale powder_ This powder lummesced green-yellow upon dlummation with UV hght The powder could be redissolved in water to yield a solution wrth the onginal spectrum m fig 1 a. The powder was stable for months The small Cd3Asz particles are prevented from getting mto close contact by the hexametaphosphate matnx which was formed upon the evaporation of the solvent Bollmg of a solution resulted in a black precipitate Usmg different amounts of arsme and different temperatures during precipitation at pH = 11.5, Cd,As, samples of yellow, orange, red and grey color (increasing particle size) were prepared m solution as well as m the sohd state. All these matenals fluoresced, the wavelengths of the emitted light being red-shifted with increasing particle size The quantum yields of fluorescence in solution were about 10%
LETTERS
25 October
1985
Using pH= 12.0 m the precipitatron procedure, solbtlons were obtamed which changed therr color withm a few hours The particles grew more rapidly in these solutions At first the spectrum of the solutlon resembled that of a solution prepared at pW= 11 5, as described above, see curve a m fig 1. The spectra at various tunes of aging are also shown in fig 1 It is seen that the mtensity of fluorescence increases up to a maximum at 1 to 4 h The lower mtenslty recorded at longer tunes IS probably due to the decreased sensrtivlty of the detection system at longer wavelength The shoulder at 660 nm present m spectrum 1 a’ grows mto a maximum upon aging and becomes stronger than the first maxlmum The absorption spectrum IS unstructured at longer times The fluorescence 1s suggested to be emitted upon the recombination of charge carriers trapped at defect sites The fact that two maxima are present may be taken as an indication for the existence of two types of defect states The Q-Cd3As2 colloids undergo many of the typical photoreactions which have been found previously m our laboratory for other semiconductor colloids with band gaps m the visible such as CdS [5-71 They are photoanodically dissolved upon ilhmunation m the presence of arr, the quantum yields being 0 0 1 to 0 025 molecules CdjAsZ decomposed per photon absorbed The products of the photoanodic drssolution arc Cd2+ and AsOzions In the presence of 3 X 1O-4 M methyl viologen, MV?+, the rate of this photo-dissolution 1s enhanced by a factor of 15. As in the case of CdS [6], a mechamsm is suggested wrth the followmg steps- (1) formation of electrons and pontive holes by light absorption; (2) reaction of the electrons with adsorbed MVz++, (3) reoxldatlon of semi-reduced methyl viologen, ML’+, by oxygen, (4) oxidation of arsenide by positive holes to yield As’radicals; (5) further oxidation of As’- by oxygen to yield AsO:The fluorescence ofCd3As2 IS quenched by methyl wologen at all wavelengths However, MV’+ IS more efficient in quenching the fluorescence of the second maximum at longer wavelengths than that of the first maximum In the case of a 3 X 10e5 M solution of orange CdjAsZ, for example, 3 0 X 10m6 and 4 5 X 10-6 M MV2+ was required to decrease the fluorescence intensities by 50% m the first and second maximum, respectively This fmdmg corroborates the 553
Volume 120, number 6
CHEMICAL
PHYSICS LETTERS
above conclusrons according to which drfferent fluorescence centres exist in the small particles. When a deaerated solution of orange Cd3As2 is illuminated no decompontion of the collard takes place. In the presence of MV2+, the blue color of MV+ appears and the colloid rs degraded The rate of reduction of MV*+ is rncreased when excess arsine is present We attnbute this to the scavengmg of posrtrve holes by arsrne The recombmatron of charge carriers is diminished m this way, ie more electrons succeed m reactmg with MV2+ molecules In addrtron, the rate of corrosion of the colloid is decreased.
554
25 October
1985
References [l] [2] [3] (41
[5] [6] [7]
H Weller, A Fojtik and A Her@&, Chem. Phys Letters 117 (1985) 485 LE. Brus, J Chem Phyr 79 (1983) 5566; 80 (1984) 4403 A Fojtik, H Weller, U Koch and A Henglein, Ber. Bunsenges Physik Chem 88 (1984) 969 Landolt-BtSmstem, Numerical data and functional rep+ tionships m sclence and technology, new series. Vol 17e (Springer, BerIm, 1983) p 201. A Henglcm, Ber Bunsenges Phystk Chem. 86 (1982) 301 A Henglem, J Phys Chem 86 (1982) 2291 A Her&in_ Pure Appl Chem 56 (1984) 1215
20
. number
6
CHEMICAL
PHYSICS
25 October
LITTERS
PHOTOCHEMISTRY OF SEMICONDUtIOR COLLOIDS. SIZE QUANTIZATION EFFEflS IN Q-CADMIUM ARSENIDE A FOJTIK
Rrcewed
‘. H WELLER
1985
-‘-
and A HENGLEIN
4 July 1985
were prrpdrsd a collolds ,n aqueous soluuon and m the sohd srate While Extremely small partrcles or Cd,As, mncrocryslallme Cd,As, IS almost a melal absorbmg m the rar-mlrared (band gap = 0 1 eV) Ihe small par~rcles have abhorpuons and fluorescences m the wslble or near-mrrared dependmg on the parwle sne The sohd parllcles can be rcdlssol\ed unhout change m we and are pholoanodlcally decomposed m the Presence ol aw Methyl tlologen is reduced by xlslble hghl m the presence of small (TeLX nm dlamc~er) pnrt~clrs of Cd3As2, lha reducrmn bang enhanced by cxccss arsme Methyl wologen qucnchcs the lluorcxcnce and promow the pholoanodlc dlssoluuon of the parrnzles
L Introduction
Cd(ClO&
It has been reported that cadmium phosphrde, Cd,P?, can be prepared as a colloid in aqueous solution m the form of extremely small particles and that these particles can also be obtamed m the solid state by careful evaporatron of the solvent [I] These small semiconductor particles have unusual optrcal properties, which are caused by the strong perturbations of the electronic energy levels due to tamer confinement [2,3] In fact, the conductron band m these particles degenerates mto a system of rather widely spaced electromc levels; these particles represent a transltlon from scmrconductor to molecular propertres We desrgnate compounds showing such size quantization effects as “Q-maten&” They generally fluoresce strongly and have photocatalytic propertres. After the preparatron and mvestigation of cadmium phosphide, it was a natural step to synthesize the homologous cadrmum arsenide The procedure which was first descrrbed for CdS [3] was apphed Hydrogen arserudc was inJected into a deaerated solution of
acrdrc medra Moreover, the alkalmity determmes the rate of the preciprtation reaction and therefore the size of the colloidal particles formed. The metaphos phate complexes cadmium ions and prevents them from being precipitated as hydroxide in alkahne solution CdaAs, 1s a black matenal in the macrocrystalline state Its energy gap is only 0 1 eV [4], i e. Cd3As2 rs almost a metal It IS shown here that the size quantization effects in thrs matenal are very pronounced as the band gap can be shifted by many electron volts through the whole range of the visible spectrum, i e samples absorbing anywhere - as desrred - rn the visible or rn the infrared can be made by changmg the size of the partrcles. As already found for Cd3P2, the small Cd,Aq particles luminesce wrth the colour of the emitted hght depending on the srze of the particles
’
Pcrmnnux address Cnxhosloxak Academy oC Science. J HeyroL4.y In~~rn~~s or PhJslwl Chemistry and Eleclrochem15~ry Prague. Cixchoslovakm
552
in the presence of metaphosphate The solution has to be alkaline as arsemdes decompose m
2 Results and dlscusson A colorless sample of Cd3As2 was prepared by inJetting half the stoichiometric amount of arsme into a deaerated 2 X lo-4 M Cd(CIO,), solution at pff= 11 5 The solution also contamed 3 X 10m4 M sodium hexametaphosphate The reaction was allowed to pro0 009-2614/85/s
(North-Holland
03.30 0 Elsevier Science Publishers B V Physics Pubhshing Division)
Volume 120, number 6
CHEMICAL
PHYSICS
Fig 1 Absmptton (full lme) and fluorescence (dashed) spcctra of Cd,A% particles of different size (a) Fresh solution; (b) after 1 h;(c) after 4 h; (6) after 1 day.
teed for 10 mm The unreacted arsme was then removed by evacuation with a water pump The amount of Cd3Asl formed was determined iodometncally. It corresponded to 40% conversion of the arsme added origmally The absorption spectrum (fig 1) contains a mzumum at 300 nm which is attnbuted to an cxcrton transItron band The fluorescence spectrum contains a maxlmum at 560 run and a shoulder at 660 nm At pY= 11 5, the absorption spectrum does not change substantially over several hours, 1-e the particles do not grow rapidly The evaporation procedure could therefore be apphed to obtain the particles in the sohd state in the form of a pale powder_ This powder lummesced green-yellow upon dlummation with UV hght The powder could be redissolved in water to yield a solution wrth the onginal spectrum m fig 1 a. The powder was stable for months The small Cd3Asz particles are prevented from getting mto close contact by the hexametaphosphate matnx which was formed upon the evaporation of the solvent Bollmg of a solution resulted in a black precipitate Usmg different amounts of arsme and different temperatures during precipitation at pH = 11.5, Cd,As, samples of yellow, orange, red and grey color (increasing particle size) were prepared m solution as well as m the sohd state. All these matenals fluoresced, the wavelengths of the emitted light being red-shifted with increasing particle size The quantum yields of fluorescence in solution were about 10%
LETTERS
25 October
1985
Using pH= 12.0 m the precipitatron procedure, solbtlons were obtamed which changed therr color withm a few hours The particles grew more rapidly in these solutions At first the spectrum of the solutlon resembled that of a solution prepared at pW= 11 5, as described above, see curve a m fig 1. The spectra at various tunes of aging are also shown in fig 1 It is seen that the mtensity of fluorescence increases up to a maximum at 1 to 4 h The lower mtenslty recorded at longer tunes IS probably due to the decreased sensrtivlty of the detection system at longer wavelength The shoulder at 660 nm present m spectrum 1 a’ grows mto a maximum upon aging and becomes stronger than the first maxlmum The absorption spectrum IS unstructured at longer times The fluorescence 1s suggested to be emitted upon the recombination of charge carriers trapped at defect sites The fact that two maxima are present may be taken as an indication for the existence of two types of defect states The Q-Cd3As2 colloids undergo many of the typical photoreactions which have been found previously m our laboratory for other semiconductor colloids with band gaps m the visible such as CdS [5-71 They are photoanodically dissolved upon ilhmunation m the presence of arr, the quantum yields being 0 0 1 to 0 025 molecules CdjAsZ decomposed per photon absorbed The products of the photoanodic drssolution arc Cd2+ and AsOzions In the presence of 3 X 1O-4 M methyl viologen, MV?+, the rate of this photo-dissolution 1s enhanced by a factor of 15. As in the case of CdS [6], a mechamsm is suggested wrth the followmg steps- (1) formation of electrons and pontive holes by light absorption; (2) reaction of the electrons with adsorbed MVz++, (3) reoxldatlon of semi-reduced methyl viologen, ML’+, by oxygen, (4) oxidation of arsenide by positive holes to yield As’radicals; (5) further oxidation of As’- by oxygen to yield AsO:The fluorescence ofCd3As2 IS quenched by methyl wologen at all wavelengths However, MV’+ IS more efficient in quenching the fluorescence of the second maximum at longer wavelengths than that of the first maximum In the case of a 3 X 10e5 M solution of orange CdjAsZ, for example, 3 0 X 10m6 and 4 5 X 10-6 M MV2+ was required to decrease the fluorescence intensities by 50% m the first and second maximum, respectively This fmdmg corroborates the 553
Volume 120, number 6
CHEMICAL
PHYSICS LETTERS
above conclusrons according to which drfferent fluorescence centres exist in the small particles. When a deaerated solution of orange Cd3As2 is illuminated no decompontion of the collard takes place. In the presence of MV2+, the blue color of MV+ appears and the colloid rs degraded The rate of reduction of MV*+ is rncreased when excess arsine is present We attnbute this to the scavengmg of posrtrve holes by arsrne The recombmatron of charge carriers is diminished m this way, ie more electrons succeed m reactmg with MV2+ molecules In addrtron, the rate of corrosion of the colloid is decreased.
554
25 October
1985
References [l] [2] [3] (41
[5] [6] [7]
H Weller, A Fojtik and A Her@&, Chem. Phys Letters 117 (1985) 485 LE. Brus, J Chem Phyr 79 (1983) 5566; 80 (1984) 4403 A Fojtik, H Weller, U Koch and A Henglein, Ber. Bunsenges Physik Chem 88 (1984) 969 Landolt-BtSmstem, Numerical data and functional rep+ tionships m sclence and technology, new series. Vol 17e (Springer, BerIm, 1983) p 201. A Henglcm, Ber Bunsenges Phystk Chem. 86 (1982) 301 A Henglem, J Phys Chem 86 (1982) 2291 A Her&in_ Pure Appl Chem 56 (1984) 1215