STUDIES OF PHOTOREDOX REACTIONS ON NANOSIZE SEMICONDUCTORS mb--q6-2 g/

Transcription

1 r i--~ 00 STUDES OF PHOTOREDOX REACTONS ON NANOSZE SEMCONDUCTORS mb--q6-2 g/ Jess P. Wicoxon Sandia Nationa Laboratories Org 52 Abuquerque NM; F. Parsapour D.F. Keey Coorado State U. Dept. of Chemistry Fort Coins Co. /8' ABSTRACT Light induced eectron transfer (ET) from nanosize semiconductors of MoS2 to organic eectron acceptors such as 22'-bipyridine (bpy) and methy substituted 44'55'-tetramethy22'-bipyridine (tmb) was studied by static and time resoved photouminescence spectroscopy. The kinetics of ET were varied by changing the nanocuster size (the band gap) the eectron acceptor and the poarity of the sovent. MoS2 is an especiay interesting semiconductor materia as it is an indirect semiconductor in buk form and has a ayered covaent bonding arrangement which is highy resistant to photocorrosion. NTRODUCTON ECEVE JAN 0 6 MoS2 is an especiay interesting semiconductor materia as it is semiconductor in buk form and has a ayered covaent bonding arrangement photocorrosion. ts most common industria appications incude therma cataysis to remove sufur compounds from crude oi and as an exceent high temperature ubricant (e.g. axe grease). n fact in buk form MoS2 in the most widey used hydrotreating catayst and thus is vita to a chemica processing for fues and feedstocks. However because MoS2 is an indirect gap back R absorbing materia it has no appication as a photocatayst. We have demonstrated that when synthesized in nanosize form the absorption edge of MoS2 can be significanty bue shifted.[] This increase in the band gap energy due to quantum confinement is accompanied by significant shifts in both conduction and vaence band energies and impies that nanosize MoS2 is capabe of ight induced eectron and hoe transfer (photoredox) reactions just as are we known direct gap materias such as CdS. Experimentay there is evidence that as MoS2 is made smaer the excitation becomes more ike a direct transition presumaby due to the increasing importance of surface and ack of ong-range transationa symmetry in the attice. n this paper we report on static and dynamic photouminesence measurements of nanosize MoS2 and show that the observed increases in the bandgap energy with decreasing nanocuster size are accompanied by increases in the conductance band potentia which permit eectron transfer eketrm to acceptor organic moecues. The rate of eectron transfer is found to be dependent on both nanocuster size and sovent poarity. The fastest rates are observed in sovents of high poarity. EXPERMENT Synthesis. Processing and Physica Characterization Nanosize MoS2 is prepared by dissoving an anhydrous M o u sat (X=C Br or ) in a water and air-free inverse micee soution. (A typica inverse micee soution woud consist of 5-0% by weight of a quaternary ammonium surfactant in an aiphatic hydrocarbon such as octane).[2] This precursor soution is then exposed to a source of sufide typicay H2S gas injected through a septum in a known amount (sighty greater than 2: S:Mo) whie rapidy stirring the soution. A brighty coored transparent soution is formed which is then purified by extraction into an oi-immisibe phase sovent typicay acetonitrie (ACN). Aternativey we have deveoped high pressure iquid chromatographic (HPLC) procedures to both separate the nanocusters from the surfactants and ionic byproducts in the soution and opticay characterize

2 DSCLAMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof nor any of their empoyees make any warranty axpressor i m p i i i or assumes any ega Eabii- ty or respoasibiityfor the acruracy competenessor usefuness of any information apparatus product or pnwmss disdosed or represents that its use woud not infringe privatey owned rights. Reference herein to any specific commercia product process or service by trade name trademark manufacturer or otherwise does not necessariy constitute or impy its endorsement recommendationor favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessariy state or refect those of the United States Government or any agency thereof.

3 Portions of this document may be iegibe in eectronic image products. mages are produced from the best avaiabe origina document

4 the purified nanocusters on-ine.[3] Gas-ChromatographyMass Spectrometry is used to ascertain that the organic byproducts have been removed from the reaction mixture. The purified MoS2 custers have been investigated using HRTEM and shown to be nanocrystaine. The arger nanocusters have been shown to have the buk hexagona attice structure by eectron diffraction but the smaest custers (diameter D<3 nm) have too few atoms to give unambiguous structura information based upon diffraction. However attice fringe images are consistent with d spacings found in the buk materia. Evidence indicates that the custers are not spherica in shape but are disc-ike. The sandwich-ike structure of MoS2 consists of successive S-Mo-S tri-ayers with a weak Van-der-Waas interaction between ayers. This graphite-ike structure is responsibe for the good ubricating properties of buk MoS2 since these panes can readiy side past one another. Dynamic ight scattering is compementary to measurements of custer cross-sectiona size by TEM since it measures the transationa diffusion of the custer in the acetonitrie soution and via Stokes aw provides an equivaent sphere hydrodynamic diameter. This diameter is comparabe to the cross-sectiona area for nanocusters with D- nm but is smaer for the D-4.5 nm custers impying the thickness of the atter custers is somewhat ess than the crosssectiona TEM measurement. MoS? - Optica ProDerties and Eectronic Structure Buk MoS2 is an indirect band-gap semiconductor with a band gap of.23 ev.[4] As in the case of other indirect materias such as Si and Ge no room temperature photouminesence can be observed. Studies of thin crystaine fims of MoS2 revea that the first direct absorbance in MoS2 occurs at.88 ev corresponding to -660 nm. n buk form the top of the vaence band is composed primariy of Mo dz2 orbitas and the bottom of the conduction band is composed of Mo Dx2-y2 and dxy orbitas. As a resut excitation to create hoe-eectron pairs is meta intraband in nature and no Mo-S bonds are weakened giving this materia strong resistance to photocorrosion. [S n contrast -V semiconductors such as CdS have vaence band orbitas consisting mainy of S 3p states so that direct excitation across the gap to conduction band Cd 5s states eads to weakening of the chemica bonds hoding the materia together and accounts for the observed significant photocorrosion in these semiconductors. As MoS2 is made smaer the momentum seection rues due to the ong-range transationa symmetry of the attice are reaxed. n effect the materia becomes more ike a direct band-gap materia. As shown in figures and 2 this resuts in increased room temperature photouminesence and arger optica extinction in the visibe absorbance. (The extinction coefficients can be obtained from these figures by mutipying by 000 since the [MoSz] x O-3M and are in the o4 to 05 cm--mo- range) A significant portion of the observed uminesence occurs to the red of the absorbance band-edge indicating that recombination is occurring primariy from sub-band-gap trapped states at the surface of the nanocuster. Addition of an eectron acceptor such as bipyridine bpy to the MoS2 nanocuster wi decrease the ifetime of these deep trap states by funneing some of the eectrons created by direct excitation to the eectron acceptor bpy from which recombination with the hoe on the MoS2 nanocuster wi aso occur. This aows us to foow the eectron transfer kinetic indirecty. A more direct aternative approach to foowing the ET kinetics invoves measurement of the change in the bpy absorbance due to ET as a function of time. We are currenty pursuing such studies. An obvious resut of making smaer MoS2 nanopartices is the bue shift of the absorbance edge shown in figures and 2 a corresponding bue shift of the emission and an increased quantum efficiency for uminesence. We can roughy estimate the conduction and vaence band energies of our nanosize MoS2 custers by using the known potentias of the buk materia of +O. V and +.33 V (vs. NHE) respectivey. f we attribute most the band-gap shift observed in figure and 2 to changes in the conduction band potentia (because of the ower eectron compared to hoe mass) then we estimate a vaue of -.66 V and -.46 V vs. NHE for the D=4.5 nm and D=3.O nm sampes.[6] -

5 By photoexciting at the direct point we produce conduction band eectrons with about 0.25 ev higher energies (more negative potentia) than the above vaues. So nanocusters wi have a significant driving force for eectron transfer compared to the buk materia waveength(nm) Figure. Absorbance (soid curve) and photouminesence (dashed curve) from D=4.5 nm MoS2 nanocusters. 4-8 * b ( ' ' 2 % 7 cg ' 3 8 _ : W " ' " A ' 300 ' '! J t;' K?. B h rn X :000 (D w :500 a ~ : " ' waveength(nm) 500 -i0 700 Figure 2. Absorbance (soid curve) and photouminesence (dashed curve) from D= nm MoS2 nanocusters. Static PL quantum Efficiency Tabe summarizes the resuts of PL integrated area PL(area) normaized by the custer absorbance (abs) at the excitation waveength indicated for one size D=3.O nm of nanocuster. Very simiar trends were observed for other sizes of custer. Aso shown in this tabe are the resuts for Coumarin 500 dye (neary 00%Q.E. for PL) under identica instrumenta conditions. We note the very significant decrease in radiative recombination (PL) as the sovent poarity decreases from ethyene gyco to o-xyene. This effect is greatest for excitation at the first direct absorbance (-340 nm see figure 2) compared to excitation at the band edge (- nm). Furthermore we observe an interesting decrease in PL by 5-fod when bpy is bound to the nanocuster and excitation is at 340 nm. Band edge excitation on the other hand actuay increases the yied by -5% when bpy is bound. Additionay changing the sovent can ater the overa waveength dependence of the PL for excitation at the first absorbance feature. (There is itte effect for excitation at the band edge) Basicay sovating the custers in very non-poar

6 sovents such as touene increases the amount of ight emitted from trapped surface states effectivey red-shifting the PL peak by -20 nm compared to poar sovents such as ethyene gyco. Tabe. Effect of Sovent Poarity on PL Quantum Efficiency for MoS2 nanocusters Size(nm) sovent h( exci t) <h(emis)> pl(area)/abs Coumarin 500 methano x 00 Ethyene Gyco DMF acn x 05 acdpy hexano O octano touene o-xyene Coumarin 500 methano x 00 3.O Ethyene Gyco dmf x 06 acn acdpy hexano x 06 3.O octano x 06 3.O touene o-xyene ET Studies The ayer-ike structure of both buk and nanosize MoS2 has important impications for its use as a photocatayst. The basa panes of sufur atoms are reativey inert and bifunctiona eectron donating igands such as bipyridine (bpy) (used in the present study) wi bind to the Mo at edge sites of the disc-ike MoS2 nanocusters. Such binding can be demonstrated to be quite strong as a MoS2hpy compex eutes as a singe entity during HPLC.[6] t has been further demonstrated that the moar ratio of Mo to bpy at fu edge site occupation is about 2: and so this was the chosen concentration for bpy and tetramethy substituted bpy (tmb) used in our studies. The actua reduction potentias of bpy and tmb are quite sensitive to the type of meta to which they are bound and the chemica environment (sovent poarity etc.). Never-the-ess the difference of potentia of 0.29 V between the two acceptors (tmb is more difficut to reduce) remains quite constant. So by adding these eectron acceptors to the two different sized MoS2 nanocuster sampes and observing the changes in the PL decay curves shown in figure 3 and 4 one can estimate this potentia. A PL reaxations were obtained with 35 nm excitation whie detection was at 420 nm for the D= nm sampe and 560 nm for the D=4.5 nm sampe. The decay times do depend on the detected waveength somewhat. Addition of tmb to the MoS2 D=4.5 nm sampe resuts in a very sma change in the PL decay curve of figure 4. and we concude that the potentias of tmb and bpy for reduction in acetonitrie are V and -.4 V vs. NHE respectivey. (The decay curve of bound tmb overaps the bare nanocuster curve of figure 4 and so is not shown). Since the decay curves obtained are very non-exponentia and represent a very wide range of decay rates one can obtain exceent fits to these data by using a stretched exponentia form Ae-(rtY from which the decay rate r can be extracted. A compicating factor is that the stretched

7 exponentia exponent p changes aong with r when bpy is bound to the custer as we as when different sovents are used. This impies that the various mechanisms for radiative recombination for a given size custer are infuenced by the detais of the custer interface. Compete detais for such energy cacuations and the underying assumptions are given esewhere [6]. From anaysis of the bue shifts of the absorbance spectra and our ET kinetics for nanosize MoS2 we have estimated the conduction band potentia of the D= and D=4.5 nm nanocusters to be -.7 V and -0.9 V respectivey in acetonitrie.[6] f we reca the estimated potentia in the buk materia is +O. V we can see that quantum size effects have made these nanocusters into very strong reducing agents t - c n L 2500 g E ; L Or - * SO00 t ime(ps) Figure 3. The decay of the photouminesence of D= nm MoS2 nanocusters for various eectron acceptors added a 2 -.C E m n time(ps) Figure 4. The decay of the photouminesence of bare D=4.5 nm MoS2 nanocusters and MoS2 with bpy. Size DeDendence Some of the resuts of our studies on ET kinetics are summarized in figure 5. We first note that for a constant sovent poarity (e.g. in acetonitrie acn) the smaest custers with the argest conduction band potentias are capabe of driving the eectron transfer to both bpy and tmb at faster rates. n fact the arger D=4.5 nm custers cannot effectivey transfer eectrons to tmb

8 the more difficut-to-reduce an eectron acceptor. We have not shown the D=4.5 nm tmb ET rates in figure 5 since they are so sow. Sovent Dependence Examination of the data of figure 5 shows that reduction of bpy to bpy radica anion is so facie for the smaest MoS2 nanocusters that sovent poarity has a very minor effect on ET kinetics. For the more difficut to reduce substrate tmb the smaer D=3.O nm custers show significanty sower ET once the moe fraction of the non-poar sovent benzene is greater than 0.5. n the case of the D=4.5 nm custers reducing the sovent poarity sows down the ET dramaticay since ower poarity sovents are ess abe to stabiize the charge separated state. n fact effectivey no ET to bpy occurs for sovent mixtures of greater than 20 moe% benzene h v a.i 0000 c) b 4 s k c) 5 k * 000 a 0 a 00 - E] - D= nm to bpy +D= nm to tmb moe fraction acetonitrie.2 Figure 5. Effect of nanocuster size and sovent in the ET decay times (acetonitriebenzene mixture). CONCLUSONS We have shown how quantum confinement in an indirect semiconductor materia can shift the conduction band potentia and aow facie eectron transfer to two types of substrate. Smaer custers were demonstrated to have improved ET rates. t was aso demonstrated that maximizing the sovent poarity increases the ET rate by stabiizing the charge separated state. ACKNOWLEDGMENT This work was supported by the U.S. dept. of Energy under contract DE-AC04-94AL Sandia is a mutiprogram ab operated by Sandia Corporation a Lockheed-Martin Company for the U.S. Dept. of Energy. REFERENCES [ 3 J.P. Wicoxon and G.A. Samara Physica Rev Rapid Comm (995). [2] J.P. Wicoxon G. Samara and P. Newcomer Proceedings of Symposium F meeting of the MRS Society Boston MA. (994) [3] J.P. Wicoxon P.P. Newcomer and G.A. Samara J. App. Phys. submitted (996). [4] B.L. Evans in Optica and Eectrica Properties (of Materias with Layered Structures) ed P.A. Lee D. Reide Pub Boston 976 Chapt. pg. [5] H. Tributsch Z. Naturforsch 32a 972 (977). [6] F. Parsapour D.F. Keey S. Craft and J.P. Wicoxon J. Chem. Phys. 04 (996).