Molekulsko modeliranje prepustnosti intrinzičnih točkovnih defektov v grafenu za difuzijo preprostih plinov

magistrsko delo

Erik Gregori (Avtor), Boštjan Genorio (Mentor), Marjan Marinšek (Član komisije za zagovor), Miran Gaberšček (Član komisije za zagovor), Anton Kokalj (Komentor)

Povzetek

Raziskav v zvezi z grafenom je že na mnogih področjih veliko. Po pregledu literature smo zasledili nekaj pomanjkljivosti na področju uporabe nepravilnega (defektnega) grafena kot separacijske membrane. Sama struktura čistega, pravilnega grafena je sestavljena iz ogljika, ki je razporejen v obliki dvo-dimenzionalnega (2D) heksagonalnega satovja in je videti kot luknjasta struktura, ki bi lahko imela potencial separacije zelo majhnih delcev. Pokazali smo, da to ne drži, saj so difuzijske bariere izredno visoke tudi za najmanjše atome v naravi. Za uporabo grafena na področju separacijskih membran je torej treba uvesti v grafen defekte. Pri tem vpliv intrinzičnih defektov v grafenu še ni popolnoma razložen, zato smo raziskali predvsem točkovne defekte oz. praznine. Z izračuni na osnovi teorije gostotnega funkcionala (ang. - Density Functional Theory, DFT) smo modelirali difuzijo H2 in O2 skozi točkovne defekte (praznine) različnih velikosti v grafenu in pokazali, da so nepasivirani defekti v izolirani plasti grafena dovolj reaktivni, da lahko razcepijo molekule. Izračunali smo aktivacijske energije za disociacijo molekul H2 , O2 in H2O. Le te so dovolj nizke, da bi te molekule pri sobni ali višji temperaturi nemudoma disocirale in s tem pasivirale praznino. V primeru kisika energijske bariere sploh ni. Na podlagi teh ugotovitev smo določili difuzijske bariere še za praznine v grafenu, ki so se pasivirale z razcepljeno molekulo O2 in H2O. Sisteme praznin smo med seboj primerjali glede na numerično izračunano van der Waalsovo površino defekta. Nazadnje smo tvorili še vrstične defekte, ki smo jih imenovali bočne reže. Difuzijske bariere, za prehod molekul preko praznin v grafenski plasti, smo tu ocenili na podlagi izkušenj, pridobljenih z računsko zahtevnimi izračuni za določaje prehodnega stanja z metodo obremenjevanja napete elastične vrvi (ang. - Nudged Elastic Band Method, NEB), z bolj enostavnimi izračuni strukturne relaksacije. Difuzijske bariere smo primerjali še glede na relativno velikost molekul, pri čemer smo med seboj primerjali difuzijske bariere za H2 , O2 in CH4 . Da bi rezultate še bolj ovrednotili, smo preko Maxwell-Boltzmannove porazdelitve aproksimativno izračunali delež delcev, ki ima dovolj visoko energijo, da lahko določeno bariero premaga (v ta namen smo obravnavali nekaj karakterističnih velikosti barier).

Ključne besede

grafen;kvantna kemija;teorija gostotnega funkcionala;DFT;difuzijska bariera;defekti;plinske molekule;magistrska dela;

Podatki

Jezik:	Slovenski jezik
Leto izida:	2021
Tipologija:	2.09 - Magistrsko delo
Organizacija:	UL FKKT - Fakulteta za kemijo in kemijsko tehnologijo
Založnik:	[E. Gregori]
UDK:	544.18(043.2)
COBISS:	85519875
Št. ogledov:	264
Št. prenosov:	53
Ocena:	0 (0 glasov)
Metapodatki:

Ostali podatki

Sekundarni jezik:	Angleški jezik
Sekundarni naslov:	Molecular modeling of permeability of intrinsic point defects in graphene for difusion of simple gasses.
Sekundarni povzetek:	Graphene has been widely investigated in many areas of research. Some studies even explored the applicability of graphene as separation molecular membranes. The structure of the perfect graphene is composed of hexagonal honeycombs and reminds of a perforated structure that could have the potential to separate very small molecules. We have shown that this is not the case, because the barriers for diffusion through an ideal graphene layer are extremely high even for the smallest atoms in nature. To utilize graphene as separation membranes, defects need to be introduced into graphene structure. The role of intrinsic defects in graphene for the diffusion of molecules through the graphene sheets has not been fully explained yet, so we have primarly investigated vacancy point defects. By calculations based on the density functional theory (DFT), we modeled the diffusion of H2 and O2 through vacancies of different sizes in graphene sheet. Our DFT calculations clearly revealed that non-passivated vacancy defects of pure graphene are so reactive as to dissociate molecules, such as H2 , O2 , and H2O. In particular, calculated dissociation barriers are low enough for the vacancy defects to quickly get passivated; O2 even dissociates without a barrier at non-passivated vacancy defects. For this reason, we also considered vacancies, passivated with dissociated O2 and H2O molecules. Diffusion barriers, calculated for various types of vacancies were then compared with respect to each other on the basis of their numerically calculated van der Waals vacancy areas. Finally, we also considered a special type of line defects, named as side slits. Here, we have estimated the diffusion barriers based on the experience gained from the time consuming transition-state nudged-elastic-band (NEB) calculations of diffusion barriers for molecular diffusion through vacancy defects in graphene sheet. The gained experience allowed us to estimate the diffusion barriers for the diffusion through the side slits by using the simplified constrained relaxation calculations. We have then compared the so calculated diffusion barriers with respect to the relative size of the molecules, whereby we made the comparision for H2 , O2 and H2O molecules. As to better appreciate the calculated diffusion barriers, we also performed an analysis on the basis of energy distribution of gas phase molecules that is based on the Maxwell-Boltzmann velocity distribution of gas-phase molecules. In this way, we calculated the fraction of molecules that have kinetic energy high enough to overcome the diffusion barrier for several characteristic diffusion barriers.
Sekundarne ključne besede:	graphene;density functional theory;diffusion barrier;defect;gas molecules;
Vrsta dela (COBISS):	Magistrsko delo/naloga
Študijski program:	1000376
Konec prepovedi (OpenAIRE):	1970-01-01
Komentar na gradivo:	Univ. v Ljubljani, Fak. za kemijo in kemijsko tehnologijo, smer Kemijsko inženirstvo
Strani:	48 str.
ID:	13454783