Synthesis of monolayer graphene on polycrystalline nickel and nickel-copper bimetallic catalyst and study toward the reuse of nickel catalyst / Seah Choon Ming
Grafena merupakan struktur karbon dengan ketebalan satu atom. Grafena
terdedah semua atomnya ke medium sekitar. Selepas penemuan grafena pada 2004,
ia menjadi subjek utama penyelidikan di seluruh dunia. Grafena mempunyai sifatsifat
yang luar biasa dari segi mekanikal, optik, haba dan elektrik. Sifat-sifat tersebut
menjadikan grafena berpotensi digunakan dalam pelbagai aplikasi. Pemendapan wap
kimia bermangkin (CVD) adalah saluran yang paling baik untuk menghasilkan
grafena berskala wafer, kerana teknik ini mempunyai kelebihan dalam proses
pemisahan grafena daripada pemangkin selepas CVD. Dengan bantuan penyejukan
pantas, grafena berlapis tunggal berjaya dibentuk pada foil nikel polibabluran
dibawah CVD tekanan atmosfera, dengan suhu 850 ºC, tekanan separa metana 0.2
atm and 5 min tempoh reaksi. Tetapi grafena berlapis tunggal gagal didentuk dengan
menggunakan foil kuprum sahaja. Penyejukan pantas selepas CVD mendorongkan
pelindapkejutan aktiviti pemangkin dan menghadkan kadar difusi karbon dalam nikel
ke permukaan nikel. Proses ini memudahkan pembentukan grafena berlapis tunggal
berskala wafer. Untuk meningkatkan keseragaman grafena berlapis tunggal, satu
teknik mudah digunakan untuk menumbuh grafena berlapis tunggal secara serentak
pada kedua-dua foil nikel polihabluran dan foil kuprum polihabluran, pada suhu 950
ºC, tekanan separa metana 0.2 atm and 5 min tempoh reaksi. Stuktur grafena yang
seragam dan berkualiti tinggi dapat dibukti dengan spektroskopi Raman dan
mikroskop transmisi electron resolusi tinggi. Sistem pemangkin dwilogam yang
dicadang membolehkan pengawalan difusi karbon ke permukaan dalam foil Ni dan
Cu. Khususnya, kebolehcapaian karbon dapat dikurangkan pada permukaan Ni
dalaman, manakala Cu memainkan peranan sebagai penghalang karbon. Mekanisme
pertumbuhan grafena berlapis tunggal dapat dibantu denagn difusi karbon melalui
bijian Ni dan sempadan bijian Ni. Daya penggerak untuk difusi karbon datang
daripada kepekatan kecerunan karbon antara permukaan yang kaya dengan karbon
dan permukaan kurang karbon. Sempadan bijian Ni telah terbukti memainkan
peranan yang penting dalam kawalan difusi karbon semasa peringkat pertumbuhan.
Dengan bantuan penyejukan pantas, proses pelindapkejut mengurangkan jumlah
atom karbon diasing dari Ni, hanya atom karbon yang terletak berhampiran
permukaan Ni mempunyai masa yang cukup untuk mengasing dan membentuk
grafena. Sementara itu difusi atom karbon dalam tengah foil Ni telah dihalang dan
lepas itu membentuk Ni3C. Ni3C dikenali sebagai perlindungan yang baik terhadap
kakisan. Kehadiran Ni3C digabungkan dengan penggunaan ferum nitrat (0.5mol/L)
sebagai bahan punar lemah semasa pemisahan grafena, foil Ni boleh digunakan
semula sehingga 6 kali tanpa menyebabkan sisihan yang besar terhadap kualiti dan
keseragaman grafena berlapis dua. Ni3C ternyata mampu untuk menghadkan kesan
punaran foil Ni. Kerja-kerja ini telah berjaya mempamerkan cara yang mudah dan
novel untuk mensintesis grafena berlapis tunggal dengan kualiti yang tinggi.
_______________________________________________________________________
Graphene is a layer of sp2 hybridized carbon atoms with a thickness of only
one atom, which exposed most of its atoms to the surrounding medium. Since the
discovery of graphene in 2004, it has become the main subject of research around the
world. The attractiveness of graphene is mainly attributed to its remarkable
mechanical, optical, thermal and electrical properties, enabling graphene to be
potentially used in various applications. To date, CVD is the promising method to
produce wafer-scale graphene, because it allows an easier separation of graphene
from the catalytic substrate. With the assist of fast cooling, monolayer graphene was
grown directly on polycrystalline Ni foil under atmospheric pressure CVD with
temperature of 850 ºC, methane partial pressure of 0.2 atm and reaction duration of 5
min. However, monolayer graphene could not be formed on Cu under the chosen
CVD conditions. Fast cooling after CVD allowed the quenching of the activity of the
catalyst and limiting diffusion of dissolved carbon to the surface of Ni, which later
facilitate the formation of predominantly wafer scale monolayer graphene. To further
improve the uniformity of monolayer graphene, a facile technique was applied to
grow monolayer graphene simultaneously on both polycrystalline Ni and Cu foils
using a Ni-Cu bilayer catalyst at temperature of 950 ºC, methane partial pressure of
0.2 atm and reaction duration of 5 min. High uniformity and quality of the crystalline
structure of the grown graphene was evidenced by Raman spectroscopy mapping and
High Resolution Transmission Electron Microscope. The straightforward bimetallic
catalytic system allows the control of carbon diffusion to the interface of Ni and Cu.
In particular, carbon accessibility is reduced at the inner Ni surface, and Cu behaves
as a carbon barrier. The growth mechanism of monolayer graphene was facilitated by
carbon diffusion through the bulk and Ni grain boundary, the driving force coming
from concentration gradient between carbon-rich surface to carbon-lacked surface.
The grain boundaries were shown to play a crucial role in carbon control during the
growth stage. Facilitated by the applied fast cooling, the quenching process reduced
the amount of carbon atoms segregated, only the carbon atoms situated near the
surface had enough time to segregate and form graphene. Meanwhile, diffusion of
carbon atoms at the middle of the Ni foil was highly inhibited; forming Ni3C. Ni3C is
known to offer good protection against corrosion. The presence of Ni3C combined
with the use of iron nitrate (0.5mol/L) as soft etchant for graphene separation, the Ni
foil could be reused again up to 6 cycles without causing a huge deviation on the
quality and the uniformity of bilayer graphene. Ni3C is indeed able to limit the
etching effect of the Ni foil. This work has successfully demonstrated a simple and
novel route to synthesize monolayer graphene with high quality.
Synthesis of monolayer graphene on polycrystalline nickel and nickel-copper bimetallic catalyst and study toward the reuse of nickel catalyst / Seah Choon Ming
Synthesis of monolayer graphene on polycrystalline nickel and nickel-copper bimetallic catalyst and study toward the reuse of nickel catalyst_Seah Choon Ming_K4_2016_MYMY