DOPING OF GRAPHENE AND ITS APPLICATION IN PHOTO ELECTROCHEMICAL WATER

SPLITTING

Seminar by,BASUDEV

BARALJUNIOR

PROJECT FELLOWCentre for Nano Science and Nano Technology

Institute of Technical Education & ResearchSiksha 'O' Anusandhan University, BBSR

Contents…… INTRODUCTION

GRAPHENE

DOPING IN GRAPHENE

APPLICATION IN ARTIFICIAL PHOTOSYNTHESIS

CONCLUSION

Introduction….

• Energy and environmental issues at a global level are important topics. It is indispensable to construct clean energy systems in order to solve the issues.

• Hydrogen will play an important role in the system because it is an ultimate clean energy and it can be used as a fuel .

• In photoelectrochemical (PEC) water splitting, hydrogen is produced from water using sunlight and specialized semiconductors called photoelectrochemical materials, which use light energy to directly dissociate water molecules into hydrogen and oxygen .

• This is a long-term technology pathway, with the potential for low or no greenhouse gas emissions.

So, what is Graphene ???? Simply graphene is …

- World’s first 2D crystal. - Latest invented allotrope of Carbon - Noble Prize for Physics 2010 - Single layer hexagonal structured compound of

carbon. - Obtained from simple and abundant form of

graphite . - Thinnest ever material in the world.

Graphene ????

Discovery…….. In 2004, Andre Geim and Kostya Novolselov

came up with an ingenious method after years of effort to isolate monolayer graphene flakes and awarded the 2010 Nobel prize in physics for this work.

Kostya Novoselov

Andre Geim

They simply stuck a flake of graphite debris onto plastic adhesive tape, folded the sticky side of the tape over the flake

and then pulled the tape apart, cleaving the flake in two. As the experimenters repeated the process, the resulting fragments grew thinner.

Structure……• It is the one-atom thick planar sheet of sp2

hybridised carbon atoms (graphite), which makes it the thinnest material ever discovered.

• 2-dimentional crystalline allotrope of carbon. • C-C Bond length is 0.142 nm.• Graphene Sheets interplanar spacing is of

0.335 nm. • It is almost completely transparent, yet so dense

that not even helium can pass through it.

Electrical and electronic properties of graphene….. High electrical conductivity. Conductivity further can be

increased by applying electric field Electrical conductivity of sheets are 10 times that of

copper. Best known conductor till now

Graphene differs from most conventional three-dimensional materials.

Intrinsic graphene is a semi-metal or zero-gap semiconductor

Graphene has a remarkably high electron mobility at room temperature

The mobility is nearly independent of temperature between 10 K and 100 K

Resistivity of the graphene sheet would be 10−6 Ω·cm.

Doping in graphene…..• Chemical doping of elements like boron and

nitrogen in graphene gives rise to useful properties.

• Chemical doping of graphene becomes necessary to create a band gap which is useful for various applications

• It is a gapless material with ballistic conduction at room temperature and high carrier mobility

• The problem with single-layer grapheneis that for many applications it is necessary to create a band-gap.

• Thus, to use graphene in nano-electronic devices, a band-gap has to be engineered which will in turn reduce its electron mobility

• Such a band-gap can be created by sur-face modification or chemical doping.

• Doping graphene with hetero atoms like N & B can effectively tune its electronic structure and other intrinsic properties

Synthesis of nitrogen-doped graphene……

Nitrogen in graphenecan be pyrrolic, pyridinic, pyrazolic and graphitic. These differenttypes of nitrogen are characterized by N (1s) X-ray photo-electron spectra

• Several methods have been employed for the synthesisof nitrogenated graphene . Some of them are,

Method of synthesis Synthetic parameters Nitrogen content (at.%)

1. Ball milling Pristine graphite in NH3 14.84

2. Microwave treatment

GO, urea, 900 W, 30 s 13—15

3. Hydrothermal GO, urea, 180◦C for 12 h 10.13

Synthesis of boron-doped graphene…..

some of the most common boron induced defects are boronic, borinic or substitutional deffects

Several variations of this method have been developed; inorder to obtain boron-doped graphene . Some of the common methods are,

Method of synthesis Synthetic parameters Boron content (at.%)

1. Thermal GO and B2O3, 1200◦C, 10 min

3.2

2. Thermal GO and H3BO3, 800◦C 2—3

3. CVD Cu foil, CH4/H2/B2H6, 1000◦C

2.5

Application in water splitting / artificial photosynthesis…..• Photocatalytic water splitting is an artificial

photosynthesis process with photocatalysis in a

photoelectrochemical cell used for the dissociation of water into its constituent parts, hydrogen (H2) and oxygen (O2) using either artificial or natural light.• Theoretically, only solar energy (photons), water, and a catalyst are needed

• Methods such as photocatalytic water splitting are being investigated to produce hydrogen, a clean-burning fuel. •Water splitting holds particular promise since it utilizes water, an inexpensive renewable resource

Photocatalytic water splitting has the simplicity of using a powder in solution and sunlight to produce H2 and O2 from water and can provide a clean, renewable energy, without producing greenhouse gases or having many adverse effects on the atmosphere.

There are several strict requirements for a photocatalyst to be useful for water splitting. The minimum potential difference (voltage) needed to

split water is 1.23V at 0 pH. visible-light-driven photocatalysts have to meet the third

qualification – band gap energy less than 3.0 eV

• in order to achieve overall water splitting, its conduction band (CB) has to be located at a more negative potential than the reduction potential of H+/H2 (0 V)

• the valence band (VB) has to be located at a more positive position than the oxidation potential of O2/H2O (1.23 V)

• Under illumination with photon energy equal to or higher than the Eg of a semiconductor photocatalyst, electrons are excited from the VB to the CB, leaving holes in the VB.

• Then, the electrons reduce water to H2 and the holes simultaneously oxidize water to O2

Advantages of doping…….

• Doping graphenewith heteroatoms can effectively tune its electronic struc-ture and other intrinsic properties

• Doping graphenewith nitrogen or boron creates a band-gap and makes it an-type or p-type material. Such chemically doped mate-rials have unique properties

• N-graphene(2 wt.% N)/CdS nanocomposites shows a higher H2 evolutionthan pure CdS and the other composites from water undervisible light irradiation

• N-graphene as a protective layer canprevent CdS from photocorrosion under light irradiation

• N-graphene/CdS composites (210 mol/h) show 5 times the H2evolution rate higher of pure CdS (40 mol/h).

• Perfectly designed doped gaphene with band gap more than 1.3eV and less than 3eV can be used for water splitting in visible light also.

Conclusion……...

• forming donor levels in the forbidden band, enhancing the valence band position, preparing solid solution, have been proven to be effective for the design of visible-light induced photocatalysts.

• In order to efficiently realize solar-to-chemical process, band engineering is the key to developing new photocatalysts