ICONN+2010+abstract

= =

Outline
=== 1. Introduction 2. Experimental 2.1 Hummers synthesis 2.2. Cell fabrication 2.3 Cell measurement 3. Results & discussion 3.1 Successful graphene reduction Reduction shown by colour change • UV-Vis • Raman? 3.2 Photocurrent enhancement Amperometry results • Minitab output 4. Conclusions TiO2 + UV reduces graphene oxide to graphene • Graphene enhances photocurrent by 2-5 times ===

5. Further work Resistivity measurement
Climate change is tending towards the worst-case estimates of the IPCC. In order to mediate the negative effects rapid decarbonisation of society is required. Energy generation accounts for the majority of global CO2 emissions; Carbon nanostructures have drawn attention because of their unusual electronic and catalytic properties

Consist of conjugated sp2-bonded carbon atoms, i.e. highly mobile delocalized electrons

2-dimensional single / bi-layer graphene sheets possess great potential

 The strong van der Waals interactions among these reduced graphene sheets results in their aggregation to form graphite.

 In order to exfoliate graphene, various methods have been developed: mechanical method and chemical oxidation-reduction method.

=
Experimental

=
Graphene oxide was prepared using the Hummers method [ref]. 23ml sulfuric acid was added to 1g natural graphite and 500mg of sodium nitrate. The solution was placed in an ice bath with stirring and 3g of potassium permanganate was added slowly such that the solution temperature didn't exceed 20 deg. After 5 min the solution was heated to ~40 deg with stirring for about 30min. The solution was then added very slowly to ~40ml distilled water. After 15min 40ml of 10% hydrogen peroxide was added to the solution, which bubbled vigorously & turned yellow-brown. After 10 min of stirring the solution was filtered with rough filter paper. The following steps were repeated 3 times to purify the graphene oxide: The filter cake was resuspended in 5% sulfuric acid & 5% hydrogen peroxide, centrifuged at 10000rpm for 20 minutes, the supernatant was removed, the solid resuspended in distilled water, and the solution filtered. After the washing step the solid was dried in vacuum for 2 days.

To make the films, the 1. Material preparation Hummers method

TiO2 naoparticles Commercial, ST-01

TiO2-graphene nanocomposites Irradiate the mixture of TiO2 and graphene oxide in methanol solution under UV light (>320nm) for 30min in N2 atmosphere. Dropcasting the TiO2 and reduced graphene on OTE glass substrate Repeat with different TiO2-graphene ratio

2. Photoelectrochemical measurement - 2-electrode system (working and counter electrode) - 1M KoH (electrolyte) in air

=
== Results & Discussion

=
==

1. TiO2-mediated reduction of graphene oxide

=
======= Graphene oxide is insulating; it must be converted to graphene to regain its conductivity. Hi-throughput. In-situ. Fewer processing steps. Well dispersed. No surfactants required. Simple. Graphene oxide must be reduced in-situ, as reduced go quickly agglomerates. Surfactants can be difficult to remove.

Successful reduction can be verified qualitatively by observing colour change from brown to black. UV analysis of TiO2 flourescence during reduction; flourescence decreses with increasing graphene concentration indicating graphene is scavenging electrons. Raman spectroscopy can be used to show the conversion from graphene oxide to graphene: primarily though the reduction of the D band relative to the G (indicates disorder reduction?) from GO to G, and a slight shift to higher wavenumber from graphite to graphene oxide.

Mechanism

From Kamat paper:

e- generated on TiO2 surface (holes are scavenged by ethanol). e- reduces hydroxyl groups (reaction?) to give reduced graphene oxide, restoring conductivity.

TiO2 + hv -> TiO2 (h+e) -> TiO2 (e) + *C2H4OH + H+ (1) TiO2 (e) + graphene oxide -> TiO2 + reduced graphene (2)

The synthesis procedure results in graphene well-dispersed throughout the TiO2 particles. The particles prevent the graphene from agglomerating. [figure].

2. Photoconductivity enhancement due to graphene

=
=========== The presence of graphene integrated using this method improves the photoconductivity by up to 5 times beyond that of TiO2 itself, as measured using amperometry (no bias was applied). We believe the graphene acts as a conducting pathway from the Tio2 to the substratre; prevents electrons flowing between particles, which reduces conductivity through interfacial trapping and defects etc. Beyond concentrations of 0.9mg the photoconductivity begins to decrease, which we attrbute to a shadowing effect by the graphene (individual graphene layers absorb 2.3% of the light that passes through it).

Images to use


= = =3.=

df

Images for use

=Conclusions=

The TiO2 photocatalytic reaction is effective in reducing graphene oxide to graphene.

Reduced graphene serves as a good support material to anchor TiO2 photocatalyst particles. Films of reduced graphene-TiO2 composite show significant enhancement (factor of 5) in the photocurrent generation when employed as photoanodes.