NBEXP0008

=NBEXP0008 — XPS analysis of graphite, graphene oxide, UV-irradiated graphene oxide and graphene/TiO2=

=Objective=

To

=Background=

XPS is a surface sensitive technique used to analyze surface chemical composition and bonding. A sample is irradiated with an X-ray beam while the number of electrons that escape and their kinetic energy are simultaneously measured. We shall use XPS to observe the degree of oxidation of graphite to graphene oxide and the reduction to graphene by measuring the relative proportion of oxygen in the various samples. Specifically, we shall be looking at the following C1s peaks:
 * C-C (~284.8eV)
 * C-N (~285.7eV)
 * C-O (286.2eV)
 * C=O (~287.8eV)
 * C(O)O (~289.1eV)

and the O1s peaks:
 * C=O (530.6eV)
 * O=C-OH (530.6eV)
 * C-OH (533eV)

(Values from [1])

During reduction the peaks relating to oxygen-containing groups should decrease relative to the C-C peak (by comparing relative FWHM peak area). Maximum reduction seems to be around ~90% reduction in amount of O in sample [2] (e.g. C:O ratio of 12.4:1 [1])



Peak shifts to lower energies after oxidation may indicate lowered pi electron density due to disruption of the lattice [3].

=Samples=

Sample 1: graphite Sample 2: graphene oxide Sample 3: UV-itrradiated graphene oxide Sample 4: Reduced graphene oxide-TiO2 Sample 5: Graphene oxide-TiO2

=Procedure=

=Results=

Sample 1: Graphite
[|NBEXP0008 Sample 1 - Graphite]

Sample 2 : graphene oxide
[|NBEXP0008 Sample 2 — graphene oxide]



Sample 3 — UV-irraidiated graphene oxide
[|NBEXP0008 Sample 3 — UV-irridiated graphene oxide]



Sample 4 — Reduced graphene oxide-TiO2
[|NBEXP0008 Sample 4 — Reduced graphene oxide-TiO2]



Sample 5 — Graphene oxide-TiO2
[|NBEXP0008 Sample 5 — graphene oxide-Tio2]



=Discussion=

Effectiveness of graphite oxidation
The C1s graphite trace shows a C-C strong peak (at 284.5), as expected, and a small peak at 286.7, which probably corresponds to C-O at the graphite flake edges. After oxidation, the intensity of the C-C peak is significantly diminished and a new peak — C-O at 287eV — appears. The relative ratio of the areas of the C-C and C-O peaks in the graphene oxide spectrum is 1:1.3 (C:O) — the graphite has been significantly oxidised.



The O1s peak shows a narrowing of the O1s peak after oxidation and intensity increase. The peak maxima remains the same (532.9eV before oxidation, 533.1 after). The peak narrowing is the opposite of what was expected; the curve should have broadened to accommodate the contribution of new C=O and C-O-OH bonds. Perhaps this means that the oxidation process creates primarily hydroxide groups which swamp the contribution from other O-containing groups?



**Effect of UV illumination on graphene oxide solution**
There is little relative change in the C1 spectra shapes. The peak maxima remain the same: 284.9eV both before and after illumination for the C-C peak, and 287 before and 287.2 after illumination for the C-O peak. The relative C:O ratios before and after illumination are 1:1.38 and 1:1.15, which may indicate some slight reduction effect.



There is a slight red-shift in the O1s spectrum — the peak shifts from 533.1eV before illumination to 532.6eV after illumination. It's hard to say whether this represents an actual chemical/structural change without additional data.

Effectiveness of TiO2-mediated graphene oxide reduction
From the C1s spectra the reduction seems to be very effective. The C:O ratio goes from 1:1.3 to 1:0.3 following reduction by TiO2 (assuming 286.46 in the G-TiO2 spectrum can be attributed to C-O — it represents a substantial blue-shift). C=O and 0=C-OH also go from 1:0.22 to 1:0.03 and 1:0.13 to 1:0.09. There is an additional peak convolution in the G-TiO2 spectrum assigned to 284.03 and I don't know what to assign to it (if it is indeed a real peak).




 * Peak assignments:**
 * |||| Graphene oxide |||| Reduced graphene oxide / TiO2 ||
 * || Binding energy (eV) || Intensity (counts /s) || Binding energy (eV) || Intensity (counts /s) ||
 * C-C || 284.97 || 1282.47 || 285.06 || 1599.34 ||
 * C-O || 286.98 || 1678.9 || 286.46 || 481.85 ||
 * C=O || 287.97 || 282.46 || 287.96 || 54 ||
 * O=C-OH || 289.26 || 171.4 || 289.11 || 145.17 ||



Comparison of graphene oxide-TiO2 and reduced graphene oxide-TiO2



 * |||| graphene oxide-TiO2 |||| reduced graphene oxide-TiO2 ||
 * || Binding energy (eV) || Intensity (counts /s) || Binding energy (eV) || Intensity (counts /s) ||
 * C-C || 284.97 || 2627.1 || 285.06 || 1599.34 ||
 * C-O || 286.63 || 742.87 || 286.46 || 481.82 ||
 * O=C-OH || 288.88 || 339.12 || 289.11 || 145.17 ||

Peak assignments:
 * |||| graphene oxide-TiO2 |||| reduced graphene oxide-TiO2 ||
 * || Binding energy || Intensity || Binding energy || Intensity ||
 * O-Ti-O || 530.38 || 23548.02 || 530.2 || 12916.17 ||
 * C=O & O=C-OH || 531.89 || 3711.67 || 531.65 || 2105.1 ||
 * C-OH || 533.2 || 2008.58 || 532.79 || 1045.08 ||

There is a slight red-shift in the binding energies after annealing, especially for C-OH. The relative intensities remain almost exactly the same before and after reduction: 0.16:1 both before & after for C=O:Ti-O2, and 0.09:1 and 0.08:1 before & after respectively for C-O:Ti-O2. The C1s spectra have shown that there is a significant reduction in the C-OH peaks after reduction, which doesn't seem to be reflected in the O1s spectra. Alternatively, the C-OH peak may have diminished, along with a proportional loss of TiO2.

=Conclusion=

1. Our oxidation procedure is very successful. The C:O ratio of the synthesised graphene oxide is 1:1.3. (Typical ratios are around 2:1 C:O).

2. The reduction of graphene oxide using the photochemical reaction of TiO is successful, I think. Comparison of XPS spectra of graphene oxide the reduced graphene oxide-TiO2 composite show a huge reduction in the C:O ratio for hydroxyl and carbonyl groups. Carboxyl groups seem to be difficult to remove. However, the comparison of the graphene oxide-TiO2 and reduced graphene oxide-TiO2 spectra show little difference, which is confusing. The samples appear very different (the non-irradiated sample is a light reddish-brown, the irradiated sample is black) which is evidence that one should be well reduced relative to the other. Perhaps the samples got mixed up?

3. Graphene oxide exposed to UV light while not in the presence of TiO2 is not reduced and seems to undergo no chemical reaction.

=Log=

=References=

[1] Dongxing Yanga, Aruna Velamakannia, Gulay Bozoklu et. al., Chemical analysis of graphene oxide films after heat and chemical treatments by X-ray photoelectron and Micro-Raman spectroscopy. Carbon 47 (2009) 145-152 [2] Héctor A. Becerril, Jie Mao, Zunfeng Liu et. al. Evaluation of Solution-Processed Reduced Graphene Oxide Films as Transparent Conductors. ACS Nano 2 (2008) 463-470 [3] Electrochem communications 3, 608

=Tags=