NBEXP0004

=NBEXP0001 Review of previous graphene/TiO2 photocurrent measurements=

Aim
To re-analyse previous photocurrent measurements of graphene/TiO2 composite films & review conclusions from experiments.

NBEXP010201
Dark and light IV curve measurement of TiO2/graphene films (0, 0.1, 0.3, 0.5, 0.7, 0.9mg graphene to 4mg TiO2). Bias was swept from -1 to -1V over a few seconds (exact duration unknown) under Xenon UV lamp. Pure TiO2 sample was destroyed so comparison was impossible. Also missing dark IV curve of 0.1mg sample & light IV curve of 0.3mg sample.

Analysis not performed - too much data missing.

NBEXP010301
NBEXP0004 - NBEXP010301 photocurrent chart

Early attempt to measure photocurrent. Potentiostat was set to a very low sensitivity so data is mostly useless.

Aim
To determine the effect, if any, of taking photocurrent measurement with and without the reference potentiostat reference electrode.

Results
NBEXP0004 - NBEXP010401 photocurrent measurements

Comparison between reference and no-reference measurements for each graphene concentration:

Comparison of photocurrent measurements, using data captured with reference electrode:



Discussion & conclusions
Using the reference electrode slightly increases the measured photocurrent, although the measurements of ITO and pure TiO2 with reference exhibit stronger baseline skew than those without the reference electrode.

Further measurements will be performed with the reference electrode in place.

Aim
To see if graphene increases the photoconductivity of TiO2.

Results
NBEXP0004 - NBEXP010701 original data NBEXP0004 - NBEXP010701 baseline corrected data NBEXP0004 - NBEXP010701 peak values

Original data exhibited severe baseline skew. Corrected baseline of each measurement using MestReNova 'multipoint' baseline correction tool. Corrected data still exhibited skew around t=0. Data from t=0 to t=15s was not plotted to show photocurrent peaks better. Instrument sensitivity seems not to have been set properly - graphs are quite noisy and some large extraneous peaks are visible. Noise has also made it difficult to accurately adjust the baseline and some areas of the graph dip below the x axis. (Presumably the same error exists in graphs above the x axis.)

Interestingly, the 0.1mg G sample exhibits lower photocurrent than the pure TiO2 sample.

Amperometry charts before and after baseline correction:

Minitab analysis shows large spreads in the data. Chart of means has fairly narrow confidence interval, which is surprising. Many outliers shown in the box plot - probably correspond to the extraneous peaks mentioned before.



From the minitab analysis of the means, the change in photocurrent with graphene concentration is as follows:
 * G conc. || Mean || Change ||
 * 0 || 5.021E-6 || - ||
 * 0.1 || 3.620E-6 || 0.72 ||
 * 0.3 || 6.497E-6 || 1.29 ||
 * 0.5 || 8.529E-6 || 1.70 ||
 * 0.7 || 19.656E-6 || 3.91 ||
 * 0.9 || 27.432E-6 || 5.46 ||

Conclusions
Higher concentrations of graphene do increase the photocurrent above that of TiO2 alone. Magnitude is hard to determine owing to the spread of the data, although a 3-5x increase for the higher concentrations (0.7 & 0.9mg) seems reasonable.

Aim
Same as for NBEXP010701

Results
NBEXP0004 - NBEXP010801 original measurements NBEXP0004 - NBEXP010801 baseline corrected values NBEXP0004 - NBEXP010801 peak values

Photocurrent measurements (before & after baseline correction):

Minitab output of 'peak' values (photocurrent measurements taken while lamp was on).

Comparison of mean values (from Minitab interval plot):
 * = **Graphene conc. (mg)** ||= **Mean photocurrent (A)** ||= **Magnitude change** ||
 * = 0 ||= 4.454E-6 ||= - ||
 * = 0.1 ||= 5.089E-6 ||= 1.14 ||
 * = 0.3 ||= 7.675E-6 ||= 1.72 ||
 * = 0.5 ||= 9.911E-6 ||= 2.225 ||
 * = 0.7 ||= 5.105E-6 ||= 1.146 ||
 * = 0.9 ||= 13.31E-6 ||= 3.00 ||

Discussion
As with NBEXP010801, there is a genuine increase in photocurrent with graphene concentration, although the magnitude of the increase (3x for 0.9mg) is not very accurate due to the spread of the data. The cause of the large slopes in the peaks — which is the main cause for the large spread in the values — is unknown. It could either be due to: the instrument; chemical reaction between the electrolyte & sample; or the film is degrading during measurement.

Conclusions
The maximum photocurrent increase — //roughly// 3x — occurrs at graphene concentrations of 0.9mg. The stability of the films in solution, the potentiostat measurement technique, and chemical compatibility of the solvent must be investigated.

Aim
Same as for NBEXP010701

Results
NBEXP0004 - NBEXP010901 original data NBEXP0004 - NBEXP010901 baseline corrected values NBEXP0004 - NBEXP010901 peak values

Photocurrent measurements before and after baseline correction:

Minitab analysis of peak values:

Comparison of mean values (from minitab interval plot):


 * = **Graphene conc. (mg)** ||= **Mean photocurrent (A)** ||= **Magnitude change** ||
 * = 0 ||= 3.00E-6 ||= - ||
 * = 0.1 ||= 3.65R-6 ||= 1.22 ||
 * = 0.3 ||= 4.85E-6 ||= 1.62 ||
 * = 0.5 ||= 5.76E-6 ||= 1.92 ||
 * = 0.7 ||= 3.20E-6 ||= 1.07 ||
 * = 0.9 ||= 6.85E-6 ||= 2.28 ||

Conclusions
The maximum photocurrent increase due to graphene is roughly 2x. The precision in the measurement is much greater than in the previous measurements, indicating that future experiments may be able to improve on earlier results.

Aim
Same as for above

Results
NBEXP0004 - Experiment 7 - original values NBEXP0004 - Experiment 7 - baseline corrected values NBEXP0004 - Experiment 7 - peak values

Photocurrent measurements, before and after baseline correction:



Minitab analysis of peak values:



Comparison of mean values (from minitab interval plot):
 * = **Graphene conc. (mg)** ||= **Photocurrent (A)** ||= **Magnitude change** ||
 * = 0 ||= 0.310E-6 ||= - ||
 * = 0.1 ||= 3.847E-6 ||= 12.40 ||
 * = 0.3 ||= 6.813E-6 ||= 21.96 ||
 * = 0.5 ||= 9.509E-6 ||= 30.65 ||
 * = 0.7 ||= 16.768E-6 ||= 54.06 ||
 * = 0.9 ||= 16.109E-6 ||= 51.93 ||

Discussion
In this experiment we got a photocurrent increase of 54 times that of TiO2 alone. This grossly contradicts the previous experiments, which found a 5x increase at most. It is possible that the sample labelled '0mg' is bare ITO rather than TiO2. During first few experiments the film and ITO layer dissolved in the electrolyte because we applied too much current through the potentiostat. The photocurrent charts don't have a large skew like the previous ones so it seems the films were stable during this set of measurements. This leads us to suspect that the skew is due to incorrect instrument setup.

Conclusions
The increase in photocurrent from the addition of 0.7mg graphene is found to be 54 times that of TiO2 films alone. However, we suspect this is an erraneous result due to incorrect experimental procedure.

Overall conclusions
The addition of graphene to films of TiO2 does increase the photogenerated current. The optimal amount of graphene is between 0.7mg and 0.9mg to 4mg of TiO2, and the resulting photocurrent increase is about 2-5 times that of TiO2 alone.

The measurements to date have shown a large amount of variation and it is uncertain whether the films are stable or chemically, given the lage baseline skew seen in experiments NBEXP010701 and NBEXP010801. Further experiments are required to verify: - reproducibilty - optimal graphene/TiO2 ratio - chemical inertness & stability

Log
2009-109-11

12:00 Imported data from 'NBEXP010201' into excel. Found no data for: pure TiO2 sample (see 'Results' above); 'dark' IV curve for 0.1mg sample; 'light' IV curve for 0.3mg G sample. 12:12 Aborted analysis of experiment - too much data missing for analysis to be useful. 12:12 Began importing data from 'NBEXP010701' into excel. 12:22 Plotted data from NBEXP010701 - severe baseline skew present. Began correcting baselines in MestReNova. 12:26 Baseline correct NBEXP010701 0mg using MestReNova 'multipoint' tool 12:28 Baseline correct NBEXP010701 0.1mg using MestReNova 'multipoint' tool. Very noisy - applied smoothing using 'moving average' with span of 2. 12:32 Baseline correct NBEXP010701 0.3mg using MestReNova 'multipoint' tool. 12:34 Baseline correct NBEXP010701 0.5mg using MestReNova 'multipoint' tool. 12:36 Baseline correct NBEXP010701 0.7mg using MestReNova 'multipoint' tool. 12:38 Baseline correct NBEXP010701 0.9mg using MestReNova 'multipoint' tool. 12:44 Imported baseline-corrected data into excel. Plotted from t>15s to remove residual baseline skew at t=0. 13:18 Finished selecting photocurrent measurements from the 'on-lamp' pulse and copying into new worksheet. 13:47 Finished analysing 'on-lamp' data in Minitab. Produced box-plots with & without outliers. 13:55 Produced scatter plot and confidence interval plots. 16:55 Baseline correct NBEXP010801 0mg using MestReNova 'multipoint' tool. Smoothed (moving average, span=2) 16:59 Baseline correct NBEXP010801 0.1mg using MestReNova 'multipoint' tool. No smoothing. 17:01 Baseline correct NBEXP010801 0.3mg using MestReNova 'multipoint' tool. Smoothed (moving average, span=2) 17:02 Baseline correct NBEXP010801 0.5mg using MestReNova 'multipoint' tool. Smoothed (moving average, span=2) 17:03 Baseline correct NBEXP010801 0.7mg using MestReNova 'multipoint' tool. Smoothed (moving average, span=2) 17:05 Baseline correct NBEXP010801 0.9mg using MestReNova 'multipoint' tool. Smoothed (moving average, span=2) 21:00 Baseline correct NBEXP010901 0mg using MestReNova 'multipoint' tool. Smoothed (moving average, span=2) 21:03 Baseline correct NBEXP010901 0.1mg using MestReNova 'multipoint' tool. Smoothed (moving average, span=2) 21:05 Baseline correct NBEXP010901 0.3mg using MestReNova 'multipoint' tool. Smoothed (moving average, span=2) 21.06 Baseline correct NBEXP010901 0.5mg using MestReNova 'multipoint' tool. Smoothed (moving average, span=2) 21:07 Baseline correct NBEXP010901 0.7mg using MestReNova 'multipoint' tool. Smoothed (moving average, span=2)