Improved electron-hole separation and migration in anatase TiO2 nanorod/reduced graphene oxide composites and their significance on photocatalytic performance

The as-synthesized TiO2 nanorods a-TNR (amorphous TiO2 layer covering the crystalline anatase TiO2 core) and TNR (fully crystalline anatase TiO2) were decorated with reduced graphene oxide (rGO) to synthesize two series of TiO2 + rGO composites with different nominal loadings of GO (from 4 to 20 wt%). The structural, surface and electronic properties of the obtained TiO2 + rGO composites were analyzed and correlated to their performance in the photocatalytic oxidation of aqueous bisphenol A solution. X-ray photoelectron spectroscopy (XPS) analyses revealed that charge separation in TiO2 + rGO composites is improved due to the perfect matching of TiO2 and rGO valence band maxima (VBM). Cyclic voltammetry (CV) experiments revealed that the peak-to-peak separations (ΔEp) are the lowest and the oxidation current densities are the highest for composites with a nominal 10 wt% GO content, meaning that it is much easier for the charge carriers to percolate through the solid, resulting in improved charge migration. Due to the high charge carrier mobility in rGO and perfect VBM matching between TiO2 and rGO, the electron-hole recombination in composites was suppressed, resulting in more electrons and holes being able to participate in the photocatalytic reaction. rGO amounts above 10 wt% decreased the photocatalytic activity; thus, it is critical to optimize its amount in the TiO2 + rGO composites for achieving the highest photocatalytic activity. BPA degradation rates correlated completely with the results of the CV measurements, which directly evidenced improved charge separation and migration as the crucial parameters governing photocatalysis.


N 2 physisorption results
The nitrogen sorption isotherms and the corresponding pore size distribution curves for pure TiO 2 and TiO 2 +rGO composites are presented in Fig. S1.According to IUPAC classification 1 , all composites correspond to type IV isotherms which are typical for mesoporous materials with pore sizes in the range from 2 to 50 nm.Typical for type IV isotherms is a hysteresis loop (type H3), which is associated with the occurrence of N 2 condensation in pores.The hysteresis loops approach P/P 0 =1 in a way, which suggests the presence of macropores in investigated samples (>50 nm).From the BJH pore size distribution plots presented in Figs.
S1b and S1d we can see that the pore size distribution is not effected with the increasing rGO loading regardless of the type of TiO 2 for synthesizing composites.

Quantification of rGO content in the TNR+rGO composites
The CHNS elemental analysis of pure GO has shown that the content of C is 42.4 wt.%, which implies that in the composites with 4, 10 and 20 wt.% of nominal GO loading, 1.7, 4.2 and 8.4 wt.% belongs to C. We performed TPO-MS analysis of a-TNR+9.2%rGO composite in order to investigate in detail combustion of the rGO phase and determine the temperature range where mass loss can be attributed to rGO oxidation.As one can see in Fig. S2 To analyze the actual composition of prepared TiO 2 +rGO composites, TGA measurements in air were conducted.Figs.S3a and S3b show TGA curves obtained for a-TNR and TNR composites containing different amounts of rGO; the TGA curve for pure GO is illustrated, too.It can be seen that in the case of pure GO major weight loss occurs at around 180 o C because of pyrolysis of oxygen containing groups; as a result, CO, CO 2 and H 2 O are produced 2 .GO was completely decomposed at temperatures above 680 o C. In the TGA curves belonging to TiO 2 +rGO composites we can observe that there is a weight loss at around 150-180 o C, and the second one in the range of 340-650 o C. No weight loss was observed above 650 o C. The residual mass remaining at 800 o C is associated with the TiO 2 phase in the composites; we can see that with the increasing rGO loading in the composites, the residual masses are decreasing due to lower content of TiO 2 .If we compare the weight losses of the composites between 340 and 800 o C, we can see that the mass losses in this temperature range increase with the increasing GO loading: in the case of TNR composites from 1.7, 5 and 8 % for composites with 4, 10 and 20 wt.% of nominal GO loading, and 2.6, 5.3 and 9.2 % for a-TNR composites with 4, 10 and 20 wt.% of nominal GO loading, respectively.These values are in rather good agreement with the TGA data reported above concerning the loss of mass in the temperature range of 340-800 o C.

Fig. S1 .
Fig. S1.N 2 adsorption-desorption isotherms and BJH pore size distributions of (a,b) a- , formation of CO as a result of the sample being exposed to diluted 5 % O 2 /He stream is negligible in the entire temperature range examined.Desorption of H 2 O takes place in two overlapping peaks, the first one centered at 70 and the second one at 220 o C, which decays slowly and reaches the baseline value at 630 o C. The low temperature peak likely originates from desorbed water, while the second one is attributed to thermal degradation and desorption of surface hydroxyl groups exhibiting a broad distribution of binding strengths.Desorption of CO 2 which indicates oxidation of rGO layers initiates at temperatures above 100 o C. The first small peak is centered at 200 o C. The CO 2 signal continues to rise at higher temperatures reaching the second, most intensive maximum at 590 o C. Afterwards, the CO 2 signal drops quickly to the baseline value at 630 o C. The O 2 signal is showing maxima in its consumption at the exact same positions at which peaks occur in the CO 2 signal.This indicates that CO 2 formation and O 2 consumption are directly linked and that CO 2 evolution is not a consequence of surface carbonate species decomposition, which would not require the presence of O 2 .The obtained results provide the basis for attributing mass loss at temperatures below 340 o C to catalyst dehydroxylation and water desorption, whereas mass loss above 340 o C primarily originates from gasification of reduced GO.The mass losses in the composites obtained in the temperature range from 340 to 800 o C provide an evidence that rGO was successfully reduced during the hydrothermal procedure.

Fig.
Fig. S4a is showing multilayered thin and dispersed sheets of graphene on amorphous carbon