# Conformal Titanyl Phosphate Surface Passivation for Enhancing Photocatalytic Activity
## 3 Results and Discussion
### Photocatalytic Performance  
The photocatalytic activity was investigated via the photodegradation experiment using rhodamine B aqueous solution. In Figure 3a, TP-TiO\({}_{2}\) shows outstanding photocatalytic activity comparing to bare TiO\({}_{2}\). In order to calculate their rate constants, natural logarithmic plots were prepared as shown in Figure 3b. By using the first order reaction rate equation (C/C\({}_{0}\)=exp(- _kt_)), the rate constants (_k_) are calculated [27]. TP-TiO\({}_{2}\) shows approximately 3-folds higher \(k\) value than bare TiO\({}_{2}\). Furthermore, Figure 3c shows the performances of photocatalytic hydrogen production. Since the surface phosphate groups can promote the formation of hydroxyl radicals in water, 1 M KHCO\({}_{3}\) aqueous solution was exploited as the electrolyte to stabilize the H\({}_{2}\)O\({}_{2}\) production and induce the 2-electron pathway for water splitting [23; 37; 38; 39; 40]. Significantly higher photocatalytic hydrogen production rate (86.7 umol h\({}^{-1}\)g\({}^{-1}\) for TP-TiO\({}_{2}\) and 46.1 umol h\({}^{-1}\)g\({}^{-1}\) for bare TiO\({}_{2}\)) was achieved. Moreover, TP-TiO\({}_{2}\) also shows much stable photocatalytic activity than bare TiO\({}_{2}\) due to the conformal passivation of titanyl phosphate. After continuous operation for 24 h, negligible changes in the production rate was observed in TP-TiO\({}_{2}\), whereas visible changes were shown by bare TiO\({}_{2}\). TEM image of TP-TiO\({}_{2}\) after 24 h of operation (Supplementary Figure S2) shows the long-term stability of titanyl phosphate layer. The surface modification using the phosphate species can generate the negative electrostatic field, which facilitates the efficient charge migration. The negative electrostatic field also can activate the formation of hydrogen bond between phosphate anion and water molecule. This promotes the adsorption of water molecules at the surface of photocatalysts and the generation of hydroxyl radical, resulting in high efficient photocatalytic properties [20; 22; 23]. Moreover, the charge recombination was suppressed since the charge trap states of TiO\({}_{2}\) were ameliorated by the surface titanyl phosphate passivation layer. It is noteworthy that no significant changes in the UV-Vis absorbance spectra were observed (Supplementary Figure S3). This implies that the surface passivation layer with the disordered structure does not influence the light harvesting property of photocatalyst [41]. To investigate the efficient charge migration properties, the photoelectrochemical (PEC) response was characterized using the three electrodes system, of which working electrode was comprised of the TiO\({}_{2}\) photocatalysts (Supplementary Figure S4). 1 M KHCO\({}_{3}\) aqueous solution (pH 7.4) was also used as the electrolyte. The significantly enhanced photocurrent density value of TP-TiO\({}_{2}\) was resulted from not only the modified surface catalytic kinetics, but also the suppressed charge recombination and promoted charge migration [41]. Additionally, the external quantum  
Figure 2: (a) Ti 2p; (b) O 1s; and (c) P 2p XPS spectra of bare and TP-TiO\({}_{2}\).  
efficiency (EQE) spectra were investigated to demonstrate the enhanced performance of TP-TiO\({}_{2}\) in Supplementary Figure S4b.