Bipolarized intrinsic faradaic layer on a semiconductor surface under illumination

Abstract Interface charge transfer plays a key role in the performance of semiconductors for different kinds of solar energy utilization, such as photocatalysis, photoelectrocatalysis, photochromism and photo-induced superhydrophilicity. In previous studies, different mechanisms have been used to understand interface charge transfer processes. However, the charge transfer mechanism at the solid/liquid interface remains a controversial topic. Here, taking TiO2 as a model, we find and prove, via experiments, the new characteristic of photo-induced bipolarity of the surface layer (reduction faradaic layer and oxidation faradaic layer) on a semiconductor for the first time. Different from energy level positions in the classic surface states transfer mechanism, the potential window of a surface faradaic layer is located out of the forbidden band. Moreover, we find that the reduction faradaic layer and oxidation faradaic layer serve as electron and hole transfer mediators in photocatalysis, while the bipolarity or mono-polarity of the surface layer on a semiconductor depends on the applied potential in photoelectrocatalysis. The new characteristic of bipolarity can also offer new insights into the charge transfer process at the semiconductor/liquid interface for solar energy utilization.


Pretreatment of single crystal TiO2
A commercial single crystal rutile (110) TiO2 (Hefei, Kejing) was calcined at 700℃ for 200 min in the mixture carrier gas with 10% H2 and 90% Ar to improve its conductivity for photoelectrochemical measurement [3]. The back side was connected to a copper rod with molten indium to form ohmic contact. Then the back side of single crystal TiO2 electrode was sealed with insulting silica glue.

Characterization of samples
The crystal structure of the samples was characterized by X-ray diffraction (XRD, smartlab, 9 kW). The morphology of the samples was investigated by scanning electron microscope (SEM, Gemini 500) with an accelerating voltage of 10 kV and transmission electron microscope (TEM, Tecnai F20). The X-ray photoelectron spectroscopy (XPS, Thermo Scientific XPS K-alpha) were performed with an Al Kα X-ray source. The binding energy of the C1s peak at 284.6 eV was used to calibrate the XPS data. Ions depth profiles in the samples were obtained by time-of-flight secondary-ion mass spectroscopy (TOF-SIMS, Ion tof Gmhb 5) with a detection mode of negative ions. A beam of 30 keV Bi + was used as primary ions, with an analyzing area of 91 * 91 μm 2 .
The Cs + ions with 1 keV were used to sputter the samples was performed in an area of 250 * 250 μm 2 .
In situ X-ray photoelectron spectroscopy (XPS, Thermofisher Escalab 250Xi) was performed in the dark and under a Xe lamp illumination for 30 min. To prepare the TiO2 sample with Mn 2+ and Ag + as electron and hole imaging agents, the solution of 5 mM CH3COOAg and 10 mM (CH3COO)2Mn was dropped onto the surface of TiO2 and dried in the dark. In situ electron paramagnetic resonance (EPR, Bruker E500) was performed in the dark and under a Xe lamp illumination for 30 min in N2.

Isotope labeling experiments
The isotope labeling experiments on TiO2 were performed in the mixture solution of D2O and H2 18 O (volume ratio of D2O/ H2 18 O, 1:1) in the dark and under a Xe lamp illumination for 2 h. In order to make D diffuse more deeply into TiO2, methanol was added into the mixture solution (volume ratio of methanol/mixed water, 1:5) as a hole scavenger. Moreover, an electrochemically reduced TiO2 was carried out at -0.4 VRHE for 1 h in 1 M phosphate buffer aqueous solution with D2O as solvent.

Photo-deposition of MnOx and Ag
A TiO2 thin film was immersed into the aqueous solution with 5 mM CH3COOAg or 10 mM (CH3COO)2Mn or both, respectively, and then they were irradiated by a Xe lamp for 5 min, 10 min and 30 s, respectively. Moreover, the co-deposition on single crystal TiO2 was carried out in the aqueous solution with 5 mM CH3COOAg and 10 mM (CH3COO)2Mn under illumination for 2 min.

(Photo-)electrochemical measurements
The (photo-)electrochemical measurement of films was investigated in a three-           Similar bipolarized intrinsic faradaic layer is also observed on hematite Fe2O3 surface ( Figure S14). New peaks of Fe 2+ reduced from Fe 3+ and decrease of H2O content are appeared under illumination compared with in the dark [4] ( Figure S13). Therefore, Fe 2+ is the RFL of Fe2O3. Since detection of Fe 4+ on Fe2O3 is already shown by FT-IR under photoelectrochemical measurement in previous study, we suggest that Fe 4+ as the oxidation product of Fe2O3 [5]. The reactions of reduction and oxidation faradaic layer and the corresponding potential window for Fe2O3 are shown in Table S1.   is about 2.0 V in the dark. In a Faradaic junction, the photovoltage means the difference between the quasi-fermi levels of electrons and holes, which can be obtained by the difference between a photo-onset potential of TiO2 and a dark onset potential of Ti foil [6]. Therefore, the photovoltage in TiO2 is about 1.7 V.