Reverse Semi‐Combustion Driven by Titanium Dioxide‐Ionic Liquid Hybrid Photocatalyst

Abstract Unprecedented metal‐free photocatalytic CO2 conversion to CO (up to 228±48 μmol g−1 h−1) was displayed by TiO2@IL hybrid photocatalysts prepared by simple impregnation of commercially available P25‐titanium dioxide with imidazolium‐based ionic liquids (ILs). The high activity of TiO2@IL hybrid photocatalysts was mainly associated to (i) TiO2@IL red shift compared to the pure TiO2 absorption, and thus a modification of the TiO2 surface electronic structure; (ii) TiO2 with IL bearing imidazolate anions lowered the CO2 activation energy barrier. The reaction mechanism was postulated to occur via CO2 photoreduction to formate species by the imidazole/imidazole radical redox pair, yielding CO and water.


Preparation of TiO2@IL catalysts
The TiO2@ [BMIm][Im] (around of 3 wt%) and [BMIm]Cl (around of 9 wt%), were prepared according to  The NMR analyses were performed on a Bruker Avance 400 spectrometer, equipped with a BBO 5 mm probe with z-gradient operating at 400 MHz for 1 H, and 100 MHz for 13 C. The spectra were obtained at 298 K unless otherwise specified. Chemical shifts are reported in parts per million (ppm, δ) referenced to D2O and DMSO-d6 as an external reference (capillary).

General Characterisation
Infra-Red (IR) analyses were conducted using a Bruker Alpha (Fourier transform infrared) FTIR spectrometer with an ATR attachment. Data collection utilised 256 cumulative scans with a resolution of 4 cm -1 . CHN Elemental analysis of the ILs immobilized on the support surfaces was carried out on a CHN Exeter Analytical CE-440 using helium (99.997% purity) as a carrier and oxygen for the combustion (99.995% purity). Thermogravimetric (TGA) analyses were performed in a TA Instruments T5500 Thermogravimetric analyser in a stepwise programmed up to 1000 ˚C (10 ˚C/min) using a nitrogen (99.995% purity) flow of 25 mL min -1 . Samples were held in Pt (hightemperature resistant) pans. Data analysis was processed using TRIOS Discovery program. N2 isotherms of the catalysts, previously degassed at 180 °C under vacuum for 18 h, were obtained using TriStar and 3Flex Micromeritics instruments. Specific surface areas were determined by the BET multipoint method, and average pore size was obtained by the BJH method.

UV-Vis Diffuse Reflectance Spectroscopy
UV-Vis diffuse reflectance was performed by using a CARY 5000 spectrophotometer. TiO2 shows an expected band gap of 3.3 eV [2] while the hybrid TiO2@IL displayed a red shift of 0.2 eV independently of the IL.

Raman Spectroscopy
Raman spectroscopy was performed by using a Horiba-Jobin-Yvon LabRAM HR spectrometer, with a laser wavelength of 785 nm operating at a power of ca. 4 mW and a 600 lines mm -1 grating. Spectra were collected by averaging 2 acquisitions of 15 s duration. The presence of Anatase phase was observed for all samples and no significantly shift/change was observed in the peak position and FWHM (143.3 ± 0.1 cm -1 and 8.8 ± 0.1 cm -1 , respectively). [3] The higher intensity displayed for the samples with IL may suggest a decrease of the scattering due to the hybrid morphology (Table S1) which might lead an enhancement of the Raman signal. Indeed, deeper investigations are necessary to fully understand the cause of the Raman signal enhancement to hybrid TiO2@IL, which will be done in the future since this is not the aim of this work.

XPS Spectroscopy
XPS probes the upper surface of a material to depths of about 10 nm with XPS signal dominated by the surface of few nm. This allows the study of interface between TiO2 and IL hybrid, and thus TiO2@IL VB. Moreover, the area of analysed was ca. 0.5 mm 2 therefore probing TiO2@IL average surface.
Powder sample were mounted on double sided tape (Sellotape) and pressed to ensure a good coverage of the tape with the powder. X-ray photoelectron spectroscopy (XPS) measurements were performed using a Kratos AXIS Ultra DLD instrument. The chamber pressure during the measurements was 5 × 10 −9 Torr. Wide energy range survey scans were collected at pass energy of 80 eV in hybrid slot lens mode and a step size of 0.5 eV.
Wide scan and high-resolution data on the C 1s, O 1s, N 1s, F 1s, S 2p, P 2p Ti 2p and VB photoelectron peaks was collected at pass energy 20 eV over energy ranges suitable for each peak, and collection times of 5 min, step sizes of 0.1 eV. The charge neutraliser filament was used to prevent the sample charging over the irradiated area.
The X-ray source was a monochromated Al Kα emission, run at 10 mA and 12 kV (120 W). The energy range for each 'pass energy' (resolution) was calibrated using the Kratos Cu 2p3/2, Ag 3d5/2 and Au 4f7/2 three-point calibration method. The transmission function was calibrated using a clean gold sample method for all lens modes and the Kratos transmission generator software within Vision II. The data were processed with CASAXPS (Version 2.3.17).
In XPS measurements an incorrect charge reference may induce misinterpretation of data, thus leading an equivocally explanation of the surface electronic structure of the materials. C 1s is the most used for charged reference XPS spectrum, however, for materials that contain a great amount of carbon in their composition (such as ionic liquids) the use of C 1s as reference is very uncertain. O 1s also can be used as charge reference, but can be very challenge to separate the contribution of the different oxidation states. Nonetheless, the C 1s and O 1s peaks are likely to contaminations such as adventitious carbon, water absorbed, etc. In this work, we have charged reference the XPS spectra using the Ti 2p at 458.9 eV [4] (Figure 7), therefore, all the shifts observed in the VBM are relative to Ti 2p. Ti 2p displayed a high number of counts/resolution and very narrow peak (around 1.2 eV for all the samples). Moreover, a non-significantly shift of Ti 2p signal for TiO2-modified surface electronic structure was reported [5] . Figure S7. XPS wide scan and C 1s spectra of (a, b) To ensure the appearance of new surfaces states found are ascribed only by the interaction between TiO2 and IL, TiO2@[BMIm][Im] was annealed at 650 °C for 2 h under air to remove the IL contribution from the TiO2 surface. In addition, bare TiO2 was also annealed at the same conditions, since at 650 °C crystalline structure rearrangement might induce changes in the VB. As can be observed in the Figure S10 the VBM of the samples annealed are located at same position (ca. 7.5 eV), which shows that the absence of IL on TiO2 surface reverses it to its bare counterpart confirming our hypothesis.

Electrochemistry measurements
All the electrochemistry measurements were performed using TiO2 and TiO2@[BMIm][Im] films deposited on fluorine doped tin oxide (FTO) substrate, with each sample being prepared and measured three times to ensure the reproducibility of the measurements. Electrochemistry experiments were performed using an Autolab (PGSTAT 100N) potentiostat. The measurements were carried out in a quartz cell using a standard three-electrode configuration cell, with a platinum wire as the counter electrode, Ag/AgCl as the reference electrode and the photocatalyst as the working electrode; and 1 M NaOH media (pH 13.8). Mott Schottky measurements were performed in dark, in the frequency range of 100 kHz to 0.1 Hz at an amplitude of 10 mV.

Photocatalytic CO2 Reduction
Typically, a Schlenk tube containing 40 mL of degassed distilled water was saturated with 50 bar of CO2 gas.      [30] * Not found  [49] * Not found