Direct Observation of Photoinduced Charge Separation in Ruthenium Complex/Ni(OH)2 Nanoparticle Hybrid

Ni(OH)2 have emerged as important functional materials for solar fuel conversion because of their potential as cost-effective bifunctional catalysts for both hydrogen and oxygen evolution reactions. However, their roles as photocatalysts in the photoinduced charge separation (CS) reactions remain unexplored. In this paper, we investigate the CS dynamics of a newly designed hybrid catalyst by integrating a Ru complex with Ni(OH)2 nanoparticles (NPs). Using time resolved X-ray absorption spectroscopy (XTA), we directly observed the formation of the reduced Ni metal site (~60 ps), unambiguously demonstrating CS process in the hybrid through ultrafast electron transfer from Ru complex to Ni(OH)2 NPs. Compared to the ultrafast CS process, the charge recombination in the hybrid is ultraslow (≫50 ns). These results not only suggest the possibility of developing Ni(OH)2 as solar fuel catalysts, but also represent the first time direct observation of efficient CS in a hybrid catalyst using XTA.


The synthesis of Al2O3 NPs.
A solvothermal method was utilized from a combination of literature procedures. [6][7][8][9] In a typical synthesis, 3.75 g of Al(NO3)3·9H2O and 0.6 g of NaOH were dissolved in 70mL of 50% ethanol separately. The combined mixture was stirred for 4 hours at room temperature followed by the addition of 5 mL of NH4OH. The resulting cloudy white hydroxide precursor precipitant was collected via centrifugation and rinsed with water. The obtained gel was then autoclaved in a Parr Teflon-lined autoclave at 200°C for 16 hours.

5.
Femtosecond absorption spectroscopy is available in our research lab. The spectrometer is based on a regenerative amplified Ti-Sapphire laser system (Solstice, 800nm, < 100 fs FWHM, 3.5 mJ/pulse, 1 KHz repetition rate). The tunable pump is generated in TOPAS which has output with tunable wavelength ranging from 254 nm to 960 nm. The tunable UV-visible probe pulses are generated by while light generation in a CaF2 window (330-720 nm) on a translation stage. The femtosecond transient absorption is performed in Helios ultrafast spectrometer (Ultrafast Systems LLC). The energy of the 600 nm pump pulse used for the measurements was 454 nJ. The sample cuvette path length was 2 mm.
6. X-ray Photoelectron Spectroscopy was performed with the use of XPS/UPS instrument at Material Science Division of Argonne National Laboratory. This home-build instrument is S3 equipped with Perkin-Elmer X-ray dual anode 04-500 gun (MgK line, h=1253.6 eV, 300W output power), hemispherical energy analyzer (VSW HA100) and 16-anode detector. The total energy resolution of the instrument in given operating mode was =0.93 eV. Energy calibration of the spectrometer is performed in routing manner with the use of freshly deposited Au standard.
The sample (NPs solution in toluene) was deposited onto Si wafer substrate (0.4 mm thickness), dried off and place into vacuum chamber with ultimate residual gas pressure about 6x10 -8 Pa.
Obtained XPS data were processed with using Casa XPS software. The pump wavelength at 527 nm was obtained from the output of second harmonic generation.
The X-ray pulse with 80 ps FWHM width at 6.5 MHz repetition rate was used as the probe. The laser pump and X-ray probe intersect at a flowing sample stream with 550 m in diameter. The Xray fluorescence signals were collected at 90˚ angle on both sides of the incident X-ray beam by two avalanche photodiodes (APDs). A soller slits/Co filter combination, which was customdesigned for the specific sample chamber configuration and the distance between the sample and the detector, was inserted between the sample stream and the APD detectors. The emitted Ni Xray fluorescence collected at 5 ns after the laser pump pulse excitation of RuN3/Ni(OH)2 hybrid was used to build the spectrum of reduced state of Ni(OH)2 NPs in advanced photon source standard operating mode. The Ni fluorescence resulting from averaging the previous 50 round trips in the storage ring prior to the laser pulse were used to construct the ground state spectrum of Ni(OH)2 NPs.

Size and Distribution by Small Angle X-ray Scattering.
To determine the size and distribution of Ni(OH)2 NPs, we performed solution small angle x-ray scattering (SAXS) at beamline 12ID-B of the Advanced Photon Source at Argonne National Laboratory. The SAXS data are displayed in Figure S1 (red curve). The data fitting was performed over the q range of 0.014-0.4 Å -1 , where q is scattering momentum transfer, = 4 .  is the Bragg angle and  is the wavelength the incident x-ray, which is set as 0.886 Å for the measurement.
The particles were approximated with spherical shape with homogeneous electron density () and a form factor of a spherical object, P (q,R), was used in the fitting, ( , ) = , where R is the radius of the sphere. 10 SAXS measures millions of nanoparticles in the x-ray beam path. The nanoparticles were assumed to follow log normal distribution with respect to particle radii. The log normal distribution function is expressed as: where R0 is the maximum population position and  describes the distribution width. The total xray scattering from the particles in the x-ray beam can be written as: ( ) = ∫ ( , ) ( , 0 , ) . (Eq. 1) SAXS data fitting using Eq. 1 was performed in Matlab by minimizing the following penalty function,  2 : where Iexp(qj) and Icalc(qj) are experimental data and scattering intensity calculated from Eq1, respectively, α is a scaling factor, and s(qj) is the value of experimental error at qj. A good fit was achieved in the whole range ( Figure 1a) and a relatively narrow distribution was yielded from the S5 fitting ( Figure S1b). The overall radius obtained from the fitting is 22.1 ± 4.0 Å, which is consistent with our TEM results ( Figure 2).