Atomic Scale Analysis of the Enhanced Electro- and Photo-Catalytic Activity in High-Index Faceted Porous NiO Nanowires

Catalysts play a significant role in clean renewable hydrogen fuel generation through water splitting reaction as the surface of most semiconductors proper for water splitting has poor performance for hydrogen gas evolution. The catalytic performance strongly depends on the atomic arrangement at the surface, which necessitates the correlation of the surface structure to the catalytic activity in well-controlled catalyst surfaces. Herein, we report a novel catalytic performance of simple-synthesized porous NiO nanowires (NWs) as catalyst/co-catalyst for the hydrogen evolution reaction (HER). The correlation of catalytic activity and atomic/surface structure is investigated by detailed high resolution transmission electron microscopy (HRTEM) exhibiting a strong dependence of NiO NW photo- and electrocatalytic HER performance on the density of exposed high-index-facet (HIF) atoms, which corroborates with theoretical calculations. Significantly, the optimized porous NiO NWs offer long-term electrocatalytic stability of over one day and 45 times higher photocatalytic hydrogen production compared to commercial NiO nanoparticles. Our results open new perspectives in the search for the development of structurally stable and chemically active semiconductor-based catalysts for cost-effective and efficient hydrogen fuel production at large scale.

process, NiO nanowires were obtained as shown in the Figure S2b. The structure of as-synthesized NiO nanowires is identified to be of the cubic symmetry with the space group of Fm3 m (225) by indexed X-ray diffraction patterns ( Figure S2c).

Different orientation study in NiO
NiO crystal structure belongs to cubic system. The angle between crystal planes (h 1 ,k 1 ,l 1 ) and (h 2 ,k 2 ,l 2 ) in cubic crystal can be calculated using: We calculated an angle of 19 o between the (110) and (120) (110) and (130) crystal planes. The (120) plane is the nearest neighboring plane to the (110) plane , and the plane of (130) is the second nearest neighboring plane.

Facet notation for high Miller index surfaces
To determine the Miller index of a surface, Somorjai et al has demonstrated a notation method for surface index of cubic material. S1 In fcc lattice, a high Miller index (hkl) of a surface with (h t k t l t ) terraces, (h s k s l s ) steps, and a step-to-terrace atom ratio of : can be written as: : : Where a t , a s are the vector decomposition coefficients of the vector (h t k t l t ) and (h s k s l s ), respectively. h t , k t , l t , must be an irreducible set of integers and so is h s , k s , l s . P t =4 when h t , k t , l t are all odd; P t =2 when h t , k t , l t are not all odd. P s =4 when h s , k s , l s are all odd; P s =2 when h s , k s , l s are not all odd. This method helps us to rapidly find the Miller index of a curved surface in an fcc lattice.

Computational methods
All the calculations were performed by using Vienna ab initio Simulation package (VASP). [S2] The frozen-core all-electron projector augmented wave (PAW) model with Perdew-Burke-Ernzerhof (PBE) function was employed to describe the interactions between core and electrons. An energy cutoff of 300 eV was used for the plane-wave expansion of the electronic wave function. The force and energy convergence criterion was set to 0.01 eV/Å and 10 -5 eV, respectively. Only Gamma point was performed for the first Brillouin zone. Atomic models were built by 5 to 9 NiO layers with the thickness of about 5 Angstrom for all of the facets, whose surfaces are exposed to the vacuum in the unit cell for VASP computations ( Figure S6).

Electrochemical characterization
All electrochemical experiments were performed in a three-electrode system at room temperature with an electrochemical analyzer (660D CH Instrument, purchased from Shanghai Chenhua Instrument Co., Ltd. for convenience. The naked GCE electrode was used as the control test. All the cyclic voltammograms were collected at 50 mV/s in a 1.0 M potassium phosphate buffer solution (Pi, pH = 7.0). The CV scans were recorded in a range of -2.0V~0 V versus Ag/AgCl electrode (3 M KCl). There was an iR drop for compensation and no stirring was used for the CV tests. All the potential in this work was referenced versus reversible hydrogen electrode (RHE).
The H 2 was detected by using gas chromatography (SP-6890, nitrogen as a carrier gas) equipped with thermal conductivity detector (TCD). The experiment was carried out in a gas-tight electrochemical cell and the solution was degassed by bubbling with high purity N 2 for 1 h with vigorous stirring. After that, 5 mL of nitrogen was The catalyst stability of the electrochemical activity for the H 2 evolution was measured by chronopotentiometry. The current density was fixed at 10 mA/cm 2 .
There was no significant change of the overpotential during the catalysis after 24 hours.

Photocatalytic hydrogen production
Different ratios of NiO nanowires (0.5% ~ 3.0%) were added to a TiO 2 suspension in deionized water, and the final mixtures were sonicated for 30 min. Photocatalytic activities for hydrogen production were carried out in a 50 mL round-bottom flask containing 20 mL of deionized water, 10 mg TiO 2 -NiO, 1 mL TEOA, 0.5 mM Eosin-Y dye. Sacrificial electron donor TEOA and photosensitizers Eosin-Y dye play important roles for the efficient photocatalytic reactions. In the presence of TEOA, it can promote the transition of fluorescent dye from singlet to triplet state, then the life of triplet state becomes longer than that of the singlet. So it is more favorable to generate the excited states of EY 2− (Eosin-Y), which enhances the utilization efficiency of excited state EY 2− species. [S3-S5] The container was capped with a rubber The dye's influence on the catalytic properties of NiO can be attributed to the heavy-atom effect of Br substituents, which can promote the efficient formation of long-lived triplet states for EY 2− from its photoexcited singlet state. [S4] Too much dye could block the active sites in NiO to contact the reactants.

Powder X-ray diffractometer (XRD)
The crystal phase and phase composition of the NiO samples were determined by powder X-ray diffraction (XRD, D/max-TTR III) using graphite monochromatized Cu Ka radiation of 1.54178 Å, operating at 40 kV and 200 mA. The scanning rate was 10˚ min -1 from 20˚ to 70˚ in 2θ.  Step I stabilization stage, step II pre-oxidation stage, step III calcination process. (b-d) XPS spectra collected during the thermal decomposition of the Ni(CH 3 COO) 2 /PVP composite nanowires. The peak at 284.9 eV observed in the sample at 500℃ was due to the contamination of the adventitious carbon during the XPS measurements. Figure S3. FTIR spectra of (a) As electrospun Ni(CH 3 COO) 2 /PVP nanowires (b) Stabilized nanowires (c) Pre-oxidized nanowires (d) Calcinated NiO nanowires  Figure S4. The linear density of exposed HIF atoms in NiO nanoparticles with a zone axis along [110] direction.   Figure S8. The evolution of hydrogen production with different composition of reactants in photocatalytic HER. In the presence of as-synthesized NiO as co-catalyst in HERs, the hydrogen production exhibits a substantial increase. Figure S9. The TEM images of (a) and (b) TiO2 nanoparticles. The TEM images of (c) and (d) TiO2/NiO photcatalysts.
The specific surface area of NiO samples was measured by Quantachrome Instrument Autosorb-iQ. The NiO nanowires were loaded into the chamber, then pretreated at 300℃ for 12h under vacuum atmosphere. After that, the Brunauer-Emmett-Teller (BET) measurements were carried out at 77K under N 2 atmosphere. Finally, the measured specific surface area were obtained as listed in the Table S2.