Phase-segregated NiPx@FePyOz core@shell nanoparticles: ready-to-use nanocatalysts for electro- and photo-catalytic water oxidation through in situ activation by structural transformation and spontaneous ligand removal† †Electronic supplementary information (ESI) available: Experimental details, additional characterization and results. See DOI: 10.1039/c8sc00420j

The phase-segregated NiPx@FePyOz core@shell NPs act as a colloidally stable, ready-to-use, and excellent OER active transition metal phosphide-based catalyst.

Bi(NO3)3·5H2O (99.9%) was purchased from Wako Pure Chemical Industries, Ltd. Di-n-octyl ether (>95%) was purchased from Tokyo Chemical Industry Co., Ltd. All reagents were used without further purification. (Hitachi). X-ray diffraction (XRD) patterns were measured on PANalytical X'Pert Pro MPD with CuKα radiation (λ = 1.542 Å) operated at 45 kV and 40 mA. X-ray photoelectron spectroscopy (XPS) measurements were conducted in an ultrahigh vacuum combined system equipped with a hemispherical electron analyzer and a Mg Kα X-ray source (1253.6 eV). The detail has been described in a previous paper [1] . Transmittance measurements were acquired on a U-3310 spectrophotometer (Hitachi). X-ray fluorescence (XRF) elemental analysis was performed on an Element Analyzer JSX-3202C (JEOL). Fourier Transform infrared (FT-IR) analyses were performed on a IR Prestige 21 (Shimadzu).

Synthesis of a-NiPx NPs
Ni(acac)2 (1 mmol), 1-octadecene (4.5 mL), and oleylamine (6.4 mL) were mixed and heated to 80 °C under a N2 atmosphere to form a blue-green transparent solution. TOP (2 mL) was injected and the mixture was heated to 230 °C at 10 °C min -1 , and maintained at that temperature 3 for 30 min. The reaction solution was then cooled to room temperature and purified with ethanol.
The precipitated solid NPs were redispersed in hexane and stored for further use.

Synthesis of NiPx@FePyOz NPs
A 34-mg portion of a-NiPx NPs was dispersed in a mixture of 1-octadecene (9 mL) and oleylamine (1.9 mL). After repeated degassing-refilling with N2, TOP (2 mL) solution containing Fe(CO)5 (0.4 mmol) was injected into the mixture at room temperature. The mixed solution was heated to 270 °C for 60 min. After the reaction, the solution was cooled to room temperature and purified with ethanol. The precipitated solid NPs were redispersed in hexane and stored for further use.

Synthesis of Ni2P NPs
a-NiPx NPs (85 mg) was dissolved in a mixture of di-n-octyl ether (9 mL) and oleylamine (1.9 mL). The mixture was heated to 270 °C at 10 °C min -1 under a N2 atmosphere. The solution was reacted at 270 °C for 60 min, cooled to room temperature, and purified with ethanol. The precipitated solid NPs were redispersed in hexane and stored for further use.

Synthesis of FeOx NPs
The 5-nm FeOx NPs were synthesized following a literature procedure [2] . Briefly, a mixture of Fe(CO)5 and oleylamine (1:1 mol/mol) was heated to 80 °C at 2 °C min -1 and held at that temperature for 30 min under a N2 atmosphere. A deep red solution of Fe(CO)5-oleylamine complex was injected into 1-octadecene at 180 °C in a N2 atmosphere. The solution was reacted at 180 °C for 30 min, cooled to room temperature, and then purified with ethanol. The precipitated solid NPs were redispersed in hexane and stored for further use.

Preparation of NP-loaded carbon powder catalysts
The electrocatalyst powder was prepared by mixing NPs and conductive carbon powder (XC-72, Cabot). Typically, XC-72 (8 mg) was dispersed in hexane (8 mL) by sonication, followed by the dropwise addition of hexane (2 mL) containing NPs (2 mg). Further sonication was 4 applied for 30 min. The catalyst powder was collected by centrifugation, washed with acetone twice, and dried under vacuum.

Preparation of carbon powder catalyst working electrodes
Catalyst powder (1 mg) was mixed with water (396 µL), 2-propanol (94 µL), and Nafion resin solution (10 µL), then sonicated for 30 min to form a homogenous catalyst slurry. The catalyst slurry was deposited on a glassy carbon rotating disk electrode 5 mm in diameter and dried by rotating at 700 rpm under ambient conditions.

Immobilization of NPs on FTO-coated glass electrodes
A 1.5 × 5 cm 2 piece of FTO-coated glass (AGC Fabritech) was washed with ethanol and acetone.
The hexane solution of NiPx@FePyOz NPs (3 mg mL -1 , 50 µL) was dropped onto the FTO coated glass and spun at 1000 rpm for 10 s (the coated area was 2.25 cm 2 ).

Acid etching of NiPx@FePyOz NPs
The NiPx@FePyOz NPs-loaded carbon powder was mixed with 0.5 M H2SO4 aqueous solution (2 mL) and sonicated for 1 h. Then the powder was collected by centrifugation and washed with water and acetone. Finally, the powder was dried under vacuum for TEM and XRF measurements.

Preparation of BiVO4 electrodes
We prepared porous BiVO4 electrodes according to the method reported by Choi et al. [3] .
Briefly, a BiOI film was deposited on an FTO coated glass substrate by immersing the substrate in a water/ethanol solution of Bi(NO3)2, KI, and p-benzoquinone, followed by the application of -0.1 V vs Ag/AgCl (3 M KCl) for 2 min (the deposited area was 1~1.5 cm 2 ). A DMSO solution of VO(acac)2 was placed on the BiOI film, followed by annealing at 450 °C for 2 h in 5 air. The annealed electrode was immersed in 1 M NaOH for 30 min to remove V2O5 by-products, washed with deionized water, and dried under ambient conditions.

NP deposition on BiVO4 electrodes
Before loading the NPs on the

Electrochemical measurements
The electrochemical measurements were performed in a three-electrode electrochemical cell with a ALS620C electrochemical analyzer (BAS) at room temperature (~25 °C). Hg/HgO (1 M NaOH) or Ag/AgCl (3 M NaCl) was used as a reference electrode in 0.1 M KOH or 0.125 M K2B4O7 electrolyte, respectively. The reference electrodes were calibrated with an unused saturated calomel electrode in saturated NaCl aqueous solution. A Pt coil was used as a counter electrode. Prior to each measurement, the electrolyte was deaerated by bubbling with Ar for 20 min to remove oxygen. Cyclic voltammograms and chronoamperometry were conducted under a continuous Ar flow. In the case of a rotating disk electrode, the working electrode was rotated at 1600 rpm during measurements with a RRDE-3A (BAS). In the case of carbon paper and FTO coated glass electrodes, the working electrode was immersed and the electrolyte solution was stirred during the measurements. All displayed voltammograms except for Figure S9 and S11b were iR-collected to account for any uncompensated resistance. The uncompensated resistance was measured with a function of the analyzer based on a previously reported 6 technique [4] . The conversion between potentials vs. reference electrode (Eref) and vs. reversible hydrogen electrode (RHE) (ERHE) was performed with the equation below:

PEC measurements
The PEC performance of the photoanodes was evaluated in a three-electrode configuration with a ALS620C electrochemical analyzer (BAS) and a light source (300 W Xe lamp, MAX-303, Asahi Spectra). Illumination was obtained by passing light from the source through a >385 nm short wavelength cut-off filter, and irradiated through the FTO side (back-side illumination).
The power density of the incident light was ~190 mW cm -2 at the BiVO4/FTO electrode measured by a thermal sensor (PS10, Coherent). The simulated sunlight was obtained using 300 W Xe lamp equipped with AM filter. Power density was adjusted to 100 mW cm -2 using

Effect of Ar bombardments
(a) Ni 2p: Before OER; after Ar bombardment, strong peaks of Ni 0 , which corresponded to Ni in the NiPx core appeared. After OER; the Ar bombarded sample also showed Ni 0 peaks, which were attributed to reduced Ni cations in NiOOH formed during the Ar bombardment.
(b) P 2p: Before OER; after Ar bombardment, P 0 peak at 130 eV became strong owing to the exposure of P 0 in NiPx cores. After OER; no peaks appeared even after Ar bombardment, indicating P was not only absent from the surface but was also eliminated from the bulk during OER.
(c) Fe 2p: Before OER; after Ar bombardment, a weak Fe 0 peak emerged at 707.5 eV, which was attributed to Fe(0) in NiPx cores. After OER; no peaks of Fe 0 appeared because most of the NiPx cores were oxidized during OER.
(d) O 1s: Before OER; after Ar bombardment, O1s peak slightly shifted. One explanation for this shift is that the outer and inner parts of the shell were mainly oxide (531 eV) and phosphate (532 eV), respectively. After OER, after Ar bombardment, a shoulder peak at a lower binding energy (530 eV) assigned to metal oxide appeared, indicating that the inner part of the sample after OER contained metal oxide and (Ni, Fe)OxH.     a PED = Photo-assisted electrodeposition b CBD = Chemical bath deposition Gray rows: BiVO4 is fully covered with robust cocatalyst layer White rows: BiVO4 is partially covered with nanosized particle or monolayer of molecule