Pd
3P was prepared by a one-step annealing reaction of Pd(Ac)
2 and NaH
2PO
2 with a mass ratio of 1:5 under Ar atmosphere. As revealed by X-ray diffraction (XRD) pattern in Fig. S2 (Supporting information), the sample synthesized at 400 ℃ has well-defined peaks of orthorhombic Pd
3P (JCPDS No. 089-3046) [
20-
22] without other impurities. Further, X-ray photoelectron spectroscopy (XPS) was performed to analyze the valence state and electronic properties of Pd
3P NPs. As shown in Fig. S3c (Supporting information), two discernible peaks of P 2p region located at 129.8 eV and 130.6 eV can be assigned to P 2p
3/2 and 2p
1/2 of Pd
3P, respectively. The peak at 134.6 eV can be attributed to P—O bond, which originates from surface oxidation of Pd
3P [
23]. Compared with the P
0 peaks at 130.1 and 131.4 eV [
20-
24], P element in Pd
3P owns a lower binding energy, indicating its slightly negative charge in Pd
3P NPs. In addition, the peaks of Pd 3d region located at 335.9 eV and 341.2 eV belong to Pd 3d
5/2 and Pd 3d
3/2, respectively. Compared with binding energies (335.2 eV and 340.5 eV) of metal Pd [
12,
25], the higher binding energies of Pd species in Pd
3P NPs indicate that Pd element is in the form of slightly positive valence state. Finally, the effect of calcination temperature on the structure and composition of as-prepared samples was explored and corresponding XRD patterns of samples synthesized at 300 ℃, 400 ℃ and 550 ℃ are shown in Fig. S2. We failed to obtain pure Pd
3P above or below 400 ℃. For example, incomplete phosphidation exists in the sample synthesized at 300 ℃, in which two peaks at 39.4° and 45.8° ascribed to Pd NPs are observed [
26]. However, PdO and other impurities such as PdP
2 and PdO are detected in the sample synthesized at 550 ℃.