The high H2 sensing performance of Pt NP assembly ultimately originates from its ability to efficiently catalyze H2 oxidation even at room temperature. To understand how Pt NPs acquire such high activity after simple assembly, we compared the structures of Pt NPs and their assembly. X-ray absorption spectroscopy (XAS) results show different Debye-Waller factors (Pt NPs: 8.2 ±0.1 and Pt NP assembly:10±0.1, unit: 10 -3 Å 2 ), suggesting that Pt NP assembly has higher local structural disorder than Pt NPs, but similar fitted Pt-Pt bond lengths (Pt NPs: 0.277 ± 0.002 nm; Pt NP assembly: 0.278 ± 0.002 nm) show no global lattice expansion or shrinkage (fig. ). High-resolution x-ray diffraction (XRD) spectra of Pt NPs and Pt NP assembly in Fig. present similar structural information as the XAS result. The XRD peaks of Pt NP assembly show broadening but no peak position change, relative to Pt NPs (Fig. , fig. ). Microstrain, crystallite size, and instrumental errors together determine XRD peak broadening but do not change the peak position. The result in Fig. suggests only increased microstrain is present in the Pt NP assembly, given the same instrumental errors and similar crystallite sizes of Pt NPs and Pt NP assembly (Fig. ). More interestingly, the XRD peak broadening for Pt NP assembly is anisotropic and asymmetric. Specifically, the peak broadening is more significant for the (111) peak (~17%) than others (e.g., 10% for (200) and 4% for (311) in Fig. ). The (111) XRD peak of Pt NP assembly was broadened by ~18% on the left side but ~10% on the right at half maximum. The anisotropy and asymmetry in peak broadening should be attributed to the dominance of Σ3 GBs that form along the {111}direction in Pt NP assembly and the tensile strain associated with these GBs. To further test this premise, we focused on the {111} planes and compared them with {200} planes using HRTEM (Fig. ). We observed a lattice expansion of ~0.1 nm (i.e., tensile strain) in the Σ3 GB region of Pt NP assembly, shown by an offset between the TEM intensity profiles along the {111} planes of Pt NP and Pt NP assembly in Fig. . In contrast, there was no apparent offset for their {200} profiles (Fig. ), possibly because {200} planes do not cross the GB. Based on TEM results, statistics of the d-spacing values for {111} and {200} planes also revealed that the d-spacing of Pt NP assembly exhibited a broader (~20%) distribution than Pt NPs, but their averaged spacings for {111} and {200} are almost the same: 0.226/0.225 nm and 0.195/0.196 nm, respectively (Fig. , fig. ), consistent with the observed XRD peak broadening for Pt NP assembly. We further quantitatively analyzed the strain field near GBs in the Pt NP assembly from atomic-resolution STEM images using a geometric phase analysis algorithm.(29) Fig. shows a tensile strain of up to 7% along the x-axis direction (i.e., the direction normal to {111} planes) is localized at a Σ3 (111) GB. Density functional theory (DFT) calculations performed on a Σ3 GB model formed between two Pt201 NPs confirmed the presence of tensile strain near the Σ3 GB (Fig. , fig. ). We further studied the two possible H2 oxidation mechanisms over Pt (111) surface: associative and dissociative pathways (fig. ).(30) DFT results in Fig. and fig. -20 show that the associative pathway dominates for Pt (111) slabs and Pt201 NP (111) facet, owing to its lower energy barrier at the rate-determining step (i.e., OOH* formation, 0.38~0.42 eV, Table ) than that of the dissociative pathway (O2* dissociation: 0.54~0.72 eV or OH* formation: 0.75~1.02 eV, Table ). Furthermore, we found a substantial enhancement of the associative H2 oxidation rate due to the tensile strain on Pt (111) surfaces (green circles in Fig. ). Such enhancement was also observed for Pt GB with tensile strain relative to isolated Pt NP (green pentagon vs triangle in Fig. ). More excitingly, we discovered the GB also extensively reduced the activation barriers of O2* dissociation to merely 0.008 eV (Table , fig. ) and subsequent hydrogenation of atomic O* to 0.60 eV (Fig. ), greatly accelerating H2 oxidation through the dissociative pathway as well (orange pentagon, Fig. ).