Size Effects in Thermal and Photochemistry of $(\mathrm{NO}{)}_2$ on Ag Nanoparticles

NO dimers adsorbed on alumina supported silver nanoparticles (Ag NPs, radii $R\sim 1–6\mathrm{nm}$) show decreasing desorption temperatures and complex behavior of photoinduced desorption with decreasing NP size. In particular, for resonant excitation of the $(1,0)$ Mie plasmon at 3.5 eV the photoinduced desorption cross section increases with $1/R$, showing a pronounced enhancement (40 times) at $R\sim 2.5\mathrm{nm}$ compared to Ag(111). At 4.7 eV the translational temperature of photodesorbed NO increases strongly with $1/R$. We discuss these trends and peculiarities in terms of the size-dependent properties of the Ag NPs.

Figure 1

(a) Temperature programmed desorption (TPD) results measured at $m/e=30$ from Ag NPs of various diameters ($D$) dosed with NO at 75 K. The dashed curve shows a rescaled TPD result from Ag(111) for comparison. (b) Desorption peak temperatures of NO (squares) and ${\mathrm{N}}_2\mathrm{O}$ (triangles) in (a) as a function of reciprocal radius, $1/R$ ($R=D/2$). The solid lines are linear fits for $1/R$ between 0 and $\sim 0.5{\mathrm{nm}}^{-1}$. The error bars indicate standard deviations in the distributions of $D$.

Figure 2

(a) Photodesorption cross sections of NO from Ag NPs as a function of $1/R$ ($R$ is the mean radius of Ag NPs) at $h\nu =2.3$ (squares, multiplied by 10), 3.5 (circles), 4.7 eV (triangles). (b) Translational temperatures ($T_t$) of NO photodesorbed from NO dimers adsorbed on Ag NPs at 2.3 (squares), 3.5 (circles), 4.7 eV (triangles). Inset: Time-of-flight spectra for 4.7 eV excitation, fitted to a sum of shifted Maxwell-Boltzmann distributions ($T_t=2700$, 750, and 70 K for peaks at 110 (in gray), 230, and $750\mu \mathrm{s}$, respectively), measured for $D=2.8$ and 6.1 nm.