Yu et al. Nanoscale Research Letters 2012, 7:234 http://www.nanoscalereslett.com/content/7/1/234 Results and discussion Figure 1 shows the AFM images of the plasma-reduced samples prepared in different pH values. The initial pH of the mixture H2PdCl4 and GO before adding any NaOH was 4 (Figure 1a). After 24 h of aging and the plasma reduction, particles were found mostly on the mica substrate and not on the GO, which means that Pd2+ ions would not link with functional groups on GO at pH 4. By adding NaOH solution until pH reaches 5 (Figure 1b), a few particles were found on GO. Increas- ing the pH to 6 (Figure 1c,f) led to the formation of uni- formly distributed fine particles anchored on GO substrates without free particles on mica substrates. To see the particles more clearly, a zoom-in image (Figure 1f) and a cross-section analysis were made. The cross-section line went through one of the highest (brightest in color) particles in the whole image and a step between the GO and the mica substrate. It showed that the GO had a thickness of 1.3 nm. The highest par- ticle was around 2 nm, and the majorities were around 1 nm in diameter. When the pH value was further increased to 7 and 10 (Figure 1d,e), large aggregations formed in the solution with little Pd loaded on GO. Page 2 of 4 Thus, the best metal distribution with the highest metal loading efficiency occurred when pH was equal to 6. This sample (denoted as Pd/GO) was further studied by the following characterizations. FT-IR analysis was employed to determine the changes of functional groups on the GO surface in the glow discharge plasma reduction. Figure 2a presents the characteristic absorption bands of GO corresponding to the C = O carbonyl stretching (1,725 cm-1), O-H bend vibration (1,402 cm-1), C-OH stretching (1,221 cm-1) and C-O stretching of epoxide (1,056 cm-1) [5,12,13]. The spectrum also presents a peak at 1,620 cm-1. This was attributed to C = C ring vibrations throughout the carbon skeleton. However, the HOH bending vibrations also exist [12]. The spectrum of the GO after the glow discharge plasma reduction (Figure 2b) shows little dif- ference compared with that of the untreated GO (Figure 2a), which proves that the glow discharge plasma has no effect on the functional groups of the GO. In the Pd/ GO spectrum (Figure 2c), all characteristic absorption bands of the GO could be found in similar strength, indicating that the GO substrate was not reduced after the reduction of Pd nanoparticles. However, there were Figure 1 Plasma-reduced Pd/GO samples prepared in different pH values. (a) pH = 4, (b) pH = 5, (c) pH = 6, (d) pH = 7, (e) pH = 10 and (f) a zoom-in image of (c) with a cross-section analysis which shows the height of graphene and peaks arising from Pd nanoparticles. The scale of images (a) to (e) are 4 μm × 4 μm, and image (f) is 400 nm × 400 nm.