UV–Vis spectra of the copper nanoparticles synthesized at different
pHs, as well as variations of the peak position (λmax) and height by pH
are presented inFig. 3. The results showed at pHs lower than 10 no obvious peak was observed which indicated that copper nanoparticles
were not synthesized at low pHs. At pH~ 10 surface plasmon resonance
peak aroundλ~ 562 nm was appeared indicating the formation of the
copper nanoparticles. By increasing the pHup to 11, the peak height in-creased without significant change of the peak position. This behavior
was due to the increase of the number of the copper nanoparticles. At
pHs higher than 11 the peaks height was decreased and the peaks position shifted gradually towards longerλwhich can be attributed to the
increase of the nanoparticles size as well as the formation of copper
oxide[27]. So, the pH ~ 11 was used as optimum value in the further
experiments