The morphology and size of nanoparticles play a crucial role in determining their photochemical properties, including their absorption and reactivity towards light. The size and shape of nanoparticles can influence their surface plasmon resonance SPR , which is the collective oscillation of electrons in response to an electromagnetic field. This phenomenon is responsible for the unique optical properties of nanoparticles, such as enhanced absorption and scattering of light.1. Size effect:As the size of nanoparticles decreases, the quantum confinement effect becomes more pronounced. This leads to an increase in the energy bandgap, which in turn affects the absorption spectrum of the nanoparticles. Smaller nanoparticles exhibit blue-shifted absorption spectra compared to their bulk counterparts due to the increased energy bandgap.Experimental evidence: In a study by Link and El-Sayed 1999 , they investigated the size-dependent optical properties of gold nanoparticles. They observed that as the size of the nanoparticles decreased, the SPR peak shifted to shorter wavelengths blue shift . This demonstrated the influence of nanoparticle size on their absorption properties.2. Morphology effect:The shape of nanoparticles also has a significant impact on their photochemical properties. Different morphologies, such as spheres, rods, cubes, and stars, exhibit distinct SPR characteristics due to the variation in their surface electron distribution.Experimental evidence: In a study by Jain et al. 2006 , they synthesized gold nanorods with varying aspect ratios and investigated their optical properties. They found that the longitudinal SPR peak of the nanorods red-shifted as the aspect ratio increased. This demonstrated the influence of nanoparticle morphology on their absorption properties.Additionally, the morphology of nanoparticles can affect their reactivity towards light. For instance, sharp edges and corners in nanoparticles can lead to a higher local electric field, which can enhance their photocatalytic activity.Experimental evidence: In a study by Tian and Tatsuma 2005 , they synthesized gold nanorods and nanospheres and compared their photocatalytic activities. They found that the nanorods exhibited higher photocatalytic activity than the nanospheres due to the enhanced local electric field at the tips of the nanorods.In conclusion, the morphology and size of nanoparticles significantly affect their photochemical properties, including their absorption and reactivity towards light. Smaller nanoparticles and those with specific shapes, such as rods or stars, can exhibit enhanced absorption and photocatalytic activity due to the quantum confinement effect and the influence of surface plasmon resonance.