In this work, different sizes of gold nanoparticles were synthesized at room temperature by using trisodium citrate as a surfactant stabilizing agent and sodium borohydride as a reducing agent. Transmission Electron Microscopy (TEM) confirmed that the samples were synthesized in spherical shapes with three different particle sizes: 4 nm, 7 nm and 11 nm. Ultraviolet-visible spectra measurements were used to analyze the way that surface plasmon bands were affected by the different particles sizes. The effect of sphere size on photocatalytic reduction of 4-Nitrophenol was then studied and the rate constant of the reduction was calculated to be 0.014 s -1, 0.0091 s -1 and 0.003 s -1 for particles sizes of 4 nm, 7 nm and 11 nm, respectively. The results obtained indicated that small particles were more active in catalytic reduction due to their high surface energy.
Nanomaterials are commonly defined as materials with an average grain size less than 100 nanometres and at least one dimension in the nanometre range [
All chemicals―hydrogen tetrachloroauratetrihydrate (HAuCl4・3H2O, 99.9%,) molecular weight: 339.79 g/mol, tri-sodium citrate dihydrate (HOC(COONa)(CH2COONa)2・2H2O) molecular weight: 294.10 g/mol, sodium borohydride (NaBH4) molecular weight: 37.83 and 4-Nitrophenol (4-NP) molecular weight: 139.11 were purchased from Sigma-Aldrich (USA) and used as received without further purification. Unless mentioned, distilled and deionized water was used as a solvent in all the preparations.
The molar concentration was calculated using the formula;
The morphology and distribution of the AuNPs samples were analysed using images from transmission electron microscopy (TEM). TEM (JEOL-JEM-1011), Japan was operated at an accelerating voltage of 120 kV. The samples for TEM were prepared by depositing a drop of colloidal AuNPs on a carbon-coated standard copper grid (300 meshes) and allowed to dry before the TEM measurements were taken. The UV-VIS absorption spectra of AuNP nanoparticles were measured at room temperature on a spectrophotometer (Thermo-scientific Evolution 220) in a 1 cm optical path quartz cuvette over wavelengths of 300 - 900 nm at a resolution of 2 nm.
The photo-catalytic reduction of 4-NP was performed in a quartz cuvette 4 cm in height with 1 cm path length. An aqueous solution of 0.03 M NaBH4 and 4-NP (2 mmol) was prepared and kept at 4˚C. The photo-catalytic reduction was studied by mixing 200 µL (2 mmol) of 4-NP with 2 mL deionized water in a cuvette and then adding 1 mL of 0.03 M NaBH4 to the mixture. The UV-VIS spectra were measured at different times. The distance between the light source and the cuvette containing the mixture was kept constant in all cases while measurements were taken. The same reaction was carried out in a cuvette again and 300 µL of colloidal AuNPs (S1) was added so that the UV-VIS spectra could be monitored at different times in situ using a UV-VIS spectrophotometer (Thermo-scientific Evolution 220). The last two colloidal AuNPs samples (S2 and S3) used the same volumes (300 µL) of the mixture and same concentrations of 4-NP and NaBH4. The spectra of these two mixtures were measured multiple times. The reaction temperature was held constant at room temperature (20˚C) to reduce thermal effects on the catalytic rate. The time the reduction started and completed varied depending on the size of the nanoparticle.
strongly suggests that the main relaxation process involves electron ± electron collisions [
atoms approximately. Also, the number of gold atoms was calculated for the last two samples S2 (7 nm) and S3 (11 nm) by the same method at 84,721 and 323,649, respectively.
The reduction reaction is carried out by the hydrogen and involves the production of hydrogen gas that is seen in the form of bubbles. The continuous reduction in the intensity of the peak at 400 nm shows the consumption of 4-NP. The reaction mechanism can be reasoned by the inherent hydrogen adsorption by AuNPs. The AuNPs shuttle the hydrogen transport between NaBH4 and 4-NP. This behaviour may be explained since AuNPs adsorb
hydrogen from the NaBH4 and efficiently release it during the reduction reaction and hence AuNPs act as a hydrogen carrier in this reduction reaction. The same behaviour was seen for the reduction of 4-NP in the presence of 300 μL of AuNPs in samples S2 and S3, except that the time of the reduction changed. In S2, as in S1, the absorption peak at 400 nm decreased with increasing time of reduction and a new band appeared at about 295 nm, indicating the formation 4-AP. In sample S3, the absorption peak at 400 nm took a long time to occur and this is may have been due to the incomplete consumption of 4-NP. In addition, the intensity of the absorption band at 295 nm was very low. These results indicate that 4-NP did not completely transform into 4-AP in S3. It is well known that, the applications of gold nanoparticles in the catalytic application depend on the number of gold atoms on surface of nanoparticle and the sample has a larger number of atoms in the surface is more catalytic activity. Sample S1 is more active due to it has a small number of gold atom (16,862) than S2 (84,721) and S3 (323,649) that mean it has a large number on the surface so that it has a catalytic activity than S2 and S3. The mechanisms and forms of reduction reaction for the transformation of 4-NP to 4-AP are shown in
where Ao is the initial absorbance of the reaction system, A is absorbance at time t, and k is the rate constant of the chemical reduction. From this kinetic curve, the rate constant (k, s−1) was calculated at 0.014 s−1, 0.0091 s−1
and 0.003 s−1 for the AuNPs of diameter 4 nm, 7 nm and 11 nm (in S1, S2 and S3, respectively). By comparing the rate constants of 4-NP (0.014 k, s−1, 0.0091 k, s−1 and 0.003 k, s−1) in the presence of different sizes of AuNPs (4 nm, 7 nm and 11 nm), it was seen that, S1, which contained 4 nm AuNPs, has more catalytic activity due to the weak interaction between 4-NP and the gold nanoparticles with higher particles sizes. The reduction of 4NP by S1 and S2 can be related to the Langmuir-Hinshelwood model of heterogeneous catalysed reduction [
In this study, we prepared AuNPs of different sizes by a simple chemical method. The results of UV-VIS measurements showed that the surface plasmon resonance bands depended on the particle size. The prepared samples were used to reduce 4-NP, confirming that smaller nanoparticles were more active in the reduction process. The rate constant of chemical reduction was calculated for each sample and this decreased with increasing particle size. The catalytic activity of the nanoparticles can be affected and increased directly by increasing the ratio of the number of atom on the surface to volume. This ratio was increased with decreasing the particle size, so the small AuNPs size was more active in the reduction process.
Rozik, A.S., Tolba, A.S. and El-Dosuky, M.A. (2016) Design and Implementation of the Sense Egypt Platform for Real-Time Analysis of IoT Data Streams. Advances in Internet of Things, 6, 65-91. http://dx.doi.org/10.4236/ait.2016.64005