Conclusions of the study of the effect of synthetic parameters on the formation of Pd-PAM NPs using DOE

The analysis of the effects of synthesis time, synthesis temperature, PAM-to-Pd molar ratio, Pd concentration and stirring rate on the formation of colloidal Pd-PAM NPs was performed by means of design of experiments methodology. More precisely, the studied responses were focused on the ability to synthesize in an efficient way spherical NPs with large size and narrow distribution. To limit to eight the number of experiments and, at the same time, be able to study the effect of five parameters, the plan of experiments had to follow a fractional factorial design that involved confusions between single factors and second-order interactions. Therefore, the interpretation of the results was performed using assumptions based on our chemical knowledge and experimental observations. From these interpretations, different combinations of factors were found for each studied response (percentage of spherical NPs, NPs size, synthesis efficiency and relative standard deviation) and are summarized in Table ‎3.10.

Table ‎3.10. Combinations of factors highlighted for each studied responses Higher percentage of spherical NPs (Y1) (-1, -1, -1, -1, ±1)

Larger mean NPs size (Y2) (-1, -1, -1, -1, +1)

Higher efficiency of the synthesis (Y3) (-1, -1, -1, -1, ±1)

Lower relative standard deviation (Y4) (-1, +1, -1, ±1, ±1) (+1, -1, -1, ±1, ±1)

(-1, -1, -1, -1, +1)

Interestingly, one combination of factors was common to the four studied responses: (-1, -1, -1, -1, +1). This combination means that high percentage of spherical NPs, large NPs, high synthesis efficiency and low relative standard deviation were obtained at short reaction time, low temperature, low PAM-to-Pd ratio, low Pd concentration and high stirring rate.

To confirm these conclusions, one additional synthesis (called RUN09) was performed using the following combination: (-1, -1, -2, -2, +2). During the OFAT and DOE studies, it was observed that the size of the NPs was not significantly modified (in the range of 3 to 5 nm). Therefore, to attempt an increase of size, it was preferred to test the conditions (-1, -1, -2, -2, +2) instead of (-1, -1, -1, -1, +1) since the three last parameters (RATIO, CONCPD and STIRRING) were significantly affecting the NPs size, thus their levels (values) were increased in absolute value to a higher step (instead of a step of 1, a step of 2 was chosen). This combination of factors corresponds to a synthesis performed during 1 h, at 50ºC with 30 eq. of PAM-to-Pd molar ratio, 7.4 mM of Pd concentration and 1100 rpm of stirring rate. The TEM micrograph and size distribution histogram obtained for the automatic measurement of more than 5000NPs are displayed in Figure ‎3.37 and Figure ‎3.38.

Figure ‎3.37. TEM micrographs of the RUN09 performed during 1h, at 50ºC with 30 eq. of PAM- to-Pd molar ratio, 7.4 mM of Pd concentration and 1100 rpm of stirring rate (combination -1, -

Figure ‎3.38. Results of RUN09. Left side: Size distribution histogram with bin size of 1 nm obtained by the fitting of one Gaussian function to the automatic measurement of 5190NPs.

Right side: Cumulative probability plot of the equivalent diameter of the spherical NPs (in black) and the equivalent diameter of all the NPs

The NPs obtained were 90% spherical which represent an increase of ca. 5% in comparison with the higher percentage of spherical NPs obtained in the experiments of the designed matrix. Moreover, the synthesis efficiency showed an increase of 25% in comparison with the best result obtained during the DOE study: ca. 77%. The NPs presented a mean size of 5.05 ± 1.36 nm. The NPs size remained constant indicating that the studied parameters did not enable the formation of NPs larger than 5 nm in the studied range of values. Other parameters could be contemplated to attempt a variation of size such as the pH, the nature of the reducing agent, the nature of the solvent or the nature of the polymer.

For example, P. Witte et al. performed a variation of Pd NPs size from 5 to 15 nm by pH modification.24 Then, Braun and co-workers reported the effect of the polymer molecular weight on NPs sizes.25 In their study the mean diameter of the NPs decreased with increasing the polymer weight. Finally Cheng and co-workers reported the preparation of colloidal Pd NPs by a reduction-by-solvent method.26 NaBH4 was

of polyvinylpyrrolidone (PVP) as stabilizer. They found that varying the amount of NaBH4, C2H5OH and PVP, different NPs sizes were obtained. In the same way, Perez-

Cordonado and co-workers reported the effect of the reducing agent nature on the NPs size and an important decrease in NPs size when the reducing agent-to-metal ratio was increased.16

To conclude, this DOE study allowed finding in which direction the synthetic parameters had to be modified to increase the fraction of spherical Pd-PAM NPs and the synthesis efficiency. Moreover it was shown that the NPs size could not be varied more than in the range of 3 to 5 nm indicating that this characteristic was not determined by the studied reaction conditions.

The use of DOE methodology helped to understand better how each parameter affect a concrete response (sphericity, size, distribution or efficiency) and to highlight important interactions. This study demonstrates that this chemical system is complex and, in most of the cases, the major part of the studied parameters and interactions highly affected the responses. Thus, the DOE methodology is a powerful methodology to guide the identification of the combinations of factors that allow obtaining the desired well-defined NPs.

The common factors studied in the OFAT and DOE structure-synthesis relationship studies were the PAM-to-Pd ratio, the temperature and synthesis time. Concerning the PAM-to-Pd ratio, higher values led to narrower size distributions in the OFAT study. In the DOE study, the opposite trend was observed but the interaction between the synthesis time and temperature appeared more significant than this ratio for the control of the relative standard deviation.

Then, in both studies, shorter times and lower reaction temperature resulted in better defined NPs.

However, a direct comparison between both studies is not straightforward. Indeed, the data analysis procedure was different in both study (manual vs. automatic measurements of the NPs). Moreover these studies were performed with three years

of difference using the same batch of PAM stabilizer since the later was not anymore commercially available. Aging effects were detected on this reagent which was affecting slightly the NPs formation. This remark is of high importance for industrial considerations. Indeed, knowing the negative effects of a long-term storage, the production schedules could be adapted to avoid this issue as much as possible.

3.5. Conclusions

In this chapter, the study of the effects of several synthetic parameters on a novel Pd- polyacrylamide NPs (Pd-PAM NPs) formed by chemical reduction in aqueous solution was described. This study was first performed using a classical OFAT approach and was then completed with eight experiments that followed a DOE matrix.

The main conclusions of the OFAT study are reminded as follow.

 No significant variations of sizes were observed from the set of experiments. Significant improvements in terms of size distribution were highlighted.

 The key parameters were the stabilizer-to-metal ratio, synthesis temperature and synthesis time.

 In contrast to the typical reported trend, when the amount of stabilizer was increased from 20 eq. to 80 eq., the NPs size did not varied significantly but their distribution drastically improved.

 An optimum seemed to appear at 50ºC leading to the narrowest NPs size distribution under the tested reaction conditions. Nevertheless, this non-linear comportment could be confirmed with DOE approach.

 Until 15.5 hours of reaction, the Pd-PAM NPs were well-formed but longer reaction times led to their decomposition in a non-stable suspension

 From this OFAT study, a recipe of well-defined NPs of ca. 3.9 nm was obtained and called Pd-PAM 6. They were synthesized at 50ºC with 40 equivalents of polyacrylamide under 3 bar of hydrogen, 700 rpm of stirring rate and during 5.5 hours. The robustness of this recipe was checked and the NPs characterization was provided.

During this study, limitations of the manual measurements of the TEM micrographs were pointed out (accuracy and lack of information of NPs shape and agglomeration). Consequently, automatic measurements of NPs were proposed combined with the use of a Matlab® routine that enabled the estimation of the fraction of spherical NPs in a sample, the efficiency of a synthesis, the mean NPs size and distribution.

An additional structure-synthesis relationship study was proposed using the DOE approach. The main conclusions these eight experiments are given as follow.

 The direction that should follow each of the studied parameters in order to increase the fraction of spherical NPs and the synthesis efficiency was determined.

 The NPs size remained equal to ca. 5 nm under the studied reaction conditions as in the previous OFAT study.

 Short synthesis time, low temperature, low PAM-to-Pd ratio, low Pd concentration and high stirring rate led to the formation of highly spherical NPs with a low fraction of agglomerates and a narrow NPs distributions.

3.6. Appendix

3.6.1. Appendix 1: NPs automatic measurement procedure for colloidal