Ligand-free platinum nanoparticles were obtained by laser ablation in liquid and characterized in their colloidal state. Figure 1 shows the particle size distributions which were measured using disc centrifugation and electron microscopy. Laser-generated platinum nanoparticles have a diameter in the range of 3–5 nm showing a monomodal particle size distribution in the particle number-weighted histograms. As there are a few nanoparticles > 5 nm, the mass-weighted particle size analysis shows a weak second mode around 10 nm. UV/Vis spectra shown in Figure 2 reveal the characteristic extinction spectra of colloidal platinum nanoparticles36,38. The zeta potential of platinum nanoparticles was measured to be –36 mV resulting in an electrostatically stable colloid that could be stored for days without any significant change. Surface charge characterization of nanoparticles and support was done by measuring the zeta potential as a function of pH which is shown in Figure 3. The isoelectric point (neutral particles) is about 2.8 for platinum nanoparticles and 6.3 for TiO2 support, respectively.
For adsorption experiments, the colloid was mixed with TiO2 as described in the experimental section. To quantify adsorption, the concentration of colloidal nanoparticles was measured by UV/Vis spectroscopy before and after the mixing. If nanoparticle adsorption takes place, the liquid gets clear and the absorption band (at 300 nm) disappears in the UV/Vis spectra which is shown in Figure 2. Using this simple method one can easily calculate nanoparticle adsorption efficiency to the support9, 24. Please note that negative values correspond to the given calculation method (see experimental section). In this case, some TiO2 is dispersed and contributes to the
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VI.i UV/Vis extinction after mixing with the colloid. Thus, equation 1 results in a negative value.
However, no adsorption of platinum nanoparticles takes place in this case which was checked by centrifugation and subsequent size analysis by ADC (data shown in the Supporting Information, Figure S4-S6).
Figure 1: Particle size of platinum nanoparticles measured by analytical disc centrifugation (ADC) and transmission electron microscopy (TEM).
As can be seen from Figure 2, all colloidal nanoparticle quantitatively (adsorption efficiency > 95%) adsorb to the white support resulting in a transparent supernatant and grey-colored TiO2 support with platinum nanoparticles (more information on this can be found in the Supporting Information).
Figure 2: UV/Vis spectra of colloidal platinum nanoparticles before (black line) and after mixing with TiO2
support (red dotted line).
For further use like for catalysis, the supported nanoparticles only have to be filtrated and dried. According to literature, the nanoparticle adsorption can be considered as an electrostatic adsorption2. The mixture of platinum nanoparticles and TiO
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VI.i means that platinum nanoparticles are negatively charged (IEP 2.3-2.8) and TiO2 support is
positively charged (IEP 6.3-6.4) resulting in an attractive Coulomb force which initiates adsorption. By adsorption of negatively charged nanoparticles to suspended support, we can clearly observe a net charge transfer to the support. This net charge transfer is caused by a shift of IEP of the TiO2 support in Figure 3. The IEP shifts from 6.4 to a lower value of 5.0 which means that a negative charge is transferred by ions after platinum nanoparticles are adsorbed on TiO2-support. In a microscopic view, this should be connected to an initial neutralization of the surface charge of TiO2. Consequently, the net-charge-transfer to the support results in less positively charged support and should margin the maximal load of nanoparticles. Accordingly, Figure 3 (bottom) demonstrates the impact of nanoparticle load on IEP of the TiO2 support (raw data can be found within the Supporting Information, Fig. S8): A higher nanoparticle load lowers the IEP of the TiO2 until it converges with the IEP of platinum nanoparticles. At this point, the charges of nanoparticles and support are almost identical to prevent further electrostatic-controlled nanoparticle adsorption. For the given system, we observe a maximal nanoparticle load up to 65 wt% which is shown within the Supporting Information (Figure S8).
Figure 3: Top: Zeta potentials and isoelectric points of colloidal platinum nanoparticles, TiO2 support and
Pt/TiO2 catalyst (1 wt%). Bottom: Isoelectric point of Pt/TiO2 as a function of nanoparticle load (the line is
just to guide the eyes).
Due to electrostatic interaction, the adsorption process is very sensitive to the pH of the dispersion. Figure 4 shows the adsorption efficiency as a function of pH using different electrolytes like phosphate buffer, carbonate buffer and citrate or sodium hydroxide solution. Low pH values (< 5.5) result in a complete adsorption of platinum nanoparticles which is initiated by an attraction between positively charged support and negatively charged platinum nanoparticles. Since the particle surface gets protonated at low acidity (pH<4), too low pH values close to the IEP of platinum nanoparticles (IEP 2.7) have to be avoided. Thereby, the negative surface charge is compensated and results in critically low zeta potential values < kT/e
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VI.i that cause agglomeration. If the pH exceeds the IEP of the support (IEP 6.4), the charge of the
support is reversed from positive to negative, causing electrostatic repulsion of negatively charged nanoparticles and negative support. Overall, this clearly reveals that adsorption of metal nanoparticles is efficient only if pH is higher than the IEP of the nanoparticle adsorbate and lower than the IEP of the adsorbent.
Figure 4: Adsorption efficiency of platinum nanoparticles to TiO2 as a function of pH. Negative values result
from dispersion of TiO2 (no nanoparticle adsorption, see Supporting Information).
In addition to electrostatic interactions, attractive forces like van der Waals forces also play a distinct role. In a recent work, we demonstrated that negatively charged silver nanoparticles could also adsorb to a slightly negatively charged barium sulfate support23. In this case, a support with lower surface charge density like a sulfate39 shows only a weak repulsive force to the colloidal nanoparticle that may be overcompensated by van der Waals forces. However, when using an oxide with high surface charge density like TiO2 as a support, electrostatic interactions are the dominant force in the adsorption process. Hence, adsorption is triggered by adjusting the pH between IEP of the nanoparticles and IEP of the support which result in oppositely charged particle species and subsequent electrostatic attraction.
To identify the adsorption mechanism, we modelled the adsorption process using Henry, Langmuir, Freundlich und Temkin isotherms which can be found within the Supporting Information (Tab. S4-5, Fig. S10). Linear regression of experimental data showed that isotherms by Henry, Freundlich and Temkin result in the best fitting result with high correlation coefficients of R2 > 0.95. This is in good agreement with recent work, where we have shown that silver nanoparticle adsorption to barium sulfate supports clearly follows a Freundlich
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VI.i adsorption isotherme23. Consequently, the Freundlich model of adsorption should be the most
appropriate model for adsorption of ligand-free nanoparticles to support surfaces.The failure of the Langmuir model indicates that there are interactions between adsorbed nanoparticles and differences in adsorption sites also might play a role.