Detecting and Quantifying TiO2 Size Distribution and Oversize in Ink Nanosuspensions by Resonance Light Scattering (RLS)

Abstract

Concentrated nanomaterials suspensions are currently being commercialized for numerous applications. To measure the properties of such suspensions, conventional characterization techniques may require extensive experimental procedures, including sample pre-treatment and/or dilution. Novel techniques need to be developed to measure the properties of concentrated suspensions sensitively and reproducibly without sample dilution. In this work, resonance light scattering (RLS) data was assessed as a promising tool to monitor the concentration and size of nanoparticles in concentrated commercial ink suspensions. This project aimed to develop a reproducible RLS method for concentrated nano-suspensions quantifying analysis using triangular or square cuvettes. Second, it used RLS to determine a simple, robust, reproducible method for assessing the size and concentration changes of TiO2 and white inks. Third, to identify a method to detect and measure oversized TiO2 nanoparticles in a matrix of smaller particles in the white ink nano-suspension. For single ink investigations, UVVIS absorption data were recorded to assess the optimum dilution factor (DF) of the TiO2 white inks. The results showed that the absorption spectra started to become smoother at a DF of 11,000, which was found to be the minimum DF required to obtain a reasonably smooth absorption spectra curve. As for DLS measurements, the value of the Z-average was exponentially correlated to the value of sample DF, with an R 2 value of 0.99. This correlation shows that DLS can be used as a reference calibration technique to track the concentrations of single ink suspensions. RLS spectra were recorded for diluted ink suspensions using two types of cuvettes, a square and a triangular cuvette. For both types of cuvettes, the maximum RLS intensity peak was recorded at a wavelength of 420 nm because such a peak originates from the emission of the bandgap transition in the TiO2 structure and does not depend on the geometry of the cuvette used. The intensity of this maximum peak was correlated to the DF values using a similar exponential equation to that obtained for the DLS data. The R 2 value of the maximum RLS intensity peak was 0.97 when the triangular cuvette was used. On the other hand, using the square cuvette led to a poor correlation, which may be explained by the overshadowing of particles to interior volumes of the concentrated suspension sample in the cuvette. The triangular cuvette was further used throughout the rest of the investigations for mixed ink suspensions. For mixed ink suspensions, a poor correlation was found between the maximum peak intensity values and the volume percent of the milled ink in the mixed ink samples. This indicates that RLS is still not a sensitive technique to differentiate between average sizes of TiO2 particles compared to DLS measurements, which showed a strong linear correlation between z-average values and the volume percent of the milled ink (R2 = 0.97). For spiked ink suspensions, the maximum peak could be correlated to the weight percent of the spiking suspension using an exponential growth function with an R 2 value of 0.94. The exponential growth curve shows a plateau beyond 0.2 wt.% for the spiking suspension. This shows that RLS can be sensitive to the presence of a few large particles in suspension; however, a II further increase in the concentration of oversized particles did not result in a corresponding increase in the value of the maximum RLS spectra intensity peak. This can be explained based on the overshadowing of larger particles to each other in suspension leading to weak or no contribution from the shadowed large particles to the RLS spectra. On the other hand, the Z-average obtained from DLS measurements showed an exponential increase in correlation with the weight percent of the spiking suspension, reflecting the sensitive nature of DLS. In summary, RLS was found to be a sensitive technique for the concentration of single ink suspensions and low weight percentages of spikes suspensions when the triangular cuvette was used. For highweight percent spiked suspensions, and mixed ink suspensions, RLS was not assensitive as DLS. To overcome the overshadowing problem in RLS, the design of the cuvette itself, the path length and probably the position of the detector needs to be investigated, which can be the topic of future studies.