Title: The synthesis and characterization of PVP coated cerium oxide nanoparticles.

Transcription

1 Supporting Information. Title: The synthesis and characterization of PVP coated cerium oxide nanoparticles. Authors: Ruth C. Merrifield 1, Zhiwei Wang 2, Richard E. Palmer 2, Jamie R. Lead 1 * 1 School of Geography, Earth and Environmental science, University of Birmingham, Birmingham, UK 2 School of Physics and Astronomy, University of Birmingham, Birmingham, UK *Corresponding author: Jamie R. Lead. JLEAD@mailbox.sc.edu. This document contains supporting information on: S1. Toxicology media composition. S2. STEM images S3. AFM sizing. S4. DLS and FFF sizing S5. Lattice spacing of ceria. S6. Changes in hydrodynamic diameter with ionic strength. S7. Ce(III) and Ce(IV) standards for EELS Including 19 pages, 2 tables and 14 figures. S1

2 S1. Toxicology media composition. This is the composition of OECD daphnia and algae media in mgl -1. The ph of the daphnia media and algae media were in the range of 6-9 and respectively. Daphnia media Compound mgl -1 Calcium chloride 294 Magnesium sulphate sodium carbonate and potassium chloride 5.75 sodium selenite Algae media di-potassium hydrogen orthophosphate 75 Potassium di-hydrogen orthophosphate 175 Magnesium sulphate 75 Sodium Nitrate 250 Calcium chloride 025 Sodium Chloride 25 Ferrous sulphate 4.98 Sulphuric acid conc. 184 (wt per ml = 1.84g) Boric acid Zinc sulphate Manganese chloride Cupric sulphate Cobaltous nitrate 0.08 Sodium molybdate S2

3 S2. STEM images. Lower resolution STEM images of samples NP a and NP d are shown in figure S2-1a and b. It can be seen that the particles are all well separated with no large aggregates present. However the particles in sample NP d are well separated so more images were needed to obtain the 100 particles needed to obtain a good size distribution. Figure S2-1. STEM images showing low resolution images of a) NP a and b) NP d Figure S2-2 shows three examples of the need to go through focus when imaging NPs so that you can decide if the structures are all crystalline. It can clearly be seen that the particle in the centre of the image that does not seem to have a crystalline structure in figure S2-2a does have a strong lattice in figure S2-2b while the other particles are now out of focus (similar patterns can be seen when you compare figures S2-2c with d). This demonstrates that you can tell whether all particles possess a crystalline structure in STEM however the same image must be taken in different focal planes to access the crystalinity of a sample. S3

4 Figure S2-2 a and c are two different examples of sample NP a in one focal plane. Figure S2-2b and d are the corresponding images in a different focal plane showing all particles to be crystalline. Figure S2-3a and b are of different particles form the NP d sample. They both show that the particles are made up of aggregates of smaller particles. All of the particles can be seen to be crystalline and have their own crystal orientation s. S4

5 Figure S2-3a and b are examples sample NP d. They show clearly that the particles are made up of smaller particles that possess their own crystal structures. S5

6 S3. AFM sizing. Figure S2-1a and b are AFM images and 3D projections of sample NP a and NP d respectively. From figure S2-2a-d it can be seen that the size distributions of the solutions from the AFM images are 5.1(1.6), 6.1(3.1), 10.0(3.0) and 16.2(6.8) for samples NP a-d respectively. Figure S3-1: AFM images and 3D projections of a) sample NP a and b) sample NP d. The scale bar in image a is 1µm (all images are of the same scale), taken at scan speeds of 0.5Hz. S6

7 Figure 3-2: Size distributions of NP a-d (A, B, C, D, respectively from TEM images (blue, lines bin, of 1nm), with corresponding Gaussian fit (red line). S7

8 S4. DLS and FFF sizing. Figure S4-1 and corresponding table (S-T1) show the typical DLS results for the four NP suspensions NP a-d. It can clearly be seen that the particle size increases with increasing PVP chain length. Figure S4-1 Typical DLS results for the four NP suspensions Solution Size(nm) Standard deviation Poly dispersive index (PDI) Standard deviation NP a NP b NP c NP d Table S4-T1. The z-averages and the poly dispersive index for the four NP suspensions. S8

9 Figure S4-2 shows the reproducibility of the particles via DLS with the z average height and PDI for each batch measured. While table S4-T2 shows the ionic concentration of each batch. It can be seen from these results that the reproducibility of these reactions. Figure S4-2.Comparison of different batches of samples NP a-d via DLS. It can be seen that there is little alteration in the primary particle size within each sample between batches. The numbers corresponding to each individual batch refer to the z-average size (PDI). suspension batch average Standard deviation % yield recovery of Ce ions NP a NP b NP c NP d Table S4-T2. ICP-MS results for multiple batches of suspensions. S9

10 It can be seen in Table S4-T3 that the z-average sizes of the four NP suspensions NP a- d are similar when diluted into ultrapure water, algae media and daphnia media. However they are larger than the original stock solutions due to the difference in viscosity between the stock solutions and the others. Suspension Z average (SD) Stock Water Algae Daphnia PDI (SD) Z average (SD) PDI (SD) Z average (SD) PDI (SD) Z average (SD) PDI (SD) NP a 5.11 (0.13) (0.05) (0.06) 0.19 (0.01) 7.06 (0.09) 0.19 (0.02) 8.73 (0.05) 0.29 (0.01) NP b 7.82 (0.03) 0.16 (0.02) (0.47) 0.16 (0.03) (0.16) 0.16 (0.2) (0.37) 0.20 (0.03) NP c (4.97) 0.41 (0.02) (0.77) 0.23 (0.01) (0.38) 0.21 (0.01) (0.53) 0.22 (0.01) NP d (0.51) 0.45 (0.02) (0.35) 0.26 (0.01) (0.88) 0.24 (0.01) (0.13) 0.25 (0.01) Table S4-T3 shows the z-average and PDI for the stock suspensions and those diluted into ultrapure wate, algae media and daphnia media. Figure S4-3 shows a typical result obtained for suspension NP a when measuring the size using FFF. The average size of particles with a passivation layer is larger when measured with FFF as, unlike TEM/AFM, the FFF measures the core and the passivation layer. Figure S4-4 is a graphical depiction of the sizes for NP a-d using different sizing techniques. S10

11 Figure S4-3. A typical graph obtained from the FFF. Figure S4-4. A comparison of different sizing techniques for the samples NP a-d S11

12 S5 Lattice spacing of ceria Cerium oxide has the same crystal structure at calcium fluorite. The unit cell consists of cerium atoms located at the face centred cubic (FFC) positions, which encase eight oxygen atoms located at the tetrahedral matrices. Cerium oxide can come in two different states with the cerium ions being in the Ce(III) or Ce(IV) oxidation states. It is able to move between these states without altering its structure. Figure S5-1a and S5-1b shows the structure of cerium oxide in the Ce(IV) and Ce(III) respectively. As oxygen is removed from the lattice the corresponding cerium ion changes to be in the cerium (III) state. This also affects the lattice parameter. For a Ce(IV) lattice the parameter in 0.54nm but as oxygen is removed and the Ce(IV) alters to Ce(III) the parameter is increased to 0.56nm 1 Figure S5-1a and b show the structure between Ce(IV) AND Ce(III) oxide respectively. The reactivity and stability of cerium NPs depend on atoms exposed on the terminating surface. Figure S5-2a-c shows the atoms on the surface when different atomic planes are exposed for the (001), (110) and (111) respectively. It can be seen that the (001) surface is purely cerium atoms exposed while the (111) (S2-2C) is oxygen terminated. This S12

13 causes the (111) surface to be the most stable of all. Whereas the (100) surface is cerium terminated making it the least stable and most reactive with the (110) surface being intermediate. (001) (110) (111) Figure S5-2a-c: show the atoms exposed when looking down different atomic plans. Black circles are cerium ions and the grey ones are oxygen atoms. Figure S5-3 shows the different atomic planes that can be observed along the (110) direction, along with the distances between the atomic planes as observed in the (110) projection. From this, the spacing and angles between planes can be calculated. Calculating the distances and angles between the atomic planes enables identification of the crystal structure and particle morphology to become possible, this is investigated by HR-TEM and STEM. The lattice parameter of 0.56 nm has been used in this case at the particles are Ce(III) as shown in the sr-eels data. S13

14 Figure S5-3: Shows the lattice spacing s expected for cerium(iii) oxide particles. Figure S5-4(a-c) show three HR-TEM images of different particles. S5-4a and c are hexagonal in shape. The lattice fringes of 0.34 nm and 0.28 nm correspond to the (111) and (110) planes, this along with the measured angles means that both of these particles are likely to be truncated octahedral in shape. The particle in figure S5b has 5 (111) planes, 1 (100) planes and 2 (110) plane present. Has an irregular shape this is due to the (100) and (110) planes becoming more energetically stable in water while the PVP will preferentially bind to the (111) plane causing both to grow and kinks to be formed at sites where the PVP attaches. These two processes cause the presence of the higher energy surfaces. S14

15 Figure S5-4(a-c): hr-tem images of cerium oxide nanoparticles capped with PVP. S15

16 S6 Changes of size with ionic strength. Figure S6-1a and b are graphical depictions of the DLS measurements of suspension NP a when exposed to varying concentrations of NaCl (0, 0.01, 0.1, 1.0, 5.0, 10, 50 and 100 mm). Figure S6-1a shows the intensity averaged data as obtained from the DLS. While Figure S6-1b is a graph showing the Z-average data with error bars. It can be seen that the data points all lie within the error of the measurements, showing there is no significant difference between the size of the particles over this range of ionic strengths. The corresponding Z-average and PDI can be found in table S6-T1. Figure S6-1a) Sample NPa exposed to different ionic strengths water between 0 and 100ppm. S6-1b shows the z-average against ionic strength. S16

17 Ionic strength (nm) Z average (nm) PDI Table S6-T1. Shows the Z-average and corresponding PDI for all ionic strengths as measured by DLS. S17

18 S7 Ce(III) and Ce(IV) standards for EELS Ce(III) and Ce(IV) standard samples were imaged and their energy loss spectra analysed in order to calibrate the spectra obtained from the NP suspensions. The EELs spectra is characterized by a steep rise near the energy of the white lines and is due to the transition of a core electron to an unbound state (3d 3/2 4f 5/2 and the 3d 5/2 4f 7/2 for the M 4 and M 5 level respectively). These lines arise from the coupling of the spin quantum number (s) and the orbital quantum number (l), resulting in a total angular momentum of j (Where j = l + s). The energy difference between the M 4 and M 5 is a measure of the spin-orbit coupling of the 3d level and for cerium is approximately 18 ev. In order to analyse whether the cerium is in the Ce(III) or Ce(IV) state the raw EELs spectra obtained need to be processed to remove the exponentially decaying background due to the tail of the strong zero loss peak, plural scattering depending on sample thickness and a contribution from the transitions to the atomic continuum 2. The data here has been processed in digital micrograph by removing the background and passed through a high pass filter (to remove phantom current points) before the second derivative of the spectra is taken to find the relative integrated intensities of the M 4 and M 5 peaks. Figure S7-1a show average energy loss spectra obtained from the Ce(III) and Ce(IV) standards along with the corresponding second derivatives. From the raw data it can be seen that the M 4 and M 5 peaks are shifted to lower energy for the Ce(III) standard (M to 883 and M to 801 respectively) and a shoulder appears on the M 4 line. The second derivatives of these spectra clearly show that the peak intensity of the white lines has switched from the M 5 line being more intense for the Ce(III) standard and the M 4 line being more intense for the Ce(IV) standard. S18

19 Figure S7-1b and S7-1d show the STEM image and corresponding line profile (S7-1c and S7-1e) and M 5 /M 4 integrated intensity (taken from the second derivative of the original data). EELs spectra were obtained every 20.8 nm and every 8.78 nm along a designated line on the image. The time taken for each measurement is kept short (3 seconds) as ceria particles are very beam sensitive, with reduction of Ce 4+ to Ce 3+ with time through irradiation-induced reduction 3. The M 5 /M 4 integrated intensity ratio for Ce(IV) is 0.83 and the Ce(III) is 1.2. Figure S7-1a) top two graphs are the EELs signals for Ce 3+ (red) and Ce 4+ (blue) and the bottom two graphs are the respective second derivatives. b and d) STEM image of bulk cerium and cerium(iii) nitrate respectively, with accompanying graph (c and e) of intensity profile of STEM image (blue), ratio of M5/M4 EELs signal (red), M5/M4 signal for Ce(IV) and Ce(III) (green and maroon respectively). S19

20 Reference List 1. Deshpande, S.; Patil, S.; Kuchibhatla, S. V. N. T.; Seal, S. Size dependency variation in lattice parameter and valency states in nanocrystalline cerium oxide. Applied Physics Letters 2005, 87 (13). 2. Williams, D. B.; Carter, C. B. Transmission Electron Microscopy: Spectroscopy; Springer: New York, 1996; Vol Garvie, L. A. J.; Buseck, P. R. Determination of Ce4+/Ce3+ in electron-beam-damaged CeO2 by electron energy-loss spectroscopy. J. Phys. Chem. Solids 1999, 60 (12), S20