Sanding
Ultrafine Sanding Paper: A Simple Tool for Creating Small Particles Helmut Schäfer,* Claudia Hess, Heinrich Tobergte, Anna Volf, Sachar Ichilmann, Henning Eickmeier, Benjamin Voss, Nikolai Kashaev, Jörg Nordmann, Wajiha Akram, Brigitte Hartmann-Azanza, and Martin Steinhart According to the new Publicly Available Specification (PAS71)[1] presenting the UK view on standards in Europe and at the international level a nanoparticle is: “a particle having one or more dimensions of the order of 100 nm or less”. For most of their potential applications nanoparticles must meet high standards regarding size, size distribution, uniformity and optical properties.[2–4] Considerable efforts have been directed to the development and optimization of synthetic approaches to nanoparticles. However, the practical applicability of most synthetic procedures thus established is restricted by high reaction temperatures, by inevitable use of ultra-pure reactants and solvents, as well as by the necessity of carrying out syntheses under inert gas atmosphere.[2–4] Here we report an amazingly simple, solvent–free approach leading to nanoparticles and sub-micron particles based on grinding bulk raw materials with ultrafine sanding paper. Top-down approaches to create small particles by mechanical forces starting from bulk materials have been explored for thousands of years; particle size reduction down to the micrometer range was performed already 8000 B.C.[5] Mechanical forces applied to bulk specimens first results in plastic deformation until strain reaches a threshold value above which cracks are formed and the material finally breaks into smaller pieces.[6] Consecutive particle size reduction steps based on mechanical impact will lead to smaller and smaller particles. However, as the particles get smaller, the efficiency of the size reductions steps decreases and the portion of the applied mechanical energy converted into heat increases.[7] Thus, top-down procedures carried out in this way mainly yield either micron-sized particles[8,9] or particle ensembles with
Dr. H. Schäfer, C. Hess, H. Tobergte, A. Volf, S. Ichilmann, H. Eickmeier, B. Voss, J. Nordmann, W. Akram, B. Hartmann-Azanza, Prof. M. Steinhart Institute of Chemistry of New Materials and Center of Physics and Chemistry of New Materials Universität Osnabrück Barbarastrasse 7, 49076, Osnabrück, Germany E-mail: [email protected] Dr. N. Kashaev Department Joining and Assessment, Institute of Materials Research Materials Mechanics, Helmholtz-Zentrum Geesthacht Max-Planck-Straße 1, 21502, Geesthacht, Germany DOI: 10.1002/smll.201303930 small 2014, DOI: 10.1002/smll.201303930
broad particle size distributions.[5,8,9] In addition, strong particle agglomeration has turned out to be a serious drawback.[5] To overcome these drawbacks, the ball milling technique was developed, which represents, to the best of our knowledge, the only top-down access to nanoparticles of acceptable quality based on application of mechanical forces on bulk materials. In this procedure powder particles are crashed by plastic deformation due to repetitive compressive loads arising from impacts between metal balls.[10,11] Ball milling is associated with high energy consumption and the properties of the material which has to be crushed have to match the properties of the material from which the balls are made. The applicability of other recently developed strategies for the preparation of small particles starting from bulk solids like electrochemical methods,[12,13] laser vaporization,[14] sonication,[15,16] or stirring and heating in ionic liquids[17] either critically depends on the nature of the bulk material itself[12–16] or on specific interactions between bulk materials and helping agents.[17] Moreover, toxic components like mercury[13] or auxiliary agents[17] are often required. Because of the significant problems associated with state-of-the-art top-down approaches to small particles, the overwhelming share of nanoparticles is synthesized by bottom-up procedures[18–25] including co-precipitation, thermal decomposition, hydro(solvo)thermal synthesis, sol– gel processing and combustion synthesis.[2,26] Some of us have recently reported a solvent-free roomtemperature synthesis for nanocrystalline hexagonal NaYF4 that involves grinding dry powders of Na2CO3, NH4F and rare earth carbonates accompanied by release of ammonia via a solid-state reaction.[27] However, the development of a simple, generic and solvent-free approach to small particles with reasonably narrow size distribution that involves direct separation of the small particles from bulk specimens using a readily available technique has remained challenging. Sanding paper is inexpensive and commercially available at large quantities with a broad range of characteristic roughnesses. To systematically explore grinding with sanding paper as access to small particles, we selected gypsum (CaSO4; Vickers hardness 36) as a relatively soft model material and fluorspar (CaF2; Vickers hardness 189) as a relatively hard model material. These model materials are representative of a wide range of frequently used salts. For instance, the important class of fluoride-based up- and down-converting salts of the type MYF4 has the same crystal structure and almost the same
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
1
communications
H. Schäfer et al.
Figure 1. Photographs of a bulk CaSO4 crystal (top left), a bulk CaF2 crystal (top right) and a conventional delta sander equipped with ultrafine sanding paper grit-60000 (green).
mechanical properties as CaF2.[28–38] Whereas several solventbased approaches to CaF2 nanoparticles were reported,[39–41] nano-crystalline CaSO4[42] has hardly been synthesized. As starting materials we used bulk CaSO4 crystals and bulk CaF2 crystals with sizes in the cm range (Figure 1). X-ray powder diffraction analysis revealed that the bulk CaSO4 crystals consisted of monoclinic CaSO4 (Figure S1) and that the bulk CaF2 crystals consisted of cubic CaF2 (Figure S2). Evaluation of the widths of the reflections using the Scherrer method[43] revealed that the peak widths were determined by device limitations and that the crystallite sizes were beyond 2 microns. The compositions of the CaSO4 and CaF2 crystals were moreover studied by X-ray spectroscopy (Table S1). The sanding paper usually used on common grinding machines like delta sanders or orbital sanders has, depending on the requirements (polishing or grinding), rough outer layers with feature sizes ranging from of a few µm up to a few hundred µm. Typical feature sizes are 40 µm (CAMI grit designation: 320; Figure S3), 30 µm (CAMI grit designation: 400; Figure S4) and around 10–20 µm (CAMI grit designation: 1000; Figure S5). Sanding paper typically used for the polishing of metals is characterized by feature sizes of ∼10 µm (CAMI grit designation: 2000; Figure S6) or of 5–10 µm (CAMI grit designation: 4000; Figure S7). Commercially available ultrafine sanding papers feature roughnesses characterized by length scales of 1–5 µm (CAMI grit designation: 12000; Figure S8), 500 nm-2 µm (grit 40000, Figure S9) and 50–300 nm (CAMI grit designation: 60000; Figure S10). Sanding of bulk CaSO4 and CaF2 crystals was conducted under ambient conditions using a delta sander (Figure 1) and
2 www.small-journal.com
grit-1000 to grit-60000 sanding papers. In general, sanding at low oscillation frequencies ranging from 180 min−1 to 1000 min−1 yielded independent of the sanding paper used particles of poor quality (large particles with diameters of several µm and broad particle size distributions). We obtained significantly better results at high oscillation frequencies in the range from 12000 min−1 to 22000 min−1. Grinding CaSO4 with grit-1000 sanding paper at an oscillation frequency of ∼12000 min−1 yielded lenticular submicron-sized nonagglomerated CaSO4 particles (Figure 2a,b). Image analysis of the scanning electron microscopy (SEM) image displayed in Figure 2a revealed an average apparent particle area of 0.30 µm2 (standard deviation 0.11 µm2; Figure 2c).The anisotropic shape of the CaSO4 particles is represented by a mean aspect ratio (ratio long axis to short axis) of 1.80 (standard deviation 0.16; Figure 2d), a mean length of the long axis of 0.82 µm (standard deviation 0.18 µm, Figure 2e) and a mean length of the short axis of 0.45 µm (standard deviation 0.08 µm; Figure 2f). Aspect ratios, long axes and short axes of the CaSO4 particles were calculated using the program ImageJ (see experimental section) based on the ellipses representing the best fits to the evaluated CaSO4 particles. We tried to reduce the gypsum particle size by using finer sanding paper. However, in the case of CaSO4 rather increasing the oscillation frequency than decreasing roughness feature sizes of the sanding papers led to particle size reduction. Sanding CaSO4 crystals with grit-1000 sanding paper at an oscillation frequency of ∼22000 min−1 yielded gypsum particles often exhibiting rectangular contours (Figure 3a–c). The apparent areas of these particles, as determined by image analysis of a SEM image, amounted to 15341 nm2 (standard deviation 5359 nm2; Figure 3d). Their shape, as apparent in the processed SEM image, was analyzed by means of their circularity. The circularity of an object is a dimensionless shape descriptor calculated as 4 • π • area/ perimeter2. The circularity for a perfect circle is 1 and for a perfect square ∼0.785, whereas infinitely elongated polygons would have a circularity of 0. Thus, the circularity of 0.71 (standard deviation 0.07; Figure 3e) obtained for the CaSO4 particles prepared by sanding with grit-1000 sanding paper at an oscillation frequency of ∼22000 min−1 indicates dominance of slightly elongated polygonal particle shapes. For the harder CaF2 mineral, grit-60000 sanding paper combined with an oscillation frequency of ∼ 12000 min−1 appeared to be the best choice. Sanding with these settings yielded CaF2 nanoparticles with diameters of a few tens of nm, as revealed by SEM investigations (Figure 4a,b) and by transmission electron microscopy (TEM) investigations (Figure 4c,d). The crystallographic phases of the CaSO4 and CaF2 particles obtained by sanding could be elucidated by X-ray diffraction. X-ray patterns obtained by theta/2theta scans in reflection of grit-1000 sanding paper coated with CaSO4 particles obtained at oscillation frequencies of 12000 min−1 and of 22000 min−1 (Figure S11) showed, besides reflections originating from the sanding paper, the reflections expected for monoclinic gypsum (as revealed by a comparison with PDF card 96–101–982). X-ray patterns obtained by theta/2theta scans in reflection of grit-60000 sanding paper coated with CaF2 particles obtained at an oscillation frequency of 12000 min−1 (Figure S12) showed,
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2014, DOI: 10.1002/smll.201303930
Ultrafine Sanding Paper: A Simple Tool for Creating Small Particles
Figure 2. CaSO4 particles obtained by grinding with grit-1000 sanding paper mounted on a delta sander at an oscillation frequency of ∼12000 min−1. a), b) SEM images; c) frequency density of the apparent areas of the CaSO4 particles displayed the SEM image of panel a); d), e) f) frequency densities of d) the aspect ratios, e) the long axes and f) the short axes of the CaSO4 particles appearing in the SEM image of panel a) calculated from the best-fitting ellipses using the program ImageJ.
again besides reflections originating from the sanding paper, the reflections expected for cubic calcium fluoride (as revealed by a comparison with PDF card 01–087–0971). Hence, the particles prepared by sanding consisted of the same phases than the bulk crystals used as starting materials. While the results summarized above were well reproducible, some questions remain open. Grinding of CaF2 at high oscillation rates of 20000 min−1 or 22000 min−1 did not lead to satisfying results. The SEM images of the particles thus obtained (Figure S13 and S14) revealed two problems. The combination of high oscillation frequency and high hardness of the sanded CaF2 crystal resulted in shear forces that partially ripped down the coating from the sanding paper (Figure S13). In addition, at early stages of the sanding process the CaF2 particles are very small (∼10 nm) and tend to agglomerate (Figure S14). The agglomeration may be related to an increase in temperature during grinding. This argument might rationalize the observation that an increase in small 2014, DOI: 10.1002/smll.201303930
oscillation frequency does not necessarily lead to a reduction in particle size. It also remains an open question why grinding with sanding papers having smaller feature sizes does not always lead to smaller and more faceted particles. Therefore, the sanding procedure needs to be optimized for any specific material. However, once a set of suitable parameters (roughness of sanding paper, oscillation frequency) has been identified, simple grinding with sanding paper is a cheap and solvent-free access to small particles with sizes down to some tens of nm and reasonably narrow size distributions.
Experimental Section CaSO4 and CaF2 bulk crystals were supplied by Treibacher Industries (9330 Althofen, Austria). X-ray powder patterns on milled CaSO4 and CaF2 bulk crystals were measured as theta/theta scans in reflection using a PANalytical X’Pert Pro diffractometer operated with Cu
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.small-journal.com
3
communications
H. Schäfer et al.
Figure 3. CaSO4 particles obtained by grinding with grit-1000 sanding paper mounted on a delta sander at an oscillation frequency of ∼22000 min−1. a), b) SEM images and c) TEM image; d) frequency density of the apparent areas of CaSO4 particles obtained by analyzing a representative SEM image and e) corresponding frequency density of the circularities of the analyzed CaSO4 particles.
Kα radiation. During the theta/theta scans the powder samples were rotated about an axis in the scattering plane halving the angle between incident ray and scattered ray. The compositions of the CaSO4 and CaF2 bulk crystals were investigated by X-ray spectroscopy with a Panalytical Axios 1 kW Spectrometer under He atmosphere. Simple wavelength-dispersive spectroscopy (WDX) allowed separating the X-ray lines; the rate of generation of secondary photons can be considered to be roughly proportional to the element concentration. However, detection of light elements by this method is problematic and absolute compositions of samples containing light elements like Ca, O or F determined in this way are somewhat inaccurate. Sanding paper with CAMI grit designations equal or smaller than 12000 was purchased from Albion Alloys Ltd., Canada. Sanding paper with CAMI grit designations equal or larger than 15000 was purchased from Alpha Abrasives, Canada. Grinding was carried out with a delta sander Skil Masters 7120 MA. Sanding paper with an area of about 6 cm2 was mounted on the vibrating plate of the delta sander using double-sided adhesive tape (Figure 1). Grinding was performed by pressing the bulk crystal for 30 seconds against the rotating sanding paper until a total area of ∼4 cm2 was covered with a white layer. X-ray patterns
4 www.small-journal.com
Figure 4. CaF2 particles obtained by grinding with grit-60000 sanding paper mounted on a delta sander at an oscillation frequency of ∼12000 min−1. a), b) SEM images; c), d) TEM images.
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2014, DOI: 10.1002/smll.201303930
Ultrafine Sanding Paper: A Simple Tool for Creating Small Particles
of native sanding paper and of sanding paper coated with either CaSO4 particles or CaF2 particles were obtained by theta/2 theta scans measured in reflection using a PANalytical X’Pert Pro MRD diffractometer equipped with an Eulerian cradle, which was operated with Cu Kα radiation at 40 kV and 40 mA. Specimens for electron microscopy investigations were prepared as follows. The sanding papers coated with CaSO4 particles or CaF2 particles were immersed into 50 ml of deionized water and sonicated for 5 min. Tests revealed that the coating of the sanding paper was stable under these conditions. Only after extended sonication times of 60 minutes and longer the coatings of the sanding papers were partially transferred into the supernatant suspension. However, material originating from the sanding paper could easily be distinguished from the sanded particles in electron microscopy images (Figure S15). The supernatant aqueous suspensions were then diluted by a factor of 10. To prepare specimens for SEM investigations, a droplet of a suspension was deposited onto a piece of a doped silicon wafer with high conductivity. To prepare specimens for TEM investigations, a droplet of a suspension was deposited onto a copper grid coated with a carbon film. SEM images were taken on a Zeiss Auriga scanning electron microscope at an accelerating voltage of 2 kV. SEM images were acquired with a secondary electron detector. For TEM investigations a JEOL JEM-2100 transmission electron microscope operated at 200 kV equipped with a charged-coupled device (CCD)-camera (Gatan) was used. Image analysis of SEM images was performed with the programs Adobe Photoshop 6.0 and ImageJ 1.48 v. At first, particles were separated from the image background using the magic wand tool of Adobe Photoshop 6.0. We employed the “tolerance” setting (maximum difference in pixel intensity of a pixel from that of the directly selected pixel that still leads to incorporation of the former pixel into the selected area) to set the boundary between particle area and image background. For the evaluation presented in Figure 2, tolerance was set to 14, for the evaluation presented in Figure 3 to 55. The image background was replaced by black background (hex color code “000000”). The image thus obtained was converted into a black-and-white image by thresholding. Objects that were no particles, particles that could not be separated from the background sufficiently and pairs of particles that could not be separated from each other were manually removed. Particle size distributions and distributions of particle shape descriptors were then obtained using the program ImageJ 1.48 v.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author. [1] Department of Trade and Industry (DTI) in collaboration with the British Standards Institution (BSI). [2] M. Haase, H. Schäfer, Angew. Chem. Int. Ed. 2011, 50, 5808. [3] F. Wang, X. Liu, Chem. Soc. Rev. 2009, 38, 976. [4] F. Vetrone, J. A. Capobianco, Int. J. Nanotechnol. 2008, 5, 1306. [5] S Deguchi, S.-A Mukai, T. Yamazaki, M. Tsudome, K. Horikoshi, Phys. Chem. C. 2010, 114, 849. [6] H.-l. Yuan, J.-P Xie, A.-Q. Wang, W.-Y. Wang, C. Wang, China Foundary 2007, 4, 194. small 2014, DOI: 10.1002/smll.201303930
[7] Y.-F Maa, S. J. Prestrelski, Curr. Pharm. Biotechnol. 2000, 1, 283. [8] J. L. Perez-Rodriguez, A. Wiewiora, V. Ramirez-Valle, A. Duran, L. A. Perez-Maqueda, J. Phys. Chem. Sol. 2007, 68, 1225. [9] X. Chen, L. Liu, Y. Dong, L. Wang, L. Chen, W. Jaing, Progr. Nat. Sci.: Mater. Int. 2012, 22, 201. [10] C. C. Koch, Annu. Rev. Mater. Sci. 1989, 19, 121. [11] K. T. Paul, S. K. Satpathy, I. Manna, K. K. Chakraborty, G. B. Nando, Nanoscale Res Lett. 2007, 2, 397. [12] Y. C. Liu, L. H. Lin, Electrochem. Comm. 2004, 6, 1163. [13] S. Neveau, R. Massart, V. Rocher, V. Cabuil, J. Phys.: Condens. Matter 2008, 20, 204104. [14] Y. B. Pithawalla, M. S. El-Shall, S. C. Deevi, V. Ström, K. V Rao, J. Phys. Chem. B. 2001, 105, 2085. [15] Z. W. Li, X. J. Tao, Y. M. Cheng, Z. S. Wu, Z. J. Zhang, H. X. Dang, Ultrason. Sonochem. 2007, 14, 88. [16] Z. W Li, X. J. Tao, Y. M. Cheng, Z. S. Wu, Z. J. Zhang, H. X. Dang, Mat. Sci. Eng. A. 2005, 407, 7. [17] B. Rodriguez-Cabo, E. Rodil, H. Rodriguez, A. Soto, A. Arce, Angew. Chem. 2012, 124, 1453. [18] X. Wang, J. Zhuang, Q. Peng, Y. Li, Nature. 2005, 437, 121. [19] Y. Yin, P. Alivisatos, Nature. 2005, 437, 664. [20] B. L. Cushing, V. L. Koleshnichenko, C. O`Connor, J. Chem. Rev. 2004, 104, 3893. [21] M.-C. Daniel, D. Astruc, Chem. Rev. 2004, 104, 293. [22] V. F. Puntes, K. M. Krishnan, A. P. Alivisatos, Science. 2001, 291, 2115. [23] T. S. Ahmadi, Z. L. Wang, T. C. Green, A. Henglein, M. A. El-Sayed, Science. 1996, 272. 1924. [24] F. Wang, Y. Han, R. R. Deng, J. Wang, Q. X. Wang, X. Y. Chen, X. Liu, Nature Mat. 2011, 10, 968. [25] B. Ungun, R. K. Prud`home, S. J. Budijono, J. N. Shan, S. F. Lim, Y. G. Ju, R. Austin, Opt. Express. 2009, 17, 80. [26] W. Lenth, R. M. Macfarlane, J. Lumin. 1990, 45, 346. [27] H. Schäfer, P. Ptacek, H. Eickmeier, M. Haase, Adv. Funct. Mater. 2009, 19, 3091. [28] Y. Zhang, L. Zhang, R. Deng, J. Tian, Y. Zong, D. Jin, X. Liu, J. Am. Chem. Soc 2014, 136, 4893. [29] P. Huang, W. Zheng, S. Zhou, D. Tu, Z. Chen, H. Zhu, R. Li, E. Ma, M. Huang, X. Chen, Angew. Chem. Int. Ed. 2014, 53, 1252. [30] D. Tu, Y. Liu, H. Zhu, R. Li, L. Liu, X. Chen, Angew. Chem. Int. Ed. 2013, 52, 1128. [31] Q. Ju, D. Tu, Y. Liu, R. Li, H. Zhu, J. Chen, Z. Chen, M. Huang, X. Chen, J. Am. Chem. Soc. 2012, 134, 1323. [32] F. Wang, R. Deng, X. Liu, Nature Protoc. 2014, 9, 1634. [33] H. Schäfer, P. Ptacek, O. Zerzouf, M. Haase, Adv. Funct. Mater. 2008, 18, 2913. [34] F. Wang, X. Liu, Acc. Chem. Res. 2014, 47, 1378. [35] H. Schäfer, P. Ptacek, K. Kömpe, M. Haase, Chem. Mater. 2007, 19, 1396. [36] H. Schäfer, P. Ptacek, K. Hickmann, M. Prinz, M. Neumann, M. Haase, Russ. J. Inorg. Chem. 2009, 54, 1914. [37] H. Schäfer, P. Ptacek, H. Eickmeier, M. Haase, J. Nanomater. 2009, 685624. [38] H. Schäfer, P. Ptacek, H. Eickmeier, M. Haase, Cryst. Growth Des. 2010, 10, 2202. [39] N.-N. Dong, M. Pedroni, A. Speghini, J. A. Capobianco, ACS Nano. 2011, 5, 8665. [40] J. Labe´gueriea, P. Gredin, M. Mortier, G. Patriarche, A. de Kozak, Z. Anorg. Allg. Chem. 2006, 632, 1538. [41] G. Wang, Q. Peng, Y. Li, J. Am. Chem. Soc. 2009, 131, 14200. [42] N. Osterwalder, S. Loher, R. N. Grass, T. J. Brunner, L. K. Limbach, S. C. Halim, W. J. Stark, J. Nanopart. Res. 2007, 9, 275. [43] A. L. Patterson, Phys. Rev. 1939, 56, 978.
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: December 27, 2013 Revised: September 1, 2014 Published online:
www.small-journal.com
5
Ultrafine Sanding Paper: A Simple Tool for Creating Small Particles Helmut Schäfer,* Claudia Hess, Heinrich Tobergte, Anna Volf, Sachar Ichilmann, Henning Eickmeier, Benjamin Voss, Nikolai Kashaev, Jörg Nordmann, Wajiha Akram, Brigitte Hartmann-Azanza, and Martin Steinhart According to the new Publicly Available Specification (PAS71)[1] presenting the UK view on standards in Europe and at the international level a nanoparticle is: “a particle having one or more dimensions of the order of 100 nm or less”. For most of their potential applications nanoparticles must meet high standards regarding size, size distribution, uniformity and optical properties.[2–4] Considerable efforts have been directed to the development and optimization of synthetic approaches to nanoparticles. However, the practical applicability of most synthetic procedures thus established is restricted by high reaction temperatures, by inevitable use of ultra-pure reactants and solvents, as well as by the necessity of carrying out syntheses under inert gas atmosphere.[2–4] Here we report an amazingly simple, solvent–free approach leading to nanoparticles and sub-micron particles based on grinding bulk raw materials with ultrafine sanding paper. Top-down approaches to create small particles by mechanical forces starting from bulk materials have been explored for thousands of years; particle size reduction down to the micrometer range was performed already 8000 B.C.[5] Mechanical forces applied to bulk specimens first results in plastic deformation until strain reaches a threshold value above which cracks are formed and the material finally breaks into smaller pieces.[6] Consecutive particle size reduction steps based on mechanical impact will lead to smaller and smaller particles. However, as the particles get smaller, the efficiency of the size reductions steps decreases and the portion of the applied mechanical energy converted into heat increases.[7] Thus, top-down procedures carried out in this way mainly yield either micron-sized particles[8,9] or particle ensembles with
Dr. H. Schäfer, C. Hess, H. Tobergte, A. Volf, S. Ichilmann, H. Eickmeier, B. Voss, J. Nordmann, W. Akram, B. Hartmann-Azanza, Prof. M. Steinhart Institute of Chemistry of New Materials and Center of Physics and Chemistry of New Materials Universität Osnabrück Barbarastrasse 7, 49076, Osnabrück, Germany E-mail: [email protected] Dr. N. Kashaev Department Joining and Assessment, Institute of Materials Research Materials Mechanics, Helmholtz-Zentrum Geesthacht Max-Planck-Straße 1, 21502, Geesthacht, Germany DOI: 10.1002/smll.201303930 small 2014, DOI: 10.1002/smll.201303930
broad particle size distributions.[5,8,9] In addition, strong particle agglomeration has turned out to be a serious drawback.[5] To overcome these drawbacks, the ball milling technique was developed, which represents, to the best of our knowledge, the only top-down access to nanoparticles of acceptable quality based on application of mechanical forces on bulk materials. In this procedure powder particles are crashed by plastic deformation due to repetitive compressive loads arising from impacts between metal balls.[10,11] Ball milling is associated with high energy consumption and the properties of the material which has to be crushed have to match the properties of the material from which the balls are made. The applicability of other recently developed strategies for the preparation of small particles starting from bulk solids like electrochemical methods,[12,13] laser vaporization,[14] sonication,[15,16] or stirring and heating in ionic liquids[17] either critically depends on the nature of the bulk material itself[12–16] or on specific interactions between bulk materials and helping agents.[17] Moreover, toxic components like mercury[13] or auxiliary agents[17] are often required. Because of the significant problems associated with state-of-the-art top-down approaches to small particles, the overwhelming share of nanoparticles is synthesized by bottom-up procedures[18–25] including co-precipitation, thermal decomposition, hydro(solvo)thermal synthesis, sol– gel processing and combustion synthesis.[2,26] Some of us have recently reported a solvent-free roomtemperature synthesis for nanocrystalline hexagonal NaYF4 that involves grinding dry powders of Na2CO3, NH4F and rare earth carbonates accompanied by release of ammonia via a solid-state reaction.[27] However, the development of a simple, generic and solvent-free approach to small particles with reasonably narrow size distribution that involves direct separation of the small particles from bulk specimens using a readily available technique has remained challenging. Sanding paper is inexpensive and commercially available at large quantities with a broad range of characteristic roughnesses. To systematically explore grinding with sanding paper as access to small particles, we selected gypsum (CaSO4; Vickers hardness 36) as a relatively soft model material and fluorspar (CaF2; Vickers hardness 189) as a relatively hard model material. These model materials are representative of a wide range of frequently used salts. For instance, the important class of fluoride-based up- and down-converting salts of the type MYF4 has the same crystal structure and almost the same
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
1
communications
H. Schäfer et al.
Figure 1. Photographs of a bulk CaSO4 crystal (top left), a bulk CaF2 crystal (top right) and a conventional delta sander equipped with ultrafine sanding paper grit-60000 (green).
mechanical properties as CaF2.[28–38] Whereas several solventbased approaches to CaF2 nanoparticles were reported,[39–41] nano-crystalline CaSO4[42] has hardly been synthesized. As starting materials we used bulk CaSO4 crystals and bulk CaF2 crystals with sizes in the cm range (Figure 1). X-ray powder diffraction analysis revealed that the bulk CaSO4 crystals consisted of monoclinic CaSO4 (Figure S1) and that the bulk CaF2 crystals consisted of cubic CaF2 (Figure S2). Evaluation of the widths of the reflections using the Scherrer method[43] revealed that the peak widths were determined by device limitations and that the crystallite sizes were beyond 2 microns. The compositions of the CaSO4 and CaF2 crystals were moreover studied by X-ray spectroscopy (Table S1). The sanding paper usually used on common grinding machines like delta sanders or orbital sanders has, depending on the requirements (polishing or grinding), rough outer layers with feature sizes ranging from of a few µm up to a few hundred µm. Typical feature sizes are 40 µm (CAMI grit designation: 320; Figure S3), 30 µm (CAMI grit designation: 400; Figure S4) and around 10–20 µm (CAMI grit designation: 1000; Figure S5). Sanding paper typically used for the polishing of metals is characterized by feature sizes of ∼10 µm (CAMI grit designation: 2000; Figure S6) or of 5–10 µm (CAMI grit designation: 4000; Figure S7). Commercially available ultrafine sanding papers feature roughnesses characterized by length scales of 1–5 µm (CAMI grit designation: 12000; Figure S8), 500 nm-2 µm (grit 40000, Figure S9) and 50–300 nm (CAMI grit designation: 60000; Figure S10). Sanding of bulk CaSO4 and CaF2 crystals was conducted under ambient conditions using a delta sander (Figure 1) and
2 www.small-journal.com
grit-1000 to grit-60000 sanding papers. In general, sanding at low oscillation frequencies ranging from 180 min−1 to 1000 min−1 yielded independent of the sanding paper used particles of poor quality (large particles with diameters of several µm and broad particle size distributions). We obtained significantly better results at high oscillation frequencies in the range from 12000 min−1 to 22000 min−1. Grinding CaSO4 with grit-1000 sanding paper at an oscillation frequency of ∼12000 min−1 yielded lenticular submicron-sized nonagglomerated CaSO4 particles (Figure 2a,b). Image analysis of the scanning electron microscopy (SEM) image displayed in Figure 2a revealed an average apparent particle area of 0.30 µm2 (standard deviation 0.11 µm2; Figure 2c).The anisotropic shape of the CaSO4 particles is represented by a mean aspect ratio (ratio long axis to short axis) of 1.80 (standard deviation 0.16; Figure 2d), a mean length of the long axis of 0.82 µm (standard deviation 0.18 µm, Figure 2e) and a mean length of the short axis of 0.45 µm (standard deviation 0.08 µm; Figure 2f). Aspect ratios, long axes and short axes of the CaSO4 particles were calculated using the program ImageJ (see experimental section) based on the ellipses representing the best fits to the evaluated CaSO4 particles. We tried to reduce the gypsum particle size by using finer sanding paper. However, in the case of CaSO4 rather increasing the oscillation frequency than decreasing roughness feature sizes of the sanding papers led to particle size reduction. Sanding CaSO4 crystals with grit-1000 sanding paper at an oscillation frequency of ∼22000 min−1 yielded gypsum particles often exhibiting rectangular contours (Figure 3a–c). The apparent areas of these particles, as determined by image analysis of a SEM image, amounted to 15341 nm2 (standard deviation 5359 nm2; Figure 3d). Their shape, as apparent in the processed SEM image, was analyzed by means of their circularity. The circularity of an object is a dimensionless shape descriptor calculated as 4 • π • area/ perimeter2. The circularity for a perfect circle is 1 and for a perfect square ∼0.785, whereas infinitely elongated polygons would have a circularity of 0. Thus, the circularity of 0.71 (standard deviation 0.07; Figure 3e) obtained for the CaSO4 particles prepared by sanding with grit-1000 sanding paper at an oscillation frequency of ∼22000 min−1 indicates dominance of slightly elongated polygonal particle shapes. For the harder CaF2 mineral, grit-60000 sanding paper combined with an oscillation frequency of ∼ 12000 min−1 appeared to be the best choice. Sanding with these settings yielded CaF2 nanoparticles with diameters of a few tens of nm, as revealed by SEM investigations (Figure 4a,b) and by transmission electron microscopy (TEM) investigations (Figure 4c,d). The crystallographic phases of the CaSO4 and CaF2 particles obtained by sanding could be elucidated by X-ray diffraction. X-ray patterns obtained by theta/2theta scans in reflection of grit-1000 sanding paper coated with CaSO4 particles obtained at oscillation frequencies of 12000 min−1 and of 22000 min−1 (Figure S11) showed, besides reflections originating from the sanding paper, the reflections expected for monoclinic gypsum (as revealed by a comparison with PDF card 96–101–982). X-ray patterns obtained by theta/2theta scans in reflection of grit-60000 sanding paper coated with CaF2 particles obtained at an oscillation frequency of 12000 min−1 (Figure S12) showed,
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2014, DOI: 10.1002/smll.201303930
Ultrafine Sanding Paper: A Simple Tool for Creating Small Particles
Figure 2. CaSO4 particles obtained by grinding with grit-1000 sanding paper mounted on a delta sander at an oscillation frequency of ∼12000 min−1. a), b) SEM images; c) frequency density of the apparent areas of the CaSO4 particles displayed the SEM image of panel a); d), e) f) frequency densities of d) the aspect ratios, e) the long axes and f) the short axes of the CaSO4 particles appearing in the SEM image of panel a) calculated from the best-fitting ellipses using the program ImageJ.
again besides reflections originating from the sanding paper, the reflections expected for cubic calcium fluoride (as revealed by a comparison with PDF card 01–087–0971). Hence, the particles prepared by sanding consisted of the same phases than the bulk crystals used as starting materials. While the results summarized above were well reproducible, some questions remain open. Grinding of CaF2 at high oscillation rates of 20000 min−1 or 22000 min−1 did not lead to satisfying results. The SEM images of the particles thus obtained (Figure S13 and S14) revealed two problems. The combination of high oscillation frequency and high hardness of the sanded CaF2 crystal resulted in shear forces that partially ripped down the coating from the sanding paper (Figure S13). In addition, at early stages of the sanding process the CaF2 particles are very small (∼10 nm) and tend to agglomerate (Figure S14). The agglomeration may be related to an increase in temperature during grinding. This argument might rationalize the observation that an increase in small 2014, DOI: 10.1002/smll.201303930
oscillation frequency does not necessarily lead to a reduction in particle size. It also remains an open question why grinding with sanding papers having smaller feature sizes does not always lead to smaller and more faceted particles. Therefore, the sanding procedure needs to be optimized for any specific material. However, once a set of suitable parameters (roughness of sanding paper, oscillation frequency) has been identified, simple grinding with sanding paper is a cheap and solvent-free access to small particles with sizes down to some tens of nm and reasonably narrow size distributions.
Experimental Section CaSO4 and CaF2 bulk crystals were supplied by Treibacher Industries (9330 Althofen, Austria). X-ray powder patterns on milled CaSO4 and CaF2 bulk crystals were measured as theta/theta scans in reflection using a PANalytical X’Pert Pro diffractometer operated with Cu
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.small-journal.com
3
communications
H. Schäfer et al.
Figure 3. CaSO4 particles obtained by grinding with grit-1000 sanding paper mounted on a delta sander at an oscillation frequency of ∼22000 min−1. a), b) SEM images and c) TEM image; d) frequency density of the apparent areas of CaSO4 particles obtained by analyzing a representative SEM image and e) corresponding frequency density of the circularities of the analyzed CaSO4 particles.
Kα radiation. During the theta/theta scans the powder samples were rotated about an axis in the scattering plane halving the angle between incident ray and scattered ray. The compositions of the CaSO4 and CaF2 bulk crystals were investigated by X-ray spectroscopy with a Panalytical Axios 1 kW Spectrometer under He atmosphere. Simple wavelength-dispersive spectroscopy (WDX) allowed separating the X-ray lines; the rate of generation of secondary photons can be considered to be roughly proportional to the element concentration. However, detection of light elements by this method is problematic and absolute compositions of samples containing light elements like Ca, O or F determined in this way are somewhat inaccurate. Sanding paper with CAMI grit designations equal or smaller than 12000 was purchased from Albion Alloys Ltd., Canada. Sanding paper with CAMI grit designations equal or larger than 15000 was purchased from Alpha Abrasives, Canada. Grinding was carried out with a delta sander Skil Masters 7120 MA. Sanding paper with an area of about 6 cm2 was mounted on the vibrating plate of the delta sander using double-sided adhesive tape (Figure 1). Grinding was performed by pressing the bulk crystal for 30 seconds against the rotating sanding paper until a total area of ∼4 cm2 was covered with a white layer. X-ray patterns
4 www.small-journal.com
Figure 4. CaF2 particles obtained by grinding with grit-60000 sanding paper mounted on a delta sander at an oscillation frequency of ∼12000 min−1. a), b) SEM images; c), d) TEM images.
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2014, DOI: 10.1002/smll.201303930
Ultrafine Sanding Paper: A Simple Tool for Creating Small Particles
of native sanding paper and of sanding paper coated with either CaSO4 particles or CaF2 particles were obtained by theta/2 theta scans measured in reflection using a PANalytical X’Pert Pro MRD diffractometer equipped with an Eulerian cradle, which was operated with Cu Kα radiation at 40 kV and 40 mA. Specimens for electron microscopy investigations were prepared as follows. The sanding papers coated with CaSO4 particles or CaF2 particles were immersed into 50 ml of deionized water and sonicated for 5 min. Tests revealed that the coating of the sanding paper was stable under these conditions. Only after extended sonication times of 60 minutes and longer the coatings of the sanding papers were partially transferred into the supernatant suspension. However, material originating from the sanding paper could easily be distinguished from the sanded particles in electron microscopy images (Figure S15). The supernatant aqueous suspensions were then diluted by a factor of 10. To prepare specimens for SEM investigations, a droplet of a suspension was deposited onto a piece of a doped silicon wafer with high conductivity. To prepare specimens for TEM investigations, a droplet of a suspension was deposited onto a copper grid coated with a carbon film. SEM images were taken on a Zeiss Auriga scanning electron microscope at an accelerating voltage of 2 kV. SEM images were acquired with a secondary electron detector. For TEM investigations a JEOL JEM-2100 transmission electron microscope operated at 200 kV equipped with a charged-coupled device (CCD)-camera (Gatan) was used. Image analysis of SEM images was performed with the programs Adobe Photoshop 6.0 and ImageJ 1.48 v. At first, particles were separated from the image background using the magic wand tool of Adobe Photoshop 6.0. We employed the “tolerance” setting (maximum difference in pixel intensity of a pixel from that of the directly selected pixel that still leads to incorporation of the former pixel into the selected area) to set the boundary between particle area and image background. For the evaluation presented in Figure 2, tolerance was set to 14, for the evaluation presented in Figure 3 to 55. The image background was replaced by black background (hex color code “000000”). The image thus obtained was converted into a black-and-white image by thresholding. Objects that were no particles, particles that could not be separated from the background sufficiently and pairs of particles that could not be separated from each other were manually removed. Particle size distributions and distributions of particle shape descriptors were then obtained using the program ImageJ 1.48 v.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author. [1] Department of Trade and Industry (DTI) in collaboration with the British Standards Institution (BSI). [2] M. Haase, H. Schäfer, Angew. Chem. Int. Ed. 2011, 50, 5808. [3] F. Wang, X. Liu, Chem. Soc. Rev. 2009, 38, 976. [4] F. Vetrone, J. A. Capobianco, Int. J. Nanotechnol. 2008, 5, 1306. [5] S Deguchi, S.-A Mukai, T. Yamazaki, M. Tsudome, K. Horikoshi, Phys. Chem. C. 2010, 114, 849. [6] H.-l. Yuan, J.-P Xie, A.-Q. Wang, W.-Y. Wang, C. Wang, China Foundary 2007, 4, 194. small 2014, DOI: 10.1002/smll.201303930
[7] Y.-F Maa, S. J. Prestrelski, Curr. Pharm. Biotechnol. 2000, 1, 283. [8] J. L. Perez-Rodriguez, A. Wiewiora, V. Ramirez-Valle, A. Duran, L. A. Perez-Maqueda, J. Phys. Chem. Sol. 2007, 68, 1225. [9] X. Chen, L. Liu, Y. Dong, L. Wang, L. Chen, W. Jaing, Progr. Nat. Sci.: Mater. Int. 2012, 22, 201. [10] C. C. Koch, Annu. Rev. Mater. Sci. 1989, 19, 121. [11] K. T. Paul, S. K. Satpathy, I. Manna, K. K. Chakraborty, G. B. Nando, Nanoscale Res Lett. 2007, 2, 397. [12] Y. C. Liu, L. H. Lin, Electrochem. Comm. 2004, 6, 1163. [13] S. Neveau, R. Massart, V. Rocher, V. Cabuil, J. Phys.: Condens. Matter 2008, 20, 204104. [14] Y. B. Pithawalla, M. S. El-Shall, S. C. Deevi, V. Ström, K. V Rao, J. Phys. Chem. B. 2001, 105, 2085. [15] Z. W. Li, X. J. Tao, Y. M. Cheng, Z. S. Wu, Z. J. Zhang, H. X. Dang, Ultrason. Sonochem. 2007, 14, 88. [16] Z. W Li, X. J. Tao, Y. M. Cheng, Z. S. Wu, Z. J. Zhang, H. X. Dang, Mat. Sci. Eng. A. 2005, 407, 7. [17] B. Rodriguez-Cabo, E. Rodil, H. Rodriguez, A. Soto, A. Arce, Angew. Chem. 2012, 124, 1453. [18] X. Wang, J. Zhuang, Q. Peng, Y. Li, Nature. 2005, 437, 121. [19] Y. Yin, P. Alivisatos, Nature. 2005, 437, 664. [20] B. L. Cushing, V. L. Koleshnichenko, C. O`Connor, J. Chem. Rev. 2004, 104, 3893. [21] M.-C. Daniel, D. Astruc, Chem. Rev. 2004, 104, 293. [22] V. F. Puntes, K. M. Krishnan, A. P. Alivisatos, Science. 2001, 291, 2115. [23] T. S. Ahmadi, Z. L. Wang, T. C. Green, A. Henglein, M. A. El-Sayed, Science. 1996, 272. 1924. [24] F. Wang, Y. Han, R. R. Deng, J. Wang, Q. X. Wang, X. Y. Chen, X. Liu, Nature Mat. 2011, 10, 968. [25] B. Ungun, R. K. Prud`home, S. J. Budijono, J. N. Shan, S. F. Lim, Y. G. Ju, R. Austin, Opt. Express. 2009, 17, 80. [26] W. Lenth, R. M. Macfarlane, J. Lumin. 1990, 45, 346. [27] H. Schäfer, P. Ptacek, H. Eickmeier, M. Haase, Adv. Funct. Mater. 2009, 19, 3091. [28] Y. Zhang, L. Zhang, R. Deng, J. Tian, Y. Zong, D. Jin, X. Liu, J. Am. Chem. Soc 2014, 136, 4893. [29] P. Huang, W. Zheng, S. Zhou, D. Tu, Z. Chen, H. Zhu, R. Li, E. Ma, M. Huang, X. Chen, Angew. Chem. Int. Ed. 2014, 53, 1252. [30] D. Tu, Y. Liu, H. Zhu, R. Li, L. Liu, X. Chen, Angew. Chem. Int. Ed. 2013, 52, 1128. [31] Q. Ju, D. Tu, Y. Liu, R. Li, H. Zhu, J. Chen, Z. Chen, M. Huang, X. Chen, J. Am. Chem. Soc. 2012, 134, 1323. [32] F. Wang, R. Deng, X. Liu, Nature Protoc. 2014, 9, 1634. [33] H. Schäfer, P. Ptacek, O. Zerzouf, M. Haase, Adv. Funct. Mater. 2008, 18, 2913. [34] F. Wang, X. Liu, Acc. Chem. Res. 2014, 47, 1378. [35] H. Schäfer, P. Ptacek, K. Kömpe, M. Haase, Chem. Mater. 2007, 19, 1396. [36] H. Schäfer, P. Ptacek, K. Hickmann, M. Prinz, M. Neumann, M. Haase, Russ. J. Inorg. Chem. 2009, 54, 1914. [37] H. Schäfer, P. Ptacek, H. Eickmeier, M. Haase, J. Nanomater. 2009, 685624. [38] H. Schäfer, P. Ptacek, H. Eickmeier, M. Haase, Cryst. Growth Des. 2010, 10, 2202. [39] N.-N. Dong, M. Pedroni, A. Speghini, J. A. Capobianco, ACS Nano. 2011, 5, 8665. [40] J. Labe´gueriea, P. Gredin, M. Mortier, G. Patriarche, A. de Kozak, Z. Anorg. Allg. Chem. 2006, 632, 1538. [41] G. Wang, Q. Peng, Y. Li, J. Am. Chem. Soc. 2009, 131, 14200. [42] N. Osterwalder, S. Loher, R. N. Grass, T. J. Brunner, L. K. Limbach, S. C. Halim, W. J. Stark, J. Nanopart. Res. 2007, 9, 275. [43] A. L. Patterson, Phys. Rev. 1939, 56, 978.
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: December 27, 2013 Revised: September 1, 2014 Published online:
www.small-journal.com
5
