Easy Way To Create Stepped Surface: A Thought from Oriented

Sep 6, 2017 - Using an oriented attachment way to create stepped surface instead of an atom-by-atom way is an interesting and promising attempt and ca...
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Easy Way To Create Stepped Surface: A Thought from Oriented Attachment Jin Chen,† Bingyu Lei,† Huan Xie,† Lei Zhou,*,†,‡ and Shenqi Wang†,‡ †

Department of Biomedical Engineering, School of Life Science and Technology, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China ‡ Advanced Biomaterials and Tissue Engineering Center, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China S Supporting Information *

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oriented attachment process. If the initial nanoparticles are too small, they may be “eaten” by large particles with losing their morphology.2−4 An optimized particle size can prevent the defects from disappearing after a long period of recrystallization and interface relaxation during the third step.6−8 In this work, we try to use mesocrystalline hydroxyapatite (HA) and TiO2 as precursors to synthesize materials with a large amount of stepped structures on the surface. This method is feasible and only needs a simple sintering process. In comparison with the usual methods, it does not need a complex control process of supersaturation. Theoretically, the method may be extended to the synthesis of other materials with stepped structure, only if we can obtain their precursors with mesocrystalline structures. The mesocrystalline HA precursors were prepared via a topotactic transformation reaction from CaHPO4 (DCPA) to HA. Experiment details are given in the Supporting Information. The homemade DCPA particles are elongated plates with a thickness of ∼400 nm and a length of ∼40 μm (Figure S1). Their surface is very smooth. There are no prealigned subunits in these plates (Figure S1c). After the topotactic reaction, the resulting product was characterized by XRD and SEM. The XRD pattern (Figure 1a) matches the standard pattern of HA (ICDD 09-0432), suggesting the product is HA. The SEM image (inset of Figure 1a) indicates the overall shape remains after transformation. High magnification SEM and TEM images (Figure 1b and Figure S2a) show that the rough surface is constructed by largely well-arranged spindle-like particles with a width of ∼100 nm. Although these spindle-like particles parallel to each other along the long axis, there are some dislocations and gaps caused by the shrinkage during the topotactic transformation. The SAED pattern (Figure S2b) confirms that the spindle-like particles are crystallographically aligned and with some small misalignments. These mismatches are essential to the formation of stepped structure.2,6,10 The details of the spindle-like particles were further investigated using TEM and SAED. The TEM image (Figure 1c) suggests a spindle-like particle is comprised of smaller and rod-like HA nanoparticles with a width of ∼10 nm and a length of ∼60 nm. The SAED pattern (Figure 1d) suggests that the HA nanocrystals are also crystallographically

riented attachment, referred as a nonclassical and particle-by-particle crystal growth pathway, usually includes three steps. The first step is the collision and rotation of adjacent nanoparticles via Brownian motion, which leads to crystallographic alignment of the nanoparticles.1−3 The second step is the attachment of the aligned nanoparticles, which leads to formation of larger particles.3−5 The third step is the further recrystallization and interface relaxation, which leads to modification of the crystal structure and elimination of the defects.6−8 Some researchers may combine the three steps into two steps.1,9 For either way, it has to be pointed out that due to rough surfaces and diverse shapes of the initial nanoparticles, the alignment during the first step is not always perfect. Small mismatches often occur, which leads to imperfect attachment during the second step1,6,10 and therefore planar defects during the third step.2,4 Many researchers have suggested and proven that oriented attachment is a promising way to produce nanostructure with planar defects.1,4,6,10 Stepped structure, also known as terrace-step structure, has very important effects on surface properties of a material (such as molecular adsorption/ desorption and electron behaviors) because of the low atomic coordination on its step edge and kink sites.11−16 Many methods have been developed to create stepped structure. Most of them are based on classical and atom-by-atom crystal growth pathways.15−18 Usually, these methods need accurate control of supersaturation for layer-by-layer growth and therefore may not be feasible for practical use and massive production. Using an oriented attachment way to create stepped surface instead of an atom-by-atom way is an interesting and promising attempt and can extend the practical usage of oriented attachment. However, the Brownian motion during the first step is random and uncontrollable,1−3 which leads to lack of a general strategy to synthesize various materials with stepped surface. Therefore, it will be better to develop a strategy that can skip over the Brownian motion and starts from crystallographically prealigned nanoparticles. Recently, a new class of hierarchically nanostructured materials, usually called mesocrystals, have drawn a lot of attention,19,20 which are built of crystallographically aligned nanoparticles and can be regarded as the intermediates of oriented attachment process.21−23 This kind of materials well meet the above requirement for the formation of stepped surface. In addition, using mesocrystals gives us more ability to tune the properties of the nanoparticles such as particle size. It has been suggested that the particle size has essential impact on the formation of planner defects during © 2017 American Chemical Society

Received: April 17, 2017 Revised: August 29, 2017 Published: September 6, 2017 7653

DOI: 10.1021/acs.chemmater.7b01522 Chem. Mater. 2017, 29, 7653−7657

Communication

Chemistry of Materials

Figure 2. (a) SEM image of the sample obtained by sintering the mesocrystalline HA precursor at 1100 °C for 1 h; inset, HRSEM image from side view. (b) AFM image. (c) AFM height profile along the dashed red line in panel b.

Figure 1. (a) XRD pattern of the mesocrystalline HA; inset, low magnification SEM image. (b) High magnification SEM image of the mesocrystalline HA; inset, simulated 3D model. (c−e) TEM analysis of a broken (or half) mesocrystalline HA bundle. (c) TEM image and simulated 3D model (inset). (d) SAED pattern. (e) HRTEM image.

to the dimension of one or half of a unit cell. For our system, the step height is too large to originate from an atom-by-atom process.16−18,26 In addition, there is no significant sign of melting at the ceramic surface to provide the necessary supersaturation that is crucial for classical crystal growth. Therefore, it is highly possible that the prealignment of the nanoparticles that compose the mesocrystalline precursors leads to the formation of stepped surface. In order to figure out whether the prealignment of nanoparticles (i.e., the unique structural feature of the mesocrystalline precursor) induces the formation of the stepped structures, nanocrystalline (Figure S5a) and monocrystalline HA (Figure S6a) precursors were synthesized and studied as well. For the nanocrystalline precursor, the building units are needle-like nanoparticles with a width of 10−20 nm and a length of 30−100 nm (Figure S5b). For the monocrystalline precursor, the building unites are micrometer-sized single crystal plates with a length to 10 μm (Figure S6b,c). Both precursors were sintered under the same conditions as those used for the mesocrystalline precursor. In order to mimic the tight assembly of nanocrystals in the mesocrystalline precursor, the nanocrystalline precursor was pressed into a compact dish (Figure S5c) before sintering. The TEM image and SAED pattern (Figure S5d,e) of a piece from the dish suggest that the dish is composed of randomly arranged nanoparticles. After sintering, the crystallization degrees of both samples increase without any sign of generation of impurity (Figures S5a and S6a). The surface of grains is smooth and without any evidence of stepped surface (Figures S5f and S6d). The nanocrystalline precursors were also sintered at different temperatures for different lengths of time. The SEM

aligned and only with small misalignments. The HRTEM image of a single HA nanocrystal (Figure 1e) indicates the elongated direction is the [001] direction of HA crystal.24,25 All above results indicate that the resulting HA plates are built of spindlelike particles, which are bundles of crystallographically prealigned nanoparticles. In order to synthesize HA materials with stepped surfaces, the mesocrystalline HA precursors were sintered at 1100 °C for 1 h. The XRD pattern (Figure S3a) indicates that the sintering process does not change the chemical composition of the precursor and only increase its crystallinity. After sintering, the overall plate-like shape remains, although the surface is rougher (Figure S3b). The rod-like poor-crystallized HA bundles (Figure 1b) in the mesocrystalline precursor disappear. Much larger grains with sizes of ∼1000 nm (Figure 2a) generate, which have regular and equilibrium shapes and are responsible for the roughness changing. Stepped structures are visible on the surface of grains (red circles). The TEM image and SAED pattern clearly indicate that these grains are single-crystallized (Figure S4). The HRSEM image (inset of Figure 2a) clearly displays the morphology of the stepped structure. The planes of each terrace are very smooth. In order to further analyze the details of the stepped surfaces, AFM was carried out. The results (Figure 2b,c) indicate that the interlayer step height is from 10 to 18 nm and the terrace width is from 50 to 100 nm. Usually, a stepped structure is often attributed to a classical crystal growth process. However, the heights of the steps may tell a different story. For the classical crystal growth progress, the step height is usually equal 7654

DOI: 10.1021/acs.chemmater.7b01522 Chem. Mater. 2017, 29, 7653−7657

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Chemistry of Materials images of the products are shown in Figure S7a−i, which clearly show that no stepped structure forms after changing the sintering conditions. The maximum sintering temperature is limited to 1250 °C, because a higher temperature (1300 °C) leads to the formation of Ca3(PO4)2 (Figure S7j). All these results imply that neither crystallinity nor crystal size of the precursor is responsible for the formation of stepped structures. We also tried to prepare HA precursors composed of similar building units with that for the mesocrystalline precursor but with different orderliness (i.e., degree of alignment) by changing the topotactic transformation conditions such as the concentration of NaOH or reaction temperature (Figure S8a,b,d,e,g,h,j,k). After sintering, only the precursors with a certain degree of orderliness can be converted to products with stepped structures (Figure S8c,f,i). When the orderliness decreases (Figure S8b,e,h,k), the stepped structures that can be observed on surface decrease as well (Figure S8c,f,i) and disappear finally (Figure S8l). The results indicate that the prealignment or orderliness of the building units is essential to the formation of the stepped structures. In order to figure out what happened on the mesocrystalline precursor during the sintering process, semi-in situ SEM experiments were carried out. Three samples were taken out when the temperature reaches 300, 700 and 1100 °C. The fourth sample was taken out after 4 h of sintering at 1100 °C. For comparison, the nanocrystalline precursors were processed at the same conditions. SEM results are shown in Figure 3h. For both precursors, with the increase of temperature, the sizes of crystallites increase as well. For each temperature point (i.e., 300, 700 or 1100 °C), the grain sizes of the samples from the mesocrystalline precursor are always much larger than that of the samples from the nanocrystalline precursor. Considering the similarity of the starting sizes between the nanocrystalline

building units for the mesocrystalline (Figure 1c) precursor and that for the nanocrystalline precursor (Figure S5b), it is suggested that for the mesocrystalline precursor, the energy barrier to form larger crystallites is much lower. The conclusion is also supported by the comparison of the linear shrinkage behaviors (Figure S9a) and DSC thermograms (Figure S9b) of the two precursors. For the mesocrystalline precursor, the obvious shrinkage starts at 500 °C; for the nanocrystalline precursor, it starts at 700 °C. In addition, the endothermic effect of the mesocrystalline precursor is lower than the nanocrystalline precursor in the early stage of sintering process (from 550 to 1100 °C). These results confirm that the crystal growth of the mesocrystalline precursor needs much lower driving force (i.e., need to overcome a much lower energy barrier). This advantage origins from the crystallographically oriented alignment of the nanoparticle.3,5 As the temperature increases to 1100 °C, highly crystallized grains with stepped structures (Figure 3g) can be easily found on the surface of the sample from the mesocrystalline precursor. The corresponding AFM result (Figure S10) shows the morphology of the stepped surface and indicates the interlayer step height is from 4.2 to 7.4 nm. After a longer time (4 h) of sintering, the stepped structure for the sample from the mesocrystalline precursor is almost eliminated (Figure 3h), which is caused by further crystallization and interface relaxation.6−8 For the sample from the nanocrystalline precursor (Figure 3a,c,d), highly crystallized grains with a similar size to that of the sample from the mesocrystalline precursor are obtained. However, no stepped structure is found during the sintering process. According to the nucleation theory and based on our results, a formation mechanism of the stepped structure via crystallographically prealigned nanoparticles is proposed and given in Figure 3i (the green route). At the initial stage, the prealigned nanoparticles attach together leaving behind various defects, such as gaps/interspaces.1,6,10 With the nanoparticles’ growth, further attachment and coarsening, the defects accumulate and finally transform to the stepped structure. Stopping sintering at a proper time can terminate the further crystallization and coarsening processes. Hence the metastable stepped structures can be preserved and exposed on the surface. It has to be pointed out that because the stepped structures are metastable, further sintering can eliminate them and result in smooth surface. Our method can be seen as a part of the classical oriented attachment process but without the first step (i.e., collision and rotation). This step is random, uncontrollable and dominated by the Brownian motion. Skipping this step makes the formation of step structure easier. When the nanocrystalline HA precursor is used (the blue route), the initial nanoparticles are crystallographically randomly aligned. The grain sizes of the initial nanoparticles increase by consuming surrounding smaller particles. Atoms or ions transfer from smaller neighbors (high chemical potential) to the large central particle (low chemical potential) through grain boundary growth and solid-state diffusion mechanisms.27 At our sintering conditions, the appropriate supersaturation that is required for the formation of stepped structure does not appear,15−18 so grains with smooth surface formed finally. Our method is demonstrated for HA; however, the used synthetic strategy and mechanism represents a general approach, suggesting that other materials with stepped surface can also be synthesized using their mesocrystalline phase as precursors. In this paper, we also tried to prepare highly crystallized TiO2 square-brick-like particles with stepped

Figure 3. SEM images of samples taken out at different temperatures during the sintering process. (a−d) For the nanocrystalline HA precursor. (e−h) For the mesocrystalline HA precursor. (i) Schematic diagrams of grain growth processes during sintering for both precursors. 7655

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among the nanoparticles. The method is very easy and theoretically applicable to synthesize other materials with stepped surface, only if we can successfully synthesize their mesocrystalline precursors. Stepped structures usually have essential influence on surface properties of a material and may leads to many uncommon chemical and electronical properties. This will help us to improve the performance of these materials in many applications and even develop novel applications.

structures on surface using their mesocrystalline precursor. The mesocrystalline TiO2 precursor was prepared according to a previous top-down method that our group developed.21,28 XRD pattern, SEM and TEM images are given in Figure S11. It is clear that the mesocrystalline TiO2 particle is built of crystallographically prealigned square-like TiO2 nanoparticles with some small misalignments. After being sintered at 800 °C for 1 h, the mesocrystalline TiO2 successfully converted to highly crystallized TiO2 (Figure 4a). The average crystallite size

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01522. Experiment methods, SEM images, TEM images, AFM images, XRD patterns, DSC analysis, linear shrinkages curves (PDF)

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AUTHOR INFORMATION

Corresponding Author

*L. Zhou. E-mail: [email protected]. ORCID

Lei Zhou: 0000-0003-4125-8502 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (No. 51172084) and the Fundamental Research Funds for the Central Universities (No. 2014QN120 and 2016YXMS259). The authors also thank the Analytical and Testing Center of Huazhong University of Science and Technology for providing SEM, TEM, AFM and XRD measurements.

Figure 4. Sample obtained by sintering the mesocrystalline TiO2 precursor at 800 °C for 1 h. (a) XRD pattern; (b) SEM image; (c) HRSEM image.

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increases from ∼25 to ∼55 nm. Under low magnification (Figure 4b and Figure S12a), the overall square shape remains after sintering. Corresponding HRTEM images of the sintered sample are shown in Figure 4c and Figure S12b. Many stepped structures are clearly shown on the surface (red circles). For comparison, commercially available nanocrystalline P25 powders were pressed in a dish and sintered at the same conditions. No stepped structure on surface is observed (Figure S13). It has to be pointed out the shape of the building units of the mesocrystalline TiO2 precursor (square-like) is different to that of the mesocrystalline HA precursor (rod-like). The results indicate that the above proposed formation mechanism of stepped surface (Figure 3i) applies not only to HA materials but also to TiO2 materials without the impact from the shapes of the building units. The unique hierarchical structure of mesocrystals (i.e., prealignment of nanoparticles) is crucial for the formation of stepped surface. In this work, we developed a new method to prepare HA and TiO2 materials with a large amount of stepped structures on the surface. First, mesocrystalline precursors (i.e., crystallographically prealigned nanoparticles) were prepared. Then, sintering is applied to promote the crystallographically oriented attachment among the building nanoparticles. Finally, the stepped structure appears on the surface due to minor misalignments

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DOI: 10.1021/acs.chemmater.7b01522 Chem. Mater. 2017, 29, 7653−7657