Nada Hiroki, Kobayashi Makoto, Kakihana Masato
National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba 305-8569, Japan.
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan.
ACS Omega. 2019 Jun 25;4(6):11014-11024. doi: 10.1021/acsomega.9b01100. eCollection 2019 Jun 30.
Control over TiO rutile crystal growth and morphology using additives is essential for the development of functional materials. Computer simulation studies on the thermodynamically stable conformations of additives at the surfaces of rutile crystals contribute to understanding the mechanisms underlying this control. In this study, a metadynamics method was combined with molecular dynamics simulations to investigate the thermodynamically stable conformations of glycolate, lactate, and 2-hydroxybutyrate ions at the {001} and {110} planes of rutile crystals. Two simple atom-atom distances were selected as collective variables for the metadynamics method. At the {001} plane, a conformation in which the COO group was oriented toward the surface was found to be the most stable for the lactate and 2-hydroxybutyrate ions, whereas a conformation in which the COO group was oriented toward water was the most stable for the glycolate ion. At the {110} plane, a conformation in which the COO group was oriented toward the surface was the most stable for all three hydroxylate ions, and a second most stable conformation was also observed for the lactate ion at positions close to the {110} plane. For all three hydroxylate ions (α-hydroxycarboxylate ions), the stability of the most stable conformation was higher for the {110} plane than for the {001} plane. At both planes, the stability of the most stable conformation was highest for the 2-hydroxybutyrate ion and lowest for the glycolate ion. Supposing that all three hydroxylate ions serve to decrease the surface free energy at the rutile surface and that a more stable conformation at the rutile surface leads to a greater decrease in the surface free energy, the present results partially explain experimentally observed differences in the changes in growth rate and morphology of rutile crystals in the presence of glycolic, lactic, and 2-hydroxybutyric acids.
使用添加剂控制TiO金红石晶体的生长和形态对于功能材料的开发至关重要。关于添加剂在金红石晶体表面热力学稳定构象的计算机模拟研究有助于理解这种控制背后的机制。在本研究中，将元动力学方法与分子动力学模拟相结合，以研究乙醇酸根、乳酸根和2-羟基丁酸根离子在金红石晶体{001}和{110}平面上的热力学稳定构象。选择了两个简单的原子间距离作为元动力学方法的集体变量。在{001}平面上，发现乳酸根和2-羟基丁酸根离子最稳定的构象是COO基团朝向表面，而乙醇酸根离子最稳定的构象是COO基团朝向水。在{110}平面上，对于所有三种羟基酸根离子，COO基团朝向表面的构象最稳定，并且在靠近{110}平面的位置，乳酸根离子还观察到第二稳定的构象。对于所有三种羟基酸根离子（α-羟基羧酸根离子），{110}平面上最稳定构象的稳定性高于{001}平面。在两个平面上，最稳定构象的稳定性对于2-羟基丁酸根离子最高，对于乙醇酸根离子最低。假设所有三种羟基酸根离子都用于降低金红石表面的表面自由能，并且金红石表面更稳定的构象导致表面自由能的更大降低，本研究结果部分解释了在乙醇酸、乳酸和2-羟基丁酸存在下金红石晶体生长速率和形态变化的实验观察差异。
