Hans Journal of Nanotechnology
Vol.07 No.02(2017), Article ID:20489,8 pages
10.12677/NAT.2017.72003

Based on Microcalor Technology to Research in Situ Growth Thermodynamics and Mechanism of CaMoO4 Nanocake

Jiangyuan Qiu1, Gaochao Fan1, Ting Wan1, Biyuan Xiao1, Zaiyin Huang1,2,3*

1College of Chemistry and Chemical Engineering, Guangxi University for Nationalities, Nanning Guangxi

2Key Laboratory of Forest Chemistry and Engineering, Guangxi University for Nationalities, Nanning Guangxi

3Key Laboratory of Food Safety and Pharmaceutical Analytical Chemistry, Guangxi Colleges and Universities, Nanning Guangxi

Received: Apr. 21st, 2017; accepted: May 5th, 2017; published: May 11th, 2017

ABSTRACT

In recent years, great progress has been made to investigate the growth process of nanomaterials, and its growth mechanism has also been expanded by various methods. However, there are few methods to acquire the instantaneous thermodynamics information via in situ. In this paper, we use a RD 496-III microcalorimeter with high precision and high sensitivity to obtain the in situ heat flow curves in the growth process of CaMoO4 nanocake. The growth parameters of CaMoO4 nanocake were acquired by thermodynamics equation to investigate the energy and morphology evolution of the CaMoO4 growth mechanism. Results showed that the growth process of CaMoO4 nanocake had gone through two stages, including the phase nucleation and the crystal growth, whose nucleation rate and crystal growth rate were 1.57 ´ 10−3 s−1 and 3.50 ´ 10−3 s−1 respectively at 298.15 K. Moreover, the relationships between the rate of heat change in situ growth process and the ion concentration, crystal growth, dissolution, gathered, and diffusion have been analyzed in-depth. This paper is aimed to provide an effective method to study the in situ crystal growth, and further to build the foundation for thermal dynamic information of non-equilibrium growth process of nanomaterials in physical chemistry development.

Keywords:Nanomaterials, Thermokinetics, In-Situ Growth, Microcalorimetry Techniques

基于微量热技术研究钼酸钙纳米饼原位生长的热动力学及机理

邱江源1,范高超1,万婷1,肖碧源1,黄在银1,2,3*

1广西民族大学化学化工学院,广西 南宁

2广西林产化学与工程重点实验室,广西 南宁

3广西高校食品安全与药物分析化学重点实验室,广西 南宁

收稿日期:2017年4月21日;录用日期:2017年5月5日;发布日期:2017年5月11日

摘 要

近年来,对纳米材料的生长过程研究取得了重大的进步,同时也发展了多种方法研究其生长机理。但鲜有方法能够原位的获取纳米材料生长过程的瞬时变化热动力学信息。本文利用高精度、高灵敏度的RD496-III型微量热计获得了CaMoO4纳米饼的生长过程的原位特征热谱信息,结合热动力学方程获取了CaMoO4纳米饼生长的热动力学特征生长参数阐述了其原位生长机理;CaMoO4纳米饼的生长过程经历了反应成核与晶体生长两个阶段,在298.15 K时其反应成核速率和晶体生长速率分别为1.57 × 10−3 s−1、3.50 × 10−3 s−1,以及深刻的诠释了原位生长过程产生的热变速率与溶液中离子浓度、晶体的成长、溶解、聚集、扩散之间的关系。本文旨在为研究纳米材料原位生长机理提供新的技术方法,进一步为纳米材料非平衡生长过程的热动力学信息的物理化学发展奠定了基础。

关键词 :纳米材料,热动力学,原位生长,微热量技术

Copyright © 2017 by authors and Hans Publishers Inc.

This work is licensed under the Creative Commons Attribution International License (CC BY).

http://creativecommons.org/licenses/by/4.0/

1. 引言

随着纳米科技的快速发展,纳米材料的制备和性能研究已取得了引人注目的成就 [1] - [8] 。但是从本质上原位研究纳米材料生长过程,高精度可控地重复长出满足特定需求的纳米材料和纳米结构,进而完善纳米材料生长过程理论仍然是一个艰巨的挑战。与普通材料相比,纳米材料有其独特的生长特点。它常常在微小的纳米受限空间非平衡态下发生,在纳米受限空间受到的热力学和动力学的共同作用大不同于无限体系;在不同常规的生长参数及环境体系中,存在取向生长、极性生长和择优生长等多种可能,体系内任何因素产生的微小扰动都有可能导致完全不同的生长结果,这完全不同于传统晶体的生长规律及特点。纳米材料最终形貌的形成必然与生长过程相联系,而生长过程的可变性导致了纳米材料维度、形貌的多样性。因此,要真正实现纳米材料的可控生长必须对纳米材料生长过程进行原位研究。

迄今,原位研究纳米材料生长过程的主要方法有:① 原位电镜技术 [9] [10] [11] [12] [13] 、②扫描隧道显微镜实时观测技术 [14] [15] 、③ 椭圆偏振技术 [16] [17] 、④ 原位X射线表征技术 [18] [19] 、⑤ 原位光谱技术 [20] [21] 、⑥ 原位石英晶体微天平技术 [22] 、⑦ 原位扫描电镜技术 [23] 、⑧ 原位透射电镜技术 [24] [25] 、⑨ 原子隧道显微镜技术 [26] 。

上述方法各有其独特的优点,但它们均不能用常规的生长参数(如浓度、温度等)来描述纳米材料非平衡态生长全过程的瞬时动态信息,包括化学反应、结晶成核、晶体长大和形貌演变等。因而无法解释生长过程的差异性与产物最终形貌之间存在的必然联系,当前,多数情况下对纳米材料生长机理的解释还只是推测的结果。因此寻找能获取纳米材料生长过程瞬时动态的热动力学信息的原位测量技术是非常必要的。

本文采用高精度、高灵敏度的RD496-III型微量热计对CaMoO4纳米饼的生长过程进行了原位在线动态监测。通过对CaMoO4纳米饼生长过程特征热谱曲线的分析,获得了其非平衡态生长过程的热力学及动力学信息。将特征热谱曲线与CaMoO4纳米饼的形貌及维度相关联,并结合热动力学信息提供的特征生长参数,对CaMoO4纳米饼的生长机理进行了诠释。

2 实验部分

2.1. 实验试剂与仪器

Na2MoO4·2H2O、CaCl2、TritonX-100、正辛醇、环己烷(A.R.,汕头西陇化工股份有限公司)。

集热式磁力搅拌器(DF-2,金坛市科析仪器有限公司),原位微量热计(RD496-III,绵阳中物热分析仪器有限公司),微量进样器(1000 μL,沈阳市北华医材有限公司)。

2.2 原位生长实验过程

1)室温搅拌条件下,将TritonX-100、正辛醇、油相环己烷按体积比3/2/8配置成透明的混合物,取浓度为0.64 M的Na2MoO4水溶液1.76 mL滴入上述透明混合物中制备成微乳液A;将TritonX-100、正辛醇、环己烷按体积比3/2/8配置成透明的混合物,取浓度为0.08 M的CaCl2水溶液1.76 mL滴入上述透明混合物中制备成微乳液B。将微乳液A和B分别封膜,待用。

2)原位生长实验在RD496-III型微量热计中进行,用微量进样器取1 mL上述制备的微乳液A和微乳液B分别移内外玻璃套管中并用保鲜膜封装,再将套管放入不锈钢水热反应釜中,放入微量热计主体,设置反应参数并恒温25℃;待基线稳定后将玻璃内管捅破,使是微乳液A和微乳液B混合发生反应,同时微量热计原位在线测量反应过程的热力学和动力学信息。反应完成后,用丙酮、无水乙醇及蒸馏水多次洗涤、分离。最后将所得产物置于真空干燥箱中室温干燥6 h后进行表征。

2.3. 表征

产物的表征分别在X射线衍射仪(XRD,Philips PW 1710,Cu Kα射线,λ = 1.5406 Å)和场发射扫描电镜(FE-SEM, JEOL JSM-6700F)上进行。

3. 结果与讨论

3.1. 产物的物相及形貌表征

图1为所制备产物的XRD图谱,XRD图中所有的衍射峰均与白钨矿结构四方晶系CaMoO4(JPCDS卡片No. 29-0351)相对应,其晶胞参数为a = 5.19 Å和c = 11.25 Å。衍射图中较高的衍射强度和尖锐的衍射峰,表明所得产物的结晶性较好。图中未见有杂质峰,表明晶型单一、晶体较纯。

图2为产物的扫描电镜照片。图2(a)为产物的低倍放大照片,由图可知产物的形貌为圆饼状,大面积的尺寸和形貌分布高度均一的这种结构被制得。图2(b)为产物的高倍放大照片,进一步证实了产物为饼状结构,其平均直径约300 nm,厚度约60 nm。

3.2. 热谱曲线与热变速率曲线

图3为298.15 K下CaMoO4纳米饼原位生长的特征热谱曲线。由图中可知,基线稳定后(即图中从t =

Figure 1. Typical XRD pattern of the as-prepared products nanocakes

图1. 所制备产物的XRD图谱

Figure 2. SEM images of the as-prepared CaMoO4

图2. 所制备的CaMoO4纳米饼的扫描电镜照片

Figure 3. Microcalorimetric heat flow curve of in situ growth for CaMoO4 nanocakes at 298.15 K

图3. 298.15 K 下CaMoO4纳米饼原位生长的特征热谱曲线

0 s至t = 1600 s),将小样品池捅破,大、小样品池里的微乳液发生混合,同时被包裹的含Ca2+的水溶液亦相互混合而开始发生反应。从t = 1600 s开始,过程瞬时放热迅速增大到第一个极大值(t = 1815 s),随后放热减缓,曲线开始下降,直到放热曲线下降至t = 2884 s时,下降趋势停止。从t = 2884 s开始,过程瞬时放热迅速增大到第二个极大值(t = 2980 s,第二个极大值的峰值远小于第一个极大值的峰值),之后曲线又缓慢下降,在t = 3500 s以后,放热速率变得平缓,曲线开始变得平稳,并且持续很长时间,最终得到平稳曲线。

图4为298.15 K下CaMoO4纳米饼原位生长的热焓变化率曲线,利用该图可分析CaMoO4纳米饼原位生长过程的热量变化速率。由图可知,反应初始阶段,放热变化速率由慢到快,该过程进行非常迅速,在t = 1815 s到达放热速率的第一个极大值,随后放热变化速率相对较快的减小直到t = 2235 s,从t = 2235 s 至t = 2884 s 放热变化速率非常缓慢的减小。从t = 2884 s开始,放热变化速率又突然增加,在t = 2980 s时到达放热速率的第二个极大值,随后放热变化速率相对较快的减小直到t = 3500 s。以后,放热速率基本保持不变并趋于零,直到整个生长过程结束。

由特征热谱曲线可知,用反相微乳液法合成CaMoO4纳米饼不非一步完成。依据微乳液微反应的反应性能,结合反应过程能量的变化,推测出该条件下用反相微乳液法合成CaMoO4纳米饼大致分为2个阶段,反应成核阶段 (t =1600 s → t = 2884 s)及晶体生长阶段 (t = 2884 s → t =7000 s)。

3.3. CaMoO4纳米饼原位生长的热动力学数据

表1为CaMoO4纳米饼生长过程中的反应成核阶段及晶体生长阶段的部分原位热动力数据。采用原位微量热法并结合热动力学理论,获取了经历时间t时,CaMoO4纳米饼生长过程的反应热分数(Hi/H)及在t时刻的热变化速率(dH/dt)。

依据原位微量热实验所得数据,利用如下热动力学方程 [27] 可求出不同阶段的反应速率常数k。

式中,H为整个反应的焓变;dHi/dt为热焓变化率;n为反应级数;Hi为反应经历时间t时的焓变;

Figure 4. Enthalpy variation curve of in situ growth for CaMoO4 nanocakes at 298.15 K

图4. 298.15 K下CaMoO4纳米饼原位生长的热焓变化率曲线

Table 1. In situ thermokinetic data of growth process of CaMoO4 nanocakes

表1. CaMoO4纳米饼生长过程的原位热动力数据

k 为用转化率表示的速率常数,其单位为s−1。用最小二乘法将反应的热动力学数据进行回归,即得到反应速率常数 k。

最终,可求得298.15 K时,CaMoO4纳米饼的反应成核速率为1.57 × 10−3 s−1 (R = 0.9999),晶体生长速率为3.50 × 10−3 s−1 (R = 0.9997)。

3.4. 钼酸钙纳米饼的原位生长机理

依据图3中的特征热谱曲线可知,CaMoO4纳米饼原位生长过程可分为两个阶段:反应成核阶段和晶体生长阶段。第一阶段从t =1600 s 至t = 2884 s,该过程一直放热,并且出现较大的放热峰,主要原因为:① 从当将小样品池捅破后,微乳液液滴之间碰撞融合而大量放热[28],同时伴随胶束融合而吸收相对较少的热量用以克服油水界面膜的强度,因此整体表现为较多的放热;② 由于胶束融合后,微乳液中包裹的水溶液相互混合,Ca2+和MoO42−迅速发生反应,生成CaMoO4分子而大量放热;③ 在布朗运动作用下水核中的产物分子逐渐达到过饱和状态,分子开始成核而吸热,因此在放热曲线上出现较大的放热峰,成核过程很快使放热减缓。第二阶段从t = 2884 s 至t = 7000 s,过程也一直放热,但放热的强度远小于第一阶段,出现较小的放热峰。主要原因为:晶体生长是典型的放热过程,但由于生成的晶核粒子大小不一,依据Ostwald熟化机制[29],小粒子的溶解度大而溶解,大粒子的溶解度小而不断长大,小颗粒溶解将产生吸热效应,而抵消部分放热效应,因此在放热曲线上出现较小的放热峰。随着结晶过程的进行,晶体开始择优生长,生长过程中晶体聚集与扩散会发生相互竞争,从而使某些晶面的生长受到了限制,减慢了晶体生长过程的进行,所以在t = 3500 s后放热速率变缓直到曲线平稳。

基于以上分析,将CaMoO4纳米饼原位生长过程的特征热谱曲线与其形貌及维度关联,提出了CaMoO4纳米饼的形貌演变可能的机理,可概括为“成核-导向连接-溶解在结晶”的动力学控制机制,过程如图5所示。过程(A):微乳液A与微乳液B混合后,Ca2+开始反应,生成CaMoO4单体。产物分子逐渐达到饱和状态后发生均相成核,并聚集成球形小颗粒。过程(B):依据Ostwald熟化机制,球形小颗粒逐渐生长变大,相邻的球形颗粒与生长速率较快的晶面相结合为主,并通过导向连接形成不

Figure 5. CaMoO4 nano cake growth mechanism schematic

图5. CaMoO4纳米饼生长机理示意图

致密的饼状前驱体。过程(C):产物溶液在温和的室温条件下陈化,饼状前驱体通过溶解再结晶过程逐步形成致密且结晶性良好的纳米饼。

纳米饼状形貌的形成主要取决于晶体生长阶段,当然也与作为软模板的微乳液相关。只有进一步对产物的晶体性质进行表征,并结合晶体学相关的机制才能推测其可能的形成机理。因此,采用原位微量热技术获得纳米材料生长过程的实时动态信息,结合原位电镜技术获取材料形貌演变过程的直观图像信息两者相结合的方法研究纳米材料原位生长机理是一个重要的途径。

4. 结论

采用高精度、高灵敏度的RD496-III型微量热计对CaMoO4纳米饼的生长过程进行了原位在线监测。通过对CaMoO4纳米饼生长过程特征热谱曲线的分析,获得了其非平衡态生长过程的热力学及动力学参数;在298.15 K时其反应成核速率和晶体生长速率分别为1.57 × 10−3 s−1、3.50 × 10−3 s−1,将特征热谱曲线与CaMoO4纳米饼的形貌及维度相关联,结合热动力学信息提供的特征生长参数,提出了CaMoO4纳米饼可能的生长机理并;CaMoO4纳米饼原位生长过程可分为两个阶段:反应成核阶段和晶体生长阶段,并对CaMoO4纳米饼的生长机理进行了诠释。

基金项目

国家自然科学基金(20963001,21273050,21573048)广西民族大学研究生教育创新项目(gxun- chxzs2016114)资助。

文章引用

邱江源,范高超,万 婷,肖碧源,黄在银. 基于微量热技术研究钼酸钙纳米饼原位生长的热动力学及机理
Based on Microcalor Technology to Research in Situ Growth Thermodynamics and Mechanism of CaMoO4 Nanocake[J]. 纳米技术, 2017, 07(02): 21-28. http://dx.doi.org/10.12677/NAT.2017.72003

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  30. NOTES

    *通讯作者。

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