Article pubs.acs.org/JPCC
Heat Transport between Au Nanorods, Surrounding Liquids, and Solid Supports Jonglo Park,*,† Jingyu Huang,‡ Wei Wang,† Catherine J. Murphy,‡ and David G. Cahill† Departments of †Materials Science and Engineering and ‡Chemistry, University of Illinois, Urbana, Illinois 61801, United States ABSTRACT: We report experimental studies of heat transport for a system of Au nanorods immobilized on a crystalline quartz support and immersed in various organic fluids. The Au nanorods are abruptly heated by a subpicosecond optical pulse; the cooling of the Au nanorods is monitored by transient absorption. We analyze the data using a three-dimensional model that describes heat flow between the nanorod and fluid with an additional interface thermal conductance added to account for heat transport between the Au nanorods and the high thermal conductivity support. For methanol, ethanol, toluene, and hexane, the thermal conductance of the nanorod/fluid interface falls within a narrow range: 36 ± 4 MW m−2 K−1 for methanol, 32 ± 6 MW m−2 K−1 for ethanol, 30 ± 5 MW m−2 K−1 for toluene, and 25 ± 4 MW m−2 K−1 for hexane.
I. INTRODUCTION The production of large temperature excursions confined to nanoscale dimensions requires both a nanometer-scale source of heat and an extremely high intensity of heat.1,2 High intensity, nanoscale heat sources are typically created by the absorption of short duration laser pulses by metallic nanoparticles. Fast temperature excursions, that is, fast time scales for the heating and cooling of the nanoparticle, localizes the temperature field to the thermal diffusion distance in the surrounding matrix. On short time scales, the cooling rate of the metal nanoparticle and the temperature rise in the surroundings are controlled by the thermal conductance G of the nanoparticle/matrix interface.3 Understanding of heat transport between a heated nanoparticle and its surroundings is a necessary first step in designing nanoscale heat sources for applications in nanotechnology and thermally based medical therapies. In the past several years, systematic studies of photothermal therapy have been carried out using various nanoparticle geometries, for example, gold-silica nanoshells, Au nanorods, and colloidal Au nanoparticles.4−6 The temperature of Au nanoparticles heated by a 1 kHz pulsed laser and phase transformation in surrounding water was investigated using synchrotron-based X-ray diffraction.7 Experiments on the thermal conductance of interfaces between solid and fluids have typically employed suspensions of metal nanoparticles in water or organic solvents.3 This approach greatly limits, however, the types of interfaces that can be investigated because of the experimental requirement of a homogeneous stable suspension. For example, metal nanoparticles are typically made soluble in water by adsorbed surfactants; the interfaces that are studied in this case are relatively thick and include the thermal conductance of the metal/surfactant interface, the surface molecules, and water added in series. The thermal conductance of the interface in this case is typically large, 100 < G < 200 MW m−2 K−1.8 More © 2012 American Chemical Society
recently, the interface thermal conductance G of Au nanorods covered with hexadecyltrimethylammonim bromide (CTAB) was reported to be 130 < G < 450 MW m−2 K−1, with G decreasing with increasing CTAB concentrations.9 A subsequent study of Au nanorods covered with various ligands, mercaptoundecanoic acid (MUDA), mercaptohexadecanoic acid (MHDA), and thiolated polyethylene glycol (PEG), found G = 175 ± 75 MW m−2 K−1, 163 ± 35 MW m−2 K−1, and ∞, respectively.10 Alkane-thiol terminations can be used to make metal nanoparticles soluble in organic solvents. For metal nanoparticles suspended in toluene, G is significantly smaller than in water, G ≈ 15 MW m−2 K−1, but the uncertainties in these measurements were large (see the discussion in section 4 of ref 8). We have previously studied the thermal conductance of a hydrophobic interface with water using a planar metal film coated by a self-assembled monolayer.11 We have not been able to use this approach to study interfaces with organic fluids because the small thermal effusivity of organic fluids, that is, the square root of the product of the thermal conductivity and heat capacity per unit volume, greatly reduces the sensitivity of the experiment to the thermal conductance of the interface. (The thermal effusivity of a typical organic solvent is a factor of ≈3 smaller than the thermal effusivity of water.) Here, we describe a new approach and provide new data for the thermal conductance of interfaces with organic fluids using CTABcoated Au nanorods dispersed on a solid, high thermal conductivity support. The high thermal conductivity support has the added benefit of reducing the accumulation of heat from the high repetition rate (80 MHz) laser that we use for the transient absorption experiments. Received: August 15, 2012 Revised: October 24, 2012 Published: November 29, 2012 26335
dx.doi.org/10.1021/jp308130d | J. Phys. Chem. C 2012, 116, 26335−26341
The Journal of Physical Chemistry C
Article
4800, Germany). The SEM sample was coated with ≈1.5 nm Pt film to avoid charging. To characterize the size and shape of the nanorods, we used transmission electron microscopy (TEM, JEOL 2100 Cryo, Japan). The TEM sample was prepared by placing a drop of Au nanorod solution diluted by a factor of 50 on a carbon-coated copper grid. For transient absorption measurements, we use a modelocked Ti:sapphire laser that produces a series of
