Applied Surface Science 347 (2015) 461–470 Contents lists available at ScienceDirect Applied Surface Science jou rn al h om ep age: www.elsevier.com/locate/apsusc Magnetic colloid by PLA: Optical, magnetic and thermal transport properties B.K. Pandey ∗ , A.K. Shahi, Ram Gopal Laser Spectroscopy and Nanomaterials Lab, Department of Physics (UGC-CAS), University of Allahabad, Allahabad 211002, India a r t i c l e i n f o Article history: Received 19 November 2014 Received in revised form 5 February 2015 Accepted 6 April 2015 Available online 28 April 2015 Keywords: Optical band gap engineering Magnetic property Ferrofluids Thermal conductivity a b s t r a c t Ferrofluids of cobalt and cobalt oxide nanoparticles (NPs) have been successfully synthesized using liquid phase-pulse laser ablation (LP-PLA) in ethanol and double distilled water, respectively. The mechanism of laser ablation in liquid media and formation process for Co target in double distilled water (DDW) and ethanol are speculated based on the reactions between laser generated highly nascent cobalt species and vaporized solvent media in a confined high temperature and pressure at the plume-surrounding liquid interface region. Optical absorption, emission, vibrational and rotational properties have been investigated using UV–vis absorption, photoluminescence (PL) and Fourier transform-infra red (FT-IR) spectroscopy, respectively. In this study optical band gap of cobalt oxide ferrofluids has been engineered using different pulse energy of Nd:YAG laser in the range of (2.80–3.60 eV). Vibrating sample magne- tometer (VSM) is employed to determine the magnetic properties of ferrofluids of cobalt and cobalt oxide NPs while their thermal conductivities are examined using rotating disc method. Ferrofluids have gained enormous curiosity due to many technological applications, i.e. drug delivery, coolant and heating purposes. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Cobalt oxide has extensively used in lithium ion batteries, heterogeneous catalysts, gas sensing, ceramic pigments, and elec- trochemical devices [1,2]. In the current research cobalt oxide has attracted new efficient catalyst for water oxidation [3]. Cobalt oxide is well known ferromagnetic semiconductor which has attracted a lot of interest due to their potential applications in spin electronics and in memory devices [4]. Nanofluids are a relatively new class of fluids which contain engineered colloidal nanoparticles (1–100 nm) dispersed in the base fluid. The base fluid is usually a conventional fluid like water, glycol, etc. Ferrofluids or magnetic nanofluids are the colloidal sus- pensions related to the surface modified magnetic nanoparticles (2–20 nm) dispersed in the base fluid. Ferrofluids have numer- ous applications in biomedical categories: the cancer treatment by hyperthermia [5], MRI contrast agent [6], drug delivery [7], DNA hybridization [8] and cell separation [9]. The thermal trans- port properties of colloidal magnetic ferrofluids nanoparticles were reported previously [10]. A small volume fraction of metal or metal ∗ Corresponding author. Tel.: +91 532 2460764; fax: +91 532 2460993. E-mail addresses: bishnu.pandey750@gmail.com, bknanophysics@gmail.com (B.K. Pandey). oxide powder in base fluid increases the thermal conductivity of the particle fluid mixtures over those of the base fluid. In the present study thermal conductivity of colloidal cobalt oxide NPs has been investigated against temperature in water as a base fluid. Thermal conductivity of nanoparticles–fluid mixtures has been reported by Ravi et al. [11]. The increased thermal con- ductivity of nanofluids has gained huge interest in industry for various applications such as heat transfer, including microelec- tronics, fuel cells, pharmaceutical processes, and hybrid-powered engines. Many chemical methods have been reported for the syn- thesis of cobalt and cobalt oxide nanoparticles, but physical route especially liquid phase pulse laser ablation (LP-PLA) has its own importance because of one step process without the aid of any expensive and toxic precursors and solvents. Here we report novel approach for the synthesis of cobalt and cobalt oxide ferrofluids in colloidal form using LP-PLA. This method became very popular due to environment friendly and high purity product. Structural investigations, band gap engineering using dif- ferent laser pulse energy, magnetic and thermal properties have been discussed in detail. 2. Experimental The experimental details for the synthesis of cobalt and cobalt oxide NPs are same as described earlier [12–15]. The slice of thin cobalt rod of high purity (99.99%) (Spec pure, Johnson Matthey), http://dx.doi.org/10.1016/j.apsusc.2015.04.045 0169-4332/© 2015 Elsevier B.V. All rights reserved.