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Layered double hydroxides as precursors of metallic nanoparticles and carbon nanostructures

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Caroline Silva de Matos
Total Authors: 1
Document type: Doctoral Thesis
Press: São Paulo.
Institution: Universidade de São Paulo (USP). Conjunto das Químicas (IQ e FCF) (CQ/DBDCQ)
Defense date:
Examining board members:
Vera Regina Leopoldo Constantino; Jivaldo do Rosario Matos; Danilo Mustafa; Jairo Tronto
Advisor: Vera Regina Leopoldo Constantino; Koiti Araki

Layered double hydroxides (LDHs) are materials which chemical versatility regarding layer composition and intercalated species allows the control of properties aiming a wide range of applications. In this way, LDHs based in transition metals are highlighted due to their catalytic and electrochemical activities, besides catalyze the growth of carbon nanostructures through thermal decomposition matrices with organic species intercalated. In this work, it was evaluated the synthesis of LDHs with Ni/Al molar ratio (R) between 2 and 4, intercalated with anions derived from terephthalic (TA) and 2- aminotherephthalic acids (ATA), and carboxymethylcellulose (CMC) polymer. It has been studied the thermal decomposition of these materials and their potential as precursors in the synthesis of composites based in carbonaceous structures, oxides and/or metallic particles. The changing in R values for LDH-TA and LDH-ATA systems modifies its chemical composition and structure, once the decreasing in layers charge density (charge nm-2) with increasing R favors a change in anions arrangement (from perpendicular to parallel) in relation to the layers. Despite this, significative changes were not observed in vibrational spectra (infrared and Raman) and thermal behavior of the materials. The pyrolysis of LDH-TA and LDH-ATA (R equal to 2) in temperature values above 600 °C has resulted in the organic species complete decomposition due to (i) thermal decomposition leading to formation of carbon oxides (CO and CO2) and volatile aromatic compounds; and (ii) carbonaceous species acting as reducing agents in nickel reduction. In turn, LDH-CMC systems characterization has indicated the successful entrapment of polymeric chains into layered matrices, although a moiety of nonintercalated polymer involves the LDH particles. It is still observed that layer charge density decrease causes the segregation of LDH phase intercalated with chloride anions. Pyrolysis of LDH-CMC with R equal 2, in temperature values above 600 °C has led tomaterials containing metallic nickel nanoparticles (Ni-NPs) and carbonaceous structures. The results reveal the presence of spherical Ni-NPs homogeneously dispersed into a carbonaceous matrix and surrounded by graphitic structures. However, in pyrolysis temperatures above 800 °C, particles homogeneity is lost due to their migration and coalescence processes. Furthermore, under pyrolysis temperature increasing, heterogeneous carbon structures were converted in more ordered graphitic forms, as nanoribbons and nano-onions, evidencing catalytic graphitization promoted by metallic nanoparticles. Essentially, LDH-CMC pyrolysis has produced nanocomposites based on nickel nanoparticles and graphitic carbon nanoforms. In this regard, the pyrolysis temperature is a pivotal parameter, once it influences carbothermal reaction and graphitization and coalescence processes. While the metallic nanoparticles catalyze carbon graphitization, the shell surrounding the Ni-NPs protects them against aggregation, preserving their homogeneity in size, shape and distribution. (AU)

FAPESP's process: 14/04816-8 - Development of hierarchical structures with nanoarchitectures based on inorganic layered units
Grantee:Caroline Silva de Matos
Support Opportunities: Scholarships in Brazil - Doctorate (Direct)