The physical and optical investigations of the tannic acid functionalised Cu-based oxide nanostructures
Nurul Akmal Che Lah, Puhanes Murthy, Mohd Nashrul Mohd Zubir
Abstract
Abstract The need for a mild, low-cost, green environment that is able to produce exotic properties of output nanostructures is appealing nowadays. Employing these requirements, the copper (Cu)—based oxide nanostructures have been successfully synthesised via one-pot reaction using biocompatible natural polyphenol, tannic acid (TA) as both the reducing agent and stabiliser at 60, 70 and 80 °C. The structural and optical studies disclosed the effect of TA on the surface morphology, phase purity, elemental composition, optical microstrain and optical intrinsic energy of this mixed Cu 2 O and CuO nanostructures. The optically based method describes the comparative details of the multi-band gap in the presence of more than one element with overlapping spectra from the first-derivative absorbance curve $$\frac{\Delta E}{\Delta A}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mfrac> <mml:mrow> <mml:mi>Δ</mml:mi> <mml:mi>E</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>Δ</mml:mi> <mml:mi>A</mml:mi> </mml:mrow> </mml:mfrac> </mml:math> and the exponential absorbance of Urbach tail energy $${E}_{U}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>E</mml:mi> <mml:mi>U</mml:mi> </mml:msub> </mml:math> towards the conventional Tauc bandgap. The $$\frac{\Delta E}{\Delta A}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mfrac> <mml:mrow> <mml:mi>Δ</mml:mi> <mml:mi>E</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>Δ</mml:mi> <mml:mi>A</mml:mi> </mml:mrow> </mml:mfrac> </mml:math> demonstrates that the pronounced effect of TA that Cu 2 O and CuO nanostructures creates much sensitive first-derivative bandgap output compared to the Tauc bandgap. The results also show that the $${E}_{U}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>E</mml:mi> <mml:mi>U</mml:mi> </mml:msub> </mml:math> reduced as the temperature reaches 70 °C and then experienced sudden increase at 80 °C. The change in the pattern is parallel to the trend observed in the Williamson–Hall microstrain and is evident from the variations of the mean crystallite size $${D}_{m}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>D</mml:mi> <mml:mi>m</mml:mi> </mml:msub> </mml:math> which is also a cause response to the change in temperature or pH. Therefore, the current work has elucidated that the structural and optical correlations on the as-synthesised Cu 2 O and CuO nanostructures in the presence of TA were the combined reaction of pH change and the ligand complexation reactions. The acquired results suggest a more comprehensive range of studies to further understand the extent relationship between the physical and optical properties of TA functionalised Cu-based oxide nanostructures.