How to set full page geometry while using pages with two columns geometry?

Question

I am using this geometry to write:

\documentclass[11pt]{book}
\usepackage[paperheight=10in,paperwidth=9in, top=1in, bottom=0.8in, twocolumn]{geometry}
\setlength{\columnseprule}{0.4pt}
\usepackage{amssymb,amsfonts}
\usepackage{mathrsfs}
\usepackage[centertags]{amsmath}
\usepackage{amsthm}
\usepackage{amsmath}
\newtheorem{theorem}{Theorem}
\usepackage{epsfig}
\usepackage{graphicx}\graphicspath{{Graphics/}}
\usepackage{mathptmx}
\usepackage[square,sort&compress]{natbib}
\usepackage{pgf,tikz,pgfplots}
\pgfplotsset{compat=1.15}
\usetikzlibrary{arrows}
\usepackage[T1]{fontenc}
\usepackage{fancyhdr}\pagestyle{fancy}
\usepackage{xcolor}
\usepackage{setspace}
\usepackage{booktabs}
\usepackage{enumitem}
\setlist[enumerate,2]{label=(\roman*).}
\usepackage{tasks}
\settasks{label=$\circ$}
%\usepackage{hyperref}
\usetikzlibrary{arrows.meta}
\renewcommand{\baselinestretch}{1.5}
\newcommand\aug{\fboxsep=-\fboxrule!!!\fbox{\strut}!!!}
\theoremstyle{definition}
\newtheorem{Thm}{Theorem}[section]
\newtheorem{lem}[Thm]{Lemma}
\newtheorem{pro}[Thm]{Proposition}
\newtheorem{de}[Thm]{Definition}
\newtheorem{re}[Thm]{Remark}
\newtheorem{ex}[Thm]{Example}
\newtheorem{cor}[Thm]{Corollary}
\numberwithin{equation}{section}
\definecolor{uuuuuu}{rgb}{0.26666666666666666,0.26666666666666666,0.26666666666666666}
\begin{document}
\frontmatter
%\include{frontmatter/colophon}
%\include{fronmatter/frontpiece}
%\include{frontmatter/dedication}
%\include{frontmatter/preface}
\twocolumn
%\tableofcontents
\mainmatter
%\include{mainmatter/Chapter-1}
%\include{mainmatter/Chapter-2}
%\include{mainmatter/Chapter-3}
%\include{mainmatter/Chapter-4}
%\include{mainmatter/Chapter-5}
\include{mainmatter/Chapter-6}
%\include{mainmatter/Chapter-7}
%\include{mainmatter/Chapter-8}
%\include{mainmatter/Chapter-9}
%\include{mainmatter/Chapter-10}
%\include{mainmatter/Chapter-11}
%\include{mainmatter/Chapter-12}
%\include{mainmatter/Chapter-13}
\include{Graphics}
\end{document}

I need to use full page instead of splitting it in two columns somewhere in this document. How to set the full page somewhere while using the same geometry with two columns. But where I want to use full page instead of splitting in two columns there should be no splitting of page in two columns.