**1. Introduction**

Graphene oxide (GO) is a sheet of graphite oxide obtained by exfoliating graphite oxide into a layered sheet that contains only one or a few layers of carbon atoms through sonication or mechanical stirring [1]. GO can be reduced partially to graphene-like sheets by removing oxygen-containing groups through the restoration of conjugated structure referred to as reduced GO (rGO). These rGO sheets are usually considered to be one type of chemically derived graphene that has similar properties to pure graphene. Graphene and GO are very different, where graphene consists only of sp2 hybridized carbon atoms, while GO has a carbon structure that is decorated by various oxygen functional groups.

The study of GO was reported in 1859 when the English chemist Brodie named it graphite acid or graphite oxide [2], which was prepared by chemical treatment of graphite with potassium chlorate (KClO3) and nitric acid (HNO3). After the graphite oxide has been prepared, GO can be obtained by exfoliating the graphite oxide into

monolayer sheets through various thermal and mechanical methods [3]. At present, the single-atom carbon layer of graphite oxide is considered graphene oxide (GO).

GO can be produced using inexpensive graphite by applying cost-effective chemical methods with simple processes and high yields. Furthermore, GO is highly hydrophilic and can form stable aqueous colloids to facilitate the assembly of macroscopic structures by a simple and low-cost solution process [4]. The conventional way to convert graphite oxide into GO is carried out by mechanically exfoliating the graphite oxide, by sonication of graphite oxide in water or a polar organic medium into completely exfoliated GO flakes [5]. In addition, through mechanical stirring of graphite oxide in water, graphite oxide can also be well exfoliated into GO [6]. The sonication and mechanical stirring methods can be combined together to exfoliate the graphite oxide producing a better efficiency than the individual methods separately.

It has been reported that there are four methods to synthesize GO including the Brodie method [2], Staudenmaier [7], Hummer and their modifications [8, 9], and Tour method [10]. Nowadays, the synthesis of GO by the modified Hummer's method has become the most common technique for its production. In the modified Hummer method, graphite is served as the main precursor for GO synthesis.

Graphite is classified into natural graphite and synthetic graphite which can be produced by heating the hydrocarbon precursors at very high temperatures. Meanwhile, the combustion of biomass waste produces charcoal which consists of a mixture of hydrocarbons that could be applied as graphite precursors using graphitization process in a lower temperature.

GO contains various reactive oxygen functional groups, which makes it a good candidate for use in many applications, such as polymer composites, materials for energy conversion and environmental applications [11], sensors, FET transistors, as well as biomedical applications, due to its excellent electrical, mechanical and thermal properties [12]. In this chapter, the synthesis of biomass waste-based graphene oxide and the application of GO in electronics, optics, optoelectronics, as well as energy conversion and storage will be discussed.
