Graphene-based semiconductor photocatalysts have been extensively studied for the photocatalytic degradation of organic compounds and have proven more efficient than the bare semiconductor photocatalysts [1-5]. However, graphene, as a “rising star” material and another allotrope of carbon (activated carbon, fullerenes, and carbon nanotubes), is it really superior to other carbon materials when combined with semiconductor photo catalysts, or just because graphene is a newly discovered “weirdie” and people are blindly keen on? To get a comprehensive but still open answer to this question, the unique properties of graphene and the physical and chemical factors that matters the photodegradation activity of carbon-semiconductor photocatalysts should be addressed. In addition, the characteristics of different carbon-semiconductor system should be emphasized.

Graphene has many exceptional properties, such as high electron mobility, theoretically high surface area of 2600 m²g^-1, and high transparency [6]. For the synthesis of graphene-based semiconductor photocatalysts, the most widely used technique to obtain graphene is Hummers’ method [7]. The exfoliated graphite oxide (GO) sheets which are the intermediate products in the synthesis process usually possess a rich assortment of oxygen-containing groups, such as carboxylic, hydroxyl, and epoxide functional groups [4]. The presence of oxygen functionalities in GO allows interactions with the cations and provides reactive sites for the nucleation and growth of nanoparticles, which results in the rapid growth of various graphene-based composites. Furthermore, high quality graphene sheets permit ballistic transport, making them potentially ideal electron sinks or electron transfer bridges [8]. Based on these intrinsic physical and chemical characteristics, the enhanced photocatalytic degradation of organic compound for Graphene-TiO₂ system could be attributed to the following reasons: (1) the interactions between organic molecules and the aromatic regions of graphene enhance the adsorption on photocatalysts, (2) the formation of the Ti–O–C chemical bonds narrow the band gap of TiO₂ and extends the photoresponse range, (3) the transfer of excited electrons from TiO₂ to graphene suppresses the charge recombination [3]. It is also found that the electrons in the upper valence band could be directly excited from graphene to the conduction band of TiO₂ under visible light irradiation [9], thus graphene acts as the sensitizer to TiO₂. It is unassailable that the photocatalytic activity of TiO₂- Graphene Composite is higher than that of TiO₂ alone. However, to compare graphene’s superiority over other carbon, on one side, it was advocated that graphene is superior than carbon nanotubes (CNTs) in carbon-TiO₂ composites [5]. On the other side, it is strongly argued that TiO₂-Graphene composite is in essence the same as other TiO₂- carbon (activated carbon, fullerenes, and carbon nanotubes) composite materials on enhancement of photocatalytic activity of TiO₂ [10].

To clarify whether graphene provides brand new insights into carbon-semiconductor photocatalysts, the role of graphene’s forebear should be addressed. Activated carbon (AC) has been extensively studied as a support for heterogeneous catalysis due to its high surface area (typically 900~1200 m²g^-1) structure over which TiO₂ particles may be distributed and immobilized [11]. AC–TiO₂ mixtures/composites can improve the photocatalytic activity due to the high adsorption capacity and ready passage of reacting species to the TiO₂ particles [12]. Besides, AC support itself is capable of a significant level of selfphotocatalytic activity, out-performing the AC–TiO₂ composite under UV light irradiation [13]. C60 has unique electronic properties and it absorbs moderately the visible light and strongly the UV light [14,15]. In C60-TiO₂ photocatalysts, the fullerene can act as an electron acceptor to minimize the electron–hole recombination or as an electron donor to sensitize TiO₂ [16]. CNTs can provide large amounts of surface reaction sites due to the large surface area (200~900 m²g^-1). In addition, the surface chemistry of CNTs may be tailored to promote specificity towards adsorbents. CNTs may act as extremely effective electron sinks since they exhibit high electrical conductivity and high electron storage capacity (one electron for every 32 carbon atoms). As a result, they can synergistically enhance the photocatalytic activity of TiO₂ through the retardation of electron–hole recombination. The presence of the C-O-Ti bond in the system has been proven to extend light absorption to longer wavelengths. CNTs may also enhance TiO₂ photocatalytic activity by acting as a photosensitizer, transferring electrons to TiO₂ [17]. The mid band-gap states introduced by defects or CNT oxidation also result in higher photocatalytic activity [9].

Based on the above discussion, it is clear that although graphene is much more popular than its forebears at present, the graphene-based semiconductor photocatalysts basically exhibit four positive factors that can potentially enhance the photocatalytic activity (provision of high quality highly adsorptive active sites, minimization of electron hole recombination, band-gap tuning, and photosensitization). All the four factors well established in other carbon semiconductor photocatalysts. Nevertheless, it is still too early to get a decisive answer for graphene’s superiority to other carbon materials when combined with other semiconductor photocatalysts. Graphene is a new carbon material applied for photocatalysis, it is still too young since it was only born in 2004. Although the number of the reports on graphene semiconductor photocatalysts is increasing unprecedentedly, the fully understanding of the role of graphene to the photocatalytic properties and mechanisms of graphene based photocatalysts still need more evidence. Furthermore, as pointed by Leary and Westwood [8] the development and endorsement of standardized testing methods is needed for the investigation of photocatalysis activity.