**1.2. Why photoactive graphene?**

Pristine graphene is made of sp2 hybrid carbon atoms with the s, px, and py orbitals on each carbon atom forming three strong σ bonds with other three surrounding atoms.[21] The formed valence and conduction bands touch at the Brillouin zone corners (so-called Dirac or neutrality points) making graphene a zero bandgap semiconductor with poor photoactive characteristic.[22, 23] Experimentally, the transmittance of the mechanically exfoliated graphene is overwhelming (97.7%), and thus it absorbs only 2.3% of light that passes through it (Figure 2).[2, 24] So it is impossible for graphene to absorb light when used as a photoactive material (Geim *et al*., 2008).

talysts. Although there is some available literature covering various aspects of photoactive graphene,[1, 5, 6] a strategic update that reflects the newest progress, growing trends, and opening opportunities of photoactive graphene is required. This chapter pays particular attention to the development of photoactive graphene, including material preparation, the photophysical progress of excited organic molecules and graphene, as well as its applications

The term "photoactive graphene" generally refers to graphene that undergo a chemical or physical reaction when interacted with sunlight and/or ultraviolet light. Unlike the transparent pristine graphene, photoactive graphene shows optical response characteristics when light

To date, two different approaches toward the preparation of photoactive graphene can be found in the literature. As shown in Figure 1 (a), the first is based on the bandgap engineering (opening and tuning) of pristine graphene,[7] and in the second strategy the photoactive graphene is obtained by chemical functionalization with photoactive moieties.[8] The most reported examples of the first route are roughly classified into four categories: (1) heteroatom doping;[9-11] (2) chemical modification;[12] (3) electrostatic field tuning;[13, 14] and (4) cutting graphene into nanoribbons.[15-19] As shown in Figure 1 (b), the second strategy for giving photoactive to graphene is chemical functionalization of graphene with photoactive units, including organic conjugated molecules and polymers, inorganic semiconductors particles and quantum dots, rare-earth metal complexes, and so on. This approach has been demon‐ strated to be a feasible route to achieve the photo-electron response of graphene to light.[8, 20] Herein, we will only discuss the progress and challenges related to the chemical function‐

**Figure 1.** Two different approaches toward the photoactive graphene. One is based on the bandgap opening and tun‐ ing of pristine graphene (a), another strategy is chemical functionalization of graphene with photoactive moieties (b).

carbon atom forming three strong σ bonds with other three surrounding atoms.[21] The

hybrid carbon atoms with the s, px, and py orbitals on each

in optoelectronics and photocatalysis.

92 Graphene - New Trends and Developments

**1.1. What is photoactive graphene?**

alization approach to preparing photoactive graphene.

**1.2. Why photoactive graphene?**

Pristine graphene is made of sp2

passes through it.

**Figure 2.** Optical adsorption property of graphene and its layer (Geim *et al*., *Science* 2008, 320, 1308).

The poor photoactive characteristic of graphene leads to some major drawbacks in photoenergy conversion applications. For photovoltaic devices, using pristine graphene as active layer may significantly reduce the open circuit voltage, compared to conversional semicon‐ ductors.[25] Moreover, graphene is unfavorable for efficient photocurrent generation in photovoltaic devices. From a dynamics point of view, to output a photocurrent in an external circuit, the photo-generated carrier needs to separate from the photo-generation region before recombination.[26, 27] However, for graphene based transistor, the lifetime of photo-generated hot carriers was cooled down within several hundred femtoseconds, followed by electron– hole recombination on a picosecond timescale.[28, 29] Therefore, in order to improve the photo-energy conversion efficiency of graphene, it is necessary to extend the lifetime of photogenerated carriers of graphene.

Recently, endowing graphene photoactive property has been viewed as an effective approach to exploit the advantages of graphene in photovoltaics and photocatalytics.[30] The photoac‐ tive graphene can play a significant role in photovoltaics and photocatalysts resulting from the synergy effect of the charge transfer based in the photoactive graphene. For photoactive graphene, the propensity of graphene to interact with excited state photoactive moiety is often involve energy- and/or charge-transfer processes, where the excited fluorescent moieties is served as excellent probes by monitoring their emission evolution.[31] Meanwhile, the high charge mobility of graphene promotes it can be used as efficient acceptors to enhance photoinduced charge transfer for improving photocurrent conversion performance. The effective nonradiative deactivation of the excited photoactive moieties on graphene surface by interfa‐ cial charge transfer is necessary for the improvement of the conversion efficiency from optoelectronic and photo-energy to electricity.

In recent years, there has been a growing interest in graphene functionalized with photoactive units owing to their significance in both fundamental research and practical applications. Recent research results have demonstrated that chemical functionalization of graphene with photoactive moieties is a necessity to harvest its full potential.[8]
