© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 3297 www.advmat.de www.MaterialsViews.com wileyonlinelibrary.com COMMUNICATION Nitrogen-Doped Graphene Oxide Quantum Dots as Photocatalysts for Overall Water-Splitting under Visible Light Illumination Te-Fu Yeh, Chiao-Yi Teng, Shean-Jen Chen, and Hsisheng Teng* quantum confinement, which becomes prominent when the sp 2 domain size is less than 10 nm. [30–32] Quantum confine- ment causes the separation of the π and π* orbitals, and creates a band gap in graphene. [27–32] Modifying graphene by oxygen adsorption forms C–O covalent bonds that damage the original orbitals and confine π electrons because of the reduction in sp 2 domain size. This modification renders the quantized discrete levels to be dictated by the nature of the sp 2 domains and asso- ciated functional groups. [33,34] GO is a p-doped material because oxygen atoms are more electronegative than carbon atoms. [35] Replacing oxygen functional groups on the GO sheet edge with nitrogen-containing groups transforms GO into an n-type sem- iconductor. [22,36–39] In addition to surface modification by addi- tion of functionalities, direct substitution with heteroatoms in the graphene lattice induces the modulation of optical and elec- tronic properties. [40,41] GO derived from extensive oxidation of graphite powders exhibits a large accessible surface in aqueous solution, which makes GO an effective medium for photocatalytic water-split- ting without the presence of noble metal co-catalysts such as Pt or Ru. [20–23] The p-type conductivity results in the formation of an accumulation layer at the GO/water interface, which is favorable for water reduction to hydrogen. [20,21] Nitrogen-con- taining GO, which exhibits n-type characteristics, promotes hole transfer for water oxidation to oxygen. [22] Modifying a graphene sheet to exhibit both p- and n-type conductivities may produce a photocatalytic medium effective for overall water-splitting into H 2 and O 2 . In addition, effective exciton separation and charge transfer are essential factors for overall water-splitting to occur. Reducing the size of the GO sheets may lower the recombina- tion probability of the photogenerated charges. Based on the structural characteristics required for photocat- alytic water-splitting, we synthesized nitrogen-doped graphene oxide-quantum dots (NGO-QDs) as the catalyst. The NGO-QDs exhibited both p- and n-type conductivities, based on the results of the electrochemical Mott-Schottky analysis. The prominent photoluminescence emission indicated that photochemical p-n diodes constituted the NGO-QDs. The diode configuration resulted in an internal Z-scheme charge transfer for effective reaction at the QD interface. Visible light (>420 nm) irradiation on the NGO-QDs resulted in simultaneous H 2 and O 2 evolu- tion from pure water at an H 2 :O 2 molar ratio of 2:1. This paper demonstrated that graphene species are promising materials for synthesizing metal-free, cost-effective, and environmen- tally-friendly catalysts for overall water-splitting under solar illumination. We synthesized NGO-QDs by treating GO in NH 3 at 500 °C, and then subjecting the NH 3 -treated GO to oxidation using a Mr. T.-F. Yeh, Mr. C.-Y. Teng, Prof. H. Teng Department of Chemical Engineering and Center for Micro/Nano Science and Technology National Cheng Kung University Tainan 70101, Taiwan E-mail: hteng@mail.ncku.edu.tw Prof. S.-J. Chen Department of Engineering Science and Center for Micro/Nano Science and Technology National Cheng Kung University Tainan 70101, Taiwan DOI: 10.1002/adma.201305299 Photocatalytic water-splitting into H 2 and O 2 using sunlight has attracted considerable attention as a renewable energy resource. [1–5] For large-scale hydrogen fuel production, pow- dered photocatalytic water-splitting, which has a large area for catalyst-water contact and an unsophisticated reactor design, is advantageous over photoelectrochemical systems. [6] To make powdered photocatalytic water-splitting sustainable, a visible- light sensitive material capable of splitting water into H 2 and O 2 is critical. Numerous studies have reported metal-containing photocatalysts with high activity for H 2 or O 2 evolution from water decomposition under visible-light irradiation, but most only executed water-splitting half-reactions with sacrificial rea- gents. [7–12] Domen et al. developed Rh 2-y Cr y O 3 /GaN:ZnO com- pounds, which contain noble metals and are so far the most active catalysts for overall water-splitting under visible light irradiation. [3,13–16] An alternative approach for cost-effective hydrogen production is to develop photocatalysts from carbon materials, which are abundant and environmentally friendly. Graphitic carbon nitride [17–19] and graphene oxide (GO) [20–24] are capable of decomposing water for H 2 evolution, if sacrifi- cial reagents are added under irradiation. Electronic structural analysis revealed that GO materials have conduction band min- imum (CBM) and valence band maximum (VBM) levels suit- able for generating H 2 and O 2 , respectively, under visible-light irradiation. [22] Developing a synthesis route that precisely tunes the electronic characteristics of GO materials is critical for GO- assisted overall water-splitting. GO is a graphene compound with a basal plane and an edge bearing oxygen functionalities. The VBM and CBM of gra- phene consist of bonding π and anti-bonding π (that is, π*) orbitals, respectively. A single sheet of graphene with an infi- nite sp 2 domain is a zero band-gap semiconductor because π and π* orbitals touch at the Brillouin zone corners. [25] Size modulation and chemical modification readily tune the elec- tronic properties of graphene. [26–29] The size effect results from Adv. Mater. 2014, 26, 3297–3303