(436e) Graphene Oxide Photocatalysts for Water Splitting and Its Upconverted Photoluminescence

Graphene-Oxide Photocatalysts for Water
Splitting and Its Upconverted Photoluminescence

Te-Fu
Yeh¹ and Hsisheng Teng^1,2,*,

¹Department of Chemical
Engineering and Research Center for Energy Technology
and Strategy, National Cheng Kung University, Tainan 70101, Taiwan

²Center for Micro/Nano
Science and Technology, National Cheng Kung University, Tainan 70101, Taiwan

*E-mail:
hteng@mail.ncku.edu.tw

Abstract

        Graphene
oxide (GO) is a p-type semiconductor with a suitable band positions for photocatalytic
watersplitting. GO sheets have a hydrophilic 2D carbon structure that allows for
extensive chemical modification. This study presents strategies of tuning the
conductivity type of GO from p-type to n-type for simultaneous evolutions of H₂
and O₂ under visible light irradiation. Further converting the n-type
graphene oxide sheets to quantum dots results in stoichiometric evolution of H₂
and O₂ at 2-to-1 ratio. The quantum dots also serve as an energy-converting
medium with upconverted photoluminescence to promote the activity of
metal-oxide photocatalyts.

Introduction

Polymeric
semiconducting molecules, which have a large contact area with water, attract significant
attention as alternatives to metal-containing photocatalysts. Graphene
oxide (GO) derived from graphite oxidation is a molecule-like semiconductor [1]. GO can be extensively dispersed in water to molecular
scale and has a large exposed area. Graphene is a zero gap semiconductor whereas
the GO gap increases with the oxidation level [2,3]. The
bonding with electron-withdrawing oxygen functionalities results in the p-type
conductivity of GO. N-type conductivity appears when
graphene covalently bonds to electron-donating nitrogen functionalities [4-6].

In
addition to chemical doping, we exploit the quantum-confinement effect to
modulate electronic properties. Unlike two-dimensional graphene, the
zero-dimensional graphene quantum dots (GQDs) have a finite band gap depending
on their size [7]. The confined p-electron in sp² domain gives the quantized discrete
levels [8]. With these features, GQDs exhibit upconvered photoluminescence
(PL) [9].

        This
study presents the potential of GO as a photocatalyst for water splitting.
P-type GO can
catalyze H₂ evolution with the presence of electron donors. Ammonia-treated GO (NGO)
exhibits n-type conductivity and promising photocatalytic activity for
generating H₂ and O₂ simultaneously, but with the ratio
below the stoichiometric value (that is, 2). Miniaturized N-doped graphene
oxide quantum dots (NGO QDs) function as photocatalysts for stoichiometric H₂
and O₂ generation from water cleavage under visible light
irradiation. We also demonstrate the upconversion ability of NGO QDs.

Experimental

        GO was prepared using a natural graphite
powder
through a modified Hummers' method. NGO was obtained
through nitridation of the as-prepared GO performed at room temperature using a
flow of NH₃ gas. NGO QDs were prepared from
oxidation of nitrogen-doped graphene. Photocatalytic reactions were conducted at
approximately 25 ¢XC in a gas-enclosed inner irradiation system or a side irradiation
system.

Result
and Discussion

        We
demonstrated the photocatalytic activity of GO in the gas-enclosed circulation
system with mercury-lamp irradiation. Continuous H₂ evolution over
metal-free GO was observed for reactions conducted in pure water and an aqueous
methanol solution [1]. The total evolution of H₂
from aqueous methanol solution far exceeds the amount of H₂ obtained
from pure water. GO cannot catalyze O₂ generation even with the
presence of the electron scavenger Ag⁺ ion.  

We
modified the as-synthesized GO photocatalyst by ammonia treatment to form NGO, which exhibited n-type
conductivity. The change of conductivity type was contributed by the
electron-donating property of the introduced amino groups. NGO effectively
catalyzes O₂ evolution under mercury-lamp irradiation [6]. The n-type conductivity of NGO attracts hole
migration toward the interface region for O₂ evolution. NGO gave
rise to simultaneous evolutions of H₂ and O₂ from the
side irradiated system under visible light irradiation [6].
The molar ratio of the evolved H₂ and O₂ was smaller than
the stoichiometric ratio 2-to-1.

Quantum
confinement effect elevates the conduction band edge, promoting the electron
injection for H₂ evolution from water. We found stoichiometric
evolutions of H₂ and O₂ from pure water over NGO QDs with
visible-light irradiation. Figure 1 shows the PL spectra of
NGO QDs excited by long-wavelength light (660-900
nm), with the upconverted emission located in the range of 450-530 nm. The upconverted PL
emission of NGO QDs excited NaTaO₃ to catalyze H₂
evolution from an aqueous methanol solution under visible light irradiation.

Figure 1. Upconverted
PL from NGO QDs. The excitation wavelength ranges from 660 nm to 900 nm. The
upconverted emission located in the range of 450-530
nm.

Conclusions

        The
p-type GO catalyzed H₂ evolution from an aqueous methanol solution under
mercury-lamp illumination. The
introduction of nitrogen functionalities converted the p-type GO to an n-type
NGO, which was capable of catalyzing simultaneous H₂ and O₂
evolutions from water under visible light. The NGO QD catalyst achieved overall
water splitting (H₂/O₂ = 2) under visible light
irradiation. The upconversion ability of NGO QDs made wide-gap photocatalysts active
under visible-light illumination.

Acknowledgements

This research is
supported by the National Science Council of Taiwan (101-2221-E-006-243-MY3 and
101-2221-E-006-225-MY3) and the "Aim for the Top-Tier University and Elite
Research Center Development Plan" of National Cheng Kung University.

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