Is Reduced Graphene Oxide Always an Enhancer in Photo(Electro)

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Energy, Environmental, and Catalysis Applications

The Importance of the Interfacial Contact: Is Reduced Graphene Oxide Always an Enhancer in Photo(Electro)Catalytic Water Oxidation? Zhirun Xie, Hui Ling Tan, Xiaoming Wen, Yoshitaka Suzuki, Akihide Iwase, Akihiko Kudo, Rose Amal, Jason Scott, and Yun Hau Ng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03624 • Publication Date (Web): 28 May 2019 Downloaded from http://pubs.acs.org on May 29, 2019

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ACS Applied Materials & Interfaces

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The Importance of the Interfacial Contact: Is

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Reduced Graphene Oxide Always an Enhancer in

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Photo(Electro)Catalytic Water Oxidation?

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Zhirun Xie,a Hui Ling Tan,†, a, * Xiaoming Wen,b Yoshitaka Suzuki,a Akihide Iwase,c Akihiko

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Kudo,c Rose Amal,a Jason Scott,a Yun Hau Ngd, * a

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Particles and Catalysis Research Group, School of Chemical Engineering, The University of

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New South Wales, Sydney, NSW 2052, Australia b

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University of Technology, Melbourne, VIC 3122, Australia c

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Centre for Micro-Photonics, Faculty of Science, Engineering and Technology, Swinburne

Department of Applied Chemistry, Tokyo University of Science, 1-3 Kagurazaka Shinjuku-ku, Tokyo 162-8601, Japan

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School of Energy and Environment, City University of Hong Kong, Kowloon, Hong Kong SAR

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ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT

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Optimizing interfacial contact between graphene and a semiconductor has often been proposed as

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essential for improving their charge interactions. Herein, we fabricated bismuth vanadate-reduced

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graphene oxide (BiVO4/rGO) composites with tailored interfacial contact extents and reveal their

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disparate behavior in photoelectrochemical (PEC) and powder suspension (PS) water oxidation

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systems. BiVO4/rGO with a high rGO coverage on the BiVO4 surface (BiVO4/rGO HC) exhibited

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an 8-fold enhancement in the PEC photocurrent density with respect to neat BiVO4 at 0 V versus

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Ag/AgCl, while BiVO4/rGO with a low rGO coverage (BiVO4/rGO LC) gave a lesser 3-fold

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enhancement. In contrast, BiVO4/rGO HC delivered a detrimental effect while BiVO4/rGO LC

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exhibited an enhanced performance for oxygen evolution in the PS system. The phenomenon is

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attributed to changes in the hydrophobicity of the BiVO4/rGO composite in conjunction with the

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interfacial contact configuration. Better BiVO4/rGO interfacial contact was found to improve the

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charge separation efficiency and charge transfer ability of the composite material, explaining the

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superior PEC performance of BiVO4/rGO HC. Additionally, optimization of the interfacial contact

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extent was revealed to further improve the energetics of the composite material, as evidenced by

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a Fermi level shift to a more negative potential. However, the high hydrophobicity of BiVO4/rGO

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HC arising from the higher rGO reduction extent triggered poor water miscibility, reducing the

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surface wettability and therefore hampering the photocatalytic O2 evolution activity of the sample.

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The study underlines water miscibility as a governing issue in the PS system.

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KEYWORDS: interfacial contact; reduced graphene oxide; photoelectrochemistry; powder

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suspension photocatalysis.

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INTRODUCTION

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Rising concern regarding the worldwide energy crisis and environmental issues, such as global

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warming and climate change, has seen calls for a necessary energy switch from traditional fossil

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fuels to clean and renewable energy.1-2 Photo(electro)catalytic water splitting, which uses

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semiconducting photocatalysts to harness solar energy for H2 production, is an ideal energy carrier

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that produces zero CO2 emissions when combusted and is a promising approach to achieve

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energetic sustainability.3-7 Due to the superior electrical properties of graphene,8-9 graphene-based

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materials are widely employed in energy conversion and storage applications, including batteries,10

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supercapacitors,11 fuel cells,12 and photo(electro)catalytic water splitting.13 In relation to

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photo(electro)catalytic applications, incorporating graphene with a suitable light-absorbing

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semiconductor photocatalyst has been extensively demonstrated to be beneficial as graphene

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exhibits superior conductivity as well as optically transparency.14 A graphene presence enhances

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electron transport, suppresses charge recombination, and improves the photoactivity of graphene-

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based semiconductors.15

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The charge interactions between graphene and a semiconductor fundamentally occur at the

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interface. While different surface structural and electronic properties are associated with the

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different crystal facets exposed on a semiconductor,16 the interfacial contact facet between

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graphene and a facet-engineered semiconductor was also demonstrated to affect the charge

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interactions due to the distinct interfacial properties. For instance, we previously revealed that the

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degree of photocurrent enhancement in a bismuth vanadate and reduced graphene oxide

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(BiVO4/rGO) composite correlated with the surface area of the {010} facet exposed on the

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BiVO4.17 The experimental results were corroborated by first principle calculations, where electron

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injection from the {010} facet of BiVO4 to rGO was predicted to be easier than from the {110}


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facet due to the electronic property differences between the two graphene/BiVO4 interfaces.

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Similar facet-dependent electron transfer has also been reported for TiO2-graphene interfaces. Liu

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et. al discovered that the interfacial charge transfer rate from the TiO2{100} facet to graphene was

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higher than from the TiO2{101} and TiO2{001} facets. The difference arose from the unique Ti-

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C bonding between TiO2{100} and graphene rather than the Ti-O-C bonding exhibited by the

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graphene/TiO2{101} and graphene/TiO2{001} interfaces.18 This was reflected in superior

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photocatalytic activity by {100}-dominant TiO2/graphene for H2 evolution.

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The typical synthesis methods (e.g., hydrothermal, self-assembly, and semiconductor-assisted

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photoreduction) for graphene-based semiconductor materials are largely based on the

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(photo)reduction of graphene oxide (GO) to rGO and involves simple mixing of GO with the

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semiconductor. These approaches lack the ability to control the GO or rGO deposition pattern on

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the semiconductor which hampers good interfacial contact between the materials and subsequently

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effective charge transfer.19-20 The obtained semiconductor/rGO composite generally consists of

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rGO sheets intertwined with semiconductor nanoparticles with poor contact. Several research

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groups have prepared semiconductor/rGO composites with a wrapped structure whereby the rGO

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sheets extensively cover the semiconductor surface. Although the rGO-wrapped semiconductors

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exhibited enhanced photoactivity compared to the bare semiconductors, the photoactivity was

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strictly focused on organic dye degradations such as Rhodamine B21 and methylene blue.22

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It is expected that optimization of semiconductor surface coverage by graphene expedites

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electron transfer at their interface. However, to the best of our knowledge, the benefits of an ideal

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contact in comparison to randomly oriented and poorly contacted graphene/semiconductor

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configurations have not been demonstrated, particularly for water splitting systems. Despite both

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the powder suspension (PS) and the photoelectrochemical (PEC) systems being viable for water


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splitting, the fundamental operating principles of the two systems are different. Recent work by

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our group revealed that charge separation overrules charge transport in a PS system and vice versa

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for a PEC system.23 While the role of graphene to enhance charge separation and transport in

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graphene/semiconductor composites is commonly acknowledged, the extent to which a tailored

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graphene/semiconductor interface will affect the charge dynamics is intriguing and continues to

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remain unknown.

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In this work, we compare the water-oxidation performance in PS and PEC water splitting

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systems for two rGO-BiVO4 composites with distinct structures: (i) low rGO coverage where the

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rGO sheets are randomly interspersed between BiVO4 particles; and (ii) BiVO4 particles

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extensively wrapped by the rGO sheets. The two BiVO4/rGO composites displayed contrary

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behaviors for the two water splitting systems. Detailed examination on the physicochemical and

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electronic properties of the BiVO4/rGO composites revealed the effects of the interfacial contact

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configuration on the carrier dynamics and the energetics of the materials. A high rGO coverage

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contributed to greater PEC photocurrent density enhancement, attributed to enhanced charge

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separation and improved interfacial charge transfer efficiency. In contrast, a low rGO coverage

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was preferable in the PS system as high rGO coverage induced greater rGO reduction level during

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GO photoreduction, which diminished the water miscibility of the BiVO4/rGO sample and

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attenuated the photocatalytic O2 evolution.

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RESULTS AND DISCUSSION

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Scheme 1. Synthesis processes for BiVO4/rGO LC and BiVO4/rGO HC samples.

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Neat BiVO4 was first prepared following a method reported previously.24 The structural and

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morphological properties of the bare BiVO4 were characterized using X-ray diffraction (XRD)

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spectroscopy and scanning electron microscopy (SEM) (Figure S1) and confirmed that the BiVO4

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is the monoclinic scheelite phase with well-defined facets.25 The successful exfoliation of GO is

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also indexed by the diffraction peak located at 10° in Figure S1a. The neat BiVO4 was then coupled

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with graphene oxide (GO) via different strategies to tune the rGO coverage in the resultant

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composites, as depicted in Scheme 1. The BiVO4/rGO with low rGO coverage (denoted as

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BiVO4/rGO LC) was obtained via one-step photodeposition. The BiVO4/rGO with high rGO

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coverage (denoted as BiVO4/rGO HC) was prepared by a sequential two-step method involving

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wrapping the GO around BiVO4 via an evaporation-induced self-assembly process in the dark


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followed by photoreduction of GO to rGO under the same illumination time as BiVO4/rGO LC.21-

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Unless otherwise stated, the GO or rGO loading in the BiVO4/graphene composites is 2.0 wt%.

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Figure 1. SEM images of BiVO4/rGO LC (a, b) and BiVO4/rGO HC (d, e); EDS-mappings of

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BiVO4/rGO LC (c) and BiVO4/rGO HC (f).

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The obtained BiVO4/rGO LC and BiVO4/rGO HC were examined under SEM and EDS, as

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shown in Figure 1. Typically, when GO is introduced into a BiVO4 suspension, the mutual

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attraction between the oxygen functional groups on BiVO4 surfaces and GO drives attachment of

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the GO sheets onto BiVO4 particles. With the intimate contact and the suitable energy alignments

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between BiVO4 and GO, the photogenerated electrons within BiVO4 are transferred to GO under

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illumination, reducing GO to rGO.19,

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consumed by the hole-scavenging ethanol. In the case of BiVO4/rGO LC, mixing GO and BiVO4

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Concurrently, the photoinduced holes are effectively


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under illumination resulted in the rGO sheets being interwoven between the BiVO4 particles, as

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shown in Figure 1a. That is, the BiVO4 particles were have aggregated and are attached to the rGO

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sheets. The high-magnification image (Figure 1b) illustrates the limited contact between BiVO4

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and rGO. The majority of the BiVO4 particle surfaces are clean and smooth signifying limited

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adhesion of rGO sheets. In contrast, the initial evaporation process when preparing BiVO4/rGO

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HC with prolonged stirring under dark ensured sufficient electrostatic-based interaction between

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BiVO4 and GO, facilitating the wrapping of the GO sheets around the BiVO4 particles. This is

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confirmed by the HR-SEM in Figure S2, where the GO sheets are seen to uniformly and

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conformally cover the BiVO4 particles in the as-produced BiVO4/GO. Subsequent light exposure

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induced the photocatalytic reduction of BiVO4/GO into BiVO4/rGO HC. As displayed in Figure

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1d, individual and free-standing rGO sheets interspersed amongst the BiVO4 particles are absent

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for BiVO4/rGO HC. The high-resolution image (Figure 1e) indicates the rGO sheets enfold the

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BiVO4 particles, similar to that of the as-produced BiVO4/GO composite. The elemental

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distributions in EDS mappings also reinforce the different interfacial contacts between BiVO4 and

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rGO in BiVO4/rGO LC and BiVO4/rGO HC. The distribution of C matches well with the profiles

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of Bi and V in BiVO4/rGO HC (Figure 1f) whereas the C distribution in BiVO4/rGO LC only

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partially overlaps with BiVO4 (Figure 1c). Both the SEM and EDS results confirm the optimized

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and maximized interfacial contact between rGO and BiVO4 in the BiVO4/rGO HC sample.


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Figure 2. UV-vis spectra (a) of BiVO4, BiVO4/rGO LC, and BiVO4/rGO HC; XPS C 1s spectra

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(b) of GO, BiVO4/rGO LC, and BiVO4/rGO HC; TGA curves (c) of BiVO4, BiVO4/rGO LC, and

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BiVO4/rGO HC.


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rGO incorporation darkened the color of the BiVO4/rGO composites relative to the neat BiVO4,

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as has also been reported in the literature.26 In particular, BiVO4/rGO HC was found to exhibit a

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darker shade (dark green) than BiVO4/rGO LC (yellowish green), as illustrated in the inset of

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Figure 2a. Accordingly, both BiVO4/rGO LC and BiVO4/rGO HC showed stronger absorption

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compared to the bare BiVO4 at wavelengths longer than 500 nm, whereby BiVO4/rGO HC

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exhibited the greatest absorption (Figure 2a). Irrespectively, the absorption edges of the three

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samples were comparable, located at around 500 nm which corresponds to the band gap energy of

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BiVO4. It is widely reported that in TiO2/graphene composites, the light absorption extension from

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UV to the visible light region is attributed to shallow doping at the TiO2 surface arising from Ti-

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O-C or Ti-C bond formation at TiO2/graphene interface.18, 27 In this study, however, no apparent

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absorption edge shift was observed after BiVO4 hybridized with rGO. This suggests that the

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interactions between BiVO4 and rGO were mainly physical, which was supported by the

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preservation of the structural phase and crystallinity of BiVO4 after rGO incorporation as shown

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by the XRD analysis (Figure S1). Despite successful rGO addition, XRD patterns of the

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BiVO4/rGO composites primarily displayed the peaks corresponding to monoclinic BiVO4. The

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untraceable carbon-related peak was possibly due to the low rGO loading.

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Figure S3a displays the X-ray photoelectron spectroscopy (XPS) survey spectra of neat BiVO4

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and the BiVO4/rGO composites. While the C 1s signal from neat BiVO4 can be ascribed to

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adventitious carbon, rGO incorporation intensifies the C 1s signals for the composite materials.

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Note that the C ls signal intensification on BiVO4/rGO HC is comparatively greater for

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BiVO4/rGO LC. In particular, the C/Bi and C/Vi ratios, as estimated from C 1s, Bi 4f 7/2, and V

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2p3/2 atomic percentages, were calculated to increase from 2.5 and 2.9 for BiVO4/rGO LC to 12.1

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and 15.3 for BiVO4/rGO HC. The enhanced C 1s signal is attributed to the presence of more carbon


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atoms on BiVO4 surface arising from the rGO’s carbon backbone and thus attests improved rGO

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surface coverage on the BiVO4/rGO HC. XPS analyses were further employed to determine the

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rGO reduction degree in BiVO4/rGO LC and BiVO4/rGO HC compared to GO. As shown in

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Figure 2b, three distinct peaks located at 284.6 eV, 286.6 eV, and 288.9 eV, which correspond to

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graphitic C-C, oxygenated C-O, and C=O bonds, respectively, were detected in the C 1s spectra

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of the samples. The reduction degrees of the graphene derivatives (i.e., GO and rGO) were

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calculated by the peak area ratio of graphitic carbon (C-C) to total carbon (C-C, C-O, and C=O).

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Given that part of the oxygen functional groups on the GO surface are removed during the

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photocatalytic reduction of BiVO4/GO, the latter consistently displays higher reduction degrees

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than GO. Interestingly, BiVO4/rGO HC gave a higher reduction degree of 78.8 % π-conjugated C-

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C restoration compared to BiVO4/rGO LC (68.6 %). The successful incorporation and the varied

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reduction degree of rGO in BiVO4/rGO composites can also be confirmed by the Raman spectra.

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As shown in Figure S4, the obvious peaks located at 1350 and 1605 cm-1 were indexed to the D

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band and G band for BiVO4/rGO composites, respectively. The quality and structural properties

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of the graphitic network in functionalized graphene materials can be identified based on the

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intensity ratio between D and G bands (ID/IG). Typically, D band relates to the disordered

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structures, while G band is associated with the graphitization degree. As displayed in Figure S4,

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the ID/IG ratios were calculated to be 0.70 for BiVO4/rGO LC and 0.86 for BiVO4/rGO HC. The

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higher ID/IG value of BiVO4/rGO HC than that of BiVO4/rGO LC is correlated to its higher

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reduction degree. During the photoreduction process, more oxygenated functional groups were

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removed from the carbon framework of rGO in BiVO4/rGO HC, thus simultaneously generating

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more defects, which is consistent with the XPS results. The higher reduction level of BiVO4/rGO

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HC infers more efficient electron transfer from BiVO4 to GO during the photocatalytic reduction


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process, corroborating the idea of charge interaction optimization by maximizing the interfacial

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contact. While the formation of a new chemical bond of Ti-O-C has been generally observed for

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TiO2/graphene composites,18 the unchanged Bi 4f and V 2p XPS spectra between the BiVO4 and

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both BiVO4/rGO composites (Figure S3b and c, respectively) suggest that neither bismuth nor

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vanadium atoms form chemical bonds with the carbon atom from the graphitic network. This again

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confirms that the contact between BiVO4 and rGO is primarily physical.

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The rGO content in the BiVO4/rGO composites was confirmed by thermogravimetric analysis

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in the air over the temperature range from room temperature to 900 °C. As shown in Figure 2c,

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bare BiVO4 did not undergo weight loss over the given temperature range, which agrees with the

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previous work that showed BiVO4 does not undergo phase transformation at temperatures below

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900 °C.28 The result infers that the mass change observed for the BiVO4/rGO composites was

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entirely due to rGO decomposition. Figure 2c indicates the mass loss of BiVO4/rGO LC began at

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around 200 °C, while BiVO4/rGO HC started at around 270 °C. The slightly higher decomposition

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temperature of BiVO4/rGO HC may be due to greater C-C bond restoration after photoreduction,

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as evidenced by the earlier XPS results. The mass conservation beginning at around 550 °C

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suggests the complete decomposition of rGO has been achieved. As seen in Figure 2c, the overall

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mass losses of BiVO4/rGO LC and BiVO4/rGO HC were 2.03% and 2.28%, respectively. The

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TGA results signify that the rGO content in both BiVO4/rGO LC and BiVO4/rGO HC is highly

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comparable and consistent with the nominal GO loading of 2.0 wt%.


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Figure 3. Linear sweep voltammetry (LSV) (a) of BiVO4, BiVO4/rGO LC, and BiVO4/rGO HC.

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Photocurrent densities (b) of BiVO4 and BiVO4/rGO composites with different rGO loading,

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measured at 0 V versus Ag/AgCl. The photoelectrochemical measurements were measured in 0.1

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M Na2SO4 solution (pH 6.8). Steady-state PL spectra (c), time-resolved PL spectra (d), Nyquist

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plots (e) and Mott-Schottky plots (f) of BiVO4, BiVO4/rGO LC, and BiVO4/rGO HC.

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The neat BiVO4 and BiVO4/rGO composite powders were immobilized onto conductive fluorine-

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doped tin oxide substrates to produce electrodes for investigating the PEC water oxidation

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performance of the materials. As an n-type semiconductor, the photoexcited electrons within

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BiVO4 or BiVO4/rGO would be withdrawn from the photoanode to the counter electrode with the

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assistance of an externally applied bias to generate a photocurrent, whereas holes diffuse to the

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semiconductor/electrolyte interface to oxidize water. Figure 3a depicts the current-potential (I-V)

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curves of the samples under on-off visible light irradiation cycles (λ > 420 nm). All the

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photoelectrodes display anodic photocurrent across the applied bias range of -0.3 to 1.0 V versus

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Ag/AgCl, in accordance with the n-type characteristic of BiVO4. As an excellent electron acceptor,

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rGO incorporation evidently improves the photocurrent of the BiVO4/rGO composites relative to

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neat BiVO4 and is consistent with other studies related to semiconductor/rGO composites.26, 29 It

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is apparent that BiVO4/rGO HC exhibited unambiguously higher photocurrent density compared

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to BiVO4/rGO LC. The higher photocurrent signifies better electron collection efficiency at the


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substrate. The superior performance of BiVO4/rGO HC over BiVO4/rGO LC in the PEC system

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was also confirmed using chronoamperometry measurements at a constantly applied bias of 0 V

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versus Ag/AgCl (Figure 3b). The photocurrent density for each sample shown in Figure 3b was

4

deduced by subtracting the dark current from the steady-state photocurrent under the visible light

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illumination. Despite the different rGO contents, the photocurrent magnitudes of BiVO4/rGO HCs

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are invariably higher than for BiVO4/rGO LCs. The optimum rGO loading for both BiVO4/rGO

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LC and BiVO4/rGO HC was found to be 1.0 wt%, giving respective 3-fold and 8-fold of

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photocurrent enhancements relative to the neat BiVO4. A similar effect was obtained in the

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chronoamperometry test under the constantly applied bias of 0.5 V versus Ag/AgCl (Figure S5).

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The findings highlight the beneficial role of maximized interfacial contact between rGO and

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BiVO4 to improve charge transfer interactions in the PEC system, as evidenced by the greater

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photocurrent enhancement portrayed by BiVO4/rGO HC. Furthermore, to investigate the

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photoelectrochemical stability of BiVO4 and BiVO4/rGO composites, their transient photocurrent

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densities with on-off illumination cycles were provided under the external bias of 0 V and 0.5 V

15

vs. Ag/AgCl, as shown in Figure S6. All of BiVO4 and BiVO4/rGO composites photoelectrodes

16

demonstrated reproducible anodic photocurrents. The neat BiVO4 photoanode exhibits a better

17

photoelectrochemical stability than the BiVO4/rGO composites with almost no photocurrent decay

18

after several illumination on-off cycles across range the external biases applied. Meanwhile, with

19

the cycled on-off illumination, both BiVO4/rGO LC and BiVO4/rGO HC suffer slight

20

photocurrents decay under different biases. Afterwards, the moderately steady-state and apparently

21

enhanced photocurrent densities were obtained for the BiVO4/rGO composites.

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To develop a deeper understanding on the influence of BiVO4/rGO interfacial contact on the

23

carrier dynamics of the material, the extent of charge recombination in the BiVO4/rGO composites

24

was compared with neat BiVO4 using steady-state photoluminescence (PL), as illustrated in Figure

25

3c. All the steady-state PL spectra exhibit a strong emission peak at around 500 nm, which matches

26

well with the band gap energy of BiVO4 (Figure 2a) and corresponds to the band edge emission of

27

BiVO4. PL quenching in the rGO presence supports the electron-accepting role of rGO, which

28

expedites electron transfer from BiVO4 and suppresses charge recombination. A more discernible


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PL quenching by BiVO4/rGO HC compared to BiVO4/rGO LC verifies the benefit of thorough

2

rGO coverage on BiVO4 to improve charge separation and minimize charge recombination in the

3

composite material. The band edge fluorescence lifetimes of BiVO4, BiVO4/rGO LC, and

4

BiVO4/rGO HC were also determined by a time-correlated single-photon counting (TCSPC)

5

approach. As shown in Figure 3d, the time-resolved PL decay curves could be well-fitted using a

6

biexponential function with the fast decay components (τ1) and slow decay components (τ2)

7

associated with the nonradiative and radiative recombination of the electron-hole pairs,

8

respectively.30 The fitted τ1 and τ2 values for all samples are listed in Table 1. Calculation of the

9

average lifetime based on the decay time constants revealed that BiVO4/rGO LC and

10

BiVO4/rGO HC have considerably shorter charge lifetimes compared to neat BiVO4. With an

11

average lifetime of 5.98 ns for neat BiVO4, rGO incorporation reduces the average lifetime to 3.17

12

ns for BiVO4/rGO LC and 2.64 ns for BiVO4/rGO HC. The shortened lifetimes are attributed to

13

efficient photogenerated electron transfer from BiVO4 in the presence of rGO,29, 31 consistent with

14

the steady-state PL results. The shortest lifetime of BiVO4/rGO HC further highlights the benefits

15

of optimizing rGO/BiVO4 interfacial contact for improved charge transfer between BiVO4 and

16

rGO.

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Table 1. Fitting parameters of time-resolved PL spectra of BiVO4 and BiVO4/rGO composites.

18

Sample

τ1 / ns

τ2 / ns

/ ns

19

BiVO4

0.94±0.002

6.00±0.245

5.98±0.245

20

BiVO4/rGO LC

0.53±0.006

3.28±0.041

3.17±0.040

BiVO4/rGO HC

0.28±0.001

2.65±0.019

2.64±0.19

21 22

Electrochemical impedance spectroscopy measurements were conducted to probe the charge

23

transfer efficiency and the donor densities of the photoelectrodes based on Nyquist and Mott-


15


Page 16 of 34

1

Schottky (MS) plots, respectively.32 The smaller arc radii of BiVO4/rGO LC and BiVO4/rGO HC

2

with respect to neat BiVO4 (Figure 3e) indicates that rGO incorporation promotes the charge

3

transfer process in the photoelectrodes. In particular, BiVO4/rGO HC displayed the greatest charge

4

transfer ability. Figure 3f provides the MS plots of the samples, measured at a frequency of 500

5

Hz. According to the x-intercepts of the MS plots, a slightly cathodic shift of flatband potential of

6

BiVO4/rGO composites was observed, among which BiVO4/rGO HC exhibited a more negative

7

trend than BiVO4/rGO LC compared to the neat BiVO4. The positive slopes displayed by the MS

8

plots confirm the n-type semiconducting characteristic of the materials and can be used to

9

determine the donor density (ND) of the photoelectrode.33-34The slopes for BiVO4/rGO LC and

10

BiVO4/rGO HC are smaller than for the neat BiVO4, indicating an increase in donor density for

11

both of the composite materials. The ND values were quantitatively estimated as 7.37 × 1018 cm-3

12

for BiVO4/rGO LC and 8.70 × 1018 cm-3 for BiVO4/rGO HC, which are twice that of neat BiVO4

13

(3.46 × 1018 cm-3). The steady-state PL, time-resolved PL, and Nyquist plot analyses jointly

14

indicate that rGO incorporation improves the charge separation and charge transfer aspects of the

15

BiVO4/rGO composites, which is supported by the increase in donor densities as verified by the

16

MS plots. Maximization of the interfacial contact between rGO and BiVO4 has an undeniably

17

positive impact on the carrier dynamics, explaining the superior performance of BiVO4/rGO HC

18

in the PEC system.

19


16

Page 17 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 2

Figure 4. UPS spectra of BiVO4, BiVO4/rGO LC, and BiVO4/rGO HC, measured relative to the

3

Fermi level.

4

As well as being a good electron mediator by accepting and shuttling photogenerated electrons

5

from a semiconductor, rGO has been demonstrated to have electron storage capabilities.35

6

Considering the thermodynamically favorable electron transfer from excited BiVO4 to rGO,

7

electron accumulation in the composite arising from charge equilibration among the two


17


Page 18 of 34

1

components will shift the Fermi level to a more negative potential. This is because the Fermi level

2

of the n-type semiconductor is directly dependent on the electron concentration.36 A negative shift

3

in Fermi level has been observed for semiconductor/metal composites due to the electron retaining

4

characteristic of the metal.31, 36-38 Ultraviolet photoelectron spectrum (UPS) was used to evaluate

5

the Fermi levels of the BiVO4/rGO composites in comparison to neat BiVO4 based on work

6

function (ϕ) estimation. Figure S7 pinpoints the secondary electron cut-off (Ecut-off), Fermi edge

7

(EFermi), and vacuum level (Evac) positions on the UPS spectrum of BiVO4 and Ag. Accordingly, ϕ

8

can be deduced by subtracting Ecut-off from the incident photon energy (21.2 eV). Based on the Ecut-

9

off

values determined from Figure 4, the ϕ of the samples is revealed to decrease from 5.20 eV for

10

neat BiVO4, to 5.15 eV for BiVO4/rGO LC and 5.09 eV for BiVO4/rGO HC, illustrating the

11

negative shift in Fermi level across the samples, which is in coincidence with the cathodic shift in

12

the MS plots. While the negative shift highlights efficient charge separation between BiVO4 and

13

rGO, and is consistent with the results discussed previously, it also indicates an improved reductive

14

power of the composite material compared to neat BiVO4. In particular, BiVO4/rGO HC is more

15

energetic than BiVO4/rGO LC.


18

Page 19 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 2

Figure 5. Photocatalytic O2 evolution with AgNO3 as the electron scavenger (a) and water contact

3

angles (b) of BiVO4, BiVO4/rGO LC, and BiVO4/rGO HC samples; O2 evolution rate (c) of BiVO4

4

and BiVO4/rGO composites with different rGO loading evaluated from the initial 1h illumination.

5

The influence of the different BiVO4 and rGO interfacial contact configurations on water

6

oxidation was subsequently evaluated in the PS system under visible light using silver ions (Ag+)

7

as the electron scavenger. Based on the improved carrier dynamics and energetics for BiVO4/rGO

8

as discussed above, enhanced photocatalytic O2 evolution by the BiVO4/rGO composites,

9

especially BiVO4/rGO HC, in comparison with neat BiVO4 may be envisaged. However, the time-

10

profiles for oxygen evolution by neat BiVO4, BiVO4/rGO LC, and BiVO4/rGO HC (Figure 5a) are


19


Page 20 of 34

1

contrary to such an expectation. BiVO4/rGO LC exhibits the highest photocatalytic O2 evolution

2

rate followed by neat BiVO4 and then BiVO4/rGO HC. The amounts of O2 evolved by bare BiVO4,

3

BiVO4/rGO LC, and BiVO4/rGO HC over the three hour reaction period were 268, 322, and 234

4

μmol, respectively. The improved performance of BiVO4/rGO LC can be ascribed to the rGO-

5

mediated effective charge separation and charge transfer combined with improved energetics of

6

the composite. However, the diminished activity of BiVO4/rGO HC indicates some alternate effect

7

overshadows the benefits of the composite material with this make-up in the PS system. To verify

8

the difference in water oxidation performance was originated from the distinct rGO coverage and

9

eliminate the potential influence from the Ag+ ion, another batch of reaction was conducted with

10

Fe3+ as electron scavenger instead, as shown in Figure S8. It is clear that the photocatalytic oxygen

11

evolution performance of BiVO4/rGO LC outperforms that of the neat BiVO4 and BiVO4/rGO HC,

12

manifesting the same trend as obtained from the Ag+ solution.

13

It was observed that, when preparing a BiVO4/rGO LC suspension, the particles were readily

14

dispersed. In contrast, when preparing a BiVO4/rGO HC suspension, a significant amount of

15

BiVO4/rGO HC powder was seen to remain floating on the solution surface despite prolonged

16

ultrasonication and constant stirring (Figure S9). Water contact angle measurements were

17

conducted to probe the difference in hydrophobicity between the samples. The water contact angle

18

(Figure 5b) of BiVO4/rGO HC (155.7 °) was found to be considerably larger than for neat BiVO4

19

(104.6 °) and BiVO4/rGO LC (111.6 °), demonstrating the highly hydrophobic nature and water

20

immiscibility of BiVO4/rGO HC. Given the hydrophobic characteristic of the carbon backbone in

21

rGO,39 it is reasonable to anticipate that rGO incorporation will increase the hydrophobicity of the

22

BiVO4/rGO composites. The significantly greater hydrophobicity of BiVO4/rGO HC compared to

23

BiVO4/rGO LC can be ascribed to the higher degree of reduction for the material, as evidenced by


20

Page 21 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

XPS (Figure 2b). On removing more hydrophilic oxygen functional groups from GO, the resulting

2

rGO can be expected to be more hydrophobic and thus more water-repellent. The poor water

3

miscibility and ensuing lack of BiVO4/rGO HC contact with water (the electron donor) delivers a

4

poorer photocatalytic water oxidation capability of the composite.

5

To suppress the large hydrophobic difference between BiVO4/rGO LC and BiVO4/rGO HC,

6

these composites with lower rGO contents (0.1, 0.5, and 1.0 wt%) were prepared and assessed for

7

their photocatalytic water oxidation activities. The hydrophobic characteristic of BiVO4/rGO HC

8

was found to be comparable to BiVO4/rGO LC when the rGO loading was 0.5 wt% or less (Figure

9

S10). Significantly, the photocatalytic activity of BiVO4/rGO HC strongly depends on its water

10

miscibility (Figure 5c). On tuning the BiVO4/rGO HC hydrophobicity to be comparable with

11

BiVO4/rGO LC, the photocatalytic water oxidation performance of the former can be improved

12

over the neat BiVO4. However, the activity by BiVO4/rGO LCs was consistently higher than the

13

neat BiVO4 and BiVO4/rGO HCs regardless of the rGO loading, which is the inverse of that seen

14

for the PEC system. In addition to water miscibility differences, another contributing effect to

15

BiVO4/rGO LC outperforming BiVO4/rGO HC for PS photocatalytic water oxidation is the

16

extensive coverage of BiVO4 by rGO in BiVO4/rGO HC may obscure reactive sites on BiVO4

17

surface and restrict water accessibility.

18 19

CONCLUSION

20

Two BiVO4/rGO composites, one with a low rGO coverage on the BiVO4 surface and the other

21

with a high rGO coverage, were fabricated such that the interaction between the two components

22

varied substantially. The tailored rGO coverage delivered opposing effects for the water

23

photooxidation performance of the composites in PS and PEC systems. The increased interfacial


21


Page 22 of 34

1

contact between BiVO4 and rGO in BiVO4/rGO HC gave a greater PEC anodic photocurrent

2

relative to BiVO4/rGO LC, which was reflected in the water oxidation reaction at the photoanode

3

surface. In contrast, the O2 evolution performance of BiVO4/rGO HC in the PS system was the

4

poorest. The superior interfacial contact between BiVO4 and rGO in BiVO4/rGO HC led to

5

improved charge separation efficiency and electron transfer between the two components,

6

delivering a better photocurrent relative to BiVO4/rGO LC Interestingly, the rGO/BiVO4

7

composites experienced a shift in Fermi level to more negative potentials (relative to the neat

8

BiVO4) with BiVO4/rGO HC undergoing the greatest shift. The shift indicates that BiVO4/rGO

9

HC was more reductive than BiVO4/rGO LC. Despite the merits of BiVO4/rGO HC, its stronger

10

hydrophobicity, which derived from the higher degree of rGO reduction and was a function of the

11

rGO loading, lowered its capacity for dispersion in the PS system. This constrained the required

12

good contact between the semiconductor and the reactant and impaired the O2 evolution rate. The

13

work highlights the contrasting effects of interfacial contact between BiVO4 and rGO on water

14

oxidation in PS and PEC systems. Hydrophobicity was identified as a critical factor in the PS

15

system such that it can overshadow any benefits greater interfacial contact can provide. It remains

16

a challenge to balance the pros and cons of graphene to justify its usefulness in developing

17

graphene-based photocatalysts with enhanced catalytic efficiency.

18 19

EXPERIMENTAL SECTION

20

Bismuth (III) oxide (Bi2O3, 99.9%), vanadium (V) oxide (V2O5, ≥99.6%), silver nitrate (AgNO3,

21

≥99.5%) graphite ( 420 nm) along with constant stirring and continuous argon (Ar) gas

14

bubbling. The color change from vivid yellow to dark yellow was observed during this process.

15

After photodeposition, the product was dried at 110 °C overnight.

16

2) BiVO4/rGO HC: 100 mg of BiVO4 and 2 mg of GO were suspended in 100 ml of ethanol and

17

50 ml of deionized water, respectively. These two suspensions were ultrasonicated to ensure a

18

homogeneous dispersion. Next, the GO-containing suspension was added dropwise into the BiVO4

19

suspension under vigorous stirring. The mixture was stirred for 12 hours without sealing and the

20

residual solution was then heated at 60 °C in an oil bath with stirring. Both the stirring and heating

21

processes were carried out under dark to prevent GO photoreduction. Upon complete evaporation

22

of the solution, the as-obtained BiVO4/GO was redispersed in 25 ml of ethanol and the

23

photocatalytic reduction of GO was performed under the same experimental conditions as


23


Page 24 of 34

1

BiVO4/rGO LC. A color change from vivid yellow to dark yellow of was observed during the

2

evaporation-induced self-assembly phase of BiVO4/GO after which the composite turned dark

3

green during GO photoreduction. The precipitate was collected and dried at 110 °C to obtain

4

BiVO4/rGO HC.

5

Powder suspension photocatalytic O2 evolution measurement:

6

Photocatalytic O2 evolution was conducted in an enclosed gas circulation system directly linked

7

with an online gas chromatograph (Shimadzu GC-8A) equipped with a TCD detector, a molecular

8

sieve 5A column and Ar as the carrier gas. 50 mg of BiVO4 or BiVO4/rGO composite was firstly

9

well-dispersed in 150 ml of 0.05 M silver nitrate (AgNO3) aqueous solution in a top-irradiation

10

reactor with a Pyrex window. Prior to irradiation, the entire system was repeatedly evacuated to

11

about 2 kPa and purged with Ar gas at least five times to remove dissolved O2 from the solution.

12

An Oriel 300 W xenon lamp with 420 nm cut-off filter was used as the visible light source. The

13

evolved O2 was collected and quantified by the online GC at a predetermined time interval.

14

Photoelectrochemical measurements:

15

The powder samples (BiVO4 and BiVO4/rGO composites) were first made into thin film

16

electrodes by separately drop-casting the powder onto a 2×1.5 cm2 area of conducting fluoride-

17

doped tin oxide (FTO) glass. The surface density was 2 mg/cm2. The electrodes were heated at

18

150 °C overnight to improve photocatalyst adhesion to the substrate.

19

PEC measurements were performed in a standard three-electrode system with the BiVO4 or

20

BiVO4/rGO photoelectrode as the working electrode, a Pt foil as the counter electrode, and a

21

saturated Ag/AgCl as the reference electrode. A 0.1 M sodium sulfate (Na2SO4) with saturated

22

nitrogen (N2) was employed as the electrolyte. An Oriel 300 W xenon lamp with 420 nm cut-off

23

filter was used as the light source. Linear sweep voltammetry (LSV), chronoamperometry, and


24

Page 25 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

electrochemical impedance spectroscopy (EIS) measurements were acquired using an Autolab

2

potentiostat (Model PGSTAT320N). The LSV and chronoamperometry were obtained with

3

backside illumination under chopped irradiation. EIS Nyquist plots were procured under the dark

4

condition over the frequency range of 10-2 to 106 Hz and the amplitude of the sinusoidal wave was

5

5 mV. Mott-Schottky measurements were also conducted under the dark with a frequency of 500

6

Hz. The donor density (ND) of the photoelectrode is inversely proportional to the slope of the MS

7

plot (

C  2 ). V 1

8

2  C  2  ND  e0 A 2  V 

9

where C is space charge layer capacitance of photoanode, ε is relative dielectric constant (~ 68

10

for BiVO4), ε0 is vacuum dielectric constant (8.85×10-12 Fm-1), e is the electronic charge (1.6×10-

11

19

12

Material characterization:

1

C), V is the applied bias, and A is the geometrical area exposed to electrolyte (1.5 cm × 2.0 cm).

13

Powder X-ray diffraction (XRD) patterns were recorded on a Phillips X'pert MPD x-ray

14

diffractometer using a Cu Kα radiation source (λ = 1.54 Å) with an operating voltage and operation

15

current of 45 kV and 40 mA, respectively. The surface microstructure and elemental distribution

16

were analyzed using an FEI NOVA 450 field emission scanning electron microscopy (FE-SEM)

17

with an operational voltage at 15 kV. A thermogravimetric TA Q5000 analyzer was used to

18

quantify the rGO content in the BiVO4/rGO composites. The sample was heated from room

19

temperature to 900 °C at a rate of 10 °C/min under 25 ml/min of air flow. X-ray photoelectron

20

spectroscopy (XPS) was undertaken using a Thermo ESCALAB250Xi instrument with a

21

monochromated Al Kα radiation source (hν = 1486.68 eV). Calibration of the binding energy was

22

based on the C 1s peak at 284.8 eV. Ultraviolet photoelectron spectroscopy (UPS) measurements


25


Page 26 of 34

1

were carried out using He excitation (hν = 21.2 eV) in the ultrahigh vacuum chamber of the XPS.

2

The analyzer was first calibrated with Ag to determine analyzer Fermi edge on the energy scale.

3

UV-visible diffusion reflectance spectra (UV-vis) were obtained from a Shimadzu UV 3600

4

spectrophotometer. The absorbance results were converted from the initial reflectance spectra via

5

Kubelka-Munk transformation. Photoluminescence (PL) spectra were attained on a Horiba

6

Fluoromax-4 spectrofluorometer with an excitation wavelength of 405 nm. Water contact angles

7

were measured on the thin film of as-prepared electrodes. An Attension Theta T200 contact angle

8

goniometer was used in conjunction with the sessile drop method. A droplet of water (~ 4 μl) was

9

deposited on the tested film and the contact angle then captured by a high-speed camera.

10

Fluorescence lifetimes were measured by time‐correlated single‐photon‐counting (TCSPC)

11

technique (Microtime‐200 system, Picoquant) following excitation with a 405 nm laser. The time-

12

resolved PL spectra curves were fitted using a biexponential function (Equation 2) and the average

13

lifetime, , was evaluated by Equation 3, where τ is the fitted lifetime and A indicates the

14

amplitude. 𝑡

𝑡

15

y = 𝐴1 e−( ⁄𝜏1) + 𝐴2 e−( ⁄𝜏2)

16

< 𝜏 >= 𝐴1 𝜏1 +𝐴2 𝜏2

2

𝐴 𝜏2 +𝐴 𝜏2 1 1

3

2 2

17 18

SUPPORTING INFORMATION

19

XRD patterns of GO, BiVO4, BiVO4/rGO LC, and BiVO4/rGO HC. SEM image of neat BiVO4

20

and BiVO4/GO. XPS survey, Bi 4f and V 2p spectra of BiVO4, BiVO4/rGO LC, and BiVO4/rGO

21

HC. Raman spectra of BiVO4/rGO LC, and BiVO4/rGO HC. Photocurrent density of BiVO4 and

22

BiVO4/rGO composites with different rGO loadings, measured at 0.5 V versus Ag/AgCl.

23

Photoelectrochemical stability of BiVO4, BiVO4/rGO LC and BiVO4/rGO HC. UPS spectra


26

Page 27 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

calibration using Ag. Photocatalytic O2 evolution in the presence of 2 mM Fe3+ (pH 2.4). Top view

2

of BiVO4/rGO LC and BiVO4/rGO HC suspensions prior to photocatalytic water oxidation

3

reactions. Water contact angles of BiVO4/rGO LC and BiVO4/rGO HC composites with varying

4

rGO loadings, namely 0.1, 0.5, and 1.0 wt%.

5

AUTHOR INFORMATION

6

Corresponding Author:

7

* E-mail: [email protected].

8

* E-mail: [email protected].

9

†Present

address:

10

Department of Applied Chemistry, Faculty of Engineering, Kyushu University, Nishi-Ku,

11

Fukuoka 819-0395, Japan

12

ORCID:

13

Zhirun Xie: 0000-0002-4864-0256

14

Hui Ling Tan: 0000-0001-6156-2857

15

Xiaoming Wen: 0000-0001-8298-483X

16

Akihide Iwase: 0000-0002-6395-9556

17

Rose Amal: 0000-0001-9561-4918

18

Jason Scott: 0000-0003-2395-2058

19

Yun Hau Ng: 0000-0001-9142-2126

20 21


27


Page 28 of 34

1

Funding:

2

This work is supported by the Australian Research Council Discovery Projects DP180102540 and

3

DP170102895.

4

Notes

5

The authors declare no competing financial interest

6

ACKNOWLEDGEMENTS

7

We thank the Australian Research Council for financial support (DP180102540 and

8

DP170102895). We acknowledge the UNSW Mark Wainwright Analytical Centre for providing

9

access to all the analytical instruments.

10


28

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TABLE OF CONTENTS GRAPHIC

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Is Reduced Graphene Oxide Always an Enhancer in Photo(Electro)

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