Nanocomposites of Three-Dimensionally-Ordered Porous TiO2

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Nanocomposites of Three-Dimensionally-Ordered Porous TiO Decorated with Pt and Reduced Graphene Oxide for the VisibleLight Photocatalytic Degradation of Waterborne Pollutants Jingwan Huo, Chris Yuan, and Yin Wang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00215 • Publication Date (Web): 17 Apr 2019 Downloaded from http://pubs.acs.org on April 18, 2019

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ACS Applied Nano Materials

1

Nanocomposites of Three-Dimensionally-Ordered Porous TiO2 Decorated with Pt and

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Reduced Graphene Oxide for the Visible-Light Photocatalytic Degradation of Waterborne

3

Pollutants

4 Jingwan Huo†, ‡, Chris Yuan§, *, Yin Wang†, *

5 6

†Department

7

Milwaukee, WI 53201, USA

8

‡Department

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53201, USA

10

§Department

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Cleveland, OH 44106, USA

of Civil and Environmental Engineering, University of Wisconsin – Milwaukee,

of Mechanical Engineering, University of Wisconsin – Milwaukee, Milwaukee, WI

of Mechanical and Aerospace Engineering, Case Western Reserve University,

12 13

*Corresponding author

14

Yin Wang, 3200 N. Cramer St., Milwaukee, WI 53211, [email protected], 414-229-3137

15

Chris Yuan, 10900 Euclid Ave., Cleveland, OH 44106, [email protected], 216-368-5191

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Revised Manuscript Submitted to ACS Applied Nano Materials

18

2019

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1 ACS Paragon Plus Environment

ACS Applied Nano Materials 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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A new photocatalyst was developed by coupling three-dimensionally ordered macroporous

22

(3DOM) TiO2 with platinum (Pt) and reduced graphene oxide (rGO) for water purification

23

applications under visible light. The rGO/Pt/3DOM TiO2 ternary photocatalyst was synthesized by

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co-decoration of 3DOM TiO2 prepared by the colloidal crystal templating approach with Pt

25

nanoparticles and rGO nanosheets via impregnation and in situ reduction methods. Modification

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with Pt and rGO reduced the band energy and significantly improved the visible light absorption

27

of the rGO/Pt/3DOM TiO2 photocatalyst. The performance of rGO/Pt/3DOM TiO2 was

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investigated for the photodegradation of methyl orange as a model dye pollutant in water under

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visible light irradiation. rGO/Pt/3DOM TiO2 enhanced the degradation rates of methyl orange by

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a factor of six, four and two, compared to bare 3DOM TiO2, Pt/3DOM TiO2 and rGO/3DOM TiO2,

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respectively. The significantly enhanced photocatalytic activity of rGO/Pt/3DOM TiO2 may be

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attributed to the synergy of the highly ordered 3DOM structure that induced the slow photon effect

33

and the decorated Pt and rGO that promoted light absorption and charge separation. The

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rGO/Pt/3DOM TiO2 ternary photocatalyst showed high stability that it can be used for several

35

cycles without reducing activity. Results suggested that rGO/Pt/3DOM may hold promises as a

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visible light photocatalyst for the removal of waterborne contaminants.

37 38

KEYWORDS: three-dimensionally ordered macroporous TiO2, inverse opal, graphene, Pt

39

nanoparticles, photocatalysis, water purification

40


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INTRODUCTION

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Semiconductor-based heterogeneous photocatalysis has been increasingly applied to

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address environmental pollution problems.1-3 Specially, photocatalytic degradation of organic

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pollutants holds great promises for water and wastewater treatment, due to its advantageous

45

features that include low cost, mild operating conditions (e.g., ambient temperature and pressure),

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and no production of secondary waste streams.4-5 Under light irradiation, a semiconductor

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photocatalyst absorbs light with energy equal to or greater than its band gap. This process excites

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an electron from the valance band (VB) to the conduction band (CB) and leaves a positively

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charged hole (h+) at the band edge of the VB. The photoinduced electron (e-) and hole can mitigate

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to the catalyst surface and react with dissolved oxygen, water and surface hydroxyl groups to

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produce reactive radicals, which can then oxidize and remove the organic pollutants.5-6

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Since the report of titanium oxide (TiO2) for photocatalytic splitting of water, TiO2 has

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been intensively investigated in energy and environmental applications and recognized among the

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most efficient photocatalysts because of its favorable properties, including chemical and biological

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inertness, high photostability, low cost, minimal toxicity and superior oxidization ability.1, 6-7 The

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photocatalytic activity of TiO2 is affected by its physicochemical properties that include crystalline

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structure, crystallite size, exposed lattice facet and surface area.8 Among various TiO2 materials,

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three-dimensionally ordered macroporous (3DOM) structure is drawing increasing attention

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because of its unique optical properties. The periodic macroporous structure of 3DOM materials

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has been found to restrict the propagation of light with certain energies due to coherent Bragg

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diffraction, resulting in stop-band reflection that increases the path length of light through slow-

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proton effect.9-12 When the photonic bandgap edges overlap with the electronic bandgap of a

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photocatalyst, the slow-proton effect at the photonic bandgap edges can significantly improve the



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light absorption and utilization efficiency, and thus lead to an improved photocatalytic activity.8,

65

13

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heterogeneous catalytic applications by promoting the immobilization and dispersion of

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nanoparticles and reducing the mass transfer limitation.14 However, TiO2 is a wide band gap

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photocatalyst that primarily utilizes UV light (i.e., 2-3% of the solar spectrum), which presents a

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major barrier for widespread technology adoption.15 Additionally, the fast electron-hole

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recombination rate within bare TiO2 also significantly reduces the photocatalytic reaction

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efficiency.7

In addition, the ordered macroporous structure of 3DOM materials is also beneficial for

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Doping TiO2 with noble metals has been recognized as an efficient approach to promote

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light harvesting and the photocatalytic activity of the material.16 Platinum (Pt) has been frequently

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used for TiO2 modification because of its excellent catalytic properties and the surface plasmon

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resonance (SPR) effect that improves both charge separation and light absorption of the catalysts.17

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Chen et al reported the synthesis of Pt-modified 3DOM TiO2 for photodegradation of dye

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pollutants (methyl orange) in water. Improved photocatalytic activity was observed because of the

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synergy of the slow photon effect induced by the 3DOM structure and the chemically amplified

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photochemistry by Pt nanoparticles.18 Liang et al also found improved catalytic performance of

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Pt/3DOM TiO2 for water-gas shift reactions, which was attributed to both the high intrinsic activity

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of Pt/TiO2 and the 3DOM porous structure that increased mass transfer.19 However, the catalytic

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activity of noble metal/TiO2 materials is still impeded by the ultrafast relaxation of hot electrons

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and the rapid charge recombination between electrons in TiO2 and holes in noble metal

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nanoparticles.20-22

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Two-dimensional graphene is an emerging class of carbon material that garnered

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increasing attention in photocatalytic applications because of its superior charge transport


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properties.23-27 Introduction of graphene has been considered an effective strategy to maximize the

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charge separation of noble metal/TiO2 materials, and several studies have reported the

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development of ternary hybrid catalysts that coupled reduced graphene oxide (rGO) or graphene

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with various structured noble metal/TiO2 materials that include TiO2 nanoparticles, nanotubes and

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nanosheets.21-22 However, despite the favorable properties of 3DOM structure, investigations on

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the synergy between graphene and noble metal loaded 3DOM TiO2 remain limited. In a recent

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pioneering work, Boppella et al reported the development of rGO and Au-incorporated 3DOM

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TiO2 photoelectrodes as photoelectrochemical water splitting devices for energy conversion

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applications.28 Pre-synthesized Au nanoparticles were used in that study with a relatively large

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particle size (~15 nm). In designing ternary catalysts for water purification applications, ideally a

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reduced noble metal particle size would be favorable to promote the degradation of pollutants. To

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the best of our knowledge, however, the development of an efficient rGO/noble metal/3DOM TiO2

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ternary nanostructured catalyst has not been explored for photodegradation of waterborne

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pollutants.

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In this contribution, we reported the design, characterization and photocatalytic activity of

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a new rGO/Pt/3DOM TiO2 ternary photocatalyst for water purification application. The catalyst

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was prepared by growing Pt nanoparticles in situ within the porous network of 3DOM TiO2,

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followed by decoration with rGO. The performance of the catalyst was investigated for the

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photodegradation of methyl orange as a model dye pollutant in water under visible light irradiation.

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rGO/Pt/3DOM TiO2 exhibited significantly enhanced photocatalytic activity, due to the

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synergistic effects of the 3DOM structure that reduced photon velocity and improved mass transfer,

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and the decorated Pt and rGO that promoted charge separation and light absorption. Stability test

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suggested that the ternary catalyst could be used for multiple times without compromising the



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photocatalytic activity. The rGO/Pt/3DOM TiO2 ternary catalyst holds great promises as a visible

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light photocatalyst in the treatment of waterborne pollutants.

112 113

EXPERIMENTAL SECTION

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Chemicals. All chemicals were reagent grade and used as received. Methyl methacrylate

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(MMA), titanium butoxide (Ti(OBu)4), acetylacetone, ethanol, chloroplatinic acid hexahydrate

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(H2PtCl6.2H2O), and methyl orange were purchased from Sigma-Aldrich. Single layer graphene

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oxide (GO) was obtained from ACS Material. Potassium persulfate (K2S2O8) was purchased from

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Alfa Aesar. Hydrochloric acid (HCl), nitric acid (HNO3), 1,4-benzoquinone (BQ), isopropanol

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(IPA), and ethylenediaminetetraacetate (EDTA) were acquired from Fisher Scientific. Ultrapure

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water (resistivity > 18.2 MΩ) was used for all experiments.

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Preparation of photocatalysts. Monodisperse poly(methyl methacrylate) (PMMA)

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nanospheres were prepared using surfactant-free emulsion polymerization based on a modification

123

of protocols reported by Zou et al.29 Briefly, ultrapure water (165 mL) and MMA (18.6 mL) was

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added into a completely stirred three-neck round-bottom flask equipped with a water-cooled

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condenser. The solution was heated to 70 ºC under nitrogen gas protection. 10 mL of an aqueous

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K2S2O8 solution (0.5 x 10-3 mol/L) was then added into the mixture as the initiator. The reaction

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was sustained for 2 hours to produce colloidal PMMA spheres. PMMA colloidal crystals were

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then obtained through a self-assembling approach via centrifuging the colloid suspension at 1500

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rpm for 24 h, removing the water and air-drying the solid for 3 days. The PMMA colloidal crystals

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were preserved for use as a hard template for the preparation of 3DOM TiO2.

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3DOM TiO2 was fabricated using a colloidal crystal templating approach based on

132

established protocols.8 Briefly, TiO2 precursor was prepared by mixing Ti(OBu)4, acetylacetone ,


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H2O and ethanol in a molar ratio of 1:1:3:20 in open air at room temperature (20 ± 1 °C) for 2

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hours.30 Dry PMMA colloidal crystals were grounded into powdered form and deposited onto a

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filter paper within a Bϋchner funnel in mm-thick layers. The precursor solution was then added

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dropwise to completely wet the PMMA layer with suction applied to the Bϋchner funnel. This

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infiltration step was repeated 5 times to allow the TiO2 precursor fulfilling the voids of the PMMA

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template. After dried at room temperature for 2 hours, the solids were calcined at 300 ºC at a step

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of 2 ºC per min for 3 hours to remove the PMMA template, and then at 450 ºC at a step of 2 ºC per

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min for 4 hours to calcine TiO2.

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Pt was loaded onto 3DOM TiO2 through an in situ growth method with the use of H2PtCl6

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as precursor.19, 31 In a typical synthesis, 1.5 mL of a 7-mM H2PtCl6 aqueous solution was added

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dropwise onto 0.1 g of 3DOM TiO2 solid. The paste was dried at 70 oC for 12 hours and then

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thermally reduced at 500 ºC in air for 3 hours. This approach provided the initial Pt content of the

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Pt/3DOM TiO2 to be ~2.0 wt%.

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To prepare rGO/Pt/3DOM TiO2, an aliquot of a GO stock solution (0.127 mL, 10 mg/mL)

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was first added into 20-mL water, followed by ultrasonication for 1 hour (Branson, 2510). Then,

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a desired amount of Pt/3DOM TiO2 (0.0636g) was added into the GO solution and ultrasonicated

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for 1 hour. The solid was filtered with FP-200 membrane (Paul Life Sciences, PVDF) and dried in

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air to obtain Pt/GO/3DOM TiO2 with the GO content of 2.0 wt%. Pt/GO/3DOM TiO2 was further

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calcined at 500 oC for 2 hours to reduce GO to rGO. rGO/3DOM TiO2 was prepared using the

152

same method by replacing Pt/3DOM TiO2 with 3DOM TiO2.

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Material characterization. The morphology and composition of the photocatalysts were

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investigated using transmission electron microscopy (TEM, Hitachi H9000NAR TEM) and

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scanning electron microscopy (SEM, Hitachi S-4800 FE-SEM) coupled with energy-dispersive X-



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ray spectroscopy (EDS). The content of Pt in Pt/3DOM TiO2 and rGO/Pt/3DOM TiO2 was

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quantified via digesting the solids in a mixture of HCl and HNO3 (volume ratio 1:4) at 100 °C,

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filtering the digestate using syringe filters (Millipore, polyethersulfone (PES) 0.22-µm), and

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measuring the concentration of Pt in the filtrates by inductively coupled plasma-optical emission

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spectroscopy (ICP-OES) (Perking Elmer Optima 2100 DV). The Brunauer-Emmet-Teller (BET)

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surface areas of the photocatalysts were determined by measuring N2 adsorption/desorption using

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a Micromeritics ASAP 2000 surface analyzer. The crystalline structure of the photocatalysts was

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investigated via powdered X-ray diffraction (XRD) using Bruker D8 Discover A25 diffractometer

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with copper Kα radiation. Raman spectra were performed to determine the functional groups of the

165

catalysts using a Reinshaw 1000B Raman spectrometer with a 632.8 nm HeNe laser source. The

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UV-Vis diffuse reflectance spectra (DRS) were obtained using a Shimadzu UV-2600 UV-Vis

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spectrophotometer, with BaSO4 as reference. X-ray photoelectron spectroscopy (XPS) was used

168

to determine the oxidation states of individual elements in the catalysts using a Perkin Elemer PHI

169

5440 ESCA system with an Al Kα X-ray source. The photoluminescence (PL) spectra of the

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catalysts were determined using a Cary Eclipse Fluorescence Spectrophotometer. Electrochemical

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impedance spectroscopy (EIS) analysis was performed using a CHI 600 Electrochemical Analyzer.

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Photocatalytic activity test. The performance of 3DOM TiO2, Pt/3DOM TiO2,

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rGO/3DOM TiO2 and rGO/Pt/3DOM TiO2 was investigated by monitoring methyl orange

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degradation under visible light irradiation. Experiments were conducted in a completely mixed

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batch reactor surrounded by a water-cooled jacket for temperature control (20 ± 1 °C). A 300-W

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Xenon lamp (CEL-HXF300, Beijing China Education Au-light Co., Ltd) with a cut-off filter

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(λ>420 nm) was applied to provide visible light irradiation and placed 10 cm over the top of the

178

reactor. In each experiment, 20 mg of photocatalyst was added into the reactor that contained 20


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mL of a 10-mg/L methyl orange solution, and the mixture was placed in dark for 30 minutes to

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achieve adsorption-desorption equilibrium prior to photocatalytic activity test. The solution pH

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was not adjusted and was stable at ~6 over the course of the experiment. During visible light

182

illumination, aliquots (1 mL) of samples were collected periodically up to 2 hours and centrifuged

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immediately to remove catalyst particles. The concentration of residual methyl orange was

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determined by monitoring its UV-visible absorption at 464 nm using a UV-vis spectrophotometer

185

(Shimadzu UV-2600).32

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The stability of rGO/Pt/3DOM TiO2 was evaluated by performing the photocatalytic

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activity tests for three cycles under the same experimental condition described above. After each

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cycle, rGO/Pt/3DOM TiO2 was separated from solution by centrifugation, washed with ultrapure

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water, and air dried. The photocatalyst was then re-dispersed in a freshly prepared methyl orange

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solution to start a new cycle.

191 192

RESULTS AND DISCUSSION

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Synthesis and characterization of photocatalysts. A three-step strategy was applied to

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synthesize the powdered rGO/Pt/3DOM TiO2 ternary photocatalyst (Figure 1), including: (1)

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preparation of colloidal crystal-templated 3DOM TiO2 with ordered macroporous structure, (2) in

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situ growth of Pt nanoparticles within the 3DOM TiO2, and (3) modification and reduction of GO

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to obtain the rGO/Pt/3DOM TiO2 composite. A combination of techniques that include colloidal

198

crystal templating, sol-gel processing, and impregnation method were used in the photocatalyst

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preparation process.

200

Surface morphology and composition of the photocatalysts were examined using SEM and

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TEM. SEM images showed that 3DOM TiO2 exhibited a highly ordered macroporous structure



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with a pore diameter of ~210 nm and a wall thickness of ~25 nm (Figure 2A). In the synthesis of

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3DOM TiO2, the PMMA template was oxidized and gasified during the calcination treatment,

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which caused a small extent of pore shrinkage, and resulted in a smaller pore size in comparison

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to the diameter of PMMA microspheres (~330 nm).11, 33-34 The small windows along the opal pores,

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together with the overlapped layers of porous structure, replicated the three-dimensional closely-

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packed PMMA opals (Figure S1 of Supporting Information), suggesting that 3DOM TiO2

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possesses a three-dimensionally well-open, ordered and interconnected macroporous network.

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Once Pt was loaded through impregnation and in situ reduction of H2PtCl6, small nanoparticles

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were observed on the porous skeleton of 3DOM TiO2, and the macroporous network was retained

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(Figure 2B). EDS spectra and elemental mapping confirmed the presence and uniform distribution

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of Pt within the network of 3DOM TiO2 (Figure S2 of Supporting Information). Analysis of the

213

digestate of the catalyst showed that the Pt loading was 1.7 wt%, which was in good agreement

214

with the theoretical value. The presence of Pt was corroborated by TEM observation showing that

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Pt nanoparticles were highly dispersed and anchored into the walls of the porous 3DOM TiO2 with

216

sizes ranging from 3 – 6 nm (Figure 2E). Both TiO2 and Pt nanoparticles were highly crystalline

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in nature, as observed by high-resolution TEM image (Figure 2F); the lattice fringes with a d-

218

spacing of 0.352 nm and 0.224 nm represented the (101) plane of anatase TiO2 and the (111) plane

219

of metallic Pt nanoparticles, respectively.21, 35 Deposition of GO was achieved via Van der Waals

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forces and electrostatic attraction with the surface of 3DOM TiO2 and Pt/3DOM TiO2.36 After in

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situ reduction, a continuous and transparent rGO film was observed on the surface of 3DOM TiO2

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(Figure 2C) and Pt/3DOM TiO2 (Figure 2D), and it had a minimal influence on the porous

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structure of the catalyst. 3DOM TiO2 and Pt/3DOM TiO2 had BET surface areas of 45.1 and 37.5

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m2 g-1, respectively. After deposition of rGO, the BET surface areas of rGO/3DOM TiO2 and


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rGO/Pt/3DOM TiO2 increased to 67.9 and 61.7m2 g-1, respectively (Figure S3 of Supporting

226

Information).

227

XRD and Raman spectroscopy were used to characterize the structure of the catalysts.

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XRD patterns suggested anatase as the predominant crystalline phase of 3DOM TiO2, evidenced

229

by the strong peaks at 2θ of 25.3°, 37.8°, 48.0°, 53.1°, 55.1°, and 62.7° that represented the (101),

230

(004), (200), (105), (211), and (204) planes of anatase (JCPDS 21-1272), respectively (Figure

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3A). Meanwhile, tiny peaks were also observed at 2θ of 27.5o and 36.1o, indicating the formation

232

of rutile (JCPDS 21-1276) as a minor phase. Result was consistent with previous studies reporting

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the formation of mixtures of anatase and rutile for 3DOM TiO2.11, 37-39 The possible formation of

234

anatase/rutile junction may in principle promote the charge separation of the photocatalyst.11, 38

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The crystalline phases of 3DOM TiO2 were retained through the deposition of Pt and/or rGO.

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Notably, the intensity of rutile peaks slightly increased after incorporation of Pt and rGO, which

237

may be due to the high temperature treatment in the material preparation process. For both

238

Pt/3DOM TiO2 and rGO/Pt/3DOM TiO2, new peaks at 2θ of 39.7o and 46.2o were observed,

239

suggesting the formation of face-centered cubic (fcc) metallic Pt crystal (JCPDS 65-2868, Figure

240

S4A of Supporting Information); the low intensity of these peaks was probably due to the low Pt

241

content of the catalysts (~1.7 wt%).22 For catalysts containing rGO, a very small peak at 2θ of

242

22.8o occurred, which can be assigned to the (002) plane of graphitic carbon (Figure S4B of

243

Supporting Information).40 The low intensity along with the broad feature of the peak suggested

244

that the rGO sheets were poorly ordered in the stack direction, which indicated that the rGO might

245

be present in single or only a few layers in the composite catalysts.41 Raman spectroscopy was

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further used to study the nature of rGO and TiO2 in the catalysts. Characteristic bands of anatase

247

were observed in all samples (Figure 3B); the peak at 146 cm-1 (Eg(1)) related to the O-Ti-O bond



248

bending vibration, and peaks at 197 cm-1 (Eg(2)), 396 cm-1 (B1g), 515 cm-1 (A1g) and 637 cm-1 (Eg(3))

249

corresponded to Ti-O-Ti bending types.1,

250

clearly identified, probably due to the low content of rutile in comparison to anatase. For

251

rGO/3DOM TiO2 and rGO/Pt/3DOM TiO2, two peaks were observed at 1340 and 1590 cm-1

252

(Figure S5 of Supporting Information), which corresponded to the D and G peaks of rGO that

253

represented the defect-induced vibration and the in-plane vibration of sp2 boned C atoms,

254

respectively. The D and G peaks had similar intensities, indicating the presence of a large number

255

of disordered sp2 carbons in rGO that may be related to the defects generated during the reduction

256

process.42-43

11

Meanwhile, peaks associated with rutile were not

257

The oxidation states and coordination environments of individual elements in

258

rGO/Pt/3DOM TiO2 were investigated using XPS. Two distinct oxygen species were observed in

259

the O 1s spectrum with binding energies of 530.1 eV that represented the lattice oxygen of TiO2

260

and 531.9 eV that corresponded to adsorbed oxygen and/or surface hydroxyl species (i.e., >Ti-

261

OH), respectively (Figure 4A).44 Two types of carbon with binding energies at 284.8 and 287.0

262

eV were identified from the C 1s spectrum, which can be assigned to C-C and C=O bonds,

263

respectively (Figure 4B).28,

264

unreduced GO (Figure S7 of Supporting Information). The lack of C-O-C bond and the reduction

265

of O-C=O to C=O bonds suggested the highly reduced nature of rGO.28, 42, 45-46 The 2p3/2 and 2p1/2

266

peaks of Ti were found at binding energies of 459.3 eV and 465.0 eV, respectively (Figure 4C),

267

both of which were slightly higher than the unmodified 3DOM TiO2 (Figure S8 of Supporting

268

Information, 458.5 eV for Ti 2p3/2 and 464.2 eV for Ti 2p1/2). The upshift of binding energy may

269

be due to the decoration of rGO and Pt that impacted the coordination environment of Ti4+.28, 36, 47

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The characteristic peaks of Pt 4f spectrum were deconvoluted to three species, with the binding

45

In contrast, C-C, C-O-C and O-C=O bonds were observed for


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energies of 70.9 eV and 74.3 eV, 72.7 eV and 75.9 eV, and 75.2 eV and 77.6 eV assigned to the

272

4f7/2 and 4f5/2 peaks of metallic Pt, Pt(II) and Pt(IV), respectively (Figure 4D).18, 31, 46 Results

273

suggested that metallic Pt was the predominant species, and the presence of small amounts of Pt(II)

274

and Pt(IV) may be attributed to the strong metal-support interaction that promotes the electron

275

transfer from Pt to TiO2, as reported in previous studies.21, 39, 46

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UV-vis DRS was employed to determine the optical property of the photocatalysts.

277

Compared to the widely reported P25,11, 37 3DOM TiO2 exhibited a slightly wider light absorption

278

region (Figure 5A), possibly due to the unique 3DOM structure that induced slow photon effect

279

and allowed for multiple light scattering to improve the harvesting efficiency of light.38-39 The light

280

reflection spectrum of 3DOM TiO2 suggested that the Bragg reflection peak value relative to the

281

photonic band gap (PBG) was ~440 nm (Figure 5A), which was close to the calculated value

282

based on the modified Bragg equation (550 nm, Section S2 of Supporting Information). At the

283

edges of PBG, light travels with a significantly reduced group velocity that results in the formation

284

of slow photons.11, 38 The ‘blue edge’ of the PBG for 3DOM TiO2 (~400 nm) was close to its

285

electronic absorption edge, which would favor the light absorption and thereby enhance the

286

material’s photocatalytic activity theoretically. Notably, the ‘red edge’ of the PBG lied in the

287

visible region, which would also facilitate visible light absorption by a visible light-active

288

photocatalyst.8

289

Modification with Pt and/or rGO significantly promoted the photoresponse of the resulted

290

composite catalysts to visible region (Figure 5A). Pt may improve the absorption of visible light

291

by TiO2 via photosensitizing and SPR effects.17, 48-49 Similar to previous studies, no SPR peak was

292

found for Pt, which may be ascribed to its small size and the high imaginary part of its dielectric

293

function.17,

48, 50

The addition of rGO may increase visible light absorption through chemical



Page 14 of 42

294

bonding between TiO2 and rGO (e.g., formation of Ti-O-C bond).23, 51 Notably, rGO/Pt/3DOM

295

TiO2 exhibited further improved visible light absorption, probably due to the overlapping of

296

extended absorption properties by rGO and Pt plasmonic absorption. The band gap energies (Eg)

297

of the phocatalysts were calculated according to the Kubelka-Munk equation:

298

(αhν)1/2=A(hν-Eg)

299

where α, h, and ν, represent absorption coefficient, Planck constant, and light frequency,

300

respectively, and A is a constant.11, 39 The calculated band gap energy of 3DOM TiO2 was 3.04 eV

301

(Figure 5B), which was smaller than that of anatase (3.2 eV).8, 52 Modification with Pt or rGO

302

decreased the band gap energies (2.87 and 2.89 eV, respectively) of the resulted catalysts, and co-

303

decoration of Pt and rGO further reduced the band energy of rGO/Pt/3DOM TiO2 to 2.82 eV.

304

Similar results were observed in previous studies showing that introduction of rGO and noble

305

metals decreased the band gap energies of TiO2-based composite materials, which may be due to

306

the interaction of noble metal and/or rGO with TiO2 that affected the coordination environment of

307

Ti.21, 53-54 The smaller band gaps of Pt/3DOM TiO2, rGO/3DOM TiO2 and rGO/Pt/3DOM TiO2

308

would extend the light absorption and utilization region, and thus in theory promote the

309

photocatalytic activity under visible light irradiation.

(1)

310

Photocatalytic degradation of methyl orange. Methyl orange was used as a model dye

311

pollutant to probe the performance of the photocatalysts under visible light irradiation. In the pure

312

3DOM TiO2 system, methyl orange concentration decreased gradually that ~20% of methyl orange

313

degraded after 120 minutes of reaction (Figure 6A). Result suggested that 3DOM TiO2 slightly

314

promoted methyl orange degradation, possibly because of the quantum size effects.8,

315

Improved performance of 3DOM TiO2 has also been reported in comparison to P25 in several

316

previous studies for the degradation of various pollutants under visible light irradiation.52,


39, 55

55

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317

However, as shown in Figure 6A, the degradation of methyl orange was still quite inefficient in

318

the pure 3DOM TiO2 system. Addition of Pt and rGO significantly facilitated methyl orange

319

degradation, and rGO/3DOM TiO2 exhibited a higher degradation efficiency of methyl orange in

320

comparison to Pt/3DOM TiO2, which might be ascribed to the enhanced charge separation ability

321

of rGO relative to Pt nanoparticles.56 Results were consistent with DRS measurements showing

322

the reduced band gap energies of 3DOM TiO2 with Pt and rGO modification. Notably, the

323

rGO/Pt/3DOM TiO2 exhibited the highest methyl orange removal efficiency among all catalyst

324

systems, suggesting the synergistic effect between Pt and rGO that promoted the photocatalytic

325

performance under visible light. Experiments performed under dark condition suggested that all

326

photocatalysts had limited sorption of methyl orange (Figure S9 of Supporting Information),

327

indicating that degradation rather than adsorption was the primary mechanism for methyl orange

328

removal in the photocatalytic process.

329

The methyl orange degradation kinetics were fitted using the pseudo-first-order kinetics

330

model (Eq 2).

331

ln

332

where C0 (mg/L) is the initial methyl orange concentration, C (mg/L) is the methyl orange

333

concentration after time t (min), and kapp (min-1) represents the pseudo-first-order rate constant.

334

Pseudo-first-order rate law has been commonly applied to describe the kinetics of a range of

335

catalytic reactions.8, 34, 57-59 Compared to 3DOM TiO2 alone, modification with Pt and/or rGO

336

significantly increased the reaction rates. In particular, rGO/Pt/3DOM TiO2 had the highest

337

reaction rate constant of ~0.013 min-1, which was 6, 4, and 2 times of those for the 3DOM TiO2,

338

Pt/3DOM TiO2, and rGO/3DOM TiO2 systems, respectively. Result was consistent with previous

339

studies showing that modification with Pt and/or rGO (or graphene) improved the activity of TiO2

𝐶0 𝐶

(2)

= 𝑘𝑎𝑝𝑝𝑡



340

for the photocatalytic degradation of various pollutants (Table S2 of Supporting Information). The

341

stability of rGO/Pt/3DOM TiO2 was further evaluated by examining the catalytic activity under

342

visible light for three cycles. Experiments were conducted for 90 min in each cycle, after which

343

the catalyst were separated, washed, dried, and then reused. High methyl orange removal

344

efficiency was achieved during each cycle, and no catalyst deactivation was observed after 3 cycles

345

(Figure 6B). The highly stable and reusable feature of rGO/Pt/3DOM TiO2, together with its

346

improved efficiency under visible light irradiation, suggested that this catalyst may be promising

347

for practical water treatment applications.

348

Discussion of possible photocatalytic mechanisms. The photocatalytic results have shown

349

the improved performance of the rGO/Pt/3DOM TiO2 ternary catalyst for methyl orange

350

degradation under visible light irradiation. The enhanced photocatalytic activity of rGO/Pt/3DOM

351

TiO2 may be attributed to the synergistic interactions of 3DOM TiO2, Pt nanoparticles and rGO

352

nanosheets that promoted absorption of visible light and improved separation efficiency of

353

electron-hole pairs. First, as demonstrated from DRS (Figure 5A), incorporation of Pt and/or rGO

354

into 3DOM TiO2 significantly enhanced visible light absorption by the composite catalysts,

355

because of the SPR effect of Pt and the electronic interaction of TiO2 with Pt and/or rGO

356

(evidenced from XPS results in Figure 4).39, 46 Additionally, the unique optical property of 3DOM

357

TiO2 may further promote visible light absorption and utilization by the composite catalysts,

358

because of the slow photon effect that may occur in the ‘red edge’ of the PBG for 3DOM TiO2 (in

359

the visible region).8, 33 Previous studies reported that compared to TiO2 without 3DOM structure,

360

slow photon effect enhanced the photocatalytic activity of 3DOM TiO2 for various applications.13,

361

18, 34, 44

Consequently, reduced band gaps along with increased methyl orange degradation


Page 16 of 42

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362

performance have been observed for Pt/3DOM TiO2, rGO/3DOM TiO2 and rGO/Pt/3DOM TiO2,

363

compared to bare 3DOM TiO2.

364

It is worth noting that similar band gap energies have been obtained for Pt/3DOM TiO2,

365

rGO/3DOM TiO2 and rGO/Pt/3DOM TiO2 (Figure 5B), but rGO/Pt/3DOM TiO2 exhibited

366

significantly higher methyl orange degradation kinetics than Pt/3DOM TiO2 (i.e., 4-fold increase)

367

and rGO/3DOM TiO2 (i.e., 2-fold increase), suggesting that mechanisms other than improved

368

visible light absorption may contribute to the enhanced performance of rGO/Pt/3DOM TiO2 as

369

well. PL spectra of the photocatalysts were obtained to investigate the charge transfer and

370

recombination properties. The 3DOM TiO2 exhibited the strongest emission peaks centered at

371

~380 nm and ~460 nm among all catalysts (Figure 7). Modification with Pt or rGO significantly

372

decreased the PL emission intensities of the corresponding photocatalysts, and the rGO/Pt/3DOM

373

TiO2 had the lowest PL emission intensity among all photocatalysts. The PL emission intensity

374

was influenced by the recombination of excited electrons and holes, and a reduced emission

375

intensity of PL spectrum indicated an improved electron-hole separation of the material.60 Result

376

suggested that co-decoration of Pt nanoparticles and rGO effectively reduced the recombination

377

of photogenerated electron-hole pairs,53 which was consistent with the highest photocatalytic

378

activity observed for the rGO/Pt/3DOM TiO2 composite material.

379

EIS measurements were conducted to further determine the charge separation and transport

380

behaviors of the photocatalysts. The arc radius in the EIS Nyquist plots of followed the order that

381

3DOM TiO2 > Pt/3DOM TiO2 > rGO/3DOM TiO2 > rGO/Pt/3DOM TiO2 (Figure 8). A small arc

382

radius indicated a reduced electron transfer resistance of the photocatalyst, which would favor the

383

charge separation and transport.61 Our result indicated that modification of 3DOM TiO2 with Pt

384

and rGO significantly facilitated the charge separation and thus reduced the recombination of



Page 18 of 42

385

electrons and holes in rGO/Pt/3DOM TiO2, which was consistent with the PL observation. In

386

particular, rGO has been widely recognized as a promising electron accepting material with high

387

electron mobility.28, 62.The reported work function of rGO (4.4 eV) is higher than the position of

388

the conduction band of TiO2 (4.29 eV, Section S4 of Supporting Information), suggesting that

389

electron transfer from TiO2 to rGO would be thermodynamically favorable.56,

390

rGO/Pt/3DOM TiO2, rGO may also promote the electron transfer from TiO2 to Pt (work function

391

of 5.64 eV) by serving as an intermediate electron shuttle that reduces the Schottky barrier of Pt

392

and TiO2.60,

393

photocatalyst through several routes that include TiO2→rGO, TiO2→Pt, and TiO2→rGO→Pt.

394

Improved electron transfer induced by rGO may efficiently reduce the electron-hole recombination,

395

and therefore significantly increase the overall photocatalytic activity of rGO/Pt/3DOM TiO2.

63-64

63-66

In

Thus, electron transfer may be achieved in the rGO/Pt/3DOM TiO2 ternary

396

To investigate the active species generated and responsible for methyl orange degradation,

397

the performance of rGO/Pt/3DOM TiO2 was determined in the presence of a series of scavengers.

398

Specifically, IPA, EDTA, and BQ were employed as scavengers of hydroxyl radical (•OH), hole

399

(h+), and superoxide radical (O2•-), respectively.67-69 The addition of IPA significantly decreased

400

the degradation rate of methyl orange that only

Nanocomposites of Three-Dimensionally-Ordered Porous TiO2

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