Z-Scheme Photocatalytic CO2 Reduction on a Heterostructure of

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Z-Scheme Photocatalytic CO2 Reduction on a Heterostructure of OxygenDefective ZnO/Reduced Graphene Oxide/UiO-66-NH2 under Visible Light Jingchai Meng, Qian Chen, Jiaqian Lu, and Hong Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14282 • Publication Date (Web): 12 Dec 2018 Downloaded from http://pubs.acs.org on December 12, 2018

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ACS Applied Materials & Interfaces

Z-Scheme Photocatalytic CO2 Reduction on a Heterostructure of Oxygen-Defective ZnO/Reduced Graphene Oxide/UiO-66-NH2 under Visible Light Jingchai Meng, Qian Chen, Jiaqian Lu and Hong Liu* Department of Chemical Engineering, School of Environmental and Chemical Engineering, Shanghai University, 99 Shangda Road, Shanghai 200444, P R China *Corresponding

Author. Tel: +86-21-66137487. Fax: +86-21-66137725. Email:

[email protected] ABSTRACT: The construction of a Z-scheme heterojunction is an effictive way to isolate photogenerated electron-holes and enhance the activity of the semiconductor photocatalysts. However, the Z-scheme heterojunctions based on metal organic framework (MOF) were rarely reported. Herein, a novel oxygen-defective ZnO (O-ZnO)/reduced graphene oxide (rGO)/UiO-66-NH2 Z-scheme heterojunction has been prepared by a facile solvothermal route. The morphologies, structures and photoelectric characteristics of the acquired materials were characterized in detail. The photocatalytic activity of the O-ZnO/rGO/UiO-66-NH2 heterostructure was assessed by photocatalytic CO2 reduction. The results indicated that the O-ZnO/rGO/UiO-66-NH2 heterostructure could efficiently reduce CO2 to CH3OH and HCOOH, and its activity was significantly superior to that of O-ZnO/UiO-66-NH2 and ZnO/rGO/UiO-66-NH2. Under illumination of visible light, the yield of CH3OH and HCOOH over O-ZnO/rGO/UiO-66-NH2 heterostructure reached 34.83 and 6.41 μmol·g-1·h-1, respectively. The high photoactivity of O-ZnO/rGO/UiO-66-NH2 heterostructure should be caused by the effective spatial separation of photogenerated 1

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electrons and holes via a Z-scheme charge transfer. This research may well present an insight into the design and fabrication of novel Z-scheme photocatalytic systems for environmental remediation and energy conversion. KEYWORDS: metal organic framework, Z-scheme, CO2 reduction, photocatalytic, oxygen-defective 1. INTRODUCTION Photocatalytic conversion of CO2 into valuable fuels (including CH4, CO, CH3OH HCHO or HCOOH) has drawn more and more attention because this strategy can not only decrease the concentration of greenhouse gas, but also alleviate the energy shortage. Since Inoue et al. first demonstrated the photoelectrocatalytic reduction of carbon dioxide into hydrocarbon fuels,1 many research have been done to develop highly efficient photocatalysts.

2, 3

A high-efficiency photocatalytic system for CO2

photoreduction requires photocatalysts with a wide absorption spectrum, high charge-separation efficiency, strong redox capability, and long-term stability. However, single-component photocatalysts are usually hard to meet all above requirements

simultaneously.

Therefore,

composited

photocatalytic

systems,

primarily classified as type-II and Z-scheme in accordance with the transfer pathway of photogenerated charges, are widely utilized to promote the charge separation. 4, 5 The redox capability of photoinduced electrons and holes in the type-II hybrid will be weakened because of the migration of the photoinduced electrons to a more positive conduction band (CB) and the transfer of the photoinduced holes to a more negative valence band (VB), respectively.5,

6

In contrary, Z-scheme composite can realize 2


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spatial separation of photoinduced charges and ensure strong redox capability simultaneously.6-16 Recently, some visible-light-responsive Z-scheme photocatalysts for reduction of CO2, like Pt@CdS/TiO2,6 CdS-WO3,7 Fe2V4O13/RGO/CdS,8 g-C3N4/ZnO,9

Au@CdS/TiO2,6,

10

g-C3N4/Bi2WO6,11

BiOI/g-C3N4,12

Ag3PO4/g-C3N413 and α-Fe2O3/Cu2O,14 have been reported. Unfortunately, a practically viable photocatalyst which possesses satisfactory activity and stability is still not been discovered till now. Therefore, developing novel highly efficient and stable Z-scheme photocatalysts is highly desirable. Metal organic frameworks (MOFs), composed of organic ligands and inorganic clusters, are a new class of porous inorganic-organic hybrid materials.17-20 Owing to their unique properties, such as high specific surface area, tailorable pore size, designed frame structure and easy functional group, MOFs have been proved to have promising applications in various fields.21-23 Recently, MOFs have been exploited as photocatalysts for pollutant degradation,18, 19, 24 water splitting,25 CO2 reduction26 and organic transformation.20, 27 Just as all single-component photocatalysts, MOFs suffer from the fast electron-hole recombination after light excitation.15, 26 Combining with other semiconductors to form heterojunctions has been demonstrated to be an viable method to address this issue,26,

28-30

consequently improving photocatalytic activity.

However, almost all of the semiconductor/MOF composites developed to date are traditional type-II heterostructures. The problem is that the reduction ability of electrons and the oxidation ability of the holes are all lower than that of the original. The Z-scheme photocatalytic system can promote the effective separation of the 3



photogenerated charges while maintaining the strong redox capability. Nevertheless, up to now, few reports are related to the MOF-based Z-scheme photocatalytic systems.31 On the other hand, ZnO has been extensively studied because of its good stability, nontoxicity, low cost and high photocatalytic activity. However, because of its wide bandgap (3.37 eV), ZnO cannot effectively utilize visible light, which limits its practical application. The introduction of oxygen defects in ZnO can extend its optical absorption edge toward visible light.32,

33

However, the oxygen-defective ZnO

(O-ZnO) still has the problem of rapid recombination of photoelectron-holes during photocatalytic process. Therefore, it is very urgent to enhance the separation efficiency of photogenerated charges and boost the photocatalytic activity of O-ZnO. In this paper, for the first time, we construct a novel three-component Z-scheme system which utilizes UiO-66-NH2 and O-ZnO as the photocatalysts, and reduced graphene oxide (rGO) as the electron mediator. UiO-66-NH2 is a Zr-containing MOF, which is characterized by a crystalline structure consisting of hexameric Zr6O32 units and 2-amino-terephthalate linkers.18 UiO-66-NH2 was selected in this study due to its visible-light responsiveness, good stability,18,

19

and suitable band edges which can

match well with O-ZnO. In this Z-scheme photocatalytic system, the photoinduced electrons in the CB of O-ZnO transferred to rGO, and then recombined with the holes in the VB of UiO-66-NH2, leading to enhanced charge separation efficiency. This three-component composite exhibited excellent photocatalytic activity for CO2 reduction to CH3OH and HCOOH under visible light illumination. Moreover, the 4


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ternary Z-scheme photocatalyst revealed good stability. This research may present a new idea for the design and fabrication of novel Z-scheme photocatalysts with superior activity and stability. 2. EXPERIMENTAL SECTION 2.1. Chemicals. 2-Aminoterephthalic acid (ATA), N,N-dimethyl formamide (DMF),

Zirconium

tetrachloride

(ZrCl4),

zinc

acetate

dihydrate

(Zn(CH3COO)2·2H2O), sodium hydroxide (NaOH), methanol, graphite powder, sulfuric acid, potassium permanganate and hydrochloric acid were obtained from Sinopharm Chemical Reagent Co., Ltd. The reagents employed in this study were all analytical grade. 2.2. Preparation of GO. The fabrication of graphene oxide (GO) was done by oxidation of natural graphite powder using a modified Hummers’ method. 34 2.3. Synthesis of ZnO. 4.0 g Zn(CH3COO)2·2H2O was added to 60 mL of anhydrous ethanol. After 60 minutes of ultrasonic dispersion, the obtained transparent solution was transferred to a 100 mL Teflon-lined autoclave and heated at 180 °C for 4 h. After the reaction, the products were centrifugally separated and washed several times using absolute ethanol and deionized water, then dried at 60 °C for 24 h in a vacuum oven. 2.4. Synthesis of Oxygen-Defective ZnO. The synthesis of O-ZnO was conducted through a solution conversion method using ε-Zn (OH)2 as precursor according to the previous literature.32

5



2.5. Preparation of O-ZnO/rGO/UiO-66-NH2 (OZ/R/U) Composite. The OZ/R/U composite was fabricated via a solvothermal method. 0.1096 g ATA and 0.1404 g ZrCl4 were added to 50 mL of DMF to obtain a solution A. Meanwhile, 0.250 g of as-prepared O-ZnO sample and 0.005 g GO were dispersed in 10 mL of DMF via sonication for 0.5 h to obtain solution B. Then solution A was gradually added to solution B and stirred for 1 h at ambient temperature. Subsequently, the mixture was transferred to a 100 mL Teflon-lined autoclave and crystallized at 120 °C for 48 h. The resulting product was centrifuged, washed using absolute ethanol and deionized water, and dried under vacuum. The mass ratio of O-ZnO to UiO-66-NH2 was 1:1, and the content of rGO was about 1.5 wt% in the obtained OZ/R/U composite. 2.6. Characterization. The microstructure and morphology of the photocatalysts were analyzed by scanning electron microscopy (SEM, JSM-6700F), transmission electron microscopy (TEM, JEOL 200CX) and the high-resolution transmission electron microscope (HRTEM, JEM-2010F) with an energy-dispersive X-ray (EDX) spectrometer. Crystal phase identification was conducted via a powder X-ray diffraction (XRD, D/MAX-2550) using CuKα radiation. X-ray photoelectron spectroscopy (XPS) tests were performed on a PHI ESCA-5000C electron spectrometer. Electron-spin-resonance (ESR) tests were carried out on a JEOL-TE300 spectrometer. Raman spectra were analyzed by a Renshaw inVia Raman spectrometer with a laser excitation at wavelength of 785 nm. The UV-vis diffuse reflectance spectra (DRS) of the photocatalysts were recorded on a Hitachi U-3010 spectrophotometer. N2 and CO2 adsorptions were measured on Micromeritics ASAP 6


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2460 adsorption apparatus. A Hitachi F-7000 fluorescence spectrophotometer was used to measure the photoluminescence (PL) spectra of the samples. Transient time-resolved PL decay measurements were performed on a FLS920 fluorescence spectrometer. 2.7. CO2 Photoreduction. The CO2 photoreduction experiment was undertaken at room temperature in a continuous-flow reaction system using a 300 W Xe lamp combined with a UV cut-off filter ( > 420 nm) as a light source. In brief, 0.1 g of photocatalyst was dispersed in 100 mL of NaHCO3 aqueous solution (0.1 M) under magnetic stirring. Prior to illumination, CO2 (99.995%) gas was bubbled through the suspension for at least 0.5 h to saturate the solution. Thereafter, the suspension was subjected to the irradiation of visible light. The liquid products were monitored by using a GC7900 gas chromatograph with a TM-PLOTU capillary column (30 m × 0.53 mm × 20 μm) and a FID detector every hour. 2.8. Electrochemical Measurements. Photocurrents were measured by a CHI-660 electrochemical workstation (Shanghai Chenhua Instrument, China) with a standard three electrode system. An Ag/AgCl electrode and a Pt foil were applied as the reference electrode and counter electrode, respectively. Photocatalyst was deposited on an indium-tin oxide (ITO) glass as the working electrode. A 300W Xe lamp with a UV cut-off filter was used as a light source and a solution of 0.5 M Na2SO4 was employed as the electrolyte. Electrochemical impedance spectroscopy (EIS) was measured under the frequency from 10-2 to 106 Hz in 0.1 M Na2SO4. Mott-Schottky plots were undertaken in the range of −1.0 to +0.5 V (vs. Ag/AgCl) at a frequency of 7



1000 Hz in 0.2 M Na2SO4. 2.9. Investigation of hydroxyl radical formation. The production of hydroxyl radicals (•OH) on the photocatalysts were determined by a PL technique. Coumarin was used as a probe molecule because it can react with •OH to generate a highly fluorescent product, 7-hydroxycoumain. The PL intensity of 7-hydroxycoumain corresponds to the amount of the hydroxyl radicals produced during the reaction. The detailed experimental procedure was described as below: 100 mg of photocatalyst were dispersed in 100 mL of 0.001 M coumarin aqueous solution. The suspension was allowed to reach an adsorption-desorption equilibrium before irradiation was administered. Next, the mixture was subjected to the irradiation of visible light and the intermittently sampling was carried out for analysis per 15 min. A Hitachi F-7000 fluorescence spectrophotometer with an excitation wavelength of 325 nm was used to measure the PL spectra of produced 7-hydroxycoumain. 3. RESULTS AND DISCUSSION 3.1 Morphologies and Phase Structures. The XRD patterns of as-synthesized ZnO and O-ZnO samples are displayed in Figure 1a. Both the XRD peaks can be indexed to the hexagonal würtzite ZnO (JCPDS 36-1451).32, 35 All samples are well crystallized with no change in crystal phase of oxygen-defective ZnO. However, the decrease and broadening of the peak intensity of O-ZnO are observed, which may be due to the existence of oxygen deficiencies.35 In order to verify the presence of oxygen defects, ESR measurements were performed. Figure 1b shows a new weak signal peak in the ESR signal peak of O-ZnO at g = 2.001. The peak at a g-factor of 8


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2.001 can be indexed to the surface oxygen defects in O-ZnO, 32, 33, 35 which is not observed for ZnO. The surface oxygen vacancies can also be analyzed by XPS. As shown in Figure 1c, two strong peaks at 529.9 and 531.7 eV appear in the O1s spectrum of O-ZnO. The peak at 529.9 eV is due to the oxygen bond of Zn-O-Zn, and the peak at 531.7 eV can be assigned to the oxygen ions in the vicinity of the oxygen vacancy. 32, 33 In the O1s spectrum of ZnO, the peak at 531.7 eV is very weak. All the results demonstrated above clearly evidence the presence of a high concentration of oxygen defects in O-ZnO. In addition, the UV-vis absorption spectrum of O-ZnO exhibits an obvious red-shift compared to that of ZnO as a result of the rich oxygen vacancy in O-ZnO (Figure 1d). 32, 33

Intensity (a.u.)

(110)

O-ZnO ZnO

(103) (112)

(102)

ZnO

(002) (101)

(b) (100)

Intensity (a.u.)

(a)

O-ZnO ZnO (JCPDS 36-1451)

10

20

30

40

50

60

70

80

g=2.001

3120

3128

2 Theta (degree)

3136

3144

3152

Magnetic field (G) (d)

(c) O 1s

1.2

Absorbance (a.u.)

529.9 531.7

Intensity (a.u.)

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


O-ZnO 529.9

ZnO

531.7

1.0 0.8 0.6 0.4 0.2 0.0 200

542 540 538 536 534 532 530 528 526

ZnO O-ZnO

400

600

800

Wavelength (nm)

Binding energy (eV)

Figure 1. (a) XRD patterns, (b) ESR spectra, (c) O1s XPS spectra and (d) UV–vis 9



diffuse reflectance spectra of O-ZnO and ZnO. (a) (110)

OZ/R/U OZ/U

O-ZnO

Intensity (a.u.)

(103) (112)

(102)

(100) (002) (101)

(b)

(006)

UiO-66-NH2

(002)

(111)

OZ/R/U (002)

Intensity (a.u.)

10

20

30

D G

rGO 40

50

60

70

80

1000

1200

1400

1600

-1

1800

2000

Shift (cm )

2 Theta (degree)

(c) Z/R/U OZ/R/U

Intensity (a.u.)

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

Page 10 of 36

g=2.001

3120

3140

3160

3180

3200

Magnetic field (G)

Figure 2. (a) XRD patterns of rGO, O-ZnO, UiO-66-NH2 and OZ/R/U, (b) Raman spectra of OZ/U and OZ/R/U and (c) ESR spectra of Z/R/U and OZ/R/U. The crystal phases of the obtained O-ZnO, UiO-66-NH2, rGO and OZ/R/U were identified by XRD as displayed in Figure 2a. The XRD pattern of pure UiO-66-NH2 is in good agreement with those reported previously.26, 36, 37 An obvious diffraction peak is present at 25.8° corresponding to (002) planes of rGO in pure rGO sample,38 suggesting that solvothermal method can effectively reduce GO to rGO. The XRD diffraction pattern of OZ/R/U composite shows the characteristic peaks of O-ZnO and UiO-66-NH2, which indicate that the composite consists of O-ZnO and UiO-66-NH2. Nevertheless, the loss of the characteristic peak of rGO may be due to the relatively 10


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low diffraction intensity and the low amount of rGO. Besides, there is no characteristic diffraction peak (~10.6°) of GO,38 demonstrating that GO is successfully reduced to rGO during the solvothermal procedure. To verify the existence of rGO, Raman tests of OZ/R/U and O-ZnO/UiO-66-NH2 (OZ/U) were implemented (Figure 2b). In the Raman spectrum of O-ZnO/rGO/UiO-66-NH2, there are two obvious Raman peaks in the 1350 cm-1 and 1600 cm-1, indicating the D band of the sp3 hybrid structure in rGO and the G band of the ordered sp2 hybrid carbon bond structure, respectively,39,

40

which proves the existence of rGO in the

O-ZnO/rGO/UiO-66-NH2 material. Figure 2c demonstrate that the O-defects still exist in the O-ZnO/rGO/UiO-66-NH2 hybrid, implying the pre-existing O-defects were not eliminated during the solvothermal process in preparing O-ZnO/rGO/UiO-66-NH2.

Figure 3. SEM images of (a) UiO-66-NH2, (b) O-ZnO and (c) OZ/R/U; (d) TEM and (e) HRTEM images and (f) EDX spectrum of OZ/R/U.

11



(a)

(b) Zn 2p

Zn 2p

1200

1022.2 1045.3

N 1s Zr 3p C 1s Zr 3d Zn 3s Zn 3p Zn 3d

1000

800

600

400

200

OZ/R/U

Intensity (a.u.)

ZnLMM O 1s ZnLMM

O KLL

Intensity (a.u.)

Zn 2p

survey

1021.9 1044.9 O-ZnO

1020

0

1030

1040

1050

Binding energy (eV)

Binding energy (eV)

(c)

(d) O 1s

Intensity (a.u.)

532.6 533.4

OZ/R/U

Intensity (a.u.)

Zr 3d

531.8

529.9 531.7

O-ZnO

544 542 540 538 536 534 532 530 528 526

184.8

182.3

OZ/R/U

184.9

182.7

UiO-66-NH2

192 190 188 186 184 182 180 178 176

Binding energy (eV)

Binding energy (eV)

(f)

(e) C 1s

N 1s

284.8 285.7

OZ/R/U

288.9 284.3

285.9 UiO-66-NH2

294

292

290

399.4

Intensity (a.u.)

Intensity (a.u.)

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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288.4

OZ/R/U 399.7

UiO-66-NH2 288

286

284

282

404

280

402

Binding energy (eV)

400

398

396

394

Binding energy (eV)

Figure 4. XPS spectra of O-ZnO, UiO-66-NH2 and OZ/R/U: (a) survey spectrum of OZ/R/U, (b) Zn 2p, (c) O 1s, (d) Zr 3d, (e) C 1s, (f) N 1s. As illustrated in Figure 3a, bare UiO-66-NH2 is a cuboid-like morphology with its size being around 100–120 nm. The pristine O-ZnO sample displays nanoplate-like morphology with its length being 100–150 nm (Figure 3b). The SEM (Figure 3c) and 12


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TEM (Figure 3d) images of OZ/R/U show that UiO-66-NH2 and O-ZnO are evenly dispersed on rGO nanosheets. Further observations also show that UiO-66-NH2, O-ZnO and rGO are in close contact. This structure is conducive to the rapid transfer of electrons between UiO-66-NH2, O-ZnO and rGO and the increase of the photo-generated charge separation rate, which in turn enhances its photocatalytic activity (Figure 3c-d). As shown in Figure 3e, the lattice fringe is able to be observed clearly in the HRTEM image of the OZ/R/U sample with a lattice spacing of 0.261nm, corresponding to the (001) plane of ZnO.32 The EDX spectrum confirms that the OZ/R/U composite contains elements including Zr, Zn, O, C and N (Figure 3f). Figure 4 is the XPS spectra of O-ZnO, UiO-66-NH2 and OZ/R/U. The survey spectrum (Figure 4a) of OZ/R/U suggests the composite mainly consists of Zn, Zr, O, C and N, which is consistent with the EDX results. The Zn 2p diagram (Figure 4b) of OZ/R/U shows that the sample has obvious absorption peaks at 1022.2 and 1045.3 eV, corresponding to the typical values of Zn2+ in ZnO.41, 42 The O 1s high-resolution XPS spectrum of OZ/R/U can be deconvoluted into three peaks centered at 531.8, 532.6 and 533.4 eV (Figure 4c), which can be indexed to the lattice oxygen in hexagonal ZnO, oxygen ions in the oxygen-deficient regions and the Zr-O bonds in MOF, respectively.18,

26

The Zr 3d peaks of OZ/R/U (Figure 4d) locate at 182.3 and 184.8

eV, indicating the existence of Zr4+.18, 26 The C 1s spectrum (Figure 4e) of OZ/R/U can be divided into three peaks centered at 284.8, 285.7 and 288.9 eV, which are originated from graphite carbon, C-NH2 and ATA carboxylic acid (-OOC) group, respectively.18, 26, 28

The N 1s XPS spectrum (Figure 4f) of OZ/R/U locates at 399.4 eV, which is assigned 13



to the -NH2 group of UiO-66-NH2. 18, 26, 28 In comparison with the bare UiO-66-NH2, the positive shift of C 1s peaks and the negative shifts of Zr 3d and N 1s peaks are observed for the OZ/R/U sample. In addition, the Zn 2p peaks and O 1s peaks of OZ/R/U shift slightly to higher binding energies in comparison to those of O-ZnO. The shifts in binding energy suggest the presence of electronic interaction among the composite components, rather than a simple physical mixture. (b)

250

Pore volume (cm3g-1nm-1)

Volume stored (cm3g-1, STP)

(a)

200 150 O-ZnO UiO-66-NH2 OZ/R/U

100 50 0 0.0

0.2

0.4

0.6

0.8

1.0

0.4 UiO-66-NH2 OZ/R/U

0.3

0.2

0.1

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Relative pressue (P/P0)

Pore diameter (nm) (d) Quantity adsorbed (cm3g , STP)

(c) O-ZnO UiO-66-NH2

0.16

-1

Pore volume (cm3g-1nm-1)

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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OZ/R/U

0.12 0.08 0.04 0.00

1

10

100

12 10 8

UiO-66-NH2 OZ/R/U Z/R/U O-ZnO

6 4 2 0

100 200 300 400 500 600 700

Absolute pressure (mmHg)

Pore diameter (nm)

Figure 5. (a) Nitrogen adsorption-desorption isotherms, (b) HK pore-size distribution curves, (c) BJH pore-size distribution curves of UiO-66-NH2, O-ZnO and OZ/R/U, and (d) CO2 adsorption isotherms of UiO-66-NH2, O-ZnO, OZ/R/U and Z/R/U. The N2 adsorption-desorption method was further used to evaluate the porosity and specific surface area of UiO-66-NH2, O-ZnO and OZ/R/U. As illustrated in Figure 14


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5a, UiO-66-NH2 exhibits a typical type I isotherm indicative of the microporous structure.26, 43 Besides, three main pores at 0.54, 0.92 and 1.3 nm are revealed in the pore-size distribution of UiO-66-NH2 (Figure5b). The prepared O-ZnO shows a type IV isotherm with a distinct H3 hysteresis loop that implies the existence of mesopores.26, 44 These mesopores are derived from the gaps and voids between the O-ZnO nanoplates. The adsorption isotherm type of the OZ/R/U sample is a mixed isotherm type of type I and IV, indicating that the composite material has both micropores and mesopores, which can be further proved by the pore-size distributions as illustrated in Figure 5b and Figure 5c. The BET specific surface area of UiO-66-NH2 and O-ZnO is 959.3 and 21.3 m2g-1, respectively. As for the OZ/R/U composite, its specific surface area is between O-ZnO and UiO-66-NH2, which is about 877.3 m2 g-1. Figure 5d shows the CO2-adsorption behavior of UiO-66-NH2, O-ZnO and OZ/R/U at ambient temperature. It is found that the maximum CO2 uptake for UiO-66-NH2, OZ/R/U and O-ZnO are 10.7 cm3g-1, 6.6 cm3g-1 and 1.3 cm3 g-1, respectively. The order of the CO2-adsorption capacity of these three samples is nearly in accordance with that of their surface area. To explore the effect of oxygen vacancies, the CO2-adsorption behavior of the Z/R/U composite was also investigated. As shown in Figure 5d, Z/R/U exhibits a CO2 adsorption capacity of 4.7 cm3g-1, which is lower than that of OZ/R/U. This indicates that the existence of oxygen defects is conducive to the CO2 adsorption.

15



(a)

(b)

1.2

O-ZnO UiO-66-NH2 OZ/R/U

1.0 0.8

O-ZnO UiO-66-NH2 2

physical mixture

h

Absorbance (a.u.)

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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0.6 0.4 0.2 0.0 200

300

400

500

600

700

800

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0

Wavelength (nm)

Band gap (ev)

Figure 6. (a) UV-Vis DRS and (b) bandgap energies of O-ZnO, UiO-66-NH2 and OZ/R/U. The optical absorption of O-ZnO, UiO-66-NH2, OZ/R/U and the physical mixture (O-ZnO+rGO+UiO-66-NH2) were tested by diffuse reflectance UV–vis spectroscopy. As presented in Figure 6a, there is a strong absorption in the range of 200–440 nm observed in pristine UiO-66-NH2. The absorption edge observed for O-ZnO is at 425 nm. As compare with pure O-ZnO, UiO-66-NH2 and the physical mixture, OZ/R/U exhibits the enhanced visible-light absorption, which is possibly caused by the electronic interaction between the two semiconductors.45, 46 Similar observations can also be observed for other MOF-based catalysts.28, 30 The improved adsorption of the ternary OZ/R/U composite in the visible light range can result in the production of more electron–hole pairs and thereby favorably enhances the activity of photocatalyt. Here, the band gap of the as-prepared UiO-66-NH2 and O-ZnO can be calculated by 1

using the following equation: 𝛼h𝑣 = 𝐴(h𝑣 - 𝐸𝑔)2,26,

32, 33

where α, h, ν and Eg

represent the absorption coefficient, Planck constant, the optical frequency, and the bandgap energy, respectively. A is a constant. From the plot of (αhν)2 versus (hν) 16


Page 17 of 36

displayed in Figure 6b, the bandgap value of UiO-66-NH2 is estimated to be 2.82 eV, close to the reported values.26, 36 Furthermore, the band gaps of O-ZnO is calculated to be 2.98 eV. The conduction bands of O-ZnO and UiO-66-NH2 were also determined by the Mott-Schottky method, as demonstrated in Figure 7. The positive slopes suggest that both samples are n-type semiconductors. From the Mott-Schottky plots, it is observed that the flat band potential of O-ZnO and UiO-66-NH2 are about −0.39 V vs. Ag/AgCl (i.e. −0.19 V vs. NHE) and −0.75 V vs. Ag/AgCl (i.e. −0.55 V vs. NHE), respectively. Because the flat band potential is about 0.10 V below the bottom of the conduction band for n-type semiconductors,28,

47

the CB positions of O-ZnO and

UiO-66-NH2 can be determined to be −0.29 and −0.65 V (vs. NHE), respectively. According to the bandgap values of O-ZnO and UiO-66-NH2 gained from DRS analysis (Figure 6b), the VB positions of O-ZnO and UiO-66-NH2 can be calculated to be +2.69 and 2.17 V (vs.NHE), respectively. (a)

(b)

20

28

1000Hz

10 (C-2/F-2)10

16

10 (C-2/F-2)10

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


12 8 -0.39 V

0 -1.00

20 16 12 8

4

1000 Hz

24

-0.75 V

4 -0.75

-0.50

-0.25

0.00

0 -1.0

0.25

-0.5

0.0

Potential (V) vs Ag/AgCl

Potential (V) vs Ag/AgCl

Figure 7. Mott-Schottky plots of (a) O-ZnO and (b) UiO-66-NH2.

17


0.5


(b)

Intensity (a.u.)

OZ/R/U OZ/U Z/R/U O-ZnO UiO-66-NH2

1


200

CH3OH

150

HCOOH

0

250

-1 CH3OH evolution (μmol g )

(a)

2

100

3

4

50 0

5

0

1

3

4

5

6

(d) 50

40

-1 CO2 reduction rate (μmol g h-1)

(c)

2

Irradition time (h)

Retention time (min)

-1 HCOOH evolution (μmol g )

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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40 30

CH3OH HCOOH

35 30 25 20

20

15

10

10

0 0

1

2

3

4

Irradition time (h)

5

6

5 0

none

trace

O-ZnO OZ/U OZ/R/U Z/R/U UiO-66-NH2

Figure 8. (a) Original chromatograms for as-synthesized samples after 6 h irradiation, (b) CH3OH evolution amount vs. illumination time, (c) HCOOH evolution amount vs. illumination time, and (d) CO2 reduction rate of the as-prepared samples. 3.2 Photocatalytic Activity. With the aim of assessing the photocatalytic activities of the as-prepared photocatalysts, we perform the photocatalytic reduction of CO2 under visible light irradiation in an aqueous system. Under the experimental conditions, only CH3OH and HCOOH are detected as liquid-phase products (Figure 8a). Control experiment indicates that no any products can be detected when high purity of N2 gas instead of CO2 was injected into the reaction system, confirming that CO2 should be the source of CH3OH and HCOOH. Figure 8b, 8c and 8d show the comparison of photocatalytic activities of different photocatalysts. It can be seen, 18


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O-ZnO is not active for photocatalytic reduction of CO2 because the conduction band position (-0.29 eV) of O-ZnO is lower than the potential of CO2 reduction to the corresponding hydrocarbon.48, 49 The previous literatures have reported UiO-66-NH2 had a certain ability to reduce CO2.50,

51

However, no detectable reduction products

are found in this experiment when UiO-66-NH2 is utilized as a photocatalyst alone. The possible reason is that the yield of reduction is too low to be detected by the instrument. After coupling with O-ZnO to form O-ZnO/UiO-66-NH2 (OZ/U) heterostructure, the photocatalytic activity is significantly enhanced. The yield of CH3OH and HCOOH on OZ/U catalyst is 19.67 and 4.94 μmol·g-1·h-1, respectively. This increase is caused by the forming of a direct Z-scheme heterojunction between O-ZnO and UiO-66-NH2 (this can be confirmed by the hydroxyl radical experiments as shown in Figure 13), thus promoting the migration and separation of photoinduced electron-hole pairs and further boosting the photocatalytic activity. Meanwhile, the photocatalytic activity of the O-ZnO/rGO/UiO-66-NH2 is obviously superior to O-ZnO/UiO-66-NH2, in which the yields of CH3OH and HCOOH reach to 34.83 and 6.41 μmol·g-1·h-1, respectively. This is owing to the rGO as a kind of effective electronic media, can not only increase the contact area and tightness of the two semiconductors, but also create a new electron transfer bridge, which is benefit to the transmission and separation of photoinduced charges, thus making the photocatalytic activity significantly increased. In addition, to study the effect of oxygen vacancies, we also tested the photocatalytic performance of ZnO/rGO/UiO-66-NH2 (Z/R/U) for comparison. It is found that the yield of CH3OH and HCOOH on Z/R/U catalyst is 19



14.86 μmol·g-1·h-1 and 4.20 μmol·g-1·h-1, only 42.7 % and 65.5 % of OZ/R/U, indicating the existence of oxygen vacancies is benefit to the enhancement of photocatalytic activity. Possible reasons are as follows: (1) the presence of oxygen vacancies can favor the CO2 adsorption on the surfaces of photocatalyst as demonstrated in Figure 5d. (2) the oxygen vacancies may act as the capture centers of the photoinduced electrons, and thus hindering the recombination of photogenerated charge carriers in ZnO and improving the photocatalytic efficiency. 52, 53 The effect of rGO content on the photoactivity of O-ZnO/rGO/UiO-66-NH2 was also studied. It can be found from Figure 9 that the rate of the photocatalytic CO2 reduction initially rises with an increase of rGO amount in the OZ/R/U composite, and then decreases with the further increment in rGO content. The decreased activity of the photocatalyst with a weight ratio higher than 1.5 wt % is probably attributed to the fact that the excessive rGO may have absorbed more visible light and competed with O-ZnO/UiO-66-NH2 at absorbing light.54 Thus, the optimum rGO content is 1.5 wt %. -1 CO2 reduction rate (μmol g h-1)

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

40

CH3OH HCOOH

35 30 25 20 15 10 5 0 1.0

2.0

1.5

2.5

rGO content in OZ/R/U composite (wt.%)

Figure 9. Effect of rGO content on the photoactivity of OZ/R/U photocatalyst. In addition to photocatalytic activity, the catalyst lifetime is also of great 20


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Page 21 of 36

significance. So the recycling experiments for O-ZnO/rGO/UiO-66-NH2 photocatalyst were performed. After six cycles, no noticeable decrease of the photocatalytic activity is observed (Figure 10a). Moreover, there is no obviously change in the crystal structure (Figure 10b) and morphology (Figure 10c) of O-ZnO/rGO/UiO-66-NH2 composite after the reaction. The aforementioned results clearly illustrate that the as-prepared OZ/R/U composite has relatively high stability during the photocatalytic CO2 reduction.

200

200

150

150

100

100

50

50

0

0

6

12

18

24

30

0 36

(b) Fresh Used

Intensity (a.u.)

-1 HCOOH evolution (μmol g )

(a)

-1 CH3OH evolution (μmol g )

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


10

20

30

40

50

60

70

80

2 Theta (degree)

Irradiation time (h)

Figure 10. (a) Cycle runs of CO2 reduction on the OZ/R/U photocatalyst, (b) XRD patterns of OZ/R/U before and after the photocatalytic reaction, and (c) SEM image of OZ/R/U after the photocatalytic reaction. 3.3 Enhancement Mechanism of Photocatalytic Activity. To fully understand the outstanding photocatalytic ability of the O-ZnO/rGO/UiO-66-NH2 heterostructure, the 21



steady-state and time-resolved transient PL, photocurrent and EIS tests were performed. Figure 11a reveals the steady-state PL emission spectra of O-ZnO, UiO-66-NH2 and the OZ/R/U composite excited at 380 nm. It is observed that the pristine O-ZnO exhibits a strong emission near 510 nm. The emitting peak at around 465 nm is observed for bare UiO-66-NH2. Compared to pure UiO-66-NH2 and O-ZnO, the OZ/R/U composite shows the weak PL intensity, indicating the efficient interfacial electron separation between UiO-66-NH2 and O-ZnO. The time-resolved transient PL spectra of O-ZnO, UiO-66-NH2 and OZ/R/U are shown in Figure 11b. Multi-exponential decays are observed from the curvatures of the plots. Two radiative lifetimes with different percentages are obtained by fitting the decay spectra using bi-exponential models (inset of Figure 11b). The two lifetimes (τ1 and τ2) of OZ/R/U are 1.03 and 6.31 ns, respectively, which are longer than the corresponding lifetimes of O-ZnO or UiO-66-NH2. Besides, OZ/R/U has a higher average fluorescence lifetime (τA ) of 5.99 ns than that of O-ZnO (1.25 ns) and UiO-66-NH2 (1.71 ns). The increased fluorescent lifetime further indicate the suppressed recombination of photo-generated carriers in the OZ/R/U composite. The efficiency of interface charge separation can be further characterized by means of the photoelectrochemical technique. The transient photocurrent responses of O-ZnO, UiO-66-NH2 and OZ/R/U composite are shown in Figure 11c. It can be found that pure O-ZnO and UiO-66-NH2 have low photocurrent intensity as a result of the rapid recombination of their photogenerated charges. The photocurrent intensity of the OZ/R/U sample is significantly higher than those of O-ZnO and UiO-66-NH2. The 22


Page 22 of 36

Page 23 of 36

result shows that the OZ/R/U sample has high charge transfer and separation capability. Figure 11d shows that the Nyquist semicircle diameter of the OZ/R/U sample is much smaller than that of O-ZnO and UiO-66-NH2. This phenomenon indicates the electrochemical impedance of the OZ/R/U sample is far less than O-ZnO and UiO-66-NH2. Therefore, all the results shown above clearly demonstrate that the OZ/R/U hybrid has more efficient transportation and separation of photoinduced carries, which should be the reason for the enhancement of photocatalytic activity. (b)

(a)

1400

1000

Counts

PL intensity (a.u.)

OZ/R/U O-ZnO UiO-66-NH2

1200

UiO-66-NH2 OZ/R/U O-ZnO

τ1

800

(ns) O-ZnO 0.31 UiO-66-NH2 0.43 OZ/R/U 1.03

600 400

A1

τ2

A2

τA

(%) 68.2 65.0 28.2

(ns) 1.64 2.18 6.31

(%) 31.8 35.0 71.8

(ns) 1.25 1.71 5.99

200 400

450

500

550

0

600

0

5

(c)

(d)

1.4

UiO-66-NH2 OZ/R/U O-ZnO

light on

1.2

light off

15

20

25

30

150 UiO-66-NH2 OZ/R/U O-ZnO

120

-Z" (ohm)

1.0 0.8 0.6 0.4

90 60 30

0.2 0.0

10

Decay time (ns)

Wavelength (nm)

Current density (A cm-2)

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


0

40

80

120

160

200

0

240

0

50

100

Time (s)

150

200

250

300

Z' (ohm)

Figure 11. (a) PL spectra, (b) time-resolved transient PL decay, (c) photocurrent responses and (d) EIS of UiO-66-NH2, O-ZnO and OZ/R/U. On the basis of the band gap structure of O-ZnO and UiO-66-NH2, there are two possible photocatalytic mechanisms in the O-ZnO/rGO/UiO-66-NH2 composite: (I) 23



Page 24 of 36

conventional type-II heterojunction mechanism and (II) Z-scheme mechanism. As displayed in Figure 12, O-ZnO and UiO-66-NH2 can be excited by visible light to generate electrons and holes. If the charge transfer in the interface phase of the OZ/R/U composites follows the conventional type-II heterojunction mechanism (Figure 12a), the photoinduced electrons on the CB of UiO-66-NH2 will transfer to the CB of O-ZnO and then to the surface of rGO. And the holes will migrate from the VB of O-ZnO to that of UiO-66-NH2. Nevertheless, the photogenerated electrons on the CB of O-ZnO can hardly reduce CO2 to CH3OH and HCOOH owing to that the CB potential of O-ZnO (−0.29 eV) is more positive compared to the CO2 reduction potential (𝐸𝐶𝑂2/𝐶𝐻3𝑂𝐻 = - 0.38 eV and 𝐸𝐶𝑂2/𝐻𝐶𝑂𝑂𝐻 = - 0.61 eV ).48,

49

In contrast, if

following by the Z-scheme photocatalytic mechanism (Figure 12b), the photoinduced electrons on the CB of O-ZnO will transfer through rGO to the VB of UiO-66-NH2 and combine with the photoinduced holes, resulting in the accumulation of the electrons on the CB of UiO-66-NH2 and the accumulation of the holes on the VB of O-ZnO. Because the CB level of UiO-66-NH2 is more negative compared to the CO2 reduction potential, the electrons accumulated on the CB of UiO-66-NH2 can easily reduce CO2 to CH3OH and HCOOH. Therefore, based on the above analysis, the electron transport between two semiconductors obeys the Z-scheme mechanism rather than the traditional type-II heterojunction mechanism. It is worth noting that this Z-scheme charge transfer process can not only decrease the recombination rate of photogenerated charge carriers, but also maintain the high reduction capacity of the photogenerated electrons on the more negative CB of the UiO-66-NH2 and the strong 24


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oxidation capacity of the photoinduced holes on the more positive VB of O-ZnO.

Figure 12. Schematic illustration of the charge transfer and separation in O-ZnO/rGO/UiO-66-NH2: (a) type-II heterojunction and (b) Z-scheme mechanism. In order to confirm the Z-scheme mechanism proposed above, hydroxyl radicals generated on the surfaces of pure O-ZnO, UiO-66-NH2 and OZ/R/U upon visible light illumination were detected by PL spectroscopy using coumarin as a probe molecule. Figure 13a shows the spectral evolution of the OZ/R/U sample against illumination time, and Figure 13b illustrates the comparison of the PL intensity around 430 nm of O-ZnO, UiO-66-NH2 and OZ/R/U. It can be observed from Figure 13b, the PL intensity of O-ZnO and OZ/R/U gradually increases with the increased illumination time, demonstrating the formation of •OH radicals under irradiation. For pure UiO-66-NH2 sample, no PL signal is detected under the same conditions, manifesting that none of hydroxyl radicals are formed. This is because the VB potential (2.17 V vs NHE) of UiO-66-NH2 is more negative than that of OH−/•OH (2.3 V vs NHE).7 The photoinduced holes on the VB of UiO-66-NH2 are unable to react with OH−/H2O to generate hydroxyl radicals. Compared with pure O-ZnO, the OZ/R/U sample displays higher fluorescence intensity. The enhanced PL intensity can be attributed to the 25



Z-scheme charge transfer over the OZ/R/U photocatalyst (Figure 12b), which results in the enrichment of holes on the VB of O-ZnO to generate more •OH radicals. If the obtained OZ/R/U composite complies with the type-II heterojunction mechanism as illustrated in Figure 12a, the photogenerated electrons are likely transferred from the CB of UiO-66-NH2 to that of O-ZnO, and the holes are transferred from the VB of O-ZnO to that of UiO-66-NH2. The holes on the VB of UiO-66-NH2 cannot react with OH−/H2O to produce •OH radicals. Consequently, the traditional OZ/R/U heterojunction should have no PL signals. Nevertheless, this hypothesis contradicts the results of the •OH experiments. Hence, the photogenerated electron-hole transfers on the OZ/R/U composite follow the Z-scheme mechanism instead of the classical heterojunction mechanism. (b)

(a) 800

800 60 min 45 min 30 min 15 min 0 min

600 400

Intensity (a.u.)

Intensity (a.u.)

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

Page 26 of 36

200 0 350

UiO-66-NH2 O-ZnO OZ/R/U

600 400 200 0

400

450

500

550

600

0

15

30

45

60

Illumination time (min)

Wavelength (nm)

Figure 13. (a) PL spectral evolution of the OZ/R/U sample against illumination time and (b) comparison of PL peak intensity around 430 nm for O-ZnO, UiO-66-NH2 and OZ/R/U. 4. CONCLUSIONS In conclusion, a novel O-ZnO/rGO/UiO-66-NH2 Z-scheme heterojunction was 26


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achieved by coupling UiO-66-NH2 and rGO with oxygen-defective ZnO. This Z-scheme photocatalyst demonstrates excellent photocatalytic activity for conversion of CO2 to CH3OH and HCOOH under visible light illumination, in which the yields of CH3OH and HCOOH reaches 34.85 μmol·g-1·h-1 and 6.40 μmol·g-1·h-1, respectively. In addition, the composite photocatalyst shows a good stability. The boosted activity of the O-ZnO/rGO/UiO-66-NH2 composite photocatalyst is ascribed to the formation of the Z-scheme photocatalytic system, which can not only inhibit the recombination of photogenerated charge carriers, but also maintain the high reduction ability of UiO-66-NH2 and strong oxidizability of O-ZnO. This study demonstrates an insight into the design and fabrication of novel MOF-based Z-scheme photocatalytic systems. ACKNOWLEDGMENTS This research was funded by the National Natural Science Foundation of China (11472164) and Innovative Research Team Project (IRT13078). REFERENCES (1) Inoue, T.; Fujishima, A.; Konishi, S.; Honda, K. Photoelectrocatalytic Reduction of Carbon Dioxide in Aqueous Suspensions of Semiconductor Powders. Nature 1979, 277, 637-638. (2) Mao, J.; Li, K.; Peng, T. Y. Recent Advances in the Photocatalytic CO2 Reduction over Semiconductors. Catal. Sci. Technol. 2013, 3, 2481-2498. (3) Marszewski, M.; Cao, S. W.; Yu, J. G.; Jaroniec, M.

Semiconductor-Based

Photocatalytic CO2 Conversion. Nanoscale Horiz. 2016, 1,185-200. (4) Sarkar, D.; Ghosh, C. K.; Mukherjee, S.; Chattopadhyay, K. K. Three 27



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Dimensional Ag2O/TiO2 Type-II (p-n) Nanoheterojunctions for Superior Photocatalytic Activity. ACS Appl. Mater. Inter. 2013, 5, 331-337. (5) Tada, H.; Mitsui, T.; Kiyonaga, T.; Akita, T.; Tanaka, K. All-Solid-State Z-Scheme in CdS-Au-TiO2 Three-Component Nanojunction System. Nat. Mater. 2006, 5, 782-786. (6) Wei, Y. C.; Jiao, J. Q.; Zhao, Z.; Zhong, W. J.; Li, J. M.; Liu, J.; Jiang, G. Y.; Duan, A. J. 3D Ordered Macroporous TiO2-Supported Pt@CdS Core-Shell Nanoparticles: Design, Synthesis and Efficient Photocatalytic Conversion of CO2 with Water to Methane. J. Mater. Chem. A 2015, 3, 11074-11085. (7) Jin, J.; Yu, J. G.; Guo, D. P.; Cui, C.; Ho, W. A Hierarchical Z-Scheme CdS-WO3 Photocatalyst with Enhanced CO2 Reduction Activity. Small 2015, 11, 5262-5271. (8) Li, P.; Zhou, Y.; Li, H. J.; Xu, Q. F.; Meng, X. G.; Wang, X. Y.; Xiao, M.; Zou, Z. G. All-Solid-State Z-Scheme System Arrays of Fe2V4O13/RGO/CdS for Visible

Light-Driving

Photocatalytic

CO2

Reduction

into

Renewable

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Z-Scheme Photocatalytic CO2 Reduction on a Heterostructure of

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