Strongly Coupled Ternary Hybrid Aerogels of N-deficient Porous

Subscriber access provided by EMORY UNIV

Communication

Strongly Coupled Ternary Hybrid Aerogels of N-deficient Porous Graphitic-C3N4 Nanosheets/N-Doped Graphene/NiFe-Layered Double Hydroxide for Solar-Driven Photoelectrochemical Water Oxidation Yang Hou, Zhenhai Wen, Shumao Cui, Xinliang Feng, and Junhong Chen Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b04496 • Publication Date (Web): 10 Mar 2016 Downloaded from http://pubs.acs.org on March 12, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Nano Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30

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

Nano Letters

Strongly Coupled Ternary Hybrid Aerogels of N-deficient Porous GraphiticC3N4 Nanosheets/N-Doped Graphene/NiFe-Layered Double Hydroxide for SolarDriven Photoelectrochemical Water Oxidation

Yang Hou,1 Zhenhai Wen,1 Shumao Cui,1 Xinliang Feng,2* and Junhong Chen1* 1

Department of Mechanical Engineering, University of Wisconsin-Milwaukee, 3200 North Cramer Street, Milwaukee, Wisconsin 53211, USA. E-mail: [email protected]

2

Center for Advancing Electronics Dresden (cfaed) & Department of Chemistry and Food Chemistry, Technische Universitaet Dresden, 01062 Dresden, Germany. E-mail: [email protected]

ABSTRACT Developing photoanodes with efficient sunlight harvesting, excellent charge separation and transfer, and fast surface reaction kinetics remains a key challenge in photoelectrochemical water splitting devices. Here we report a new strongly coupled ternary hybrid aerogel that is designed and constructed by in-situ assembly of Ndeficient porous carbon nitride nanosheets and NiFe-layered double hydroxide into a 3D N-doped graphene framework architecture using a facile hydrothermal method. Such a 3D hierarchical structure combines several advantageous features, including effective light-trapping, multidimensional electron transport pathways, short charge transport time and distance, strong coupling effect, and improved surface reaction kinetics. Benefiting from the desirable nanostructure, the ternary hybrid aerogels exhibited remarkable photoelectrochemical performance for water oxidation. Results included a record-high photocurrent density that reached 162.3 µA cm−2 at 1.4 V

1

ACS Paragon Plus Environment

Nano Letters

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

versus the reversible hydrogen electrode with a maximum incident photon-to-current efficiency of 2.5% at 350 nm under AM 1.5G irradiation, and remarkable photostability. The work represents a significant step towards the development of novel 3D aerogel-based photoanodes for solar water splitting. KEYWORDS: 3D aerogel, Hybrid nanosheets, Photoanode, Photoelectrochemical water splitting

TOC Graphic

DPCN CB

O2

2.58 eV

VB

H2O NiFe-LDH

NRGO

FTO

2


Page 2 of 30

Page 3 of 30

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

Nano Letters

Sunlight-driven photoelectrochemical (PEC) water splitting to produce hydrogen and oxygen has attracted much attention due to its environmental friendliness and costeffectiveness.1-4 A key objective in PEC research continues to be the development of novel and efficient photoelectrodes. Despite the significant recent progress in the development of highly active photocathodes,5,6 the more challenging task is to develop efficient and stable photoanodes that can oxidize water into oxygen under sunlight.7 Among various photoanodes, 2D metal-free graphitic carbon nitride (C3N4) remains a favorable choice as photoelectrode material for PEC water oxidation thanks to its suitable band gap, appropriate band positions, and low cost.8,9 However, due to its low light-harvesting ability, poor charge transport property, and slow interfacial kinetics for water oxidation, the solar-to-energy conversion efficiency of C3N4 is far from satisfactory.10 Although various efforts such as doping with foreign elements, loading co-catalysts, and constructing heterojunctions have been made to overcome these disadvantages,11-14 realizing high-efficiency C3N4-based photoanodes with desirable stability for PEC water oxidation remains a significant challenge. For efficient photoanodes, three main determining factors, including excellent light-trapping, efficient separation and transportation of photogenerated charge carriers, and fast oxygen evolution reaction (OER) kinetics, are extremely important.15-17 Comprehensive consideration of the above three factors is the key to addressing the limitations of current materials. Self-doping/oxygen vacancy is an effective strategy to increase solar light harvesting capability by narrowing the band gap and modifying the electronic structure.18 In particular, recent investigations on nitrogen vacancies have triggered keen interest in the fabrication of N-deficient C3N4 (DCN) as it can extend the light absorption range by efficiently tuning the band

3


Nano Letters

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

position of C3N4.19 However, few studies have focused on building a 3D electrodeform of DCN (or C3N4) rather than its powder form that suffers from a complicated recycling process.20-24 In addition, the catalytic activity towards the real PEC water oxidation has rarely been investigated. Functionalization of the photoanode with a suitable OER catalyst is equally important because it can improve the PEC activity by accelerating the surface reaction kinetics and suppressing photocorrosion.7,25-29 Nickel-iron-layered double hydroxide (NiFe-LDH) is probably one of the most ideal candidates owing to its extraordinary inherent catalytic activity for OER, associated with a unique layered structure.30-35 Despite widespread investigations of the NiFe-LDH as standalone electrocatalysts, only a few have been integrated into photoanodes.36,37 The effective integration of OER catalysts with photoanode could potentially be a major challenge due to the inefficient charge transfer across interfaces, synthetic difficulties, and their chemical incompatibility and stability issues. The introduction of 2D graphene or its derivatives as an efficient electron mediator that can promote the photogenerated charge separation and transfer and minimize recombination losses is an effective strategy to optimize the interaction between the photoanode and the OER catalyst. Besides the strong and stable interfacial contact, the layered heterostructure between DCN, graphene, and NiFe-LDH could shorten the charge transport time and distance compared with bulk counterparts,11,38 thereby enabling a convenient charge transfer to the electrode/electrolyte interface where the oxidation of water occurs. Compared with 2D graphene, 3D graphene aerogels can offer more advantages, sparked by their unique architecture that integrates advantages of multidimensional electron transport pathways, hierarchically porous structure, and good electrical conductivity,39-42 which are beneficial for improving electron transfer 4


Page 4 of 30

Page 5 of 30

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

Nano Letters

and charge separation.43,44 Meanwhile, their macroscopic block morphology makes them easily and conveniently recyclable from reaction.23,45 Despite little progress in the development of graphene aerogel-based materials for photochemical and electrochemical applications,46,47 up to now there has been no report on controllable synthesis of 3D ternary aerogel hybrid as a photoanode for PEC water oxidation. The controllable integration of N-deficient porous C3N4 nanosheets (DPCN) and layered NiFe-LDH into 3D N-doped graphene (NRGO) framework architectures with the expected synergetic photoelectrocatalytic activity for water oxidation may open up new opportunities for improving the overall efficiency of solar energy conversion. In this work, we report a novel strongly coupled ternary hybrid aerogel photoanode for solar-driven PEC water oxidation, in which both DPCN and NiFe-LDH were encapsulated and anchored on a 3D interconnected NRGO nanosheet skeleton. The NRGO networks worked as an electron mediator to shuttle electrons/holes between DPCN and NiFe-LDH, leading to effective separation and transfer of photogenerated charge

carriers.

The

DPCN/NRGO/NiFe-LDH

hybrid

offers

an

optimum

configuration of a photoanode for PEC water oxidation, as it incorporates the merits of each constituent and possesses the advantages of 3D aerogels. AM 1.5G photocurrent densities of 162.3 µA cm−2 at 1.4 V and 72.9 µA cm−2 at 1.22 V versus the reversible hydrogen electrode (RHE) for water oxidation were obtained with an incident photon-to-current efficiency (IPCE) of 2.5% at 350 nm. This is the best performance for PEC water oxidation among all C3N4-based materials reported to date. The performance is even better than that of some Fe2O3, WO3, Cd2SnO4, and FeVO4 metal oxide photoelectrodes (Table S1).

5


Nano Letters

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

The proposed mechanism for PEC water oxidation is studied in detail to understand the origin of the synergistic effect of the ternary hybrid aerogel. In this unique 3D nanostructure, the pivotal improvement is the effectiveness of this novel aerogel photoanode based on (1) constructing a high surface area of 2D DPCN with deficient structures for improving the light-harvesting efficiency; (2) developing a 3D NRGObased aerogel for efficient photogenerated charge separation; and (3) achieving a strong coupling effect and interfacial contact among 2D DPCN, NRGO nanosheets, and NiFe-LDH to facilitate the charge transfer and the PEC surface reaction process. An overview of the synthesis procedure for the 3D DPCN/NRGO/NiFe-LDH is illustrated in Figure 1a. First, DPCN was fabricated by combining the pyrolysis of urea and hydrogenation treatment (Scheme S1). Meanwhile, the layered NiFe-LDH was obtained via a liquid exfoliation process using NiFe-LDH nanosheets as the precursor (Scheme S2). Then, the resulting DPCN and NiFe-LDH were mixed with graphene oxide (GO) and ammonia solutions after sonication, followed by a selfassembly process under hydrothermal conditions that enabled the formation of a 3D DPCN/NRGO/NiFe-LDH aerogel (details in the Experimental Section). In this way, the DPCN and NiFe-LDH were encapsulated and anchored on NRGO nanosheets with the simultaneous reduction of GO and the incorporation of nitrogen species into the RGO frameworks. Photographs of the reaction solution and the obtained aerogel products are shown in Figure 1b. We employed field-emission scanning electron microscopy (FESEM) to study the microstructure of the DPCN/NRGO/NiFe-LDH aerogel. The FESEM image of DPCN/NRGO shows that the NRGO nanosheets are anchored on the surface of DPCN (Figure S1). Compared with DPCN/NRGO, a well-defined and interconnected

6


Page 6 of 30

Page 7 of 30

3D hierarchical structure can be clearly observed in the DPCN/NRGO/NiFe-LDH (Figure 1c). The hybrid possesses a porous structure with micro-sized pores, favoring mass transfer and reducing the transport limitation of electrolytes.48 Since the layered morphology of the DPCN/NRGO/NiFe-LDH hybrid makes it difficult to distinguish DPCN, NRGO, and NiFe-LDH, energy dispersive X-ray (EDX) spectroscopy and elemental mapping measurements were carried out (Figure 1d and Figure S2). The results confirmed the formation of DPCN/NRGO/NiFe-LDH hybrids, with C, N, Ni, Fe, and O as the principal elemental composition. Surprisingly, the hybrid aerogel can tolerate an appropriate compressive strength without obvious damage (Figure S3).

Ammonia Hydrothermal GO

DPCN

(a) (c)

NiFe-LDH

C

NRGO LDH DPCN

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

Nano Letters

(d)

Si

N Ni

O

Ir

Fe NRGO

0.0

0.6

1.2 1.8 X-ray energy (keV)

2.4

3.0

(b) Freeze Drying

3D DPCN/NRGO/NiFeLDH Aerogels

Hydrothermal

DPCN/NRGO/NiFeLDH Hydrogels

DPCN/GO/NiFeLDH Suspension

Figure 1. (a) Schematic illustration for the process to synthesize DPCN/NRGO/NiFeLDH. (b) Photographs of the obtained aerogel products. (c) FESEM image of

7


Nano Letters

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

DPCN/NRGO/NiFe-LDH. Scale bar, 1 µm. (d) Corresponding EDX spectrum from (c). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of DPCN/NRGO/NiFe-LDH (Figure 2 and Figure S1) further revealed that the 3D structure of the ternary hybrid aerogels was constructed with multiple overlapping nanosheets, in which crumpled NRGO nanosheets with a thickness of ∼ 6.07 nm (Figure S4) were dispersed on the surface of DPCN (Figure 2a-2b). The measured height of the DPCN by atomic-force microscopy (AFM) topological height analysis is around 18 nm (Figure S5), corresponding with a few dozen stacked layers.10 The well-distributed pores in the DPCN created by gas release during the pyrolysis process would allow for the efficient diffusion/mass transfer of electrolyte.49 The laminar-structured NiFe-LDH, with an interlayer separation of 0.26 nm,50 was grown intimately on the edge of DPCN/NRGO (Figure 2c-2f), which was preserved even after a long period of ultrasonic treatment and overnight stirring in ethanol, confirming the strong coupling among DPCN, NRGO, and NiFe-LDH (Figure S6).51 A typical HRTEM image of the hybrid proves the intimate interfacial contact among three components (Figure 2f). No obvious crystal fringe was observed for DPCN due to its amorphous structure (Figure 2f and Figure S1).52 The close contact is beneficial for increasing the contact area and shortening the transfer distance of photogenerated charge carriers, thus effectively suppressing the recombination of photogenerated electron-hole pairs,38 which can be further confirmed by photoluminescence (PL) studies (see below). The DPCN, NRGO, and NiFe-LDH account for 67.6, 25.4, and 7.0 wt.% of the DPCN/NRGO/NiFe-LDH, respectively, which are calculated from thermogravimetric analysis (TGA, Figure S7-S8).

8


Page 8 of 30

Page 9 of 30

(b)

(a)

50 nm (c)

100 nm Porous (e)

NRGO

LDH 50 nm 2 nm

(d) NiFe-LDH (012) 0.26 nm

(f)

0.34 nm NRGO (002)

(g)

(110)

(012)

(006)

(003)

5 nm NiFe-LDH

DPCN

Overloaded-NiFe-LDH /NRGO/DPCN (002)

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

Nano Letters

LDH (012) 0.26 nm

DPCN/NRGO/NiFe-LDH DPCN

2 nm 10

20

30 40 50 2 Theta (degree)

60

70

Figure 2. TEM images of the (a) DPCN, (b) DPCN/NRGO, and (c) DPCN/NRGO/NiFe-LDH. (d) HRTEM image of DPCN/NRGO/NiFe-LDH from the area labeled by the rectangular frame in (c). (e-f) HRTEM images of

9


Nano Letters

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 30

DPCN/NRGO/NiFe-LDH. (g) XRD patterns of DPCN, DPCN/NRGO/NiFe-LDH, NiFe-LDH, and overloaded-NiFe-LDH/NRGO/DPCN. The crystal structure of the obtained samples was investigated by X-ray diffraction (XRD). After hydrogenation, the intensity of the (002) peak at 26.7° significantly decreases and the (100) peak at 13.1° disappears in the DPCN (Figure S9), indicating a much lowered long-range order in the atomic arrangements in DPCN. This result could be attributed to the decreased planar size and structural defects.53,54 The DPCN/NRGO/NiFe-LDH shows a much stronger and broader peak at 27.1° compared with DPCN (Figure 2g), due to the overlap from two diffraction peaks of DPCN (002) and NRGO (002).11 However, it is hard to identify the phase of the NiFe-LDH by observing several weak diffraction peaks due to the relatively low content and ultrathin feature of NiFe-LDH in the present XRD pattern.55 In order to better reveal the existence of NiFe-LDH in the hybrid, we quadrupled the NiFe-LDH precursor feed and followed the same preparation process of DPCN/NRGO/NiFe-LDH. The XRD result of the product clearly demonstrated the increase of relative intensity of (003), (012), and (110) planes of NiFe-LDH compared with DPCN/NRGO/NiFe-LDH, without the detection of any new diffraction peaks, confirming the successful introduction of NiFe-LDH in the DPCN/NRGO/NiFe-LDH. Coupled with AFM (Figure S10), the thickness of the NiFe-LDH was ∼ 7.69 nm, corresponding with fewer than 10 layers.56 Raman spectra of DPCN and PCN show similar patterns, indicating there was no significant damage to the PCN skeleton after hydrogenation (Figure S11). For DPCN/NRGO/NiFe-LDH (Figure 3a), besides the peaks at 467 and 556 cm−1, which are ascribed to NiFe-LDH,57 two broad peaks around 1,352 and 1,590 cm−1 were assigned to the disordered carbon (D band) and ordered graphitic carbon

(G

band),

respectively.58

An

increased

D/G

10


intensity

ratio

of

Page 11 of 30

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

Nano Letters

DPCN/NRGO/NiFe-LDH (1.02 vs. 0.98 of GO) with a down-shift of G bands from 1,599 (GO) to 1,590 cm−1 was observed, demonstrating a decrease in the average size of the sp2 domains due to the partial removal of oxygen functional groups and an increase in the structural defects after N-doping.59 The strong coupling and hybridization among DPCN, NRGO, and NiFe-LDH is confirmed by Fourier transform infrared (FTIR) spectra (Figure 3b-3c), in which the characteristic breathing mode of triazine units at 805 cm−1 in DPCN/NRGO/NiFe-LDH exhibits apparent shifts compared with those of DPCN/NRGO (808 cm−1) and DPCN (810 cm−1).60 Xray photoelectron spectroscopy measurement reveals the presence of C, N, O, Fe, and Ni in DPCN/NRGO/NiFe-LDH with an Ni/Fe atomic ratio of ∼ 3.0, confirming the composition of NiFe-LDH (Figure 3d).61 The high-resolution Fe 2p and Ni 2p spectra confirm the Fe3+ and Ni2+ oxidation states in NiFe-LDH (Figure S12-S13).61,62 In comparison with the XPS peak centered at 856.0 eV assigned to Ni 2p3/2 in NiFeLDH (Figure S13), an obvious up-shift of the corresponding peak in DPCN/NRGO/NiFe-LDH to 857.1 eV is identified, suggesting the strong electron transfer from NiFe-LDH to DPCN/NRGO.63,64 The high-resolution C 1s and N 1s spectra substantiate the successful doping of N in the NRGO (Figure S12 and Figure 3e). The deconvoluted N 1s spectrum shows the presence of five types of N species, including 6% N-graphene (397.5 eV), 48% pyridinic N (398.8 eV), 40% pyrrolic N (400.0 eV), 4% graphitic N (401.2 eV), and 2% oxidized N (404.3 eV), respectively.65 The appearance of an N-graphene peak could be attributed to the formation of C–N–C bonding, which serves to link DPCN with NRGO and is in agreement with previous reports.66 The DPCN/NRGO/NiFe-LDH hybrid shows type IV isotherms with a Brunauer–Emmett–Teller (BET) surface area of 85 m2 g−1 (Figure 3f), which is much higher than those of PCN (31 m2 g−1), DPCN (43 m2 g−1), and DPCN/NRGO (69 m2

11


Nano Letters

g−1). The high BET surface area would allow water molecules to easily penetrate the inside of the hybrid for efficient PEC water oxidation.30

GO

DPCN

812

DPCN DPCN/NRGO DPCN/NRGO/NiFe-LDH

NiFe-LDH

1200

1600

2000 4000

3000

-1

805

N-Graphene

Oxidized N

405

402

399

Binding energy (eV)

396

810

O

Fe

N

Fe

(d) C

140

DPCN/NRGO/NiFe-LDH

900

800 -1

Wavenumbers (cm )

(f)

3

Pyridinic N

Graphitic N

820

1000

210

-1

N 1s

Volume adsorbed (cm g )

DPCN/NRGO/NiFe-LDH

(e)

-1

Wavenumbers (cm )

Raman shift (cm )

Pyrrolic N

2000

NiFe-LDH DPCN/NRGO/NiFe-LDH DPCN/NRGO DPCN PCN

Absorbance (a.u.)

800

Ni

O-H M-O

400

XPS Survey

(c) Intensity (a.u.)

G

(b)

Transmittance (a.u.)

D

PCN

(a) Transmittance (a.u.)

Intensity (a.u.)

DPCN/NRGO/NiFe-LDH NiFe-LDH

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 12 of 30

70

600

300

0

Binding energy (eV) NiFe-LDH DPCN/NRGO/NiFe-LDH DPCN/NRGO DPCN PCN

1.6

1.2

(g) 0.8

0.4 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0)

300

400

500

600

700

Wavelength (nm)

Figure 3. (a) Raman spectra, (b) FTIR spectra, (c) Enlarged FTIR spectra from (b), (d) The XPS survey spectrum, (e) High-resolution N 1s XPS spectrum, (f) Nitrogen adsorption-desorption isotherm curves, and (g) UV-Vis diffuse reflectance spectra of DPCN, DPCN/NRGO, and DPCN/NRGO/NiFe-LDH. Data for NiFe-LDH, GO, and PCN are also shown. All Raman spectra were obtained by deducting the background of DPCN due to its strong photoluminescence effect during characterization. The UV-Visible diffuse reflectance spectra show that, besides enhancing absorption in the visible-light region, the absorption edge of the PCN red-shifts from 445 nm to 482 nm after hydrogenation (Figure 3g and Figure S14), which could be ascribed to the introduction of nitrogen vacancies in the DPCN, thus resulting in a narrowed band gap.67,68 The loss of some extra N-atoms is supported by the variation of the atomic ratio of C to N (the atomic ratio of C to N in PCN is much lower than that of DPCN) determined by XPS analysis (Figure S15). Notably, the loss of these nitrogen atoms

12


Page 13 of 30

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

Nano Letters

did not significantly change the basic characteristics of the layered structure of C3N4 (Figure S9 and Figure S11). The flat-band potentials of PCN and DPCN were estimated to be -0.66 and -0.53 V from Mott-Schottky results (Figure S16),69 respectively. These data, combined with the band gaps obtained from UV-Visible spectra,70 allow us to calculate the valence band potentials of the PCN and DPCN at about 2.13 and 2.05 V, respectively. Further incorporation of NRGO nanosheets and NiFe-LDH into the DPCN not only increases the UV-Visible absorption over the entire wave-length range investigated, but it also extends the absorption edge and thus contributes to the high light-harvesting efficiency of the DPCN/NRGO/NiFe-LDH hybrid. Next, the PEC water oxidation performance of the DPCN/NRGO/NiFe-LDH was evaluated by measuring photocurrent density–potential curves using a standard threeelectrode configuration under chopped AM 1.5G irradiation at 100 mW cm−2. In order to better compare the PEC performance of DPCN/NRGO/NiFe-LDH with other reference electrodes, the hybrid aerogel was ground, and the obtained powder was deposited on the conducting glass support as a photoanode (Figure 4a). All potentials used refer to an RHE via calibration (Figure S17). As expected, the photocurrent density of the DPCN is higher than that of the PCN in the entire potential range, suggesting that the existence of nitrogen vacancies can effectively improve the PEC activity of PCN. After the introduction of NRGO, the DPCN/NRGO yielded a photocurrent density of 73.1 µA cm−2 at 1.4 V, which is approximately 1.84 times higher than that of the DPCN (39.7 µA cm−2) at the same potential, due to the improved electron transfer and enhanced light absorbance (Figure 3g). In contrast, the DPCN/NRGO/NiFe-LDH exhibited the highest photocurrent density of 162.3 µA cm−2 at 1.4 V, remarkably higher than those of the DPCN/NRGO, DPCN, and NiFe13


Nano Letters

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

LDH (near zero), respectively. This result suggests a positive role of NiFe-LDH for PEC activity, which not only can facilitate a fast charge carrier transfer, but also can act as a co-catalyst for improved surface reaction kinetics.35 The nearly disappearing PL signal (Figure 4b) of DPCN/NRGO/NiFe-LDH demonstrates that the recombination of photogenerated electron-hole pairs is effectively suppressed through the modification of NRGO and NiFe-LDH. This is attributed to the efficient charge transfer from DPCN to NiFe-LDH through the NRGO networks (detailed analysis is shown below). The efficient transfer prevents the direct recombination of electrons and holes and accelerates the PEC water oxidation reaction. Note that no saturation of photocurrent was observed in the entire potential scan range, indicating efficient charge separation in the DPCN/NRGO/NiFe-LDH.71 To the best of our knowledge, this photocurrent density value (162.3 µA cm−2 at 1.4 V) of DPCN/NRGO/NiFe-LDH represents by far the highest photocurrent density compared with other C3N4-based photoelectrodes reported, such as the benchmark value of 120 µA cm−2 at 1.55 V (Na2S-based electrolyte) previously reported for a C3N4-based photoanode72 for PEC water splitting under the same light irradiation condition. As we know, the photocurrent density of photoelectrodes increases with increasing positive potential during the PEC water oxidation process.73,74 In other word, the DPCN/NRGO/NiFeLDH hybrid exhibited a much higher photocurrent density of 162.3 µA cm−2 at a low potential (1.4 V) than what the best C3N4-based photoanode exhibit at a high potential (120 µA cm−2 at 1.55 V), further confirming the excellent catalytic performance of the DPCN/NRGO/NiFe-LDH for water oxidation. Likewise, the values (162.3 µA cm−2 at 1.4 V and 72.9 µA cm−2 at 1.22 V) are even higher than those of some metal oxide photoelectrodes reported in the literature (Table S1), including Fe2O3 nanorod array (20 µA cm−2 at 1.23 V),75 pristine WO3 (100 µA cm−2 at 1.61 V),76 orthorhombic 14


Page 14 of 30

Page 15 of 30

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

Nano Letters

Cd2SnO4 (80 µA cm−2 at 1.60 V),77 and FeVO4 (96 µA cm−2 at 1.61 V).78 The high photocurrent density could be attributed to the unique 3D nanostructure and modification of the NRGO and NiFe-LDH, which contributes to the improved light absorption, enhanced charge separation, and decreased over-potential. The PEC performance for water oxidation can be considered more impressive by correcting the raw data with iR losses (Figure S18). Although the absolute value of photocurrent density for DPCN/NRGO/NiFe-LDH is still inferior to some state-of-the-art photoelectrodes (e.g., Fe2O3-based (record-breaking photocurrent density of 4.32 mA cm−2 at 1.23 V)79 and BiVO4-based (highest photocurrent density of 3.6 mA cm−2 at 1.23 V)80 photoanodes), there is considerable room for improvement in the activity of the photoanode to achieve higher performance by further optimizing the ternary materials (composition and microstructure). Moreover, the deposition of NRGO and NiFe-LDH on DPCN led to a considerable cathodic shift in the onset potential of 0.11 V from 0.59 V for DPCN to 0.53 V for DPCN/NRGO and 0.48 V for DPCN/NRGO/NiFe-LDH (Figure 4a), indicating more accumulation of electrons and a decreased charge recombination in the photoanode. This result also highlights the effect of NiFe-LDH, which led to a cathodic shift of ∼ 0.05 V. The effect is similar to that of reducing the kinetic over-potential with the application of co-catalysts.81 Note that the value of the onset potential (0.48 V) for DPCN/NRGO/NiFe-LDH was even comparable with that of the typical Fe2O3-based photoanode of NiFeOx-decorated hematite (0.45 V).74 This result is consistent with the Mott–Schottky analysis (Figure 4c), where the flat-band potential of the DPCN/NRGO/NiFe-LDH is 0.16 and 0.08 V lower than those of DPCN and DPCN/NRGO, respectively. The cathodic shift of the flat-band potential strengthened the band bending at the DPCN/NRGO/NiFe-LDH//electrolyte interface, which is 15


Nano Letters

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

favorable for transferring the photogenerated holes to the surface.82 By comparing the changes of onset potential and flat-band potential of DPCN/NRGO before and after the introduction of NiFe-LDH, one can conclude that the NiFe-LDH can not only effectively decrease the electron-hole recombination near the flat-band potential,83,84 but also can accelerate the water oxidation kinetics of DPCN/NRGO.85 Thus, more holes avoid surface recombination and promote the PEC water oxidation process. Notably, the flat-band potential is seen to be more largely shifted to the negative side (0.08 V shift from DPCN/NRGO to DPCN/NRGO/NiFe-LDH) in comparison with the onset potential (0.05 V shift from DPCN/NRGO to DPCN/NRGO/NiFe-LDH). This indicates that the band bending at the electrode/electrolyte interface created by the NiFe-LDH leads to a larger degree of separation of photogenerated electrons and holes,86 compared with that of the NiFe-LDH acting as an OER catalyst due to the strong coupling effect among components in the hybrid. Moreover, the DPCN/NRGO/NiFe-LDH shows a relatively smaller slope than those of DPCN and DPCN/NRGO, indicating a faster charge transfer and a higher donor density.87 Furthermore, we conducted the IPCE measurement on DPCN/NRGO/NiFe-LDH at 1.22 V. The hybrid possesses a maximum IPCE value of 2.5% at 350 nm (1.96% at 420 nm), and it remains robust at a longer wavelength (∼ 575 nm) (Figure 4d). In contrast, the IPCE spectra of both DPCN and DPCN/NRGO show very low photoresponse in the 500-575 nm range. Although the absolute value of IPCE for the DPCN/NRGO/NiFe-LDH was not significantly high, it was already 179% and 119% higher than that of the DPCN (1.4% at 350 nm) and DPCN/NRGO (2.1% at 350 nm), respectively. It is also considerably higher than that of the previously reported C3N4based composite, such as g-C3N4/Fe2O3 (maximum IPCE of 1.5% at 420 nm),88 ITO/rpg-C3N4 (maximum IPCE of 1.1% at 420 nm),89 Alg-5-CN (maximum IPCE of 16


Page 16 of 30

Page 17 of 30

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

Nano Letters

1.2% at 420 nm),90 and best g-CN film72 (maximum IPCE of 1.2% at 420 nm and 12.5% at 360 nm; note that the high IPCE value was obtained by using the mixture of an aqueous solution of Na2S and Na2SO3 as the sacrificial reagent for evaluation of PEC water oxidation). By integrating the IPCE spectrum measured at 1.22 V using the AM 1.5G spectrum (Figure S19), the obtained photocurrent density of 70.2 µA cm−2 for DPCN/NRGO/NiFe-LDH agrees well with the value (72.9 µA cm−2 at 1.22 V) measured from the current-potential curve shown in Figure 4a. The discrepancy between the calculated and measured photocurrent values might be caused by the slight difference between the simulated light and the standard AM 1.5 G. To

further

investigate

the

photoelectric

response

behavior

of

the

DPCN/NRGO/NiFe-LDH and to understand the role of NRGO nanosheets and NiFeLDH in the hybrid, the transient photocurrents of the samples were recorded at 1.34 V with repeated light on/off cycles (Figure 4e). Upon irradiation, a spike in the photoresponse was observed for all the samples because of the rapid effect upon power excitation, and then it quickly returned to the steady state. Both DPCN/NRGO/NiFe-LDH and DPCN/NRGO show much higher photocurrent densities of 120.6 and 65.5 µA cm−2 than those of the materials without NRGO loading. The photocurrent densities of the photoanodes with NRGO loading are about 162.7% and 67.5% higher than those of DPCN/NiFe-LDH (45.9 µA cm−2) and DPCN (39.1 µA cm−2), respectively. The enhancement can be attributed to the fact that the introduction of NRGO nanosheets effectively improves the transport of charges between DPCN and NiFe-LDH so that the electron-hole recombination rate is greatly reduced.91,92 Clearly, the DPCN/NRGO/NiFe-LDH gave the highest photocurrent density among the five samples tested. The value is even larger than the sum of photocurrent density for the DPCN/NRGO and NiFe-LDH (nearly zero), indicating 17


Nano Letters

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

that a cooperative interaction should exist between them. The introduction of NiFeLDH into the DPCN/NRGO could simultaneously enhance the light absorption (Figure 3g) and surface reaction kinetics of water oxidation, and the recombination of photogenerated electron-hole pairs could be greatly suppressed due to the formation of strong coupling interactions among DPCN, NRGO, and NiFe-LDH (Figure 2f, Figure 3b, and Figure S13),11 thereby leading to a more efficient interface charge transfer. Since the three components are inter-related in the hybrid and they originate from different processes and mechanisms,7 the quantitative evaluation on the contributions from each component is difficult. One can conclude that the superior PEC water oxidation activity should be attributed to the synergistic effect among DPCN, NRGO, and NiFe-LDH, and the detailed contributions for which is currently being investigated. Moreover, the photocurrent density of the DPCN/NRGO/NiFe-LDH is much higher than that of the 2D DPCN/NRGO/NiFe-LDH (74.7 µA cm−2) due to the more efficient charge transport through 3D conductive networks and larger contact area with the electrolyte,45 as evidenced by the reduced charge transfer resistance (Figure S20), fast charge transfer across the interface, and high BET surface area (Figure S21), thereby effectively suppressing the recombination of photogenerated electron-hole pairs. The DPCN/NRGO/NiFe-LDH aerogel configuration integrates the advantages of enhanced light-harvesting, efficient charge separation and transport, a strong coupling effect, and improved surface reactions, which boost the PEC performance. Notably, when bias values of 0.8, 1.1 or 1.4 V were applied to the hybrid, transient photocurrents also exhibit good switching behavior (Figure 4f).

18


Page 18 of 30

-2

-2

0.69

0.92

PL intensity (a.u.)

1.15

1.38

IPCE (%) at 1.22 V vs. RHE

10 2

(c)

0.9

480

0.0 -0.5

0.0

0.5

1.6

0.8

0.0 400

1.0

500

-2

off

off

(e) 2 Dark

Dark

Dark

Dark

3 4 5 on

0

on

100

on

200

Photocurrent density (µA cm )

-2


1.34 V 1

200

off

150

1.4 V 100

300

50

1.1 V 0.8 V

0 0

(g)

-2

Dark

Dark Dark

6.0x10

4

4.0x10

4

2.0x10

4

AM 1.5 AM 1.5

AM 1.5

0.0 0

8000

16000


4

700

on DPCN/NRGO/NiFe-LDH

(f)

100

Time (s) 8.0x10

600

Wavelength (nm)

-2

off

640

DPCN/NRGO/NiFe-LDH DPCN/NRGO (d) DPCN

2.4

Potential (V) 20 µA cm

560

Wavelength (nm)

DPCN DPCN/NRGO DPCN/NRGO/NiFe-LDH

2.7

1.8

400

Potential (V)

-2

4

Light on

Light off

0.59 V

PCN DPCN DPCN/NRGO NiFe-LDH 2D DPCN/NRGO/ NiFe-LDH DPCN/NRGO/ NiFe-LDH

(b)

0.48 V 0.53 V

3.6

1/C × 10 (F cm )

50 µA cm

DPCN/NRGO/NiFe-LDH DPCN/NRGO DPCN PCN (a) NiFe-LDH

0.46

-Z‘’ (ohm)

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

Nano Letters


Page 19 of 30

24000

100

75

200

300

Time (s) Light on

(h)

1.22 V DPCN/NRGO/NiFe-LDH

DPCN/NRGO

50

25

DPCN/NiFe-LDH DPCN

0 500

Z' (ohm)

1000

1500

2000

Time (s)

Figure 4. (a) Variation of photocurrent density versus applied voltage plots for PCN, DPCN, DPCN/NRGO, DPCN/NRGO/NiFe-LDH, and NiFe-LDH with chopped AM 1.5G irradiation. (b) Photoluminescence spectra of PCN, DPCN, DPCN/NRGO, 19


Nano Letters

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 20 of 30

NiFe-LDH, 2D DPCN/NRGO/NiFe-LDH, and DPCN/NRGO/NiFe-LDH. (c) Mott– Schottky plots. (d) IPCE spectra of DPCN, DPCN/NRGO, and DPCN/NRGO/NiFeLDH measured at 1.23 V in 0.01 M Na2SO4 electrolyte under AM 1.5G irradiation. (e) Transient photocurrent density versus time. Number labels (1), (2), (3), (4), and (5) data

represent

the

DPCN/NRGO/NiFe-LDH,

2D

DPCN/NRGO/NiFe-LDH,

DPCN/NRGO, DPCN/NiFe-LDH, and DPCN, respectively. (f) Photocurrent densities vs. time of DPCN/NRGO/NiFe-LDH being applied with different bias potentials under AM 1.5G irradiation. (g) EIS Nyquist plots of DPCN (□, ☆), DPCN/NRGO (△, ○), and DPCN/NRGO/NiFe-LDH (▽, ◇) at a bias of 0.9 V under dark (□, △, ▽) and AM 1.5G irradiation (☆, ○, ◇). (h) Transient photocurrent responses of DPCN, DPCN/NiFe-LDH, DPCN/NRGO, and DPCN/NRGO/NiFe-LDH under AM 1.5G irradiation at 1.22 V. To further understand the charge transport behavior of the hybrid, electrochemical impedance spectroscopy (EIS) measurements were conducted (Figure 4g). The smallest arch of the DPCN/NRGO/NiFe-LDH both in the dark and under irradiation indicates that the introduction of NRGO nanosheets and NiFe-LDH significantly reduced the charge-transfer resistance. This result suggests that an effective separation of photo-generated electron-hole pairs and faster interfacial charge transfer occurred on the DPCN/NRGO/NiFe-LDH interface.93 The NRGO networks worked as the electron mediator, allowing for a much faster interfacial electron transfer between DPCN and NiFe-LDH. Besides further enhancing the separation efficiency of charge carriers, the decrease in resistance after the decoration of NiFe-LDH is evidence against the surface-state passivation mechanism.94

20


Page 21 of 30

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

Nano Letters

Figure 4h shows the time course of the photocurrent density for the samples at 1.22 V. No significant photocurrent decay is observed for DPCN/NRGO/NiFe-LDH after 2,000 s of irradiation. In contrast, both DPCN and DPCN/NRGO without NiFe-LDH can only maintain less than 36.2% and 84.3% of the initial photocurrents after testing, respectively, indicating that the decoration of NRGO and NiFe-LDH efficiently improves the transport of photogenerated charge carriers and prevents self-oxidation of the DPCN caused by an accumulation of photogenerated holes at the surface of DPCN.2 The results further confirm that the NiFe-LDH not only improves the kinetic transport of photogenerated charge carriers away from the DPCN surface, but also efficiently

prevents

its

photocorrosion.

The

long-term

stability

of

DPCN/NRGO/NiFe-LDH also was studied for practical applications (Figure S22). About 98.6% of the initial photocurrent was maintained after continuous irradiation for 10 h at 1.22 V, proving its long-term stability for PEC water oxidation. The corresponding Faradaic efficiency of the DPCN/NRGO/NiFe-LDH was calculated to be 96.3% (Figure S23),95 which indicates that the amount of O2 evolved was slightly less than the theoretical amount (100%), possibly due to the unwanted backward reaction at the counter electrode.96 Based on the above results, a possible charge transfer mechanism for the ternary hybrid aerogels is proposed in Scheme S3. Upon irradiation, the electrons are promoted from the valence band of DPCN to the conduction band. The photogenerated electrons easily migrated to the 3D NRGO networks from the conduction band of DPCN due to the NRGO nanosheets behaving as a conductive electron transport “highway,” and then were collected by the current collector and ultimately transferred to the counter electrode to reduce water. Meanwhile, the holes left on the valence band of DPCN can be transferred to the surface of NiFe-LDH 21


Nano Letters

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

through the NRGO interlayer according to the potential difference of DPCN and NiFe-LDH and the strong coupling effect among the components.32,97 Of course, the direct transfer of some holes from DPCN to NiFe-LDH is possible. The unique layered aerogel feature among DPCN, NRGO, and NiFe-LDH not only increases the contact areas and the transport pathways for efficient charge transfer across the interface, but also shortens the charge transport time and distance, thereby promoting an efficient spatial separation of electrons and holes.38 The oxygen evolution was produced by the oxidation of water using the trapped holes on the surface of the NiFeLDH; therefore, by integrating DPCN, NRGO, and NiFe-LDH, the synergetic catalytic effect through the band-gap engineering, charge transfer networks design, and interfacial modulation contributes to the high PEC activity for water oxidation. In summary, we designed and developed a novel strongly coupled ternary hierarchical architecture by encapsulating DPCN and NiFe-LDH into the 3D porous interconnected framework of NRGO aerogels for efficient solar conversion. In this design, defect engineering optimized the band structure of DPCN for the enhanced light absorption. 3D NRGO-based aerogels led to a unique charge transport property and a hierarchical porous structure. A close-contacting NiFe-LDH introduced the spatial separation interfaces required for fast separation and effective kinetic transportation of photogenerated charge carriers. The strong coupling effects of three components contributed to the high stability; thus, our system achieved the simultaneous optimization of these determining factors for greatly improving the energy-conversion efficiency. To the best of our knowledge, this is the first report on PEC water oxidation by a 3D aerogel-based hybrid system. Benefiting from the desirable nanostructure, the DPCN/NRGO/NiFe-LDH hybrid exhibited significantly enhanced PEC water oxidation performance, including a record-high photocurrent 22


Page 22 of 30

Page 23 of 30

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

Nano Letters

density of 162.3 µA cm–2 at 1.4 V and remarkable photostability (＞ 10 h) under AM 1.5G irradiation. These results could open up an avenue to develop a new class of 3D layered aerogel-based photoelectrodes for efficient PEC water-splitting applications. ACKNOWLEDGMENT This work was financially supported by the U.S. Department of Energy (DEEE0003208), the Research Growth Initiative Program of the University of WisconsinMilwaukee (UWM), and an ERC Grant on 2DMATER. SUPPORTING INFORMATION AVAILABLE Experimental

details

for

the

material

preparation,

characterization,

and

photoelectrochemical property testing, along with additional supporting data, are available free of charge via the Internet at http://pubs.acs.org. NOTES The authors declare no competing financial interest. REFERENCES (1)

Hisatomi, T.; Kubota, J.; Domen, K. Chem. Soc. Rev. 2014, 43, 7520-7535.

(2)

Li, Y.; Zhang, L.; Torres-Pardo, A.; González-Calbet, J. M.; Ma, Y.;

Oleynikov, P.; Terasaki, O.; Asahina, S.; Shima, M.; Cha, D.; Zhao, L.; Takanabe, K.; Kubota, J.; Domen, K. Nat. Commun. 2013, 4, 2566. (3)

Liu, Q.; He, J.; Yao, T.; Sun, Z.; Cheng, W.; He, S.; Xie, Y.; Peng, Y.; Cheng,

H.; Sun, Y.; Jiang, Y.; Hu, F.; Xie, Z.; Yan, W.; Pan, Z.; Wu, Z.; Wei, S. Nat. Commun. 2014, 5, 5122. (4)

Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q. X.;

Santori, E. A.; Lewis, N. S. Chem. Rev. 2011, 111, 5815-5815. (5)

Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.;

Santori, E. A.; Lewis, N. S. Chem. Rev. 2010, 110, 6446-6473. 23


Nano Letters

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

(6)

Huang, Q.; Ye, Z.; Xiao, X. J. Mater. Chem., A 2015, 3, 15824-15837.

(7)

Sun, J.; Zhong, D. K.; Gamelin, D. R. Energy Environ. Sci. 2010, 3, 1252-

1261. (8)

Wang, X.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.;

Domen, K.; Antonietti, M. Nat. Mater. 2009, 8, 76-80. (9)

Liu, J.; Liu, Y.; Liu, N.; Han, Y.; Zhang, X.; Huang, H.; Lifshitz, Y.; Lee, S.

T.; Zhong, J.; Kang, Z. Science 2015, 347, 970-974. (10)

Wang, Y.; Wang, X.; Antonietti, M. Angew. Chem. Int. Ed. 2012, 51, 68-89.

(11)

Hou, Y.; Wen, Z.; Cui, S.; Guo, X.; Chen, J. Adv. Mater. 2013, 25, 6291-6297.

(12)

Sun, J.; Zhang, J.; Zhang, M.; Antonietti, M.; Fu, X.; Wang, X. Nat. Commun.

2012, 6, 1139. (13)

Zhang, G.; Zhang, M.; Ye, X.; Qiu, X.; Lin, S.; Wang, X. Adv. Mater. 2014,

26, 805-809. (14)

Hong, J.; Wang, Y.; Wang, Y.; Zhang, W.; Xu, R. ChemSusChem 2013, 6,

2263-2268. (15)

Qiu, W.; Xu, M.; Yang, X.; Chen, F.; Nan, Y.; Zhang, J.; Iwai, H.; Chen, H. J.

Mater. Chem. 2011, 21, 13327-13333. (16)

Cho, I. S.; Lee, C. H.; Feng, Y.; Logar, M.; Rao, P. M.; Cai, L.; Kim, D. R.;

Sinclair, R.; Zheng, X. Nat. Commun. 2013, 4, 1723. (17)

Kim, D.; Sakimoto, K. K.; Hong, D.; Yang, P. Angew. Chem. Int. Ed. 2015,

54, 3259-3266. (18)

Chen, X.; Liu, L.; Yu, P. Y.; Mao, S. S. Science 2011, 331, 746-750.

(19)

Niu, P.; Yin, L. C.; Yang, Y. Q.; Liu, G.; Cheng, H. M. Adv. Mater. 2014, 26,

8046-8052. (20)

Zhao, Y.; Zhao, F.; Wang, X.; Xu, C.; Zhang, Z.; Shi, G.; Qu, L. Angew.

24


Page 24 of 30

Page 25 of 30

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

Nano Letters

Chem. Int. Ed. 2014, 53, 13934-13939. (21)

Han, Q.; Wang, B.; Zhao, Y.; Hu, C.; Qu, L. Angew. Chem. Int. Ed. 2015, 54,

11433-11437. (22)

Wang, X.; Wang, L.; Zhao, F.; Hu, C.; Zhao, Y.; Zhang, Z.; Chen, S.; Shi, G.;

Qu, L. Nanoscale 2015, 7, 3035-3042. (23)

Liang, Q.; Li, Z.; Yu, X.; Huang, Z. H.; Kang, F.; Yang, Q. H. Adv. Mater.

2015, 27, 4634-4639. (24)

Zhang, Y.; Liu, J.; Wu, G.; Chen, W. Nanoscale 2012, 4, 5300-5303.

(25)

Lee, J. T.; Zhao, Y.; Thieme, S.; Kim, H.; Oschatz, M.; Borchardt, L.;

Magasinski, A.; Cho, W. I.; Kaskel, S.; Yushin, G. Adv. Mater. 2013, 25, 4573-4579. (26)

Kim, T. W.; Choi, K. S. Science 2014, 343, 990-994.

(27)

Kenney, M. J.; Gong, M.; Li, Y.; Wu, J. Z.; Feng, J.; Lanza, M.; Dai, H.

Science 2013, 342, 836-840. (28)

Smith, R. D. L.; Prévot, M. S.; Fagan, R. D.; Zhang, Z.; Sedach, P. A.; Siu, M.

K. J.; Trudel, S.; Berlinguette, C. P. Science 2013, 340, 60-63. (29)

Long, X.; Xiao, S.; Wang, Z.; Zheng, X.; Yang, S. Chem. Commun. 2015, 51,

1120-1123. (30)

Tang, C.; Wang, H. S.; Wang, H. F.; Zhang, Q.; Tian, G. L.; Nie, J. Q.; Wei,

F. Adv. Mater. 2015, 27, 4516-4522. (31)

Nayak, S.; Mohapatra, L.; Parida, K. J. Mater. Chem., A 2015, 3, 18622-

18635. (32)

Luo, J.; Im, J. H.; Mayer, M. T.; Schreier, M.; Nazeeruddin, M. K.; Park, N.

G.; Tilley, S. D.; Fan, H. J.; Grätzel, M. Science 2014, 345, 1593-1596. (33)

Long, X.; Li, J.; Xiao, S.; Yan, K.; Wang, Z.; Chen, H.; Yang, S. Angew.

Chem. 2014, 126, 7714-7718.

25


Nano Letters

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

(34)

Gong, M.; Dai, H. Nano Res. 2015, 8, 23-39.

(35)

Gong, M.; Li, Y.; Wang, H.; Liang, Y.; Wu, J. Z.; Zhou, J.; Wang, J.; Regier,

T.; Wei, F.; Dai, H. J. Am. Chem. Soc. 2013, 135, 8452-8455. (36)

Youn, D. H.; Park, Y. B.; Kim, J. Y.; Magesh, G.; Jang, Y. J.; Lee, J. S. J.

Power Sources 2015, 294, 437-443. (37)

Long, X.; Wang, Z.; Xiao, S.; An, Y.; Yang, S. Mater. Today, 2016, DOI:

10.1016/j.mattod.2015.10.006. (38)

Low, J.; Cao, S.; Yu, J.; Wageh, S. Chem. Commun. 2014, 50, 10768-10777.

(39)

Zhao, Y.; Hu, C.; Song, L.; Wang, L.; Shi, G.; Dai, L.; Qu, L. Energy Environ.

Sci. 2014, 7, 1913-1918. (40)

Biener, J.; Stadermann, M.; Suss, M.; Worsley, M. A.; Biener, M. M.; Rose,

K. A.; Baumann, T. F. Energy Environ. Sci. 2011, 4, 656-667. (41)

Nardecchia, S.; Carriazo, D.; Ferrer, M. L.; Gutierrez, M. C.; del Monte, F.

Chem. Soc. Rev. 2013, 42, 794-830. (42)

Wu, Z. S.; Yang, S.; Sun, Y.; Parvez, K.; Feng, X.; Müllen, K. J. Am. Chem.

Soc. 2012, 134, 9082-9085. (43)

Zhu, C.; Han, T. Y. J.; Duoss, E. B.; Golobic, A. M.; Kuntz, J. D.; Spadaccini,

C. M.; Worsley, M. A. Nat. Commun. 2015, 6, 6962. (44)

Han, W.; Zang, C.; Huang, Z.; Zhang, H.; Ren, L.; Qi, X.; Zhong, J. Int. J.

Hydrogen Energy 2014, 39, 19502-19512. (45)

Fan, Y.; Ma, W.; Han, D.; Gan, S.; Dong, X.; Niu, L. Adv. Mater. 2015, 27,

3767-3773. (46)

Han, W.; Ren, L.; Gong, L.; Qi, X.; Liu, Y.; Yang, L.; Wei, X.; Zhong, J. ACS

Sustainable Chem. Eng. 2014, 2, 741-748. (47)

Qiu, B.; Xing, M.; Zhang, J. J. Am. Chem. Soc. 2014, 136, 5852-5855.

26


Page 26 of 30

Page 27 of 30

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

Nano Letters

(48)

Hou, Y.; Zhang, B.; Wen, Z.; Cui, S.; Guo, X.; He, Z.; Chen, J. J. Mater.

Chem., A 2014, 2, 13795-13800. (49)

Yin, Y. X.; Xin, S.; Guo, Y. G.; Wan, L. J. Angew. Chem. Int. Ed. 2013, 52,

13186-13200. (50)

Shan, Y.; Yu, C.; Yang, J.; Dong, Q.; Fan, X.; Qiu, J. ACS Appl. Mater.

Interfaces 2015, 7, 12203-12209. (51)

Zhou, J.; Qin, J.; Zhang, X.; Shi, C.; Liu, E.; Li, J.; Zhao, N.; He, C. ACS

Nano 2015, 9, 3837-3848. (52)

Hou, Y.; Li, J.; Wen, Z.; Cui, S.; Yuan, C.; Chen, J. Nano Energy 2014, 8,

157-164. (53)

Zhao, F.; Cheng, H.; Hu, Y.; Song, L.; Zhang, Z.; Jiang, L.; Qu, L. Sci. Rep.

2014, 4, 5882. (54)

Zhou, Z.; Shen, Y.; Li, Y.; Liu, A.; Liu, S.; Zhang, Y. ACS Nano 2015, 9,

12480-12487. (55)

Song, F.; Hu, X. Nat. Commun. 2014, 5, 4477.

(56)

Abellan, G.; Coronado, E.; Marti-Gastaldo, C.; Pinilla-Cienfuegos, E.; Ribera,

A. J. Mater. Chem. 2010, 20, 7451-7455. (57)

Oliver-Tolentino, M. A.; Vázquez-Samperio, J.; Manzo-Robledo, A.;

González-Huerta, R. D. G.; Flores-Moreno, J. L.; Ramírez-Rosales, D.; GuzmánVargas, A. J. Phys. Chem., C 2014, 118, 22432-22438. (58)

Jiang, Y.; Li, X.; Yu, S.; Jia, L.; Zhao, X.; Wang, C. Adv. Funct. Mater. 2015,

25, 2693-2700. (59)

Hou, Y.; Huang, T.; Wen, Z.; Mao, S.; Cui, S.; Chen, J. Adv. Energy Mater.

2014, 4, 1400337. (60)

Ma, T. Y.; Dai, S.; Jaroniec, M.; Qiao, S. Z. Angew. Chem. Int. Ed. 2014, 53,

27


Nano Letters

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

7281-7285. (61)

Abellán, G.; Carrasco, J. A.; Coronado, E.; Prieto-Ruiz, J. P.; Prima-García,

H. Adv. Mater. Interfaces 2014, 1, 1400184. (62)

Liu, X.; Chang, Z.; Luo, L.; Xu, T.; Lei, X.; Liu, J.; Sun, X. Chem. Mater.

2014, 26, 1889-1895. (63)

Zheng, Y. R.; Gao, M. R.; Gao, Q.; Li, H. H.; Xu, J.; Wu, Z. Y.; Yu, S. H.

Small 2015, 11, 182-188. (64)

Gao, M. R.; Xu, Y. F.; Jiang, J.; Zheng, Y. R.; Yu, S. H. J. Am. Chem. Soc.

2012, 134, 2930-2933. (65)

Lai, L.; Potts, J. R.; Zhan, D.; Wang, L.; Poh, C. K.; Tang, C.; Gong, H.; Shen,

Z.; Lin, J.; Ruoff, R. S. Energy Environ. Sci. 2012, 5, 7936-7942. (66)

Liu, Q.; Zhang, J. Langmuir 2013, 29, 3821-3828.

(67)

Niu, P.; Liu, G.; Cheng, H. M. J. Phys. Chem., C 2012, 116, 11013-11018.

(68)

Wang, Z.; Yang, C.; Lin, T.; Yin, H.; Chen, P.; Wan, D.; Xu, F.; Huang, F.;

Lin, J.; Xie, X.; Jiang, M. Adv. Funct. Mater. 2013, 23, 5444-5450. (69)

Zhang, J.; Zhang, M.; Lin, L.; Wang, X. Angew. Chem. Int. Ed. 2015, 54,

6297-6301. (70)

Zhang, J.; Chen , X.; Takanabe, K.; Maeda, K.; Domen, K.; Epping, J. D.; Fu,

X.; Antonietti, M.; Wang , X. Angew. Chem. Int. Ed. 2010, 49, 441-444. (71)

Hou, Y.; Zuo, F.; Dagg, A.; Feng, P. Angew. Chem. Int. Ed. 2013, 52, 1248-

1252. (72)

Bian, J.; Li, Q.; Huang, C.; Li, J.; Guo, Y.; Zaw, M.; Zhang, R. Q. Nano

Energy 2015, 15, 353-361. (73)

Li, T.; He, J.; Peña, B.; Berlinguette, C. P. Angew. Chem. Int. Ed. 2016, 55,

1769-1772.

28


Page 28 of 30

Page 29 of 30

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

Nano Letters

(74)

Jang, J. W.; Du, C.; Ye, Y.; Lin, Y.; Yao, X.; Thorne, J.; Liu, E.; McMahon,

G.; Zhu, J.; Javey, A.; Guo, J.; Wang, D. Nat. Commun. 2015, 6, 7447. (75)

Fu, L.; Yu, H.; Zhang, C.; Shao, Z.; Yi, B. Electrochim. Acta 2014, 136, 363-

369. (76)

Wang, G.; Ling, Y.; Wang, H.; Yang, X.; Wang, C.; Zhang, J. Z.; Li, Y.

Energy Environ. Sci. 2012, 5, 6180-6187. (77)

Kelkar, S. A.; Shaikh, P. A.; Pachfule, P.; Ogale, S. B. Energy Environ. Sci.

2012, 5, 5681-5685. (78)

Biswas, S. K.; Baeg, J. O. Int. J. Hydrogen Energy 2013, 38, 14451-14457.

(79)

Kim, J. Y.; Magesh, G.; Youn, D. H.; Jang, J. W.; Kubota, J.; Domen, K.; Lee,

J. S. Sci. Rep. 2013, 3, 2681. (80)

Abdi, F. F.; Han, L.; Smets, A. H. M.; Zeman, M.; Dam, B.; van de Krol, R.

Nat. Commun. 2013, 4, 2195. (81)

Zhong, D. K.; Sun, J.; Inumaru, H.; Gamelin, D. R. J. Am. Chem. Soc. 2009,

131, 6086-6087. (82)

Hou, J.; Cheng, H.; Takeda, O.; Zhu, H. Angew. Chem. Int. Ed. 2015, 54,

8480-8484. (83)

Zhong, M.; Li, Y.; Yamada, I.; Delaunay, J. J. Nanoscale 2012, 4, 1509-1514.

(84)

Ai, G.; Mo, R.; Li, H.; Zhong, J. Nanoscale 2015, 7, 6722-6728.

(85)

Zhou, M.; Bao, J.; Bi, W.; Zeng, Y.; Zhu, R.; Tao, M.; Xie, Y. ChemSusChem

2012, 5, 1420-1425. (86)

Iandolo, B.; Zhang, H.; Wickman, B.; Zoric, I.; Conibeer, G.; Hellman, A.

RSC Adv. 2015, 5, 61021-61030. (87)

Li, L.; Yan, J.; Wang, T.; Zhao, Z. J.; Zhang, J.; Gong, J.; Guan, N. Nat.

Commun. 2015, 6, 5881.

29


Nano Letters

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

(88)

Liu, Y.; Yu, Y. X.; Zhang, W. D. Int. J. Hydrogen Energy 2014, 39, 9105-

9113. (89)

Zhang, Y.; Antonietti, M. Chem. Asian J. 2010, 5, 1307-1311.

(90)

Zhang, Y.; Schnepp, Z.; Cao, J.; Ouyang, S.; Li, Y.; Ye, J.; Liu, S. Sci. Rep.

2013, 3, 2163. (91)

Hou, J.; Yang, C.; Cheng, H.; Jiao, S.; Takeda, O.; Zhu, H. Energy Environ.

Sci. 2014, 7, 3758-3768. (92)

Iwashina, K.; Iwase, A.; Ng, Y. H.; Amal, R.; Kudo, A. J. Am. Chem. Soc.

2015, 137, 604-607. (93)

Hou, Y.; Li, X.; Zhao, Q.; Quan, X.; Chen, G. Environ. Sci. Technol. 2010, 44,

5098-5103. (94)

Hou, J.; Wang, Z.; Yang, C.; Cheng, H.; Jiao, S.; Zhu, H. Energy Environ. Sci.

2013, 6, 3322-3330. (95)

Hou, Y.; Zuo, F.; Dagg, A.; Feng, P. Nano Lett. 2012, 12, 6464-6473.

(96)

Shi, X.; Choi, I. Y.; Zhang, K.; Kwon, J.; Kim, D. Y.; Lee, J. K.; Oh, S. H.;

Kim, J. K.; Park, J. H. Nat. Commun. 2014, 5, 4775. (97)

Wang, L.; Dionigi, F.; Nguyen, N. T.; Kirchgeorg, R.; Gliech, M.; Grigorescu,

S.; Strasser, P.; Schmuki, P. Chem. Mater. 2015, 27, 2360-2366.

30


Page 30 of 30

Strongly Coupled Ternary Hybrid Aerogels of N-deficient Porous

Recommend Documents