Ultralight, Flexible, and Semi-Transparent Metal Oxide Papers for

Subscriber access provided by UNIV OF CALIFORNIA SAN DIEGO LIBRARIES

Article

Ultralight, Flexible and Semi-Transparent Metal Oxide Papers for Photoelectrochemical Water Splitting Minwei Zhang, Chengyi Hou, Arnab Halder, and Qijin Chi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14036 • Publication Date (Web): 10 Jan 2017 Downloaded from http://pubs.acs.org on January 10, 2017

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.

ACS Applied Materials & Interfaces 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 26

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

ACS Applied Materials & Interfaces

Ultralight, Flexible and Semi-Transparent Metal Oxide Papers for Photoelectrochemical Water Splitting Minwei Zhang †,§, Chengyi Hou †, ‡, §, Arnab Halder †, and Qijin Chi †,* †

Department of Chemistry, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark.

‡

The State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China.

ABSTRACT: Thanks to their versatile functionality, metal oxides (MOs) constitute one of the key family materials in a variety of current demands for sensor, catalysis, energy storage and conversion, optical electronics and piezoelectric mechanics. Much effort has focused on engineering specific nanostructure and macroscopic morphology of MOs that aims to enhance their performances, but the design and controlled synthesis of ultrafine nanostructured MOs in a cost-effective and facile way remains a challenge. In this work, we have exploited the advantages of intrinsic structures of graphene oxide (GO) papers, serving as a sacrificial template, to design and synthesize two-dimensional (2D) layered and free-standing MO papers with ultrafine nanostructures. Physicochemical characterizations showed that these MO materials are nanostructured, porous, flexible and ultralight. The as-synthesized materials were tested for their potential application in photoelectrochemical (PEC) energy conversion. In terms of PEC water splitting, copper oxide papers were used as an example and exhibited excellent performances with an extremely high photocurrent-to-weight ratio of 3 A cm-2 g-1. We have also shown that the synthesis method is generally valid for many earth-abundant transition metals including copper, nickel, iron, cobalt and manganese. _______________________________________________________________________ KEYWORDS: 2D nanomaterial, graphene oxide paper, templated synthesis, ultralight metal oxide, photoelectrochemical water splitting, hydrogen generation.

ACS Paragon Plus 1 Environment


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 2 of 26

1. INTRODUCTION Due to their versatile functionality, metal oxides (MOs) have become one of the key family materials for a range of increasing demands from sensor, catalysis, energy storage and conversion, optical electronics to piezoelectric mechanics.1-6 The fact that

many transition metals are earth abundant is offering advantageous

opportunity for scale-up production of MO nanomaterials. It has been noticed that physicochemical properties of MOs, their electronic and optical features in particular, are significantly dependent on grain size and morphology.7,

8

For

example, compared to their bulk materials nanostructured MOs with large surfaceto-volume

ratio

and

photoelectrochemical

nanometer

grain

performance.9,

10

size The

can

display

bottom-up

much

assembly

enhanced of

MO

nanostructures into a macroscopic form is needed for many practical applications of MO materials.11,12 However, nanoscale MO building blocks such as nanoparticles, nanosheets, nanorods and nanotubes often undergo densification and suffer significant loss of specific surface areas during the assembly process. To overcome these critical challenges, recent efforts have focused on the development of controlled assembly methods. For example, Dou and coworkers reported an impressive approach, based on employing lamellar reverse micelle templates, to generalize template-induced self-assembly of scalable two-dimensional (2D) transition MO nanosheets.13 Another example was demonstrated by Hwang and coworkers, in which they used graphene nanosheets as a platform for the 2D ordered assembly of TiO2 nanopartciles.14 These templating methods have in general inspired other researches to create macroscopic 2D or three-dimensional (3D) MO materials.15-19 The exploration of graphene nanosheet derived materials as soft templates to prepare MO nanosheets is rapidly emerging recently, because it could offer new and


Page 3 of 26

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


efficient approaches.14, 20-23 For instance, ordered meso-structured MO nanosheets can be synthesized using graphene nanosheet templating and have potentials for a wide range of applications. However, these MO nanosheets still need to be assembled into macroscopic constructs for practical applications. In addition, the formation mechanism of macroscopic MO structures is far from fully understood, though some MO assemblies were systematically characterized.22 A deep investigation into the formation mechanisms and gaining the ability to tailor MO structures are thus highly desirable. Furthermore, the ability to assemble ultralight, free-standing, semi-transparent and flexible macroscopic MO materials with ultrafine nanostructures is rarely reported. Such MO materials are increasingly demanded by fabrication of lightweight and flexible electrode materials for advanced solar energy utilization. In this work, we have explored the structural advantages of graphene oxide (GO) paper as a sacrificial template, which has a well-defined layer structure, to simultaneously synthesize and assemble ultrafine nanostructured MOs (with the research focused on CuO, ZnO, Co3O4, NiO, Fe2O3 and Mn3O4). We have successfully prepared free-standing MO papers that consist of ultrathin and porous 2D MO monolayers. The obtained products are pure MO in the form of papers, and they are completely different from graphene-MO composites and individual MO nanosheets. We also provide a general understanding of how MO clusters grow into porous nanosheets. The formation mechanism is found to be very different as well, in which we have introduced non-conventional oriented-attachment mechanism to account for the present case. The unique nanostructured MO papers have an ultralow density and desirable ability facilitating ion and/or solute diffusion in electrochemical or photoelectrochmeical (PEC) environments. CuO papers were used as a main target for the proof-of-concept tests of lightweight applications. The ACS Paragon Plus 3 Environment


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

CuO paper based photocathodes have shown high efficient in PEC water reduction, reflected by a high solar conversion efficiency and an extremely high photocurrent density-to-weight ratio. These observations could lay the basis for cost-effective lightweight applications of these newly synthesized MO materials.

Figure 1. Top panel: the sketch illustrating the overall synthesis procedure of MO papers and the photographs showing material appearances (blue powders and solution: Cu2SO4, black powders and papers: GO, semi-transparent light brown papers: CuO); Bottom panel: the 10 photographs successively recorded within 0.35 s were superimposed to demonstrate lightweight materials (including both CuO and GO papers) being blow-away (see Movie S1 for dynamics, Supporting Information).

2. RESULTS AND DISCUSSION 2.1 Bottom-up method preparation of metal oxide papers The overall preparation procedure of MO materials is schematically illustrated in Figure 1 (the top panel). GO paper used as a starting material was prepared according to our previously reported procedure.24-26 Both the graphitic planes and edges of individual GO nanosheets possess a notable number of oxygen-containing groups (OCGs) (Figures S1 and S2).27 Following the immersion of GO paper in


Page 4 of 26

Page 5 of 26

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


water, the d-spacing between the layers is approximately 1.2 nm, which is suitable for accommodation of some hydrated metal ions (Figure S3). This unique structure of GO papers can offer ideal chemically active sites for adsorbing and confining different metal ions. Our experiments have proven that the GO paper can capture a variety of metal ions (e.g. Fe3+, Zn2+, Cu2+, Co2+, Mg2+ and Ni2+) into its interlayers via electrostatic attraction between OCGs and metal cations. In a typical preparation procedure, we first immersed a piece of GO paper in a solution containing a specific metal ion salt for 48 hours (the detail described in the Experimental Section). After the absorption of metal ions, the Mn+-GO paper was

then removed from the salt solution, followed by rinsing with water for several times to remove those loosely (or physically) adsorbed ions. The Mn+-GO paper was

finally annealed in air (or nitrogen atmosphere) for nucleation, crystallization as well as growth of pure MO material and simultaneously the carbon components were sacrificed. As a result of space-confined and templated synthesis, the MO papers exhibit the same morphology of the GO papers used (e.g. those MO papers in Figure 2a, 2d, 2g and 2j), except for shrinking their size by approximately 10-14% due to the decomposition of GO layers. MO papers prepared by the present method are free-standing, and their apparent color depends on the type of metals and the thickness papers. For example, while the ZnO paper is almost fully transparent with a light grey-white color (Figure 2a), the Fe2O3 paper exhibits a reddish brown color (Figure 2j). The CuO papers have either a light brown (ultrathin, Figure 1) or grey black (thicker papers, Figure 2d). These MO papers are ultralight, surprisingly even much lighter than GO papers or GO nanopowders, as demonstrated by a simple blow-away test with a normal balloon (see the bottom panel in Figure 1 and Movie S1).



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

Figure 2. Photographs (top panel: (a), (d), (g) and (j)) and microscopy images (middle panel: SEM images (b), (e), (h) and (k); bottom panel: TEM images (c), (f), (i) and (l)) of the as-synthesized metal oxide papers. (a‒c) ZnO, (d‒f) CuO, (g‒i) Co3O4 and (j‒l) Fe2O3 papers. Scale bars: 10 µm (b), 100 nm (c); 15 µm (e), 500 nm (f); 3 µm (h), 100 nm (I); 5 µm (k), 100 nm (l).

2.2 Morphology and structural characterization We have performed systematic analysis on the morphology and microscopic structures of MO materials using a number of microscopy and spectroscopy techniques, including scanning electron microscope (SEM), transmission electron microscope (TEM), atomic force microscopy (AFM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). For example, digital photographs, SEM images and TEM images for a series of MO papers are shown in Figure 2 (with more results provided in Supporting Information, Figures S4 and S5). Thanks to the templating effect of GO paper, the MO materials retain the free-standing paper form and have a graphite-like layered structure. The XRD and XPS analyses reveal that ACS Paragon Plus 6 Environment

Page 6 of 26

Page 7 of 26

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


ultrapure MO crystals were produced (Figures S6 and S7). The size of MO grains is in the range of 7 to 50 nm, depending on the type of metals (Figure S5). The variation of grain sizes most likely originates in different crystallization kinetic constants of metals that are associated directly with the activation energy of MO formation. Overall, all MO papers have a graphite-like layered structure as revealed by the SEM and TEM images. They also maintain the macroscopic pattern of GO paper templates (Figure S8). By controlling the thickness of GO papers, the MO papers with a distinct thickness of 4-16 µm (but not limited to this range) can be steadily prepared (Figure S9). In control experiments, graphite-like MO structures did not form when no template was employed, or either GO nanosheets or GO aerogel was used as a template (Figure S10). These observations indicate that GO paper plays an essential role in the formation of layered MO papers. With six different types of metal oxides as examples, this space-confined and templated method could introduce a general strategy for the design and synthesis of freestanding 2D porous MO material.

2.3 Growth mechanism of metal oxide papers The MO paper is composed of individual monolayers. Intriguingly, MO monolayers have an ultrathin nanoporous structure as revealed by high-resolution TEM (HRTEM) images (Figure 2 and Figure S5). It is known that the growth of MO nanocrystals normally follows

the Ostwald ripening kinetics and is largely a random nucleation process, when the synthesis is processed in the absence of a template. The morphology of MO crystals is thus hardly to be controlled. To overcome this problem, surfactants are widely used to control the growth of MO nanocrystals. However, the Ostwald ripening process is driven mainly by the minimization of total surface free energy. As a consequence, the formation of spherical particles with relatively large sizes (from tens of nm to hundreds of nm) are ACS Paragon Plus 7 Environment


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 8 of 26

predominantly favored over ultrafine nanocrystals.28-30 In contrast, we are able to obtain an ultrafine grained nanostructure (sub-10 nm) and a well-defined macroscopic morphology in the form of 2D free-standing MO paper using the present synthesis method. In the rest of this work, our efforts are devoted to the CuO papers as an example for studying the formation

mechanism

of

such

a

unique

structure

and

for

evaluating

their

photoelectrochemical water splitting. A combination of thermogravimetric analysis (TGA) and microscopy imaging can offer some crucial clues for understanding the formation mechanisms of MO papers. For the TGA analysis, both dried Cu2+-GO and pure GO papers were heated under the same experimental conditions. For example, the samples were heated in air from room temperature (RT) to 600oC. The differential scanning calorimetry (DSC) measurements show that both samples exhibit a similar exothermic peak around 210 oC (Figure 3a), attributed to the decomposition of OCGs on GO paper,31,

32

but the Cu2+-GO paper

occurred at a notably lower decomposition temperature by approximately 10oC (i.e.

∆T≈10oC) and with a less weight loss (~3.1%) compared to the pure GO paper. For further comparisons, we performed the similar TGA and DSC measurements in a N2 atmosphere instead of air. To our surprise, the similar phenomenon was observed for both pure GO and Cu2+-GO paper samples (Figure S11). This observation suggests that external oxygen source from air is not essential for the formation of MO papers in the present case. Instead, the GO nanosheets can serve as the oxygen donor in addition to their templating function, while the absorbed Cu2+ ions act as the oxygen acceptor during the thermal reduction of GO. As a result, CuxO (x = 1 or 2, which is identified by XPS spectra and discussed later) nanoclusters were crystallized between the GO layers (TEM images in the insets of Figure 3a and Figure S11) and were growing by expanding the interplanar spacing of GO paper. The TGA curve shows that the first-phase weight loss of Cu2+-GO paper started at 180oC, in which the CuxO clusters formed and attached to GO layers as revealed by the TEM ACS Paragon Plus 8 Environment

Page 9 of 26

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


image (inset i in Figure 3a). The CuxO clusters maintained their size around 5 nm until the annealing temperature was elevated to 400oC (insets ii and iii in Figure 3a), in which the Cu2+-GO paper was subjected to the second-phase weight loss. The second exothermic process was invoked by the decomposition of carbon components in the GO paper. The decomposition of Cu2+-GO paper started at much lower temperatures (∆T≈100 oC) and was significantly faster than pure GO paper (Figure 3a), largely because the crystallization and growth of CuxO nanoparticles in the Cu2+-GO paper disrupted the ordered interlayer structure of GO paper. Owing to the sacrifice of GO layers after annealing at 400 oC or higher temperatures, CuxO clusters enabled to start growing and attaching each other to form a 2D network. The formation of cupric oxide (CuO) networks composed of around 15 nm nanocrystals was completed after annealed at 500 oC (Figures 3b‒d). The network consists of the monolayers that have a thickness of ~10-15 nm as measured by AFM (Figure 3e) and the average pore size of ~10 nm determined by the nitrogen adsorptiondesorption isotherms (based on the Brunauer-Emmett-Teller (BET) method). The BET surface area of CuO networks is estimated as 61.1 ± 0.5 m2 g-1 (Figure S12). These results confirm the polycrystalline nature and nanoporous structure of CuO networks. Owing to these structural features, the CuO papers have an ultralow apparent density of approximately 65 ± 4 mg cm-3, which is calculated on the basis of the precise weight determination by a quartz crystal microbalance (QCM). This density is at least one order of magnitude lower than the density of bulk CuO powders composed of similar size particles,33 and the CuO paper is even more than 27 times lighter than pure GO papers.34



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

Page 10 of 26

Page 11 of 26

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


Figure 3. (a) TGA (solid lines) and DSC (dash lines) analyses of Cu2+-GO (red lines) and pure GO (blue lines) papers annealed in air. Insets are the TEM images of the Cu2+-GO paper after heated at different temperatures, corresponding to the points marked on the TGA curve; scale bars: 20 nm (i), 40 nm (ii) and 15 nm (iii). (b‒d) TEM images and corresponding selected area electron diffraction (SAED) pattern of Cu2+-GO paper annealed at 500 oC; the scale bar in (b) is 40 nm. The SAED pattern indicates the presence of CuO crystalline structures. (e) AFM image and a corresponding height profile of the 2D CuO monolayered sheets.

Based on the experimental observations, we further discuss the formation mechanisms of nanoporous MO networks. With CuxO as an example, we propose a growth procedure of the MO crystals in Figure 4. The crystallization and growth of CuxO nanocrystals, as well as the enlargement and disruption of GO layers are schematically illustrated in Figure 4a‒c. During the nucleation and crystallization of nanoclusters (Figure 4b), Cu2+ ions accepted limited amount of oxygen species likely only from GO layers, leading to a partial aggregation of

Cu2+ ions into Cu2+‒O‒Cu2+ complexes. Meanwhile, the progressive

thermal activation caused the Cu2+→Cu+ reduction with oxygen elimination.35 Cuprous oxide (Cu2O) was therefore identified by XRD at this stage, and the generated Cu2O clusters undergoing spontaneous self-organization/self-recrystallization. As disclosed by the TEM image (the inset in Figure 4c and Figure S13), the 5-nm Cu2O clusters attached to each other along the common crystallographic orientations (highlighted by the parallel yellow and blue lines in the inset of Figure 4c) to form larger clusters. This observation strongly suggests that the growth of Cu2O clusters follows an Oriented-attachment mechanism (a non-conventional crystallization process).36-38 During the coarsening process, GO paper was gradually destroyed so that Cu2O clusters were further exposed to dioxygen (O2). Cu2O was consequently fully oxidized into CuO, as evidenced by the Cu 2p XPS spectra and XRD patterns (Figure 4c). In the following stage, the 15-nm CuO particles attached to their neighbors in the confined nanoscale space and form a network. However, because the aggregation had already reached saturation, further reorientation and self-



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

organization/self-recrystallization of nanocrystals were restricted. Given the facts that metal ions are confined within GO paper and their total amount is constant, it is ruled out that the MO could undergo diffusion-controlled Ostwald ripening growth process. In other words, the solo-oriented-attachment mechanism is solely responsible for the growth of 2D nanoporous MO monolayer crystals that further connects each other to form porous MO paper.37 This crystallization process results in a significant difference in structures between the MO samples synthesized by GO paper template and reference MO samples prepared by other templates (Figure S10). In short, the solo-oriented-attachment mechanism can well explain the formation of nanoporous MO monolayers. As pointed out by Dove and coworkers in a recent review,38 however, while the crystallization-by-particle-attachment (CPA) mechanism has widely been observed experimentally, many fundamental aspects still remain to be understood, particularly concerning the interplay of solution structure, interfacial forces and particle motion. In the present case, the nanoparticle growth and attachment occurred during the high-temperature annealing process. This has posed a tremendous challenge for in-situ monitoring of the crystallization processes, but it is an interesting issue for ongoing efforts.

2.4 Light-weight photoelectrochemical applications The nanoporous and layered structure of MO papers could play a key role in providing shorter diffusion paths and rapid mass transport for electrolyte ions in electrochemical or photoelectrochemical environment. The as-prepared MO papers are thus expected to exhibit high electrochemical performances. Among these MOs, CuO is one of the earliest, most earth-abundant and cheapest semiconductor materials that have been extensively interested in energy related technological applications. For example, CuO has been recognized as a versatile material that holds promising for photocatalysis and supercapacitors.39-42 ACS Paragon Plus 12 Environment

Page 12 of 26

Page 13 of 26

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


Figure 4. Schematic illustration of the proposed oriented-attachment mechanism for the growth of MO crystals. (a) A side view of the interlayer structure of Cu2+-GO paper before annealing. (b) The expansion of the interlayers upon the formation of 5 nm MO nanocrystals in Cu2+-GO paper after annealed at 180 oC. (c) HRTEM image (sample treatment, ST: 300 oC), XPS spectra and XRD patterns (ST: 180 and 400 oC) indicating the self-recrystallization via the oriented attachment

mechanism and full oxidation of Cu2O clusters into CuO. The raw TEM image is provided in Supporting Information (Figure S13). Scale bar is 5 nm. (d) Illustration and TEM image (ST: 500 o

C) of the as-prepared CuO crystal annealed upon 400 oC. Scale bar: 10 nm. Not drawn to scale.

To demonstrate the application perspective of the as-synthesized materials, we have used the CuO paper directly as the photocathode for photoelectrochemical (PEC) water



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

splitting. Nanocrystallined CuO is an attractive p-type semiconductor for PEC applications. CuO has a conduction band edge with more negative potential than that needed for the reduction of water to generate hydrogen, which makes it a promising photocathode material for water splitting application. The indirect transition bandgap of the CuO paper is estimated from the Tauc plot as ≈1.34 eV (Figure S14), further making it a good candidate for solar water splitting as the CuO paper can adsorb photon energy from the solar spectrum. In addition, the band gap of 1.34 eV is higher than the thermodynamic potential for water splitting (1.23 eV). The fabrication of CuO based photocathode was achieved by direct casting of the CuO paper on an ITO electrode, which was tested for water splitting. As shown in Figure 5a, CuO paper can be wet-casted on various substrates such as soft human fingers or hard ITO owing to the high flexibility of nanoporous CuO layers (TEM image in the inset and Figures S15‒S17). Therefore, the CuO paper could be integrated with a wide range of current collector materials including ITO (used in this work, Figure 5a), rough copper mesh, porous carbon and flexible conductive polymers. Furthermore, the transparency of CuO papers can be adjusted by controlling the thickness of GO paper template. In other words, the amount/weight/thickness of the as-prepared MO papers can be largely controlled. Figure 5b shows the transmission spectra of a series of CuO paper/ITO photocathodes (named no.2‒no.8) with different thickness. Their transmission in the visible-light region varies from 50% to 5%. The semi-transparent photoelectrodes are highly appropriate in photovoltaic applications.43,44 The CuO paper/ITO photocathodes with an active surface area of 0.8 cm2 were employed in the one-step excitation PEC water splitting (Figure 5c, light irradiation: 100 mW cm-2, λ > 400 nm). The mean photocurrent density measured for photocathodes no.2 ‒ no.8 is compared in Figure 5d. The highest net photocurrent generated (by photocathode No.5) can be up to 210 µA cm-2. Since the ITO surface was also exposed to the electrolyte ACS Paragon Plus 14 Environment

Page 14 of 26

Page 15 of 26

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


due to the nanoporous nature of CuO paper, it is necessary to measure the photocurrent generated by the bare ITO electrode as a background current. However, the result shows that bare ITO electrodes can only generate negligible photocurrent compared to CuO paper coated ITO electrodes (Figure S18). The systematical measurements were performed at photocathode No.5, with the key results shown in Figure 5e and 5f. Linear sweep voltammogram (LSV) and net I-t curves were collected in the dark and under visible-light illumination (100 mW cm-2, λ > 400 nm) in a 0.5 M Na2SO4 electrolyte solution. Upon light illumination, a considerable photocurrent was rapidly generated, and the electrode exhibited fast and stable On-Off switching (Figure 5f). The photocatalytic activity of photocathode No.5 is also superior to that of reference photocathodes made by CuO nanoparticles (see Figures S10 and S19), suggesting a beneficial effect of the unique structure of CuO papers on photocatalytic activity. We had also observed a transient photocurrent in response to chopped illumination (Figure 5f). The instantaneous photocurrent (I0) generated immediately upon the visible-light illumination is associated with a surface electron-hole recombination. After the transient phase, the steady-state photocurrent (Iss) arising from the flux of electrons that are transferred successfully to the electrolyte was obtained.45 The charge transfer efficiency (ƞtrans) can be estimated from the ratio of transient to steady photocurrents (i.e. ƞtrans= Iss/I0). The ƞtrans calculated from Figure 5f is about 70%, indicating a good electrode/electrolyte contact even without employing co-catalyst such as Pt nanoparticles.46 This facilitation is attributed to the nanoporous structure of CuO papers. We further calculated the solar conversion efficiency (ƞconv), or the efficiency of photon-to-hydrogen generation, based on the measured Iss values according to the following equation47,48

ƞconv = Iss(1.23-V)/Jlight ACS Paragon Plus 15 Environment

(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

where V is the applied potential, Jlight is the intensity of light irradiation. The conversion efficiency of photocathode No.5 is as high as 0.36%, which is a considerable improvement compared to those previously reported at co-catalyst free photoelectrodes (for example, 0.15% for ZnO:N nanowires,48 0.1% for gold nanorods,49 0.12 for Si and TiO2 nanotrees,50 0.1% for ZnO thin film,51 and 0.1% for TiO2 nanorods52). In addition, the conversion efficiency of CuO papers could be further improved by using co-catalysts or conductive chemical agents, though it is beyond the scope of the present work but should constitute an interesting ongoing research effort. More importantly, it is worth noting that only ≈70 µg CuO paper was used in the construction of photocathode No.5. In most previous reports, in contrast, the amount of photoactive materials (m) needed for the construction of a photoelectrode with the similar efficiency is normally in the range from tens of milligrams to several grams. To date, the current density-to-active material weight ratio has rarely been reported in literatures. In response to current environmental and energy concerns, however, lightweight materials could offer unique advantages to meet special needs in some areas such as wearable devices, spaceship, and army or astronaut clothing. In fact, lightweight materials have especially been identified as necessary and important by the U.S. Department of Energy (DOE), in order to fulfill the Energy Efficiency programs.53 In the present case, we calculated the Iss-m ratio of photocathode No.5 to be as high as 3.0 A cm-2 g-1, which is more than one order of magnitude higher than the value estimated from previously reported solar water splitting materials (0.125,50 0.25,54 ≈0.2 A cm-2 g-1 55).


Page 16 of 26

Page 17 of 26

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


Figure 5 (a) Photographs of CuO papers wet-casted on a finger and an ITO. Inset is a TEM image demonstrating a highly flexible nanoporous CuO. Scale bar: 400 nm. (b) Transmission spectra of a serie of CuO paper/ITO photocathodes. Spectrum 1 was obtained at bare ITO electrodes as a reference. CuO paper/ITO photocathodes with decreasing transparency in the visible light range upon increasing the film thickness are continuously numbered top-down. (c) Schematic illustration of the operation principles of light-driven water splitting for a PEC cell based on CuO paper/ITO photocathode; e- and h+ denote electron and hole, respectively. (d) Current density of a serie of CuO paper/ITO photocathodes as a function of their transmittance at 800 nm. The photocurrent density was recorded at -0.5 V (vs SCE) in 0.5 M Na2SO4 under light illumination of 100 mW cm-2, where a Pt wire and saturated calomel electrode (SCE) were used as a counter electrode and a reference



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 18 of 26

electrode, respectively. The current density-weight (I-m) ratio obtained at photocathode No.5 is highlighted. (e) Linear sweep voltammograms collected at a scan rate of 10 mV s-1 from photocathode No.5 at 100 mW cm-2. (f) Amperometric I-t curves of the photocathode No.5 at an applied voltage of -0.5 V upon light on/off cycles.

3. CONCLUSIONS In conclusion, we present here a general method for the high-yield and possibly large-scale synthesis of ultralight, free-standing, flexible and nanoporous MO papers. Six types of MO papers including ZnO, CuO, Co3O4, Fe2O3, Mn3O4 and NiO are demonstrated as examples. The MO papers consist of nanoporous monolayers, attributed to the confined synthesis facilitated by the sacrificial GO paper as a template and the CPA growth mechanism. We discuss the possible formation mechanisms of the unique nanostructured MO papers in detail, on the basis of the analyses by TGA, TEM and XPS. We suggest a non-conventional oriented attachment mechanism that can well explain the formation of the unique structured MO materials. In our proof-of-concept studies, the CuO papers are directly used to fabricate photocathodes for solar water splitting. These CuO paper based

photocathodes have

demonstrated

clear

advantages

for lightweight

electrochemical applications. Straightforward structural improvement to a material could lead to a significant enhancement in the photocurrent-weight ratio. In short, ultralight, flexible, and semi-transparent MO paper based materials could add new perspective for lightweight energy applications in terms of energy efficiency.

4. EXPERIMENTAL SECTION In this section, we present a brief summary of the key experimental materials and instrumental methods. More details are provided in the Supporting Information (SI). 4.1 Synthesis of MO papers.


Page 19 of 26

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 as-prepared GO papers were immersed in a 0.1 M metal salt (Fe3+, Zn2+, Cu2+, Co2+, Mg2+, and Ni2+) solution for 48 h. After being carefully washed with Milli-Q water, the Mn+-GO papers were heated from room temperature to 180, 300, 400, and 500oC at a rate of 10oC min-1, and kept at the final temperature for 4.5 h in air or under nitrogen atmosphere. In the reference experiments, different annealing temperatures and protective gas were used for comparisons. In the control experiments, we used Cu(CH3COO)2 powders, GO/Cu(CH3COO)2 mixture powders (GO: Cu = 10:1 wt.), and GO aerogels (immersed in Cu2+ solution) as starting materials for annealing. We compared the morphologies of as-synthesized CuO materials to investigate the GO template effects. 4.2 Instrumental methods for materials characterization. SEM characterizations were carried out on an FEI Quanta FEG 200 ESEM. A 5500 AFM system (Agilent Technologies) was used for AFM imaging. UV-vis spectra of the materials were recorded using an 8453 spectrophotometer from Agilent Technologies. XPS (Thermo Scientific) was performed to analyse the compositions of samples, with Al-Kα (1486 eV) as the excitation X-ray source. Samples were measured after 20 s ion beam etching. TEM imaging and electron diffraction were performed on Tecnai G2 T20 TEM from FEI Company. Thermogravimetric analysis (Netzsch STA 409PC) was carried out in air or under N2 atmosphere, at a relatively slow heating rate of 5 °C min−1. The weights of the MO papers were determined using a quartz-crystal microbalance (QCM200) with a sensitivity of 10 ng. The density (apparent density) of the MO papers was measured on the basis of the ISO 845:2006 standard. The Brunauer-Emmett-Teller (BET) surface area was measured at 77 K with a Micromeritics ASAP 2020 analyzer. The samples were degassed in vacuum at 200oC for 3 h. Optical microscope images were taken with a Nikon Eclipse L200N optical microscope. 4.3 Photoelectrochemical measurements. ACS Paragon Plus 19 Environment


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

Linear scan voltammetry (LSV) and photocurrents were recorded by a CHI760 electrochemical station. CuO papers were wetted in water and attached to the ITO surface to obtain the CuO paper/ITO photocathode. A saturated calomel reference electrode (SCE) was employed along with a platinum coiled wire counter electrode during all the measurements. The electrolyte was 0.5 M Na2SO4 solution. A solar-mimic white light (100 mW cm-2, λ > 400 nm) was used to illuminate the photocathodes. All electrochemical measurements were performed at room temperature. Argon was used to purge O2 from the electrolyte solutions.

ASSOCIATED CONTENT Supporting Information: More details of experimental procedures, SEM, TEM and AFM images, XPS spectra, electrochemical data, supporting movie and references.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (Q.C.) Notes §

These authors contribute equally to this work.

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by DFF-FTP (the Danish Research Council for Technology and Product Science, to Q.C., Project No. 12-127447). C.H. is grateful for financial supports by the Ørsted-Marie-Curie Cofunded postdoc fellowship (Project ID: 609405), the Shanghai ChenGuang Program (15CG33), the Shanghai Natural Science Foundation


Page 20 of 26

Page 21 of 26

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


(16ZR1401500), the Shanghai Sailing Program (16YF1400400), and the NSF of China (51603037). M.Z. acknowledges the CSC PhD scholarship (No. 201306170047).

REFERENCES [1] Rodriguez, J. A.; Garcia, M. F. Synthesis, Properties and Applications of Oxide

Nanomaterials; John Wiley and Sons, Hoboken: New Jersey, 2007. [2] Jolivet, J. P. Metal Oxide Chemistry and Synthesis from Solution to Solid State; Wiley Chester, 2000. [3] Zarur, A. J.; Ying, J. Y. Reverse Microemulsion Synthesis of Nanostructured Complex Oxides for Catalytic Combustion. Nature 2000, 403, 65‒67. [4] Reddy, M. V.; Rao, G. V.; Chowdari, S. B. V. R. Metal Oxides and Oxysalts as Anode Materials for Li Ion Batteries. Chem. Rev. 2013, 113, 5364‒5475. [5] Kim, H. J.; Lee, J. H. Highly Sensitive and Selective Gas Sensors Using p-Type Oxide Semiconductors: Overview. Sens. Actuat. B-Chem. 2014, 192, 607‒627. [6] Athar, T.; Khan, A. A.; Vishwakarma, S. K.; Razzaq, A.; Ahemd, A. A Review on Synthesis of Nanostructured Metal Oxide Powder via Soft Chemical Approach. Rev.

Adv. Sci. Eng. 2015, 4, 258‒270. [7] Barth, S.; Hernandez-Ramirez, F.; Holmes, J. D.; Romano-Rodriguez, A. Synthesis and Applications of One-Dimensional Semiconductors. Prog. Mater. Sci. 2010, 55, 563‒627. [8] Zhang, Q. B.; Zhang, K. L.; Xu, D. G.; Yang, G. C.; Huang, H.; Nie, F. D.; Liu, C. M.; Yang, S. H. CuO Nanostructures: Synthesis, Characterization, Growth Mechanisms, Fundamental Properties, and Applications. Prog. Mater. Sci. 2014, 60, 208‒337. [9] Osterloh, F. E. Inorganic Nanostructures for Photoelectrochemical and Photocatalytic Water Splitting. Chem. Soc. Rev. 2013, 42, 2294‒2320. [10] Devan, R. S.; Patil, R. A.; Lin, J. H.; Ma, Y. R. One-Dimensional Metal-Oxide Nanostructures: Recent Developments in Synthesis, Characterization, and Applications. Adv. Funct. Mater. 2012, 22, 3326‒3370. [11] Ling, D.; Lee, N.; Hyeon, T. Chemical Synthesis and Assembly of Uniformly Sized Iron Oxide Nanoparticles for Medical Applications. Acc. Chem. Res. 2015, 48, 1276‒1285.



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] Sun, Z.; Liao, T.; Dou, Y. H.; Hwang, S. M.; Park, M. S.; Jiang, L.; Kim, J. H.; Dou, S. X. Generalized Self-Assembly of Scalable Two-Dimensional Transition Metal Oxide Nanosheets. Nat. Commun. 2014, 5, 3813. [13] Lee, J. M.; Kim, I. Y.; Han, S. Y.; Kim, T. W.; Hwang, S.-J. Graphene Nanosheets as a Platform for the 2D Ordering of Metal Oxide Nanoparticles: Mesoporous 2D Aggregate of Anatase TiO2 Nanoparticles with Improved Electrode Performance.

Chem. Eur. J. 2012, 18, 13800‒13809. [14] Cao, H. L.; Zhou, X. F.; Zheng, C.; Liu, Z. P. Two-Dimensional Porous Micro/Nano Metal Oxides Templated by Graphene Oxide. ACS Appl. Mater. Inter. 2015, 7, 11984‒11990. [15] Moosavifard, S. E.; El-Kady, M. F.; Rahmanifar, M. S.; Kaner, R. B.; Mousavi, M. F. Designing 3D Highly Ordered Nanoporous CuO Electrodes for HighPerformance Asymmetric Supercapacitors. ACS Appl. Mater. Inter. 2015, 7, 4851‒ 4860. [16] Hou, C. Y; Zhang, M. W; Kasama, T.; Engelbrekt, C.; Zhang, L. L; Wang, H. Z.; Chi, Q. Reagent-Free Synthesis and Plasmonic Antioxidation of Unique Nanostructured Metal-Metal Oxide Core-Shell Microfibers. Adv. Mater. 2016, 28, 4097‒4104. [17] Chen, H.; Muller, M. B.; Gilmore, K. J.; Wallace, G. G.; Li, D. Mechanically Strong, Electrically Conductive, and Biocompatible Graphene Paper. Adv. Mater. 2008, 20, 3557‒3561. [18] Hou, C. Y.; Wang, H. Z.; Zhang, Q. H.; Li, Y. G.; Zhu, M. F. Highly Conductive, Flexible, and Compressible All-Graphene Passive Electronic Skin for Sensing Human Touch. Adv. Mater. 2014, 26, 5018‒5024. [19] Yeh, C. N.; Raidongia, K.; Shao, J. J.; Yang, Q. H.; Huang, J. X. On the Origin of the Stability of Graphene Oxide Membranes in Water. Nat. Chem. 2015, 7, 166‒169. [20] Takenaka, S.; Miyake, S.; Uwai, S.; Matsune, H.; Kishida, M. Preparation of Metal Oxide Nanofilms Using Graphene Oxide as a Template. J. Phys. Chem. C 2015, 119, 12445‒12454. [21] Boston, R.; Bell, A.; Ting, V. P.; Rhead, A. T.; Nakayama, T.; Faul, C. F. J.; Hall, S. R. Graphene Oxide as a Template for a Complex Functional Oxide.

CrystEngComm 2015, 17, 6094‒6097.


Page 22 of 26

Page 23 of 26

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


[22] Li, X.; Qi, W.; Mei, D.; Sushko, M. L.; Aksay, I.; Liu, J. Functionalized Graphene Sheets as Molecular Templates for Controlled Nucleation and Self-Assembly of Metal Oxide-Graphene Nanocomposites. Adv. Mater. 2012, 24, 5136‒5141. [23] Wang, H.; Fu, Y.; Wang, X.; Gao, J.; Zhang, Y.; Zhao, Q. Preparation and Performance of Tubular Nanoflaky (Ni, Co, Mn) Oxides with Hierarchical Mesoporous Structure. J. Alloy Compd. 2015, 639, 352‒358. [24] Zhang, M. W; Halder, A.; Hou, C. Y; Ulstrup, J.; Chi, Q. Free-Standing and Flexible Graphene Papers as Disposable Non-Enzymatic Electrochemical Sensors.

Bioelectrochemistry 2016, 109, 87‒94. [25] Zhu, N.; Han, S.; Gan, S.Y; Ulstrup, J.; Chi, Q. Graphene Paper Doped with Chemically Compatible Prussian Blue Nanoparticles as Nanohybrid Electrocatalyst.

Adv. Funct. Mater. 2013, 23, 5297‒5306. [26] Mu, J. K; Hou, C. Y; Wang, H. Z; Li, Y. G; Zhang, Q. H; Zhu, M. F. OrigamiInspired Active Graphene-Based Paper for Programmable Instant Self-Folding Walking Devices. Sci. Adv. 2015, 1, e1500533. [27] Kudin, K. N.; Ozbas, B.; Schniepp, H. C.; Prud'homme, R. K. I.; Aksay, A.; Car, R. Raman Spectra of Graphite Oxide and Functionalized Graphene Sheets. Nano Lett. 2008, 8, 36‒41. [28] Yang, H. G.; Zeng, H. C. Preparation of Hollow Anatase TiO2 Nanospheres via Ostwald Ripening. J. Phys. Chem. B 2004, 108, 3492‒3495. [29] Hou, C. Y.; Yu, H.; Zhang, Q. H.; Li, Y. G.; Wang, H. Z. Preparation and Magnetic Property Analysis of Monodisperse Co–Zn Ferrite Nanospheres. J. Alloy. Compd. 2010, 491, 431‒435. [30] Wang, Z. Y.; Zhou, L.; Lou, X. W. Metal Oxide Hollow Nanostructures for Lithium-Ion Batteries. Adv. Mater. 2012, 24, 1903‒1911. [31] Zeng, C. F.; Tang, Z. H.; Guo, B. C.; Zhang, L. Q. Supramolecular Ionic Liquid Based on Graphene Oxide. Phys. Chem. Chem. Phys. 2012, 14, 9838‒9845. [32] Ye, S. B.; Feng, J. C. A New Insight into the In Situ Thermal Reduction of Graphene Oxide Dispersed in a Polymer Matrix. Polym. Chem. 2013, 4, 1765‒1768. [33] http://www.us-nano.com/inc/sdetail/485 [34] Dikin, D. A.; Stankovich, S.; Zimney, E. J.; Piner, R. D.; Dommett, G. H. B.; Evmenenko, G.; Nguyen, S. T.; Ruoff, R. S. Preparation and Characterization of Graphene Oxide Paper. Nature 2007, 448, 457‒460.



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

[35] Xamena, F. X. L.; Fisicaro, I. P.; Berlier, G.; Zecchina, A.; Palomino, G. T.; Prestipino, C.; Bordiga, S.; Giamello, E.; Lamberti, C. Thermal Reduction of Cu2+Mordenite and Re-Oxidation Upon Interaction with H2O, O2, and NO. J. Phys.

Chem. B 2003, 107, 7036‒7044. [36] Zhang, J.; Huang, F.; Lin, Z. Progress of Nanocrystalline Growth Kinetics Based on Oriented Attachment. Nanoscale 2010, 2, 18‒34. [37] Niederberger, M.; Colfen, H. Oriented Attachment and Mesocrystals: NonClassical Crystallization Mechanisms Based on Nanoparticle Assembly. Phys.

Chem. Chem. Phys. 2006, 8, 3271‒3287. [38] De Yoreo, J. J.; Gilbert, P. U.; Sommerdijk, N. A.; Penn, R. L.; Whitelam, S.; Joester, D.; Zhang, H.; Rimer, J. D.; Navrotsky, A.; Banfield, J. F.; Wallace, A. F.; Michel, F. M.; Meldrum, F. C.; Cölfen, H.; Dove, P. M. Crystallization by Particle Attachment in Synthetic, Biogenic, and Geologic Environments. Science 2015, 349, 498 (DOI: 10.1126/science.aaa6760). [39] Barreca, D.; Fornasiero, P.; Gasparotto, A.; Gombac, V.; Maccato, C.; Montini, T.; Tondello, E. The Potential of Supported Cu2O and CuO Nanosystems in Photocatalytic H2 Production. ChemSusChem 2009, 2, 230‒233. [40] Kargar, A.; Jing, Y. S.; Kim, J.; Riley, C. T.; Pan, X. Q.; Wang, D. L. ZnO/CuO Heterojunction Branched Nanowires for Photoelectrochemical Hydrogen Generation. ACS Nano 2013, 7, 11112‒11120. [41] Wang, G. L.; Huang, J. C.; Chen, S. L.; Gao, Y. Y.; Cao, D. X. Preparation and Supercapacitance of CuO Nanosheet Arrays Grown on Nickel Foam. J. Power.

Sources. 2011, 196, 5756‒5760. [42] Patake, V. D.; Joshi, S. S.; Lokhande, C. D.; Joo, O. S. Electrodeposited Porous and Amorphous Copper Oxide Film for Application in Supercapacitor. Mater. Chem.

Phys. 2009, 114, 6‒9. [43] Sun, Y. F.; Sun, Z. H.; Gao, S.; Cheng, H.; Liu, Q. H.; Piao, J. Y.; Yao, T.; Wu, C. Z.; Hu, S. L.; Wei, S. Q.; Xie, Y. Fabrication of Flexible and Freestanding Zinc Chalcogenide Single Layers. Nat. Commun. 2012, 3, 1057 [44] Brillet, J.; Yum, J. H.; Cornuz, M.; Hisatomi, T.; Solarska, R.; Augustynski, J.; Graetzel, M.; Sivula, K. Highly Efficient Water Splitting by a Dual-Absorber Tandem Cell. Nat. Photonics 2012, 6, 823‒827. [45] Dunn, H. K.; Feckl, J. M.; Muller, A.; Fattakhova-Rohlfing, D. S.; Morehead, G.; Roos, J.; Peter, L. M.; Scheu, C.; Bein, T. Tin Doping Speeds Up Hole Transfer


Page 24 of 26

Page 25 of 26

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


During Light-Driven Water Oxidation at Hematite Photoanodes. Phys. Chem. Chem.

Phys. 2014, 16, 24610‒24620. [46] Paracchino, A.; Laporte, V.; Sivula, K.; Gratzel, M.; Thimsen, E. Highly Active Oxide Photocathode for Photoelectrochemical Water Reduction. Nat. Mater. 2011,

10, 456‒461. [47] Parkinson, B. On the Efficiency and Stability of Photoelectrochemical Devices. Acc.

Chem. Res. 1984, 17, 431‒437. [48] Yang, X. Y.; Wolcott, A.; Wang, G. M.; Sobo, A.; Fitzmorris, R. C.; Qian, F.; Zhang, J. Z.; Li, Y. Nitrogen-Doped ZnO Nanowire Arrays for Photoelectrochemical Water Splitting. Nano Lett. 2009, 9, 2331‒2336. [49] Mubeen, S.; Lee, J.; Singh, N; Kramer, S.; Stucky, G. D.; Moskovits, M. An Autonomous Photosynthetic Device in Which All Charge Carriers Derive From Surface Plasmons. Nat. Nanotech. 2013, 8, 247‒251. [50] Liu, C.; Tang, J. Y.; Chen, H. M.; Liu, B.; Yang, P. D. A Fully Integrated Nanosystem of Semiconductor Nanowires for Direct Solar Water Splitting. Nano

Lett. 2013, 13, 2989‒2992. [51] Wolcott, A.; Smith, W. A.; Kuykendall, T. R.; Zhao, Y. P.; Zhang, J. Z. Photoelectrochemical Study of Nanostructured ZnO Thin Films for Hydrogen Generation From Water Splitting. Adv. Funct. Mater. 2009, 19, 1849‒1856. [52] Wolcott, A.; Smith, W. A.; Kuykendall, T. R.; Zhao, Y. P.; Zhang, J. Z. Photoelectrochemical Water Splitting Using Dense and Aligned TiO2 Nanorod Arrays. Small 2009, 5, 104‒111. [53] http://energy.gov/eere/office-energy-efficiency-renewable-energy [54] Jia, Q.; Iwashina, X. K.; Kudo, A. Facile Fabrication of an Efficient BiVO4 Thin Film Electrode for Water Splitting Under Visible Light Irradiation. Proc. Natl. Acad.

Sci. 2012, 109, 11564‒11569. [55] Liao, L. B.; Zhang, Q. H.; Su, Z. H.; Zhao, Z. Z.; Wang, Y. N.; Li, Y.; Lu, X. X.; Wei, D. G.; Feng, G. Y.; Yu, Q. K.; Cai, X. J.; Zhao, J. M.; Ren, Z. F.; Fang, H.; Robles-Hernandez, F.; Baldelli, S.; Bao, J. M. Efficient Solar Water-Splitting Using a Nanocrystalline CoO Photocatalyst. Nat. Nanotech. 2014, 9, 69‒73.



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

TOC Graphic:


Page 26 of 26

Ultralight, Flexible, and Semi-Transparent Metal Oxide Papers for

Recommend Documents