Universal Synthesis of Porous Inorganic Nanosheets via Graphene

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Functional Inorganic Materials and Devices

Universal Synthesis of Porous Inorganic Nanosheets via Graphene-Cellulose Templating Route Ji-Soo Jang, Seunghee Cho, Hyeuk Jin Han, Seok-Won Song, Sang-Joon Kim, Won-Tae Koo, Dong-Ha Kim, Hyeon Su Jeong, Yeon Sik Jung, and Il-Doo Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b11124 • Publication Date (Web): 22 Aug 2019 Downloaded from pubs.acs.org on August 24, 2019

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ACS Applied Materials & Interfaces

Universal

Synthesis

of

Porous

Inorganic

Nanosheets via Graphene-Cellulose Templating Route

Ji-Soo Jang, †, ‡ Seunghee Cho, †, ‡ Hyeuk Jin Han, †, ‡ Seok-Won Song, †, ‡ Sang-Joon Kim, †, ‡

Won-Tae Koo, †, ‡ Dong-Ha Kim, †, ‡ Hyeonsu Jeong, ¶ Yeon Sik Jung†, ‡ and Il–Doo Kim†,

*

‡,

†Department

of Materials Science and Engineering, Korea Advanced Institute of Science and

Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea ‡Advanced

Nanosensor Research Center, KI Nanocentury, KAIST, 291 Daehak-ro, Yuseong-gu,

Daejeon 34141, Republic of Korea Institute of Advanced Composite Materials, Korea Institute of Science and Technology

¶

(KIST), Chudong-ro 92, Bongdong-eup, Wanju-gun, Jeonrabuk-do, 565-905, Republic of Korea

1

ACS Paragon Plus Environment

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

*E-mail: [email protected]

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Abstract Two-dimensional (2D) inorganic nanomaterials have attracted enormous interest in diverse research areas because of their intriguing physicochemical properties. However, reliable methods for the synthesis and composition manipulation of polycrystalline inorganic nanosheets (NSs) are still considered grand challenges. Here, we report a robust synthetic route for producing various kinds of inorganic porous NSs with desired multiple components and precise compositional stoichiometry by employing tunicin, i.e., cellulose extracted from earth-abundant marine invertebrate shell waste. Cellulose fibrils can be tightly immobilized on graphene oxide (GO) NSs to form stable tunicin-loaded GO NSs, which are used as a sacrificial template for homogeneous adsorption of diverse metal precursors. After a subsequent pyrolysis process, 2D metallic or metal oxide NSs are formed without any structural collapse. The rationally designed tunicin-loaded GO NS templating route paves a new path for the simple preparation of multi-compositional inorganic NSs for broad applications, including chemical sensing and electrocatalysis.

Keywords: Tunicin; Graphene oxide; Nanosheet; Sensor; Catalyst

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INTRODUCTION Two-dimensional (2D) nanomaterials have unique and intriguing physicochemical characteristics that offer fascinating opportunities for both fundamental research and various practical applications.1-4 For instance, due to their high surface-to-bulk ratio, mechanical flexibility, and quantum confinement effect, 2D materials have been successfully applied in optoelectronics, energy storage, field-effect transistors, and chemical sensors.2, 5-7 General strategies for fabricating these 2D materials are usually classified into gas-phase and liquid-phase syntheses.8 The gas-phase synthetic strategy is very powerful in achieving high-quality 2D materials with large lateral dimensions; however, most gas-phase approaches suffer from relatively high cost and low yields.9 The liquid-phase synthetic routes, including ion-exchange exfoliation10-12, on the other hand, enable the formation of a broad range of 2D materials, such as metal oxides (MOs), metal chalcogenides, metal halides, and metal phosphates.13 However, exfoliation processes require an as-synthesized 2D-layered bulk material as a crucial starting precursor, and thus, there is a limitation in the choice of synthesizable materials. 4


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Recently, a graphene oxide (GO)-assisted templating method has been introduced to fabricate porous inorganic nanosheets (NSs), especially for MOs.14 AbdelHamid et al. described that the oxygenated groups of the GO NSs form strong interactions with MO precursors (M), which lead to uniform decoration of salt precursor ions on GO surfaces (M-GO).14 Subsequent pyrolysis of M-GO enables the formation of holey MOs NSs. However, when multiple precursors are simultaneously loaded on the GO NSs, inhomogeneous binding of certain precursors impedes the formation of target phase materials with the desired stoichiometry. Furthermore, the spontaneous and inhomogeneous reducing characteristics of GO for noble metal precursors (e.g., K2PtCl4, HAuCl3, K2PdCl4) cause unwanted aggregation of metal particles on the GO surface, which makes it difficult to prepare densely packed continuous metal sheets15 (Figure S1). In this regard, it is imperative that we develop a powerful and straightforward synthetic strategy that can induce homogeneous binding of multiple inorganic precursors to GO templates and prevent nonuniform aggregation of noble metal clusters.

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Among various candidates, tunicin (animal-type cellulose) is a promising material to prevent metal particle aggregation since tunicin possesses abundant negatively charged carboxyl and hydroxyl groups along with ultrahigh loading capacity for metal ions. More interestingly, tunicin comes from sea squirt shell waste, which is an earth-abundant and recyclable natural resource.16 Taking advantage of tunicin, cellulose can effectively serve as a sacrificial template for tailoring nanostructures, e.g., MO nanofibers.17 However, the development of desired 2D materials using disposed sea squirt shell-derived cellulose has never been reported. Here, we report on a highly robust and scalable synthetic strategy for the preparation of

multifunctional

inorganic

NSs

composed

of

single

metals,

bimetals,

and

multicomponent MOs (a total of 10 inorganic NS species). We employed the earthabundant tunicin to uniformly capture various metal ions on the GO NSs, i.e., salt-type precursors dissolved in aqueous solutions18, providing homogeneous and continuous 2D sheet structures even after pyrolysis. Controlled pyrolysis of metal/metal oxide precursorloaded tunicin-GO NSs (M_tunicin-GO NSs) in oxidizing and reducing atmospheres 6


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results in the formation of porous metallic or MO NSs, respectively. We demonstrate the dual-functional gas sensing characteristics of thin-walled metallic NSs, which possess both chemiresistive and surface-enhanced Raman spectroscopy (SERS) sensing characteristics toward toxic and/or explosive gases (H2, H2S, toluene, and NO2). In addition, the enhanced catalytic characteristics of tunicin-GO NS-templated perovskite LaMnO3 (LMO) and LaCoO3 (LCO) NSs for the oxygen reduction reaction (ORR) are discussed. RESULTS AND DISCUSSION Due to the self-reducing nature of noble metal precursors on GO surfaces (Figure S1), noble metal salts tend to aggregate on GO NSs. In particular, the irregularly distributed functional groups on GO NSs induce nonuniform dispersion of noble metal particles. For example, Pt precursors (K2PtCl4) nonuniformly aggregate on GO NSs (Pt-GO NSs) and form precipitated Pt particles (10–100 nm) on a GO surface upon stirring.

19, 20

Thus, the

synthesis of structurally stable 2D Pt sheets after pyrolysis using conventional GO templates is difficult. The key strategy for a facile and scalable synthesis of noble metal 7



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and/or multicomplex MO NSs is illustrated in Figure 1. We employed tunicin, a unique animal-type cellulose nanofiber that is derived from abandoned shells of tunicate, as an adsorption layer for aqueous metal/MO precursors. Note that the fabrication of tunicin from the shells of tunicate can be achieved by multiple chemical steps, as shown in the previous literature.21 As illustrated in Figure 1a, the abundant hydroxyl groups (-OH) on tunicin easily bind with various oxygenated groups (-COOH, -OH, and R-O-R’) on GO NSs; thus, tunicin can be chemically assembled on the surface of GO NSs (tunicin-GO NSs) via a simple stirring process at room temperature (R.T). Due to the outstanding waterabsorbing capability of tunicin, metal/MO precursors dissolved in aqueous solutions are homogeneously loaded onto the tunicin layer. Various

aqueous

metal/MO

precursors

[e.g.,

K2PtCl4,

K2PdCl4,

H2AuCl4,

La(NO3)‧6H2O, Mn(NO3)‧6H2O, Co(NO3)‧6H2O, SnCl4‧5H2O, Ni(NO3)2‧6H2O, Sr(NO3)2] are successfully decorated onto tunicin-GO NSs (M_tunicin-GO NSs). Then, subsequent pyrolysis of M_tunicin-GO NSs resulted in porous noble metal or MO NSs (Figure 1a). Due to the random distribution of one-dimensional (1D) nanofibrous tunicin on the GO 8


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NSs (porous network structure), mesopores (2–50 nm) can be naturally developed on metal/MO NSs after thermal decomposition of tunicin during calcination. In particular, since the tunicin layer on GO NSs enables high loading of given aqueous precursors, uniform decoration of metal/MO precursors on GO NSs can be achieved. This unique advantage ensures the formation of multicomplex metal or MO NSs with the desired target stoichiometry, such as single-metal NSs, bimetal NSs, ternary MO NSs (e.g., ABO3 perovskite NSs and (AxBy) oxide), and even up to five-element MO NSs (e.g., LaxSr1xMnyNi1-yO3)

(Figure 1b–1e). In this regard, the tunicin layer serves not only as an

adsorption layer but also as a mesopore-generating layer, hence endowing the conventional GO NSs with a bifunctional templating capability. To directly observe the microstructure of tunicin-GO NSs and various M_tunicin-GO NSs, transmission electron microscopy (TEM) analysis was carried out. The nanofibrous tunicin forms a net-like porous structure (30–100 nm pore size) on GO NSs while still maintaining sheet-like 2D structures (Figure 2a). The raw tunicin fibers show a 6–16 nm diameter and are a few microns in length (Figure S2a–b). The selected area diffraction 9



(SAED) ring pattern in the inset of Figure 2a clearly shows the presence of GO NSs in the tunicin-GO composite network. Due to the excellent water wettability of the tunicin layer on the GO NSs, the aqueous single-metal precursors (Pt, Pd, and Au) and bimetal precursors (Pt-Pd, Pt-Au, La-Co, and La-Mn) were successfully loaded onto tunicin-GO NSs without a severe change in the morphology of the pure tunicin-GO NSs (Figure 2b– 2h). Moreover, we further extended the number of elements coated on the tunicin-GO NSs (e.g., Ni/Sn with a 0.1:0.9 ratio for ternary (Sn0.9Ni0.1)O2 NSs and La/Sr/Mn/Ni for fiveelement perovskite oxide NSs) to demonstrate the excellent generality of our processing strategy (Figure S3a–b). As shown in Figure S4, the precipitated Pt and Pd nanoparticles (NPs) can be observed due to the slight reduction of Pt and Pd precursors on the surface of the tunicin-GO NSs. Likewise, in case of bimetallic case, PtAu and LaCo components were also slightly reduced on the tunicin-GO NSs, thus forming the metallic NPs on surface of tunicin-GO NSs. Since the reduced metallic NPs on the tunicin-GO NSs exhibited an ultra-small size (< 2 nm) and high uniformity, calcination of those composite NSs resulted in the successful formation of metallic 2D sheets consisting of NPs networks. 10


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To confirm the chemical binding state between the metal/MO precursors and GO NSs, we carried out Fourier transform infrared spectroscopy (FT-IR) analysis on 10 samples, including GO NSs, tunicin, tunicin-GO NSs, and various M_tunicin-GO NSs. By comparing the FT-IR peaks of pristine GO NSs, tunicin, and tunicin-GO NSs, we could observe peak shifts of C=O (stretching) from 1604 to 1634 cm-1 and O-H (stretching) from 3340 to 3450 cm-1, indicating hydrogen bonding between the hydroxyl group (-OH) and ether group (RO-R’) in tunicin and the various oxygenated functional groups (-COOH, -OH, and R-O-R’) in the GO NSs. The chemical bonding states between the noble metal/MO precursors and tunicin-GO NSs were also investigated by comparing the FT-IR peaks of tunicin-GO NSs and M_tunicin-GO NSs. The chemical forms of noble metal precursors are commonly part of the metal chloride series, i.e., potassium tetrachloroplatinate (K2PtCl4), gold chloride trihydrate

(HAuCl4‧3H2O),

and

potassium

tetrachloropalladate

(K2PdCl4);

these

precursors are ionized to [MCl4]2-/[MCl4]- coordination bonds in aqueous systems, and [MCl4]2-/[MCl4]- continuously formed [MCl4(OH)2]4-/[MCl4(OH)2]3-, respectively (Equation 1 or Equation 2)22. Thus, the -OH groups in both the tunicin layer and ionized noble metal 11



precursors ([MCl4(OH)2]x- (x = 3 or 4 in this study) exhibit strong hydrogen bonding interactions (Figure 2j)17. As a result, an intermolecular bond-derived O-H stretching peak (3250–3500 cm-1) shift was observed (orange region in Figure 2i). 2K+ + [MCl4]2- + 3H2O → 2K+ + 2H+ + [MCl4(OH)2]4-

Eq. (1)

H+ + [MCl4]- + 2H2O → 3H+ + [MCl4(OH)2]3-

Eq. (2)

In the case of MO precursors such as the nitrate series, lanthanum nitrate hexahydrate (La(NO3)3‧6H2O), manganese nitrate hexahydrate (Mn(NO3)2‧6H2O), and cobalt nitrate hexahydrate (Co(NO3)2‧6H2O) are fully ionized to M3+ or M2+ ions, thus creating noncovalent bonding (electrostatic interaction) between the negatively charged oxygen in the ether group (R-O-R) of tunicin and the positively charged M3+ or M2+ ions. The change in the bonding state of the ether (R-O-R) on tunicin induces the FT-IR peak shift from 813 to 818 cm-1 or 738 to 745 cm-1 of 1,3-disubsitituted C-H bonding (yellow region in Figure 2i). Hydrogen bonding between metal ions and tunicin also contributed to the high loading of metal ions on tunicin, as shown in the peak shift of the O-H stretching peak (3250–3500 cm-1). Based on the FT-IR and TEM analysis, one can expect that various metal and/or 12


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MO precursors dissolved in aqueous systems can be easily functionalized on tunicin-GO NSs through hydrogen bonding and/or electrostatic bonding. The pyrolysis of M_tunicin-GO NSs results in the formation of porous noble metal (e.g., Pt, Pd, Au, PtPd, PtAu), transition MO (SnO2, (Sn0.9Ni0.1)O2) and perovskite MO (LCO, LMO, La0.76Sr0.19Mn0.9Ni0.1O3 (LSMN)) NSs. Given that the tunicin-GO NSs thermally decompose at 450 ℃ (Figure S5), we calcined M_tunicin-GO NSs at 500 ℃. On the other hand, the calcination of perovskite oxide NSs was carried out at 700 ℃ to form a singlephase perovskite oxide. Each type of calcined M_tunicin-GO NS exhibits porous metallic and/or MO NS morphologies (Figure 3a–3j). However, the heat treatment of metal precursor-loaded tunicin without using GO templates induces 1D rod-like structures (Figure S6); this result indicates that the use of tunicin combined with GO NSs is essential to obtain 2D porous inorganic NSs. Since the polycrystalline 2D NSs consist of nanoscale metal/MO (5 nm–25 nm) and perovskite MO (20 nm–75 nm) grains, the abundant mesopores are well developed between the multiple nanograins. More quantitatively, as a case study, tunicin-GO NS-derived SnO2 NSs showed much higher BET surface area 13



(70.43 m2/g) and mesopore volume than those of SnO2 nanoparticles (6.67 m2/g), SnO2 nanofibers (8.585 m2/g) and nanotubes (15.72 m2/g) that we previously reported (Figure S7). Through high-resolution TEM (HRTEM) analysis, we clearly confirmed that the single noble metal NSs, namely, the porous Pt, Pd, and Au NSs, revealed lattice distances of 0.223 nm, 0.225 nm and 0.235 nm, which correspond to Pt (111), Pd (111) and Au (111), respectively (Figure 3a–3c). Based on X-ray photoelectron spectroscopy (XPS) analysis, we confirmed that the metallic Pt and Au NSs were generated by simple calcination of Au_tunicin-GO NSs and Pt_tunicin-GO NSs even in ambient air, while calcination of Pd_tunicin-GO NSs resulted in the formation of porous PdO NSs (Figure S8); these features agree well with the previous literature.23 To obtain metallic Pd NSs, we further carried out calcination at 350 ℃ in a H2 atmosphere for 1 h. As shown in Figure S8c and S8f, the major XPS peaks of the porous Pd NSs change from Pd2+ to metallic Pd without a distinct morphological change. In the case of the bimetallic NSs such as porous PtPd and PtAu NSs, we observed lattice distances of 0.22 nm and 0.264 nm, which correspond to the PtPd phase and PdO phase in the PtPd NSs, respectively, while the PtAu NSs 14


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show two distinctive Pt and Au lattice distances (0.225 nm and 0.235 nm, corresponding to Pt (111) and Au (111), Figure 3k and 3l). The phase separation of Pt and Au in the PtAu NSs corresponds well to previous reports24, 25. In addition, MO NSs including transition metal oxide and perovskite oxide NSs clearly show crystalline lattice fringes (0.34 nm, 0.264 nm, 0.272 nm, 0.268 nm, and 0.38 nm, corresponding to SnO2 (110), (Sn0.9Ni0.1)O2 (101), LMO (104), LCO (104), and LSMN (012), respectively, bottom figures of Figure 3f–3j) To further investigate the crystalline structures of the prepared inorganic NSs, we carried out X-ray diffraction (XRD) analysis. The XRD peaks of single-metal NSs show metallic Pd, Pt, and Au cubic crystal structures (Figure 3k). The calcination at an elevated temperature over 500 ℃ induces excessive grain growth of porous Pd NSs, causing structural collapse of the 2D NSs (Figure S9). The XRD peaks of the porous PtPd NSs and PtAu NSs reveal that the porous PtPd NSs show both a PtPd alloy phase and a slightly oxidized Pd phase, while the porous PtAu NSs exhibit separate Pt and Au phases (Figure 3k). Based on these results, we concluded that the high-temperature (500 ℃)

15



pyrolysis of bimetallic M_tunicin-GO NSs leads to the formation of an alloy phase (e.g., PtPd) or separate Pt and Au phases. We investigated heterogeneous Ni-doped SnO2 NSs (Sn0.9Ni0.1O2) as an example of a transition metal oxide. The tetragonal SnO2 crystalline phase was not affected by the addition of 10 at % Ni content (JCPDS#77–0452) (Figure 3l). Furthermore, Ni and Sn elements on porous Sn0.9Ni0.1O2 NSs were uniformly distributed (Figure S10). Since heterogeneous MO precursors (e.g., binary La/Mn ions and La/Co ions or quaternary (La/Sr)(Mn/Ni) ions) with the desired molar ratio (e.g., 1:1) can be homogeneously adsorbed onto hydrophilic tunicin, subsequent pyrolysis at 700 ℃ (La/Mn and La/Co cases) and 900 ℃ (La/Sr/Mn/Ni case) in ambient air produced perovskite (ABO3) oxide NSs with the desired stoichiometry. The SAED patterns of porous LMO and LCO NSs showed clear lattice distances of 0.272 nm and 0.268 nm, corresponding to hexagonal LMO (104) and cubic LCO (104), respectively (Figure S11). Additionally, we confirmed that the porous LCO and LMO NSs show LaCoO3 and La0.96Mn0.96O3 perovskite structures, respectively, without any secondary phase (JCPDS#48–0123 and JCPDS#75–0440) 16


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(Figure 3l). In addition, five-element LSMN oxide NSs also showed perovskite crystalline structures with a uniform distribution of the five components (Figure S12). Porous 2D materials offer high gas accessibility, abundant reaction sites stemming from high surface area, and open porosity, which enable outstanding gas sensing characteristics. We tested porous Au, Pt, Pd, and PtPd NSs as chemiresistive R.T. gas sensors to detect flammable and toxic gas molecules. We conducted sensing tests with 2D inorganic NSs drop-coated on sensing substrates that had an interdigitated Au electrode (width = 25 μm, gap size = 150 μm). The porous single metal NSs, i.e., Pt, Pd, and Au NSs, show reversible hydrogen (H2) gas sensing behaviors at R.T. (Figure S13 and Figure 4a). In particular, compared to porous Pt and Au NSs, porous Pd NSs show a much enhanced H2 response in terms of the sensing response ([Rair-Rgas]/Rair x 100), sensing speed and limit of detection (LOD). Note that compared to the Au and Pt NSs (22 % and 14 %, respectively), the porous Pd NSs manifest the highest H2 response (44.13 %) at [H2] of 4 %. Reverse H2 sensing behavior was observed in the porous Pd NSs depending on [H2]. Based on the H2 sensing mechanism (Equation 3), the metallic Pd phase changed to α-PdHx (x < 0.017) when dissociated H2 gas (H) (< 1 % of [H2]) permeates into interstitial sites of Pd; this phenomenon leads to a 17



decrease in the conductivity of Pd26. This sensing behavior corresponds to the blue region in Figure 4a. On the other hand, the conductivity of porous Pd NSs actually improves in the range of [H2] > 2 %. Since exposure of Pd NSs to over 2 % [H2] induces the formation of β-PdHx (x > 0.58), which exhibits volume expansion characteristics (3.5 % lattice constant expansion, from 0.3889 nm for Pd to 0.403 nm for β-PdH0.7),26 each Pd NP on the porous Pd NSs expands so that the interconnections of Pd NPs on the Pd NSs can be increased. The interconnected Pd NPs on the porous Pd NSs provide improved electron pathways, so a noticeable resistance drop can be observed when porous Pd NSs (yellow green side of Figure 4a and Figure 4b) are exposed to over 2 % H2 gas. The reverse H2 sensing kinetics of porous Pd NSs depending on [H2] are attributed to their porous structure and a multigrain effect. Furthermore, the porous Pd NSs, as well as other single-metal (Pt and Au) NSs, show high selectivity for H2 over carbon monoxide (CO), nitrogen dioxide (NO2), and hydrogen sulfide (H2S). Note that the single-metal NSs exhibit a negligible response (noise level) to interfering gas species (e.g., CO, NO2, and H2S, Figure 4c). In terms of the material stability, Pt showed reliable H2 sensing behavior even 18


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after 1 month meaning that our porous inorganic NSs show stable physicochemical properties (Figure S14). Pd + H2(g) → PdHx (α phase: x < 0.017, β: x > 0.58)

Eq. (3)

On the other hand, an exceptionally reversible response to 1 ppm NO2 and H2S gases was achieved by using bimetallic NSs (e.g., porous PtPd NSs) even at R.T. (Figure 4d and Figure S15a–b). Previous studies reported that the PtPd alloy phase possesses much higher catalytic activity than single metallic Pt with respect to oxygen reduction27, 28. Thus, more ionized oxygen species (O-) and physisorbed oxygen molecules can be adsorbed on the porous PtPd NSs via the so-called “spillover” process29. Since adsorbed oxygen species induce electron scattering at the metal surface30 that disturbs the electrical signal path, desorption of numerous oxygen species on porous PtPd NSs occurs when the porous PtPd NSs are exposed to NO2, leading to a dramatic resistance drop (Figure S15c). However, single-metal Pt NSs do not show a reversible NO2 sensing reaction due to their deficient catalytic effect compared to that of bimetallic PtPd NSs (Figure S16). On the other hand, H2S often induces sulfur poisoning of PtPd NSs31, 32 and thus leads to an 19



increase in resistance when PtPd NSs (Figure S15d) are exposed to H2S; the gradual increase in baseline resistance during H2S sensing can be indirect evidence for partial sulfur poisoning. Thus, porous PtPd NSs show intriguing selectivity for ppm-level NO2 and H2S gases while showing poor cross-sensitivity toward H2 and CO gases (Figure 4e). To further improve the selectivity characteristics, we designed four sensor arrays assembled with porous PtPd, Pt, Pd, and Au NSs and carried out principal component analysis (PCA), which is widely used as a pattern recognition tool.23 As a result, the sensor arrays successfully distinguished the H2S, CO, NO2, and H2 gas molecules without any overlap (Figure 4f). SERS sensing has been considered an accurate sensing tool for the recognition of biological signals and for selective detection of gas-phase molecules with a low LOD range33. In particular, Au-based nanomaterials have been expansively studied as promising SERS materials due to their excellent surface plasmon resonance effect34, 35. To investigate the SERS characteristics of porous Au NSs, we directly coated the porous Au NSs onto a Si substrate and evaluated the SERS spectra in the presence of toxic gas 20


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molecules (e.g., toluene and ethanol). The porous Au NS spectra clearly exhibit Raman peaks corresponding of toluene and ethanol gases at 140 ppm (Figure 4g). This result reveals that porous Au NSs prepared by the tunicin-GO NS templating route can distinguish ppm-level gas–phase molecules with high selectivity. According to previous studies, the Raman signal enhancement factor can be dramatically increased by decreasing the size of the nanogaps of Au-based nanostructures36, 37. In this sense, the outstanding SERS characteristics of the porous Au NSs are attributed to the abundant nanogaps between Au nanograins that constitute the porous Au NSs. In contrast, a reference Au thin film (1 μm thickness) grown by e-beam evaporation on a Si wafer could not detect 140 ppm toluene and ethanol gases (Figure 4g and Figure S17); this result supports that the porous and multigrain 2D NS morphology is highly advantageous for SERS gas sensing. Through the distinguishable normalized intensity of toluene and EtOH at different Raman shifts, the recognition of EtOH and toluene is easily achieved (Figure 4h). Note that the normalized SERS peak intensity was calculated by the relative peak intensity ratio (Igas/Ibase, where Igas is the peak intensity at the target Raman shift and Ibase 21



is the baseline peak intensity). Based on these relative peak intensity results, PCA, which is dependent on the peak intensity at each Raman shift, was further conducted to visualize the gas detection capability of porous Au NSs. The porous Au NSs successfully visualized the patterns of two gas molecules (Figure 4i). Although metallic NSs such as Pd, Pt, and PtPd showed reasonably good chemiresistive sensing performance toward target gas molecules (e.g., H2 and NO2), the detection of various toxic gas species with high selectivity is still challenging through a single-type chemiresistor. On the other hand, the SERS-type gas sensor using Au NSs can compensate for the disadvantage of metallic NS-based chemiresistive sensors. Thus, we propose a dual-sensing system using sensor arrays assembled with highly robust 2D NSs, such as chemiresistive-type (e.g., Pd, and PtPd NSs) and SERS-type (e.g., Au NSs) materials, for the accurate detection of various toxic gas molecules, complementing respective disadvantages (e.g., limited number of detectable gas species for chemiresistive sensors and poor quantitative analysis for SERS sensors).

22


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In addition to gas sensing applications, porous 2D NS structures are highly desirable for electrocatalytic reactions38,

39.

Among the various types of electrocatalyst materials

(e.g., noble metal and transition metal oxides), MOs with perovskite structures (ABO3, where the A site is a rare-earth metal and the B site is a transition metal) have been considered outstanding catalysts due to (i) their high catalytic activities for facilitating the oxygen reduction reaction (ORR), (ii) easy compositional control, and (iii) cost effectiveness in comparison to noble metal catalysts40. To investigate the electrocatalytic activities of porous perovskite LCO NSs, we carried out electrochemical characterization in a 0.1 M KOH solution with a rotating disk electrode (RDE). As counter samples, commercial 30 wt% Pt/C (Vulcan XC-72 carbon), Ketjen black (KB), and LCO particles (300–400 nm, Figure S19) were compared. As shown in the linear sweep voltammogram (LSV) curves for the ORR region (Figure 5a), porous LCO NSs and the reference LCO powder showed similar onset potentials of approximately -0.165 – -0.189 V and half-wave potentials (E1/2) of -0.268 V and -0.23 V (vs Hg/HgO), respectively, due to the inherent catalytic nature of LCO. However, with the Tafel slopes (Figure 5b), we clearly 23



demonstrate that the ORR over porous LCO NSs (79 mV/decade) is more active and quicker than that over the LCO particles (101 mV/decade) and even Pt/C (86 mV/decade). The number of transferred electrons (n) per oxygen molecule for the ORR was calculated from the Koutecky-Levich (K-L) equation41. As shown in the K-L plots (Figure 5c and 5d), the n value of the LCO particles is 2.19, following a two-electron pathway for the ORR, while that of the porous LCO NSs is 3.44. This result indicates that porous LCO NSs exhibit almost four-electron reaction kinetics, facilitating markedly enhanced ORR activities. These electrochemical results prove that porous and thin sheet-like morphologies (e.g., porous LCO NSs) offer great potential for application in ORR catalysts. Through the tunicin-GO NS templating route, one can expect further optimization of electrocatalysts by rationally controlling the composition of the NSs. CONCLUSIONS We first demonstrated that porous inorganic NSs with desirable multiple elemental components and precise compositional stoichiometry can be simply obtained via the tunicin-GO NS templating route. Flat random networks of tunicin, i.e., sea-squirt shell

24


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waste-derived cellulose nanofibers, tightly immobilized on both surfaces of GO NSs act as metal ion acceptors and effectively prevent excess reduction of noble metal precursors. Subsequent pyrolysis of the metal salt-loaded tunicin-GO NSs leads to the formation of various inorganic NSs consisting of metals (Pt, Pd, Au, PtPd, PtAu) and/or MOs (LCO, SnO2, (SnxNiy)O2, LMO and LSMN NSs). In this work, we further demonstrated the potential suitability of porous inorganic NSs for broad applications: two types of sensitive gas sensors (chemiresistive- and SERS–type sensors) derived from porous metallic NSs and porous perovskite NS-based ORR catalysts. In terms of gas sensing applications, single-metal NSs such as Pd, and Pt exhibit highly selective detection capability toward H2 molecules. Furthermore, owing to the highly porous nature of Pd NSs, we observed switchable H2 sensing kinetics, enabling the easy recognition of H2 molecules, especially in the 1–2 % [H2] range. In addition to H2 detection, other gas molecules (e.g., NO2, H2S, EtOH, and toluene) can be successfully detected by using the bimetallic NS-based chemiresistor as well as porous Au NS-based SERS

25



sensors. Furthermore, complex porous oxide NSs such as perovskite NSs were rationally fabricated as potential alternative high-performance electrocatalysts to noble metals. Looking ahead, there are many more opportunities and challenges with respect to material choice, morphology, high surface area and porosity, and scaling-up of our simple synthetic approach. Rational engineering of the tunicin-loaded GO NS templating route as a powerful synthetic method for porous 2D inorganic NSs (e.g. single element metal/metal oxide to multi-elements metal/metal oxide) will open new avenues for broad applications, including chemical sensors, catalysts, and electrochemical devices such as batteries and supercapacitors.

EXPERIMENTAL SECTION Materials. To prepare porous inorganic nanosheets (NSs), graphene oxide (GO) aqueous solution (2 mg/ml), K2PtCl4, K2PdCl4, H2AuCl4, La(NO3)‧6H2O, Mn(NO3)‧6H2O, Co(NO3)‧6H2O, SnCl4‧5H2O, Ni(NO3)2‧6H2O, and Sr(NO3)2 were purchased from Sigma Aldrich. For tunicin preparation, 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO, 98 %), sodium bromide

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(NaBr), sodium hypochlorite (NaClO, 12 %) and sodium hydroxide (NaOH, 97 %) were all purchased from Sigma Aldrich. All chemicals were used without further purification. Synthesis of tunicin nanocellulose. Tunicate cellulose (tunicin) was prepared by a well-known TEMPO-mediated oxidation1. Briefly, tunicin was isolated from Halocynthia roretzi (HR) through a series of alkaline treatments and bleaching processes to remove protein, lipids, and other polysaccharides from HR as described in a previous study.21 After isolation, a tunicin suspension (10 g, 100 mL) in water was mixed with TEMPO (0.016 g, 0.1 mmol) and sodium bromide (0.1 g, 1 mmol), followed by adding sodium hypochlorite (5 mmol) at R.T. The pH of the reaction was adjusted to 10 via the addition of 0.5 M sodium hydroxide for 1 h. Subsequently, the oxidized cellulose was obtained by filtration and then washed with deionized water (DI water). A tunicin aqueous suspension (0.5 wt%) was obtained by ultrasonication at 750 W with 50 amplitude for 1 h (VCX-740, Sonics & Materials Inc. USA). Synthesis of single-metal porous NSs. 27



The 3 g of GO solution (2 mg/ml) and 10 ml of aqueous tunicin solution (tunicin-GO solution) were mixed in a glass vial and vigorously stirred with a magnetic bar at 150 rpm for 1 h. Meanwhile, 0.2 g of noble metal precursors (e.g., K2PtCl4, K2PdCl4, and H2AuCl4‧xH2O) were dissolved in 24 ml of aqueous solution. The as-prepared aqueous solution of the noble metal precursor was added to tunicin-GO solution and stirred at 300 rpm for 3 h. After the stirring process, the M_tunicin-GO NS solution was purified by centrifugation at 3000 rpm for 5 min. The purified M_tunicin-GO NSs were semidried at 50 ℃ for 6 h, and gel-like moist M_tunicin-GO NSs were calcined at 500 ℃ for 1 h with a ramping rate of 5 ℃/min. The dark (Pt and Pd) or yellow (Au) powders were collected in glass vials. Synthesis of bimetallic porous NSs. Similar to the synthesis of single-metal porous NSs, tunicin-GO solution was prepared. Then, four different aqueous solutions of the PtPd precursors (0.08 g K2PtCl4 and 0.06 g K2PdCl4), PtAu precursors (0.1 g K2PtCl4 and 0.08 g H2AuCl4‧xH2O), LaCo precursors (0.216 g La(NO3)3‧6H2O, and 0.145 g Co(NO3)2‧6H2O), and LaMn precursors (0.216 g 28


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La(NO3)3‧6H2O, and 0.143 g Mn(NO3)2‧6H2O) were prepared by dissolving them in 24 ml of DI water. Each of the four aqueous solutions was added to the tunicin-GO solution and mixed at 300 rpm for 3 h. After the stirring process, the prepared solutions were each purified by centrifugation at 3000 rpm for 5 min. The purified bimetallic precursor-loaded GO NSs were semidried at 50 ℃ for 6 h, and gel-like moist bimetallic M_tunicin-GO NSs were calcined at 500 ℃ for 1 h with a ramping rate of 5 ℃/min. In the case of perovskite LMO and LCO NSs, calcination was carried out at 700 ℃ for the formation of the perovskite phase. The prepared powders were each collected in a glass vial. Characterization. Transmission electron microscopy (TEM 300 kV, Techni) was carried out to observe the microstructures of various 2D inorganic NSs. The crystal structures of the inorganic NSs were observed by X-ray diffraction (XRD, Rigaku D/MAX-RB; with Cu Kα radiation (λ=0.154 nm)). Fourier transform infrared spectroscopy (FT-IR) analysis was conducted to investigate the chemical bonding state of M_tunicin-GO NSs. X-ray photoelectron spectroscopy (XPS, Sigma Probe, Thermo VG Scientific) with Al Kα radiation (1486.6 eV) 29



was conducted to characterize the chemical bonding states. Raman spectroscopy was measured on a Raman spectrometer with a 514 nm laser (FT-Raman, Bruker) for SERS sensing measurements. Sensor fabrication and gas sensing measurement. The responses of sensors were evaluated by resistance changes (S=[Rgas-Rair]/Rair × 100 %) in sensing layers (porous Pt, Pd, and Au NSs). The resistance of the sensing layers was measured by a data acquisition system (34972A, Agilent). Note that Rair is the sensor baseline resistance upon exposure to air and Rgas is the resistance upon exposure to the target gas. The chemiresistive sensing materials (e.g., Pt, Pd, Au, and PtPd NSs) were dispersed in ethanol and subsequently sonicated for 1–2 min to uniformly disperse the sensing materials. These dispersed solutions were separately drop-coated onto Al2O3 sensing substrates (2.5 mm × 2.5 mm size) that have interdigitated gold electrodes (width = 25 μm, gap size = 150 μm) for measuring the resistance variations. The sensing temperature was controlled by a Pt microheater located on the back side of the Al2O3 sensing substrate. All of the sensors were stabilized in dry air (30 % RH). The sensors 30


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were exposed to various toxic gas molecules (NO2, H2S, CO, and H2) with concentrations ranging from 1 ppm to 5 ppm for NO2, H2S, and CO and from 0.1 % to 4 % for H2. The on/off interval of the exposed gases was 10 min. For the SERS sensing measurement, we prepared 1.05 μm thick porous Au NSs dispersed in ethanol, which were drop coated onto a Si wafer (1.5 cm × 1.5 cm). As a control sample, a 1.00 μm thick Au thin film was e-beam evaporated onto a Si wafer under identical conditions as the aforementioned Si wafer. High-resolution dispersive Raman spectroscopy (ARAMIS, Horiba) with a 514 nm excitation laser was used for SERS measurements. The gas concentration for SERS measurement was controlled by the volume of evaporated solvents (toluene and ethanol) in a sealed room. Electrochemical characterization. Electrochemical measurements were carried out in a three-electrode system on a potentiostat (ZIVE SP1, Wonatech). To fabricate the working electrode for electrochemical testing, porous LaMnO3 (LMO) and LaCoO3 (LCO) NSs were mixed with Ketjen black (KB) at a weight ratio of 1:3 in 1 ml of DI water/IPA (v/v, 3:1), respectively. Then, 80 μl of Nafion 31



solution (5 wt %, Sigma-Aldrich) was added to the catalyst ink. The active ink suspension was sonicated for 30 min and dropped on a glassy carbon electrode (GCE), including 0.285 mg cm-2. The as-prepared catalyst-loaded GCE was used as the working electrode, and Pt coil and Hg/HgO (filled with 1 M NaOH, E0Hg/HgO = 0. 140 V vs RHE) were used as the counter and reference electrode, respectively. The oxygen reduction reaction (ORR) measurement was performed in 0.1 M KOH solution with O2 purging for 1 h. Linear sweep voltammetry (LSV) was recorded at a scan rate of 5 mV s-1 in the voltage range of -0.9 ~ 0.2 V for the ORR.

ASSOCIATED CONTENT

Supporting information Supporting Information is available online from the http://pubs.acs.org or from the author. Supporting Information included the additional TEM, AFM, SEM, TGA, XPS, sensing results, electrocatalysts results and EDS mapping analysis. The authors declare no competing financial interest. AUTHOR INFORMATION 32


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Corresponding Author *E-mail: [email protected] ORICD Il-Doo Kim: 0000-0002-9970-2218

ACKNOWLEDGMENT

This work was supported by Wearable Platform Materials Technology Center (WMC) funded by National Research Foundation of Korea (NRF) Grant of the Korean Government (MSIP) (No. 2016R1A5A1009926). This research was also supported by the Nano Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2016M3A7B4905609). This work was supported by the Ministry of Science, ICT & Future Planning

as

Biomedical

Treatment

Technology

Development

Project

(2015M3A9D7067418). Received: ((will be filled in by the editorial staff)) Revised: ((will be filled in by the editorial staff)) Published online: ((will be filled in by the editorial staff))

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Figure 1. Schematic illustration of (a) synthetic process for the inorganic NSs by using the tunicin-GO NSs templating route, and synthesized (b) single metallic NSs, (c) binary metallic NSs, (d) ternary oxide NSs, (e) five-component oxide NSs

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Figure 2. TEM images of (a) bare tunicin-GO NSs, (b) Pt_tunicin-GO NSs, (c) Pd_tunicinGO NSs, (d) Au_tunicin-GO NSs, (e) PtPd_tunicin-GO NSs, (f) PtAu_tunicin-GO NSs, (g) LaCo_tunicin-GO NSs, (h) LaMn_tunicin-GO NSs, (i) FT-IR peaks of various samples including (GO, tunicin, tunicin-GO NSs, and M_tunicin-GO NSs), (j) schematic images for illustrating formation mechanism of noble metallic M_tunicin-GO NSs. Scale bars: (a) 200 44


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nm and 5 1/nm (inset), (b) 100 nm, (c) 100 nm, (d) 200 nm, (e) 100 nm, (f) 100 nm, (g) 100 nm, (h) 100 nm

Figure 3. TEM and corresponding high-resolution TEM images of (a) porous Pt NSs, (b) porous Pd NSs, (c) porous Au NSs, (d) porous PtAu NSs, (e) porous PtPd NSs, (f) porous 45



SnO2 NSs, and (g) porous (Sn0.9Ni0.1)O2 NSs, (h) porous LMO NSs, (i) porous LCO NSs, (j) porous LSMN NSs, XRD peaks of (o) porous single metallic NSs, (p) porous bi-metallic NSs, (q) porous oxide NSs. Scale bars: (a) 500 nm (top) and 10 nm (bottom), (b) 200 nm and 5 nm, (c) 100 nm and 5 nm, (d) 200 nm and 5 nm, (e) 200 nm and 10 nm, (f) 100 nm and 5 nm, (g) 100 nm and 5 nm, (h) 500 nm and 5 nm, (i) 400 nm and 5 nm, (j) 500 nm and 5 nm, XRD peaks of (k) metallic NSs, and (l) oxide NSs

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Figure 4. (a) dynamic resistance transition of porous Pd NSs, (b) schematic illustration for phase transition of porous Pd NSs depending on the hydrogen concentration, (c) selective detection properties of porous Pd, Au, and Pt NSs, (d) dynamic response transition of porous PtPd NSs, (e) selective detection properties of porous PtPd NSs, (f) pattern recognition analysis using sensor arrays assembled with porous Pt, Pd, Au, and PtPd NSs, (g) Raman signal spectra obtained from the Au NSs with 140 ppm toluene and ethanol gases, respectively, (h) Normalized SERS signal intensity of Au NSs vs. Raman shift toward Toluene and EtOH, (i) SERS signal dependent pattern recognition analysis using Au NSs SERS sensor

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Figure 5. (a) Linear sweep voltammetry (LSV) curves tested in ORR region of KB, porous LCO NSs, LCO powder, and 30 wt% Pt/C based air electrodes in O2 saturated 0.1 M KOH. Note that the rotating speed for ORR was 1600 rpm. (b) Tafel plots of ORR current of porous LCO NSs, LCO powder, and 30 wt% Pt/C obtained at near the onset potentials, rotating-disk voltammograms of (c) porous LCO NSs, and (d) LCO powder in O2 saturated 0.1 M KOH solution at various rotation speed (e.g. 400, 625, 900, 1225, and 1600 rpm). Note that the insets of (c) and (d) exhibit corresponding Koutecky-Levich (K-L) plots at different potentials. 48


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Universal Synthesis of Porous Inorganic Nanosheets via Graphene

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