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Cite This: Chem. Mater. 2018, 30, 5108−5115
Surface-Mediated Chemical Dissolution of Two-Dimensional Nanomaterials toward Hole Creation Guijian Guan,*,†,‡ Mingda Wu,‡ Yongqing Cai,† Shuhua Liu,‡ Yuan Cheng,† Si Yin Tee,‡ Yong-Wei Zhang,*,† and Ming-Yong Han*,‡ †
Institute of High Performance Computing, A*STAR, 1 Fusionopolis Way, Singapore 138632 Institute of Materials Research and Engineering, A*STAR, 2 Fusionopolis Way, Singapore 138634
‡
Chem. Mater. 2018.30:5108-5115. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/06/18. For personal use only.
S Supporting Information *
ABSTRACT: Chemically engineered holes on two-dimensional (2D) nanomaterials may significantly increase the number of edge sites to tune their intrinsic properties to achieve promising performance. Here, we report a general and mild approach to the convenient creation of holes on atomically thin nanosheets for engineering bandgaps and enhancing properties of 2D materials. Through surface blocking, controlled dissolution, and chemical stabilization, WO3 nanosheets are readily treated to create holes in the presence of bovine serum albumin (BSA) via the reaction of WO3 with OH− ions at pH 8. Arising from the increased bandgaps and more edge sites as demonstrated experimentally and theoretically, the resulting holey WO3 nanosheets exhibit enhanced photocurrents and much better performance during selective adsorption and photocatalytic degradation compared with those of bulky WO3 and nonporous nanosheets. Also, this approach is further extended to the convenient creation of holes on more 2D nanomaterials such as MoS2 and C3N4 nanosheets, which are facilely made in aqueous solutions of diluted H2O2 and HCl, respectively. Overall, this work not only demonstrates a surface-mediated chemical dissolution strategy for generating holes on various ultrathin nanosheets but also provides new opportunities to exploit exotic properties and novel applications of geometrically constructed 2D nanomaterials.
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INTRODUCTION Two-dimensional (2D) nanomaterials, in particular, transition metal dichalcogenides and oxides in a single layer or a few layers, have been attracting a great deal of attention because of their greatly increased surface-to-volume ratio, inherent confinement effect, and sizable bandgap in a wide range from the near infrared and visible regions to the ultraviolet region.1−13 Recent research has also started to reveal that the peripheral edge sites of nanosheets instead of their basal planes often act as active centers in enhanced applications.14 For instance, the peripheral edge sites of MoS2 and MOOHx (M = Ni/Fe, Co, or Mn) nanosheets have been demonstrated to have high activities toward hydrogen evolution reaction15,16 and oxygen evolution reaction,17,18 respectively. As such, it is quite important in exposing more active edge sites via structural manipulation to enhance the outstanding performance of 2D nanomaterials. Typically, the successful fabrication of holey MoS2 and TaS2 nanosheets was achieved by the oxygen plasma etching method,19,20 outperforming intact ones in energy storage, catalysis, and molecular transport/separation. Pore engineering on WO 3 nanosheets was also demonstrated by topological transformation of WO3·2H2O at high temperatures21 or partial oxidation of WS2 films by a laser treatment.22 Clearly, the successful fabrication of holes in the © 2018 American Chemical Society
interior of nanosheets will yield abundant edge atoms and functional groups around the holes,23 which will in turn generate new and enhanced properties for exploiting novel applications. Most recently, Yu’s group successfully synthesized various holey 2D transition metal oxide, selenide, and phosphide nanosheets (e.g., Fe2O3, Co3O4, Mn2O3, NiCo2Se4, etc.) by using graphene oxide as a sacrificial template.24−29 Herein, we will demonstrate a general and mild approach to the convenient creation of holes on atomically thin nanosheets directly in the liquid phase. Besides the development of effective methodologies for readily preparing transition metal dichalcogenide nanosheets,30 recent efforts have focused on preparing transition metal oxide nanosheets that are environmentally friendly, chemically stable, and inexpensive.31−34 For example, layered WO3 intermediates with weaker interlayer interaction were mechanically cleaved or chemically exfoliated into nanosheets.31−33 Alternatively, monoclinic WO3 crystals with strong covalent binding between adjacent layers were directly and mildly exfoliated into nanosheets in high yields in our recent work via an Received: April 13, 2018 Revised: July 2, 2018 Published: July 3, 2018 5108
DOI: 10.1021/acs.chemmater.8b01540 Chem. Mater. 2018, 30, 5108−5115
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Figure 1. Exfoliation of WO3 nanosheets in a pH 4 solution and subsequent hole creation on resulting nanosheets in a pH 8 solution. (A) Schematic illustration from the WO3 bulk to WO3 nanosheets to porous nanosheets. (B) pH-dependent evolution of the absorption intensity of WO42− ions after WO3 powder had been dissolved in aqueous solutions at different pH values for 24 h (see Figure S1). A distinct boundary at pH ∼5 was observed to divide the regions for chemical stabilization and dissolution of WO3. Below this value (e.g., pH 4), WO3 nanosheets are chemically stable because of the presence of a surface water layer. When pH is adjusted from 4 to 8, the surface water layers are dissociated (①) to expose the W atom for subsequent nucleophilic attack with OH− ions to produce WO42− and H+ ions (②), leading to the formation of holes across the WO3 nanosheets. The pH was then adjusted back to 4 to stabilize the resulting porous WO3 nanosheets with the surface water layer (③).
from the reaction of WO3 with OH− ions.35 The optical photograph of the supernatant and transmission electron microscopy (TEM) image of dried WO42− on a copper grid are shown in Figure S1B. Interestingly, the high pH values for all these solutions (pH 6, 8, 10, and 12) decreased to 5 after complete reaction, indicating of the chemical stability of WO3 at pH ≤5. The chemical dissolution of WO3 under a basic condition inspired us to develop a facile and mild method for chemically creating holes on ultrathin WO3 nanosheets. As shown in Figure 1A, WO3 in bulk is directly exfoliated into nanosheets in a pH 4 solution of BSA via an electrostatically driven process,34 which are chemically dissolved on their basal planes immediately after pH is adjusted to 8, leading to the expected creation of holes on nanosheets. With the increase in pH from 4 to 8, the surface water layer is dissociated first, and subsequently, OH− ions react with the exposed tungsten atom to release WO42− ions into solution (Figure 1B).35 This benefits from the higher energy of binding of OH− ions to tungsten atom than other ligands such as -OH, -O, and H2O (Table S1). With a continuous vertical dissolution of WO3 unit cells, a hole can be formed across the WO3 nanosheets. The pH of the solution is then adjusted back to 4 to stabilize this porous structure through the surface protective water layer.36 The chemical dissolution on WO3 nanosheets at pH 8 was demonstrated by successive additions of NaOH upon sonication for 24 h (Figure 2A). The color of the milky white solution of nanosheets became more and more shallow until it was transparent after the additions of NaOH for four rounds, indicating complete dissolution of nanosheets into
electrostatically driven process.34 In this work, we further demonstrated a surface-mediated chemical dissolution for convenient creation of holes on atomically thin WO 3 nanosheets that are partially coated with bovine serum albumin (BSA) through strong electrostatic interaction. Consequently, the resulting porous nanosheets exhibit not only greatly increased bandgaps but also substantially enhanced photocurrents together with much better performance during selective adsorption and photocatalytic degradation of substances. The increased bandgaps were also demonstrated in the holey graphite-like C3N4 nanosheets prepared in this study from their ultraviolet−visible (UV−vis) absorption and fluorescence spectra. Overall, this research develops a new strategy for fabricating various holey nanosheets (e.g., WO3, MoS2, and C3N4), engineering bandgaps across a wide spectrum, and enhancing the physicochemical properties of 2D nanomaterials for applications in photocatalysis, photoelectrocatalysis, opto/electrochromics, and electronic/optoelectronic devices.
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RESULTS AND DISCUSSION To assess the chemical dissolution of WO3, we suspended a larger amount of WO3 powder in aqueous solutions at pH 0, 2, 4, 6, 8, 10, and 12. Upon sonication for 24 h and centrifugation at 5000 rpm for 15 min, a series of colorless supernatants were collected to measure their UV−vis absorption spectra (Figure S1A). At pH 8, there is a very strong absorption peak at 260 nm, attributed to the production of more WO42− ions 5109
DOI: 10.1021/acs.chemmater.8b01540 Chem. Mater. 2018, 30, 5108−5115
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Chemistry of Materials
Figure 2. Chemical dissolution of exfoliated WO3 nanosheets toward porous WO3 nanosheets (i.e., creation of holes) in a pH 8 solution. (A) Optical images of a WO3 solution after chemical dissolution upon the successive additions of a NaOH solution (in each round, the pH was adjusted to 8; after incubation in a sonic bath for 24 h, the pH was reduced to 5). (B and C) TEM images of WO3 nanosheets before and after the creation of holes, respectively (insets are the corresponding size distributions of nonporous and porous WO3 nanosheets). (D) High-resolution TEM image of the treated WO3 nanosheet with holes as highlighted with a white circle.
Figure 3. Bandgap engineering of porous WO3 nanosheets before and after the creation of holes. (A) Normalized UV−vis absorption spectra of non-exfoliated WO3 (black), nonporous WO3 nanosheets (red), and porous WO3 nanosheets (green). The bandgaps are estimated via the corresponding tangential lines of absorption spectra. (B) Band structures and bandgaps for the three types of WO3 materials as simulated with the HSE functional in the VASP suite.
WO42− ions. After the first cycle of dissolution, the morphology of the WO3 product was examined by TEM as compared to the as-exfoliated nanosheets (Figure 2B,C). The product remained the structure of nanosheets, similar to the intact nanosheets before dissolution. In contrast, there are many “brighter” spots on the treated nanosheets with an average size of ∼15 nm, showing the production of porous WO3 nanosheets. As measured by dynamic light scattering (insets in panels B and C of Figure 2), the porous nanosheets have an average size of ∼200 nm, similar to that of the nonporous ones but with a wider size distribution. These results indicate that the chemical dissolution at basal planes is faster than that at edges for drilling holes through WO3 nanosheets, which is understood from the binding energies of BSA at the edge being higher than those on the basal plane (Table S2). The porous feature is further supported by a highresolution TEM image as depicted in Figure 2D (the hole is
highlighted with a white circle). The lattice fringe of 0.37 nm is assigned to (020) planes of WO3.37 After two cycles of dissolution, the size and morphology of the WO3 product were also examined by dynamic light scattering and a TEM image (Figure S2). The smaller nanosheets (∼150 nm) were obtained with larger holes ∼25 nm in size, indicating that the dissolution of WO3 occurred on both the peripheral edges of nanosheets and the internal edges around the early created holes after further introduction of OH− ions. The optical properties of porous WO3 nanosheets were first investigated. Their Raman modes exhibit a broadening effect with a shift toward lower wavenumbers as compared to bulky WO3 and as-exfoliated WO3 nanosheets (Figure S3), ascribed to the phonon softening and enhanced electron−phonon coupling in a few-layer regime.21 The absorption spectra of these WO3 samples at 0.25 mg/mL were collected as shown in Figure S4. It is seen clearly that the absorption intensities of 5110
DOI: 10.1021/acs.chemmater.8b01540 Chem. Mater. 2018, 30, 5108−5115
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Figure 4. Enhanced applications of porous WO3 nanosheets. (A) Photocurrents of thin films of different WO3 materials at applied potentials. (B) Temporal evolution of the percentage of adsorption for propazine (Pr) by the different WO3 products. (C) Selective adsorption of Pr on porous WO3 nanosheets for 24 h. The inset shows the calculated binding energies of various herbicides on a WO3 layer. Py, Ca, and 2,4-DA represent pyrifenox, carbofuran, and 2,4-dichlorophenoxyacetic acid, respectively. (D) Photocatalytic degradation of crystal violet with porous WO3 nanosheets in the presence of 4 mM H2O2. The concentrations of herbicides and crystal violet are 0.1 mM and 5 μM, respectively.
The photoelectrochemical (PEC) properties of porous WO3 nanosheets were further investigated by using thin films of porous WO3 nanosheets (Figure 4A), which were fabricated by casting the nanosheets on the fluorine-tin-oxide-coated glass substrates as a photoanode in a 0.5 M Na2SO4 electrolyte. The dark current is negligible under the applied potentials between 0 and 1.2 V versus Ag/AgCl for different WO3 samples. Under the irradiation of a solar simulator, the porous WO3 nanosheets displayed a pronounced photocurrent of 41 μA at 1.0 V, which is ∼5 and ∼3 times higher than those of the bulky WO3 and intact WO3 nanosheets, respectively. The greatly enhanced photocurrent arises from the stronger oxidation ability of porous WO3 nanosheets with a larger bandgap, which signifies the excellent PEC conversion efficiency of porous WO3 nanosheets. The adsorption properties of porous WO3 nanosheets were also investigated by surface binding of herbicide. Temporal adsorption of propazine (Pr) was performed by monitoring its absorption peak at ∼220 nm (Figure 4B and Figure S5). With the increase in incubation time, the absorption intensity of propazine decreased rapidly in the first 10 h. After that, the absorption intensity of propazine decreased gradually and leveled off at 15 h. At 24 h, ∼88% of propazine in a 0.1 mM propazine solution was adsorbed by porous WO3 nanosheets (0.2 mg/mL), exhibiting adsorption that was 2 and 4 times higher than that of intact WO3 nanosheets and bulky WO3,
porous and intact WO3 nanosheets are much higher than that of bulky WO3 because of their smaller size, fewer layers, and better dispersity.30 As revealed in the normalized spectra (Figure 3A), the creation of holes also leads to a significant blue-shift in the absorption peak. As such, the absorption spectrum of the surface-bound BSA at 278 nm was merged with that of porous WO3 nanosheets to form a wide absorption band. From their absorption edges, we can also calculate the corresponding bandgaps of 1.67, 2.32, and 2.83 eV for bulky WO3, intact nanosheets, and porous nanosheets, respectively. As compared to those of intact WO3 nanosheets, electrons are effectively squeezed in localized areas among holes of porous WO3 nanosheets, leading to a quantum confinement effect for an increase in the bandgap by shifting “valence band maximum” and “conduction band minimum” potentials in reverse directions (i.e., more positive Ve and more negative Ve, respectively). This increasing trend in bandgaps agrees well with that as simulated by density functional theory (DFT) using the HSE functional (Figure 3B). The simulated band structures show bandgaps of 1.71, 2.25, and 2.92 eV for fivelayer, single-layer, and porous nanosheets of WO3, respectively. The experimental evidence and DFT simulation indicate that the generation of holes greatly increases the bandgaps of porous WO3 nanosheets together with more edges and/or active centers for enhancing their applications. 5111
DOI: 10.1021/acs.chemmater.8b01540 Chem. Mater. 2018, 30, 5108−5115
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rather than decreased because of the frequently produced WO42− ions with strong absorption at a shorter wavelength at ∼260 nm. We noted that there is a plateau at short wavelengths in the absorption spectra for the porous WO3 nanosheets (pH 8 and 9), implying the formation of such structures. The evolution of UV−vis absorption spectra and optical images of exfoliated WO3 nanosheets in a pH 8 solution were further performed at different concentrations of BSA [0, 0.5, 1, 2, 4, 6, and 8 mg/mL (Figure S14)]. After sonication for 24 h, the absorption intensity was decreased more and faster at lower concentrations of BSA (0−2 mg/mL) compared to higher ones of ≤8 mg/mL. That is to say, there is a smaller amount of dissolution at a higher concentration of BSA because of its effective surface blocking by the greater adsorption of BSA on nanosheets. As such, there is an appropriate amount of BSA adsorption on WO3 nanosheets achieved at 1 mg/mL, which was demonstrated as the optimum condition for producing porous WO3 nanosheets via surface-mediated dissolution. It is clear that chemical dissolution is blocked in BSA-adsorbed areas of WO3 nanosheets while it proceeds readily on the exposed areas to form holes across the nanosheets. Therefore, it is very important to control the “sharp” starting point of chemical dissolution on the exposed areas of WO3 nanosheets to improve the size distribution of holes (Figure S15A). In addition, the uniformity of holes on WO3 nanosheets can be clearly improved by using NH3·H2O rather than NaOH (Figure S15B). Besides the requirements of a pH condition and a BSA concentration, we further studied the direct dissolution of bulky WO3 with thick plates in a pH 8 solution (Figure S16A). The dissolution preferentially occurred on the sharp edges with a higher reactivity to form round plates together with small particles (Figure S16B). Compared to the heat topological transformation at high temperatures,21 the OH−-driven dissolution of WO3 was performed at room temperature in a weakly basic solution. As an extension, the chemical dissolution of graphite-like C 3N 4 was also demonstrated to produce porous C3N4 nanosheets in an acidic solution, which is in contrast to the case for WO3 materials in a basic solution as described above. After careful grinding of the as-synthesized C3N4 solid via hightemperature treatment of melamine (Figure S17),42 the obtained C3N4 powder was cleaved to form porous C3N4 nanosheets because of its chemical dissolution in a pH 0 solution in the presence of 1 mg/mL BSA (Figure S18A),43 whereas it was only cleaved to form nonporous C 3N4 nanosheets in a pH 10 solution (Figure S18B). As indicated by DFT simulation (Table S3), the binding energies of the nonpolar benzene rings and amide groups on the C3N4 layer are 0.51 and 0.50 eV, respectively, which are much larger than those of polar groups, including carboxyl (0.31 eV), amino (0.25 eV), and hydroxyl groups (0.23 eV). As such, the benzene rings and peptide bonds can more strongly bind on the C 3N 4 nanosheets via hydrophobic interaction for exfoliation. The absorption spectra of C3N4 products were collected and normalized to show a clear blue-shift in the absorption peak to 325, 315, and 310 nm for bulky C3N4, nonporous C3N4 nanosheets, and porous C3N4 nanosheets, respectively (Figure S19), exhibiting increased bandgaps after the creation of holes, similar to the characteristic of porous WO3 nanosheets. On the basis of their absorption edges, the bandgaps are calculated to be 1.92, 2.67, and 2.77 eV for bulky
respectively. This is because of its much higher binding capacity for target accumulation arising from their larger surface-to-volume ratio and higher site accessibility to the target analyte.38,39 Meanwhile, the selective surface adsorption on porous WO3 nanosheets was demonstrated by binding a series of herbicides in solution (Figure S6). Compared to Pr, pyrifenox (Py), carbofuran (Ca), and 2,4-dichlorophenoxyacetic acid (2,4-D) have much less adsorption on the nanosheets, and the surface-adsorbed amounts decreased in the following order: Pr > Py > Ca > 2,4-D (Figure 4C). This order indicates the selective binding toward Pr. Their DFTsimulated binding energies on WO3 layers exhibit the same trend, as well, suggesting that the high selectivity of Pr is due to its highest binding energy. Furthermore, the photocatalytic performance of porous WO3 nanosheets was investigated in a pH 4 solution. With a pKa1 of 5.31, crystal violet is not ionizable at pH 4 and thus an ideal candidate for studying photocatalytic performance because of its weak adsorption on the negatively charged WO3 nanosheets.40 After incubation for 24 h, only ∼20% of crystal violet was bound as estimated from the corresponding UV−vis absorption spectra (Figure S7). Under radiation of a UV lamp, the absorption peak of crystal violet gradually decreased with time (Figure 4D) and became very weak at 40 min, indicating the almost complete photocatalytic degradation of crystal violet by porous WO3 nanosheets. This was accompanied by a color change back to milky white (Figure S8A). Meanwhile, the photocatalytic performance of porous WO3 nanosheets is much better than that of the other two WO3 materials (Figure S8B), attributed to the presence of more active sites around holes. It is noted that the photocatalytic degradation of crystal violet was achieved only in the presence of WO3 nanosheets and H2O2 rather than individual WO3 nanosheets and H2O2 (Figure S9). To investigate the in-depth mechanism for hole formation, the dissolution kinetics of WO3 nanosheets in a pH 8 solution was examined by using UV−vis absorption spectroscopy (Figure S10). The absorption intensity of nanosheets gradually decreased and leveled off after incubation for 20 h, accompanied by a blue-shift in absorption edges. The pHdependent evolution of UV−vis absorption spectra was further determined under different pH conditions (Figure S11). The chemical dissolution of WO3 was not observed distinctly at pH 6, as evidenced by their absorption spectrum being similar to that of intact WO3 nanosheets. In contrast, there was a clear decrease in the absorption intensity at pH 7, indicating the obvious chemical dissolution of WO3 in the pH 7 solution. However, this level of dissolution produced only pits and pores on WO3 nanosheets rather than holes across the nanosheets (Figure S12). At pH 8, the facile creation of holes on WO3 nanosheets (Figure 2) was achieved readily via more chemical dissolution in the presence of more OH− ions for drilling through the sheet, leading to the further decrease in absorption intensity (Figure S11). With the increase in pH to 9, the absorption intensity slightly decreased while the shape of the absorption peak was maintained. Interestingly, there were many more holes observed at the central region of WO3 nanosheets with serrated edges in a squarelike shape (Figure S13). This indicates that the dissolution of WO3 at the edges leaves more stable [100] and [010] facets (both of them have the same surface energy but a value lower than that of the [001] facet41). With the continuous increment of pH to 10 and 11, the absorption intensity at ∼310 nm greatly increased 5112
DOI: 10.1021/acs.chemmater.8b01540 Chem. Mater. 2018, 30, 5108−5115
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Figure 5. Hole creation on MoS2 nanosheets via chemical dissolution in a H2O2 solution. (A) Schematic illustration for hole creation on a MoS2 nanosheet via reaction with H2O2. (B) UV−vis absorption spectra of MoS2 nanosheets in a 50 mM H2O2 solution at different incubation times. (C) Temporal evolution of the absorption intensity at 666 nm after the addition of 50 mM H2O2 to the MoS2 solution. Insets are optical images of mixed solutions at different incubation times. (D) Low-resolution and (E) high-resolution TEM images of MoS2 nanosheets with holes, which were obtained via reaction of MoS2 nanosheets with 50 mM H2O2 for 2 h.
displays the features of holes more clearly on nanosheets. Meanwhile, the spacing fringes were 0.27 and 0.16 nm, assigned to (100) and (110) planes of MoS2,45 respectively. In the absence of BSA, MoS2 was completely dissolved after 2 h in a pH 4 solution containing 50 mM H2O2 (Figure S21). In the presence of BSA, it took a longer period of ≤10 h for the complete dissolution of MoS2 (Figure 5B), indicating the surface-blocked role of BSA in dissolution. In summary, we have developed a surface-mediated chemical dissolution strategy for the controlled creation of holes on ultrathin nanosheets, which provides more active edges for rationally tuning the intrinsic properties and facilely achieving a more enhanced performance. Experimentally, the creation of holes on WO3 nanosheets proceeded in a 1 mg/mL BSA solution via the reaction of exposed WO3 with OH− ions after pH had been adjusted from 4 to 8. Experimental evidence and DFT simulations indicated that the creation of holes can alter the bandgaps of porous nanosheets, resulting in photocurrents that are enhanced by 5- and 3-fold compared to those of bulky WO3 and nonporous WO3 nanosheets, respectively. An increased number of active centers around holes also endows porous nanosheets with better performance in selective adsorption and photocatalytic degradation of substances. Importantly, the chemical dissolution for the creation of holes was further extended to prepare porous C3N4 and MoS2 nanosheets in pH 0 and 50 mM H2O2 solutions, respectively. Overall, this work reports a general and convenient strategy for the facile creation of holes on ultrathin nanosheets and brings new opportunities to exploit more outstanding properties of 2D nanomaterials for enhanced applications.
C3 N4 , nonporous nanosheets, and porous nanosheets, respectively. Similarly, their fluorescence spectra were further collected to show a clear blue-shift in the fluorescence peak to 453, 443, and 438 nm excited at 320 nm for bulky C3N4, nonporous C3N4 nanosheets, and porous C3N4 nanosheets, respectively (Figure S20), reflecting their increased bandgaps via the creation of holes. Besides the chemical dissolution under simple basic or acidic conditions, the chemical dissolution may also proceed to create holes on other ultrathin nanosheets under different conditions. Upon sonication of bulky MoS2 in a pH 4 solution containing 1 mg/mL BSA (as reported, a strong hydrophobic interaction exists between benzene rings of BSA and MoS2 layers for effective exfoliation30), the exfoliated MoS2 nanosheets were first investigated in acidic and basic solutions, but no dissolution was observed. Interestingly, the BSA-adsorbed MoS2 nanosheets were readily dissolved in a pH 4 solution containing 50 mM H2O2 to create holes on nanosheets accompanied by the production of MoO22+ and SO42− (Figure 5A).44 Upon incubation, the absorption intensity of MoS2 nanosheets gradually decreased with an increase in time (Figure 5B) until the disappearance of all the absorption peaks, indicating the complete dissolution of nanosheets. Correspondingly, the greenish-brown color of the solution faded over time until the solution became colorless as shown in the optical image of Figure 5C. Temporal evolution of the absorption intensity at 666 nm shows the fast decrease in the first 4 h and then a plateau after that. At 2 h, the lowresolution TEM image of the MoS2 product shows the formation of holes on nanosheets (Figure 5D). The corresponding high-resolution TEM image (Figure 5E) 5113
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Heterostructures of WS2/WO3·H2O Through Selective Oxidation of Monolayer WS2. Angew. Chem., Int. Ed. 2015, 54, 15226−15230. (12) Bessonov, A. A.; Kirikova, M. N.; Petukhov, D. I.; Allen, M.; Ryhänen, T.; Bailey, M. J. A. Layered Memristive and Memcapacitive Switches for Printable Electronics. Nat. Mater. 2015, 14, 199−204. (13) Yin, Z.; Zhang, X.; Cai, Y.; Chen, J.; Wong, J. I.; Tay, Y. Y.; Chai, J.; Wu, J.; Zeng, Z.; Zheng, B.; Yang, H. Y.; Zhang, H. Preparation of MoS2-MoO3 Hybrid Nanomaterials for Light-Emitting Diodes. Angew. Chem., Int. Ed. 2014, 53, 12560−12565. (14) Lu, Z.; Chen, G.; Li, Y.; Wang, H.; Xie, J.; Liao, L.; Liu, C.; Liu, Y.; Wu, T.; Li, Y.; Luntz, A. C.; Bajdich, M.; Cui, Y. Identifying the Active Surfaces of Electrochemically Tuned LiCoO2 for Oxygen Evolution Reaction. J. Am. Chem. Soc. 2017, 139, 6270−6276. (15) Jaramillo, T. F.; Jørgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.; Chorkendorff, I. Identification of Active Edge Sites for Electrochemical H2 Evolution from MoS2 Nanocatalysts. Science 2007, 317, 100−102. (16) Hinnemann, B.; Moses, P. G.; Bonde, J.; Jørgensen, K. P.; Nielsen, J. H.; Horch, S.; Chorkendorff, I.; Nørskov, J. K. Biomimetic Hydrogen Evolution: MoS2 Nanoparticles as Catalyst for Hydrogen Evolution. J. Am. Chem. Soc. 2005, 127, 5308−5309. (17) Louie, M. W.; Bell, A. T. An Investigation of Thin-Film Ni−Fe Oxide Catalysts for the Electrochemical Evolution of Oxygen. J. Am. Chem. Soc. 2013, 135, 12329−12337. (18) McCrory, C. C.; Jung, S.; Peters, J. C.; Jaramillo, T. F. Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 16977−16987. (19) Ye, G.; Gong, Y.; Lin, J.; Li, B.; He, Y.; Pantelides, S. T.; Zhou, W.; Vajtai, R.; Ajayan, P. M. Defects Engineered Monolayer MoS2 for Improved Hydrogen Evolution Reaction. Nano Lett. 2016, 16, 1097− 1103. (20) Li, H.; Tan, Y.; Liu, P.; Guo, C.; Luo, M.; Han, J.; Lin, T.; Huang, F.; Chen, M. Atomic-Sized Pores Enhanced Electrocatalysis of TaS2 Nanosheets for Hydrogen Evolution. Adv. Mater. 2016, 28, 8945−8949. (21) Liu, Y.; Liang, L.; Xiao, C.; Hua, X.; Li, Z.; Pan, B.; Xie, Y. Promoting Photogenerated Holes Utilization in Pore-Rich WO3 Ultrathin Nanosheets for Efficient Oxygen-Evolving Photoanode. Adv. Energy Mater. 2016, 6, 1600437. (22) Ma, S.; Zeng, L.; Tao, L.; Tang, C. Y.; Yuan, H.; Long, H.; Cheng, P. K.; Chai, Y.; Chen, C.; Fung, K. H.; Zhang, X.; Lau, S. P.; Tsang, Y. H. Enhanced Photocatalytic Activity of WS2 Film by Laser Drilling to Produce Porous WS2/WO3 Heterostructure. Sci. Rep. 2017, 7, 3125. (23) Liu, Y.; Xiao, C.; Li, Z.; Xie, Y. Promoting Photogenerated Holes Utilization in Pore-Rich WO3 Ultrathin Nanosheets for Efficient Oxygen-Evolving Photoanode. Adv. Energy Mater. 2016, 6, 1600436. (24) Peng, L.; Xiong, P.; Ma, L.; Yuan, Y.; Zhu, Y.; Chen, D.; Luo, X.; Lu, J.; Amine, K.; Yu, G. Holey Two-dimensional Transition Metal Oxide Nanosheets for Efficient Energy Storage. Nat. Commun. 2017, 8, 15139. (25) Fang, Z.; Peng, L.; Qian, Y.; Zhang, X.; Xie, Y.; Cha, J. J.; Yu, G. Dual Tuning of Ni−Co-A (A = P, Se, O) Nanosheets by Anion Substitution and Holey Engineering for Efficient Hydrogen Evolution. J. Am. Chem. Soc. 2018, 140, 5241. (26) Fang, Z.; Peng, L.; Lv, H.; Zhu, Y.; Yan, C.; Wang, S.; Kalyani, P.; Wu, X.; Yu, G. Metallic Transition Metal Selenide Holey Nanosheets for Efficient Oxygen Evolution Electrocatalysis. ACS Nano 2017, 11, 9550. (27) Peng, L.; Fang, Z.; Li, J.; Wang, L.; Bruck, A. M.; Zhu, Y.; Zhang, Y.; Takeuchi, K. J.; Marschilok, A. C.; Stach, E. A.; Takeuchi, E. S.; Yu, G. Two-Dimensional Holey Nanoarchitectures Created by Confined Self-Assembly of Nanoparticles via Block Copolymers: From Synthesis to Energy Storage Property. ACS Nano 2018, 12, 820. (28) Peng, L.; Fang, Z.; Zhu, Y.; Yan, C.; Yu, G. Holey 2D Nanomaterials for Electrochemical Energy Storage. Adv. Energy Mater. 2018, 8, 1702179.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b01540. Experimental section, observation of chemical dissolution of WO3, characterization of porous WO3 nanosheets, application of porous WO3 nanosheets, effect of different conditions on the creation of holes, exfoliation of porous C3N4 nanosheets, and chemical dissolution of MoS2 (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Yuan Cheng: 0000-0003-0061-3934 Ming-Yong Han: 0000-0002-7519-6779 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partly supported by the Science and Engineering Research Council via Grants 1527000017 and 1527200023. The computational resources were provided by the A*STAR Computational Resource Centre and National Supercomputing Centre, Singapore.
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REFERENCES
(1) Geim, A. K.; Grigorieva, I. V. Van der Waals Heterostructures. Nature 2013, 499, 419−425. (2) Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L. J.; Loh, K. P.; Zhang, H. The Chemistry of Two-Dimensional Layered Transition Metal Dichalcogenide Nanosheets. Nat. Chem. 2013, 5, 263−275. (3) Rao, C. N. R.; Ramakrishna Matte, H. S. S.; Maitra, U. Graphene Analogues of Inorganic Layered Materials. Angew. Chem., Int. Ed. 2013, 52, 13162−13185. (4) Nicolosi, V.; Chhowalla, M.; Kanatzidis, M. G.; Strano, M. S.; Coleman, J. N. Liquid Exfoliation of Layered Materials. Science 2013, 340, 1226419. (5) Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Two-Dimensional Atomic Crystals. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 10451−10453. (6) Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Atomically Thin MoS2: a New Direct-Gap Semiconductor. Phys. Rev. Lett. 2010, 105, 136805. (7) Cao, T.; Wang, G.; Han, W.; Ye, H.; Zhu, C.; Shi, J.; Niu, Q.; Tan, P.; Wang, E.; Liu, B.; Feng, J. Valley-Selective Circular Dichroism of Monolayer Molybdenum Disulphide. Nat. Commun. 2012, 3, 887. (8) Huang, Z. F.; Song, J.; Pan, L.; Zhang, X.; Wang, L.; Zou, J. J. Tungsten Oxides for Photocatalysis, Electrochemistry, and Phototherapy. Adv. Mater. 2015, 27, 5309−5327. (9) Zhu, T.; Chong, M. N.; Chan, E. S. Nanostructured Tungsten Trioxide Thin Films Synthesized for Photoelectrocatalytic Water Oxidation: A review. ChemSusChem 2014, 7, 2974−2997. (10) Yun, J. M.; Noh, Y. J.; Lee, C. H.; Na, S. I.; Lee, S.; Jo, S. M.; Joh, H. I.; Kim, D. Y. Exfoliated and Partially Oxidized MoS2 Nanosheets by One-Pot Reaction for Efficient and Stable Organic Solar Cells. Small 2014, 10, 2319−2324. (11) Zhou, P.; Xu, Q.; Li, H.; Wang, Y.; Yan, B.; Zhou, Y.; Chen, J.; Zhang, J.; Wang, K. Fabrication of Two-Dimensional Lateral 5114
DOI: 10.1021/acs.chemmater.8b01540 Chem. Mater. 2018, 30, 5108−5115
Article
Chemistry of Materials (29) Zhu, Y.; Peng, L.; Fang, Z.; Yan, C.; Zhang, X.; Yu, G. Structure Engineering of 2D Nanomaterials for Energy Storage and Catalysis. Adv. Mater. 2018, 30, 1706347. (30) Guan, G.; Zhang, S.; Liu, S.; Cai, Y.; Low, M.; Teng, C. P.; Phang, I. Y.; Cheng, Y.; Duei, K. L.; Srinivasan, B. M.; Zheng, Y.; Zhang, Y. W.; Han, M. Y. Protein Induces Layer-by-Layer Exfoliation of Transition Metal Dichalcogenides. J. Am. Chem. Soc. 2015, 137, 6152−6155. (31) Kalantar-zadeh, K.; Vijayaraghavan, A.; Ham, M.-H.; Zheng, H.; Breedon, M.; Strano, M. S. Synthesis of Atomically Thin WO3 Sheets from Hydrated Tungsten Trioxide. Chem. Mater. 2010, 22, 5660−5666. (32) Liang, L.; Zhang, J.; Zhou, Y.; Xie, J.; Zhang, X.; Guan, M.; Pan, B.; Xie, Y. High-Performance Flexible Electrochromic Device Based on Facile Semiconductor-to-Metal Transition Realized by WO3·2H2O Ultrathin Nanosheets. Sci. Rep. 2013, 3, 1936. (33) Waller, M. R.; Townsend, T. K.; Zhao, J.; Sabio, E. M.; Chamousis, R. L.; Browning, N. D.; Osterloh, F. E. Single-Crystal Tungsten Oxide Nanosheets: Photochemical Water Oxidation in the Quantum Confinement Regime. Chem. Mater. 2012, 24, 698−704. (34) Guan, G.; Xia, J.; Liu, S.; Cheng, Y.; Bai, S.; Tee, S. Y.; Zhang, Y. W.; Han, M. Y. Electrostatic-Driven Exfoliation and Hybridization of 2D Nanomaterials. Adv. Mater. 2017, 29, 1700326. (35) Qi, H.; Wang, C.; Liu, J. A Simple Method for the Synthesis of Highly Oriented Potassium-Doped Tungsten Oxide Nanowires. Adv. Mater. 2003, 15, 411−414. (36) Anik, M.; Cansizoglu, T. Dissolution Kinetics of WO3 in Acidic Solutions. J. Appl. Electrochem. 2006, 36, 603−608. (37) Chen, D.; Gao, L.; Yasumori, A.; Kuroda, K.; Sugahara, Y. Sizeand Shape-Controlled Conversion of Tungstate-Based InorganicOrganic Hybrid Belts to WO3 Nanoplates with High Specific Surface Areas. Small 2008, 4, 1813−1822. (38) Guan, G.; Zhang, Z.; Wang, Z.; Liu, B.; Gao, D.; Xie, C. SingleHole Hollow Polymer Microspheres toward Specific High-Capacity Uptake of Target Species. Adv. Mater. 2007, 19, 2370−2374. (39) Guan, G.; Liu, R.; Mei, Q.; Zhang, Z. Molecularly Imprinted Shells from Polymer and Xerogel Matrices on Polystyrene Colloidal Spheres. Chem. - Eur. J. 2012, 18, 4692−4698. (40) Fisk, J. D.; Batten, R.; Jones, G.; O’Reilly, J. P.; Shaw, A. M. pH Dependence of the Crystal Violet Adsorption Isotherm at the Silica− Water Interface. J. Phys. Chem. B 2005, 109, 14475−14480. (41) Zhang, D.; Wang, S.; Zhu, J.; Li, H.; Lu, Y. WO3 Nanocrystals with Tunable Percentage of (001)-Facet Exposure. Appl. Catal., B 2012, 123−124, 398−404. (42) Yan, S. C.; Li, Z. S.; Zou, Z. G. Photodegradation Performance of g-C3N4 Fabricated by Directly Heating Melamine. Langmuir 2009, 25, 10397−10401. (43) Zhou, Z.; Shen, Y.; Li, Y.; Liu, A.; Liu, S.; Zhang, Y. Chemical Cleavage of Layered Carbon Nitride with Enhanced Photoluminescent Performances and Photoconduction. ACS Nano 2015, 9, 12480−12487. (44) Guan, G.; Liu, S.; Cheng, Y.; Zhang, Y. W.; Han, M. Y. BSAcaged Metal Clusters to Exfoliate MoS2 Nanosheets towards Their Hybridized Functionalization. Nanoscale 2018, 10, 10911−10917. (45) Ou, J. Z.; Chrimes, A. F.; Wang, Y.; Tang, S. Y.; Strano, M. S.; Kalantar-zadeh, K. Ion-Driven Photoluminescence Modulation of Quasi-Two-Dimensional MoS2 Nanoflakes for Applications in Biological Systems. Nano Lett. 2014, 14, 857−863.
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DOI: 10.1021/acs.chemmater.8b01540 Chem. Mater. 2018, 30, 5108−5115
