Surface-Mediated Chemical Dissolution of 2D Nanomaterials towards

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Surface-Mediated Chemical Dissolution of 2D Nanomaterials towards Hole Creation Guijian Guan, Mingda Wu, Yongqing Cai, Shuhua Liu, Yuan Cheng, Si Yin Tee, Yong-Wei Zhang, and Ming-Yong Han Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01540 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 5, 2018

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Chemistry of Materials

Surface-Mediated Chemical Dissolution of 2D Nanomaterials towards Hole Creation Guijian Guan,*,†,‡ Mingda Wu,‡ Yongqing Cai,† Shuhua Liu,‡ Yuan Cheng,† Si Yin Tee,‡ YongWei 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

ABSTRACT: Chemically engineered holes on 2D nanomaterials may significantly increase the number of edge sites to tune their intrinsic properties for achieving 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 pH8. 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 in the selective adsorption and photocatalytic degradation compared with bulky WO3 and non-porous 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 to generate holes on various ultrathin nanosheets, but also brings new opportunities to exploit exotic properties and novel applications of geometrically constructed 2D nanomaterials.

INTRODUCTION Two-dimensional (2D) nanomaterials in particular, transition metal dichalcogenides and oxides in single- and fewlayers have been receiving extreme attention due to greatly increased surface-to-volume ratio, inherent confinement effect and sizable bandgap in a wide range across near infrared, visible to ultraviolet region.1‒13 Recent researches also start to realize 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, Mn) nanosheets have been demonstrated to render their high activities towards hydrogen evolution reaction15,16 and oxygen evolution reaction,17,18 respectively. As such, it is of key importance in exposing more active edge sites via structural manipulation for enhancing the outstanding performance of 2D nanomaterials. Typically, the successful fabrication of holey MoS2 and TaS2 nanosheets was achieved by oxygen plasma etching method,19,20 outperforming intact ones in energy storage, catalysis and molecular transport/separation. The pore engineering on WO3 nanosheets was also demonstrated by topological transformation of WO3•2H2O at high temperature21 or partial oxidation of WS2 films by laser treatment.22 Clearly, the successful fabrication of holes in the interior of nanosheets will exhibit 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, et al.) 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 liquid phase. Besides the development of effective methodologies to readily prepare transition metal dichalcogenide nanosheets,30 recent efforts have been focusing on preparing transition metal oxide nanosheets with environmental friendliness, chemical stability and low cost.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 was directly and mildly exfoliated into nanosheets at high yield in our recent work via an electrostatically driven process.34 In this work, we further demonstrated a surfacemediated chemical dissolution for convenient creation of holes on atomically thin WO3 nanosheets that are partially coated with BSA through strong electrostatic interaction. Consequently, the resulting porous nanosheets exhibit not only greatly increased bandgaps but also substantially

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Figure 1. Exfoliation of WO3 nanosheets in pH4 solution and subsequent hole creation on resulting nanosheets in pH8 solution. (A) Schematic illustration from WO3 bulk, WO3 nanosheets to porous nanosheets. (B) pH-dependent evolution of absorption intensity of WO42‒ ions after dissolving WO3 powder in aqueous solutions at different pH for 24 h (refer to Figure S1). A distinct boundary at ~pH5 was observed to divide the regions for chemical stabilization and dissolution of WO3. Below this value (e.g. pH4), WO3 nanosheets are chemically stable due to the presence of a surface water layer. When adjusting pH from 4 to 8, the surface water layers are dissociated (①) to expose W atom for subsequent nucleophilic attack with OH‒ ions for producing WO42‒ and H+ ions (②), leading to the formation of holes across the WO3 nanosheets. The pH was then adjusted back to 4 for stabilizing the resulting porous WO3 nanosheets with the surface water layer (③).

natants, indicating that the dissolution of WO3 was not observed. At pH higher than 8, there is a very strong absorption peak at 260 nm, attributed to the production of more WO42– ions from the reaction of WO3 with OH– ions.35 The optical photograph of supernatant and 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 chemical stability of WO3 at pH5 or lower. The chemical dissolution of WO3 in a basic condition inspired us to develop a facile and mild method for chemically creating holes on ultrathin WO3 nanosheets.

enhanced photocurrents together with much better performance in 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 UV-vis absorption and fluorescence spectra. Overall, this research develops a new strategy to fabricate various holey nanosheets (e.g. WO3, MoS2 and C3N4), engineer bandgaps across wide spectrum and enhance physiochemical properties of 2D nanomaterials for applications in photocatalysis, photoelectrocatalysis, opto/electrochromics and electronic/optoelectronic devices.

As shown in Figure 1A, WO3 in bulk is directly exfoliated into nanosheets in pH4 solution of BSA via an electrostatically driven process,34 which are chemically dissolved on their basal planes immediately after adjusting pH to 8, leading to the expected creation of holes on nanosheets. With the increase of 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 binding

RESULTS AND DISCUSSION To determine the chemical dissolution of WO3, we suspended a larger amount of WO3 powder into aqueous solutions at pH values from 0, 2, 4, 6, 8, 10 to 12. Upon sonication for 24 h and centrifugation at 5000 rpm for 15 min, a series of colorless supernatants were collected for measuring their UV-vis absorption spectra (Figure S1A). At pH lower than 6, there is no absorption peak from the super-

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Figure 2. Chemical dissolution of exfoliated WO3 nanosheets towards porous WO3 nanosheets (i.e. the creation of holes) in pH8 solution. (A) Optical images of WO3 solution after chemical dissolution upon the successive additions of NaOH solution (in each round, pH value was adjusted to pH8. After incubation in sonic bath for 24 h, the pH value was reduced to 5). (B,C) TEM images of WO3 nanosheets before (B) and after (C) the creation of holes (insets are the corresponding size distribution of non-porous and porous WO3 nanosheets). (D) High-resolution TEM image of the treated WO3 nanosheet with holes as highlighted with a white circle.

energy of OH‒ ions with tungsten atom than other ligands such as ‒OH, ‒O and H2O (Table S1). With a continuous dissolution of WO3 unit cells vertically, a hole can be formed across the WO3 nanosheets. The pH of solution is then adjusted back to 4 to stabilize this porous structure through the surface protective water layer.36

results indicate that the chemical dissolution at basal planes is faster than the one at edges for drilling holes through WO3 nanosheets, which is understood from the higher binding energies of BSA at the edge than those on basal plane (Table S2). The porous feature is further supported by high-resolution TEM image as depicted in Figure 2D (the hole is highlighted with a white circle). The lattice fringe of 0.38 nm is assigned to (020) planes of WO3.37 After two cycles of dissolution, the size and morphology of WO3 product was also examined by dynamic light scattering and TEM image (Figure S2). The smaller nanosheets of ~150 nm in size were obtained with larger holes of ~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 introducing OH‒ ions.

The chemical dissolution on WO3 nanosheets at pH8 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 transparent after the additions of NaOH for four rounds, indicating complete dissolution of nanosheets into WO42‒ ions. After the first cycle of dissolution, the morphology of WO3 product was examined by TEM as compared to the as-exfoliated nanosheets (Figure 2B and 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, exhibiting the production of porous WO3 nanosheets. As measured by dynamic light scattering (insets in Figure 2B and C), the porous nanosheets have an average size of ~200 nm similar to the non-porous ones but with a wider size distribution. These

The optical properties of porous WO3 nanosheets were first investigated. Their Raman modes exhibit a broadening effect with a shift towards lower wavenumber 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-layered regime.21 Their absorption spectra of these WO3 samples at



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Figure 3. Bandgap engineering of porous WO3 nanosheets before and after the creation of holes. (A) The normalized UV-vis absorption spectra of non-exfoliated WO3 (black line), non-porous WO3 nanosheets (red line) and porous WO3 nanosheets (green line). The bandgaps are estimated via the corresponding tangential line of absorption spectra. (B) The band structures and band gaps for the three types of WO3 materials as simulated with the HSE functional in VASP suite. 0.25 mg/mL were collected as shown in Figure S4. It is seen clearly that the absorption intensities of porous and intact WO3 nanosheets are much higher than that of bulky WO3 due to their smaller size, fewer layer number and better dispersity.30 As revealed in the normalized spectra (Figure 3A), the creation of holes also leads to a significant blueshift 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 and porous nanosheets, respectively. As compared to 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 of bandgap by shifting “valence band maximum” and “conduction band minimum” potentials in reverse directions (i.e. more +Ve and more –Ve), respectively. This increasing trend in bandgaps agrees well with that as simulated by density functional theory (DFT) using HSE functional (Figure 3B). The simulated band structures show the bandgaps of 1.71, 2.25 and 2.92 eV for five layer, 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/active centers for enhancing their applications.

excellent PEC conversion efficiency of porous WO3 nanosheets.

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-tinoxide-coated glass substrates as a photoanode in 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 compared to 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

Furthermore, the photocatalytic performance of porous WO3 nanosheets was investigated in pH4 solution. With a pKa1 of 5.31, crystal violet is not ionizable at pH4 and thus an ideal candidate for studying photocatalytic performance due to 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 with a color change

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 increment of incubation time, the absorption intensity of propazine decreased rapidly in first 10 h. After that, the absorption intensity of propazine decreased gradually and levelled off at 15 h. At 24 h, ~88% of propazine in 0.1 mM propazine solution was adsorbed by porous WO3 nanosheets (0.2 mg/mL), exhibiting 2 and 4 times adsorption compared to intact WO3 nanosheets and bulky WO3, 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 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 an order of Pr > Py > Ca > 2,4-D (Figure 4C), indicating the selective binding towards Pr. Their DFTsimulated binding energies on WO3 layers have the same trend as well, suggesting that the high selectivity of Pr is due to its highest binding energy.


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Figure 4. Enhanced applications of porous WO3 nanosheets. (A) The photocurrent of thin films of different WO3 materials at applied potentials. (B) The temporal evolution of the percentage of adsorption for propazine (Pr) by the different WO3 products. (C) The selective adsorption of Pr on porous WO3 nanosheets for 24 h. Inset is the calculated binding energies of various herbicides on WO3 layer. Py, Ca and 2,4-DA represent pyrifenox, carbofuran and 2,4Dichlorophenoxyacetic acid, respectively. (D) The photocatalytic degradation of crystal violet with porous WO3 nanosheets in the presence of 4 mM H2O2. The concentration of herbicides and crystal violet are 0.1 mM and 5 µM, respectively. a clear decrease in the absorption intensity at pH7, indicating the obvious chemical dissolution of WO3 in the pH7 solution. However, this level of dissolution only produced pits/pores on WO3 nanosheets rather than holes across the nanosheets (Figure S12). At pH8, 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 of pH to 9, the absorption intensity slightly decreased while maintaining the shape of absorption peak. Interestingly, there were many more holes observed at the central region of WO3 nanosheets with serrated edges in a square-like 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 lower than that of [001] facet41). With the continuous increment of pH to 10 and 11, the absorption intensity at ~310 nm greatly increased rather than decreased due to largely produced WO42‒ ions with strong absorption at shorter

back to milky white (Figure S8A). Meanwhile, the photocatalytic performance of porous WO3 nanosheets is much better than 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 only achieved in the presence of WO3 nanosheets and H2O2 rather than individual WO3 nanosheets and H2O2 (Figure S9). To investigate in-depth mechanism for hole formation, the dissolution kinetics of WO3 nanosheets in pH8 solution was examined by using UV-vis absorption spectroscopy (Figure S10). The absorption intensity of nanosheets gradually decreased and levelled off after incubation for 20 h, accompanying with a blue-shift in absorption edges. The pH-dependent evolution of UV-vis absorption spectra was further performed at different pH conditions (Figure S11). The chemical dissolution of WO3 was not observed distinctly at pH6, evidenced from their similar absorption spectrum to intact WO3 nanosheets. In contrast, there was


Chemistry of Materials 1.6

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Figure 5. Hole creation on MoS2 nanosheets via chemical dissolution in H2O2 solution. (A) Schematic illustration for hole creation on MoS2 nanosheet via reaction with H2O2. (B) UV-vis absorption spectra of MoS2 nanosheets in 50 mM H2O2 solution at different incubation times. (C) Temporal evolution of absorption intensity at 666 nm after adding 50 mM H2O2 into the MoS2 solution. Insets are optical images of mixed solution 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. nanosheets can be clearly improved by using NH3•H2O rather than NaOH (Figure S15B). Besides the requirement of pH condition and BSA concentration, we further studied the direct dissolution of bulky WO3 with thick plates in pH8 solution (Figure S16A). The dissolution preferentially occurred on the sharp edges with higher reactivity to form round shaped plates together with small particles (Figure S16B). Compared to the heat topological transformation at high temperature,21 the OH––driven dissolution of WO3 was carried out at room temperature in weak basic solution.

wavelength at ~260 nm. We noted that there is a plateau at short wavelength 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 pH8 solution were further performed at different concentrations of BSA ranging from 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 to 2 mg/mL) compared to higher ones up to 8 mg/mL. That is to say, there is a less amount of dissolution at a higher concentration of BSA due to its effective surface blocking by the more 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 to produce 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 for improving size distribution of holes (Figure S15A). In addition, the uniformity of holes on WO3

As an extension, the chemical dissolution of graphenelike C3N4 was also demonstrated to produce porous C3N4 nanosheets in an acidic solution, which is in contrast to WO3 materials in a basic solution as described above. After careful grinding of the as-synthesized C3N4 solid via high temperature treatment of melamine (Figure S17),42 the obtained C3N4 powder was cleaved to form porous C3N4 nanosheets due to its chemical dissolution in pH0 solution in the presence of 1 mg/mL BSA (Figure S18A),43 whereas it was only cleaved to form non-porous C3N4 nanosheets in pH10 solution (Figure S18B). As indicated by DFT simulation (Table S3), the binding energies of the nonpolar benzene rings and amide groups on C3N4 layer are 0.51 and 0.50 eV, which are much larger than those of polar groups


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Chemistry of Materials in a BSA solution at 1 mg/mL via the reaction of exposed WO3 with OH‒ ions after adjusting pH from 4 to 8. Experimental evidences and DFT simulations indicated that the creation of holes can alter the bandgaps of porous nanosheets, resulting in enhanced photocurrent by 5 and 3 folds compared to bulky WO3 and non-porous WO3 nanosheets, respectively. An increased number of active centers around holes also endow 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 pH0 and 50 mM H2O2 solution, respectively. Overall, the present 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.

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 C3N4 nanosheets via hydrophobic interaction for exfoliation. The absorption spectra of C3N4 products were collected and normalized to show a clear blue-shift in absorption peak from 325, 315 to 310 nm for bulky C3N4, non-porous and porous C3N4 nanosheets, respectively (Figure S19), exhibiting the increased bandgaps after the creation of holes, similar to the characteristic of porous WO3 nanosheets. Based on their absorption edges, the bandgaps are calculated to be 1.92, 2.67 and 2.77 eV for bulky C3N4, non-porous and porous nanosheets, respectively. Similarly, their fluorescence spectra were further collected to show a clear blue-shift in fluorescence peak from 453, 443 to 438 nm excited at 320 nm for bulky C3N4, non-porous and porous C3N4 nanosheets, respectively (Figure S20), reflecting their increased bandgaps via the creation of holes.

ASSOCIATED CONTENT

Besides the chemical dissolution under simple basic or acidic condition, the chemical dissolution may also proceed to create holes on other ultrathin nanosheets under different conditions. Upon the sonication of bulky MoS2 in pH4 solution containing 1 mg/mL BSA (as reported, a strong hydrophobic interaction exist between benzene rings of BSA and MoS2 layers for the 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 pH4 solution containing 50 mM H2O2 to create holes on nanosheets accompanying with the production of MoO22+ and SO42‒ (Figure 5A).44 Upon the incubation, the absorption intensity of MoS2 nanosheets gradually decreased with the increase of time (Figure 5B) until the disappearance of all the absorption peaks, indicating the complete dissolution of nanosheets. Correspondingly, the greenish brown color of solution became more and more shallow over time until colorless as given in the optical image of Figure 5C. Temporal evolution of the absorption intensity at 666 nm exhibits the fast decrease in the first 4 h and then levels off after that. At 2 h, low-resolution TEM image of MoS2 product shows the formation of holes on nanosheets (Figure 5D). The corresponding high-resolution TEM image (Figure 5E) 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 pH4 solution containing 50 mM H2O2 (Figure S21). In the presence of BSA, it took a longer period of time up to 10 h for the complete dissolution of MoS2 (Figure 5B), indicating the surface-blocked role of BSA in the dissolution.

Supporting Information. Experimental Section, observation on chemical dissolution of WO3, characterization on 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. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * (G. G.) E-mail: [email protected]. * (Y. W. Z.) E-mail: [email protected]. * (M. Y. H.) E-mail: [email protected].

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was partly supported by the Science and Engineering Research Council with Grant No. 1527000017 and 1527200023. The computational resources were provided by A*STAR Computational Resource Centre and National Supercomputing Centre, Singapore.

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Table of Contents artwork Porous nanosheet

Bulky WO3

pH4 Exfoliation 40

pH8 Pore creation

Photocurrent (A)

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Non-porous

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Bulk

Porous

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Surface-Mediated Chemical Dissolution of 2D Nanomaterials towards

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