2D Single Crystal WSe2 and MoSe2 Nanomeshes with QuantifiableHigh Exposure of Layer Edges from 3D Mesoporous Silica TemplateWeiming Xu,*,† Kejie Chai,† Yi-wen Jiang,† Jianbin Mao,† Jun Wang,† Pengfei Zhang,†

and Yifeng Shi*,‡

†College of Material Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou 311121, China‡Hangzhou Nanosemi Nanomaterials Co., Ltd., Hangzhou, Zhejiang 310010, China

*S Supporting Information

ABSTRACT: The design and fabrication of layered transition metalchalcogenides with high exposure of crystal layer edges is one of the keypaths to achieve distinctive performances in their catalysis and electro-chemistry applications. Two-dimensional WSe2 and MoSe2 nanomesheswith orderly arranged nanoholes were synthesized by using a mesoporoussilica material KIT-6 with three-dimensional mesoporous structure as ahard template via a nanocasting strategy. Each piece of the nanomesh is asingle crystal, and its c axis is always perpendicular to the nanomesh plane.The highly porous structure brings these nanomeshes extremely highexposure of layer edges, and the well-defined nanostructure provides anopportunity to quantitatively estimate the specific length of the crystal layeredges for the WSe2 and MoSe2 nanomeshes synthesized in this work, which are estimated to be 3.8 × 10

10 and 6.0 × 1010 m g−1,respectively. The formation of a 2D sheet-like nanomesh structure inside a 3D confined pore space should be attributed to thesynergistic effect from the crystal self-limitation growth that is caused by their layered crystal structures and the space-limitationeffect coming from the unique pore structure of the KIT-6 template. The catalytic activities of the nanomeshes in anelectrocatalytic hydrogen evolution reaction were also investigated.

KEYWORDS: transition metal chalcogenides, nanomesh, nanocasting, mesoporous silica KIT-6, hydrogen evolution reaction,active sites

■ INTRODUCTIONLayered transition metal dichalcogenides (TMDs) hasattracted lots of research interest in the last two decadesbecause of their great potential in sensor, Li/Na/Mg ionbattery, semiconductor device, and catalysis applications.1−5

TMDs consist of layered crystal structures with strong in-planecovalent bonding and weak out-of-plane van der Waals forces.6

Their edge planes possess much higher surface energy than thebasal planes, because all the crystal layer edges are full ofdangling bonds.7 Therefore, it was believed for a long time thatthe layer edges of TMD crystals provide active sites for therelated catalytic reactions, including hydrodesulfurization,hydrogen evolution reaction (HER), oxygen reductionreaction (ORR), methane conversion, etc.7−10 Recently, boththeoretical calculation and experimental analysis providedmore and more solid evidences for this conjecture.11−13

Consequently, it became a well-tried effective strategy toimprove their catalytic activity by fabricating nanostructuredTMDs with higher exposure of layer edges to the greatestextent possible.8,14,15 Besides the catalysis application, a higherexposure of layer edges also favors the kinetics of theelectrochemical process in the case of TMDs used as electrodematerials for Li, Na, and Mg ion batteries.16−19 The layer edgeexposure ratio becomes a decisive factor in pursuing a better

performance in these fields.20 However, it was limited by thespontaneous trend of reducing surface energy during thecrystal growth, and therefore, the task of increasing layer edgesis still a great challenge.Unlike the acid−base catalysts, the active site number

estimation is much hard for redox-type catalysts, and therefore,the specific surface area is a commonly used index in this case.However, the extremely anisotropic crystal structure of TMDsmakes this index lose its validity for TMDs. For example, as thesizes were decreased into a submicrometer scale, TMDmaterials tend to form fullerene-like nanostructures, such asnanotubes and nano-onions, in which the crystal layers wererolled and pieced together, and thereafter, the layer edges werethus mostly eliminated.21 As a consequence, these materialscontain almost no layer edge even though they possess a veryhigh specific surface area.22 In addition, TMDs tend to formsheet-like particles with the c axis perpendicular to the sheetplane, in which the layer edges only exist on the side surface,that is, edge planes, with a very tiny occupation ratio of theentire surface.23,24 In an extreme case, if a large TMD single

Received: February 24, 2019Accepted: April 19, 2019Published: April 19, 2019

Research Article

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© 2019 American Chemical Society 17670 DOI: 10.1021/acsami.9b03435ACS Appl. Mater. Interfaces 2019, 11, 17670−17677

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crystal was perfectly exfoliated into single layers, the surfacearea will dramatically increase for several orders, while the totallength of the layer edges will be kept unchanged at all.Although lots of efforts have been made in fabricating variousnanostructured TMDs with high exposure of layer edges in thepast decade, the difficulty in quantitative estimation of layeredges in these materials holds back the fundamental research inthis field.8,15,20,25,26

Recently, Cui and co-workers reported a synthesis of MoS2,MoSe2, and WSe2 films with vertically aligned crystal layers onvarious substrates, which provided a very high density of layeredges on top of the films and therefore possessed excellentcatalytic performance in HER.14,27 However, the utilization ofthe substrate and the need of depositing a W/Mo thin layerwith a controlled thickness on the substrate in the first stepboth make the synthesis hard to scale up or extend to preparepowder form catalysts with a large quantity. Large-scalefabrication of TMD materials with quantifiable high exposureof layer edges is still a great challenge.15

In this paper, we report a novel and large-scale synthesis of2D MSe2 (M: W, Mo) nanomeshes with orderly arrangednanoholes by using a 3D mesoporous silica KIT-6 as a hardtemplate via a nanocasting strategy. Each piece of thenanomeshes fabricated in this work is a single crystal ofhexagonal phase MSe2, and its c axis is always perpendicular tothe nanomeshes’ plane. This special structural characteristicand the highly porous structure provide an extremely highexposure of crystal layer edges. The specific length of theexposed layer edges for WSe2 and MoSe2 nanomeshes can bedirectly estimated from TEM images to be 3.8 × 1010 and 6.0× 1010 m g−1, respectively. Thanks to the utilization of a 3Dmesoporous material as a hard template, more than 10 g of theproduct can be produced in a one-batch synthesis in a smalltube furnace (inner diameter: 30 mm). The well-definedstructure and the high exposure of active sites make it apromising model material for fundamental research in theirapplications.

■ RESULTS AND DISCUSSIONMesoporous silica material KIT-6 with a 3D double-gyroidpore structure (Figure 1a) was prepared according to the

literature.28 Its XRD pattern (Figure S1) and TEM images(Figure S2) clearly show that the KIT-6 hard templatepossesses a highly ordered mesostructure with a cubic Ia3dsymmetry.29 The cell parameter of the mesostructureestimated from its XRD pattern is 21.1 nm. Nitrogen sorptionanalyses (Figure S3) reveal that the specific surface area, porevolume, and mean pore size of the KIT-6 template are 635 m2

g−1, 1.12 cm3 g−1, and 8−10 nm, respectively. All these values

are very close to those reported in the literature, indicating thatthe template synthesized in this work possesses the samemesostructure as described in the literature.28

Phosphotungstic acid (PTA), the metal precursor, wasimpregnated into the pore space of the KIT-6 hard templateunder the help of an ethanol solvent as described elsewhere.30

Then, the PTA@KIT-6 intermediate was mixed with Sepowder and loaded into a quartz boat. The mixture was rapidlyheated up to a desired temperature in a tube furnace bydirectly putting the tube into a hot tube furnace, which hasbeen preheated to 300−800 °C. This special operation broughtan ultrafast heating rate; thus, the PTA@KIT-6 and Se powdermixture reached the reaction temperature in less than 5 min.The initial heating rate was even higher than 200 °C min−1 inthe first 1−2 min. Selenium melted and reacted with a H2 gasflow inside the tube, forming a high concentration of H2Seinside the tube chamber. The PTA precursor was converted tocrystalline WSe2 by reacting with the H2Se gas.

31 In the finalstep, the silica template was removed by HF aqueous solutionas described in the Experimental Section.XRD analysis reveals that the synthesized WSe2 nanomesh

material (denoted as NSM-1) prepared from 400 to 700 °C(Figure S5) possesses a crystal structure of hexagonal phaseWSe2 (JCPDS 71-0600) without any detectable impurity,indicating that the PTA precursor was successfully convertedto crystalline WSe2 by H2Se after the high temperaturetreatment as in similar syntheses reported in the literature.32 Italso shows that a lower temperature of 300 °C leads to aninsufficient conversion of PTA to WSe2, and a highertemperature of 800 °C causes a little over-reduction of PTAto metal tungsten by a H2 gas flow (Figure S5).SEM and TEM images clearly show that the final product

NSM-1 possesses a well-defined two-dimensional sheet-likemorphology with regularly arranged nanoholes (Figure 2).This is not a rationally designed outcome but an unexpectedresult. In a conventional nanocasting synthesis, the replicaproducts are expected to copy the intricate 3D structure of theKIT-6 hard template’s pore space and possess a complex 3D

Figure 1. Structure models of mesoporous silica template KIT-6 andits possible replicas.

Figure 2. SEM, TEM, HRTEM images, and the SAED pattern ofWSe2 nanomesh NSM-1 synthesized at 600 °C.

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structure as that illustrated in Figure 1c.33,34 However, in thiscase, the obtained NSM-1 possesses a well-defined nanomeshstructure with perfect two-dimensional sheet-like morphology.The macroscale particle morphology and mesoscale porestructure are both completely different from the KIT-6template. Through careful investigation, we found that theholes in all NSM-1 nanomeshes are always arranged in acentered rectangular lattice configuration with same cellparameters (a = ∼25 nm, b = ∼43 nm) (Figure 2b,d). Theb:a ratio is close to √3. All these geometry features indicatethat the WSe2 nanomesh only replicated the 2D {110} planesof the 3D pore space of the KIT-6 template in this specialnanocasting synthesis.28,35 This will be further discussed laterin more detail.The hole size, wall thickness, and layer thickness of all NSM-

1 nanomeshes are approximately 14−16, 8−10, and 8−10 nm,respectively, measured from SEM and TEM images. HRTEManalysis reveals that each piece of an NSM-1 nanomesh iscomposed of a single crystal of hexagonal phase WSe2 (Figure2e,f and Figure S6). More interestingly, the c axis of the WSe2crystal is always perpendicular to the nanomesh sheet plane(Figure 2f). The SAED pattern shown in Figure 2h was takenfrom a single piece of NSM-1 nanomesh, and it shows a clearhexagonal patterned spot array, which confirms that thenanomesh is a single crystal, and the c axis is perpendicular tothe nanomesh plane. All checked nanomeshes showed thesame single crystal nature with the same crystal orientation,indicating that it was not just brought by luck.As mentioned above, the structural characteristic of the

nanomesh product indicates that a WSe2 sheet-like crystalexclusively grew along the {110} planes of the 3D mesoporespace of the KIT-6 template, which means that the c axis of theguest WSe2 crystal is always perpendicular to the {110} crystalplanes of the host templates’ mesopore space. This destinedcrystal orientation relationship between the guest crystal andthe mesostructure of the host template is another unexpectedresult. Few single crystal 3D mesoporous replica materials havebeen synthesized by a nanocasting method, including Co3O4,Fe2O3, In2O3, Cr2O3, etc.

36 However, their crystal orientationsdo not show any deterministic correlations with theorientations of their mesostructure, which should be ascribedto the huge difference in their periodic cell parameters betweenthe crystal structure of the guest material and themesostructure of the host template.As in other typical nanocasting syntheses, the wall thickness,

size of the holes, and distance between the holes aredetermined by the pore size, wall thickness, and cell parameterof the KIT-6 template, respectively.34 Therefore, the structuralparameters of NSM-1 can be finely tuned by adjusting thestructural parameters of the KIT-6 template. As a demon-stration, a KIT-6 with a smaller cell parameter and smaller poresize (4−5 nm) was synthesized by further heating the originalKIT-6 template up to 900 °C for 2 h (Figure S4). By using thismesoporous silica as the hard template, an NSM-1 nanomeshwith a smaller hole size (11.5 nm), thinner wall thickness (4−5nm), and thinner layer thickness (4−5 nm) was successfullyprepared (Figure 2g).The NSM-1 nanomesh could also be analogously synthe-

sized by using ammonium tungstate as an alternative metalprecursor, without making any change in the synthetic processand operation parameters. SEM and TEM observations bothproved that a high quality WSe2 nanomesh was alsosuccessfully fabricated in this case (Figure 3). The mesoscale

structure parameters of the NSM-1 material synthesized fromthe ammonium tungstate precursor do not show anynoticeable difference from that synthesized from the PTAprecursor, indicating that it is a general synthesis method. Inaddition, thanks to the utilization of a 3D mesoporous materialas a hard template, this synthesis method can be easily scaledup. More than 10 g of the product can be produced in a one-batch synthesis in a small tube furnace (inner diameter: 30mm), and the final products show similar structure quality.This is a huge advantage comparing to those previouslyreported methods which need a 2D substrate to construct 2Dnanomesh structures.By using phosphomolybdic acid (PMA) as a metal

precursor, a MoSe2 nanomesh (denoted as NSM-2) can alsobe analogously prepared (Figure S5b). The structureparameters of NSM-2 are all very similar to that of NSM-1,because they used the same KIT-6 material as the hardtemplate. HRTEM and SAED patterns clearly show that NSM-2 possesses a single-crystal nature with its c axis perpendicularto the nanomesh plane as the NSM-1 material (Figure 4). This

result clearly indicates that it is a general synthesis method forTMD nanomeshes. It should be noticed that NSM-2 showedlower crystallinity than NSM-1, as revealed by their XRDpattern (Figure S5). TEM observation also found that theparticle size of the NSM-2 nanomesh was much smaller thanthat of NSM-1. This may be explained by the fact thatmolybdenum is much easier to be reduced, which leads to anincrease of crystal nucleation sites and thus reduces the crystalsize of the product. All of the above results clearlydemonstrated that TMDs will form a 2D nanomesh structureinside the 3D mesopore space of KIT-6 by this synthesismethod.It is quite unusual that 2D ultrathin WSe2 and MoSe2 single-

crystal nanomeshes formed inside a host template with a 3Dbicontinuous cubic-phase pore space. The mesopore space ofKIT-6 is always considered to be approximately isotropic in alldirections due to its extremely high symmetry (space group

Figure 3. (a, b) SEM and (c) TEM images of NSM-1 synthesized at600 °C using ammonium tungstate as a precursor.

Figure 4. (a) TEM, (b) HRTEM, and (c) the SAED pattern ofMoSe2 nanomesh material NSM-2 synthesized at 600 °C.

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Ia3d, space group number 230) (Figure 1a).35 In mostpublished reports, when KIT-6 was used as a hard template fornanocasting synthesis, the guest materials usually tend to growuniformly in all directions inside the pore space of the KIT-6template, and as a result, the final products usually possessspherical particle morphologies.36,37 In this work, the nano-mesh products possess a two-dimensional sheet-like morphol-ogy with the sheet size even larger than 700 nm, while thethickness is kept at only 8−10 nm. It is undoubted that itshould be first attributed to the anisotropic layered crystalnature of TMDs. It has been widely reported that TMDs tendto form a sheet-like morphology to reduce the specific layeredge length, that is, reduce the edge plane surface percentage,and thus reduce the surface energy.23,24 However, several factsindicate that the details of the mesopore space structure alsoplay very important roles: (1) only one type of porearrangement was observed for all nanomeshes, which meansthat the guest TMD crystals do not randomly grow alongdifferent directions inside the 3D-ordered mesopore space ofthe host template, but only along the {110} planes of the KIT-6 template. (2) The thickness of the sheet is always uniformlyaround 10 nm, which is similar to the pore diameter of theKIT-6 template. These two facts both indicate that the {110}planes of mesoporous silica KIT-6 must possess some uniquegeometrical characteristics.KIT-6 has a 3D bicontinuous mesopore structure (Figure

1a), which was denoted as a double-gyroid structure.38,39 Thisspecial structure has attracted lots of research interest due to itsextremely high symmetry and advantage in mass transfer. Itscavity network is separated into two sets of channels by thesilica walls (Figure 1b).40 Both sets of channels impenetratethe whole particle while they are nonintersecting between eachother (Figure 1c).35 These two sets of channels are mirror-symmetric with each other, and they accordingly have exactlythe same geometrical characteristics.40 Therefore, we onlydiscuss the 3D structure of one set of the channels in thefollowing sections.Figure 5a displays a 3D structure model for one set channel

of the KIT-6 template viewed from the [001] direction. If we

look at the structure along the [110] direction as shown inFigure 5b, the periodic 3D channel structure can be describedas a stacking structure built by two types of structural units(Figure 5c). One is a 2D continuous space layer A unit, andthe other is an isolated pillar layer B unit composed of a

cylindrical pore array (Figure 5c). The thickness of layer A isapproximately same with the diameter of the mesopore. If wecut the 3D channels at the center of layer A (Figure 5d), thecross-sectional pattern is a perfect nanomesh structure withcentered rectangular patterned pores (Figure 5e). The modelof the layer A without layer B (Figure 1d) shows exactly thesame structural characteristics with those of our nanomeshproducts, strongly indicating that MSe2 crystals tend toexclusively grow inside the mesopore space of layer A in oursynthesis, leaving the space of layer B unoccupied.By carefully inspecting the mesopore structure of the KIT-6

template along different directions (Movies S1-S3), it is foundthat only the {110} crystal planes can provide such acontinuous 2D space with uniform thickness for the growthof TMD crystals with sheet-like morphology (Figure S7). Forexample, if we cut the pore space along the directions,the cross sections are either rodlike islands or zigzag strips(Figure S8). That is to say, if the TMDs are trying to form athin sheet-like morphology along the {100} crystal planes ofKIT-6, only nanorods or zigzag nanostrips will be producedafter the removal of the silica template. If we cut the pore spacealong the directions, a triangle−star array is the mainpattern of the cross sections (Figure S9). The detailedinformation about the shapes of the cross sections alongdifferent directions can be found in the SupportingInformation. Further detailed information of the double-gyroidstructure can be found in the literature.40 In conclusion, onlywhen the sheet-like MSe2 crystal grows along the {110} planesinside the layer A space, a continuous 2D nanomesh productcan be formed with a uniform thickness of 8−10 nm.Otherwise, the products should be isolated small nanoparticles(

the PTA precursor to WSe2 products was accompanied withhuge volume shrinkage. Consequently, in each domain, onlyabout 52% of the mesopore space of the KIT-6 template willbe occupied by the WSe2 crystal; in spite of that, the entirepore space of this domain is filled with the PTA precursorbefore the reaction. In addition, the ultrafast heating rate leadsto an ultrafast reaction rate, and therefore, it prohibits the longdistance migration of metal species, limiting the reaction in anapproximately in situ conversion process. The huge volumeshrinkage and the in situ conversion make the TMD crystalsunable to fill up the entire pore space in any domain, whichexplained the partial occupation result. This is very importantbecause a full occupation will inevitably lead to a 3D replicastructure.It has been proven that inside a small cylindrical pore space,

the TMD crystal layer tends to grow along the pore axisbecause it can reduce the layer edge exposure in this way.30 Ifthe WSe2 single-crystal growth into the pillar space in the layerB unit without a change in the crystal orientation, the crystallayer of the TMDs will become perpendicular to the pore axisof the cylindrical pores in the layer B unit, this situation willsignificantly increase the surface energy.30 On the contrary, it isa better choice for WSe2 crystals to exclusively grow inside thecontinuous layer A unit space to decrease the edge planepercentage as well as the surface energy. We found that if theheating rate is decreased to 1 °C min−1 to make the longdistance migration of metal species possible, the obtainedsamples no longer possess a 2D nanomesh structure butreplicate the entire 3D structure of the double-gyroid porespace as illustrated in Figure 1c and Figures S10−S12.Thisresult indicates that the ultrafast heating rate is the keysynthetic parameter and the huge volume shrinkage plays animportant role in this general TMD nanomesh synthesismethod.

The single crystal TMD nanomeshes synthesized in thiswork possess well-defined nanostructures with regularlyarranged holes, and the c axis of the crystal is alwaysperpendicular to the nanomesh plane. Therefore, the innerside surface of each hole is exclusively composed of crystallayer edges, known as the edge plane in the literature.41 Thetop and bottom surfaces of the nanomeshes are exclusivelycomposed of basal planes. The size of the holes, distributiondensity of the holes, and thickness of the sheets are determinedby the structural characteristics of the KIT-6 template, whichall can be easily estimated by SEM or TEM observation.Owing to this well-defined structure, the total length of thelayer edges in these nanomeshes can be quite preciselyestimated. The specific lengths of layer edges for NSM-1 andNSM-2 synthesized in this work are estimated to be 3.8 × 1010

and 6.0 × 1010 m g−1, respectively. This value is equivalent tothe specific layer edge length of a thin film of vertically alignedWSe2 and MoSe2 with a thickness of only ∼4 nm. Theultrahigh exposure of layer edges should be attributed to thesmall diameter (8−10 nm) of the KIT-6 template, which leadsto a very small spacing between those holes. This featuremakes it a good candidate as a model material for layer edge-related applications.The electrocatalytic HER activities of the WSe2 and MoSe2

nanomeshes in acidic solution were investigated as ademonstration for these special nanostructured TMDs. Theas-prepared NSM-1 and NSM-2 samples were dispersed in anethanol/water mixture and then dropped onto a glass carbonelectrode (GCE) with a 3 mm diameter. The linear sweepvoltammetry (LSV) polarization curves were measured inacidic solution (0.5 M H2SO4) with a potential scan rate of 1mV s−1 using a typical three-electrode setup, and they are allpresented in Figure 6. The onset potential of the polarizationcurves of the WSe2 nanomesh NSM-1 and MoSe2 nanomeshNSM-2 are estimated to be −237 and −146 mV (Figure 6a

Figure 6. The LSV polarization curves of (a) glass carbon electrode (GCE), Pt/C, WSe2 and MoSe2 nanomeshes measured in N2-purged 0.5 MH2SO4 solution and (b) the corresponding Tafel plots. (c, d) LSV polarization curves of the MoSe2 nanomesh with different loading amounts.

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inset, derived from the log j vs V curves), respectively. Theircurrent densities increase rapidly under more negative overpotential and reach 10 mA cm−2 at −315 and −264 mV forNSM-1 and NSM-2. The corresponding Tafel slopes of theWSe2 and MoSe2 nanomeshes are 71 and 69 mV per decade(Figure 6b). These values are close to the state-of-the-artsamples.14,27,42−45 Since these two materials possess samemesoscale structure parameters, these results clearly show thatMoSe2 is a better candidate for the HER reaction asdemonstrated in the literature.27,45 Both of these two materialspossess outstanding chemical stabilities. After they were storedin air for half a year, the materials showed the same activity inHER measurements.The polarization curve of MoSe2 nanomesh NSM-2 with

different loading amounts is listed in Figure 6c, which exhibitsa significant difference in their HER polarization curves whenthe currents were normalized to the projected area of theelectrode. The overpotentials at the current density of 10 mAcm−2 decreased from ∼740 mV (roughly estimated from theextended line of the polarization curve) to 435, 322, 282, 264,and 250 mV as the loading amounts were increased from 1.0 to3.0, 10, 30, 50, and 60 μg (Figure 6c). If we fixed the voltage at−0.300 V versus RHE, the recorded corresponding currentdensities are 0.73, 2.00, 6.20, 16.57, 24.34, and 31.05 mA cm−2,respectively. Although these apparent polarization curves showhuge differences as described above, they show quite similarbehavior if we plot them in the form of log (current density)versus V as in Figure 6d. These curves clearly show that all ofthem possess similar onset potentials and similar Tafel slopes.This should be attributed to the fact that all of them weremeasured from the same material and their active sites possesssimilar intrinsic catalytic reactivity. The significant improve-ment of the apparent HER performance in Figure 6c should beexclusively attributed to the increasing number of activity sites.Since the specific layer edge length can be estimated from

the structure parameters, 6.0 × 1010 m g−1 for the MoSe2nanomesh, and the active sites for the HER reaction is onlylocated in the crystal layer edges for a well-crystallizednondoping TMD material, the TOF values can be calculatedand described as the frequency per nanometer of layer edges.For the abovementioned loading amount, from 1.0 to 3.0, 10,30, 50, and 60 μg, their TOF values are estimated to be 2.7,2.5, 2.3, 2.0, 1.8, and 1.9 per nanometer of layer edges at−0.300 V. The slightly decreasing of TOF can be explained bythe potential drop from the bottom of the active material layerto the top layer caused by resistance, and the thicker activematerial layer may have slightly affected the mass transfer.Therefore, the highest TOF value, 2.7 S−1 nm−1, represent theinstinct catalytic activity of this material. Comparing to thepolarization curve presented in the form of current per projectarea of the electrode as in most of the literature, these TOFvalues represent a more precise data for the evaluation of theintrinsic catalytic activity of this kind of materials.It is worth mentioning that our samples do not show the

best HER performances among all kinds of WSe2- or MoSe2-based materials in the literature, but they are comparable tothose state-of-the-art powder-form pure-phase WSe2 andMoSe2 materials.

42−45 Therefore, we believe our resultsdemonstrated that a high exposure of active layer edges dobenefit electrocatalytic applications, which is one of theadvantages of this highly porous nanomesh structure. Besidethe structure control, various strategies have been developed toimprove the performance of TMDs in catalysis applications,

such as metal or nonmetal doping, making multiphasecomposite structures with carbon/graphene, with directgrowth of active TMDs on the electrode substrate ratherthan loading the powder form sample onto the electrodesubstrate by polymer binders, etc.14,20,27,42−44,46−49 The first-rate materials are always combined using these strategies intoone material to achieve the optimized performance. In most ofthe literature, when a change of apparent HER performance isrecorded after a different synthesis condition was taken, it ishard to analyze whether the improvement comes from thechange in the number of active sites or from the change ofintrinsic activity of the active sites. Actually, the 3Dmesoporous MoSe2 and WSe2 show similar performances inHER with the 2D nanomeshes, because they also possess ahigh exposure of layer edges due to the ultrathin diameter ofthe framework (

powder was mixed with 1.3 g of selenium powder. The mixture wasthen transferred into a quartz boat and put in the middle of a quartztube at room temperature. A constant hydrogen gas flow (200 mLmin−1) was passed by the tube as the reducing agent and protectiveatmosphere. After 30 min, the quartz tube, with the quartz boat insideit, was directly put into a tube furnace, which had already beenpreheated up to 600 °C. The quartz tube was heated up to 600 °C inless than 5 min. After keeping the reaction system at this temperaturefor 4 h, it was took out of the tube furnace and cooled down to roomtemperature. During the entire heating and cooling processes, aconstant hydrogen gas flow (200 mL min−1) was passed by the tube.Two drexel bottles with 2 M NaOH aqueous solution were connectedto the outlet pipeline of the tube furnace to absorb the residual H2Sein the H2 gas flow. To avoid the threat of H2Se leakage and hydrogenaccumulation, the entire equipment was placed inside a fume hood.The silica template was removed by 10% HF aqueous solution for 4 h.Before the silica template removal, a purification process was carriedout. The details about the procedure and result of the purificationprocess are described in the Supporting Information. The MoSe2nanomesh was similarly prepared using PMA (H3PMo12O40·6H2O) asthe precursor.Characterization. The X-ray diffraction (XRD) pattern was

recorded on a Bruker D8 Advance diffractometer with Cu Kαradiation (40 kV, 40 mA). Transmission electron microscopy (TEM)images were collected on a FEI-G2 microscope. Scanning electronmicroscopy (SEM) was performed on FEI-quanta-200F. Nitrogenadsorption−desorption isotherms were measured at 77 K on aMicromeritics TriStar 3020 porosimeter apparatus. Before taking themeasurement, the sample was degassed at 160 °C in a vacuum for 12h. The surface area was calculated by the Brumauer−Emmett−Tellermethod using the adsorption point in a relative pressure from 0.05 to0.35; the pore size distribution was calculated with the adsorptionbranch of the isotherm using the density functional theory (DFT)method. The total pore volumes, Vp, were estimated from saturationadsorption points at a relative pressure P/P0 = 0.99.The electrochemical measurements were performed in a PGSTAT

302 N Autolab Potentiostat/Galvanostat (Metrohm) in a standardthree-electrode cell system. A graphite rod and Ag/AgCl (in 3 M KClsolution) electrodes were used as the counter and reference electrodesrespectively. However, all of the potentials were converted andreported relative to the reversible hydrogen electrode (RHE) in thiswork. The HER activity was evaluated by measuring polarizationcurves with linear sweep voltammetry (LSV) at a scan rate of 1 mVs−1 in N2 saturated 0.5 M H2SO4 (pH = 0.34) solutions. Acommercially available 20 wt % Pt/C (Alfa Aesar) was used as thestate-of-the-art electrocatalyst for comparison. Typically, in thepreparation of the working electrode, 1.0 mg of the tested catalystnanocrystals was dispersed in a mixture of 0.5 g of ethanol and 0.5 mLof water and was sonicated for 30 min to make the catalyst ink. Analiquot of the catalyst ink and ∼10 μL of 0.1% Nafion solution inethanol were drop-coated on an ∼3 mm glassy carbon electrode anddried at room temperature.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.9b03435.

SAXRD, TEM, nitrogen sorption isotherms, and thecorresponding pore size distribution curve of themesoporous silica KIT-6; nitrogen sorption isothermsand the corresponding pore size distribution curves ofthe KIT-6 template that has been further calcined at 900°C; XRD patterns of WSe2 and MoSe2 nanomeshesprepared at different temperature from 300 to 800 °C;HRTEM image of WSe2 nanomesh NSM-1; typical crosssections of KIT-6’s pore space cut along the , and < 111> directions; XRD, SEM, TEM andHRTEM images of 3D mesoporous WSe2 and MoSe2;

SEM images of as-made, intermediate and final products(PDF)Mesopore structure of the KIT-6 template along the directions (MP4), the directions (MP4)and the directions (MP4)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: wmxu@zju.edu.cn (W.X.).*E-Email: shiyifeng@gmail.com (Y.S.).ORCIDWeiming Xu: 0000-0003-3257-0078Pengfei Zhang: 0000-0001-9859-0237Author ContributionsThe manuscript was written through contributions of allauthors. All authors have given approval to the final version ofthe manuscript.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by funding from the Natural ScienceFoundation of China (nos. 21871071 and 21673167), MajorScientific and Technological Innovation Project of Zhejiang(2019C01081), and Climbing Project of Hangzhou NormalUniversity. We thank Professor Su for the helpful discussionsand suggestions.

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