This journal is©The Royal Society of Chemistry 2019 J. Mater. Chem. C, 2019, 7, 10173--10178 | 10173

Cite this: J.Mater. Chem. C, 2019,

7, 10173

Defect-engineered MoS2 with extendedphotoluminescence lifetime for high-performancehydrogen evolution†

Sangmin Kang,‡ab Ja-Jung Koo, ‡a Hongmin Seo,c Quang Trung Truong,d

Jong Bo Park,a Seong Chae Park,e Youngjin Jung,a Sung-Pyo Cho,f Ki Tae Nam, c

Zee Hwan Kim *a and Byung Hee Hong *acd

It has been reported that defects in molybdenum disulfide (MoS2)

enable the hydrogen evolution reaction (HER). The most widely

employed method of argon-plasma treatment for defect generation

suffers from poor material stability and loss of conductivity. Here,

we report a new method to synthesize highly polycrystalline

molybdenum disulfide MoS2 bilayers with enhanced HER perfor-

mance and material stability. This new method is based on metal

organic chemical vapor deposition (MOCVD) followed by UV/ozone

treatment to generate defects. The defect densities on MoS2 were

identified by the increase in lifetime (B76%) and intensity (B15%) in

photoluminescence (PL) as compared to those of pristine MoS2. Our

fabrication and characterization methods can be further applied to

optimize defect densities for catalytic effects in various transition

metal dichalcogenide (TMDC) materials.

Introduction

The development of renewable and sustainable green energy isa crucial issue to overcome the current energy crises.1–4 Amongvarious possible alternative energy resources, hydrogen holdstremendous promise because of its high mass energy densityas well as environmental friendliness,5–9 and water splittingis one of the most common and outstanding methods for

hydrogen production.10 In general, precious metals suchas platinum, rhodium and palladium have been usedas catalysts for the hydrogen evolution reaction (HER); how-ever, their scarcity and high cost have limited practicalapplications.11–13

Among the alternatives to noble metal catalysts, molybdenumdisulfide (MoS2), a hexagonally packed layered structure oftransition metal dichalcogenide (TMDC) with weak van derWaals interactions, has received tremendous attention in recentyears because of its relatively low cost and earth-abundancy.14,15

In addition, its stability under acidic conditions as well as highelectrochemical reactivity have enabled its application as a HERcatalyst.16,17 Unlike bulk MoS2, nanostructured MoS2 with alarge number of edge sites is expected to show a significantenhancement in HER performance, which has been theoreticallypredicted and experimentally confirmed.18–23 Compared to the2H-semiconducting phase, the 1T-metallic phase shows moreefficient HER activity in both the basal plane and edge sites, butthe stability is too poor to be used for common electrochemicalcatalysis.24–26

Increasing the sulfur vacancy by electrochemical sulfurizationwas also useful to enhance the HER,27,28 which has been realizedby complicated structural engineering. The sulfur vacanciescan be also generated by argon plasma.29 However, the largedecrease in the stability and the conductivity of the plasma-treated monolayer MoS2 practically limits its application to theHER. In addition, the thickness and coverage of MoS2 thinfilms are hardly controllable in the conventional solid-sourcegrowth method. Thus, a new approach to synthesize andmodify MoS2 is required for its practical application in theHER, and the electrochemical origin of the enhanced HER alsoneeds to be investigated to better understand the roles ofvacancies, edges, and defects in the surface modified MoS2.Here, we demonstrate a method to synthesize highly poly-crystalline MoS2 bilayers with enhanced reactivity and stabilityby metal organic chemical vapor deposition (MOCVD) and theirdefect-engineering by UV/ozone, leading to considerable HERperformance.

a Department of Chemistry, Seoul National University, Seoul 08826, Korea.

E-mail: zhkim@snu.ac.kr, byunghee@snu.ac.krb Department of Electrical and Computer Engineering,

University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USAc Department of Materials Science and Engineering, Seoul National University,

Seoul 08826, Koread Graphene Research Center & Graphene Square Inc., Advanced Institute of

Convergence Technology, Seoul National University, Suwon 16229, Koreae Graduate School of Convergence Science and Technology,

Seoul National University, Seoul 08826, Koreaf National Center for Inter-University Research Facilities, Seoul National University,

Seoul 08826, Korea

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9tc02256b‡ These authors contributed equally to this work.

Received 28th April 2019,Accepted 14th July 2019

DOI: 10.1039/c9tc02256b

rsc.li/materials-c

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ExperimentalPreparation of UV–ozone treated bilayer MoS2 by theMOCVD method

The detailed synthesis method is recorded in the ESI.† Molyb-denum hexacarbonyl (Mo(CO)6) (Sigma Aldrich, 499.9%) andhydrogen sulfide (H2S) (diluted in 5% in argon) were used asthe Mo and S precursors, respectively. One hundred sccm argonwas used as the carrier gas during growth. The growth substrateof SiO2 (300 nm)/Si wafer is loaded at the center of the quartztube vertically with a quartz wafer carrier. The furnace temperaturewas increased up to 600 1C in 15 min and kept stable at 600 1C for120 min. After growth, the furnace was left to cool down naturally.The samples were irradiated with a UV light source (UV/ozonecleaner, Bioforce) before characterization.

Material characterization

Optical microscopy was performed by Nikon ECLIPSE LV100ND.The AFM image was measured in noncontact mode (ParkSystem, XE-100). The Raman spectra were recorded using aRenishaw Invia Raman Microscope with a 1 mW 514 nm Arcw laser with the spot size of 2 mm. We measured PL using anNA 0.95 objective lens and CCD (iDus 401 BV) with a 24 mW473 nm cw laser. The microstructures and morphologies wereinvestigated by field-emission scanning electron microscopy(FESEM, AURIGA Carl Zeiss) and tunneling electron microscopy(TEM, JEOL ARM200F). XPS analysis was carried out with aKRATOS AXIS-His model at the Research Institute of Advancedmaterials. We measured fluorescence lifetime using a Pico-Quant, Micro Time-200 with a 470 nm pulsed laser.

Electrochemical measurements

The electrochemical sample was prepared through a PMMA-assisted transfer method. The MoS2/SiO2/Si substrate was firstspin-coated by a PMMA thin film. Then, 1 M KOH solution at90 1C was used as the etchant to etch away the SiO2 layer. Afterthat, the MoS2 on the PMMA was rinsed with DI water and thentransferred onto a clean glassy carbon electrode. Finally, thesample was baked at 70 1C in 1 hour and then the PMMAwas washed off with acetone and iso-propanol. In the case ofUV–ozone treatment samples, UV–ozone was irradiated ontothe samples after all transfer processes.

All electrochemical experiments were conducted in a three-electrode system. Ag/AgCl/3 M NaCl, Pt and glassy carbon coveredby MoS2 film were used as a reference electrode, a counterelectrode and a working electrode, respectively. Electrochemicaltests were carried out at room temperature using a potentiostatsystem (CHI 600D, CH instruments). The electrode potential vs.Ag/AgCl was converted to the RHE scale, using the followingequation: E(RHE) = E(Ag/AgCl) + 0.197 + 0.0592 � pH V. Theelectrolyte was sulfuric acid with 500 mM ionic strength andscan rate was 50 mV s�1. Additionally, overpotential values werecalculated by the difference between the iR-corrected potential(V = Vapplied – iR) and the thermodynamic potential of thehydrogen evolving reaction.

Results and discussion

The MoS2 layers are synthesized via MOCVD as described inFig. 1a. (More details can be found in the Experimental sectionand ESI†).31 The MoS2 films on a SiO2/Si wafer grow from thehigh-density nucleation of small triangular islands (B250 nm)(Fig. S1, ESI†), which maximizes the edge sites advantageousfor the HER.32 By optimizing the pressure and concentration ofthe source materials, the bilayer MoS2 films are homogeneouslygrown on a 3 inch SiO2/Si wafer (Fig. 1b), which is evidenced byRaman, AFM and TEM analyses (Fig. 1c–f). As shown in Fig. 1c,the Raman spectra (A1g and E2g

1) measured in the marked areasin Fig. 1b indicate the typical in-plane and out-of-plane vibrationmodes of MoS2.33 The frequency differences between A1g and E2g

from the different areas are B21 cm�1, indicating that the bilayerMoS2 films are homogeneously formed.34 The atomic forcemicroscopy (AFM) image showing the thickness of B1.4 nm(Fig. 1d) and the TEM image showing the spacing of B0.7 nm atthe folded edge (Fig. 1e and f) also support that the MOCVDgrown MoS2 films are dominantly bilayers. In addition,the composition of molybdenum and sulfur is confirmed byenergy dispersive X-ray spectroscopy (EDX) (Fig. S2, ESI†) andX-ray photoemission spectroscopy (XPS) (Fig. S3, ESI†), andphotoluminescence (PL) techniques are used to characterize

Fig. 1 (a) Schematic illustration of the preparation of UV–ozone engineeredbilayer MoS2 films synthesized by MOCVD. (b) A photograph of the MoS2 filmgrown on a 3 inch SiO2/Si wafer. (c) Raman spectra of the bilayer MoS2. (d) AnAFM profile of the MOCVD-grown bilayer MoS2 with a thickness of B1.4 nm,which is measured along the line in the inset AFM image. (e and f) TEMimages and diffraction patterns showing the characteristic planar lattices andthe interlayer spacing of B6.7 Å, corresponding to the bilayer MoS2.

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the chemical and optical properties of the MOCVD-grown films,indicating that Mo–S binding energy is that of MoS2 layers.35

The PL spectral mapping for l = 650–700 nm also shows atypical bilayer MoS2 emission peak (Fig. S4, ESI†).36 Thus, it canbe concluded that bilayer MoS2 films are homogeneouslysynthesized by MOCVD on a SiO2/Si substrate.

The UV–ozone treated MoS2 samples are analyzed by micro-Raman spectroscopy (Fig. 2a).37–39 After the UV–ozone expo-sure, the in-plane vibration mode E2g

1 is red-shifted and theout-of-plane vibration mode A1g is blue-shifted, and the peakfrequency difference between A1g and E2g modes is changedfrom B21 to B19 cm�1. The shifts of both peaks are originatedfrom the oxygen incorporation and the peak frequency differenceis due to the partial layer decrease and lattice distortion byUV–ozone and oxygen incorporation, which are confirmed byatomic structure analysis described later.40 The electronicstructure of the MoS2 layer allows two types of excitons fromdirect (K point) and indirect transitions (G point).41 The inten-sity and the position of the PL peaks also vary with UV–ozoneexposure time (Fig. 2b). The initial treatment could improve theconductivity of the MoS2 films by oxygen doping which resultsin an increased PL intensity and red shift of the direct PLpeak.30,42 The red shift is also attributed to these oxygen defects,which alter or lead to additional radiative recombination of theelectrons at shallow defect states and holes in the valenceband.43,44 However, further treatment eventually breaks the

lattice structures of MoS2 and decreases the conductivity, whichcan lead to efficiency degradation as electrochemical catalysts.29,44

Thus, the UV–ozone exposure time needs to be optimized tomaximize the electrochemical performance that is possibly relatedto the PL intensity.

The high-angle annular dark field (HAADF) aberration-correctedscanning transmission electron microscope (CS-STEM) image(Fig. 2c) shows lattice distortion and oxygen incorporation afterUV–ozone, where the bright and dark areas correspond to thebilayer and monolayer, respectively. Thus, some parts brokendown to monolayers by UV–ozone show their converted FFTfrom dodecagonal to hexagonal as shown Fig. 2c inset image.Fig. 2d shows the EELS of MOCVD based MoS2 films on theTEM grid before/after UV–ozone treatment processes. There isno oxygen peak in pristine MoS2, while the appearance of a newpeak in UV–ozone treated MoS2 directly indicates the formationof oxygen bonding. Not limited to a temporary site, such EELSspectrum can be observed throughout the entire regions(Fig. S5, ESI†). Fig. 2e and f show the XPS spectra of Mo and Sbefore and after UV–ozone treatment, respectively. The Mo 3dpeak positions at 229.8 and 232.5 eV are almost unchanged afterUV–ozone treatment (Fig. 2e), but a new peak corresponding toS–O at 529.8 eV appeared for S 2p (Fig. 2f), confirming that theattachment of oxygen atoms mainly occurs at the dangling orsubstituting atoms as like previous reports.42,45 Not only S 2pbut also the O 1s spectrum also shows the oxygen incorporationin the UV–ozone treated MoS2 films in Fig. S6 (ESI†).

We further analyzed the PL intensity variation as a functionof the applied laser exposure time to investigate the oxygenincorporation effect. When exposed to a laser source, theUV–ozone treated bilayer MoS2 produces more carriers thanpristine MoS2 due to oxygen doping (Fig. 3a and b). The time-dependent relative PL intensity (I/I0) of the UV–ozone treatedMoS2 shows the ratio of B1.38 higher PL intensity, indicating

Fig. 2 (a) Raman spectra of the bilayer MoS2 films before/after UV–ozone.(b) PL spectra of the bilayer MoS2 with increasing UV–ozone treatment.(c) CS-STEM HAADF images after UV–ozone. The inset is an FFT patterncorresponding to hexagonal lattices. (d) EELS spectra of the bilayer MoS2

before/after UV–ozone treatment. (e and f) Mo 3d and S 2p XPS spectra ofthe bilayer MoS2 before/after UV–ozone treatment, respectively.

Fig. 3 (a and b) PL intensity variation of UV–ozone treated bilayer MoS2

depending on laser exposure time. l = 473 nm. (c) The ratio of in situ PLintensity (I) to initial PL intensity (I0) after 125 s laser exposure. (d) Emissionlife-time spectra of the bilayer MoS2 synthesized by MOCVD depending onUV–ozone treatment time.

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that more defects are generated on MoS2 basal planes (Fig. 3c).We also measured the emission lifetime of our MoS2 samplesusing typical time-resolved fluorescence confocal microscopytechniques with a 470 nm pulse laser (20 MHz, 30 mW, 16 psresolution) and avalanche photodiode (APD) detector. Thebilayer MoS2 in our experiment shows the exciton lifetime ofB1.34 ns at room temperature, which is similar to the theoreticalexciton lifetime calculated by density functional theory (DFT).46

Fig. 3d depicts that the carrier lifetime, which is maximized asthe dopant density is optimized by UV–ozone treatment, and finallydecreases (see also the ESI,† Table S1) because of degradation ofMoS2.47,50,51 We expect that the elongation of the carrier lifetimeinduced by defects will enhance catalytic reactivity.

In order to investigate the effects of grain size and oxygendoping on the catalytic performance of MoS2 basal planes,hydrogen evolving activity of bilayer MoS2 before and afterUV–ozone treatment was evaluated by cyclic voltammetry (CV)in 0.5 M sulfuric acid electrolyte using a rotating disk electrode(RDE). To diminish the contribution of non-faradaic current,CV curves were polarization-corrected. Based on PL intensitydepending on laser exposure time, we expected that UV–ozonetreatment for 5 seconds exhibited the optimal HER performancedue to its exciton lifetime and lattice variation characterization.Thus, the catalytic activity of MoS2 with a 5 second UV–ozoneprocess was compared with that of pristine MoS2. As shownin Fig. 4a, current density at �0.4 V vs. RHE was �3.82 and

�15.2 mA cm�2 for the pristine electrode and UV–ozone treatedelectrode, respectively. We found that the UV–ozone process onbilayer MoS2 synthesized by MOCVD markedly enhanced theHER electrocatalytic activity. The overpotential at the currentdensity of 10 mA cm�2 was 362.4 mV. These results representconsiderable catalytic activity among only the pure monolayerMoS2 basal plane excluding chemically exfoliated MoS2 combinedwith other materials and functional groups, as shown in Fig. 4b andESI,† Table S2. Tafel plots were obtained from the polarized-corrected CV curves (Fig. 4c). Tafel slopes for pristine and UV–ozonetreated MoS2 were measured as 231 and 135 mV dec�1, respectively.The lower value of the Tafel slope indicates that the kinetics ofelectrochemical HER occur briskly. Based on the electro-kineticstudy, UV–ozone treated MoS2 showed lower overpotential and Tafelslope compared to pristine MoS2 for HER due to defect generationand oxygen doping induced by the UV–ozone process.

As shown in Fig. 4d, catalytic stability was evaluated via bulkelectrolysis at �0.4 V vs. RHE. Nearly constant current densityof UV–ozone treated MoS2 was maintained at 10.4 mA cm�2 for3500 s, suggesting the excellent stability during HER catalysis.The superior catalytic activity as well as stability of the MOCVDbased bilayer MoS2 catalysts with UV–ozone treatment can beattributed to three aspects, as shown in Fig. 4e. First, very smallgrain size could give a possibility of active sites for the HER.Second, UV–ozone treatment provides edge sites and oxygenincorporation in MoS2, which enhances the intrinsic activityof MoS2 because of the increased conductivity by the initialUV–ozone treatment and reaction sites. Last, UV–ozone treatedbilayer MoS2 has good structural stability and exciton productioncompared with a UV–ozone treated monolayer, which shows poorstability and sensitivity to external circumstances like UV–ozone,O2 plasma, and annealing.29,30,48,49

Conclusions

In summary, we demonstrate the synthesis of highly polycrystalllineMoS2 bilayers with enhanced reactivity and stability by metal organicchemical vapor deposition (MOCVD) and their defect-engineering byUV/ozone, leading to a considerable increase of the HER perfor-mance. The time-dependent photoluminescence (PL) study revealsthat the PL lifetime and the intensity of UV/ozone treated MoS2 isincreased by B76% and B15%, respectively, compared to thepristine MoS2. This implies that the increase in exciton populationand PL lifetime would be the key feature to predict the higherperformance in the HER, which can be utilized to further under-stand the electrochemical and photoelectrochemical origin of thecatalytic effects in various transition metal dichalcogenide (TMDC)materials, which would be useful to develop high-performancecatalysts based on various two-dimensional materials that can besynthesized by MOCVD on a large scale.

Authors contributions

B. H. H. and Z. H. K. conceived and supervised the project. S. K.and J.-J. K. led the project. B. H. H., Z. H. K., K. T. N., S. K., J.-J. K.,

Fig. 4 (a) Polarized CV curves show the cathodic sweep of the first cycle.Enhanced activity is observed after UV–ozone treatment. (b) HER over-potential values of MoS2 electrocatalysts at 5 mA cm�2 are compared.References were focused on film types excluding multilayer MoS2 derivedfrom chemical exfoliation or composites with other materials. (c) Tafelplots show the improved electrochemical activity of the bilayer MoS2

catalyst with UV–ozone treatment. (d) Stability test for the bilayer MoS2

on glassy carbon. (e) The mechanism illustration of UV–ozone treatedMoS2 HER compared with pristine MoS2.

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H. S., and S. C. P. wrote the manuscript. J. B. P. and S.-P. C.carried out TEM imaging and analysis. S. K., Q. T. T., Y. J., andS. C. P. synthesized materials and assisted materials char-acterization. S. K. and J.-J. K. measured optical properties ofthe materials. H. S. and K. T. N. performed electrochemicalmeasurements.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Nano Material TechnologyDevelopment Program through the National Research Foundation ofKorea (NRF) funded by the Ministry of Science (2016M3A7B4910458)and the Industrial Strategic Technology Development Program(10079969, 10079974) funded by the Ministry of Trade, Industry &Energy (MOTIE, Korea). This work was also supported by theBioNano Health-Guard Research Center funded by the Ministry ofScience, ICT and Future Planning (MSIP) of Korea as Global FrontierProject, H-GUARD_2013M3A6B2078947 and by the NRF grant(NRF-2016M3A7B4909776, Nano R&D program).

Notes and references

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