Damien Voiry | Rutgers, The State University of New Jersey (original) (raw)

Papers by Damien Voiry

Materials for Renewable and Sustainable Energy, 2015

Carbon dioxide (CO 2 ) is one of the main gases produced by human activity and is responsible for... more Carbon dioxide (CO 2 ) is one of the main gases produced by human activity and is responsible for the green house effect. Numerous routes for CO 2 capture and reduction are currently under investigation. Another approach to mitigate the CO 2 content in the atmosphere is to convert it into useful species such as hydrocarbon molecules that can be used for fuel. In this view, copper is one of the most interesting catalyst materials for CO 2 reduction due to its remarkable ability to generate hydrocarbon fuels. However, its utilization as an effective catalyst for CO 2 reduction is hampered by its oxidation and relatively high voltages. We have fabricated hybrid materials for CO 2 reduction by combining the activity of copper and the conductivity of reduced graphene oxide (rGO). Cu nanoparticles (CuNPs) deposited on rGO have demonstrated higher current density and lower overpotential compared to other copper-based electrodes that we have tested. The CuNPs on rGO also exhibit better stability, preserving their catalytic activity without degradation for several hours.

Chemical Society reviews, Jan 20, 2015

Transition metal dichalcogenides (TMDs) represent a family of materials with versatile electronic... more Transition metal dichalcogenides (TMDs) represent a family of materials with versatile electronic, optical, and chemical properties. Most TMD bulk crystals are van der Waals solids with strong bonding within the plane but weak interlayer bonding. The individual layers can be readily isolated. Single layer TMDs possess intriguing properties that are ideal for both fundamental and technologically relevant research studies. We review the structure and phases of single and few layered TMDs. We also describe recent progress in phase engineering in TMDs. The ability to tune the chemistry by choosing a unique combination of transition metals and chalcogen atoms along with controlling their properties by phase engineering allows new functionalities to be realized with TMDs.

Nature nanotechnology, Jan 23, 2015

Efficient intercalation of ions in layered materials forms the basis of electrochemical energy st... more Efficient intercalation of ions in layered materials forms the basis of electrochemical energy storage devices such as batteries and capacitors. Recent research has focused on the exfoliation of layered materials and then restacking the two-dimensional exfoliated nanosheets to form electrodes with enhanced electrochemical response. Here, we show that chemically exfoliated nanosheets of MoS2 containing a high concentration of the metallic 1T phase can electrochemically intercalate ions such as H(+), Li(+), Na(+) and K(+) with extraordinary efficiency and achieve capacitance values ranging from ∼400 to ∼700 F cm(-3) in a variety of aqueous electrolytes. We also demonstrate that this material is suitable for high-voltage (3.5 V) operation in non-aqueous organic electrolytes, showing prime volumetric energy and power density values, coulombic efficiencies in excess of 95%, and stability over 5,000 cycles. As we show by X-ray diffraction analysis, these favourable electrochemical propert...

Nanoscale, Jan 21, 2013

Chemical doping of graphene with small boron nitride (BN) domains has been shown to be an effecti... more Chemical doping of graphene with small boron nitride (BN) domains has been shown to be an effective way of permanently modulating the electronic properties in graphene. Herein we show a facile method of growing large area graphene doped with small BN domains on copper foils using a single step CVD route with methane, boric acid powder and nitrogen gas as the carbon, boron and nitrogen sources respectively. This facile and safe process avoids the use of boranes and ammonia. Optical microscopy confirmed that continuous films were grown and Raman spectroscopy confirmed changes in the electronic structure of the grown BN doped graphene. Using XPS studies we find that both B and N can be substituted into the graphene structure in the form of small BN domains to give a B-N-C system. A novel structure for the BN doped graphene is proposed.

Molybdenum disulfide (MoS2) multilayers with functional polyelectrolyte nanospacing layers are pr... more Molybdenum disulfide (MoS2) multilayers with functional polyelectrolyte nanospacing layers are presented. Taking advantage of the facile method of layer-by-layer (LbL) assembly, individual chemically exfoliated MoS2 layers are not only effectively isolated from interlayer coupling but also doped by functional polymeric layers. It is clearly demonstrated that MoS2 nanosheets separated by polymeric trilayers exhibit a much larger increase in photoluminescence (PL) as the number of layers is increased. The enhanced PL has been correlated to the ratio of excitons to trions with the type of polymeric spacers. Because uniform heterogeneous interfaces can be formed between various transition metal dichalcogenides and other soft materials, LbL assembly offers possibilities for further development in the solution-processable assemblies of two-dimensional materials.

Industrial, CVD multi-walled carbon nanotubes are shown to form true solutions by reduction with ... more Industrial, CVD multi-walled carbon nanotubes are shown to form true solutions by reduction with alkali metals and subsequent exposure to polar solvents such as DMSO. Addition of an electrophile such as bromo-decanoic acid allows functionalization of the tubes. The whole process, dissolution and functionalization includes no harsh treatment such as acid treatment and/or sonication and allows to saturate the outer surface of the MWCNTs with functional groups. Comparison is made with previous work on single-walled carbon nanotubes.

Covalent modifications of single-walled carbon nanotubes, while being useful for their manipulati... more Covalent modifications of single-walled carbon nanotubes, while being useful for their manipulation and functionalization, alter the electronic properties through disruption of the nanotube p electronic system. To avoid such negative impact, we demonstrate here that carbon nanotubes can be alkylated in a controlled manner by first preparing and isolating reduced nanotube salts with varied charge/C ratios. The reduction reaction, in the present work performed in THF with a K/naphthalene salt, is almost quantitative and the KC x salt can be isolated with a wide range of x values ranging from 10 to 370, while remaining soluble in DMSO. The reaction of these salt solutions with two alkyl bromide reagents yields functionalized SWCNTs, as demonstrated by combined IR, UV-vis-NIR and Raman spectroscopies, XPS and thermogravimetric measurements. In particular, quantification of the number of functional groups grafted shows a direct correlation with the charge/C ratio of the initial salts. Detailed analysis of Raman spectra confirms this control over the extent of covalent functionalization and shows it is not selective towards any type of SWCNT. This latter observation is ascribed to the isolation of the intermediate salts as solids, resulting in homogenization of charge densities on the reduced CNTs.

The oxygen reduction reaction (ORR)-one of the two half-reactions in fuel cells-is one of the bot... more The oxygen reduction reaction (ORR)-one of the two half-reactions in fuel cells-is one of the bottlenecks that has prevented fuel cells from finding a wide range of applications today. This is because ORR is inherently a sluggish reaction; it is also because inexpensive and sustainable ORR electrocatalysts that are not only efficient but also are based on earth-abundant elements are hard to come by. Herein we report the synthesis of novel carbon-based materials that can contribute to solving these challenges associated with ORR. Mesoporous oxygen- and nitrogen-doped carbons were synthesized from in situ polymerized mesoporous silica-supported polyaniline (PANI) by carbonization of the latter, followed by etching away the mesoporous silica template from it. The synthetic method also allowed the immobilization of different metals such as Fe and Co easily into the system. While all the resulting materials showed outstanding electrocatalytic activity toward ORR, the metal-free, PANI-derived mesoporous carbon (dubbed PDMC), in particular, exhibited the highest activity, challenging conventional paradigms. This unprecedented activity by the metal-free PDMC toward ORR was attributed to the synergetic activities of nitrogen and oxygen (or hydroxyl) species that were implanted in it by PANI/mesoporous silica during pyrolysis.

Composite materials: Tungsten disulfide and WS2 /reduced graphene oxide (WS2 /rGO) nanosheets wer... more Composite materials: Tungsten disulfide and WS2 /reduced graphene oxide (WS2 /rGO) nanosheets were fabricated by hydrothermal synthesis using tungsten chloride, thioacetamide, and graphene oxide (GO) as starting materials. The WS2 nanosheets are efficiently templated on the rGO layer. The WS2 /rGO hybrid nanosheets show much better electrocatalytic activity for the hydrogen evolution reaction than WS2 nanosheets alone.

Replacing rare and expensive metal catalysts with inexpensive and earth-abundant ones is currentl... more Replacing rare and expensive metal catalysts with inexpensive and earth-abundant ones is currently among the major goals of sustainable chemistry. Herein we report the synthesis of N-, O-, and S-tridoped, polypyrrolederived nanoporous carbons (NOSCs) that can serve as metal-free, selective electrocatalysts and catalysts for oxygen reduction reaction (ORR) and alcohol oxidation reaction (AOR), respectively. The NOSCs are synthesized via polymerization of pyrrole using (NH 4 ) 2 S 2 O 8 as oxidant and colloidal silica nanoparticles as templates, followed by carbonization of the resulting S-containing polypyrrole/ silica composite materials and then removal of the silica templates. The NOSCs exhibit good catalytic activity toward ORR with low onset potential and low Tafel slope, along with different electron-transfer numbers, or in other words, different ratios H 2 O/H 2 O 2 as products, depending on the relative amount of colloidal silica used as templates. The NOSCs also effectively catalyze AOR at relatively low temperature, giving good conversions and high selectivity.

2 Encapsulation of electronic devices based on organic materials that are prone to degradation ev... more 2 Encapsulation of electronic devices based on organic materials that are prone to degradation even under normal atmospheric conditions with hermetic barriers is crucial for increasing their lifetime.

Nanoscale, Jan 7, 2014

The production of two-dimensional rhenium disulfide (ReS2) nanosheets by exfoliation using lithiu... more The production of two-dimensional rhenium disulfide (ReS2) nanosheets by exfoliation using lithium intercalation is demonstrated. The vibrational and photoluminescence properties of the exfoliated nanosheets are investigated, and the local atomic structure is studied by scanning and transmission electron microscopy. The catalytic activity of the nanosheets in a hydrogen evolution reaction (HER) is also investigated. The electrochemical properties of the exfoliated ReS2 nanosheets include low overpotentials of ∼100 mV and low Tafel slopes of 75 mV dec(-1) for HER and are attributed to the atomic structure of the superlattice 1T' phase. The presence of bandgap photoluminescence demonstrates that the nanosheets retain their semiconducting nature. ReS2 nanosheets produced by this method provide unique photocatalytic properties that are superior to those of other two-dimensional systems.

Nature Materials, 2014

Ultrathin molybdenum disulphide (MoS2) has emerged as an interesting layered semiconductor becaus... more Ultrathin molybdenum disulphide (MoS2) has emerged as an interesting layered semiconductor because of its finite energy bandgap and the absence of dangling bonds. However, metals deposited on the semiconducting 2H phase usually form high-resistance (0.7 kΩ μm-10 kΩ μm) contacts, leading to Schottky-limited transport. In this study, we demonstrate that the metallic 1T phase of MoS2 can be locally induced on semiconducting 2H phase nanosheets, thus decreasing contact resistances to 200-300 Ω μm at zero gate bias. Field-effect transistors (FETs) with 1T phase electrodes fabricated and tested in air exhibit mobility values of ~50 cm(2) V(-1) s(-1), subthreshold swing values below 100 mV per decade, on/off ratios of >10(7), drive currents approaching ~100 μA μm(-1), and excellent current saturation. The deposition of different metals has limited influence on the FET performance, suggesting that the 1T/2H interface controls carrier injection into the channel. An increased reproducibility of the electrical characteristics is also obtained with our strategy based on phase engineering of MoS2.

Nature Materials, 2013

The ability to efficiently evolve hydrogen via electrocatalysis at low overpotentials holds treme... more The ability to efficiently evolve hydrogen via electrocatalysis at low overpotentials holds tremendous promise for clean energy [1-10]. Hydrogen evolution reaction (HER, 2H + + 2e -→ H 2 ) can be easily achieved from water if a voltage above the thermodynamic potential of the HER is applied. Large overpotentials are energetically inefficient but can be lowered with expensive platinum based catalysts. Replacement of Pt with inexpensive, earth abundant electrocatalysts would be significantly beneficial for clean and efficient hydrogen evolution. Towards this end, promising HER characteristics have been reported using 2H (trigonal prismatic) XS 2 (where X = Mo or W) nanoparticles with a high concentration of metallic edges [11] as electrocatalysts [3,4,6]. The key challenges for HER with XS 2 are increasing the number and catalytic activity of active sites [2]. Here we report atomically thin

Nano Letters, 2013

We report chemically exfoliated MoS2 nanosheets with a very high concentration of metallic 1T pha... more We report chemically exfoliated MoS2 nanosheets with a very high concentration of metallic 1T phase using a solvent free intercalation method. After removing the excess of negative charges from the surface of the nanosheets, highly conducting 1T phase MoS2 nanosheets exhibit excellent catalytic activity toward the evolution of hydrogen with a notably low Tafel slope of 40 mV/dec. By partially oxidizing MoS2, we found that the activity of 2H MoS2 is significantly reduced after oxidation, consistent with edge oxidation. On the other hand, 1T MoS2 remains unaffected after oxidation, suggesting that edges of the nanosheets are not the main active sites. The importance of electrical conductivity of the two phases on the hydrogen evolution reaction activity has been further confirmed by using carbon nanotubes to increase the conductivity of 2H MoS2.

Nature Chemistry, 2014

Chemical functionalization of low-dimensional materials such as nanotubes, nanowires and graphene... more Chemical functionalization of low-dimensional materials such as nanotubes, nanowires and graphene leads to profound changes in their properties and is essential for solubilizing them in common solvents. Covalent attachment of functional groups is generally achieved at defect sites, which facilitate electron transfer. Here, we describe a simple and general method for covalent functionalization of two-dimensional transition metal dichalcogenide nanosheets (MoS₂, WS₂ and MoSe₂), which does not rely on defect engineering. The functionalization reaction is instead facilitated by electron transfer between the electron-rich metallic 1T phase and an organohalide reactant, resulting in functional groups that are covalently attached to the chalcogen atoms of the transition metal dichalcogenide. The attachment of functional groups leads to dramatic changes in the optoelectronic properties of the material. For example, we show that it renders the metallic 1T phase semiconducting, and gives it strong and tunable photoluminescence and gate modulation in field-effect transistors.

Materials for Renewable and Sustainable Energy, 2015

Carbon dioxide (CO 2 ) is one of the main gases produced by human activity and is responsible for... more Carbon dioxide (CO 2 ) is one of the main gases produced by human activity and is responsible for the green house effect. Numerous routes for CO 2 capture and reduction are currently under investigation. Another approach to mitigate the CO 2 content in the atmosphere is to convert it into useful species such as hydrocarbon molecules that can be used for fuel. In this view, copper is one of the most interesting catalyst materials for CO 2 reduction due to its remarkable ability to generate hydrocarbon fuels. However, its utilization as an effective catalyst for CO 2 reduction is hampered by its oxidation and relatively high voltages. We have fabricated hybrid materials for CO 2 reduction by combining the activity of copper and the conductivity of reduced graphene oxide (rGO). Cu nanoparticles (CuNPs) deposited on rGO have demonstrated higher current density and lower overpotential compared to other copper-based electrodes that we have tested. The CuNPs on rGO also exhibit better stability, preserving their catalytic activity without degradation for several hours.

Chemical Society reviews, Jan 20, 2015

Transition metal dichalcogenides (TMDs) represent a family of materials with versatile electronic... more Transition metal dichalcogenides (TMDs) represent a family of materials with versatile electronic, optical, and chemical properties. Most TMD bulk crystals are van der Waals solids with strong bonding within the plane but weak interlayer bonding. The individual layers can be readily isolated. Single layer TMDs possess intriguing properties that are ideal for both fundamental and technologically relevant research studies. We review the structure and phases of single and few layered TMDs. We also describe recent progress in phase engineering in TMDs. The ability to tune the chemistry by choosing a unique combination of transition metals and chalcogen atoms along with controlling their properties by phase engineering allows new functionalities to be realized with TMDs.

Nature nanotechnology, Jan 23, 2015

Efficient intercalation of ions in layered materials forms the basis of electrochemical energy st... more Efficient intercalation of ions in layered materials forms the basis of electrochemical energy storage devices such as batteries and capacitors. Recent research has focused on the exfoliation of layered materials and then restacking the two-dimensional exfoliated nanosheets to form electrodes with enhanced electrochemical response. Here, we show that chemically exfoliated nanosheets of MoS2 containing a high concentration of the metallic 1T phase can electrochemically intercalate ions such as H(+), Li(+), Na(+) and K(+) with extraordinary efficiency and achieve capacitance values ranging from ∼400 to ∼700 F cm(-3) in a variety of aqueous electrolytes. We also demonstrate that this material is suitable for high-voltage (3.5 V) operation in non-aqueous organic electrolytes, showing prime volumetric energy and power density values, coulombic efficiencies in excess of 95%, and stability over 5,000 cycles. As we show by X-ray diffraction analysis, these favourable electrochemical propert...

Nanoscale, Jan 21, 2013

Chemical doping of graphene with small boron nitride (BN) domains has been shown to be an effecti... more Chemical doping of graphene with small boron nitride (BN) domains has been shown to be an effective way of permanently modulating the electronic properties in graphene. Herein we show a facile method of growing large area graphene doped with small BN domains on copper foils using a single step CVD route with methane, boric acid powder and nitrogen gas as the carbon, boron and nitrogen sources respectively. This facile and safe process avoids the use of boranes and ammonia. Optical microscopy confirmed that continuous films were grown and Raman spectroscopy confirmed changes in the electronic structure of the grown BN doped graphene. Using XPS studies we find that both B and N can be substituted into the graphene structure in the form of small BN domains to give a B-N-C system. A novel structure for the BN doped graphene is proposed.

Molybdenum disulfide (MoS2) multilayers with functional polyelectrolyte nanospacing layers are pr... more Molybdenum disulfide (MoS2) multilayers with functional polyelectrolyte nanospacing layers are presented. Taking advantage of the facile method of layer-by-layer (LbL) assembly, individual chemically exfoliated MoS2 layers are not only effectively isolated from interlayer coupling but also doped by functional polymeric layers. It is clearly demonstrated that MoS2 nanosheets separated by polymeric trilayers exhibit a much larger increase in photoluminescence (PL) as the number of layers is increased. The enhanced PL has been correlated to the ratio of excitons to trions with the type of polymeric spacers. Because uniform heterogeneous interfaces can be formed between various transition metal dichalcogenides and other soft materials, LbL assembly offers possibilities for further development in the solution-processable assemblies of two-dimensional materials.

Industrial, CVD multi-walled carbon nanotubes are shown to form true solutions by reduction with ... more Industrial, CVD multi-walled carbon nanotubes are shown to form true solutions by reduction with alkali metals and subsequent exposure to polar solvents such as DMSO. Addition of an electrophile such as bromo-decanoic acid allows functionalization of the tubes. The whole process, dissolution and functionalization includes no harsh treatment such as acid treatment and/or sonication and allows to saturate the outer surface of the MWCNTs with functional groups. Comparison is made with previous work on single-walled carbon nanotubes.

Covalent modifications of single-walled carbon nanotubes, while being useful for their manipulati... more Covalent modifications of single-walled carbon nanotubes, while being useful for their manipulation and functionalization, alter the electronic properties through disruption of the nanotube p electronic system. To avoid such negative impact, we demonstrate here that carbon nanotubes can be alkylated in a controlled manner by first preparing and isolating reduced nanotube salts with varied charge/C ratios. The reduction reaction, in the present work performed in THF with a K/naphthalene salt, is almost quantitative and the KC x salt can be isolated with a wide range of x values ranging from 10 to 370, while remaining soluble in DMSO. The reaction of these salt solutions with two alkyl bromide reagents yields functionalized SWCNTs, as demonstrated by combined IR, UV-vis-NIR and Raman spectroscopies, XPS and thermogravimetric measurements. In particular, quantification of the number of functional groups grafted shows a direct correlation with the charge/C ratio of the initial salts. Detailed analysis of Raman spectra confirms this control over the extent of covalent functionalization and shows it is not selective towards any type of SWCNT. This latter observation is ascribed to the isolation of the intermediate salts as solids, resulting in homogenization of charge densities on the reduced CNTs.

The oxygen reduction reaction (ORR)-one of the two half-reactions in fuel cells-is one of the bot... more The oxygen reduction reaction (ORR)-one of the two half-reactions in fuel cells-is one of the bottlenecks that has prevented fuel cells from finding a wide range of applications today. This is because ORR is inherently a sluggish reaction; it is also because inexpensive and sustainable ORR electrocatalysts that are not only efficient but also are based on earth-abundant elements are hard to come by. Herein we report the synthesis of novel carbon-based materials that can contribute to solving these challenges associated with ORR. Mesoporous oxygen- and nitrogen-doped carbons were synthesized from in situ polymerized mesoporous silica-supported polyaniline (PANI) by carbonization of the latter, followed by etching away the mesoporous silica template from it. The synthetic method also allowed the immobilization of different metals such as Fe and Co easily into the system. While all the resulting materials showed outstanding electrocatalytic activity toward ORR, the metal-free, PANI-derived mesoporous carbon (dubbed PDMC), in particular, exhibited the highest activity, challenging conventional paradigms. This unprecedented activity by the metal-free PDMC toward ORR was attributed to the synergetic activities of nitrogen and oxygen (or hydroxyl) species that were implanted in it by PANI/mesoporous silica during pyrolysis.

Composite materials: Tungsten disulfide and WS2 /reduced graphene oxide (WS2 /rGO) nanosheets wer... more Composite materials: Tungsten disulfide and WS2 /reduced graphene oxide (WS2 /rGO) nanosheets were fabricated by hydrothermal synthesis using tungsten chloride, thioacetamide, and graphene oxide (GO) as starting materials. The WS2 nanosheets are efficiently templated on the rGO layer. The WS2 /rGO hybrid nanosheets show much better electrocatalytic activity for the hydrogen evolution reaction than WS2 nanosheets alone.

Replacing rare and expensive metal catalysts with inexpensive and earth-abundant ones is currentl... more Replacing rare and expensive metal catalysts with inexpensive and earth-abundant ones is currently among the major goals of sustainable chemistry. Herein we report the synthesis of N-, O-, and S-tridoped, polypyrrolederived nanoporous carbons (NOSCs) that can serve as metal-free, selective electrocatalysts and catalysts for oxygen reduction reaction (ORR) and alcohol oxidation reaction (AOR), respectively. The NOSCs are synthesized via polymerization of pyrrole using (NH 4 ) 2 S 2 O 8 as oxidant and colloidal silica nanoparticles as templates, followed by carbonization of the resulting S-containing polypyrrole/ silica composite materials and then removal of the silica templates. The NOSCs exhibit good catalytic activity toward ORR with low onset potential and low Tafel slope, along with different electron-transfer numbers, or in other words, different ratios H 2 O/H 2 O 2 as products, depending on the relative amount of colloidal silica used as templates. The NOSCs also effectively catalyze AOR at relatively low temperature, giving good conversions and high selectivity.

2 Encapsulation of electronic devices based on organic materials that are prone to degradation ev... more 2 Encapsulation of electronic devices based on organic materials that are prone to degradation even under normal atmospheric conditions with hermetic barriers is crucial for increasing their lifetime.

Nanoscale, Jan 7, 2014

The production of two-dimensional rhenium disulfide (ReS2) nanosheets by exfoliation using lithiu... more The production of two-dimensional rhenium disulfide (ReS2) nanosheets by exfoliation using lithium intercalation is demonstrated. The vibrational and photoluminescence properties of the exfoliated nanosheets are investigated, and the local atomic structure is studied by scanning and transmission electron microscopy. The catalytic activity of the nanosheets in a hydrogen evolution reaction (HER) is also investigated. The electrochemical properties of the exfoliated ReS2 nanosheets include low overpotentials of ∼100 mV and low Tafel slopes of 75 mV dec(-1) for HER and are attributed to the atomic structure of the superlattice 1T' phase. The presence of bandgap photoluminescence demonstrates that the nanosheets retain their semiconducting nature. ReS2 nanosheets produced by this method provide unique photocatalytic properties that are superior to those of other two-dimensional systems.

Nature Materials, 2014

Ultrathin molybdenum disulphide (MoS2) has emerged as an interesting layered semiconductor becaus... more Ultrathin molybdenum disulphide (MoS2) has emerged as an interesting layered semiconductor because of its finite energy bandgap and the absence of dangling bonds. However, metals deposited on the semiconducting 2H phase usually form high-resistance (0.7 kΩ μm-10 kΩ μm) contacts, leading to Schottky-limited transport. In this study, we demonstrate that the metallic 1T phase of MoS2 can be locally induced on semiconducting 2H phase nanosheets, thus decreasing contact resistances to 200-300 Ω μm at zero gate bias. Field-effect transistors (FETs) with 1T phase electrodes fabricated and tested in air exhibit mobility values of ~50 cm(2) V(-1) s(-1), subthreshold swing values below 100 mV per decade, on/off ratios of >10(7), drive currents approaching ~100 μA μm(-1), and excellent current saturation. The deposition of different metals has limited influence on the FET performance, suggesting that the 1T/2H interface controls carrier injection into the channel. An increased reproducibility of the electrical characteristics is also obtained with our strategy based on phase engineering of MoS2.

Nature Materials, 2013

The ability to efficiently evolve hydrogen via electrocatalysis at low overpotentials holds treme... more The ability to efficiently evolve hydrogen via electrocatalysis at low overpotentials holds tremendous promise for clean energy [1-10]. Hydrogen evolution reaction (HER, 2H + + 2e -→ H 2 ) can be easily achieved from water if a voltage above the thermodynamic potential of the HER is applied. Large overpotentials are energetically inefficient but can be lowered with expensive platinum based catalysts. Replacement of Pt with inexpensive, earth abundant electrocatalysts would be significantly beneficial for clean and efficient hydrogen evolution. Towards this end, promising HER characteristics have been reported using 2H (trigonal prismatic) XS 2 (where X = Mo or W) nanoparticles with a high concentration of metallic edges [11] as electrocatalysts [3,4,6]. The key challenges for HER with XS 2 are increasing the number and catalytic activity of active sites [2]. Here we report atomically thin

Nano Letters, 2013

We report chemically exfoliated MoS2 nanosheets with a very high concentration of metallic 1T pha... more We report chemically exfoliated MoS2 nanosheets with a very high concentration of metallic 1T phase using a solvent free intercalation method. After removing the excess of negative charges from the surface of the nanosheets, highly conducting 1T phase MoS2 nanosheets exhibit excellent catalytic activity toward the evolution of hydrogen with a notably low Tafel slope of 40 mV/dec. By partially oxidizing MoS2, we found that the activity of 2H MoS2 is significantly reduced after oxidation, consistent with edge oxidation. On the other hand, 1T MoS2 remains unaffected after oxidation, suggesting that edges of the nanosheets are not the main active sites. The importance of electrical conductivity of the two phases on the hydrogen evolution reaction activity has been further confirmed by using carbon nanotubes to increase the conductivity of 2H MoS2.

Nature Chemistry, 2014

Chemical functionalization of low-dimensional materials such as nanotubes, nanowires and graphene... more Chemical functionalization of low-dimensional materials such as nanotubes, nanowires and graphene leads to profound changes in their properties and is essential for solubilizing them in common solvents. Covalent attachment of functional groups is generally achieved at defect sites, which facilitate electron transfer. Here, we describe a simple and general method for covalent functionalization of two-dimensional transition metal dichalcogenide nanosheets (MoS₂, WS₂ and MoSe₂), which does not rely on defect engineering. The functionalization reaction is instead facilitated by electron transfer between the electron-rich metallic 1T phase and an organohalide reactant, resulting in functional groups that are covalently attached to the chalcogen atoms of the transition metal dichalcogenide. The attachment of functional groups leads to dramatic changes in the optoelectronic properties of the material. For example, we show that it renders the metallic 1T phase semiconducting, and gives it strong and tunable photoluminescence and gate modulation in field-effect transistors.