due to its distinctive layer structure and electronic properties , the inorganic fullerene - like structure of ms2(m = mo , w ) has been attracting considerable attention and has been investigated extensively for various applications .
the properties of the metal disulfide nanomaterials with novel morphology are better than those of their bulk forms . ws2 and
mos2 nanomaterials have shown excellent properties and have potential applications in various fields , such as scanning probe microscopy , solid - state lubricants , heterogeneous catalysis , and electrochemical hydrogen storage . in order to investigate their properties and applications in these fields systematically ,
several methods have been reported to prepare nanoscale ms2 , such as gas  solid or gas phase reaction , pulsed laser deposition , a self - transformation process , and sulfidation of the moo3 nanofibers under a h2s / h2 steam at 350600c . however , almost all the methods are based on utilizing high temperature reactions or dangerous gases , such as h2 and h2s . on the other hand , the formation of nanosized mos2 or ws2 polyhedral or onion crystals is , similar to the case of carbon onion crystals , a rare event . from this point of view
, it is interesting and valuable to find a new large - scale approach to synthesize metal disulfide nanoparticles .
since the 1970s , lamellar disulfide has been intensively studied , especially via the intercalation technique [ 10 - 12 ] . during intercalation ,
the coordination patterns of lamellar disulfide change greatly , and numerous defects are introduced , which enable doping of other ions . on the other hand ,
after transition metal cations have entered the ws2 or mos2 matrix , there are changes in the bonding properties of the matrix and the s  m 
golub et al .   described room temperature preparation of new ternary metal sulfides mxmos2 ( m = cu , ag ) by exfoliation 
first , the mos2 powder is exfoliated into lamellae or monolayers by n - butyllithium to form lixmos2 .
they supposed that ternary metal sulfides mxmos2 could be obtained using ( mos2 ) single layers dispersed in aqueous media and salts of metals which could favor the formation of m  s rather than m 
thus , the ( mos2 ) single layers in the dispersions have henceforth to be considered not only as macroanions but also as reducing agents . for the mos2 , the negative charge of ( mos2 ) and consequently the cluster structure of s  mo
mo metal bonding can be effectively stabilized in restacked compounds by introduction into the structure of counterions covalently bonded to the matrix .
so it is possible the same reason for the ws2 , the introduction of cobalt ion into the matrix of ws2 can stabilize the ws2 .
utilized these bond changes and fabricated transition metal - doped disulfide nanotubes successfully by exfoliation 
intercalation through hydrothermal adulteration , using lamellar metal disulfide as the precursors . ws2 and mos2 could be electrode materials for rechargeable batteries due to the existence of van der waals forces across the gaps between the s  m
 s sheets , which can provide the space for guest reactants in intercalation reactions .
a few attempts have been made to investigate the electrochemical activity of ms2 ( m = mo , w , etc . )
for example , feng   reported that ws2 nanoflakes prepared by a rheological phase reaction had better lithium intercalation / deintercalation behavior than other forms of ws2 .
wang   prepared ws2 nanotubes and reported that the ws2 nanotube electrode delivered a lithium insertion capacity of 915 mah g , which is much higher than for ws2 powder .
as other approach to explore new electrode materials for li - battery , nanosized transition metal oxides have been attracting much attention in recent years .
reported the high performance as anode materials in lithium ion batteries for nanosized transition metal oxides such as nickel oxide , cobalt oxide , and iron oxide . among them , cobalt oxide demonstrated the best electrochemical properties . in this
regard , co - containing species can be promising intercalants to modify the lamellar disulfides . in this paper , bamboo - like co -
doped ws2 nanorods were synthesized on a large scale by an approach involving exfoliation  intercalation 
hydrothermal treatment , using lamellar tungsten disulfide as the precursor , and the morphology , chemical components , and raman spectrum are reported .
the cobalt - doped ws2 nanorods can reversibly store lithium with a capacity of 568 mah g over a voltage range of 0.013.0 v versus li / li and show good cycling stability . to the best of our knowledge ,
this is the first time that transition metal - doped ws2 nanorods have been synthesized and their applications in lithium ion batteries systematically investigated .
under n2 atmosphere at room temperature for a week , 8.2 g of ws2 powder was exfoliated by 2.9 m of n - butyllithium ( 15 ml ) in a container , rinsed in hexane six times , and vacuum dried at 80c overnight . then
, 0.6 g of lixws2 was put into a 50 ml autoclave with a teflon - lined inner wall , and 30 ml of 0.2 m cocl2 aqueous solution was added into the autoclave and stirred for 15 min .
the temperature of the autoclave was raised to 180c , and it was maintained at that temperature for 48 h before being cooled down to room temperature .
the products were filtrated , rinsed in deionized water several times , and dried at 80c overnight .
x - ray diffraction ( xrd ) was performed with a rigaku x - ray diffractometer at room temperature , using a quartz monochromator cu k radiation source (  = 0.1541 nm ) under a voltage of 40 kv and a current of 30 ma .
transmission electron microscopy ( tem ) was performed with a fei / tecnai g microscope . before characterization
, the samples were dispersed ultrasonically in ethanol and dropped onto a cu grid coated with a carbon membrane .
energy - dispersive x - ray spectroscopy ( edx ) was performed on an s-4800 scanning electron microscope .
raman spectra were collected under ambient conditions using a rm 2000 microscope confocal raman spectrometer equipped with a microscope ( renishaw plc , england ) .
electrochemical measurements were carried out using a 2032-type coin cell fabricated in an argon - filled glove box ( mbraun , germany ) .
the working electrode was fabricated in the ratio of 70:20:10 ( w / w ) active material / carbon black / polyvinylidene difluoride , while lithium foil served as counter and reference electrode .
the electrolyte was 1 m lipf6 dissolved in 1:1 ethylene carbonate ( ec ) and dimethyl carbonate ( dmc ) .
constant current charge / discharge cycling was conducted on a land battery tester ( ct2001a ) in the voltage range between 3.0 and 0.01 v. cyclic voltammetry ( cv ) was performed on a chi660c electrochemical workstation at a scan rate of 0.1 mv s.
under n2 atmosphere at room temperature for a week , 8.2 g of ws2 powder was exfoliated by 2.9 m of n - butyllithium ( 15 ml ) in a container , rinsed in hexane six times , and vacuum dried at 80c overnight . then
, 0.6 g of lixws2 was put into a 50 ml autoclave with a teflon - lined inner wall , and 30 ml of 0.2 m cocl2 aqueous solution was added into the autoclave and stirred for 15 min .
the temperature of the autoclave was raised to 180c , and it was maintained at that temperature for 48 h before being cooled down to room temperature .
the products were filtrated , rinsed in deionized water several times , and dried at 80c overnight .
x - ray diffraction ( xrd ) was performed with a rigaku x - ray diffractometer at room temperature , using a quartz monochromator cu k radiation source (  = 0.1541 nm ) under a voltage of 40 kv and a current of 30 ma .
transmission electron microscopy ( tem ) was performed with a fei / tecnai g microscope . before characterization
, the samples were dispersed ultrasonically in ethanol and dropped onto a cu grid coated with a carbon membrane .
energy - dispersive x - ray spectroscopy ( edx ) was performed on an s-4800 scanning electron microscope .
raman spectra were collected under ambient conditions using a rm 2000 microscope confocal raman spectrometer equipped with a microscope ( renishaw plc , england ) .
electrochemical measurements were carried out using a 2032-type coin cell fabricated in an argon - filled glove box ( mbraun , germany ) .
the working electrode was fabricated in the ratio of 70:20:10 ( w / w ) active material / carbon black / polyvinylidene difluoride , while lithium foil served as counter and reference electrode .
the electrolyte was 1 m lipf6 dissolved in 1:1 ethylene carbonate ( ec ) and dimethyl carbonate ( dmc ) .
constant current charge / discharge cycling was conducted on a land battery tester ( ct2001a ) in the voltage range between 3.0 and 0.01 v. cyclic voltammetry ( cv ) was performed on a chi660c electrochemical workstation at a scan rate of 0.1 mv s.
figure 1 shows the xrd patterns of the raw ws2 powder and the co - doped ws2 nanorods .
all the peaks are agreement with the bragg positions for a hexagonal structure ( jcpds card 08 - 0237 ) for both samples .
the co - doped ws2 nanorods , however , show significantly broadened peak shapes , indicating that the crystallite size is much smaller than that of the raw ws2 powder . the edx analysis ( fig .
2 ) indicates that these nanorods have a co : w : s atomic ratio of 3.54:31.4:58.03 . taking into account the error of the measurement ,
xrd patterns of the raw ws2 powder ( a ) and the co - doped ws2 sample ( b ) edx results on co - doped ws2 nanorods .
the silicon peak comes from silicon wafer supporting the sample figure 3a shows a tem image of the raw ws2 particles .
the selected area electron diffraction ( saed ) pattern reveals that the raw ws2 consists of single crystals of 2h - ws2 . as shown in fig .
3b , the morphology of the co - doped ws2 particles is that of bamboo - like nanorods with a diameter of about 20 nm and a length of around 200 nm , so the aspect ratio of these nanorods is about 10 .
the corresponding saed pattern reveals that the nanorods are polycrystalline 2h - ws2 , which is different from the raw ws2 particles .
the nanorods have somewhat irregular shape , in which the ends of a nanorod are smaller in diameter than its middle part , as shown in fig .
tem images of the raw ws2 ( a ) , the co - doped ws2 ( b ) , and the co - doped ws2 at higher magnification ( c , d ) .
the insets in a and b show the corresponding saed patterns the raman spectrum of the sample was collected using a he / ne laser with a wavelength of 632.8 nm , which is capable of supplying power of 1 mw . the raman spectrum of the co - doped ws2 nanorods displays two peaks around 348.6 and 419.6 cm , and no other additional peaks are observed , as shown in fig .
this result is similar to that for ws2 bulk material and is consistent with reference , indicating the local lattice structure of the co - doped nanorods is comparable with that of the bulk ws2 crystals in spite of its poor crystallinity .
raman spectrum of co - doped ws2 nanorods the electrochemical properties of the co - doped ws2 nanorods were compared with those of the raw ws2 powder .
figure 5a , 5b show cyclic voltammograms for the two samples . in the first cycle ,
a reduction peak is observed at ~0.55 v and an oxidation peak at ~2.4 v for both samples .
the reduction and the oxidation peaks correspond to li lithium intercalation / deintercalation behavior in ws2 for two samples , respectively , as described in eq .
. cyclic voltammograms of a coin cell ( vs. li ) for selected cycles of the raw ws2 electrode ( a ) , the co - doped ws2 electrode ( b ) , and the co - doped ws2 at different potential scan rates  ( v s ) ( c ) , ( d ) show dependence of the peak height on the square root of the potential scan rate for the oxidation peak in the second cycle , the reduction peak at 0.55 v has disappeared , while new reduction peaks appear in the potential range from 1.62.2 v. this change can be explained by the formation of a gel - like polymeric layer .
there was no significant change in the potentials of the redox peaks afterward for co - doped ws2 .
the main difference between two cyclic voltammograms is the relative change in the redox current with the cycle number ; the current of the redox peak for the co - doped ws2 electrode decreases more slowly than that of the raw ws2 electrode . as a result
, a better cycling performance can be obtained from the co - doped ws2 electrode , as shown in fig .
6a . typical first and second charge / discharge curves of the co - doped ws2 electrode ( a ) , typical cycling performance of the precursor ws2 and the as - prepared co - doped ws2 electrode ( b ) and coulombic efficiency versus cycle number of the raw and co - doped ws2 ( c ) .
current density : 50 ma g figure 5c shows cyclic voltammograms of the co - doped ws2 at potential scan rates ,  , from 20 to 500 v s. the relationship between the oxidation current peak values and  is depicted in fig .
the peak currents are proportional to  , which may indicate a semi - infinite diffusion mechanism on the electrode .
the relationship to peak current ip can be expressed by the classical sevcik equation : where n is the number of electrons per species reaction , a is the apparent surface area of the electrode , d0 is the diffusion coefficient of li in the solid state ,  is the scan rate , and c0 is the change in li concentration in the active material .
figure 6a shows the charge / discharge curves of the electrode containing the co - doped ws2 nanorods . in the first cycle , the ws2 electrode delivered a lithium insertion capacity of about 668 mah g when the discharge / charge current density was 50 ma g. the first discharge curve shows the insertion plateaus at 1.0 v and 0.75 v. in the second cycle , a lithium insertion plateau at ~2.2 v was observed , as well as a slope starting from 1.75 v and running down to the cut off voltage of 0.01 v. there is a clear distinction in the charge  discharge profile between 02 v ( ~300 mah g ) and 23 v ( ~250 mah g ) .
thus , this material can be regarded as a possible anode material for li - ion battery in the 02 v range and as a cathode material in li - metal cell technology .
the charge capacity obviously decreases with cycling , from 520 mah g for the first cycle to 287 mah g at the 40th cycle .
( the capacity retention is ~55% after 40 cycles . ) for the co - doped ws2 electrode , an irreversible capacity of 668 mah g is observed in the first cycle , along with a reversible capacity of 568 mah g , which is reduced to 446 mah g after 20 cycles and 380 mah g after 40 cycles .
the co - doped ws2 electrode shows a better cycling stability compared with the ws2 precursor electrode .
the first cycle coulombic efficiency of the raw ws2 is 81.5% , and it increases with the cycle number , reaching 98.2% by the 40th cycle . for the co - doped ws2 ,
the coulombic efficiency of the first cycle is 84.7% and over 98% from the 4th cycle , shown in fig .
the improved electrochemical performance of the co - doped ws2 sample may be attributed to the small particle size and its one - dimensional structure
. however , the charge / discharge capacities of the co - doped ws2 electrode are lower than those reported for ws2 nanoflakes   and ws2 nanotubes .
the formation mechanism for the nanorods is similar to the synthesis of transition metal - doped moxw1xs2 nanotubes .
first , the ws2 powder is exfoliated into lamellae or monolayers by n - butyllithium to form lixws2 , with the process introducing a large amount of defects , along with large changes in the coordination patterns of the lamellar disulfide , which enables doping with other ions . then
, the cocl2 aqueous solution is added into the above mixture , a dispersion of single - layer ws2 is formed by the reaction of lixws2 with water .
after co cations enter into the ws2 matrix , the bonding properties of the matrix and the bond angles of s  w  s are changed , especially on the sides of layers , which causes the lamellar structure to curl into bamboo - like nanorods .
the decreased capacity of this transition metal - doped ws2 electrode may be because the intercalation of transition metal ions into the ws2 matrix can decrease the distance between layers , which limits the amount of lithium ions electrochemically inserted into the co - doped ws2 .
based on previous research [ 19 - 21 ] , lithium ions intercalate into ws2 nanoclusters and defect sites in ws2 nanorods , but these sites are partly saturated with co , so the amount of intercalation of lithium ions will be decreased , and the capacity could thus also be decreased .
it can be deduced that for ws2 samples without transition metal - doping or mesoporous ws2 , this exfoliation  restacking method will both enhance the charge / discharge capacity and achieve better ws2 cycling stability .
further work is needed in order to further improve the electrochemical performance of ws2 samples via exfoliation and restacking , as well as other modifications .
bamboo - like co - doped ws2 nanorods were synthesized by an exfoliation  intercalation  hydrothermal method . ws2
co cations were doped into the ws2 matrix through a hydrothermal process in cocl2 aqueous solution .
this method is expected to be applied to other layered materials to obtain the desired morphologies .
the co - doped ws2 nanorods can reversibly store lithium with a capacity of 568 mah g over a voltage range of 0.013.0 v versus li / li .
the co - doped ws2 electrode shows much better cycling stability compared with the raw ws2 electrode .
this work was supported partially by the state key laboratory breeding base of green chemistry - synthesis technology , zhejiang university of technology and the priority research centers program through the national research foundation of korea ( nrf ) funded by the ministry of education , science and technology ( 2009 - 0094047 ) .
this article is distributed under the terms of the creative commons attribution noncommercial license which permits any noncommercial use , distribution , and reproduction in any medium , provided the original author(s ) and source are credited .
this article is distributed under the terms of the creative commons attribution noncommercial license which permits any noncommercial use , distribution , and reproduction in any medium , provided the original author(s ) and source are credited .
 cobalt - doped tungsten disulfide nanorods were synthesized by an approach involving exfoliation , intercalation , and the hydrothermal process , using commercial ws2 powder as the precursor and n - butyllithium as the exfoliating reagent . 
 xrd results indicate that the crystal phase of the sample is 2h - ws2 . 
 tem images show that the sample consists of bamboo - like nanorods with a diameter of around 20 nm and a length of about 200 nm . the co - doped ws2 nanorods exhibit the reversible capacity of 568 mah g1 in a voltage range of 0.013.0 v versus li / li+ . as an electrode material for the lithium battery , 
 the co - doped ws2 nanorods show enhanced charge capacity and cycling stability compared with the raw ws2 powder . 
Introduction
Experimental
Sample Preparation
Characterization
Results and Discussions
Conclusions
Acknowledgments
Open Access