Template conversion of MoO3 to MoS2 nanoribbons: synthesis and electrochemical properties

Laboratory of Solid State Chemistry (LQE Campinas (UNICAMP), Campinas, São Pa fraunhofer.de; oalves@iqm.unicamp.br Dept. Qúımica Fundamental, Instituto de Q Paulo, Brazil Laboratório de Materiais Funcionais Avan Universidade Federal do Ceará, Fortaleza, B † Electronic supplementary informa 10.1039/c8ra05988h ‡ Current address: Fraunhofer Institu Biotechnology IGB, Assistant Bio, Electro branch, Schulgasse 11a, 94 315 Straubing Cite this: RSC Adv., 2018, 8, 30346


Introduction
Materials with the capacity of simultaneously changing the electrical and optical properties upon insertion or removal of ions are known as electrochromic materials. These materials have promising applications such as electrochromic windows, lithium-ion batteries, catalysis and sensors. [1][2][3][4] For instance, tungsten oxides have been widely investigated for their electrochromic properties. 1 Likewise, molybdenum oxides also show pronounced electrochemical 5 and electrochromic properties. 1 Orthorhombic a-MoO 3 , for example, consists of a layered structure of covalently bonded MoO 6 octahedra connected at the edges and corners forming channels, 6 which allow the intercalation of ions to form molybdenum bronzes M x MoO 3 (M ¼ H + , Li + , Na + , K + , Mg 2+ ). 6 This ion insertion in the a-MoO 3 channels is accompanied by a change in coloration from pale to dark-blue. 7 Nanostructured a-MoO 3 with morphologies such as nanoparticles 8 and nanorods [9][10][11] are expected to have an improved electrochromic response due to their smaller diffusion path 3 and increased surface area. 5 Molybdenum suldes have been noted for their superior performance for energy storage applications such as lithium-ion batteries (LIB). [12][13][14] Different nanostructures including nanorods 15 and single layered MoS 2 (ref. 16) have been investigated with respect to their electrochemical properties. Therefore, several attempts of preparing uniform MoO 3 nanorods as precursors for MoS 2 with the same morphology have been reported. 15,[17][18][19][20][21][22][23] One of the methods to prepare nanostructured MoS 2 is the solid-gas reaction of bulk a-MoO 3 in an H 2 S/H 2 /N 2 atmosphere at 800 C. 24 The oxide nanoparticles are formed in situ in the gas phase and the oxide-to-sulde reaction takes place from the outer layer to the inner core of the oxide nanoparticles. 25 Consequently, the size and shape of the oxide normally determine the morphology of the nal sulde materials. [25][26][27][28] Hence, oxide nanorods usually generate sulde nanotubes (NT) whereas spherical nanoparticles generate inorganic fullerene-like (IF) MoS 2 . 28 Nevertheless, because the preparation of the molybdenum oxide nanoparticle takes place in the gas-phase at 800 C, 25 it is rather challenging to control the morphology of the oxide and consequently, that of the sulde. 24 Thus, for most of the solid-gas preparation methods described so far, a mixture of nanotubes and spherical fullerene-like MoS 2 nanoparticles is obtained. 28 Alternatively, several attempts of preparing uniform MoO 3 nanorods as precursors for MoS 2 with the same morphology have been reported. 15,[17][18][19][20][21][22][23] Hydrothermal treatment of sodium molybdate 15,17,18 or acidied ammonium heptamolybdate 22,23 led to uniform MoO 3 nanorods. Thermolysis of ammonium molybdate generated a mixture of spherical and rod-shaped MoO 3 . 21 These oxide nanoparticles were converted to MoS 2 using H 2 as a reducing agent and H 2 S 19-23 or S 15,17 as the sulfur source. However, many of the attempts of suldizing MoO 3 nanorods was not successful for the homogeneity of the nal product, resulting in mixtures of MoS 2 nanotubes and fullerene-like nanoparticles, 21 mixtures of MoS 2 nanorods with nanoparticles, 19 MoS 2 nanorods with an oxide core due to incomplete suldization 23 and in certain cases the sudization led to a complete loss of the original oxide morphology. 19,20,29 In this work, we contribute to the development of novel synthetic methods for the preparation of MoS 2 with a high morphological yield of nanoribbons. a-MoO 3 -NR synthesized by a hydrothermal method was used as the precursor for the synthesis of MoS 2 -NR at 800 C under a stream of H 2 S and H 2 / N 2 since the beginning of the heating ramp. With this strategy, we were able to retain the oxide nanoribbon morphology in the sulde product. A mechanism for the oxide-to-sulde conversion has been proposed. We investigated the electrochemical behavior toward lithium insertion and removal of both oxide and sulde nanostructures and we show that the MoO 3 nanoribbons have a pronounced electrochromic behavior, whereas MoS 2 present a reversible electrochemical behavior, making the oxide and sulde good candidates for electrochromic devices and LIBs, respectively.

Preparation of MoO 3 nanoribbons
MoO 3 nanoribbons were prepared by a previously described hydrothermal method. 30 In a typical procedure, 310 mg of previously prepared molybdic acid 31 was added to 0.7 mL of glacial acetic acid (Chemco, 99.7%) and 1.8 mL of deionized water in a 45 mL capacity Teon-sealed stainless steel autoclave. Hydrothermal treatment was performed at 180 C for 7 days. Aer cooling to room temperature, the product was ltered and washed stepwise with water, ethanol and ether. The pale-blue powder obtained was vacuum dried with a nal yield of 93%.

Preparation of MoS 2 nanoribbons
The prepared MoO 3 nanoribbons were dispersed carefully in a boat quartz plate and subsequently placed in a tubular furnace. Aer purging with N 2 (100 mL min À1 ), the gas stream was replaced by 5/95% H 2 /N 2 (96 mL min À1 ) and 99.9% H 2 S (6 mL min À1 ). Subsequently, the quartz tube was heated to 800 C with a heating ramp of 30 C min À1 and kept at the nal temperature for 30 min. Aer cooling to room temperature, a dark powder of MoS 2 was removed from the oven.

Electrochemical characterization
Glasses covered with a conductive coating of indium-tin oxide (ITO, Delta Technologies, sheet resistance 15-25 U sq À1 ) with dimensions of 7 Â 50 Â 0.7 mm were used as substrates for electrochemical measurements. A dispersion of the MoO 3 nanoribbons was prepared by sonicating 1 mg of MoO 3 in 1 mL of methanol for 10 min. Thin lms of the oxide were prepared by drop-casting this dispersion onto the substrates. A platinum sheet and silver wire were used as counter and quasi-reference electrode, respectively. The electrolyte was 1 mol L À1 LiClO 4 in propylene carbonate (PC).
Electrochemical measurements were carried out using an autolab PGSTAT 30 potentiostat/galvanostat (Eco Chemie). Simultaneous transmittance measurements were recorded at 660 nm using a solid-state light source (World Precision Instruments). The light passed through an electrochemical cell and was transported with optical bers to a photodiode amplier PDA1 (World Precision Instruments), linked to the ADC port in the potentiostat. UV-Vis spectra at different applied potentials were registered with an HP8453 Spectrophotometer.
The dispersion of MoS 2 was prepared by sonicating 1 mg of MoS 2 in 1 mL of acetonitrile. Thin lms were also prepared onto ITO by drop-casting. Electrochemical measurements of the MoS 2 lm were carried out in an argon-lled glove box hooked up to an autolab PGSTAT 30 (Eco Chemie). Lithium sheets were used as the reference and counter electrode in an electrolyte of 1 mol L À1 bis(triuoromethane)-sulfonimide lithium (LiTFSI) in PC.

Nanoparticles characterization
X-ray powder diffraction (XRD) patterns were obtained using a Shimadzu XRD7000 diffractometer, operating with CuKa radiation, at 30 mA and 40 kV and a 1 min À1 scan rate. Scanning electron microscope (SEM) images were obtained using a JEOL 6360LV instrument and transmission electron microscope (TEM) images were obtained using a Carl Zeiss CEM-902. Thermogravimety and differential thermal analysis (TG and DTA) were carried out using a TA equipment, model SDTQ600. Fourier transform infrared (FTIR) spectroscopy of the sample prepared as KBr wafers were recorded on a Bomen FTLA 2000 spectrophotometer. Each spectrum was measured with a total of 32 scans and a resolution of 4 cm À1 . Raman spectra were recorded at an ambient temperature on a Renishaw system 3000 Raman imaging microscope (ca. 1 mm spatial resolution) using a He-Ne laser (1.96 eV) with a 632.8 nm excitation line. The laser power density was optimized in order to avoid overheating of the nanoparticle samples by the laser beam.  (Fig. SI1 †) show a high morphological yield of nanoribbons ( Fig. 1a and b), with an average diameter of 150 nm and length of 3 to 8 mm (Fig. SI2 †). The nanostructures have an orthorhombic a-MoO 3 phase (Fig. 1c, Pbnm, ICDS 36167), which consists of corner and edge sharing MoO 6 octahedra chains, forming stacked layers held together by weak van der Waals forces. 32 The oxide crystals grow anisotropically, with a preferential orientation in the [010] direction, 33 as seen in the intense signals for the (0l0) diffraction planes.
During the hydrothermal synthesis, MoO 3 nanostructures precipitate from MoO 3 $2H 2 O dissolved in acidic media, without the formation of intermediate phases. 34 The complete conversion of MoO 3 $2H 2 O to a-MoO 3 is supported by the TG and DSC prole (Fig. 1d). The release of adsorbed water occurs until 100 C and the absence of any signal related to coordinated waterthat should appear up to 400 C (ref. 35) -conrms the formation of the stable orthorhombic structure.
The morphology of the MoO 3 nanoribbons is quite sensitive to the annealing temperature. At around 550-600 C the nanoribbons can collapse forming large plates. 36 The DTA curve shows an endothermic wave at around 550 C (Fig. 1c), which could be correlated with this morphology loss, also visible by Raman spectroscopy. 36 The collapsing of the nanoribbons morphology is a critical point for converting the oxides to suldes while maintaining the morphology. For a successful template suldization, a rst outer sulde layer must be formed below 500 C, as it will be discussed in the following session. Sublimation of the oxide starts aer 700 C, noticed by the abrupt weight loss and the endothermic peak at 795 C. Almost all of the oxide mass is volatilized at 850 C.
Both Raman and infrared spectra (Fig. 2) show characteristic stretching modes of the crystalline orthorhombic a-MoO 3 . 36-41 a-MoO 3 consists of distorted MoO 6 octahedra with the Mo-O bond length varying between 167 and 233 pm. 38 The oxygen atoms in the MoO 6 octahedra can be divided into three types: (i) terminal Mo-O from unshared oxygen, (ii) Mo 2 -O edge-shared oxygen in common with two or three octahedra, i.e. bound to two metal atoms and (iii) Mo 3 -O, an oxygen atom bound to 3 metals. 40,41 These three stretching modes are observed in both Raman and IR spectra (Fig. 2) (Fig. 3a and b) show that the nanoribbon morphology of the oxide precursor is retained in the nal product. The XRD pattern (Fig. 3c) resembles the characteristic reections for 2H-MoS 2 (hexagonal MoS 2 , space group P6 3 / Raman spectra of nanoribbons, nanotubes and bulk MoS 2 (ref. 43) showed that the nanoribbon Raman bands are shied downward and more similar to the spectrum of the bulk sulde when compared to the Raman spectrum of MoS 2 nanotubes. The wavenumbers, as well as the band relative intensities for the Raman spectrum of the prepared material (Fig. 3c) are consistent with the spectrum reported for MoS 2 nanoribbons. 43 The in-plane displacement E 1 2g of Mo-S is observed at 375 cm À1 whereas the out-of-plane A 1g has a very intense band at 403 cm À1 . 44 The weak band at 450 cm À1 corresponds to the second order zone-edge phonon 2LA(M) of MoS 2 . 44,45 Moreover, no bands of MoO 2 or MoO 3 were observed, consistently with the XRD observations.
The mechanism of suldization of MoO 3 to MoS 2 -IF and -NT described in the literature [24][25][26][27] consists of a gas-phase reaction involving three steps: rstly MoO 3 powder is sublimed at 800 C under N 2 atmosphere generating oxide nanoparticles (5-300 nm). Subsequently, the oxide vapor is reduced to MoO 3Àx under a ow of 5% H 2 /95% N 2 at 820 C. Finally, the sub-oxide is converted to sulde upon a stream of H 2 S at 840 C, generating MoS 2 inorganic fullerenes (IF) and/or nanotubes (NT). According to this mechanism, the size and shape of the original oxide nanoparticle is maintained aer the conversion to the sulde. 25,26 Yet, as this method for the preparation of the oxide nanoparticle takes place in the gas-phase, it is rather difficult to control the morphology of the oxide and consequently of the sulde. Hence, a mixture of IF and NT-MoS 2 is obtained.
In this work, we suggest a new synthetic approach for preparing MoS 2 with nanoribbon morphology. It involves rst preparing the precursor materialmolybdenum oxidealready with the desired nal morphology of nanoribbons. Then, we provide a sulfur source for the conversion of MoO 3 to MoS 2 from a stream of H 2 S, and we thermally treat the material heating from room temperature up to 800 C. By streaming H 2 S from the beginning of the heating ramp, the collapsing of the MoO 3 -NR is successfully avoided and MoS 2 with nanoribbons morphology is obtained (Fig. 4). The formation of an outer-shell of MoS 2 onto the MoO 3 -NR is likely to be formed immediately during heating of the oxide and hinders the coalescenceat 500 C (ref. 36)and the sublimation of the oxide nanoparticle. Experiments performed with an H 2 S stream that starts only when the heating treatment reaches 400 C exhibited loss of nanoribbon morphology (Fig. SI4 †), similarly to the results reported elsewhere for suldization of MoO 3 nanoribbons under H 2 S. 20 Our results suggest that the reaction takes place from the outer shell to the inner core of the nanoparticle. The reaction is limited by the slow diffusion of H 2 S into the oxide core 27 and the nanoribbon morphology of the oxide precursor is well maintained in the nal sulde (Fig. 4). MoS 2 nanoparticles obtained have 1D morphology, with 200 nm diameter and up to several mm lengths. The obtained 1D-MoS 2 have no oxide or hollow core, different from the reports where nanotubes 17,21 or nanorods with an oxide core 23 were produced from the suldization of molybdenum oxide nanorods. Starting at open circuit potential (+0.4 V) with a negative sweep direction, three well-dened reduction peaks are observed at À0.25, À0.37 and À0.65 V vs. Ag/Ag + (Fig. 5a). These cathodic peaks are characteristics for Li + intercalation into the molybdenum oxide structure, forming molybdenum bronze (Li x MoO 3 ). 6 In the anodic scan, de-intercalation of lithium is identied by the oxidation wave at À0.3 V, followed by two small anodic peaks at À0.1 and +0.05 V vs. Ag/Ag + . Multiple reduction peaks in a-MoO 3 lms are attributed to the insertion of Li + in distinct energetic sites from the oxide structure. 46,47 However, part of the inserted Li + ions can be trapped into the cavities of the layered structure 6 and, consequently, the intercalation/deintercalation redox reaction is not fully reversible. 7 In this case, the current diminishes in the subsequent voltammetric cycles, as observed from the rst to the third cycles (Fig. 5a). The coulombic efficiency is also quite low: 85, 70 and 52% from the rst to the third voltammetric cycle, respectively. The background CV of an ITO lm without MoO 3 shows that the electro-decomposition of the PC electrolyte in this potential region is minimal. Thus, the low efficiency should be mostly attributed to the irreversible formation of bronzes such as Li x Mo IV-VI O 3 . 7 This irreversible intercalation can be even more drastic with ions larger than Li + such as Mg 2+ . 6 Voltammetric experiments in a PC solution containing MgClO 4 (Fig. SI3 †) showed that the insertion of Mg 2+ is totally irreversible with no anodic peaks at all, due to the entrapment of Mg 2+ ions within the oxide interlayer space. 6 The electrochemical insertion of Li + with the formation of Li x MoO 3 bronzes (eqn (1)) is accompanied by a change in coloration from pale-blue MoO 3 to dark-blue Li x MoO 3 . 1 The absorbance change in the visible region was monitored as a function of the applied potential (Fig. 5b). When stepping the potential down, from +0.4 to À0.7 V, a signicant increase in absorbance is observed. This absorbance change is corresponding to the change of color in the oxide from pale to dark blue. Moreover, as already pointed out that the electrochemical lithium insertion is not totally reversible, the absorbance change is consistently not completely reversible when stepping back to the initial potential (+0.4 V).
The transmittance change at a single wavelength (660 nm) was monitored simultaneously with cyclic voltammetry (Fig. 4). I/E (Fig. 6a) and A/E (Fig. 6b) potentio-dynamic proles of a MoO 3 lm were started at +0.8 V with a negative sweep direction. A remarkable increase in absorption is observed at potentials more negative than À0.1 V (Fig. 6b). Scanning back to positive potentials causes the absorption to decrease, yet not to the starting point. This hysteresis can be correlated with an irreversible lattice expansion during insertion and kinetically slow de-intercalation of lithium during bleaching. 7 The differential curve of absorbance versus potential (dA/dt vs. E, Fig. 6c) shows two changes of optical density with potential, which take place simultaneously with two of the three cathodic peaks. The simultaneous coloration and electrochemical change, occurring at the same rate, indicate the  presence of two chromogenic species in the molybdenum oxide, which are correlated to the intercalation of Li + at a different energetic interlayer spacing of the MoO 6 octahedron layers. 7 As for the oxidation, only one chromogenic process is observed in the differential spectrum, which superposes the main oxidation wave at À0.35 V.
3.2.2 Electrochemical lithium intercalation in MoS 2 -NRs. MoS 2 has been investigated for its superior properties as an electrode for lithium-ion batteries. 48 Here, the voltammetric behavior of the synthesized MoS 2 -NR was investigated in a 1 mol L À1 LiTFSI/PC solution. Because this material showed sensitiveness to humidity, the electrochemical measurements of the sulde were performed in an argon-lled glove box.
The CV of the MoS 2 -NR (Fig. 7) starting at +1.2 V with a cathodic scan direction, shows two cathodic peaks rising at +0.7 and +0.3 V. On the anodic scan, two well-dened oxidation peaks are observed at +0.4 and +0.7 V, followed by a broad wave from +1.3 to +2.5 V centered at ca. +1.5 V. The coulombic efficiency of the lithium insertion/de-insertion is 95.5% (whole anodic over cathodic charge), which is considerably higher than that observed for lithium intercalation/de-intercalation in the molybdenum oxide (85%).
The electrochemical lithium insertion in MoS 2 -NRs is consistent with the CVs described in the literature for MoS 2 with different morphologies: nanorods, 15 nanoakes, 49 nano-owers, 50,51 MoS 2 /graphene composites, 52,53 mesoporous MoS 2 (ref. 54) and commercial bulk MoS 2 . 14 The rst cathodic peak at +0.7 V is attributed to the Li + insertion in the interlayer sites of MoS 2 forming Li x MoS 2 , 15 with the correspondent dislodging at +0.7 V. 15 The second and more negative peak at +0.3 V can be attributed to the formation of Mo and Li 2 S from Li x MoS 2 , as suggested from in situ XRD of bulk MoS 2 in 1 mol L À1 LiPF 6 1 : 1 ethylene carbonate : dimethyl carbonate, 14 with the correspondent anodic process taking place at +0.4 V. The small cathodic wave at +0.2 V could be attributed to the more negative reduction of Li 2 S to Li and S. 15   This journal is © The Royal Society of Chemistry 2018 a-MoO 3 nanoribbons with high purity of phase and morphology were prepared by a hydrothermal method from MoO 3 $2H 2 O. The molybdenum oxide was converted into MoS 2 retaining the same nanoribbons morphology by solid-gas reaction with H 2 and H 2 S. Streaming H 2 S from the beginning of the thermal treatment was a key factor in maintaining the nanoribbons morphology from the oxide in the nal MoS 2 product. A protective MoS 2 outer layer is formed in the oxide preventing the collapsing of the nanoribbons morphology. MoO 3 nanoribbons showed distinct electrochromic performance for Li + insertion, changing coloration from pale to dark blue upon changing the potential negatively. Spectroelectrochemical measurements with a laser in 660 nm showed the presence of two chromophores correspondent to three cathodic peaks and the formation of Li x MoO 3 . Lithium removal from the oxide is however not totally reversible and a hysteresis in absorption as well as in the charge consumed is observed. The voltammetric behavior of MoS 2 shows that lithium can be intercalated within the interlayer spacing this material forming Li x MoS 2 , which can be removed in the anodic scan. This template synthesis is a simple method to obtain MoS 2 nanoribbons with a controlled morphology (100% of nanoribbons).

Conflicts of interest
The authors declare no conicts of interest.