Article pubs.acs.org/JPCC
Template Electrochemical Growth and Properties of Mo Oxide Nanostructures Letteria Silipigni,† Francesco Barreca,† Enza Fazio,† Fortunato Neri,† Tiziana Spanò,‡ Salvatore Piazza,‡ Carmelo Sunseri,‡ and Rosalinda Inguanta*,‡ †
Dipartimento di Fisica e di Scienza della Terra, Università di Messina, Viale F. Stagno D’Alcontres, 98166 Messina, Italy Laboratorio di Chimica Fisica Applicata, Dipartimento di Ingegneria Chimica Gestionale Informatica Meccanica, Università di Palermo, Viale delle Scienze, 90128 Palermo, Italy
‡
ABSTRACT: This work is aimed at studying the growing process of nanostructures electrodeposited from molybdate aqueous solutions at different pH values into pores of polycarbonate membrane templates. The challenging issue was the opportunity to investigate a rather complex deposition process in a confined ambient, where electrochemical conditions are quite different from those usually established for deposition on a flat substrate. Nanostructures were grown from a bath containing Mo7O246− (NH4)6Mo7O24·4H2O) at different concentrations (50−100 g/L), at a constant cathodic current density of 2 mA/cm2 (electrodeposition area ∼8 cm2). Nanostructured deposit was characterized by XRD, EDS, Raman, XPS, and photoelectrochemical spectroscopy. Several parameters have been investigated, such as electrodeposition time, concentration of Mo precursor, and pH. These parameters do not influence the nature of deposit that always consists of mixed valence molybdenum oxides, whereas the nanostructure morphology changes with pH. Nanostructures were found amorphous and consisting mainly of MoO2 underneath α-MoO3. Photoelectrochemical spectroscopy revealed n-type conductivity and two linear regions in the (Iph·hν)0.5 vs hν plot, giving optical gaps of 2.5 and 3.2 eV, typical of MoO2 and αMoO3, respectively. To our best knowledge, this is the first paper dealing with the fabrication of a uniform array of MoO2/MoO3 core−shell nanowires by template electrosynthesis.
1. INTRODUCTION Mo and Mo oxides have widespread applications in several technological fields.1−3 Due to its specific properties (high thermal and electrical conductivity, low coefficient of thermal expansion, strong bonding with glass), molybdenum metal is extensively used in lighting, electrical and electronic devices, medical equipment, materials processing equipment, thermal spray coatings, and aerospace components.2 Besides, Mo thin film is used as back contact in CIGS and CZTS based solar cells, owing to its low electrical resistivity and high thermal stability. In addition, reaction of Mo with Se to form a thin MoSe2 layer improves electrical contact4 with CGIS or CZTS absorbers. Also the two stable molybdenum oxides, i.e. MoO2 and MoO3, find extensive technological applications because of their structural, electronic, and optical properties. For instance, they are used in gas sensors for NOx, CO, H2, and NH3, in Liion battery, in catalysis and electrocatalysis, and in large area displays and smart windows.5 In particular, MoO2 is an n-type semiconductor with a gap of about 2.5 eV and it is a promising material for optoelectronic devices, photocatalysis, and photovoltaic and photoelectrochemical cells.6,7 Besides, MoO2 shows good performance in electrochemical supercapacitors5 where specific capacity can be stored as high as 477 F g−1; this value can be enhanced up to 597 F g−1 using composite electrodes of MoO2/single-walled nanotubes.8 Also the applications of MoO3 are of high interest. For instance, Hariharan et al.9 tested this © XXXX American Chemical Society
oxide as anode material for sodium-ion batteries with good results. Recently, high-efficiency inverted tandem polymer solar cells with an Al doped MoO3 interconnection layer have been proposed by Liu et al.10 A MoO3 thin film was also used as hole-injection layer (HIL) for poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl-C61-butyric acid methyl ester (PC61BM) based organic bulk heterojunction photovoltaic.11 The specific properties of Mo and MoOx are enhanced at a nanosize scale. Zach et al.12 showed that Mo nanowires have good conductivity and strong mechanical resiliency. Nanostructured MoO2 has excellent field-emission characteristics and can be used as field-emitter and scanning tunneling microscopy tip.13 Also the exceptional catalytic properties of MoO3 nanowires were scrutinized.14 Considering the valuable properties of Mo and its oxides, and their potential technological applications, it is of high interest to develop a fabrication method that is simple to conduct and easily scalable. From this point of view, electrochemical deposition is a very suitable technique because it satisfies both these requirements, as confirmed by its extensive use in thin film and nanostructure synthesis.15,16 In the case of Mo, the electrodeposition from aqueous electrolytes is rather difficult,owing to (i) the quite negative standard reduction Received: June 11, 2014 Revised: August 25, 2014
A
dx.doi.org/10.1021/jp505819j | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
(NH4)6Mo7O24·4H2O) at different concentrations (50−100 g L−1). The process was conducted for different times at a constant cathodic current density of 2 mA/cm2 (electrodeposition area ∼8 cm2) using a PAR Potentiostat/Galvanostat (mod. PARSTAT 2273). It was not possible to employ higher deposition current because, according to refs 19 and 23, electrodeposition occurs under vigorous hydrogen evolution. This last must be limited because not only does it severely reduce deposition efficiency but it also causes detachment of the polycarbonate membrane from the Ni layer. Solution pH was adjusted at 2.7, 5.5, and 8.5 by addition of acetic acid, sulfuric acid, and sodium hydroxide. The last two solutions were colorless, while that at pH 2.7 had a light-blue color, probably due to the formation of Mo2O5·xMoO3, which is a blue and very soluble in water compound.28 All baths were prepared in ultrapure distilled water (18 MΩ cm) by using an AQUA Max system (JOUNGLIN, Basic 360 and Ultra 370). For each experiment, a freshly prepared solution was employed. After deposition, nanostructures were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), RAMAN spectroscopy, X-ray photoelectron spectroscopy (XPS), and photoelectrochemical measurements. The crystallographic structure of the nanostructures was investigated by X-ray diffractometry (Italstructures, mod. APD2000). Morphology was investigated using a scanning electron microscope (FEI-QUANTA 200) equipped with an Xray energy dispersive spectrometer (EDS). SEM analysis was carried out after dissolution of the polycarbonate membrane in pure CHCl3. The elemental composition of the nanostructures was analyzed by (EDS). Raman spectra were obtained at room temperature using a Renishaw (inVia Raman Microscope) spectrometer. The excitation was provided by the 633 nm line of a He:Ne laser. To verify sample uniformity, micro-Raman spectroscopy was performed on several points. Before analysis, the system was calibrated by means of the Raman peak of polycrystalline Si (520 cm−1). XPS analysis, used to investigate the changes in the molybdenum oxide nanostructure elemental composition from surface down to deeper layers, was performed at room temperature in a Thermo Scientific instrument (K-Alpha X-ray Photoelectron Spectrometer System) equipped with a monochromatic Al Kα source (hν = 1486.6 eV) and an hemispherical analyzer (spherical sector 180°) operating in the constant-pass energy (CAE) mode. The constant-pass energy was set at 200 eV for survey scans and at 50 eV for the XPS core level spectra. Progressive removal of material layers was carried out using a scanning 3 keV Ar+ ion gun, with a raster area of about 4 mm × 2 mm, until no substantial variations in the surface chemical composition were observed. XPS spectra were taken in ultrahigh-vacuum conditions (≈10−9 Torr). During XPS spectra acquisition, the X-ray source spot size was set at 400 μm. Etching cycles of 10, 30, 90, and 270 s were performed on samples. After every etching level, the ion beam was blanked, and after a 60 s time, a set of XPS spectra (survey scans and core level spectra) was recorded to detect all possible elements. A charge neutralizer (low energy e-beam 0.2 V at 100 μA) was used for charge compensation during the XPS spectra acquisition, and if necessary, binding energy shifts were calibrated keeping the C 1s position fixed at 285.0 eV. For the deconvolution of the XPS spectra, a mixed Gaussian− Lorentzian sum function with inelastic Shirley-type background
potential, (ii) multiple valence states, and (iii) complex oxyanion solution chemistry.1 Many papers describe the electrodeposition of Mo metal from nonaqueous electrolyte,17 while results concerning the use of aqueous electrolyte are rather contradictory. Several authors report the formation of a mixed oxide,12,18,19 in contrast with a few works showing the formation of Mo metal.20,21 Ivanova et al.22 claim the electrodeposition of 3−5 μm Mo films, without giving any experimental evidence of the metallic phase formation, while Morley e al.23 showed the formation of a ∼20 μm thick Mo film, with very low efficiency (more than 98% of the deposition current was lost for H2 evolution). Very interesting is the approach followed by Falola et al.,24 who used a simple galvanic deposition to obtain a 9 μm thick Mo film. On the contrary, the electrochemical deposition of Mo oxides is very simple,5,6,12,18,19 even if the literature does not clarify fully the stoichiometry of the electrodeposited product. Most of the previous works revealed mixed valence molybdenum oxide formation, together with the presence of nonstoichiometric oxides.1 This work is focused on the electrochemical deposition from molybdate aqueous solutions at different pH values to grow nanostructures into pores of polycarbonate membrane templates. The relevance of this investigation concerns the study of a rather complex deposition process in a confined ambient, i.e. inside template nanochannels, where electrochemical conditions are quite different from those usually established for deposition on a flat substrate. In particular, this work was undertaken for investigating the possibility to deposit in a confined ambient a regular array of Mo metal nanowires to be used as back contact for arrays of CIGS25 and CZTS nanowire26 absorbers in photovoltaic cells. Here, we will show that MoO2/MoO3 nanowires are obtained under all the experimental conditions. This finding appears interesting because, as reported above, Mo oxides have many appealing applications, especially in the nanostructured form. To our best knowledge, this work is the first paper reporting the fabrication of a uniform array of MoOx nanowires by template electrodeposition. Up to date, only dispersed nanowires14 and nanofibers,27 nanorods,13 or flattened wires12,18 were proposed. In this work, different characterization techniques have been employed, all showing formation of ordered arrays of Mo oxide nanostructures.
2. EXPERIMENTAL SECTION Commercial track-etched polycarbonate membrane (Whatman, Cyclopore 47), having a nominal pore diameter and a thickness of 200 nm and 20 μm, respectively, and a pore density of about 1012 pores m−2, was used as template. Initially, a gold layer was sputtered (EMITECH, mod. K575X) onto a surface of the membrane in order to make it conductive; then a Ni film was electrodeposited on the gold, and finally nanowires were grown into the polycarbonate channels. Ni film plays a key role because it acts both as current collector and mechanical support for nanowires. It was deposited potentiostatically at −1.25 V vs a standard calomel electrode (SCE, E° = +0.24 V vs normal hydrogen electrode (NHE)) on an area of about 11 cm2, up to a thickness of about 35 μm (about 90 min) from a Watt’s bath at room temperature. Electrodeposition was conducted in a three electrode cell with a Pt wire and SCE as counter and reference electrodes, respectively. Nanostructures were grown according to Ivanova et al.22 by electrodeposition from a bath containing Mo 7 O 24 6− B
dx.doi.org/10.1021/jp505819j | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
Figure 1. FEG-ESEM images, taken after template dissolution, of Mo oxide nanowires obtained in an unstirred solution with 50 g L−1 (NH4)6Mo7O24 and H2SO4 at pH 5.5 for 30 min: (a) cross-sectional view, (b) top-view, (c) tilted-view, and (d) high-magnification of the top-view.
was used. Then the so-determined photoemission peak areas were converted into atomic concentrations using Scofield sensitivity factors.29 Photoelectrochemical investigation was carried out in a 0.1 M Na2SO4 solution (pH = 5.6) at room temperature, using a cell having flat quartz windows for sample illumination. The experimental setup is described elsewhere.25 A three-electrode cell was employed, with a platinum wire as counter electrode and a saturated mercury sulfate electrode (MSE, E° = +0.66 V/ NHE) as reference. The photocurrent spectra reported below were corrected for the photon emission at each wavelength of the lamp/monochromator system, previously detected by a calibrated thermopile (Newport mod. 818P-12).
3. RESULTS AND DISCUSSION After 30 min deposition from an unstirred solution with 50 g L−1 of (NH4)6Mo7O24 at pH 5.5, the morphology of Figure 1 was obtained. Here, SEM images show the cross section of the nanostructured array (a) and the top-view of the nanowires at different magnifications (b−d) after dissolution of the polycarbonate template in pure CHCl3. Figure 1a and b clearly show the Ni support, uniformly covered by electrodeposited nanowires (NWs). In Figure 1a, Ni film appears continuous, uniform, and compact. The image of Figure 1b shows that NWs are firmly connected to the current collector, while micrographs c and d detail NW morphology, featured by a cylindrical shape fairly regular and slightly wrinkled. Interconnections between different wires, due to the typical morphology of the polycarbonate template channels, are also clearly visible. Figure 1d shows the top-view of the NWs, where different diameters
Figure 2. Nanowires length vs electrodeposition time for nanostructures obtained in unstirred solution with 50 and 100 g L−1 of (NH4)6Mo7O24 at pH 5.5.
are clearly visible. From this image, we evaluated an average diameter of 190 ± 40 nm, while the mean wire length was 1 ± 0.15 μm. This value is reported in Figure 2, where the effect of electrodeposition time on NWs length is shown: the mean length increases with time, even if it is evident that nanostructures length is not uniform (see the deviation bars). Figure 2 also shows the influence of the Mo 7 O 24 6− concentration on the nanostructures length. Up to 60 min of electrodeposition, little differences in NW mean length was observed, while, after 120 min, NWs deposited from 100 g L−1 (NH4)6Mo7O24 were about 0.7 μm longer than those obtained from 50 g L−1 solution. It is also evident that the length C
dx.doi.org/10.1021/jp505819j | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
Figure 3. FEG-ESEM images, taken after template dissolution, of Mo oxide nanowires obtained in a stirred solution with 50 g L−1 of (NH4)6Mo7O24 and H2SO4 at pH 5.5 for 30 min: (a and b) top-view, (c) tilted-view, and (d) high-magnification of the tilted top-view.
Figure 5. X-ray patterns of Ni-coated template before and after the electrodeposition of nanostructures inside the channels of polycarbonate membrane.
Figure 4. Nanowires length vs electrodeposition time and solution pH for nanostructures obtained in stirred solution with 50 g L−1 of (NH4)6Mo7O24. Insets show morphology of nanostructures.
To favor H2 bubbles removal from template channels, the electrochemical setup was modified, providing a vigorous stirring of the solution with a rotary shaker at 600 rpm. Of course, the morphology of the nanostructured array was practically unchanged as shown in Figure 3, where SEM images of a deposit obtained from a 50 g L−1 (NH4)6Mo7O24 stirred solution after 30 min of electrodeposition are reported. Parts a and b of Figure 3show that the randomly interconnected NWs are well distributed over the entire surface of the Ni support and that NWs did not collapse after the complete dissolution of
uniformity was not changed, indicating that Mo 7 O 24 6− concentration has modest influence on the growth rate of the nanostructures. The very low uniformity of NWs length may be attributed to both the interference of hydrogen evolution, whose random bubble accumulation inside membrane pores inhibits deposition, and to the nonidentical electrochemical conditions at the pore bottom due to a nonuniform coverage by the sputtered gold film.30 D
dx.doi.org/10.1021/jp505819j | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
Figure 6. Typical EDS spectrum of Mo oxide nanostructures.
electrodeposition process from Mo7O246− solution leads to the formation of a scarcely conductive phase. This issue was further scrutinized starting from the findings by Morley et al.,23 who showed the dramatic effect of pH on deposition efficiency, nature, and microstructure of the deposit. In particular, below pH 5.5 they did not obtain metallic deposit, while above pH 8.5 a compact Mo layer, well adherent to the cathode, was deposited. Accordingly, we conducted electrodeposition at different pH for 30 min in a stirred solution, and we found that solution pH affects both the growth rate of the nanostructures and their morphology, as shown in Figure 4. Longer nanostructures were obtained at the lowest pH, but the insets of Figure 4 show the change of the morphology with pH. In particular, nanostructures obtained at pH 2.7 consist only of nanotubes with a smooth surface and a wall thickness of 37 ± 11 nm, while at pH above 5.5, NWs were formed. Besides, at pH 8.5, a nonuniform distribution of nanowires was found, probably due to a slow deposition kinetics as reported in ref 19. As for nanotubes obtained at pH 2.7, we observed they collapse in many support areas after the dissolution of the polycarbonate template. This was probably due to the thin wall of the nanotubes determining a poor mechanical strength. The preferential formation of nanotubes instead of nanowires at low pH is likely due to the vigorous H2 evolution, clearly visible at the electrode surface during the electrodeposition process. H2 gas builds up inside pores, confining the deposition only in the gap between the pore wall and gas bubbles.31,32 In practice, H2 bubbles act as a local template for nanostructures. In all the deposition conditions, after total dissolution, the template samples exhibit a blue-violet color that is typical of MoO2. In order to identify deposit composition, we have carried out a complete characterization of our nanostructures using different techniques. By comparing in Figure 5 the XRD patterns of the polycarbonate template before and after electrodeposition, we were able to conclude that deposit was amorphous, in agreement with the literature.5,6,33 Before electrodeposition of the nanostructures, XRD patterns show peaks of Ni, covering the template (ICDD34 card 4-850). Besides, due to the very thin thickness of gold sputtered film (few nm, not measurable by SEM), only a small peak of Au was detected in correspondence of the highest intensity diffraction
Figure 7. Raman spectra of Mo oxide nanostructures obtained at different solution pH values.
the template but remained tightly attached to the support. The cross-sectional views (Figures 3c and d) reveal that NWs are perfectly cylindrical, with a smooth wall, and have a mean diameter of 220 ± 20 nm. Unlike the arrays obtained in unstirred solution (Figure 1), NW length appears quite uniform, as confirmed by the very small deviation bar of Figure 4. This finding confirms that the random accumulation of H2 bubbles inside the template channels was the prevalent reason determining the poor length uniformity of the NWs grown in an unstirred solution. In addition, NWs grown in a stirred solution (Figure 4), but under otherwise identical conditions, are about 1 μm longer than those of Figure 1. Figure 4 also confirms that NWs mean length increases with the electrodeposition time, while the mean growth rate decreases from about 0.07 nm min−1 after 30 min to 0.03 nm min−1 after 120 min of deposition. The most probable cause of this behavior may be the low electrical conductivity of the deposit determining a progressive increase of ohmic drop that, in turn, modifies the current distribution favoring the secondary reaction of H2 evolution. From this point of view, this finding is of some value because it suggests that the E
dx.doi.org/10.1021/jp505819j | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
Figure 9. Evolution of the Mo 3d core level XPS spectra as a function of etching time for Mo oxide nanostructures prepared at pH 2.7 (a), 5.5 (b), and 8.5 (c).
Figure 8. XPS atomic percent−etching time profiles for Mo oxide nanostructures grown at pH 2.7 (a), 5.5 (b), and 8.5 (c).
To confirm the presence of molybdenum oxide, Raman spectroscopy was performed (Figure 6). Many peaks are due to α-MoO3 (MoO3 can exist in two phases: α-MoO3 that is thermodynamically stable, and β-MoO3 that is metastable). The weak peaks at 422 and 737 cm−1 are relative to MoO2, while the Raman mode at 485 cm−1 is due to the presence of the suboxide35,36 MoO3−x. In the region 870−980 cm−1, some peaks due to different hydrated MoO3·xH2O37,38 are present. Changing solution pH, oxides with different hydration degrees were formed. A less hydrated oxide was likely obtained at pH 5.5, as revealed by the presence of modes at 905 and 935 cm−1 due to the stretching vibration of OMo in MoO3·(1/3)H2O
plane (ICDD34 card 4-784). After electrodeposition, only nickel peaks and a Au peak were again detected, but with weakened intensity. Peak lowering was due to the screening effect of the deposit inside polycarbonate channels overlying the support layer. Figure 6 shows a typical EDS spectrum of nanostructures, revealing that deposit consists of a molybdenum oxide. Mo and oxygen are detected together with Ni and Au, arising respectively from the nickel support and from the gold sputtered film. The carbon peak is due to residual polycarbonate. F
dx.doi.org/10.1021/jp505819j | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
Table I. Mo 3d5/2 Best Fit Parameters and Their Attributions Relative to Mo Oxide Nanostructures Grown at pH 5.5 Etching time (s)
Mo oxidation state
Mo 3d5/2 (eV)
Height (CPS)
fwhm (eV)
Atomic %
0 0 0 30 30 30 30 270 270 270 270
5+ 6+ 0 4+ 5+ 6+ 0 4+ 5+ 6+ 0
231.3 232.6 228.0 229.5 231.3 232.5 227.7 229.4 231.3 232.8 228.0
1917.75 14899.40 24.98 21884.09 12357.76 5637.26 103.23 36499.76 14205.36 5505.18 1097.53
2.06 2.06 2.06 1.84 1.84 1.84 1.84 1.85 1.85 1.85 1.85
11.4 88.5 0.1 54.7 30.9 14.1 0.3 63.7 24.8 9.6 1.9
because, according to ref 37, a progressive transformation of MoO2 in MoO3 can occur under laser irradiation. With the aim of both better clarifying this aspect and evaluating the distribution of the different oxides, XPS analyses were carried out on the Mo oxide nanostructures obtained by changing the solution pH. Figure 8 shows the XPS atomic percent vs etching time profiles for the Mo oxide nanostructures prepared at 2.7, 5.5, and 8.5 solution pH, respectively. In all cases, carbon contamination of the surface was found to decrease with etching time. On the nanostructures top layer, a small silicon impurity has also been detected at pH 2.7 (Figure 8a), while at pH 5.5 (Figure 8b) small impurities of copper and sulfur, arising from synthesis, are present. As shown in Figure 8a and c, nickel (from the electrodeposited film on the gold layer), gold (from the sputtered layer on the membrane surface), and sodium (coming from the electrodeposition bath, only at pH 8.5) are also present, and their concentrations increase with the etching time. Initially, in all oxide nanostructures, Mo and O concentrations increase with etching time up to a constant value. In order to detect the oxidation state of molybdenum ions into deposits, evolution of the Mo 3d core level was examined as a function of etching time for the three investigated nanostructures, as illustrated in Figure 9. A change in the Mo oxidation state is noted as the etching time increases. In fact, as shown in Figure 9, initially (0 s etching time) the XPS Mo 3d core level spectrum is dominated by two peaks at about 232.6 and 235.7 eV, corresponding to the Mo 3d5/2 and 3d3/2 spin− orbit components, respectively. According to the literature,38−40 these binding energy positions correspond to the +6 oxidation state of Mo. Therefore, on the nanostructures surface the MoO3 phase, in which Mo is present as Mo6+, dominates. At pH 8.5 (Figure 9c) a shoulder located at about 230.9 eV, typical of the 5+ oxidation state,39,40 is also evident after 10 s of ion bombardment, the previous XPS structures widen, and the MoO2 phase, characterized by the presence of the Mo4+ ion, emerges out with its dominating structure at 229.5 eV.38,40 The width of the structures lets us suppose the presence of both the +5 oxidation state for Mo, just evident at the 0 s etching time at 8.5 pH, and a small amount of metallic Mo. To clearly see the different Mo oxidation states, a Gaussian− Lorentzian peak fit has been carried out as the ion bombardment proceeded up to 270 s. Figure 10 shows the peak fit after (a) 0 s, (b) 30 s, and (c) 270 s of ion bombardment for the Mo nanostructures grown at pH 5.5.
Figure 10. Mo 3d doublet XPS spectra at (a) the top surface, (b) after 30 s etching time, and (c) after 270 s etching time, for Mo oxide nanostructures grown at pH 5.5. Spectra are deconvoluted into characteristic lines assigned to the different Mo oxidation states.
and MoO3·H2O, respectively. Deposit formed at pH 2.7 and 8.5 shows modes at 920 and 948 cm−1 that can be attributed36 to MoO3·H2O. Spectra also show a low-frequency mode at 312 cm−1 that is due to the stretching vibration37 of Mo−OH2 in MoO3·(1/2)H2O. From Raman analysis (Figure 7), it can be concluded that, in under conditions, the nanostructures consist of mixed MoO3 and MoO2 with the presence also of different hydrated oxide. The stronger intensity of the α-MoO3 peaks may suggest that this phase is predominant in comparison with MoO2. However, this hypothesis needs further investigation G
dx.doi.org/10.1021/jp505819j | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
Figure 11. (a) Photocurrent action spectrum of Mo oxide nanowires and (inset) current transients upon illumination at different wavelengths. (b and c) determination of the optical gap.
At 0 s etching time, the Mo6+ state largely prevails with 88.5% of contribution, while Mo5+ and Mo0 contribute with 11.4% and 0.1%. At the 30 s etching time, the Mo4+ state dominates with 54.7% of total contribution while Mo5+, Mo6+, and Mo0 contribute with 30.9%, 14.1%, and 0.3%. In the nanostructures etched for 270 s, the contribution from Mo4+ is increased up to 63.7%, whereas the others are lowered correspondingly. All these results are summarized in Table I. Thus, the XPS data curve fitting procedure reveals that the Mo 3d level comprises many components corresponding to different Mo oxidation states (4+, 5+, 6+, 0); some of them become more evident as the ion bombardment proceeds. In particular, with increasing the etching time, the intensity of the Mo4+ component increases while the Mo6+ intensity decreases, confirming thus that the phase MoO2 prevails on the MoO3 one on going from the surface to the nanostructure deeper layers.
In order to fully characterize the nature of the nanostructures, we analyzed the deposit also by photoelectrochemical measurements carried out in 0.1 M Na2SO4 aqueous solution. We found that nanostructured deposits are photoactive, as revealed by Figure 11a, reporting the photocurrent action spectrum for the NW array of Figure 3; the spectrum was recorded at −0.15 V/MSE, which is slightly anodic with respect to the open-circuit potential (−0.35 V/MSE). It must be emphasized that photocurrent density refers to the geometrical area of the membrane, because it is difficult to estimate correctly the real nanostructure surface exposed to the light. Current transients, recorded during manual chopping of the light beam of different wavelengths, always revealed n-type photocurrent (inset of Figure 11a). In addition, the absence of photocurrent spikes, usually due to recombination effects at the nanostructure surface, was observed. According to the results obtained in ref 6, where the best fitting of (Iph·hν)n vs hν plots was obtained with n values H
dx.doi.org/10.1021/jp505819j | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
stirring of the electrolyte is necessary during the electrodeposition, in order to remove quickly H2 bubbles from template channels. The structure and composition of deposits was identified employing XRD, EDS, Raman, XPS, and photoelectrochemical measurements. From XRD analysis, we have found that nanostructures consist of an amorphous phase, identified by Raman as a mixture of MoO2 and MoO3 with the presence of some hydrated oxide. XPS analysis revealed that the main phase is MoO2 underlying MoO3. The presence of both these oxides has also been confirmed by photoelectochemical experiments: deposits exhibit anodic photocurrent and two optical gaps at 2.5 and 3.2 eV, corresponding to MoO2 and MoO 3, respectively. All these results are independent of electrolyte pH, and no remarkable evidence of the formation of a metallic phase has been found in all the growth conditions. Considering the interesting properties of the molybdenum oxides, these findings appear of some value because they show that using a simple method it is possible to obtain uniform arrays of nanostructures to be used without any further treatments. From a scientific point of view, there is clear evidence that only Mo oxides can be deposited electrochemically in a confined ambient such as channels of a nanostructured template
ranging between 0.395 and 0.642, we assumed nondirect optical transitions for processing the spectra. In the (Iph·hν)0.5 vs hν plots, two different linear regions can be envisaged (Figures 11b and c). Extrapolation to zero photocurrent from the higher photon energy range gives an optical gap close to 2.5 eV, close to that reported for crystalline MoO2 films6 (2.3−2.53 eV). The second extrapolation, in the region of lower photon energy, gives optical gap values of about 3.2 eV; this value supports the presence of α-MoO3, whose band gap is reported in the range 3.1−3.3 eV.41 Also in this case, optical transitions are reported to be nondirect for the amorphous MoO3 films and indirect for the annealed MoO3 films.37 A similar behavior was observed also for nanostructures grown at pH 2.7 and 8.5. These results, together with those of RAMAN and XPS, clearly indicate the formation by electrochemical deposition of mixed valence molybdenum nanostructures, independently of solution pH, electrodeposition time, and MoO42− concentration. The independence of the deposit chemical nature from solution pH is probably due to the parasitic H2 evolution reaction. It is wellknown that the occurrence of this reaction leads to an increase of the interfacial pH up to very basic values42 (about 10.7) and that this phenomenon is emphasized in a confined ambient43 such as channels of polycarbonate membrane. This means that after a short time of electrodeposition, the pH at the NW/ solution interface is very basic, and then the deposition process is not influenced by solution bulk pH. It must be highlighted that, in all experimental conditions investigated, we never found the formation of a pure metallic deposit; only XPS revealed the presence of small traces of metallic Mo. On the basis of all above results, the possible reactions justifying the formation of core−shell nanostructures are Mo7O624− + 20H+ + 14e− → 7MoO2 + 10H 2O
(1)
Mo7O624− + 3H 2O → 7MoO3 + 6OH−
(2)
■
AUTHOR INFORMATION
Corresponding Author
*Tel.: +39-0912386567232. Fax: +39-09123860841. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was partially funded by the European Community through the Programma Operativo Nazionale Ricerca e Competitività 2007−2013 (PON02_00355_3391233 Project).
■
Thus, the electrochemical process (reaction 1) leads to MoO2 nanowire deposition. Since the interfacial pH is alkaline, due to the simultaneous H2 evolution, the outer layer of MoO2 nanostructures was covered by MoO3 according to reaction 2.
REFERENCES
(1) Saji, V. S.; Lee, C.-W. Molybdenum, Molybdenum Oxides, and their Electrochemistry. ChemSusChem 2012, 5, 1146−1161. (2) Shields, J. A. Application of Molybdenum Metal and its Alloys; IMOA: London, 2013; ISBN 978-1-907470-30-1). (3) Ellefson, C. A.; Marin-Flores, O.; Ha, S.; Norton, M. G. Synthesis and Applications of Molybdenum (IV) Oxide. J. Mater. Sci. 2012, 47, 2057−2071. (4) Wu, H.-M.; Liang, S.-C.; Lin, Y.-L.; Ni, C.-Y.; Bor, H.-Y.; Tsai, D.C.; Shieu, F.-S. Structure and Electrical Properties of Mo Back Contact for Cu(In,Ga)Se2 Solar Cell. Vacuum 2012, 86, 1916−1919. (5) Farsi, H.; Gobal, F.; Raissi, H.; Moghiminia, S. On the Pseudocapacitive Behavior of Nanostructured Molybdenum Oxide. J. Solid State Electrochem. 2010, 14, 643−650. (6) Dukstiene, N.; Sinkeviciute, D. Photoelectrochemical Properties of MoO2 Thin Films. J. Solid State Electrochem. 2013, 17, 1175−1184. (7) Jubault, M.; Ribeaucourt, L.; Chassaing, E.; Renou, G.; Lincot, D.; Donsanti, F. Optimization of Molybdenum Thin Films For Electrodeposited CIGS Solar Cell. Sol. Energy Mater. Sol. Cells 2011, 95, S26− S31. (8) Gao, F.; Zhang, L.; Huang, S. Fabrication Horizontal Aligned MoO2/Single-Walled Carbon Nanotube Nanowires for Electrochemical Supercapacitor. Mater. Lett. 2010, 64, 537−540. (9) Hariharan, S.; Saravanan, K.; Balaya, P. α-MoO3: A High Performance Anode Material for Sodium-Ion Batteries. Electrochem. Commun. 2013, 31, 5−9. (10) Liu, J.; Shao, S.; Fang, G.; Wang, J.; Meng, B.; Xie, Z.; Wang, L. High-Efficiency Inverted Tandem Polymer Solar Cells with Step-Al-
4. CONCLUSIONS In this work, we have shown that it is possible to grow amorphous molybdenum oxide nanostructures by template electrodeposition inside the channels of polycarbonate membranes. To our best knowledge, this is the first paper dealing with the fabrication of a uniform array of MoO2/ MoO3 core−shell nanowires by template electrosynthesis. A systematic investigation has been carried out to correlate some of the electrodeposition parameters to the growth of nanostructures and to their composition. We have found that electrodeposition time, concentration in solution of Mo precursor, and solution pH did not influence the composition of the nanostructures. By adjusting electrodeposition time, it is possible to control the length of the nanostructures, while by adjusting pH it is possible to change morphology from nanotubes (at lower pH) to nanowires (for pH above 5.5). This is due to the vigorous H2 evolution, mainly at low pH, with gas bubbles acting as template, since they occlude polycarbonate channels and confine the deposition process into the gap between gas bubbles and pore wall. It has also been found that for growing nanostructures with uniform length a continuous and vigorous I
dx.doi.org/10.1021/jp505819j | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
doped MoO3 Interconnection Layer. Sol. Energy Mater. Sol. Cells 2014, 120, 744−750. (11) Wang, G.; Jiu, T.; Li, P.; Li, J.; Sun, C.; Lu, F.; Fang, J. Preparation and Characterization of MoO3 Hole-Injection Layer for Organic Solar Cell Fabrication and Optimization. Sol. Energy Mater. Sol. Cells 2014, 120 (Part B), 603−609. (12) Zach, M. P.; Ng, K. H.; Penner, R. M. Molybdenum Nanowires by Electrodeposition. Science 2000, 290, 2120−2123. (13) Kumari, L.; Ma, Y.-R.; Tsai, C.-C.; Lin, Y.-W.; Wu, S. Y.; Cheng, K.-W.; Liou, Y. X-ray Diffraction and Raman Scattering Studies on Lange-area Array and Nanobranched Structure of 1D MoO2 Nanorods. Nanotechnology 2007, 18, 115717. (14) Dong, W.; Huang, H.; Zhu, Y.; Li, X.; Wang, X.; Li, C.; Chen, B.; Wang, G.; Shi, Z. Room-Temperature Solution Synthesis of Ag Nanoparticle Functionalized Molybdenum Oxide Nanowires and their Catalytic Applications. Nanotechnology 2012, 23, 425602. (15) Inguanta, R.; Garlisi, C.; Spanò, T.; Piazza, S.; Sunseri, C. Growth and Photoelectrochemical Behaviour of Electrodeposited ZnO Thin Films for Solar Cells. J. Appl. Electrochem. 2013, 43, 199−208. (16) Inguanta, R.; Sunseri, C.; Piazza, S. Photoelectrochemical Characterization of Cu2O-Nanowire Arrays Electrodeposited into Anodic Alumina Membranes. Electrochem. Solid State Lett. 2007, 10, K63−K66. (17) Nakajima, H.; Nohira, T.; Hagiwara, R. Electrodeposition of Metallic Molybdenum Films in ZnCl2-NaCl-KCl-MoCl3 at 250°. Electrochim. Acta 2006, 51, 3776−3780. (18) Zach, M. P.; Inazu, K.; Ng, K. H.; Hemminger, J. C.; Penner, R. M. Synthesis of Molybdenum Nanowires with Millimeter-Scale Lengths Using Electrochemical Step Edge Decoration. Chem. Mater. 2000, 14, 2120−2123. (19) McEvoy, T. M.; Stevenson, K. J. Elucidation of the Electrodeposition Mechanism of Molybdenum Oxide from Iso- and Peroxo-Polymolybdate Solutions. J. Mater. Res. 2004, 19, 429−438. (20) Ksycki, M. J.; Yntema, L. F. The Electrodeposition of Molybdenum from Aqueous Solutions. J. Electrochem. Soc. 1949, 96, 48−56. (21) Daolio, S.; Fleischmann, M.; Pletcher, D. A Study of the Deposition of Molybdenum at a Vitreous Carbon Cathode from an Aqueous Citrate Bath. J. Electroanal. Chem. 1981, 130, 269−275. (22) Ivanova, N. D.; Ivanov, S. V.; Boldyrev, E. I.; Standnik, O. A. Electrodeposition of Metal Molybdenum from Electrolytes Containing Hydrofluoric Acid. Prot. Met. 2006, 42, 354−358. (23) Morley, T. J.; Penner, L.; Schaffer, P.; Ruth, T. J.; Bernard, F.; Asselin, E. The Deposition of Smooth Metallic Molybdenum from Aqueous Electrolytes Containing Molybdate Ions. Electrochem. Commun. 2012, 15, 78−80. (24) Falola, B. D.; Krishnamurthy, A.; Radhakrishnam, R.; Suni, I. I. Galvanic Deposition of Mo Atop Al 6061 Alloy. ECS Electrochem. Lett. 2013, 2, D37−D39. (25) Inguanta, R.; Livreri, P.; Piazza, S.; Sunseri, C. Fabrication and Photoelectrochemical Behavior of Ordered CIGS Nanowire Arrays for Application in Solar Cells. Electrochem. Solid State Lett. 2010, 13, K22− K25. (26) Battaglia, M.; Piazza, S.; Sunseri, C.; Inguanta, R. CuZnSnSe Nanotubes and Nanowires by Template Electrosynthesis. Adv. Sci. Technol. 2014. (27) Taurino, A. M.; Forleo, A.; Francioso, L.; Sicilianio, P.; Stalder, M.; Nesper, R. Synthesis, Electrical Characterization, and Gas Sensing Properties of Molybdenum Oxide Nanorods. Appl. Phys. Lett. 2006, 88, 152111. (28) Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solutions; Pergamon Press: Oxford, 1966; p 272. (29) Scofield, J. H. Hartree-Slater Subshell Photoionization CrossSections at 1254 and 1487 eV. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 129−137. (30) Inguanta, R.; Piazza, S.; Sunseri, C. Influence of the Electrical Parameters on the Fabrication of Copper Nanowires into Anodic Alumina Templates. Appl. Surf. Sci. 2009, 255, 8816−8823.
(31) Fukunaka, Y.; Konishi, Y.; Ishii, R. Producing Shape-Controlled Metal Nanowires and Nanotubes by an Electrochemical Method. Electrochem. Solid State Lett. 2006, 9, C62−C64. (32) Inguanta, R.; Ferrara, G.; Livreri, P.; Piazza, S.; Sunseri, C. Ruthenium Oxide Nanotubes via Template Electrosynthesis. Curr. Nanosci. 2011, 11, 210−219. (33) Patil, R. S.; Uplane, M. D.; Patil, P. S. Structural and Optical Properties of Electrodeposited Molybdenum Oxide Thin Films. Appl. Surf. Sci. 2006, 252, 8050−8056. (34) International Centre for Diffraction Data, Power Diffraction File (2007), Pennsylvania, USA (card 4−850 for Ni). (35) Camacho-Lopez, M. A.; Escobar-Alarcon, L.; Picquart, M.; Arroyo, R.; Cordoba, G.; Haro-Poniatowski, E. Micro-Raman Study of the m-MnO3 to α-MnO3 Transformation Induced by cw-Laser Irradiation. Opt. Mater. 2011, 33, 480−484. (36) Seguin, L.; Figlarz, M.; Cavagnat, R.; Lassègues, J.-C. Infrared and Raman Spectra of MoO3 Molybdenum Trioxides and MoO3· xH2O Molybdenum Trioxide Hydrates. Spectrochim. Acta, Part A 1995, 51, 1323−1344. (37) Camacho-Lopez, M. A.; Haro-Poniatowski, E.; Lartundo-Rojas, L.; Livage, J.; Julien, C. M. Amorphous-Crystalline Transition Studied in Hydrated MoO3. Mater. Sci. Eng. B 2006, 135, 88−94. (38) Grünert, W.; Stakheev, A. Y.; Feldhaus, R.; Anders, K.; Shpiro, E. S.; Minachev, K. M. Analysis of Mo3d XPS Spectra of Supported Mo Catalysts: An Alternative Approach. J. Phys. Chem. 1991, 95, 1323−1328. (39) Bhosle, V.; Tiwari, A.; Narayan, J. Growth of Epitaxial MoOx (2 < x
