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Beyond mono-, di- and trisulfides: synthetizing vanadium tetrasulfide (VS) films for energy conversion 4
Eduardo Flores, Esmeralda Muñoz-Cortés, Julio Bodega, Olga Caballero-Calero, Marisol S. Martin-Gonzalez, Carlos Sanchez, Jose Ramon Ares, and Isabel J. Ferrer ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00449 • Publication Date (Web): 03 May 2018 Downloaded from http://pubs.acs.org on May 3, 2018
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ACS Applied Energy Materials
Beyond mono-, di- and trisulfides: synthesizing vanadium tetrasulfide (VS4) films for energy conversion Eduardo Flores1, Esmeralda Muñoz-Cortés2, Julio Bodega1, Olga Caballero-Calero3, Marisol Martín-González3, Carlos Sánchez1, José R. Ares1, and Isabel J. Ferrer1. 1
MIRE-group, Dpto. de Física de Materiales, UAM, Cantoblanco, E-28049, Madrid (Spain).
2
Grupo de Óptica no lineal, Dpto. de Física de Materiales, UAM, Cantoblanco, E-28049, Madrid (Spain).
3
FINDER-group, Instituto de Micro y Nanotecnología, IMN-CNM, CSIC (CEI UAM+CSIC) Isaac Newton, 8, E-28760, Tres Cantos, Madrid (Spain). KEYWORDS: VS4, patronite, metal sulfides, hydrogen photogeneration, solar energy conversion, low dimensional chalcogenides. ABSTRACT: Vanadium tetrasulfide (VS4, called patronite as mineral) is a one-dimensional compound with promising properties for energy conversion applications. However, it has been scarcely investigated due to its complex synthesis. In this work, we report a detailed investigation about the formation mechanism of VS4 (V4+(S22-)2) as well as its structural, transport and photoelectrochemical properties. To this aim, VS4 films were grown by a solid-gas reaction process between vanadium films and sulfur at temperatures between 350 and 450 ºC during different reaction times. Films´ characterization (X-ray diffraction, energy dispersive analysis of X-ray, micro-Raman spectroscopy and scanning electron microscopy) reveals the formation of monoclinic VS4 nanorods (I2/C) as single crystalline phase since very short reaction times (t425 ºC) lead to a formation of vanadium oxides (Figure 2a) that could be explained by the V-S phase diagram23 and kinetics considerations. This phase diagram indicates that VS4 is not stable at T> 400 ºC because it melts incongruently into liquid sulfur and V1+xS2 which is formed in an extremely slow process24 enhancing the reaction of vanadium with residual oxygen to form vanadium oxides11,14,25. In conclusion, 400 °C seems to be the optimal sulfuration temperature to grow crystalline VS4 as the diffraction pattern reveals (Figure 2c).
Vanadium tetrasulfide films (VS4) have been obtained by solid-gas reaction between vanadium (Alfa Aesar, 99.7%) thin films (thermally evaporated by electron beam on both conductive Ti and insulating glass substrates) and sulfur
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V2O3
VxOy
V2O3
VxOy
(011) (002)
(011) (002)
VS4
425ºC b)
VS4
400ºC c)
10
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80h
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20h
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(002)
(011) (002)
(011) (002)
metallic V film d)
V metallic
(011) (002)
(011)
450ºC a)
V2O3 Intensity (arb. units)
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Intensity (arb. units)
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350ºC d) 50
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glass substrate 10
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2. Influence of sulfuration time on structural, compositional and morphological properties of VS4. Figure 3a-d shows the XRD diffractograms of metallic vanadium before and after sulfuration at 400 ºC during 5, 20 and 80h. Longer sulfuration times, ts, (170h) leads to decomposition of VS4 into VxOy (JCDPS 01-077-0050)(see Figure S2). After sulfuration, monoclinic VS4 (𝐼2/𝑐) was identified according to JCPDS-ICDD 087-0603 (Figs. 3a-c) as a single crystalline phase even at the shortest sulfuration time (5 hours). The diffraction peaks related to (011) and (002) planes are significantly higher than the rest of the peaks suggesting the existence of a preferential orientation growth in those films as will be discussed further. Finally, figure 3d show the XRD of initial metallic vanadium film (d = 300 nm). A sole peak is observed corresponding to the (110) crystalline plane according to cubic Vanadium, JCPDS-022-1058.
40
50
60
2 (deg)
2 (deg) Figure 2. XRD diffractograms of V thin films (300nm) sulfurated during 20h at a) 450 °C, b) 425 °C, c) 400 °C and d) 35 °C. Both (011) and (002) diffraction peaks correspond to polycrystalline VS4.
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Figure 3. X-ray diffraction patterns of sulfurated V films at 400ºC under different sulfuration times (a-c) and initial V film (d). Red bars in (a-c) correspond to the reference pattern of synthetic patronite. Red bars in (d) correspond to the reference pattern of vanadium. Structural parameters obtained from the analysis of the XRD pattern of VS4 grown at 400ºC during 20h are shown in Table 1. It is remarkable the good agreement between the obtained lattice parameters and the reported values of synthetic bulk VS4. An average crystallite size of 200 nm has been estimated by Scherrer equation applied to the (011) and (002) diffraction peaks. From the analyses of all XRD spectra no influence of ts on lattice volume, as well as on crystallite size, has been observed. Table 1: Values of the lattice parameters and crystallite sizes (Dv) obtained from XRD pattern of VS4 film sulfurated at 400ºC during 20h. Lattice parameters reported in the literature are included for comparison purposes.
VS4 Experimental
Literature4,14
a (Å) (± 0.005)
11.919
b (Å) (± 0.005)
10.360
c (Å) (± 0.005)
6.781
𝜷 (degrees) (± 0.005)
100.840
Vcell (Å3) (± 0.05)
824.40
12.089 12.110 10.353 10.42 6.749 6.775 100.627 100.800 826.63 839.77 ----
(𝟏𝟏𝟎)
(𝟎𝟐𝟎)
𝑫𝑽 / 𝑫𝑽 (± 10) nm
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ACS Applied Energy Materials VS4 films show a particular morphology composed by elongated particles that exhibits nanorod shapes as Figure 4 shows and as reported in 11,14. Individual nanorods were analyzed by EDX exhibiting a stoichiometric ratio of 𝑆⁄𝑉 = 4.0 ± 0.1, i.e., nanorods are composed by VS4. Unlike structural properties, nanorod size could be tunned by sulfuration time. For instance, whereas films sulfurated during 10h show rods of diameters of around 500 nm and lengths of 2 μm, films sulfurated during 80h exhibit bigger sizes i.e. diameters of around 1μm and lengths higher than 10μm. Moreover, they are mostly oriented with their axial direction parallel to the substrate surface confirming the preferential orientation observed in the XRD patterns shown in Figure 3.
a)
b)
4
3 S/V
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2
1
0 0
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sulfuration time (h) Figure 5: Influence of sulfuration time on film stoichiometric ratio (analysis area of 3mm x 2mm). Vanadium films were sulfurated at 400ºC from 5h to 120h of sulfuration time.
c)
d)
Figure 4. SEM images of V films sulfurated at 400ºC during a) 10, b) 20 and c) 80 hours, as indicated over the images. EDX analysis of individual nanorods shown in d) was performed in the red points. Concerning film stoichiometry an influence of ts on S/V ratio is shown in Figure 5. S/V values increase with sulfuration time exhibiting higher values (3.2± 0.2) than those (1.2) reported by Weimer20 but, surprisingly, it does not reach the stoichiometric value of 4, even at the longest sulfuration times (t =120h).
This discrepancy could be understood by considering that the films are formed by VS4 and non-crystalline metallic vanadium based phases, due to the incomplete sulfuration of the original vanadium films because of the extremely slow formation kinetics. This explanation would be consistent with the lowest values of S/V measured at shortest sulfuration times, in which the V(metallic)/V(as VS4) proportion would be the highest. The maximum S/V ratio obtained has been around 3.2. Assuming our hypothesis, this value hints that the 80% of vanadium has been sulfurated forming VS4 and the 20% of vanadium remains as non-crystalline metallic V. The V(metallic)/V(as VS4) proportion would be 0.25 at the longest times of sulfuration. At the same time, the value obtained at 5 hours of sulfuration (S/V=1.75) would lead to films formed by 44% of VS4 and 56% of vanadium, i.e. the mentioned proportion would be around 1.27. Additionally, we have performed micro-Raman measurements in order to determine the existence of non-crystalline phases in the films. Figure 6 shows the Raman spectra of vanadium films sulfurated at 400ºC during 5 and 20h (Raman spectra of individual nanorods are shown in Figure S3).
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with the formation mechanism of other sulfides (CuInS2, CdS, PbS, etc.)31,32.
a)
VS4 film, 20h 345 218 490 191
counts (arb. u.)
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269
554
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VS4 film, 5h
b)
342 216 284
193
500nm
553
490
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Figure 7. SEM-FEG image at edge position of VS4 rod showing its nucleation centers along different crystalline planes.
-1
Raman shift (cm ) Figure 6. Raman spectra of VS4 films grown at 400°C during: a) 20h and b) 5h. All marked peaks correspond to VS4 except the 490 cm-1 that can be attributable to SiO2 (polymorph) vibrational mode from glass substrate26,27. In both cases, the obtained spectra are in agreement with the first principles DFT calculations for non-resonant modes and the experimental values reported by Weimer20 for VS4, confirming the existence of VS4 as sole phase at sulfuration times of 5h and 20h, as observed by XRD. This results suggest that obtained values of film stoichiometry lower than nominal values are due to existence of metallic vanadium because no Raman peaks related to V-O and VS phases are observed and supporting a sluggish formation kinetics14,16. The observed preferential growth at VS4 is usually observed in other sulfides (GaS, ZnS, TiS3, NbS3) which exhibit morphologies such as nanowires, nanoribbons etc. Those morphologies are commonly explained by a vapor-solid (VS) mechanism of synthesis28,29 with the growth oriented along the more unstable crystallographic plane i.e. higher free surface energy (011). However, at VS4, the growth plane, (001), shows a lower atomic density that other planes (See Table S1) and the plane shape of the tip of the nanorod discarding this mechanism. In fact, high resolution SEM image (Figure 7) shows that rods exhibit nucleation centers along different crystalline faces. Moreover, SEM-EDX analyses performed in sulfurated thin vanadium films (50 nm) reveals the existence of VS4 bunchs surrounded by regions with scarce material (Figure S4) suggesting the existence of a growth mechanism based on a Kinkerdall effect due to a faster V diffusion into VS4 rod than that of sulfur30 which is also habitually related
3. Transport and optical properties. Figure 8 shows the influence of sulfuration time on transport properties of the V films sulfurated at 400ºC. Before sulfuration V films exhibit the expected thermoelectric coefficient values for metallic vanadium33 ( 3 µV/K) increasing up to 25 µV/K during formation of VS4 (Figure 8a). The smaller Seebeck coefficient value than this obtained at mineral patronite17 points out the existence of residual metallic vanadium in the film as EDX measurements indicate. It is also remarkable that the sign of Seebeck coefficient reveals that films possess p-type character, opposite to the n-type conductivity observed in mineral patronite17. A similar discrepancy has been previously reported in other sulfides, which is usually attributed to the impurities contained in the mineral counterpartner18,34. Further investigation is necessary to clarify this point. Finally, resistivity measurements (Figure 8b) also confirm that the metalsemiconductor transition occurred during the sulfuration up to 80h by increasing the value from 10-4 ·cm to 1000 ·cm. The Seebeck coefficient values of VS4 films are smaller than those previously reported (measured35 and theoretically predicted36) from metallic trisulfides as TiS3 and ZrS3 (~ hundreds of µV/K). On the contrary, they are closer to those corresponding to metallic disulfides as FeS218 and Co-doped FeS219 (tens of µV/K).
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zone (E2) at energies in the range of 1.67-2.33 eV, approximately. Values of Eg1 and E2 are included in Figure 9. (Graph of 𝛼 vs. ℎ𝜈 is available in Figure S5).
a)
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metallic film
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1.8x10 2
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()
Seebeck Coefficient (V/K)
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-10
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1.2x10 E 1.35 0.05 eV g1 8
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b) 10000 Resistivity (cm)
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E21.67 0.05 eV
1000
0.0 0.5
100
1
1.5
2.0
2.5
3.0
Figure 9. Pankove plot of VS4 film (sulfurated at 400ºC during 20h) besides the corresponding linear fitting. The energy values of the two direct transitions are included.
0.1 0.01 1E-3
1.0
h (eV)
10
metallic film
1E-4 1E-5 0.1
1
10
100
sulfuration time (h) Figure 8. Evolution of the transport properties: Seebeck coefficient (a) and resistivity (b) vs. sulfuration time of V films sulfurated at 400ºC. The resistivity curve hint a regime of transformation from metallic V to semiconducting VS4 films. VS4 films synthesized on glass were also used to optically characterize patronite. Tansmittance (T) and reflectance (R) spectra (range from 300 to 2500 nm) of VS4 were obtained at room temperature. From T and R data the optical absorption coefficient (𝛼) was obtained by the following equation37:
𝑇=
(1 − 𝑅)2 ∙ 𝑒 −𝛼𝑑 1 − 𝑅2 ∙ 𝑒 −2𝛼𝑑 [1]
where d is the VS4 film thickness. Obtained absorption coefficient is around 104 cm-1 at h>1.6 eV. Moreover, the bandgap energy was estimated by plotting (𝛼)𝑛 vs. ℎ𝜈 ( photon energy, eV) according to Pankove formalism38, being n=1/2 for indirect and n= 2 for direct allowed transitions, respectively. The linear fitting of Pankove plots is presented in Figure 9 and hints two direct transitions in different ranges of photon energy. The first one (Eg1) appears at the lowest energies (1.35-1.62 eV) and the second
Optical results shown in Figure 9 clearly suggest that the VS4 films present two direct transitions. The lowest energy one can be considered as the optical band gap 𝐸𝑔1 = ~1.35 ± 0.05 𝑒𝑉 and its value is slightly higher than the value previously reported by Lui11 (~1.13 eV) and similar to that experimentally determined by Rout6 with VS4 particles dispersed on reduced graphene oxide (VS4/rGO hybrids). A second direct transition of 𝐸2 = ~1.67 ± 0.05 𝑒𝑉 is reported in this work by first time. The existence of a second direct allowed transition at energies higher than the reported in the literature has been reported in some TMTs37,39, as TiS3.
4. Photoelectrochemical characterization. In order to perform the photoelectrochemical characterization in PEC, the VS4 films were synthesized on Ti substrates ( = 15 mm, to get a good electrical contact) by sulfuration of V thin films deposited on Ti disks. This preparation method also causes the change in the conductivity type of patronite films due to the sulfuration process, in which Ti from the substrate diffuses through the VS4 leading to n-type patronite film. The effect of Ti diffusion in the conductivity type has been previously observed in other sulfides40. Figure 10 shows the photopotential curve of VS4 photoanodes. The n-type conductivity is confirmed by the negative sign of photopotential, as expected from the doping of VS4 with diffused Ti atoms.
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6.0x109
Energy of electronvs vacuum (eV)
C-2 (1/F2) sc
4.0x109
0
Vfb=-0.86 0.02 V (Ag/AgCl)
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1000Hz 500Hz 100Hz 0.0 -1.5
pH=9.5
-4
-1
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NbS3
TiS3
+
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FeS2
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-7
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Figure 10. Photopotential curve of VS4 photoanode under 200 mW/cm2 white light illumination in aqueous =0.5 M Na2SO3.
Figure 11. Mott-Schottky plot of VS4 film in Na2SO3 electrolyte (pH=9.5) at different modulation frequencies. Flat band potential value is included.
In order to describe the energy band position at the VS4/ electrolyte interface, the VS4 flatband potential (Vfb) was determined using the Mott-Schottky method, which uses the space charge layer capacitance, 𝐶sc, obtained from the impedance (𝑍𝑖𝑚 ) , according to the equation41:
Band edges can be derived experimentally from the flatband potential measurement for a great variety of metal oxides and sulfides assuming the empirical relationship between the flat band potential and the conduction band energy level (𝑉𝑓𝑏 ≈ 𝐸𝐶𝐵)43. Valence band energy level is then estimated by the bandgap energy. Fig 12 shows the energy levels scheme for the VS4/electrolyte interface at pH=9.5 compared with those of other investigated sulfides. As can be observed the conduction band energy level of VS4 is placed above the reduction potential of H+, hinting the suitability of VS4 to generate H2.
[2] where 𝑤 is the modulation frequency. Zim was measured by AC modulated cyclic voltage scans from -1.5 V to 0.3 V vs. (Ag/AgCl) at different frequencies from 100 Hz to 1000 Hz. The relationship between Csc and bias potential (Vbias) is described by the Mott-Schottky equation42: 1 2 𝑘𝐵 𝑇 ) (𝑉𝑏𝑖𝑎𝑠 − 𝑉𝑓𝑏 − ) 2 = ( 2 𝐶𝑆𝐶 𝜀𝑠𝑐 𝜀0 𝐴 𝑒𝑁𝐷 e [3] where, A is the working electrode (photoanode) area, e is the electronic charge, 𝜀𝑠𝑐 is the dielectric constant of VS4, 𝜀0 is the permittivity of vacuum, ND is the number of donors, Vbias is the applied bias potential in volts, kB is Boltzmann´s constant, and T is the temperature in absolute units (298K). Correspondingly, a 𝐶𝑆𝐶−2 vs Vbias plot should yield a straight line and from its interception with the Vbias axis41 the flat band potential could be estimated. Figure 11 shows the Mott-Schottky plots of VS4 at frequencies of 100 Hz, 500 Hz and 1000 Hz, respectively. The flat band potential of a VS4 film (sulfurated at 400ºC during 20h on Ti substrate) is 𝑉𝑓𝑏 = −0.86 ± 0.02 𝑉 (𝐴𝑔/𝐶𝑙𝐴𝑔) = −0.67𝑉 ± 0.02 𝑉(𝑁𝐻𝐸). The factor (− because of its low value.
𝑘𝐵 𝑇 𝑒0
) in [3] has been neglected
-220 Potential vs Ag/AgCl (mV)
1 = 𝐶𝑠𝑐 𝑤𝑍𝑖𝑚
Potential, E/VNHE
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Vph=VOff-VOn
-230
Vph= 30mV -240
-250 ON
0
Light 500
OFF
1000
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Figure 12. Conduction and valence band energy levels scheme at the VS4/electrolyte interface. Other metal sulfides, previously investigated in our group at the same PEC conditions, are included for comparison purposes. Current densities of VS4 nanorods used as photonanodes at dark and under illumination conditions are shown in Figure 13a in which values of several mA measured in the range from -0.2 V to 0.7 V of bias potential (vs Ag/AgCl) are
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Current density (mA/cm2)
5
a)
illumination darkness
4
0.25 Light
H2 flux (mol/min*cm2)
obtained. Figure 13b shows the photocurrent obtained from the difference between the current density under illumination and the current density at dark. A maximum in the photocurrent density at V=0.5V (Ag/AgCl) is observed which could be related with the photogenerated hole trapping in surface states at the interface at V > 0.5 V (Ag/AgCl)44. This behavior has also been previously reported in PEC with TiS3 as photoanode39. Moreover, as it can be observed, the photocurrent onset potential is close to -0.1 V (vs. Ag/AgCl).
Off
0.20 0.23mol/min is the maximun instantaneous rate of generation after 30 min of lighting
0.15
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Vbias=0.3V 0.05 On
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0.00 0
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Figure 14. Hydrogen photogenerated flux in PEC using VS4 as photonanode at 0.3 V vs. (Ag/AgCl) and 200 ± 20 mW/cm2 of white light illumination.
0 -0.2
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0.8
b)
1.2
0.8
Similar experiments have been carried out with several metal sulfides as photoanodes in a PEC reporting hydrogen evolution fluxes near to that of VS4 here presented (see Table 2). Only the light power density is rather lower in this work than that reported. Bias potential (0.3V, Ag/AgCl) and electrolyte (0.5 M Na2SO3) are the same. VS4 (200 mW/cm2)
0.4
0.23
0.0 -0.2
0.0
0.2
0.4
0.6
0.8
Vbias vs Ag/AgCl
Figure 13. a) Linear Sweep Voltammetry (LSV) at dark and under illumination using VS4 as photoanode, b) VS4 photocurrent density obtained from the difference between the current density under illumination and the current density at dark.
TiS3 39 (270 mW/cm2) 1.94
Ti-FeS2 (270 mW/cm2) 0.91
45
PdS 22 (270 mW/cm2) 0.04
Table 2. Hydrogen evolution fluxes in mol/min·cm2 obtained with different metal sulfides photoanodes in 0.5M Na2SO3 at 0.3V Ag/AgCl under illumination of white light during times 20
