3.2. Deposition of the MoSx~2+δ/Mo Films by on-Axis PLD
Figure 3a shows the experimental and model RBS spectra for a thin MoS
x film obtained on a Si substrate over 1 min of on-axis PLD at an Ar pressure of 8 Pa. According to the SIMNRA simulation, the composition of this film was MoS
2.7±0.2. The loadings of the model thin-film catalyst were ~6.6 × 10
15 Mo atom/cm
2 (1.1 μg/cm
2) and ~17.8 × 10
15 S atom/cm
2 (0.95 μg/cm
2). At a density of 5.06 g/cm
3, the film thickness was ~4 nm.
Notably, the actual load was greater, due to the Mo nanoparticles, which caused a long “tail” in the RBS spectrum (channels 240–315). Calculation of the “Mo peak/Mo tail” intensity ratio revealed that the Mo content in the thin MoSx film was approximately equal to that of the Mo nanoparticles. The total MoS2.7/Mo catalyst load was estimated to be ~3.4 μg/cm2, corresponding to a deposition rate of ~3.4 μg/cm2/min. For the used energy of the analyzed ion beam, the imposition of the RBS peak from the Mo atoms on the RBS peak from the S atoms occurred when the He ions were scattered by Mo particles larger than 230 nm. The use of the buffer gas reduced the deposition rate of the atomic component of the laser plume; however, this allowed for an increase in the S concentration in that component of the hybrid MoSx~2+δ/Mo films, which was formed, due to the deposition of the atomic flux.
Figure 3b shows the RBS spectra for the MoS
x~2+δ/Mo films deposited on the Si/SiO
2 substrate under different conditions for 4 min. These spectra indicated that the relative contribution of the Mo peak can be attributed to “Mo atoms in the MoS
x~2+δ film”, which reduced with increasing Ar pressure, where the yield of He ions in the range of channels 275 to 240 did not depend on the Ar pressure, which implies that the deposition flux of Mo particles persisted unchanged under various conditions of the on-axis PLD.
Due to the greater deposition rate of atomic flux during vacuum on-axis PLD, the RBS spectrum for a corresponding MoSx~2+δ film could be adequately processed by SIMNRA if the model object consisted of a homogeneous MoSx=2.1-0.2 film on a SiO2 substrate. The catalyst loading included ~1.7 × 1017 Mo atom/cm2 (27.3 µg/cm2) and ~3.6 × 1017 S atom/cm2 (19.4 µg/cm2). The deposition rate for the vacuum on-axis PLD of the MoSx/Mo catalyst was 11.7 μg/cm2/min.
Figure 3c shows the dependence of the loading of the MoS
x~2+δ/Mo catalyst on the time of its on-axis PLD in Ar at a pressure of 8 Pa. A monotonic growth of the catalyst loading was observed with increases in deposition time. The shapes of the RBS spectra are strongly distorted, due to the accumulation of Mo particles. An example of the mathematical fitting of the RBS spectra is shown in
Figure 3c for a sample obtained after 32 min of on-axis PLD deposition. A fairly good match was obtained for a model that contained a thin surface layer with the composition MoS
x~1.8 and a thicker underlayer with the composition MoS
x~1.4. A satisfactory fit was achieved, due to the assumption of large surface roughness for the model film. The model film contained ~4.8 × 10
17 Mo atom/cm
2 (76 μg/cm
2) and ~7.5 × 10
17 S atom/cm
2 (44 μg/cm
2). The calculated deposition rate of the MoS
x~2+δ/Mo film was 3.75 µg/cm
2/min, which correlated well with the data regarding the deposition rate at the initial stage (i.e., for 1 min).
The spectra of the MoS
x~2+δ/Mo films obtained with longer deposition times were difficult to process with SIMNRA (
Figure 3c). Below, we show that the optimum catalyst loading was formed with an on-axis PLD time of 64 min. We assumed that, for this time of on-axis PLD, the catalyst loading increased to 240 µg/cm
2 and included ~9.6 × 10
17 Mo atom/cm
2 (152 µg/cm
2) and ~1.5 × 10
18 S atom/cm
2 (88 µg/cm
2). After an increase in deposition time of up to 128 min, the RBS spectrum of the MoS
x~2+δ/Mo film corresponded to a thick layer of a homogeneous mixture of Mo and S, and the atomic ratio of these components was S/Mo ≤ 1.
The formation of Mo nanoparticles during the pulsed laser ablation of MoS
2 targets and their transfer to the surfaces of the MoS
x~2+δ/Mo films during the on-axis PLD was confirmed by the TEM, and MD results from a thin film deposited on NaCl (
Figure 4). At low magnification, round-shaped dark particles were detected. The sizes of these particles varied in the range of 10–200 nm. The MD pattern of these particles corresponded to the cubic lattice of Mo. In high-resolution TEM imaging of a separate Mo nanoparticle, atomic planes with an interplanar distance of 0.22 nm, characteristic of Mo (110), were observed. The Mo particles were surrounded by an amorphous ~5 nm-thick shell. The MoS
x~2+δ matrix of the MoS
x~2+δ/Mo film was amorphous. The TEM image of this film contained rounded nanosized areas with lighter contrast. These areas probably corresponded to the sites at which the Mo nanoparticles were localized. However, when manipulating the film for the TEM studies, these particles were removed, due to weak adhesion to the thin MoS
x~2+δ matrix.
Figure 5a shows SEM images of a relatively thin MoS
x~2+δ/Mo film deposited by on-axis PLD onto the smooth surface of the Si/SiO
2 substrate. Here, the deposition time was 4 min, and at this stage of film growth, the film had a relatively dense structure. This structure was formed from sufficiently small Mo nanoparticles and a scattered flux of Mo and S atoms. The sizes of most of the Mo nanoparticles did not exceed 200 nm, and the scattered flux of Mo and S atoms provided a conformal deposition of a MoS
x~2+δ shell on these particles and enveloped them with approximately the same efficiency over the entire rounded surface.
Figure 5a shows that larger particles occasionally appeared in the surface of the MoS
x~2+δ/Mo film. The sizes of these particles reached several fractions of a micrometer. The deposition of Mo particles of a submicron size had a significant effect on the formation of the morphologies of the thicker films.
Figure 5b shows SEM images of the MoS
x~2+δ/Mo film deposited on the Si/SiO
2 substrate for 64 min. The large conglomerates are formed from submicro- and nano-sized particles, and the structure of the film became porous here. The film thickness, as estimated based on the SEM images of the cleaved Si/SiO
2 substrate covered with this film, was ~1.5 μm.
An SEM image of the MoS
x~2+δ/Mo catalyst obtained under the same conditions by on-axis PLD on the glassy carbon is shown in
Figure 5c. The image indicates that the catalyst consisted of loose packaging of nano- and sub-micro-particles. The catalytic film covered the surface of the glassy carbon with a continuous layer. The GC substrate had noticeable roughness, and the lateral sizes of the surface cavities on this substrate were approximately several μm, and their depths varied in the range of ~0.1–1 μm. Due to the roughness of the glassy carbon substrate, the local loading of the MoS
x~2+δ/Mo catalyst was somewhat less than 240 µg/cm
2. EDS analysis of the sample, shown in
Figure 5c, indicated that the concentrations of the Mo, S, O, and C atoms were 10.6, 10.2, 5.2, and 74%, respectively. A surface area of 10 × 10 μm
2 was analyzed by EDS. The content of O atoms was practically unchanged when the EDS analysis was performed for a pure polished GC substrate. This result indicated that the O atom concentration in the films did not exceed several percent. The result of the EDS measurement of the ratio S/Mo in the thick MoS
x~2+δ/Mo catalytic film coincided well with the result of its measurement by the RBS.
Figure 6a shows the micro-Raman spectrum for a MoS
x~2+δ/Mo catalyst obtained on glassy carbon by on-axis PLD for 4 min in Ar at 8 Pa. For this spectrum, there were no peaks characteristic of the MoS
2 compound. Measurements of the Raman spectrum for the MoS
2 target showed that the characteristic and most intense E
2g1 and A
1g peaks were located at frequencies of 383 cm
−1 and 408 cm
−1, respectively. This spectrum confirms the amorphous structure of the catalytic layer, which is characterized by the presence of several broad vibrational bands near the frequencies of 200, 330, 450, and 540 cm
−1. The same Raman data, with four very weak and broad bands, were obtained from MoS
x films grown by traditional PLD elsewhere [
42]. McDevitt et al. [
42] proposed that this Raman spectrum indicates that the laser-deposited films represent a mixture of small domains of MoS
2 and amorphous sulfur. It is more reasonable to use a more recent model of a cluster-based polymeric structure that consists of Mo
3-S clusters with some different configurations of S ligands [
4,
16,
18,
27]. In the frame of this model, the characteristic Raman spectrum of amorphous MoS
x contains the following vibration modes: ν(Mo-Mo) at ~200 cm
−1, ν(Mo-S)
coupled at ~320 cm
−1, ν(Mo-S
apical) at ~450 cm
−1, ν(S-S)
terminal at ~520 cm
−1, and ν(S-S)
bridging at 540 cm
−1. The Raman spectra of the MoS
x~2+δ/Mo catalyst contained bands that could be attributed to all these modes. However, the vibration peaks of the bridging S
22− and terminal S
22 moieties overlapped, which caused the appearance of a broadened band in the 500–560 cm
−1 range. This finding indicated a weak order of atom packing in the local regions, comparable to the sizes of the Mo
3-S clusters.
The spectrum of the film obtained by on-axis PLD contained no peaks that are characteristic of nanocrystalline molybdenum oxides. In the case of the formation of MoO
3 nanocrystals, the distinctive peaks at ~820 and ~990 cm
−1 were observed [
18]. For the MoO
2 nanocrystals, vibrations at ~205, 229, 345, 365, 498, 572, and ~745 cm
−1 are characteristic [
43]. However, the appearance in the spectrum in
Figure 6a, with wide vibration bands at ~820 and ~950 cm
−1, indicates the formation of disordered MoO
3-y clusters in the MoS
x~2+δ/Mo film.
This conclusion was confirmed by the results of the XPS studies of the MoS
x~2+δ/Mo film, which are shown in
Figure 6b, c. The measurements of the XPS spectra were performed after the prolonged exposure of the film in the air (for approximately six months). The film was prepared by the on-axis PLD method in Ar at 8 Pa for 64 min. In the spectrum of Mo 3d, in addition to the doublet Mo 3d
5/2‒Mo 3d
3/2, which corresponds to the chemical bonding of Mo with S (Mo
4+, the binding energy E
B of Mo 3d
5/2 is 229.7 eV), there was a doublet found that was attributable to Mo oxide (Mo
6+, Mo 3d
5/2 E
B~232.8).
The XPS studies of the MoS
2 target indicated that, in the case of effective Mo-S bond formation, the surface of the compound had a higher resistance to oxidation in the air (results not shown). The formation of the molybdenum oxide nanophase could have resulted from the ineffective interaction of Mo and S atoms during the film deposition. Unsaturated Mo bonds in the local structure of the MoS
x~2+δ film interacted with O atoms when the sample was exposed to air. Another mechanism of molybdenum oxide formation is the oxidation of the surface of the Mo particles that were uncoated with the MoS
x~2+δ thin shell. The former mechanism seems to be more likely, because amorphous MoS
x films obtained by electrochemical deposition (i.e., those without Mo nanoparticles) also undergo a slow transformation from Mo
4+ to Mo
6+ under atmospheric conditions [
18]. This process could have partially occurred between the preparation and characterization of these MoS
x thin films.
The XPS spectrum of S 2p for the same MoS
x~2+δ/Mo film is shown in
Figure 6c. This figure reveals the presence of different S ligands that were considered in the Mo
3-S cluster-based model of amorphous MoS
x. The S ligands with an S 2p
3/2 peak at 162.3 eV were assigned to the S
22− terminal or unsaturated S
2− entities in the amorphous MoS
x and S
2− in the crystalline MoS
2. The S 2p
3/2 peak at 163.7 eV corresponded to the bridging S
22− and apical S
2− ligands of the Mo
3-S cluster [
17,
18,
37,
44,
45]. This result agrees well with the abovementioned Raman spectra of the MoS
x~2+δ/Mo films (
Figure 6a). The oxidative process of the MoS
x~2+δ/Mo film in the air involved, to some extent, the S atoms. Consequently, a broad band at ~169 eV appeared on the XPS spectrum, and this binding energy was assigned to the S-O bonds [
17,
18].
Quantitative compositional analysis by XPS indicated that the S content in the surface layer of the MoS
x~2+δ/Mo films was slightly higher than in the bulk of the films. The
x value measured by XPS was 5–10% larger than that measured by RBS. This could be due to the adsorption of sulfur atoms on the surface of the films from the residual atmosphere in the deposition chamber after the PLD process finished. A similar result was earlier revealed in Reference [
35]. The O concentration in the surface layer of the films did not exceed 10 at % after prolonged exposure in the air. The O concentration was reduced to 3 at % after ion sputtering of the layer of surface contamination for 30 s.
3.3. Deposition of the MoSx~3+δ Films by off-Axis PLD
Figure 7 shows the RBS spectrum of a thin MoS
x film deposited on a Si/SiO
2 substrate for 1 min using the off-axis mode of PLD in Ar at a pressure of 8 Pa. Fitting of the spectrum revealed that the experimental RBS spectrum coincided well with the model RBS spectrum that was calculated for a continuous/smooth thin film with a composition of MoS
x~3.9 (results not shown). The thin film contained ~2.6 × 10
16 Mo atom/cm
2 (~4 μg/cm
2) and ~1 × 10
17 S atom/cm
2 (6 μg/cm
2). Increases in deposition time caused increases in catalyst loading with a sublinear dependence. To fit the experimental RBS spectrum from a thicker film obtained by off-axis PLD for 20 min, a two-layer film model was necessary (
Figure 7). A layer of MoS
4.1 was formed on the surface of this film, and a sublayer was composed of MoS
3.8. The film contained ~4.2 × 10
17 Mo atom/cm
2 (65 μg/cm
2) and ~1.6 × 10
18 S atom/cm
2 (98 μg/cm
2). Thus, the catalyst loading was increased to 163 μg/cm
2.
RBS studies have shown that with increasing target ablation time, the composition of the laser plume can be altered to some extent. As a rule, the surface composition and roughness of the transition metal dichalcogenide target are significantly modified by prolonged pulsed laser irradiation [
41]. However, an accumulation of sulfur vapor in the film production chamber is also possible with an increase in deposition time. The effects of these factors were also observed during the formation of a thicker MoS
x/Mo film by off-axis PLD.
Notably, upon calculating the model spectrum of a thicker film, the spectrum was found to be well-matched to the experimental spectrum in the channel range that corresponded to He ion scattering from Mo and S atoms. The carbon concentration in the film was set to no more than 20 at %. This value was determined by the X-ray energy dispersive spectroscopy of this film (results not shown). A disagreement between the model and experimental spectra was observed in the range of channels (less than 200), in which the ions yielded from the Si/SiO2 substrate were accumulated. This result could be due to the structural features of a thicker film. Below, we demonstrate that the microstructure of the film (micro-crack formation) allowed for the “channeling” of ions through the film. It is difficult to achieve a good fit result for either the film or the substrate under these conditions.
The TEM and SAED studies revealed that the use of the off-axis PLD mode substantially decreased the Mo nanoparticle deposition on the surface of the MoS
x~3+δ film (
Figure 8). The TEM image contrast and SAED pattern indicated the amorphous and quite homogeneous structure of the thin film. At low magnification, only individual Mo nanoparticles were observed on the TEM image. The high resolution TEM image of the MoS
x~3+δ film differed from that of the MoS
x~2+δ/Mo film in terms of a pronounced contrast that contained nanosized threads of dark and light tones. This result could be due to the different natures of the local packings of atoms in the films obtained by on- and off-axis PLD.
SEM studies of the MoS
x~3+δ films revealed the formation of a relatively dense thin film material with a quite smooth surface, and the morphology was slightly dependent on both the deposition time and the nature of the substrate (
Figure 9). The main effect on the growth of the films during the off-axis PLD was the fragmentation of the films, which caused the formation of micro-blocks that were separated by grooves (micro-cracks). An SEM study of the cross section of the films revealed that the micro-cracks could have been formed, due to the development of a columnar structure in the films. The columnar units originated on the substrate-film interface and grew up to the surface of the film. This growth was characteristic of chemical compound films with a cauliflower structure in which bushes are formed, due to the deposition of the scattered flux of atoms and/or clusters of atoms of the laser plume [
46].
The separate rounded particles of submicron size present on the surfaces of these films (
Figure 9) could have been formed by the deposition of Mo nanoparticles that subsequently grew in size, due to the deposition of a vapor. The needle-like submicroparticles that appeared on the film surface could have been formed, due to the spreading and solidification of larger liquid droplets that were ejected from the target at high speed and slid over the film surface.
The results of the MRS and XPS studies, shown in
Figure 10, did not reveal essential differences in the local structural or chemical states of the catalysts formed by the on- and off-axis PLD. Indeed, the Raman spectra of the MoS
x~3+δ catalysts were wholly like those of the MoS
x~2+δ/Mo catalysts (
Figure 6a). The XPS Mo 3d spectrum for the catalyst obtained by off-axis PLD for 20 min consisted of two doublets that corresponded to Mo
4+ and Mo
6+. Regarding the XPS S 2p spectrum of the MoS
x~3+δ catalyst, the intensity of the doublet with high binding energy was greater than that with low binding energy. Similar results were obtained for the MoS
x~2+δ/Mo catalyst. However, in the S 2p spectrum of the MoS
x~3+δ film, the relative intensity of the band corresponding to the S-O bonds was noticeably larger than that in the spectrum of the MoS
x~2+δ/Mo film. This finding suggests that, at higher S concentrations in the catalyst, not all S atoms formed perfect chemical bonds with Mo or other S atoms included in the Mo
3-S clusters. The S atoms possessing unsaturated bonds were subject to oxidation in the air environment.
3.4. Electrocatalytic Performances of the MoSx~2+δ/Mo and MoSx~3+δ Films Prepared by on-Axis and off-Axis PLD
Figure 11 shows the results of an electrochemical study of the dependence of the electrocatalytic properties of the MoS
x~2+δ/Mo films on the buffer Ar gas pressure. The results of LV measurements indicate that the smallest overpotential of HER was found for films deposited at pressures of 4 and 8 Pa (
Figure 11a). A current density of 10 mA/cm
2 was achieved at a voltage of
U10 ~206 mV. The Tafel slope was ~53.6 eV/dec. The MoS
x~2+δ/Mo films deposited in a vacuum and at a higher Ar pressure (16 Pa) required much more overvoltage to achieve a current density of 10 mA/cm
2, and their Tafel slope was as large as 56.7 mV (
Figure 11b). Additional anodic CV measurements and TOF calculations (
Figure 11c,d) revealed that the relatively good performance of the catalyst, deposited at a pressure of 4 Pa, was caused by a larger loading compared to that for the catalyst obtained at 8 Pa. Indeed, for the film obtained at 8 Pa, the TOF value at the voltage of −200 mV was ~0.023 s
−1, and for the film deposited at 4 Pa, the TOF was ~0.014 s
−1. The films deposited in Ar at 16 Pa also had relatively large TOF (~0.023 s
−1). However, at the pressure of 16 Pa, the MoS
x~2+δ/Mo film deposition rate was the lowest. These results were used to choose the Ar pressure of 8 Pa for the PLD of amorphous MoS
x films in the present work.
Comparison of the shapes of the anodic CV curves (anodic stripping voltammograms) for the MoS
x~2+δ/Mo films deposited at different Ar pressures indicated that an increase in Ar pressure of up to 8 Pa led to the formation of a curve in which a broad peak at ~700 mV was dominant (
Figure 11c). Other peaks at higher voltages, which were present on the CV curves for the MoS
x~2+δ/Mo films deposited in a vacuum and at Ar at 4 Pa, disappeared. This finding suggests that Ar pressure increases during the on-axis PLD, resulting in the formation of a homogeneous local structure of MoS
x~2+δ (0.7 ≤ δ ≤0.9) catalysts, in which all active sites possess an identical nature and participate in the HER. Notably, the oxidation of active sites in the amorphous MoS
x catalysts obtained by electrodeposition was registered at ~900 mV [
18,
37]. This finding indicates a possible difference in the local atomic structure of the active sites formed during the PLD and electrochemical deposition of amorphous MoS
x films.
Figure 12 shows the results of the studies of the electrocatalytic properties of the MoS
x~2+δ/Mo films that were deposited by on-axis PLD onto the glassy carbon in Ar at 8 Pa for different times. The polarization curves (
Figure 12a) show the benchmark activities of the MoS
x~2+δ/Mo films, where both the apparent geometric area and the catalyst loading are known. The LV measurements indicated that a noticeable catalytic effect from a thin film was observed for very short deposition times (~2 min). With MoS
x~2+δ/Mo catalyst loading of 6.8 μg/cm
2, an overpotential of −222 mV was required to achieve a current density of 10 mA/cm
2. With an increase in the deposition time to 64 min, the
U10 value decreased (in absolute value) to −154.5 mV. A significant time increase up to 128 min caused only a slight decrease of the
U10 to −150 mV. The Tafel slopes of the linear portions of the LV curves decreased from 53.7 to 50 mV/dec.
All anodic CV curves, shown in
Figure 12b, had approximately identical shapes in which a broad peak dominated. This peak shifted from ~700 to ~800 mV under loading growth. The intensity of the peak revealed an outrunning growth with catalyst loading increase that caused a change of the TOFs for these films. The highest TOF value, −200 mV (equal to 0.026 s
−1), was found for the catalyst with minimal loading. As the loading increased, the TOFs decreased, and the TOF values were in the range of 0.01 ± 0.002 s
−1 for the films with higher loadings.
The results of the study of the electrocatalytic properties of the MoS
x~3+δ films deposited on glassy carbon by off-axis PLD in Ar at 8 Pa for different times are shown in
Figure 13. For the lowest deposition time of 1 min (MoS
x~3+δ catalyst loading 10 μg/cm
2), an overpotential of −209 mV was required to achieve a current density of 10 mA/cm
2. An increase in the deposition time, up to a certain point (20 min), caused a monotonic decrease of the
U10 to −165.5 mV (
Figure 13a). The Tafel slope decreased from 48.2 to 44.4 mV/dec. Further time increases caused only a slight decrease of
U10 to −162 mV. The Tafel slopes of 50 and 44.4 mV/dec indicated that the HER was actually proceeded by a mechanism that was identical for the films obtained by on- and off-axis PLD.
The LV measurements indicated that the maximum achievable catalytic performance for the MoS
x~3+δ films was less than that for the MoS
x~2+δ/Mo films. This finding contradicted the results of the TOF measurements, which revealed that the TOFs were higher for the MoS
x~3+δ films (
Figure 13c) than for the MoS
x~2+δ/Mo films (
Figure 12c). Indeed, the highest TOF of ~0.05 s
−1 (at −200 mV) was found for the thinnest MoS
x~3+δ film that was deposited for 1 min. An increase in loading caused a decrease in the TOF of the MoS
x~3+δ film, which was ~0.024 s
−1 for the film deposited for ≥20 min. The larger TOFs for the MoS
x~3+δ films were due to both a lower current density during anodic striping CV and a narrower shape of the main peak, which is ascribable to the active site oxidation (
Figure 13b). The narrower CV peak located at ~710 mV indicated a uniform chemical state of the atoms in the local areas of the catalytically active sites on the surfaces of the MoS
x~3+δ films.