Molybdenum Diselenide and Tungsten Diselenide Interfacing Cobalt-Porphyrin for Electrocatalytic Hydrogen Evolution in Alkaline and Acidic Media

Easy and effective modification approaches for transition metal dichalcogenides are highly desired in order to make them active toward electrocatalysis. In this manner, we report functionalized molybdenum diselenide (MoSe2) and tungsten diselenide (WSe2) via metal-ligand coordination with pyridine rings for the subsequent covalent grafting of a cobalt-porphyrin. The new hybrid materials were tested towards an electrocatalytic hydrogen evolution reaction in both acidic and alkaline media and showed enhanced activity compared to intact MoSe2 and WSe2. Hybrids exhibited lower overpotential, easier reaction kinetics, higher conductivity, and excellent stability after 10,000 ongoing cycles in acidic and alkaline electrolytes compared to MoSe2 and WSe2. Markedly, MoSe2-based hybrid material showed the best performance and marked a significantly low onset potential of −0.17 V vs RHE for acidic hydrogen evolution reaction. All in all, the ease and fast modification route provides a versatile functionalization procedure, extendable to other transition metal dichalcogenides, and can open new pathways for the realization of functional nanomaterials suitable in electrocatalysis.


Introduction
Transition metal dichalcogenides (TMDs) have taken centerstage during the past decade due to their interesting properties stemming from their two-dimensional structure [1]. However, a key challenge remains the adaptation of versatile functionalization routes for TMDS en route to the preparation of functional materials, especially efficiently performing in electrocatalysis. TMDs are great alternatives to expensive and finite-source platinum-based electrocatalysts, due to their low-cost, earth abundance, and intrinsic activity toward hydrogen evolution reaction (HER) [1]. Specifically, metallic TMDs (1T type) show intrinsic reduced charge-transfer resistance compared to semiconducting (2H type) ones, which is favorable in electrocatalysis. Interestingly, transition metal diselenides are most understudied among the TMDs family, compared to molybdenum disulfide (MoS 2 ), with tungsten diselenide (WSe 2 ) being less explored compared to its Mo counterpart molybdenum diselenide (MoSe 2 ). This is quite odd since it seems that heavier chalcogens provide better electrical conductivity, which is critical for energy applications [2,3] while tungsten is more affordable and has a more benign nature in comparison with its molybdenum correspondent [4]. TMDs combined with other species, [5][6][7][8][9][10][11] or creating interesting architectures [12][13][14] have shown increased electrocatalytic activity in acidic and alkaline

Materials and Methods
General. All chemicals, reagents, and solvents were purchased from Sigma-Aldrich (Burlington, MA, USA) and used without further purification unless stated otherwise.
Characterization techniques. Raman measurements were acquired with a Renishaw in Via Raman spectrometer, equipped with a CCD camera and a Leica microscope at 514 nm. Renishaw Wire and Origin software were used to record and analyse the data, respectively. Mid-infrared spectra, in the region of 500-4500 cm −1 were obtained on a Fourier Transform IR spectrometer (Equinox 55 from Bruker Optics (Ettlingen, Germany)) equipped with a single reflection diamond ATR accessory (DuraSamp1IR II by SensIR Technologies (Danbury, CT, USA)). Scanning transmission electron microscopy (STEM) imaging and electron energy loss spectroscopy (EELS) techniques have been developed using a probe-corrected Titan low-base (Thermo Fisher Scientific, Waltham, MA, USA) working at 120 kV. This microscope is equipped with a high-brightness field-emission gun (X-FEG) and with a Gatan Image Filter (GIF) Tridiem ESR 866 spectrometer for EELS acquisitions. XPS data were recorded using a Kratos Axis Supra spectrometer equipped with a monochromated Al Kα X-ray source using an analyzer pass energy of 160 eV for Nanomaterials 2023, 13, 35 3 of 13 survey spectra and 20 eV for the core level spectra. Spectra were recorded by setting the instrument to the hybrid lens mode and the slot mode providing approximately a 700 x 300 µm 2 analysis area using charge neutralization. Regions have been calibrated using the reference value BE(C 1s sp2) = 284.5 eV. All XPS spectra were analyzed using CASA XPS software. The XPS peaks were fitted to GL(70) Voigt lineshape (a combination of 70% Gaussian and 30% Lorentzian character), after performing a Shirley background subtraction. TGA Q500 V20.2 Build 27 instrument by TA in nitrogen (purity > 99.999%) inert atmosphere was used for thermogravimetric analysis.
Electrochemical measurements. Autolab PGSTAT128N potentiostat/galvanostat was used for the electrochemical measurements. A standard three-compartment electrochemical cell was used equipped with an RDE with a glassy carbon disk (geometric surface area: 0. 196 cm 2 ) as a working electrode, graphite rod as a counter-electrode, and Hg/HgSO 4 (0.5 M K 2 SO 4 ) or Hg/HgO (0.1 M KOH) as reference electrodes. HER LSV measurements were performed in N 2 -saturated aqueous 0.5 M H 2 SO 4 solution or 0.1 M KOH at room temperature. For preparing the catalyst ink, catalytic powder (4.0 mg) was dissolved in a mixture (1 mL) of deionized water, isopropanol, and 5% Nafion (v/v/v = 4:1:0.02) followed by sonication for 30 min before use. The working electrode was polished with 1, 3, and 6 mm diamond pastes, washed with deionized water, and finally sonicated in double-distilled water before casting 8.5 µL aliquots of the electrocatalytic ink on the electrode's surface. Finally, electrochemical impedance spectroscopy (EIS) measurements were acquired from 10 5 to 10 −1 Hz with an AC amplitude of 0.01 V.
Preparation of WSe 2 . Tungsten hexacarbonyl (3 mmol) and selenium powder (5.8 mmol) were dissolved in dry p-xylene (145 mL) and the resulting suspension was transferred into a 300 mL Teflon-lined stainless-steel autoclave reactor and heated at 250 • C for 24 h. After the autoclave was cooled to room temperature, the resulting suspension was filtrated through a PTFE membrane filter (0.2 µm) to remove unreacted species and rinsed with acetone and dichloromethane to remove unreacted species to obtain WSe 2 (790 mg).
Preparation of MoSe 2 . Molybdenum hexacarbonyl (3 mmol) and selenium powder (5.8 mmol) were dissolved in dry p-xylene (145 mL) and the mixture was transferred to a 300 mL Teflon-lined stainless-steel autoclave reactor and held at 250 • C for 24 h. The resulting suspension was filtrated through a PTFE membrane filter (0.2 µm) to remove unreacted species and rinsed with acetone and dichloromethane to get MoSe 2 (810 mg).
Preparation of f-WSe 2 . In a round bottom flask, WSe 2 (30 mg) was dispersed in H 2 O (30 mL). After sonication, 4-aminopyridine (0.3375 mmol) was added to the mixture and sonicated for 60 s. The resulting suspension was filtrated through a PTFE membrane filter (0.2 µm) and rinsed with water to obtain f-WSe 2 (48 mg).
Preparation of f-MoSe 2 . In order to obtain f-MoSe 2 , a similar procedure was followed. 5-(4-carboxyphenyl)-10,15,20-triphenylporphyrinCo(II). CoCl 2 (54 mg, 0.22 mmol), 5-(4-carboxyphenyl)-10,15,20-triphenylporphyrin (10 mg, 0.015 mmol) and DMF (5 mL) were added in a round bottom flask and the reaction mixture was heated to reflux for 4 h. After, DMF was distilled; the residue was dissolved in a mixture of CH 2 Cl 2 together with two drops of methanol. The organic layer was washed two times with water and dried over Na 2 SO 4 followed by filtration and evaporation. Finally, column chromatography (SiO 2 , CH 2 Cl 2 /MeOH, 100:4) and reprecipitation of the product with CH 2 Cl 2 and cold hexane furnished CoP as a dark red-brown solid (9 mg, 83%). The 1 H-NMR spectrum showed broad peaks due to the presence of paramagnetic Co(II). FT-IR: materials 2022, 12, x FOR PEER REVIEW 3 of 12 Spectra were recorded by setting the instrument to the hybrid lens mode and the slot mode providing approximately a 700 x 300 µm 2 analysis area using charge neutralization. Regions have been calibrated using the reference value BE(C 1s sp2) = 284.5 eV. All XPS spectra were analyzed using CASA XPS software. The XPS peaks were fitted to GL(70) Voigt lineshape (a combination of 70% Gaussian and 30% Lorentzian character), after performing a Shirley background subtraction. TGA Q500 V20.2 Build 27 instrument by TA in nitrogen (purity > 99.999%) inert atmosphere was used for thermogravimetric analysis. Electrochemical measurements. Autolab PGSTAT128N potentiostat/galvanostat was used for the electrochemical measurements. A standard three-compartment electrochemical cell was used equipped with an RDE with a glassy carbon disk (geometric surface area: 0. 196 cm 2 ) as a working electrode, graphite rod as a counter-electrode, and Hg/HgSO4 (0.5 M K2SO4) or Hg/HgO (0.1 M KOH) as reference electrodes. HER LSV measurements were performed in N2-saturated aqueous 0.5 M H2SO4 solution or 0.1 M KOH at room temperature. For preparing the catalyst ink, catalytic powder (4.0 mg) was dissolved in a mixture (1 mL) of deionized water, isopropanol, and 5% Nafion (v/v/v = 4:1:0.02) followed by sonication for 30 min before use. The working electrode was polished with 1, 3, and 6 mm diamond pastes, washed with deionized water, and finally sonicated in double-distilled water before casting 8.5 μL aliquots of the electrocatalytic ink on the electrode's surface. Finally, electrochemical impedance spectroscopy (EIS) measurements were acquired from 10 5 to 10 −1 Hz with an AC amplitude of 0.01 V.
Preparation of WSe2. Tungsten hexacarbonyl (3 mmol) and selenium powder (5.8 mmol) were dissolved in dry p-xylene (145 mL) and the resulting suspension was transferred into a 300 mL Teflon-lined stainless-steel autoclave reactor and heated at 250 °C for 24 h. After the autoclave was cooled to room temperature, the resulting suspension was filtrated through a PTFE membrane filter (0.2 μm) to remove unreacted species and rinsed with acetone and dichloromethane to remove unreacted species to obtain WSe2 (790 mg).
Preparation of MoSe2. Molybdenum hexacarbonyl (3 mmol) and selenium powder (5.8 mmol) were dissolved in dry p-xylene (145 mL) and the mixture was transferred to a 300 mL Teflon-lined stainless-steel autoclave reactor and held at 250 °C for 24 h. The resulting suspension was filtrated through a PTFE membrane filter (0.2 μm) to remove unreacted species and rinsed with acetone and dichloromethane to get MoSe2 (810 mg).
Preparation of f-WSe2. In a round bottom flask, WSe2 (30 mg) was dispersed in H2O (30 mL). After sonication, 4-aminopyridine (0.3375 mmol) was added to the mixture and sonicated for 60 s. The resulting suspension was filtrated through a PTFE membrane filter (0.2 μm) and rinsed with water to obtain f-WSe2 (48 mg).
Preparation of WSe2-CoP. CoP (7.3 mg) was added to dry dichloromethane (15 mL) and sonicated for 30 min under an N2 atmosphere. Next, EDCI (13.42 mg, 0.07 mmol) was added at 0 °C and the mixture was left to stir for 1 h, followed by the addition of f-WSe2 (8 mg), HOBt (13.51 mg, 0.1 mmol), and N,N-diisopropylethylamine (20 μL). The dispersion was then left to stir for five days at room temperature. Finally, the resulting Preparation of WSe 2 -CoP. CoP (7.3 mg) was added to dry dichloromethane (15 mL) and sonicated for 30 min under an N 2 atmosphere. Next, EDCI (13.42 mg, 0.07 mmol) was added at 0 • C and the mixture was left to stir for 1 h, followed by the addition of f-WSe 2 (8 mg), HOBt (13.51 mg, 0.1 mmol), and N,N-diisopropylethylamine (20 µL). The dispersion was then left to stir for five days at room temperature. Finally, the resulting suspension was filtrated through a PTFE membrane filter (0.2 µm) and rinsed with dichloromethane, methanol and acetone to obtain 15 mg of WSe 2 -CoP.
Preparation of MoSe 2 -CoP. A similar procedure was followed for the preparation of MoSe 2 -CoP.

Results and Discussion
Firstly, MoSe 2 and WSe 2 were prepared by a one-pot reaction in an autoclave [32]. Heating of molybdenum hexacarbonyl (Mo(CO) 6 ) (tungsten hexacarbonyl (W(CO) 6 ) alternatively) and Se powder in p-xylene produced grams of 1T-MoSe 2 (and 1T-WSe 2 , respectively). Afterward, sonication (ca. 1 min.) of the aqueous dispersion of TMDs and aminopyridine yielded amino terminated groups to the transition metal diselenides, via the formation of a metal-ligand bond between Mo or W and the pyridinic nitrogen, furnishing functionalized MoSe 2 (f-MoSe 2 ) and WSe 2 (f-WSe 2 ), respectively. Next, a condensation reaction between amino-modified transition metal diselenides and carbonyl-terminated CoP led to the formation of MoSe 2 -CoP and WSe 2 -CoP, as seen in Figure 1. suspension was filtrated through a PTFE membrane filter (0.2 µm) and rinsed with dichloromethane, methanol and acetone to obtain 15 mg of WSe2-CoP. Preparation of MoSe2-CoP. A similar procedure was followed for the preparation of MoSe2-CoP.

Results and Discussion
Firstly, MoSe2 and WSe2 were prepared by a one-pot reaction in an autoclave [32]. Heating of molybdenum hexacarbonyl (Mo(CO)6) (tungsten hexacarbonyl (W(CO)6) alternatively) and Se powder in p-xylene produced grams of 1T-MoSe2 (and 1T-WSe2, respectively). Afterward, sonication (ca. 1 min.) of the aqueous dispersion of TMDs and aminopyridine yielded amino terminated groups to the transition metal diselenides, via the formation of a metal-ligand bond between Mo or W and the pyridinic nitrogen, furnishing functionalized MoSe2 (f-MoSe2) and WSe2 (f-WSe2), respectively. Next, a condensation reaction between amino-modified transition metal diselenides and carbonyl-terminated CoP led to the formation of MoSe2-CoP and WSe2-CoP, as seen in Figure 1. The morphology, structure, and chemical composition of these hybrids were analyzed via scanning transmission electron microscopy (STEM). Figure 2a shows that flakes of around one micron were obtained. They are similar to the ones of the starting MoSe2 material ( Figure S1). High-resolution high-angle annular dark field (HAADF) STEM images of these flakes of MoSe2-CoP show that the TMD material corresponds to the 1T phase (Figure 2b,c). Similar STEM-HAADF imaging analyses were also performed on WSe2-CoP ( Figure S2). Electron energy-loss spectroscopy (EELS) in the spectrum-line model [33,34] was employed to locally investigate the chemical nature of these nanostructures and in particular to identify the CoP moieties. An EELS spectrum-line was recorded in the highlighted area of Figure 2c. From this EELS spectrum-line, 15 EEL spectra, recorded in the red-marked line of Figure 2c, were selected and added. Two different energy regions of the EEL spectra are displayed in Figure 2d,e. The detection of C, N, and Co in this area confirms the presence of the CoP entities at the surface of MoSe2. The morphology, structure, and chemical composition of these hybrids were analyzed via scanning transmission electron microscopy (STEM). Figure 2a shows that flakes of around one micron were obtained. They are similar to the ones of the starting MoSe 2 material ( Figure S1). High-resolution high-angle annular dark field (HAADF) STEM images of these flakes of MoSe 2 -CoP show that the TMD material corresponds to the 1T phase (Figure 2b,c). Similar STEM-HAADF imaging analyses were also performed on WSe 2 -CoP ( Figure S2). Electron energy-loss spectroscopy (EELS) in the spectrum-line model [33,34] was employed to locally investigate the chemical nature of these nanostructures and in particular to identify the CoP moieties. An EELS spectrum-line was recorded in the highlighted area of Figure 2c. From this EELS spectrum-line, 15 EEL spectra, recorded in the red-marked line of Figure 2c, were selected and added. Two different energy regions of the EEL spectra are displayed in Figure 2d In order to follow the modification that occurred on MoSe2 and WSe2, w ultimate goal to verify the successful formation of MoSe2-CoP and WSe2-CoP, spectroscopic, thermal, and microscopy imaging techniques were employed. For typical bands of pyridine rings are seen in the IR spectrum of f-MoSe2 due to the su coordination of the former at the metal of MoSe2, at 3430 (N-H), 1651 (C-N) and 1 (C-H (Figure 3a). On the other hand, CoP shows the carbonyl vibration mode at 16 due to the surface -COOH units. The IR spectrum of MoSe2-CoP is occupied characteristic band at 1641 cm −1 owned to the carbonyl amide stretching du successful hybridization, while N-H vibrational features are evident in MoSe2-C similar manner, the effective grafting of amino groups onto WSe2 is obvious characteristic bands due to the pyridine ring at 3430 (N-H), 1646 and 1329 (C-N), a cm −1 (C-H) ( Figure S3a). Additionally, the coupling of amino-modified WSe2 w carbonyl terminated CoP is shown at 1650 cm −1 , due to the carbonyl amide stretc seen in Figure S3a.  Figure S3a). Additionally, the coupling of amino-modified WSe 2 with the carbonyl terminated CoP is shown at 1650 cm −1 , due to the carbonyl amide stretching, as seen in Figure S3a. similar manner, the effective grafting of amino groups onto WSe2 characteristic bands due to the pyridine ring at 3430 (N-H), 1646 and 1 cm −1 (C-H) ( Figure S3a). Additionally, the coupling of amino-modi carbonyl terminated CoP is shown at 1650 cm −1 , due to the carbonyl a seen in Figure S3a. Next, Raman spectra were recorded upon excitation at 514 nm. Specifically, the spectrum of MoSe 2 is guided by the A 1g -LA(M), A 1g , E 1 2g , and 2LA(M) bands at 171, 238, 285, and 450 cm −1 respectively. Meanwhile, bands located at 118, 137, and 223 cm −1 correspond to J 1 , J 2, and J 3 phonon modes, respectively, and are indicative of the 1T octahedral phase of MoSe 2 [35]. In the spectrum of f-MoSe 2 , the suppression of the 2LA(M) band, which is associated with chalcogen vacancies and defect sites, compared to MoSe 2 is indicative of the metal-ligand coordination that took place at the defect sites. The fact that there is no alteration in the intensity of the 2LA(M) mode in MoSe 2 -CoP indicates that the covalent grafting of CoP onto MoSe 2 does not affect the basal plane of MoSe 2 (Figure 3b). Accordingly, the spectrum of WSe 2 shows apparent prominent peaks at 175, 237, and 256 cm −1 corresponding to A 1g -LA(M), E 1 2g , A 1g , and 2LA(M) modes, while E 1 2g , and A 1g modes overlap [36][37][38]. Additional J 1 , J 2 , and J 3 bands located at 112, 148, and 216 cm −1 , respectively, are fingerprint phonon modes of the metallic phase of WSe 2 [34]. As mentioned previously, the decrease in the intensity of the 2LA(M) band of f-WSe 2 reveals the successful functionalization, while intensity remained the same for WSe 2 -CoP ( Figure S3b).
Insight into the MoSe 2 and WSe 2 phases was given from X-ray photoelectron spectroscopy (XPS) measurements. Figure 4a depicts the deconvoluted Mo 3d spectra, showing doublets at 228.6 eV for Mo 3d 5/2 and 231.8 eV for Mo 3d 3/2 , revealing the existence of the 1T phase in MoSe 2 . The doublet at higher energies, 232.9 eV for 3d 5/2 and 235.7 for 3d 3/2 , corresponds to Mo 6+ [39,40]. Similarly, deconvoluted W 4f spectra are shown in Figure 4b. The apparent doublet of the 4f W +4 peaks at 31.7 for W 4+ 4f 7/2 and 33.9 eV for W 4+ 4f 5/2 is assigned to the 1T phase, while signals at 36.04 and 38.22 eV indicate the presence of W 6+ species within WSe 2 [40,41]. The Se 3d peak profile of MoSe 2 can be fitted into two sets of doublet peaks at higher energies (55.0 eV for Se 3d 3/2 and 54.2 eV for Se 3d 5/2 ) for the 2H phase and another couple at lower energies (54.4 eV for Se 3d 3/2 and 53.6 eV for Se 3d 5/2 ) for the 1T phase (Figure 4c) [39,40]. The same applies to the Se 3d peak profile of WSe 2 ; peaks at 55.3 eV for Se 3d 3/2 and 56.1 eV for Se 3d 5/2 for the 2H phase and 54.7 eV for Se 3d 3/2 and 53.7 eV for Se 3d 5/2 for the 1T phase (Figure 4a) [40,41]. The above results are aligned with the Raman data suggesting the strong presence of the 1T phase in MoSe 2 and WSe 2 .  Thermogravimetric analysis (TGA) was employed to gather information about the thermal stability of the hybrid materials under an N2 atmosphere. It can be seen in Figure  5 that MoSe2 losses an 14% mass up to 550 °C, attributed to the generation of defects during the solvothermal preparation ( Figure 5). A higher mass loss of up to 21% is shown for the f-MoSe2 up to the same temperature region, revealing the thermal decomposition of the incorporated organic species onto MoSe2. Finally, MoSe2-CoP presents an additiona mass loss of 9% due to the successful grafting of CoP to f-MoSe2. Rest on the above, the loading in the MoSe2-CoP material was calculated to be one CoP for every 16 f-MoSe2 units Furthermore, Figure S4 shows the thermographs of WSe2-based materials. Accordingly, the loading in the WSe2-CoP was calculated to be one CoP for every 16 f-WSe2 units. Thermogravimetric analysis (TGA) was employed to gather information about the thermal stability of the hybrid materials under an N 2 atmosphere. It can be seen in Figure 5 that MoSe 2 losses an 14% mass up to 550 • C, attributed to the generation of defects during the solvothermal preparation ( Figure 5). A higher mass loss of up to 21% is shown for the f-MoSe 2 up to the same temperature region, revealing the thermal decomposition of the incorporated organic species onto MoSe 2 . Finally, MoSe 2 -CoP presents an additional mass loss of 9% due to the successful grafting of CoP to f-MoSe 2 . Rest on the above, the loading in the MoSe 2 -CoP material was calculated to be one CoP for every 16 f-MoSe 2 units. Furthermore, Figure S4 shows the thermographs of WSe 2 -based materials. Accordingly, the loading in the WSe 2 -CoP was calculated to be one CoP for every 16 f-WSe 2 units.  Steady-state electronic absorption spectroscopy provided further proof for the formation of MoSe2-CoP and WSe2-CoP (Figures 6 and S5, respectively). The absorption spectra for intact MoSe2 and WSe2 exhibit strong and broad absorption in the visible region without any distinct features, typical for TMDs of 1T-phase derived from bottom-up approaches [36,37]. Meanwhile, in the absorption spectrum of MoSe2-CoP, the Soret band at 410 nm is evident, also visible in the UV-Vis spectrum of CoP (Figure 6), whereas the band of MoSe2 centered at 537 nm is blue-shifted at 502 nm indicating the electronic communication of MoSe2 and CoP within MoSe2-CoP hybrid at the ground state. Moreover, the UV-Vis spectrum of WSe2-CoP ( Figure S5) shows a distinct absorption band at 410 nm, deriving from the successful conjugation of f-WSe2 with CoP.  Steady-state electronic absorption spectroscopy provided further proof for the formation of MoSe 2 -CoP and WSe 2 -CoP ( Figure 6 and Figure S5, respectively). The absorption spectra for intact MoSe 2 and WSe 2 exhibit strong and broad absorption in the visible region without any distinct features, typical for TMDs of 1T-phase derived from bottom-up approaches [36,37]. Meanwhile, in the absorption spectrum of MoSe 2 -CoP, the Soret band at 410 nm is evident, also visible in the UV-Vis spectrum of CoP (Figure 6), whereas the band of MoSe 2 centered at 537 nm is blue-shifted at 502 nm indicating the electronic communication of MoSe 2 and CoP within MoSe 2 -CoP hybrid at the ground state. Moreover, the UV-Vis spectrum of WSe 2 -CoP ( Figure S5) shows a distinct absorption band at 410 nm, deriving from the successful conjugation of f-WSe 2 with CoP.  Steady-state electronic absorption spectroscopy provided further proof for the formation of MoSe2-CoP and WSe2-CoP (Figures 6 and S5, respectively). The absorption spectra for intact MoSe2 and WSe2 exhibit strong and broad absorption in the visible region without any distinct features, typical for TMDs of 1T-phase derived from bottom-up approaches [36,37]. Meanwhile, in the absorption spectrum of MoSe2-CoP, the Soret band at 410 nm is evident, also visible in the UV-Vis spectrum of CoP (Figure 6), whereas the band of MoSe2 centered at 537 nm is blue-shifted at 502 nm indicating the electronic communication of MoSe2 and CoP within MoSe2-CoP hybrid at the ground state. Moreover, the UV-Vis spectrum of WSe2-CoP ( Figure S5) shows a distinct absorption band at 410 nm, deriving from the successful conjugation of f-WSe2 with CoP. Next, electrocatalytic acidic HER for hybrids and reference materials was evaluated by recording linear sweep voltammetry (LSV). Notably, significant low onset potential was recorded for MoSe2-CoP at −0.17 V vs RHE, lower than those recorded for MoSe2, f-MoSe2, and CoP at −0.22, −0.35 and −0.28 V vs RHE, respectively (Figure 7a). The beneficial role of CoP was further revealed by evaluating the HER at −10 mA/cm 2 . In fact, MoSe2-CoP operates HER at −10 mA/cm 2 at −0.31 V vs RHE, ca. 100 mV lower than that of MoSe2, at −0.41 V vs RHE. For f-MoSe2 and CoP, higher onset potential values are noted at −0.47 and −0.52 V, respectively. At the same time, hydrogen evolution for WSe2-CoP starts at lower potentials, at −0.22 V vs RHE compared to reference materials, while the lowest potential value was required to drive HER at a reference current density of −10 mA/cm 2 , Next, electrocatalytic acidic HER for hybrids and reference materials was evaluated by recording linear sweep voltammetry (LSV). Notably, significant low onset potential was recorded for MoSe 2 -CoP at −0.17 V vs RHE, lower than those recorded for MoSe 2 , f-MoSe 2 , and CoP at −0.22, −0.35 and −0.28 V vs RHE, respectively (Figure 7a). The beneficial role of CoP was further revealed by evaluating the HER at −10 mA/cm 2 . In fact, MoSe 2 -CoP operates HER at −10 mA/cm 2 at −0.31 V vs RHE, ca. 100 mV lower than that of MoSe 2 , at −0.41 V vs RHE. For f-MoSe 2 and CoP, higher onset potential values are noted at −0.47 and −0.52 V, respectively. At the same time, hydrogen evolution for WSe 2 -CoP starts at lower potentials, at −0.22 V vs RHE compared to reference materials, while the lowest potential value was required to drive HER at a reference current density of −10 mA/cm 2 , at −0.33 V i.e., 100 mV lower compared to WSe 2 (Figure 7d). The significantly lower overpotential value for driving protons reduction to molecular hydrogen for hybrid materials is justified by the presence of CoP, while the covalent grafting between CoP and MoSe 2 and/or WSe 2 promotes charge transfer and current flow driving the overall reaction to lower potentials. Nanomaterials 2022, 12, x FOR PEER REVIEW 9 of 12 at −0.33 V i.e., 100 mV lower compared to WSe2 (Figure 7d). The significantly lower overpotential value for driving protons reduction to molecular hydrogen for hybrid materials is justified by the presence of CoP, while the covalent grafting between CoP and MoSe2 and/or WSe2 promotes charge transfer and current flow driving the overall reaction to lower potentials. Tafel slope values were extracted to gain information on the reaction mechanism. In acidic HER protons are initially adsorbed onto the electrode surface via a reduction process (Volmer step). Afterward, the desorption of adsorbed hydrogen atoms onto the electrode (Heyrovsky step) or the recombination of two adsorbed protons (Tafel step) follows generating molecular H2. With all that said, the smooth current flow within the MoSe2-CoP is confirmed by the lower Tafel slope value of 114 mV/dec, compared to the higher Tafel slope value of MoSe2 (375 mV/dec), f-MoSe2 (123 mV/dec) and CoP (288 mV/dec), suggesting that the release of the molecular hydrogen onto the electrode is the rate-limiting step (Figure 7b). The same applies to WSe2-CoP which displays the lowest Tafel slope value (133 mV/dec) among Wse2-based electrocatalysts (Figure 6e).
Additional insight into HER kinetics is obtained by electrochemical impedance spectroscopy (EIS) assays. In Figure 7c, MoSe2-CoP shows the smaller frequency semicircle in the Nyquist plot compared to MoSe2 and f-MoSe2, corresponding to a smaller charge transfer resistance (Rct) value of 53 Ω than the much higher Rct values of ca. 63, 74 and 84 Ω for MoSe2, f-MoSe2, and CoP respectively. Similarly, WSe2-CoP shows a lower Rct value of 75 Ω compared to WSe2 (92 Ω) and f-WSe2 (119 Ω), (Figure 7f). The lower Rct values for hybrid materials reflect higher conductance and more facile electron transfer kinetics in MoSe2-CoP and WSe2-CoP due to the covalent linkage of the TMDs with the cobalt-metallated porphyrin.
The stability of hybrids was assessed, after performing 10,000 ongoing electrocatalytic cycles in acidic media as shown in Figures 7a and 6d, respectively. Interestingly, MoSe2-CoP and WSe2-CoP exhibited extremely high stability as they showed negatively shifted LSV curves of only 10 mV after continuous cycling. Tafel slope values were extracted to gain information on the reaction mechanism. In acidic HER protons are initially adsorbed onto the electrode surface via a reduction process (Volmer step). Afterward, the desorption of adsorbed hydrogen atoms onto the electrode (Heyrovsky step) or the recombination of two adsorbed protons (Tafel step) follows generating molecular H 2 . With all that said, the smooth current flow within the MoSe 2 -CoP is confirmed by the lower Tafel slope value of 114 mV/dec, compared to the higher Tafel slope value of MoSe 2 (375 mV/dec), f-MoSe 2 (123 mV/dec) and CoP (288 mV/dec), suggesting that the release of the molecular hydrogen onto the electrode is the rate-limiting step (Figure 7b). The same applies to WSe 2 -CoP which displays the lowest Tafel slope value (133 mV/dec) among Wse 2 -based electrocatalysts (Figure 6e).
Additional insight into HER kinetics is obtained by electrochemical impedance spectroscopy (EIS) assays. In Figure 7c, MoSe 2 -CoP shows the smaller frequency semicircle in the Nyquist plot compared to MoSe 2 and f-MoSe 2 , corresponding to a smaller charge transfer resistance (R ct ) value of 53 Ω than the much higher R ct values of ca. 63, 74 and 84 Ω for MoSe 2 , f-MoSe 2 , and CoP respectively. Similarly, WSe 2 -CoP shows a lower R ct value of 75 Ω compared to WSe 2 (92 Ω) and f-WSe 2 (119 Ω), (Figure 7f). The lower R ct values for hybrid materials reflect higher conductance and more facile electron transfer kinetics in MoSe 2 -CoP and WSe 2 -CoP due to the covalent linkage of the TMDs with the cobalt-metallated porphyrin. The stability of hybrids was assessed, after performing 10,000 ongoing electrocatalytic cycles in acidic media as shown in Figures 7a and 6d, respectively. Interestingly, MoSe 2 -CoP and WSe 2 -CoP exhibited extremely high stability as they showed negatively shifted LSV curves of only 10 mV after continuous cycling. Table S1 sums up the electrocatalytic acidic HER data before and after 10,000 cycles for hybrids and reference materials and Pt/C.
The superior electrocatalytic performance of MoSe 2 -CoP and WSe 2 -CoP was also observed towards alkaline HER, as seen in Figure S6a,d, respectively, as hybrid materials note significantly lower overpotentials compared to reference materials. Additionally, lower Tafel slope values were calculated for MoSe 2 -CoP ( Figure S6b) and WSe 2 -CoP ( Figure S6d) along with high R ct values, Figure S6c,f, respectively, reflecting the easier current flow within the hybrids. However, it should be noted that acidic HER is less complicated compared to alkaline HER. In the former case, the reduction of hydronium ions (H 3 O + ) to gaseous dihydrogen (H 2 ) occurs during water electrolysis, while in alkaline HER extra energy is needed to produce the protons by breaking the water molecule, affecting the overall reaction rates [42]. Additionally, in alkaline HER Volmer and Heyrovsky steps are likely to include a water-dissociation step [43] due to the vast decrease in proton concentration as detailed below: Indeed, the above are depicted in the higher overpotentials and Tafel slope values compared to acidic HER (Table S1). Finally, the higher stability for both electrocatalysts is shown in Figure S6a,d, while relative data are summarized in Table S1.
All in all, the advantageous role of CoP in electrocatalytic HER is depicted in both acidic and alkaline media within the hybrid materials. MoSe 2 -CoP and WSe 2 -CoP show improved performance compared to just MoSe 2 and WSe 2 as they achieve lowered overpotentials against HER, possess easier reaction kinetics and improved conductivity featuring the beneficial presence of CoP along with the effective conjugation with the TMDs.

Conclusions
The adaptation and exploration of versatile modification routes for TMDs toward the realization of novel electrocatalysts is a hot issue. In this work, we have demonstrated an easy and fast functionalization approach, via a metal-ligand covalent bond, for the introduction of amino-terminated pyridine rings onto MoSe 2 and WSe 2 and the subsequent covalent linkage with carbonyl-terminated cobalt(II) porphyrins. The new hybrids exhibited improved HER activity in acidic and alkaline media and showed easier reaction kinetics, higher conductivity, and stability compared to bare transition metal diselenides. Noticeably, MoSe 2 -CoP showed the best performance and reached a low onset potential of −0.17 V vs RHE towards acidic HER. The successful incorporation of cobalt-porphyrin onto MoSe 2 and WSe 2 speeded up the initiation of HER, while covalent linkage improved charge delocalization and transfer in neighboring species leading to high electrocatalytic activity for the reduction of protons to molecular hydrogen. Finally, the facile functionalization approach can be applied for the modification of other transition metals dichalcogenides and may open new routes for the realization and exploration of functional electrocatalysts.