Optimizing MoS2‑Graphene Hybrid Crystallization for Enhanced Electrocatalysis

Background

Two‑dimensional (2D) hybrid nanomaterials are rapidly emerging across photovoltaics, water splitting, sensors, and energy storage. Their exceptional surface‑area‑to‑volume ratios and tunable electronic properties make them ideal for heterojunctions or 3D frameworks that integrate transition‑metal dichalcogenides (TMDs) with conductive graphene. In particular, MoS₂/graphene hybrids combine the high surface reactivity of MoS₂ with the exceptional electron transport of graphene, yielding catalysts that rival Pt in performance while dramatically reducing cost.

While pristine MoS₂ suffers from low intrinsic conductivity and graphene alone exhibits modest redox activity, their synergistic assembly creates a network of active sites and continuous charge pathways. Prior work has shown that hydrothermal routes—leveraging graphene as a crystallization scaffold—offer a low‑cost, scalable approach to fabricate these hybrids. However, the precise influence of hydrothermal temperature on MoS₂ crystallization and phase evolution in the presence of graphene remains underexplored.

Here we systematically vary the hydrothermal temperature (150–240 °C) to elucidate how growth conditions shape the morphology, crystal phase, and electrocatalytic performance of MoS₂/graphene hybrids. By integrating structural characterization with DSSC and HER testing, we identify 180 °C as the optimal temperature that balances high crystallinity, defect‑mediated activity, and electrical conductivity.

Methods

Material Preparation and Characterization

Hydrothermal synthesis of MoS₂/graphene hybrids was performed using microwave‑exfoliated graphene oxide (MEGO) as the substrate. MEGO was produced by 900 W microwave irradiation of graphite oxide for 90 s under argon, simultaneously reducing the material. 2.8 mg of MEGO was dispersed in 20 mL deionized water, followed by sequential addition of 42 mg sodium molybdate dihydrate and 84 mg thiourea (the excess thiourea further reduces MEGO). The suspension was transferred to 50 mL autoclaves and heated at 150, 180, 210, or 240 °C for 24 h, yielding MG‑150, MG‑180, MG‑210, and MG‑240, respectively. The solids were collected, washed, and dried under vacuum at 70 °C overnight.

Morphology and composition were examined by FE‑SEM (Hitachi S‑4800) and EDS mapping (Bruker detector). TEM and HRTEM (Hitachi H 9000 NAR) probed crystal structure, while XRD (Bruker D8 Discover) and Raman spectroscopy (Renishaw 1000B, 633 nm laser) assessed phase and crystallinity. XPS (VG ESCA 2000, Mg Kα) quantified Mo and S oxidation states and phase ratios.

DSSC Fabrication and Tests

FTO glass was sequentially cleaned with acetone, isopropanol, and DI water. A TiO₂ photoanode was assembled by doctor‑blading a commercial TiO₂ paste and sintering to 500 °C over 30 min. The TiO₂ layer was then sensitized with a 0.5 mM N719 ethanolic solution for 24 h. Counter electrodes were fabricated by doctor‑blading the hybrid slurries (20 mg sample + 5 µL Triton X‑100 in 500 µL DI water) onto FTO, followed by annealing at 500 °C for 30 min in argon. Pt counter electrodes were prepared similarly using a 0.01 M H₂PtCl₆ ethanolic solution. DSSCs were assembled with commercial electrolyte injection and sealed with thermoplastic film.

Photovoltaic performance was measured under AM 1.5G (100 mW cm^–2) illumination using a Keithley 2420 source meter. The system was calibrated with a Si reference cell. Electrochemical impedance spectroscopy (EIS) spanned 0.1–10 kHz under one‑sun illumination at 0.7 V bias, recorded on a CHI 760D workstation.

Electrochemical Measurements

HER and electrochemical tests employed a standard three‑electrode glass cell with a saturated Ag/AgCl reference (converted to RHE). All measurements were conducted in 0.5 M H₂SO₄ using a CHI 760D. Working electrodes were glassy‑carbon discs (3 mm diameter) coated with 5 mg of hybrid dispersed in 50 µL 5 % Nafion ethanol + 450 µL DI water, followed by drying. Linear sweep voltammetry (LSV) ran from 0.2 to –0.8 V vs. Ag/AgCl at 5 mV s^–1; Tafel plots were derived from LSV. Cyclic voltammetry (CV) scanned –1 to 1 V at 50 mV s^–1. EIS covered 0.1–10 kHz at 0.5 mV bias. Stability was evaluated over 20,000 s at –0.5 V vs. Ag/AgCl.

Results and Discussion

FE‑SEM images (Figure 1a–h) reveal that MoS₂ nucleates as perpendicularly oriented flower‑like nanoflakes across all temperatures. As synthesis temperature rises, the flake size enlarges, and the MG‑240 sample transitions from layered to nanoparticle morphology, indicating a loss of layer‑by‑layer growth at the highest temperature. The increased branching at mid‑range temperatures is advantageous because edges and defects act as catalytic active sites.

Morphology of MoS₂/graphene hybrids. SEM images of hybrids at 150 °C (a, e), 180 °C (b, f), 210 °C (c, g), and 240 °C (d, h); TEM/HRTEM of MG‑180 (i, j). Inset of (j) shows the SAED pattern (dashed circle). Lattice spacings are annotated in (j).

TEM/HRTEM of the MG‑180 sample confirms the intimate interfacial contact between MoS₂ lamellae and graphene. The 0.65 nm spacing matches the 2H‑MoS₂ (002) plane, while the 0.23 nm spacing corresponds to the graphene zig‑zag chain. SAED patterns corroborate the 2H‑MoS₂ crystallinity and the seamless stitching of graphene, indicating efficient electron transfer across the interface. Across all hybrids, higher temperatures yield sharper lattice fringes and better crystallinity.

XRD and Raman spectroscopy (Figure 2) further illustrate the temperature‑driven evolution. MG‑150 displays weak MoS₂ peaks, reflecting limited crystallization. At 180 °C, the (103) and (105) 2H peaks sharpen, and an additional (006 + 104) feature suggests the emergence of a 1T phase. The (100) peak broadening and shift confirm lattice rearrangement. Raman A_1g and E_2g intensities rise with temperature, and the high A_1g intensity in MG‑210 and MG‑240 indicates a perpendicularly oriented MoS₂ arrangement. The D/G ratio of graphene also increases, signaling stronger van der Waals coupling with MoS₂ at elevated temperatures.

Crystallization comparison of MoS₂/graphene hybrids. (a) XRD spectra of hybrids prepared at 150, 180, 210, and 240 °C versus MEGO. (b) Raman spectra of hybrids and MEGO. 2H peaks of MoS₂ are labeled.

XPS analysis (Figure 3) confirms a temperature‑dependent phase transition. Peak sharpening from MG‑150 to MG‑240 indicates progressive crystallization. The Mo 3d binding energy shifts ~0.63 eV from 180 °C to 240 °C, consistent with a 1T→2H transition. Deconvolution yields 2H:1T ratios of 4.84:1 (MG‑150), 3.01:1 (MG‑180), 13.7:1 (MG‑210); MG‑240 shows only 2H. Thus, optimal performance requires a balanced 1T content to provide metallic conductivity without excessive defect‑induced charge trapping.

Binding analysis of MoS₂/graphene hybrids. XPS spectra of hybrids at 150, 180, 210, and 240 °C. (a) Mo 3d orbits; (b) S 2p orbits.

Electrochemical testing reveals that the MG‑180 hybrid delivers the best catalytic performance. In DSSCs, all hybrids maintain a V_OC of ~0.7 V, but MG‑180 achieves the highest short‑circuit current (i_sc) and power conversion efficiency, approaching that of Pt counter electrodes. MG‑210 and MG‑240 exhibit reduced i_sc due to diminished charge transport stemming from over‑stacked MoS₂ layers. EIS data confirm the lowest charge‑transfer resistance for MG‑180, aligning with its superior performance.

DSSC schematic and performance. (a) Schematic of DSSC with MoS₂/graphene counter electrode. (b) J–V curves for hybrids; Pt delivers the highest efficiency, while MG‑180 closely follows. V_OC declines as synthesis temperature increases beyond 210 °C.

HER testing shows MG‑180 also outperforms its counterparts. Onset potentials are –176 mV (MG‑150), –179 mV (MG‑180), and –287 mV (MG‑210). The MG‑180 Tafel slope (74.5 mV dec^–1) is markedly lower than MG‑150 (137 mV dec^–1) and MG‑210 (not shown), indicating faster kinetics. EIS confirms a smaller semicircle for MG‑180, reflecting efficient interfacial charge transfer. BET analysis yields a specific surface area of 73.5 m² g^–1 for MG‑180, comparable to MG‑210 but far exceeding MG‑150 (49.5 m² g^–1), corroborating the branched morphology observed by SEM.

HER performance comparison. (a) Polarization curves (IR‑corrected). (b) Tafel plots for hybrids at 150, 180, and 210 °C. (c) Stability test of MG‑180 over 20,000 s at –0.5 V. (d) Overpotential comparison at 10 mA cm^–2 versus Pt/C, exfoliated MoS₂, and amorphous MoS₂.

The MG‑180 hybrid demonstrates robust stability, maintaining performance over 20,000 s at –0.5 V, and outperforms commercial Pt/C and exfoliated MoS₂ at the same current density. These results confirm that a hydrothermal temperature of 180 °C optimizes the defect density, 1T fraction, and morphological branching necessary for high catalytic activity.

Conclusions

By correlating structural evolution with electrocatalytic performance, this work establishes that MoS₂ crystallization on graphene is highly temperature‑dependent. At < 180 °C, the hybrid retains a 1T‑rich, defect‑laden phase; above 210 °C, the material transitions to the more stable 2H phase with diminished defects. The 180 °C hybrid exhibits the optimal blend of high crystallinity, active edge sites, and robust charge transport, delivering DSSC efficiencies and HER kinetics that rival Pt‑based catalysts. These insights provide a clear design principle for tailoring 2D hybrid electrocatalysts via controlled hydrothermal synthesis.

Abbreviations

2D

Two‑dimensional

3D

Three‑dimensional

BET

Brunauer‑Emmett‑Teller

CV

Cyclic voltammetry

DFT

Density functional theory

DSSC

Dye‑sensitized solar cell

EDS

Energy‑dispersive X‑ray spectroscopy

EIS

Electrochemical impedance spectroscopy

FE‑SEM

Field‑emission scanning electron microscope

FF

Fill factor

GCE

Glassy carbon electrode

HER

Hydrogen evolution reaction

HRTEM

High‑resolution transmission electron microscope

i_sc

Short‑circuit current

LSV

Linear sweep voltammetry

MEGO

Microwave‑exfoliated graphene oxide nanosheets

RHE

Reversible hydrogen electrode

SAED

Selected area electron diffraction

TEM

Transmission electron microscope

TMD

Transition metal dichalcogenide

V_OC

Open‑circuit voltage

XPS

X‑ray photoelectron spectroscopy

XRD

X‑ray diffraction