High-performance SrO–Co₃O₄ nanoparticles anchored on rGO for clean energy: hydrogen generation from formic acid and photocatalytic dye degradation

Abstract

SrO–Co₃O₄ nanoparticles loaded on reduced graphene oxide (SrO–Co₃O₄@rGO) were synthesized through a facile impregnation-reduction route. Physicochemical characterization was conducted to demonstrate the successful synthesis and the structure-property relationship of the catalyst. The catalysts were characterized using UV-Vis spectroscopy, from which their optical band gap was estimated. The SrO–Co₃O₄@rGO composite also exhibits enhanced visible-light absorption behavior compared with GO, implying greater visible-light adsorption capability. Moreover, the maximum absorption peak shifted from 270 to 260 nm, indicating that there was a little blue shift, which could be due to the result of electronic interactions between rGO and SrO–Co₃O₄ species. The crystalline phases of SrO, Co₃O₄, and rGO were characterized by XRD, and the well-distributed nanoparticle distribution on rGO sheets was determined by SEM-EDX. FT-IR confirmed the presence of metal-anchoring functional groups, and TGA showed good thermal stability of the graft. The Brunauer–Emmett–Teller (BET) test showed a mesoporous structure, a large surface area, and an appropriate pore-size distribution suitable for improved mass diffusion. Catalytic analysis of formic acid (FA) dehydrogenation yielded enhanced hydrogen evolution rates, and the highest apparent turnover frequency (TOF) was 5881.1 h⁻¹ under mild conditions. The reaction kinetics followed pseudo-first-order behavior, and the activation energy (E_a) was determined to be 37.34 kJ mol⁻¹. Moreover, the SrO–Co₃O₄@rGO photocatalyst was also more active towards methylene blue (MB) degradation and followed pseudo-first-order kinetics with an activation energy of 10.8 kJ mol⁻¹. The results of the radical scavenging experiments suggested that O₂˙⁻ and photogenerated holes h⁺ were the main reactive species. The rGO support may facilitate interfacial electron transfer, thereby facilitating directional charge transport and reducing recombination. The combined effects of SrO and Co₃O₄ present promising dual-function catalytic properties in SrO–Co₃O₄@rGO for clean energy generation and wastewater treatment.

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1. Introduction

Due to the growth of industrialization, urbanization, and agricultural development, the global water pollution crisis is becoming one of the most pressing environmental and public health issues of the 21st century.¹ The introduction of toxic organic wastes, especially synthetic colorants from the textile, leather, and paper sectors, into freshwater reservoirs poses a serious threat to aquatic life and human health, as these substances are non-biodegradable, extremely toxic, and may undergo bioaccumulation.² The most common of these is the cationic thiazine dye, methylene blue (MB), which is commonly found in industrial effluents.³ The fact that it remains in water despite its low concentration can lead to respiratory distress and methemoglobinemia, as well as long-term ecological damage, which are compelling reasons for the urgent need for effective and sustainable remediation technologies.⁴

Among the wide variety of remediation strategies, solar-light-driven photocatalysis has attracted significant interest due to its environmental friendliness, low energy requirements, and ability to completely mineralize organic pollutants. In this respect, photocatalytic transformation of organic pollutants using solar light as the energy source has emerged as a green and cost-effective method for complete mineralization of organic and inorganic pollutants.⁵ In photocatalysis, semiconductors are excited by light to generate electron–hole pairs and reactive oxygen species (ROS) that can oxidize pollutants into less harmful products such as CO₂ and H₂O.⁶ The development of visible-light-active photocatalysts is therefore a major focus of contemporary research.⁷ To enhance the utilization of visible light and inhibit charge recombination, carbon-based conductive supports such as graphene oxide (GO) and reduced graphene oxide (rGO) have been extensively used in semiconductor photocatalysts.⁸ Numerous studies have shown that GO-modified semiconductors (GO/TiO₂ and GO/ZnO composites) exhibit improved photocatalytic activity compared with their pristine counterparts, attributed to enhanced charge separation and improved visible-light absorption.⁹

Many advanced materials have been reported for photocatalytic and adsorption applications in recent literature. Recently, organo–inorganic 6,13-pentacenequinone/TiO₂ (PQ/TiO₂) nanocomposites have been studied and shown to increase the visible-light-driven photocatalytic degradation of methylene blue through better light absorption and interfacial charge separation.¹⁰ Similarly, 6,13-pentacenequinone (PQ) has also been reported to exhibit visible-light-driven photocatalytic activity for hydrogen production and methylene blue degradation under natural sunlight.¹¹ Likewise, by altering the electronic structure of ZnO and enabling photovoltaic conversion of incident solar energy, PQ-ZnO composites have also demonstrated improved photocatalytic effectiveness.¹² Biomass-derived adsorbents (e.g., Bael fruit shell) have also been investigated as low-cost, environmentally friendly materials for Cu and Pb removal owing to their porous structures and abundant functional groups.¹³ In addition, Fe-doped hydroxyapatite encapsulated in alginate matrices has also been studied due to its enhanced surface activity and structural stability, which provide high potential for adsorption performance.¹⁴ Due to the persistence and bioaccumulation of persistent organic pollutants (POPs) in aquatic environments, large-scale applications of efficient catalytic and photocatalytic wastewater remediation technologies are in demand.¹⁵ In addition, hierarchical ZnO/WO₃ heterostructures have attracted considerable interest for their enhanced charge separation at the heterojunction, thereby improving photocatalytic activity.¹⁶ Moreover, Layered MoS₂ systems and nano-α-Fe₂O₃ photocatalysts have also shown noticeable performance for the methylene blue degradation due to their increased absorption in the visible light spectral range and high surface reactivity.¹⁷ Ag-doped Fe₂O₃ nanostructures have emerged as promising systems with enhanced visible-light-driven photocatalytic dye-degradation capability compared to pristine Fe₂O₃, owing to improved light harvesting and efficient charge separation.¹⁸

Similarly, the Ni-modified ZnIn₂S₄/In(OH)₃ heterostructure systems have also demonstrated improved photocatalytic hydrogen evolution behavior due to enhanced charge separation and electron transfer.¹⁹ Recently, polyquaternium (PQ) – modified BiOI nanocomposites have shown improved visible-light-driven photocatalytic degradation of crystal violet dye by suppressing charge recombination and enhancing visible-light absorption.²⁰ In the same way, the chemical precipitation route synthesized CuO nanoparticles exhibit visible-light-driven photocatalytic degradation of methylene blue and other organic dyes, demonstrating the potential of transition metal oxide photocatalysts for wastewater remediation.²¹

To enhance photocatalytic performance and charge transport, carbon-based conductive supports have also been widely investigated. Advances in GO/rGO-based photocatalytic systems (2024–2026) have further demonstrated their potential for dye degradation.²² Incorporation of rGO has been demonstrated to enhance photocatalytic performance by improving charge separation, facilitating electron transport, and increasing the generation of reactive oxygen species in a variety of systems, including ZnO/GO, NiO/GO ternary composites, and rGO-supported ferrite.²³ Specifically, in these systems, the rGO acts as a conductive support to promote interfacial charge transfer and minimize electron–hole recombination. Additionally, diverse modification strategies for g-C₃N₄-based photocatalytic systems have been developed to further enhance charge separation and visible-light absorption via heterojunction construction, morphology control, and defect engineering.²⁴ However, even with these improvements, the systems reported still do not resolve issues related to light harvesting and the effective utilization of photogenerated charge carriers. Thus, further design of advanced oxide-rGO hybrid systems with interfacial interactions has become an important research focus.

Based on this background and recent developments, this research aims to fabricate a new, multi-purpose nanocatalyst for clean energy generation and water purification. Although mixed metal oxide systems based on transition metals such as cobalt (Co), have shown notable catalytic performance, the combined use of SrO and Co₃O₄ oxide phases on a conductive rGO support has not been extensively explored for simultaneous formic acid (FA) dehydrogenation and photocatalytic degradation of dyes. To fill this gap, a very recent study has also explored the potential of SrO–Co₃O₄ nanoparticles supported on ZnO for similar dual applications. Here, we describe the simple preparation of a SrO–Co₃O₄@rGO nanocomposite. Structural, optical, and textural characteristics of the material were described. It was tested in two different applications systematically: (i) hydrogen production through selective dehydrogenation of formic acid under mild conditions, and (ii) the photocatalytic degradation of methylene blue under solar-light conditions. In addition, the possible roles of the SrO–Co₃O₄ oxide phases and rGO support were investigated through reaction kinetics, thermodynamic parameters, and catalytic studies.

2. Experimental

2.1. Materials

Cobalt nitrate, strontium nitrate, graphite powder (99.9%), sulphuric acid, phosphoric acid, hydrochloric acid, formic acid, sodium formate, dimethylformamide, hydrogen peroxide, ethanol, and sodium borohydride (98%) are the chemicals and reagents used in this work. All reagents and chemicals, except organic solvents, were purchased from Sigma-Aldrich. Organic solvents were Lab Scan. Distilled water was used during the experiment.

2.2. Methods

2.2.1. Synthesis.
2.2.1.1. Synthesis of GO. GO was synthesized using the Tour method, a permanganate-based oxidative method.²⁵ In this process, phosphoric acid and sulfuric acid are mixed in a 1 : 9 volume ratio. The acidic medium is then cooled in an ice bath, and graphite flakes and potassium permanganate are added to a weight ratio of 6 : 1, respectively, to prevent excessive heat generation. KMnO₄ is used as an oxidizing agent and fits between the graphite layers, adding oxygen-containing functional groups such as hydroxyl, epoxide, and carboxyl groups on the sides of the graphite sheet as well as on the basal planes. This oxidation increases interlayer spacing, enabling exfoliation of graphite into individual GO sheets. The reaction mixture will then be heated at 50 °C and stirred for 12 h to ensure complete oxidation. The reaction is stopped after cooling by adding ice and 30% H₂O₂ to reduce any residual permanganate and manganese dioxide, turning the product vivid yellow or brown, indicative of fully oxidized GO. Phosphoric acid can not only regulate exothermic reactions but also improve oxidation efficiency and act as a dispersant, ensuring that GO exhibits greater structural regularity and oxidation degree.

2.3. Preparation of SrO–Co₃O₄ decorated on rGO

SrO–Co₃O₄ nanoparticles supported on rGO were synthesized by chemical reduction. To synthesize the SrO–Co₃O₄ nanocomposite supported on rGO, a 36 mM stock solution of Co(NO₃)₂ was prepared by dissolving 212.2148 mg of Co(NO₃)₂ in 100 mL of water. The SrO–Co₃O₄ catalyst was prepared by using Co(NO₃)₂, Sr(NO₃)₂, and rGO as support. The Sr : Co (molar ratio) of the precursor was 2 : 8, a total of 6.19 × 10⁻⁴ mol. In the SrO–Co₃O₄@rGO composite, the total loading of SrO and Co₃O₄ was fixed at 2 wt%. For a Sr : Co ratio of 2 : 8, 6.07 µL of Co(NO₃)₂ stock solution, 02.99 mg of Sr(NO₃)₂, 100 mg of rGO support, and 10 mL of distilled water were added to a round-bottom flask. After ultrasonication for 5 min, the mixture was stirred at 1000 rpm for 30 min. The mixture was stirred for an additional 24 h. Add 25 mg of NaBH₄ to 1 mL of distilled water in the above solution and mix for an additional 30 min after the initial 24 h. After 30 min, the aforementioned solution was centrifuged at 1000 rpm for 15 min. Following that, wash three times with distilled water and let the mixture dry overnight at 60 °C. SrO–Co₃O₄@rGO samples with different Sr : Co molar ratios (Sr : Co = 0 : 10, 1 : 9, 2 : 8, 3 : 7, 4 : 6, 5 : 5, 6 : 4, 7 : 3, 8 : 2, 9 : 1, 10 : 0) were prepared in the same way. During the reduction process, GO was partially reduced to rGO, thereby forming the SrO–Co₃O₄@rGO nanocomposite.

2.4. Analyses and characterization

Various characterizations were conducted to verify successful synthesis and structural integrity of rGO-supported SrO, Co₃O₄, and SrO–Co₃O₄ nanocomposites. The optical properties and electronic transitions of the materials were characterized by recording UV-Vis absorption spectra of the synthesized materials in the 200–800 nm wavelength range using a Shimadzu UV-1800 spectrophotometer (Japan). The functional groups were identified by processing the FTIR spectra of the samples in the 500–4000 cm⁻¹ range using the KBr pellet technique on a Shimadzu FTIR spectrophotometer (Japan) to reveal effective interactions between the metal precursors and the rGO support.

The SEM at 20 kV was used to assess the morphology and surface texture of the prepared catalysts. An SEM equipped with an EDX was used to analyze the elemental distribution and composition of the metal. XRD patterns in powder form were collected on a JEOL X-ray Diffractometer (JDX-3532) using Cu Kα radiation, and the spectra were scanned over a 2θ range of 5–100°. Crystalline structure and phase identification were carried out.

Thermal stability and decomposition behavior of the nanocomposites were studied by TGA under a nitrogen atmosphere over the temperature range from room temperature to 800 °C. Moreover, BET, which is based on nitrogen adsorption–desorption isotherms, was performed using a Micromeritics TriStar II 3020 to determine specific surface area, pore size distribution, and sample amount. All these characterizations of the series of catalysts revealed that the prepared catalysts were effectively synthesized, with unchanged composition, favorable morphology, good thermal stability, and a porous structure, all of which are essential to catalytic functionality.

2.5. H₂ gas production through degradation of FA

The conventional water displacement method has been used to assess the catalytic performance of the prepared SrO@rGO, Co₃O₄@rGO, and SrO–Co₃O₄@rGO nanocomposites for hydrogen evolution using FA. The equipment consisted of a two-necked round-bottom flask in a water trough, with an inverted graduated gas burette that enabled accurate reading of the volume of evolved gas. One of its necks was connected to the water-filled cylinder via an airtight rubber tube, the other to the inlet of reactants and catalyst.

In every experiment, a 10 mg catalyst (SrO@rGO, Co₃O₄@rGO, or SrO–Co₃O₄@rGO) was added to the flask along with 5 mL of the aqueous solution. The addition of a mixture of FA (72.80 µL, 0.0016 M) and SF (0.11 g) into the flask initiated the reaction. A homogeneous flow of the reacting mixture was maintained within the flask by magnetic stirring at 500 rpm. It was carried out isothermally in an oil bath at 60 °C in the flask. FA was removed by distillation as it was formed, and dehydrogenation of the FA by catalytic action split it into the slow evolution of H₂ and CO₂.

The displacement of water in the volumetric measuring device attached to the vessel determined the volume of the evolved gas. A daily measurement of the gas produced was recorded, and the net quantity of H₂ was calculated as the difference between the membrane water level at the start and end. To measure the selectivity of catalysts to dehydrogenation (as opposed to dehydration), gaseous products were determined on a Shimadzu GC-2014 gas chromatograph equipped with a Carboxen-1000 column and Thermal conductivity detectors.

2.5.1. Turnover frequency (TOF) calculation. Catalytic performance of SrO@rGO, Co₃O₄@rGO, and SrO–Co₃O₄@rGO nanocomposites was estimated based on TOF, expressed as moles of hydrogen gas produced per mole of active metal sites per unit time. TOF was calculated using the following formula:n_H2 is the number of moles of H₂ evolved (mol), n_metal is the number of moles of total active metal (Sr, Co, or both) present in the catalyst used (mol), and t is the reaction time in hours.

The evolved hydrogen gas was measured by the water displacement method and then converted to moles of hydrogen using the ideal gas law at standard conditions, unless adjusted for pressure and temperature. The metal stoichiometry in each catalyst was quantified according to the synthesis procedure or confirmed by EDX. In the case of SrO–Co₃O₄@rGO, the total metal was calculated as the sum of moles of SrO and Co₃O₄ present in the catalyst mass used for the reaction. Such TOF calculations enable comparisons of catalytic activity across various compositions and experimental conditions on a normalized basis by accounting for the number of active catalytic reaction centers.

It is important to note that this TOF value is calculated from the total moles of metal (Sr + Co) in the catalyst, assuming all metal atoms are catalytically active. In reality, only surface-accessible metal atoms participate in the reaction. Therefore, this value represents an upper-limit estimate or apparent TOF rather than the true intrinsic turnover frequency. Accurate determination of site-specific TOF would require normalization to surface metal atoms measured by chemisorption or electrochemically active surface area (ECSA) analysis, which will be addressed in future work.

2.6. Photocatalytic degradation of methylene blue

The photocatalytic activity of the prepared SrO–Co₃O₄@rGO nanocomposite was evaluated by monitoring MB degradation under sunlight, following procedures similar to those reported in the literature. In the experiment, 5 mg of SrO–Co₃O₄@rGO was suspended in 20 mL of a 10 ppm MB solution in a glass vial. To establish a stable adsorption–desorption system between the catalyst surface and the dye molecules, the suspension was stirred magnetically in the dark at room temperature for 3 min before irradiation. After this equilibration, the suspension was placed in natural sunlight under ambient conditions of moderate humidity and a UV index of 7. The qualitative sign of dye degradation was observed during irradiation: a progressive loss of the characteristic blue color of the MB solution.²⁶ Every 15 minutes, 2 mL samples were taken from the reaction mixture and processed by centrifugation to sediment the catalyst particles. The samples were then analyzed on a UV-Vis spectrophotometer by measuring absorbance at λ = 664 nm, the peak absorption wavelength of MB.

The photocatalytic experiments were carried out on clear, sunny days (between 11:00 AM and 2:00 PM) in March–April 2025 in Lahore, Pakistan (latitude 31.52° N). A digital lux meter (TES-1339, accuracy + −5%) and a UV power meter (Lutron UV-340A) were used to estimate solar irradiance, with the digital lux meter showing an average sunlight intensity of 98 000 + −5000 lux and the UV index of 7. Each experiment was repeated 3 times on different days under similar solar conditions (with less than a 10 percent difference). The degradation was negligible (less than 3 percent over 60 minutes) in control experiments conducted in the dark, supporting the notion that degradation is photo-driven.

The percentage degradation efficiency was calculated using the following formula:

where:

C₀ is the initial concentration of MB (at time t = 0), and C_t is the concentration of MB at time t. The obtained data were then plotted to form degradation curves and assess the photocatalytic effectiveness of the SrO–Co₃O₄@rGO catalyst over time. All experiments were conducted under the same lighting and environmental conditions to improve reproducibility.

3. Results and discussion

3.1. Synthesis of GO and SrO–Co₃O₄ nanoparticles supported on rGO

GO was synthesized via a well-known permanganate oxidation route, using the approach described by Tour, which yields high oxidation and structural uniformity, as shown in Fig. 1. H₂SO₄ and phosphoric acid (9 : 1 v/v) produced a strongly acidic, thermally controlled medium, and KMnO₄ served as the oxidizing agent; intercalation and functionalization of the graphite layers were easily carried out. The oxidation reaction produced an enormous amount of oxygen functional groups, e.g., hydroxyl, epoxide, and carboxyl groups, on basal planes and edges of graphite, which enable exfoliation of graphite to obtain few-layered GO flakes. The quenching reaction product, formed in the presence of ice and H₂O₂, turned yellow-brown in color, indicating that GO was formed. The presence of phosphoric acid also served a dual role, suppressing violent exothermic reactions and increasing the extent of oxidation, thereby producing GO that is more dispersible and structurally regular.

Fig. 1 Schematic illustration of the synthesis of SrO–Co₃O₄@rGO.

It was facile to deposit SrO–Co₃O₄ nanoparticles onto the surface of rGO via a simple impregnation and chemical reduction method. Co(NO₃)₂ aqueous solution and Sr(NO₃)₂ aqueous solution were taken as metal precursors, combined with 100 mg of prepared rGO, and then sonicated and stirred magnetically at low speed over time to ensure that the distribution of metal ions was uniform. NaBH₄ was used as a reducing agent, which facilitated the in situ reduction of metal ions on the rGO surface to form SrO and Co₃O₄ nanoparticles. The black precipitate was obtained by centrifugation and thoroughly washed to remove residual ions, then dried overnight at 60 °C. The procedure resulted in the formation of SrO–Co₃O₄@rGO nanocomposites with balanced particle dispersion and secure anchoring to the rGO sheet, supported by electrostatic and coordination forces between residual oxygen on the rGO and the metal ions.

Specifically, SrO–Co₃O₄@rGO and its molar ratio proved to be the most optimum composition, where the total loading of metals was 2 wt% relative to that of rGO. Multiple Sr : Co molar ratios (0 : 10–10 : 0) were used as a systematic approach to explore the occurrence of mixed oxide interactions and the impact of composition on catalytic performance. The attachment of SrO and Co₃O₄ to the rGO surface is essential for preventing particle agglomeration and maximizing dispersion, all of which are required to ensure high surface area and the availability of active sites. The chemical interaction between the metal nanoparticle and the rGO substrate enhances electron mobility and structural stability, both of which are major considerations for ensuring effective catalytic dehydrogenation of formic acid.²⁷

3.2. Characterization and analyses

3.2.1. UV-Vis spectroscopic analysis and band gap characterization. Fig. 2A shows the UV-Vis absorption spectra of GO and SrO–Co₃O₄@rGO. The UV-Vis spectra of GO reveal a distinctive strong absorption band assigned to the π → π* transition of the aromatic C=C bonds and an additional weak shoulder corresponding to n → π* transitions originating from oxygen-containing functional groups. The broader, extended absorption profile of the SrO–Co₃O₄ supported on rGO suggests enhanced visible-light absorption compared to GO alone (out to ≈600 nm). The origin of this broad absorption can be attributed to the combined contributions of restored sp² carbon domains in rGO, Co²⁺/Co³⁺ charge-transfer transitions in Co₃O₄, and interfacial electronic coupling between SrO–Co₃O₄ nanoparticles and the rGO sheets.

Fig. 2 (A) UV-Vis absorption spectra of GO and SrO–Co₃O₄@rGO, (B) the associated Tauc plots employed for the estimation of the optical band gap. The linear edge-only region was fitted with a tangent line and extrapolated to the photon–energy axis. (C) FTIR spectra of GO and SrO–Co₃O₄@rGO and (D) XRD patterns of GO, SrO@rGO, Co₃O₄@rGO, and SrO–Co₃O₄@rGO.

The optical band gap was estimated using the Tauc relation shown in Fig. 2B. The linear segment was selected exclusively from the absorption-edge region of the Tauc analysis, and the extrapolation was performed down to the photon energy axis. The apparent optical transition energy of the SrO–Co₃O₄@rGO nanocomposite was estimated at approximately 2.75 eV, indicating enhanced visible-light absorption compared to GO. The generation of the SrO–Co₃O₄@rGO heterostructure with a smaller crystal size, accompanied by a simultaneously longer absorption tail, suggests possible interfacial electronic interactions that may facilitate charge separation, thereby improving catalytic/photocatalytic performance.

The UV-Vis absorption peaks and apparent optical transition energies of GO and SrO–Co₃O₄@rGO are shown in Table 1. These results suggest that incorporating SrO and Co₃O₄ into rGO not only tunes the band structure but also enhances the material's optical response and electronic properties.

Table 1 UV-Vis absorption peaks and apparent optical transition energies of GO and SrO–Co₃O₄@rGO

Sample	Absorption peak (nm)	Apparent optical transition energies (eV)	Shift compared to rGO
GO	270 ± 2	4.90 ± 0.10	Reference
SrO@rGO	265 ± 2	2.25 ± 0.08	Blue shift
Co3O4@rGO	263 ± 2	2.45 ± 0.08	Blue shift
SrO–Co3O4@rGO	260 ± 2	2.75 ± 0.08	Blue shift

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The values obtained from the Tauc analysis, therefore, are referred to as apparent optical transition energies rather than exact intrinsic band-gap energies because of the heterogeneous electronic structures exhibited by GO/rGO systems that make them nonideal crystalline semiconductors.

3.2.2. FTIR analysis. The FTIR of GO and SrO–Co₃O₄@rGO in Fig. 2C gives information on the functional groups on the surface and the effective addition of the metal oxides onto the graphene structure. The GO spectrum shows a broad band at approximately 3400 cm⁻¹ due to the OH stretch vibrations of hydroxyl groups and adsorbed water, with a peak at approximately 1620 cm⁻¹ due to C=C stretching of graphitic domains. Other bands present in the region of about 1100–1200 cm⁻¹ are associated with C–O–C and C–O vibrations, indicating that the GO surface still contained oxygen-containing functional groups. When the SrO–Co₃O₄@rGO nanocomposite is formed, there are visible variations in the FTIR spectrum. The decrease in intensity of the oxygen-containing functional groups indicates a partial reduction of GO, and residual oxidized species are likely interacting with metal oxide species.

The FTIR spectra for GO and the SrO–Co₃O₄@rGO composite are presented in Fig. 2C. Well-established peaks representative of GO (C–O stretching 1303 cm⁻¹ and C=C skeletal vibrations 1590 cm⁻¹) are referred to in the literature; however, these features are strongly modified within the current system. The lower intensity of the peak around 1303 cm⁻¹ could be related to the partial removal of oxygen functional groups during GO reduction and also possible interaction between residual moieties and metal oxide species.

Likewise, the =intense band at ∼1589 cm⁻¹, typically observed as graphitic C–C vibrations in GO, is not clearly visible or can be shifted in the composite, which could be related to the structural recovery of rGO and possible interfacial interactions between rGO and SrO–Co₃O₄ nanoparticles. The above changes provide evidence of the GO-to-rGO transformation and confirm the successful formation of the composite structure. Interestingly, the emergence of absorption bands in 500–700 cm⁻¹ due to metal–oxygen vibrations (Sr–O and Co–O) suggests that metal oxides have been incorporated onto rGO. These spectral changes support the effective incorporation of metal oxide species onto the rGO framework.

3.2.3. XRD analysis. The XRD patterns in Fig. 2D reveal that the characteristic broad diffraction peak of GO is in the range of 10–12°, indicating the interlayer spacing of oxidized graphene sheets. This peak is much diminished or broadened in the composite specimens, indicating partial reduction of GO to rGO and regeneration of graphitic structures, consistent with the layered structure of the graphene support. In Co₃O₄@rGO, the discrete diffraction peaks were recorded at about 31°, 36°, 44° degree, 59° degree and 65° indexed to (220), (311), (400), (511) and (440) planes of Co₃O₄, thus, confirming the presence of crystalline cobalt oxide nanoparticles Likewise, the SrO phases have characteristic peaks in the SrO@rGO sample, which point to a successful incorporation of strontium oxide species within the rGO-supported composite.

In the case of SrO–Co₃O₄@rGO composite, the XRD pattern distinctly reveals the presence of two phases, both SrO and Co₃O₄, with peaks corresponding to planes such as (200), (220), (311), (111), and so on, which indicates the formation of a mixed SrO–Co₃O₄ oxide composite on rGO support. The composite exhibits somewhat wider peaks than the monometallic samples, which are attributed to nanoscale dispersed particles on rGO. The absence of additional impurity peaks further suggests high purity and successful synthesis of the catalyst, thereby supporting the synthesis of the composite material. In addition, these small variations might also cause minor shifts in peak position or broadening, which might correspond to the dispersion of nanoscale particles and structural defects in the composite.

3.2.4. SEM analysis. The obvious microstructure of the SrO–Co₃O₄@rGO is shown in Fig. 3. It shows a fibrous matrix with many crystalline or granular aggregates distributed on the surface. These surface features indicate successful in situ growth or deposition of inorganic particles on the structured support, most likely due to metal or metal-oxide nanoparticles grafted onto a porous organic or carbonaceous support.²⁸ The SEM images were taken using the instrument, displaying the original scale bars and magnification without any post-imaging modifications that would affect dimensional understanding.

Fig. 3 SEM images of (A) GO and (B) SrO–Co₃O₄@rGO nanocomposite, showing layered GO sheets and uniform dispersion of SrO–Co₃O₄ nanoparticles on rGO with a porous surface morphology and EDS spectra of (C) GO showing dominant C and O peaks and (D) SrO–Co₃O₄@rGO composite showing additional SrO and Co₃O₄ peaks, confirming successful incorporation without detectable impurities.

The uniformly distributed small particles indicate high surface area and porosity, which are beneficial for catalytic or adsorption applications.^29,30 The presence of the visible voids between the fibrous structure and embedded particles indicates the presence of interstitial pores that also bind up as a part of the total pore volume.

Fig. 3B is an SEM image taken at 1000× magnification. Also, here, the surface is heterogeneous, but the presence of a few spherical, irregularly shaped, very large or very small agglomerated particulates indicates the diversity in particle sizes. The differences in morphology at comparable scale bars are due to imaging distinct areas of the specimen and, hence, to local heterogeneity, nanoparticle agglomeration, and non-uniform distribution, rather than to inconsistencies in magnification.

This smoother matrix and larger particulates might indicate a comparatively lower surface area than in Fig. 3A. However, the uneven surface and the clear interparticle spaces indicated the presence of macro- and mesopores, which would also be conducive to diffusion and surface access. The presence of larger grains could be due to agglomerates of metal oxides or other crystalline phases.³¹

Both samples exhibit microstructures characteristic of porosity. While SEM alone does not provide precise pore sizes and volumes, the observed surface roughness, particle spacing, and distribution of embedded particles suggest a hierarchical pore structure, with likely contributions at the micro-, meso-, and macro-scales. The morphology of the pores is only qualitatively described by SEM, with indicative characterization provided by complementary BET and BJH analyses; pore structure cannot be inferred from SEM observations alone.

This morphology is characteristic of materials designed to enhance interaction with target molecules or ions, for adsorption, sensing, or catalytic applications.³² An accurate estimation of the pore size distribution and volume would require the utilization of BET and BJH analyses, but the morphology obtained by SEM strongly suggests a porous, reactive surface for the synthesized materials.³³

3.2.5. EDX compositional analysis. The EDX spectra given in Fig. 3 are of the elemental study of two samples. EDX can be used to identify the qualitative and semi-quantitative elemental composition of a sample by measuring the X-rays emitted by the sample in response to electron bombardment.³⁴

As shown in Fig. 3C, the EDS spectrum of GO shows that carbon and oxygen were very strong in the peaks, indicating that the graphite had been effectively oxidized using the process invented by Tour. The impurity was insignificant, indicating that the produced GO is phase-pure.

Comparatively, the SrO–Co₃O₄@rGO composite Fig. 3D possesses additional peaks, which prove the existence of SrO and Co₃O₄, and that they were effectively deposited onto the rGO surface. There is no complementary elemental indication to validate the composite's purity and the strength of the synthesis protocol. The small artifacts of impurity observed earlier were attributed to instrumental and environmental effects and have been removed in the re-analysis.

3.2.6. BET analysis. Nitrogen adsorption–desorption isotherms at 77 K were obtained to determine the specific surface area and porosity of the synthesized SrO–Co₃O₄@rGO catalyst (Fig. 4A–D). Isotherm profiles (linear and multipoint BET, BJH, and DFT) were used to investigate textural properties, including surface area, total pore volume, and pore-size distribution.^33,35

Fig. 4 (A) Nitrogen adsorption–desorption isotherms of SrO–Co₃O₄@rGO nanocomposite at 77 K, (B) multipoint BET linear plot for SrO–Co₃O₄@rGO sample, (C) BJH desorption pore size distribution curve of SrO–Co₃O₄@rGO, (D) DFT-based pore width distribution of SrO–Co₃O₄@rGO nanocomposite, and (E) TGA curves of GO and SrO–Co₃O₄@rGO nanocomposites under nitrogen atmosphere.

Nitrogen adsorption–desorption isotherms Fig. 4A reveal a type IV isotherm with an H3-type hysteresis loop at a relative pressure (P/P₀) ranging from 0.4 to 1.0, indicating the existence of a mesoporous structure with slit-like pores, which can often be observed in layered materials, for example, reduced graphene oxide. The high increase at higher P/P₀ values represents capillary condensation in the mesopores.³⁶

Multipoint BET analysis (Fig. 4B) yielded a specific surface area of 157.87 m² g⁻¹, indicating high porosity and a homogeneous distribution of the SrO–Co₃O₄ nanoparticles on the rGO support. This increased surface area provides more active sites for catalytic reactions such as H₂ generation and dye degradation.³⁷

Fig. 4C presents the pore size distribution calculated using the Barrett–Joyner–Halenda (BJH) model from the desorption branch, with a dominant pore size of approximately 16.0 nm, characteristic of mesoporousity. The total pore volume was 0.8780 cm³ g⁻¹, indicating significant internal voids that could facilitate the diffusion and adsorption of reactant molecules (e.g., formic acid and methylene blue dye).³⁸

Moreover, DFT in Fig. 4D exhibited a broader pore-size distribution, with pores as large as ∼20 nm. BET Surface Area, pore volume, and average pore diameter of GO and SrO–Co₃O₄@rGO are presented in Table 2.

Table 2 BET surface area, pore volume, and average pore diameter of GO and SrO–Co₃O₄@rGO

Parameter	Value	Method
BET surface area	157.87 m^2 g^−1	BET (multipoint)
Pore volume	0.878 cc/g	Isotherm/BJH
Average pore diameter	∼16–20 nm (160–200 Å)	BJH/DFT
Pore width distribution range	2–50 nm	DFT
Hysteresis loop type	H3	Isotherm
Isotherm type	IV	IUPAC

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3.2.7. Thermogravimetric analysis. Fig. 4E shows the TGA curves of pure GO and the SrO–Co₃O₄@rGO nanocomposite, which are plotted to illustrate their thermal stability and degradation properties. The TGA profiles show characteristic decomposition trends for the two materials in 30–800 °C range while the GO sample shows a narrow weight loss starting at 150 °C and then continuously decreases to 700 °C with an overall mass decrease of about 60%, in this temperature region, which can generally be attributed to the decomposition of residual oxygen-containing functionalities (OH, COOH and epoxy groups) resulting in the evolution of gases at medium to high temperatures.³⁹ It is also likely that this loss is due to the pyrolysis of carbonaceous structures in the presence of oxygen.

In comparison, the SrO–Co₃O₄@rGO nanocomposite exhibits markedly improved thermal stability. The onset of the major weight loss shifts to higher temperatures, and the total weight loss at 700 °C is much lower, approximately 40%.⁴⁰ This increase in thermal stability could be due to the presence of SrO and Co₃O₄ metal oxides in the rGO matrix, which provide structural support and suppress the volatility of thermally sensitive groups. The lower decomposition rate observed in SrO–Co₃O₄@rGO indicates that metal components have been successfully incorporated into the rGO framework and that stable metal–oxygen-carbon bonds form during the thermal decomposition. These stabilization effects are consistent with the TGA results for rGO decorated with metal oxide nanocomposites.^41,42

Such investigations not only demonstrate the enhanced thermal stability of SrO–Co₃O₄@rGO composites but also support their promising applications in catalysis, adsorption, energy storage, and conversion at high temperatures, as thermal stability is particularly important for these applications. The TGA results indirectly support the effectiveness of the synthesis method in loading metal species onto the rGO support. Decomposition ranges and total weight loss are expressed in Table 3.

Table 3 TGA results of GO and SrO–Co₃O₄@rGO

Sample	Decomposition range (°C)	Total weight loss (%)	Interpretation
GO	150–600	∼60	Decomposition of oxygen groups & carbon
SrO–Co3O4@rGO	200–700	∼40	Improved thermal stability due to metal oxide bonding

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3.2.8. Mott–Schottky (MS) analysis. To explore the electronic characteristics and density of charge carriers of the prepared materials, the Mott Schottky (MS) analysis was conducted in Fig. S1(A). The plots show positive slopes in both GO and SrO–Co₃O₄@rGO, suggesting semiconductor-like electronic behavior with electrons as the predominant charge carriers. The difference in the slope of GO as well as SrO–CO₃O₄@rGO points to some modifications in the interfacial electronic behavior that take place after incorporation of SrO–Co₃O₄ species onto the rGO framework. This behavior can be rationalized by interfacial interactions between the SrO–Co₃O₄ oxide species and the conductive carbon matrix, thereby enabling charge transfer during catalysis. The flat-band potential (E_fb) of this SrO–Co₃O₄@rGO composite was also calculated and found to be equal to ≈ −0.36 V (vs. Ag/AgCl). The negative flat-band potential suggests that electron transfer to the catalyst for catalytic reactions takes place under thermodynamically favorable conditions.

3.2.9. Photoluminescence (PL) analysis. The photoluminescence (PL) spectra were measured to analyze the recombination of the photogenerated charge carriers, as shown in Fig. S1(B). The GO sample exhibits a broad, strong emission band centered at 530–550 nm, attributed to radiative recombination via defect states and oxygen-functional groups. Conversely, the SrO–Co₃O₄@rGO composite exhibits a significant reduction in the intensity of the PL, which suggests that the recombination between electrons and holes is suppressed significantly. This quenching effect is achieved by the efficient transfer of electrons between SrO–Co₃O₄ active sites and the rGO support, which also acts as an electron acceptor and facilitates charge separation. The low recombination rate increases the charge-carrier lifetime, thereby enhancing overall catalytic efficiency. These findings demonstrate that the introduction of SrO and Co₃O₄ species is a feasible means of controlling the electronic framework and facilitating interfacial charge transfer in the composite system.

3.3. Catalytic dehydrogenation of formic acid

Catalytic dehydrogenation of HCOOH using SrO@rGO, Co₃O₄@rGO, and SrO–Co₃O₄@rGO catalysts Fig. 5A is dependent on the metal used and the mixed oxide combination. These catalysts, with respect to formic acid dehydrogenation, are not included in Table S1, which concerns photocatalytic dye degradation. It was found that SrO–Co₃O₄@rGO, among the three catalysts, had the best catalytic performance, fastest H₂ evolution, and the peak apparent TOF (5881.1 h⁻¹). SrO@rGO and Co₃O₄@rGO, on the other hand, had smaller TOFs of 1001.79 and 957.08 h⁻¹, respectively.⁴³ This enhancement of the SrO–Co₃O₄@rGO is attributed to the interfacial interaction between SrO and Co₃O₄ species, promoting electron mobility and generating active sites. It has been found that customized interfacial composition and electronic interactions are key to enhancing catalyst performance, as confirmed by many past studies in which cooperative metal interactions have improved catalytic performance.^44–46

Fig. 5 The plots of evolved gas (CO₂ + H₂) versus time for (A) SrO@rGO, Co₃O₄@rGO, and SrO–Co₃O₄@rGO catalysts with 30 mg dosage, in aqueous media, at pH 4, FA/SF (5 : 1) and 333 K temperature (B) different metal ratios in SrO–Co₃O₄@rGO catalyst at optimum conditions, (C) various dosage of SrO–Co₃O₄@rGO nanocomposite in aqueous media, at pH 4, FA/SF (5 : 1) and 333 K (D) various solvents using SrO–Co₃O₄@rGO catalyst along with their corresponding apparent TOF values shown in the insets.

3.3.1. Effect of metal ratios. Fig. 5B shows the impact of various molar ratios of Sr and Co in SrO–Co₃O₄ at the rGO catalyst. SrO–Co₃O₄@rGO composition possessed effective catalytic activity with apparent TOF = 5881.1 h⁻¹ compared to those of the other compositions. This response also indicates the need for an optimal system for the SrO–Co₃O₄ species to achieve a maximum combined surface effect and access to catalytic sites. Sr₁₀ Co₉₀ and Sr₉₀Co₁₀ both showed poorer catalytic activity, pointing to the fact that the excess presence of SrO or Co₃O₄ led to dominating the mono-metallic activity and hence a lack of cooperative mixed oxide catalysis. This is consistent with literature results indicating that an appropriate metal balance can promote better active-site behavior and surface redox characteristics.⁴⁷

3.3.2. Effect of catalyst dosage. The catalytic activity was shown to depend on the quantity of catalyst in the dehydrogenation reaction,⁴⁸ as depicted in Fig. 5C; increasing the ading (from 10 mg to 40 mg) leads to better H₂ production. The apparent TOF data suggest 30 mg of optimal loading, which offers an apparent TOF of 5881.1 h⁻¹. Beyond this point, TOF will decrease, likely due to agglomeration or deactivation of catalyst particles, or clogging of active sites, thereby reducing area and accessibility to the catalyst.⁴⁹ Consequently, an increased catalyst mass yields more active sites, and overloading can hinder mass transport and pore diffusion, thereby undermining per-site performance, as observed in research on metal-doped carbons.

3.3.3. Effect of solvent. From the solvent effect in Fig. 5D, it is obvious that the solvent has the most significant influence on catalytic activity. In water, the reaction exhibits the highest rates of hydrogen evolution and apparent turnover frequency (5881.1 h⁻¹) compared with those in DMF and ethanol.⁵⁰ This enhanced solubility and activation might occur because water is a polar protic solvent, which leads to improved interaction between the surface and the water of the catalyst. Ethanol and DMF, in turn, may occlude catalytic surface exposure or stabilize intermediates. This result aligns with earlier studies showing that water-rich settings boost catalytic dehydrogenation by shuttling protons and stabilizing unstable reaction intermediates.

3.3.4. Effect of pH. The pH exerts a significant influence on the activity of the catalyst, and the reaction is dependent on the acidic nature,^51,52 as illustrated in Fig. 6A. The value of the maximum hydrogen evolution rate and apparent TOF (5881.1 h⁻¹) appeared at pH 4, but it significantly declined at higher pH values. As the pH was adjusted to 10, the TOF was found to be far lower at 200.2 h⁻¹, indicating that an alkaline environment strongly inhibited catalytic activity. Greater activity at acidic conditions is due to the greater availability of protons and the strong binding of formate intermediates under acidic conditions to ensure that dehydrogenation is done effectively.⁵³ These findings are consistent with the findings in the literature, which imply a vital role of medium acidity in the regulation of the reaction kinetics as well as on the surface interactions on the catalyst.⁵⁴

Fig. 6 The plots of evolved gas (CO₂ + H₂) using SrO–Co₃O₄@rGO versus time for (A) pH 4, 6, 8, and 10 at 30 mg catalyst dosage, in aqueous media, at 333 K and FA/SF ratio 5/1 (B) various FA/SF ratio at 30 mg catalyst dosage, in aqueous media, pH 4, and 333 K temperature (C) 298, 313, 323, and 333 K temperature with 10 mg catalyst dosage, in aqueous media solvent at pH 4, and FA/SF ratio 5/1 along with their corresponding apparent TOF shown in insets (D) Arrhenius plot for determination of activation energy (E_a = 37.34 kJ mol⁻¹).

3.3.5. Effect of (FA : SF) ratios. The hydrogen evolution rate is plotted on the molar ratio of formic acid to sodium formate (FA : SF) as shown in Fig. 6B. The highest catalytic performance was afforded at a ratio of FA : SF of 5 : 1, providing the maximum apparent TOF of 5881.1 h⁻¹. Poor activity was observed in lower (1 : 1) and higher (7 : 1) ratios. It is agreed that an adequate quantity of SF is required to deprotonate FA and generate intermediate species⁵⁵ an excessive base could hinder the catalytic turnover due to excessive ionic interactions or by hindering active sites. Similar tendencies are achieved in base-promoted catalytic regimes when the formate ion plays a central role in the reaction mechanism.^56,57

3.3.6. Effect of temperature. The catalytic activity at different temperatures is depicted in Fig. 6C. The increase in temperature from 298 to 333 K increased the quantity of hydrogen that was produced.⁵⁸ The apparent TOF values changed from 1675.5 h⁻¹ at 298 K to 5881.1 h⁻¹ at 333 K, which is also consistent with the common Arrhenius kinetic characteristic of thermally activated reactions. High temperature is employed to increase the frequency of molecular collisions and to lower the activation energy of the reaction.⁵⁹ This enhancement demonstrates the value of thermal energy in boosting reaction rates and is consistent with catalytic systems that augment hydrogen evolution at elevated temperatures.

3.3.7. Estimation of activation energy. Fig. 6D shows the Arrhenius plot of ln(TOF) versus 1/T, revealing an apparent activation energy of 37.34 kJ mol⁻¹ for the reaction. The dehydrogenation of FA over SrO–Co₃O₄@rGO requires relatively low activation energy. Such a low energy barrier suggests favorable interaction between the catalyst and FA, facilitating good reaction kinetics and product desorption.⁴⁴

3.3.8. GC-TCD analysis. According to the GC-TCD chromatogram of the evolved gases during FA decomposition under optimized conditions (Fig. 7A), only H₂ and CO₂ are detected as products; carbon monoxide and methane are not detected. This establishes that the SrO–Co₃O₄@rGO catalyst follows a selective dehydrogenation pathway rather than the dehydration pathway, which typically produces CO. Specifically, the absence of CO is also important because CO is a common catalyst poison that can inactivate active catalytic sites during HER. The stoichiometric evolution of CO₂ and H₂ corresponds to the dehydrogenation reaction of FA, indicating the great catalytic activity and selectivity of the SrO–Co₃O₄@rGO system.²⁶

HCOOH → H₂ + CO₂ (3)

Fig. 7 (A) GC-TCD chromatogram of gas evolved during catalytic dehydrogenation of FA using 20 mg Sr₈₀–Co₂₀ with FA/SF (5 : 1) in aqueous media at a temperature of 333 K and pH 4, (B) mechanistic illustration of catalytic dehydrogenation of FA on SrO–Co₃O₄@rGO nanocomposite.

This elevated activity can be attributed to the synergistic effect of SrO and Co₃O₄ nanoparticles loaded on conductive rGO, which enhances electron transfer and β-elimination and reduces the amount of CO generated via dehydration. Also, the absence of secondary methanation and reduction reactions under the conditions used is evidenced by the presence of CO₂ but not CH₄.^60,61 The results indicate high selectivity of SrO–Co₃O₄@rGO for hydrogen production and a potential candidate for clean energy applications, where CO-free gas is applicable to fuel cell technologies.

3.3.9. Mechanism of catalytic dehydrogenation of FA. Fig. 7B depicts the possible process of the catalytic dehydrogenation of FA over the SrO–Co₃O₄@rGO catalyst in the performance trend of SF. The reaction follows a formate-anion pathway and proceeds through several consecutive steps. Originally, during the heterolytic splitting of the OH bond in HCOOH, there is the formation of a proton (H⁺) and a formate anion (HCOO⁻), which is adsorbed by the active surface on the SrO–Co₃O₄@rGO catalyst. In the following step, the formate species reaches a critical stage in the adsorption mode.⁶² The fact that the formate (Sr/Co anion to formate is bidentate) causes the C–H bond to be far away from the catalyst surface is not conducive to bond breaking. So, the formate should be in a monodentate conformation, placing the C–H bond near the active sites and facilitating cleavage of the catalyst. After this structural rearrangement, the system suffers a β-hydride abstraction to the surface-bound monodentate formate intermediate. The C–H bond is then cleaved, producing CO₂ gas and a surface-bound hydride (Sr/Co-[H]⁻), as the second step. The last step is through the combinative desorption of the surface hydride with the earlier adsorbed proton (H⁺), thereby forming molecular hydrogen (H₂) and re-forming the catalyst surface.^44,63–65 The introduction of SF is considered a key factor that substantially accelerated this transformation, due to the scission of the O–H bond and the increase in the steady-state concentration of the formate anion in the reaction medium. This mechanistic route indicates that SrO–Co₃O₄ active centers anchored on rGO likely contribute to the selective dehydrogenation of FA under mild conditions by stabilizing key intermediates and activating chemical bonds in each reaction.

3.3.10. Post-catalysis characterization. Post-catalysis XRD and FTIR analyses were conducted to assess the structural integrity and chemical stability of the SrO–Co₃O₄ annealed rGO catalyst following the catalytic reaction, as shown in Fig. S2.

The post-catalysis XRD pattern (Fig. S2(A)) shows that the typical diffraction peaks of SrO and Co₃O₄ phases are still intact following the reaction, which means that the crystalline structure of the catalyst is maintained. No further impurity peaks or phase changes are observed, indicating the material's structural integrity.⁶⁶ The slight broadening of the minor peaks and the slight loss of intensity indicate that restructuring of the catalyst surface occurred, or that dispersion was enhanced during catalysis, rather than degradation. These findings affirm that the active metal oxide phases are stable and do not readily leach under reaction conditions. Small shifts or broadening in the post-catalysis XRD peaks are attributed to adsorption-induced lattice strain, minor surface restructuring, and possible defect/oxygen-vacancy generation during catalysis. The retention of the major SrO and Co₃O₄ diffraction peaks suggests that the crystalline phases remain intact after reaction. Therefore, the observed peak shifts support dynamic surface participation of SrO–Co₃O₄@rGO during FA dehydrogenation rather than phase transformation or catalyst collapse.

On the other hand, the post-catalysis FTIR spectra, as shown in Fig. S2(B), indicate that the primary functional groups and M–O (Sr–O and Co–O) vibrations in the 500–700 cm⁻¹ range are retained, which proves that the chemical bonding in the catalyst is preserved. A minor drop in the intensity of oxygen-containing groups indicates their potential involvement in the catalytic process, likely as interaction sites for intermediates. Furthermore, there is a minor shift in peak locations following catalysis due to changes in the electronic and chemical environments of the active sites. The origin of such shifts is likely due to strong interfacial interactions between SrO–Co₃O₄ and rGO, leading to charge redistribution during catalytic processes. In addition, the adsorption of reaction intermediates and the possible modulation of oxidation states (e.g., Co²⁺/Co³⁺) may also contribute to this spectral behavior, thereby demonstrating the active nature of catalytic surfaces without structural degradation. In FTIR, shifts and intensity changes in oxygen-containing groups are attributed to the adsorption and activation of formate species at SrO–Co₃O₄@rGO active sites. During the reaction, formic acid first undergoes O–H bond cleavage, generating surface-bound formate species. These intermediates can interact with residual –OH, C–O/C–O–C, and metal–oxygen sites, leading to slight changes in vibrational frequency. The attenuation or displacement of C–O/C–O–C bands indicates participation of rGO oxygen groups in activating surface intermediates, while changes in the low-frequency metal–oxygen region suggest temporary modification of Sr–O and Co–O bonding environments during Co²⁺/Co³⁺ redox cycling and β-hydride elimination. Importantly, no new strong bands associated with irreversible organic residues or decomposition products are observed, confirming the catalyst's chemical stability.

Notably, no peaks of the degradation products are observed, indicating chemical stability. On the whole, the results demonstrate that the catalyst has both structural and surface stability, which helps it remain durable and retain its catalytic activity.

3.4. Photocatalytic degradation of the methylene blue

3.4.1. UV-Vis monitoring of methylene blue degradation. The result of the catalytic degradation of MB via SrO–Co₃O₄@rGO was under the observation of UV-Vis spectroscopy at time intervals of 0, 5, 10, 15, 20, 25, and 30 minutes, as demonstrated in Fig. 8A. The typical absorption peak of MB was gradually reduced as the time of the reaction increased, which pointed to the gradual destruction of the molecules of the dye.⁶⁷ It recorded a significant decrease in peak intensity, even during the initial 25 minutes, and nearly 100 percent decolorization was achieved at 30 minutes. Such a time-dependent decline confirms the activity of the SrO–Co₃O₄@rGO catalyst in assisting the oxidative degradation of MB under ambient conditions.⁶⁸ This enhanced catalytic effect may be ascribed to a combined effect between rGO-supported NPs of SrO and Co₃O₄. Strontium increases surface alkalinity and promotes the production of hydroxyl radicals, while cobalt furnishes redox-active sites that catalyze electron transfer and activate molecular oxygen.⁶⁹ The rGO matrix contributes to two important aspects: it stabilizes the nanoparticles and also renders charge mobility on the catalytic surface fast.^70,71 This is not just a combination that improves MB adsorption on the catalyst, but it also facilitates enhanced degradation through oxidative pathways. The combination of metal-oxide interfacial interactions and a high-surface-area suspension yields faster reaction kinetics and enhanced degradation compared with their unsupported and monometallic counterparts.⁷² Catalyst composition plays a key role in photocatalytic and oxidative activity, as indicated in these findings. The SrO–Co₃O₄@rGO complex provides an effective, robust, and reusable procedure for wastewater treatment, and its MB-degradation performance is highly efficient and stable.

Fig. 8 (A) UV-Vis absorbance spectra of MB solution under photocatalytic degradation using SrO–Co₃O₄@rGO catalyst at different time intervals (0–50 min). (B) Degradation efficiency (%) of methylene blue as a function of time at various temperatures (298 K, 313 K, 323 K, 343 K), showing increased degradation with temperature. (C) First-order kinetic plots (ln(A₀/A_t) vs. time) for methylene blue degradation at different temperatures, confirming pseudo-first-order reaction kinetics. (D) Comparison of degradation efficiency with standard deviation bars under different thermal conditions.

3.4.2. Degradation efficiency of methylene blue. Fig. 8B shows that the degradation efficiency of MB depended on reaction time across different temperatures (298 to 343 K). The degradation efficiency increased with increasing temperature, and the highest degradation rate was observed at 323 K within the 30 min interval. This reaction is thermally activated, as indicated by this behavior. Here, high temperature increases molecular mobility, thereby enhancing interactions between dye molecules and the catalyst's active sites.⁷³ Furthermore, increased kinetic energy at high temperatures facilitates the formation and diffusion of reactive species, such as hydroxyl and superoxide radicals, which play essential roles in the oxidative degradation of MB.⁷⁴ The SrO–Co₃O₄@rGO catalyst is stable, both in performance and structure, across all tested temperatures, thereby explaining its thermal stability and adaptability under different working conditions.

3.4.3. Comparative performance with literature reports. The photocatalytic activity of SrO–Co₃O₄@rGO for MB degradation was compared with that of recently reported GO/rGO-based photocatalysts. Each catalyst reported here is consistently listed in Table S1 in the SI, along with its corresponding precipitation conditions. Rather than detailing the particulars of each study, these reported systems include an aggregated performance trend. TiO₂-PRGO continued to degrade (100% in 90 min),⁷⁵ and GO-ZnO-Ag continued to degrade (100% in 40 min).⁷⁶ WN-C-RGO⁷⁷ and GO-SnO₂ (ref. 78) had 90% (UV) and 89% in 120 min and 60 min, respectively. MIL-100(Fe)/GO took 210 min to degrade 95 percent,⁷⁹ and GO/ZnO/lignin took 98 percent in 30–60 min.⁸⁰ Note that these studies were conducted under dissimilar experimental conditions, particularly with respect to the source of light (UV vs. sunlight), which can affect degradation efficiency and time. These results suggest that most of the reported catalysts used longer reaction times and, in some cases, specific irradiation conditions (e.g., UV light) to achieve high degradation efficiency.

Our SrO–Co₃O₄@rGO catalyst achieves 96% MB degradation within 30 min under natural sunlight, exhibiting relatively rapid degradation performance under these conditions. As such, this indicates improved reaction kinetics under more realistic (solar) scenarios.

This is due to: (i) interfacial SrO–Co₃O₄ heterojunctions that enhance charge separation; (ii) conductive rGO support that facilitates the movement of electrons; (iii) high surface area (157.87 m² g⁻¹); and (iv) apparent optical transition energy (2.75 eV) that allows the separation of charges and transport of electrons, respectively, and the visible-light harvesting. This combination facilitates effective electron–hole separation, fast charge transfer, and improved visible-light absorption. The high degradation rate indicates that SrO–Co₃O₄@rGO can be applied in real wastewater treatment. As a result, the current catalyst exhibits competitive photocatalytic performance in environmental real-world applications, despite some differences in experimental conditions. So the enhanced performance results not only from elevated efficiency but also from reduced response time enabled by abundant solar light, making it applicable to practical green technologies.

3.4.4. Pseudo-first-order kinetic analysis. To examine the reaction kinetics, experimental studies on photocatalytic degradation of MB were carried out using SrO–Co₃O₄@rGO at four distinct temperatures (25, 40, 50, and 70 °C) for 80 min under natural sunlight irradiation.²⁶ The absorbance was measured at varying time intervals, and the apparent rate constant (k) was estimated by the pseudo-first-order kinetic model:where A₀ and A_t represent the zero-time absorbance of MB and absorbance at time t, and k is the rate constant. The straight line of ln(A₀/A_t) versus time at various temperatures, as shown in Fig. 8C proves that the rate of degradation of MB in solution occurs under pseudo-first-order conditions, and the correlation coefficient (R²) value approaches unity.

3.4.5. Energy of activation and temperature-dependent kinetics. The effects of temperature on MB degradation were examined to determine the activation energy and the thermodynamic parameters (ΔH, ΔS, and ΔG). The activation energy was calculated from the Arrhenius plot of ln(k/T) versus 1/T (K⁻¹), showing characteristic thermally activated behavior in the photocatalytic degradation of methylene blue with the SrO–Co₃O₄@rGO catalyst, as shown in Fig. 9A. The activation energy was calculated using the Arrhenius equation.where k is the rate constant, R is the general gas constant (8.314 J mol⁻¹ K⁻¹), and T is the absolute temperature. The linear plot of ln k vs. 1/T in Fig. 9B yielded an activation energy of about 10.8 kJ mol⁻¹, indicating that the degradation process occurs with a lower than average activation energy. ΔH and ΔS were calculated by the Eyring–Polanyi equation:

Fig. 9 (A) Arrhenius plot of ln(k/T) versus 1/T (K⁻¹) for the photocatalytic degradation of methylene blue using SrO–Co₃O₄@rGO catalyst, indicating thermally activated behavior, (B) Arrhenius plot of ln k versus 1/T (K⁻¹) used to calculate the activation energy (E_a = 10.8 kJ mol⁻¹), (C) the plot of reusability performance of SrO–Co₃O₄@rGO catalyst during photocatalytic degradation activity. (D) Degradation efficiency of Methylene blue using different scavengers.

Eqn (1) was used to calculate the Gibbs free energy change:

ΔG = ΔH − TΔS (7)

The positive ΔH value implies that the reaction was endothermic, and the negative ΔS indicated a decrease in disorder at the transition. The positive values of ΔG at all observed temperatures indicate that the reaction is nonspontaneous. These findings are consistent with the trend toward more effective degradation at elevated temperatures, indicating that the heating energy facilitates photoactivation.

3.4.6. Mechanism of photocatalytic degradation of methylene blue. Photocatalytic degradation of organic dyes such as MB is mainly catalyzed by the formation of reactive oxygen species (ROS) a, including hydroxyl radicals (OH˙), superoxide anion radicals (O₂⁻˙), H₂O₂, and valence-band holes (h⁺), as shown in Fig. S3. Such species are important in the oxidation and subsequent mineralization of dye molecules. A radical scavenging experiment was carried out with four target specific reagents: disodium ethylenediamine acetate (Na₂EDTA) to identify the prevailing reactive species role in the degradation pathway; p-benzoquinone (p-BQ) to determine O₂⁻˙, isopropanol (IPA) to determine OH˙, and L-Ascorbic acid (L-AA) to determine H₂O₂.⁸¹

In all trials, a 0.2 mM solution of the corresponding scavenger was pipetted into a 10 ppm MB dye solution containing 20 mg of the SrO–Co₃O₄@rGO catalyst. The efficiency of dye degradation was significantly reduced in the presence of any scavenger, suggesting that all four ROS species were involved in the solar-light-driven degradation process. The strongest inhibition, however, was observed in the presence of p-BQ and Na₂EDTA. The decomposition process is central to the superoxide radicals (O₂⁻˙), and valence band holes (h⁺) as depicted in Fig. 9D.

To further examine the possible electron-transfer pathway, potentials of the conduction band and valence band of the catalyst were also computed with the Butler–Ginley equations (eqn (8) and (9)):

E_CB = X − E_C − 0.5 E_g (8)

E_VB = E_CB + E_g (9)

In this, E_C, E_g, and X show the energy of free electrons (4.5 eV), the apparent optical transition energy obtained from the Tauc plot, and Mulliken's average absolute electronegativity (2.75 eV), respectively. The values of E_CB and E_VB were found to be −1.755 and 0.995 eV, respectively. Since the reduction potential of O₂/O₂˙⁻ is approximately −0.33 eV and the conduction band is at −1.755 eV, this suggests that superoxide radicals are generated, consistent with the experimental observation that O₂˙⁻ is the dominant species in this process. The mechanism of azo dye degradation was proposed based on the role of reaction radicals in the process, as shown in eqn (5)–(13). According to the proposed mechanism, the catalyst's valence electrons are excited by natural sunlight and promoted into the conduction band, generating electron–hole pairs. Dissolved oxygen molecules trap the excited electrons and result in the formation of active reactive superoxide radicals (O₂⁻). Such radicals also react with several oxygen species, forming reactive oxygen species, which are likely involved in dye degradation.⁸²

i. The reaction starts when the SrO–Co₃O₄@rGO catalyst is activated by absorbing light energy from sunlight, as shown in Fig. S3. This light energy must be equal to or greater than the band gap of the catalyst. As a result, the electrons present in the valence band get excited and shift to the conduction band, leaving a hole in the valence band. This process may initiate photocatalytic degradation as shown in eqn (10):

Sr–Co@rGO + hv(UV) → Sr–Co@rGO[h⁺(VB) + e⁻(CB)] (10)

ii. The conduction band electron experiences a reduction reaction along with dissolved oxygen, resulting in the formation of superoxide radical as shown in eqn (11):

O₂ + Sr–Co@rGO[e⁻(CB)] → O₂ (11)

iii. Valence band holes oxidize water molecules or hydroxyls, attributed to surface-adsorbed molecules, which form hydroxyl radicals as in eqn (12):

H₂O_(ads) + Sr–Co@rGO[h⁺(VB)] → HO_(ads)⁻ + H⁺ (12)

iv. The produced protons react in an unfavorable reversible reaction with O₂˙⁻ to produce hydroperoxyl radicals as shown in eqn (13):

O_2(ads)⁻ + H⁺ € HOO_(ads)⁻ (13)

v. The hydroperoxyl radicals combine to produce molecular oxygen and hydrogen peroxide as depicted in eqn (14):

2HOO_(ads)⁻ → H₂O₂ + O₂ (14)

vi. Breakdown of hydrogen peroxide results in the formation of hydroxyl free radicals as depicted in eqn (15):

H₂O_2(ads) → 2HO_(ads)⁻ (15)

vii. MB dye molecules may undergo reduction by conduction band electrons, resulting in the formation of reduced products as shown in eqn (16):

MBDye + Sr–Co@rGO[e⁻(CB)] → Reductionproducts (16)

viii. Likewise, MB dye molecules are oxidized by valence band holes, resulting in the formation of oxidized products as described in eqn (17):

MBDye + Sr–Co@rGO[h⁺(VB)] → Oxidationproducts (17)

ix. In its turn, the produced hydroxyl radicals are expected to contribute to the oxidation of MB dye and its intermediates, carbon dioxide, and water, as shown in eqn (18):

MBDye + 2HO⁻ → Intermediates → CO₂ + H₂O (18)

3.4.7. Catalyst recyclability and stability. The main inference on the reusability of the SrO–Co₃O₄@rGO catalyst for MB degradation was drawn from five consecutive cycles, and the results are shown in Fig. 9C. The catalyst's degradation efficiency in the first cycle was high, at about 78%. In the second cycle, the degradation was slightly reduced to 74%, and by the fifth cycle, it was 60%. This gradual loss of activity has been attributed to partial deactivation of the active sites, mild nanoparticle aggregation, or minor surface fouling from the accumulation of reaction intermediates on the catalyst surface. The catalyst exhibits favorable recyclability and operational stability, with over 75% of the initial activity maintained after three cycles and over 60% after five cycles. The stability of catalytic activity across subsequent cycles indicates the strong structural integrity of the SrO–Co₃O₄@rGO composite, in which the rGO-based support helps disperse metal nanoparticles and prevents leaching. The combined contribution of SrO and Co₃O₄ oxide phases is another aspect, as SrO can enhance structural support and Co₃O₄ redox cycling, even after repeated use. The findings validate the practicality of SrO–Co₃O₄@rGO as a recyclable nano-catalyst for wastewater treatment, exhibiting high initial activity and acceptable durability upon repeated use.

4. Conclusion

In this study, SrO–Co₃O₄ NPs dispersed on rGO were effectively prepared and characterized using UV-Vis spectroscopy, FTIR, XRD, SEM-EDX, TGA, and BET. UV-Vis and Tauc plot analyses indicated that SrO–Co3O4@rGO exhibits improved visible-light absorption characteristics and is active in photocatalysis. BET analysis showed a high surface area and mesoporous structure, providing a large surface area and improved catalytic activity. TGA confirmed the thermal stability of the catalyst, and SEM-EDX indicated that SrO and Co₃O₄ are evenly distributed on the rGO surface. The catalyst showed enhanced HER performance via selective dehydrogenation of FA under its optimum condition: Sr : Co ratio, catalyst loading 30 mg, the solvent was water, FA/SF ratio was 5 : 1, pH was 4, and the temperature was 333 K, at which the apparent frequency (TOF) was 5881.1 h⁻¹ and recovered all gas in 6 minutes. The selectivity of the catalyst in the dehydrogenation pathway was evidenced by GC-TCD analysis which only showed the release of CO₂ and H₂ but no CO. Photocatalytic degradation of MB represented the effective and fast activity of the catalyst in photocatalytic degradation of the quality standard, under the optimum conditions as 96% degradation of the MB was achieved within 30 min using catalysts dose of 5 mg, 10 ppm MB solution and natural sunlight irradiation. Kinetics indicated a pseudo-first-order model, whereas thermodynamic values showed an endothermic, non-spontaneous reaction. The radical-scavenging and band-structure results suggest a possible Z-scheme-like charge-transfer pathway, in which photoexcited electrons and holes produce reactive oxygen species (˙OH, O₂˙⁻, H₂O₂), which degrade the dyes into minerals. Altogether, the combined effect of SrO and Co₃O₄ oxide phases, as well as the high conductivity and surface functionality of rGO, helps to increase the efficiency of both HER and photocatalytic dye degradation. The study on SrO–Co₃O₄@rGO presents a promising catalyst capable of clean hydrogen production and wastewater decontamination, representing a promising future in sustainable energy and environmental protection. Future work will focus on determining the true surface site-specific TOF using chemisorption techniques to complement the apparent TOF values reported here. We appreciate that the exploitation of natural sunlight, though environmentally sensitive, introduces variation in light intensity. The next round of work will involve a calibrated solar simulator (AM 1.5 G filter, 100 mW cm⁻²) to ensure homogenized photocatalytic performance and a more reliable comparison with results reported in the literature.

Author contributions

Sayyar Ali Shah: methodology, investigation, formal analysis, writing – original draft. Shah Faisal Mohammad Hanbing Song: writing – review & editing, formal. Imtiaz Hussain: writing – review & editing, formal analysis. Anisa Arif: writing – review & editing, formal analysis. Muhammad Saad Riaz: writing – review & editing, formal analysis. Ibrahim A. Shaaban: writing – review & editing, formal analysis. Akhtar Hayat: writing – review & editing, formal analysis. Umar Nishan: writing – review & editing, formal analysis. Hanbing Song: writing – review & editing, formal analysis. Azhar Abbas: conceptualization, supervision, project administration, writing – review & editing, formal analysis.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

The data supporting the findings of this study are openly available in Figshare at https://doi.org/10.6084/m9.figshare.32365377. The repository includes supplementary characterization data, mechanistic illustrations, and additional analyses of catalytic performance associated with this work. Supplementary information (SI): physicochemical characterization, mechanistic investigations such as Mott–Schottky and photoluminescence studies, post-catalytic XRD & FTIR stability tests, a proposed photocatalytic degradation mechanism and comparative performance study against recent generations of photocatalysts. See DOI: https://doi.org/10.1039/d6ra00101g.

Acknowledgements

The authors extend their appreciation to the Deanship of Research and Graduate Studies at King Khalid University, Saudi Arabia, through Large Research Project under grant number RGP-2/304/47. We are also grateful to Shandong Xiehe University for the partial support of this work.

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