Regulation and enhancement of the optical performance of graphene oxide through Ni/Fe/Ag nanoparticles doping

As the most prevalent derivative of graphene, graphene oxide (GO) comprises a layered sp²–sp³ carbon network modified by oxygen-containing functional groups. Because of its exceptional mechanical properties, high carrier mobility, and superior thermal conductivity, GO has extensive applications in gas sensors, supercapacitors, field-effect transistors, nonvolatile memory devices, flexible electronic devices, and various other domains.^1–6 Also, GO has outstanding linear optical (LO) properties due to its oxygen-containing functional groups. Numerous studies^7–10 have indicated that GO has a broad optical band gap, with its distinctive sp²–sp³ carbon hybrid serving as the foundation for the adjustable broadband gap of GO, making it a promising candidate for manufacturing optoelectronic devices.

Besides intriguing LO properties, the 2D structure of GO films combined with the aforementioned tunable optical band gap results in exceptional nonlinear optical (NLO) properties, characterized by strong nonlinear absorption (NLA) and significant Kerr nonlinearity.¹¹ The NLO absorption of GO fluctuates with light intensity and can manifest in various forms, ranging from saturation absorption (SA) to reverse saturation absorption (RSA) depending on the excitation wavelength and band gap of the specific GO film. SA has extensive applications in pulse compression, Q switching, and mode-locking, whereas RSA is used in optical limiting and the prevention of high-power laser damage.^12–14 Jiang et al.¹⁵ investigated the optical limiting behavior of GO thin films for femtosecond laser pulses at 800 and 400 nm, a property that could be adjusted by regulating the degree of reduction. Enhancing the NLO response of GO is very important for broadening its use in optical shutters, light limiting, information processing, and various photonic applications.

Recently, the NLO properties of GO and its composites have attracted significant research attention within the GO domain, leading to a plethora of pertinent literature. Ma et al.¹⁶ prepared reduced GO films via photochemical and photothermal reduction to acquire reduced GO, subsequently evaluating their NLO response. The results showed that reduced graphene oxide (rGO) exhibits significant optical nonlinearity, particularly in terms of two-photon absorption (TPA); an exceptionally high TPA coefficient of 10⁵ cm/GW was achieved via two photo-reduction mechanisms, revealing a substantial enhancement in the TPA of graphene-based materials. Hao et al.¹⁷ investigated the NLO properties of CuS/rGO composites using the hydrothermal method; the results showed that as the graphene content in CuS/rGO nanocomposites increases, a transition from RSA to SA occurs, suggesting that the enhancement in nonlinearity is due to the synergistic effect among multiple materials. Fraser et al.¹⁸ doped Au nanoparticles (NPs) into GO films acquired via vacuum filtration to achieve tunable third-order optical nonlinearity at varying doping concentrations. Kimiagar and Abrinaei¹⁹ synthesized MgO–GO nanocomposites using the hydrothermal method and determined their NLA coefficient β and nonlinear refraction (NLR) index n₂ at a laser intensity of 1.1 × 10⁸ W/cm² to be 10⁻⁷ cm/W and 10⁻¹² cm²/W, respectively.

At present, the NLO properties of GO composites predominantly encompass composites of GO and organic molecules, metal sulfides, metal nanoparticles (MNPs), or metal oxides. Metal materials have been extensively researched and utilized because of their high specific strength, high specific modulus, high corrosion resistance, exceptional electric heating properties, and superior machining characteristics. The combination of MNPs with GO induces a synergistic effect that enhances energy transfer. GO–MNP composites exhibit superior NLO properties compared to pristine GO, thereby offering increased potential for applications in photoelectric materials, optical limiting, and NLO theory research. However, the investigation of the NLO properties of composites of GO and precious-metal or metal-oxide NPs is currently incomplete, with no examination of the NLO properties of GO composites containing other MNPs such as Fe and Ni.

In the study reported herein, Ni, Fe, and Ag NPs were used for doping GO, and the LO and NLO properties of composites of GO with Ni, Fe, or Ag NPs were investigated systematically. The surface morphology and structure of these composites were analyzed using laser scanning confocal microscopy (LSCM), SEM, energy dispersive spectroscopy (EDS), XRD, and Raman spectroscopy. The optical absorption properties were analyzed using UV–visible absorption spectra, and the optical band gaps of both GO and its composites were determined from Tauc plots constructed from those spectra. Also, the NLO properties of the thin films were investigated using femtosecond laser Z-scanning.

The modified Hummer’s method was used to prepare an aqueous solution of GO with a concentration of 1 mg/ml via oxidation, sonication, and centrifugation (see the supplementary material for details). With a particle size of ∼500 nm and sourced from Beijing Ryubon New Material Technology Co., Ltd., Ni, Fe, or Ag powder was prepared as an aqueous solution with a concentration of 5 mg/ml and then subjected to sonication for 1 h. Next, 1 ml of the GO aqueous solution was combined with 0.1 ml of the MNP aqueous solution, followed by ultrasonic treatment for 30 min to ensure thorough and uniform mixing of the components. After that, the films were fabricated via vacuum filtration. As shown in Fig. 1, an organic nylon filter membrane (thickness: 60 μm; diameter: 25 mm) was initially placed in the liquid intake area of a suction filter bottle, and a glass rod was used to introduce the diluted solution into the suction filter bottle. The suction pump was then activated, and ca. 1–2 h were allowed for the GO film to attach to the filter membrane. The materials were maintained at a consistent temperature of 25 °C for 24 h, then the filter membrane was immersed in deionized water, causing the GO film and filter membrane to separate automatically and become suspended on the water surface. The prepared film was then transferred onto conductive glass. Finally, after drying for 24 h at 25 °C, the material was ready for experimental use. Films of GO, GO + Ni NPs, GO + Fe NPs, and GO + Ag NPs were prepared using vacuum filtration and exhibited excellent flatness and high uniformity.

The surface morphologies of the samples were examined using a laser scanning confocal microscope (OLS4100; Olympus) and a field emission scanning electron microscope (S-4800; Hitachi), and their components were analyzed using an energy-dispersive x-ray spectrometer (JSM6360A; JEOL). XRD measurements were conducted using a Rigaku MiniFlex 600 with Cu Kα radiation (λ = 1.54 Å). A Horiba LabRAM Odyssey spectrometer was used for Raman spectrum characterization, with a 532-nm excitation source. A 50× objective lens (NA = 0.75; Olympus) was used, and a CCD was used to capture the scattered signal from the 600-gr/mm grating. Before the experiment, the spectrometer was calibrated meticulously using the Si Raman peak at 520.7 cm⁻¹. During the measurement process, the laser power was ∼10 mW, the spectral integration time was set to 4 s, the integration was repeated twice, and the spectral range was 800–3500 cm⁻¹. UV-visible absorption spectra were acquired in the range of 200–800 nm using a Lambda 750S spectrometer (Perkin Elmer, Inc.).

The NLO properties of the thin films of GO, GO + Ni NPs, GO + Fe NPs, and GO + Ag NPs were characterized using Z-scan technology, and Fig. 2 shows the specific experimental equipment. This Z-scan system uses an ultrafast fiber laser (Keplin Photoelectric Technology Co., Ltd.) emitting at a central wavelength of 515 nm with a pulse duration of 285 fs and a repetition rate of 200 kHz. After passing through the spectroscope, the laser beam is divided into two parts: one part is captured by power meter D₂ as the experimental reference light (in the vertical direction); the other serves as the experimental excitation light (in the original direction). The excited beam is directed through a convex lens (f = 150 mm), while the sample is displaced along the z axis using an electric displacement platform and the beam is focused on it with a beam waist radius of 5.9 μm. Both the open and closed aperture signals are captured by power meter D₁. The transmittance of the sample varies with the input energy, which is modulated by moving in and out of the beam focus along the z axis.

Figures 3(a)–3(d) show LSCM images of the GO, GO–Ni-NP, GO–Fe-NP, and GO–Ag-NP thin films, respectively. LSCM can be used to characterize the surface morphology, dispersion uniformity, and roughness of GO films. Figure 3 shows that the surface of the prepared GO film appears relatively flat. However, the surface morphology of the GO film undergoes significant changes when loaded with MNPs. Also, the uniform color of the image suggests good uniformity in the prepared film.

The materials were characterized using SEM to assess the impact of MNP loading on the GO morphology. Figures 4(a)–4(d) show SEM images of GO, GO + Ni NPs, GO + Fe NPs, and GO + Ag NPs, respectively. Wrinkles and folds are evident on the surface of GO. These features can be attributed to the formation of sp³ carbon and oxygen-containing functional groups on the base surface, along with the presence of various structural defects in GO.^20,21 Figures 4(b)–4(d) show that the GO was decorated with MNPs without any noticeable aggregation. The insets in Fig. 4 indicate that the particle size is ∼500 nm.

The combination of SEM and EDS enables the simultaneous analysis of both the morphology and microscopic elements of the product. Figures 5(a)–5(d) show the EDS results for the major elements of GO, GO + Ni NPs, GO + Fe NPs, and GO + Ag NPs, respectively. The primary characteristic of the GO-based structure is the presence of observable C and O atoms.²² In addition, the characteristic peaks of Ni atoms at ∼0.85 keV, Fe atoms at ∼0.7 keV, and Ag atoms at ∼3 keV can also be seen. The concentrations of the main elements are given in Table I; as can be seen, the concentration of oxygen in GO is 43.01 wt. %, while the addition of MNPs leads to a notable decrease in the concentration of oxygen in GO composite films (GO + Ni NPs: 28.57 wt. %; GO + Fe NPs: 30.69 wt. %; GO + Ag NPs: 30.51 wt. %). Because of the combination of MNPs with oxygen-containing functional groups in GO, the content of oxygen elements is reduced, suggesting that MNPs exhibit a reduction effect on GO.²³ 

XRD is frequently used to analyze the phase and crystal structure of materials, and Fig. 6 shows the XRD patterns of the GO, GO–Ni-NP, GO–Fe-NP, and GO–Ag-NP films. As can be seen, the original GO sample exhibits a diffraction peak at 2θ = 11.3°, which corresponds to the (001) plane of GO. In the XRD pattern of the GO–MNP composite films, the diffraction peak of the MNPs is also observed in addition to the diffraction peak of GO. GO + Ni NPs exhibit diffraction peaks at 2θ = 44.5°, 52.1°, and 76.7°, corresponding to the (111), (200), and (220) planes of Ni NPs (JCPDS No. 04-0850), respectively.²⁴ GO + Fe NPs exhibit diffraction peaks at 2θ = 44.7° and 65.0°, corresponding to the (110) and (200) planes of Fe NPs (JCPDS No. 99-0064), respectively.²⁵ GO + Ag NPs exhibit diffraction peaks at 2θ = 38.5°, 44.7°, 64.8°, and 77.7°, corresponding to the (111), (200) (220), and (311) planes of Ag NPs (JCPDS No. 01-887-0597), respectively.²⁶ Therefore, the successful synthesis of GO–MNP composite films is further demonstrated by the XRD patterns.

Raman spectroscopy was used to characterize the GO, GO–Ni-NP, GO–Fe-NP, and GO–Ag-NP thin films. Graphene and graphene-based materials exhibit three main characteristic peaks, i.e., the D band at ∼1350 cm⁻¹, the G band at ∼1585 cm⁻¹, and the 2D band at ∼2700 cm⁻¹. The D band represents a defect-induced Raman peak associated with the vibration of sp³ hybrid carbon atoms, revealing disorder within the graphene system and serving to characterize structural defects or edges in graphene samples. The G band is the Raman characteristic peak of all sp² carbon materials, depicting the vibration mode of first-order Raman scattering resulting from the stretching vibration between C–C bonds in the plane. The 2D band represents a second-order Raman mode of sp² carbon, originating from double-resonance Raman scattering; its strength and shape vary with the number of graphene layers. Moreover, the Raman peak at ∼2950 cm⁻¹ is associated with the combination of D+G and is also induced by disorder.^27–30 

Figure 7 shows the typical Raman spectra of the GO, GO–Ni-NP, GO–Fe-NP, and GO–Ag-NP composite films, alongside the corresponding data given in Table II. As can be seen, a frequency shift occurs in the D band of the GO film doped with MNPs, and the half-peak width (FWHM) increases, while the G band does not change. The D band of GO + Ni NPs is shifted by 5 cm⁻¹ to a higher frequency, with a corresponding 13% increase in FWHM. Similarly, the D band of GO + Fe NPs exhibits a 3-cm⁻¹ shift to a higher frequency, accompanied by a 9% increase in FWHM. Furthermore, the D band of GO + Ag NPs experiences an 8-cm⁻¹ shift to a higher frequency, coupled with a 22% increase in FWHM. The variation in the amplitude of the D-band frequency shift is associated with the ionization energy of the MNPs.³¹  Figure 8 shows the relationship between the frequency shift of the D band and the ionization energy of the metal. Note that the ionization energy follows the order of Ag < Ni < Fe, with respective values of 7.5, 7.6, and 7.9 eV. The amplitude of the D-band shift decreases as the metal ionization energy increases, and a similar trend is observed for the half-peak width of the D band. A higher ionization energy in the metal results in tighter electron binding, thereby restricting the charge transfer between the MNPs and GO. Because the charge transfer itself is related to the ionization energy and electron affinity of the metal particles, the present Raman spectroscopy shows that there is a significant electronic interaction between the MNPs and the GO.^32,33

Figure 9 shows the normalized 2D peak strength of the GO, GO–Ni-NP, GO–Fe-NP, and GO–Ag-NP films. As can be seen, the 2D peak intensity of GO films doped with MNPs decreases, while the half-peak width increases. Inserting MNPs diminishes the intermolecular forces among GO sheets, and given that the 2D peak typically corresponds to the vibrational mode between the graphene layers, the diminished intermolecular force induces a shift in the vibration mode of GO. This results in a reduction of the 2D peak intensity and an increase in the half-peak width observed in Raman spectroscopy.³⁴ 

Figure 10 shows the UV–visible absorption spectra of the GO, GO–Ag-NP, GO–Fe-NP, and GO–Ni-NP films. The data represent the average of 10 distinct measurements, all conducted under identical conditions. As can be seen, the UV–visible spectrum of GO exhibits a characteristic absorption band of double absorption peaks, with a strong absorption peak at 230 nm, corresponding to the π → π* transition of the C=C bond; there is a weak absorption peak at 300 nm that corresponds to the n → π* transition of the C=O bond.^19,35 The absorption spectra of the three GO–MNP composite films differ from that of GO. The GO–Ag-NP thin film exhibits a distinct and well-defined absorption edge in the UV region, with the characteristic absorption peak shifting to 252 nm, representing a redshift of 22 nm compared to GO. This redshift phenomenon indicates that adding Ag NPs causes the reduction of GO, thereby restoring the π-electron conjugation within the GO sheet.^19,36,37 At the same time, the absorption of the GO–Ag-NP film is notably enhanced in the visible region (400–780 nm), with the absorption band around 420 nm corresponding to the distinctive surface plasmonic resonance (SPR) of Ag NPs. The absorption of GO + Fe NPs in the visible region (400–780 nm) is between that of GO + Ag NPs and GO + Ni NPs. Fe NPs typically exhibit absorption peaks in the range of 330–350 nm, resulting in GO + Fe NPs exhibiting significantly broad peaks spanning 300–500 nm due to SPR induced by Fe NPs.³⁸ The UV–visible absorption spectrum of GO + Ni NPs closely resembles that of GO, with the characteristic absorption peak at 230 nm being redshifted to 234 nm. This shift suggests that adding Ni NPs restores π-electron conjugation in the GO sheet. In addition, the weak absorption peaks at 362 nm and ∼490 nm are attributed to the SPR absorption of Ni NPs between 300 and 400 nm. Jaleh et al. calculated that the SPR absorption of Ni NPs is at ∼350 and 490 nm.³⁹ The slight redshift of the absorption band is related to the size of the NPs.⁴⁰ The optical absorption of GO + Ag NPs is most prominent in the UV–visible region, making them an excellent option for photosensitive materials and for fabricating devices with high absorptivity. These properties are highly advantageous in a range of optical applications, including solar absorbers and photocatalysis. Also, GO + Fe NPs and GO + Ni NPs exhibit significant potential for advancement in the development of integrated optoelectronic devices such as photodetectors and optical sensors, attributed to their broad absorption range within the visible light spectrum.

The results of the OA and CA experiments are summarized in Table III. As can be seen, for the GO–MNP composite films, $Iχ3$ is two orders of magnitude larger than $Rχ3$⁠, which means that the absorption effect is stronger than the refraction effect.⁵¹ In addition, the RSA behavior and self-defocusing effects of all GO–MNP composite films are improved compared with the original GO. The β and n₂ values for GO + Ni NPs are 7.8 × 10⁻⁶ cm/W and −1.06 × 10⁻¹² cm²/W, respectively, and the NLA and self-defocusing effects are increased by 2.3 times and 1.5 times, respectively. The β and n₂ values for GO + Fe NPs are 8.9 × 10⁻⁶ cm/W and −1.48 × 10⁻¹² cm²/W, respectively, and the NLA and self-defocusing effects are increased by 2.6 times and 2 times, respectively. The β and n₂ values for GO + Ag NPs are 9.9 × 10⁻⁶ cm/W and −1.19 × 10⁻¹² cm²/W, respectively, and the NLA and self-defocusing effects are increased by 2.9 times and 1.7 times, respectively. The enhancement of nonlinear RSA and NLR indicates the existence of light-induced electron transfer and energy transfer between the GO and MNPs. This leads to an exceptionally robust synergistic coupling effect, thereby bolstering the interaction between light and matter. The electron transfer pathways from MNPs to GO are shown by the energy band diagram in Fig. 15. GO contains the sp² matrix of graphene and the sp³ matrix of oxygen-containing groups, while MNPs generally attach to it through oxygen-containing groups. Upon irradiation of the GO–MNP composite with a femtosecond laser pulse of 515-nm wavelength, the metal’s irradiated region generates free electrons that are subsequently transferred to GO via the interface charge. The free carrier lifetime is increased, which further enhances the TPA in the composite compared to that in GO, thus exhibiting enhanced nonlinear properties. To the best of our knowledge, there have been no research reports on the NLO properties of GO–Ni-NP and GO–Fe-NP composites. The results for the NLO parameters presented herein are very satisfactory, and compared with previous studies, the GO–Ag-NP composites presented herein exhibit notable enhancements in NLA.

In this work, we systematically investigated the LO and NLO properties of GO–Ni-NP, GO–Fe-NP, and GO–Ag-NP composite films. LSCM, SEM, EDS, and XRD data were analyzed to confirm the successful loading of MNPs on GO films. The electron interactions between MNPs and GO were studied by Raman spectroscopy. The results of UV–visible spectra showed that the different electronic interactions between different metal particles and GO can effectively regulate the band gap of GO. By adjusting the band gap of GO, the properties of GO composites can be controlled, which is of great significance for photovoltaic devices, photoelectric sensors, and optical communications and provides a theoretical basis for the preparation of integrated optoelectronic devices.

The NLO properties of GO and its composite films under femtosecond pulsed laser excitation were studied by the Z-scan technique. The optical limiting properties of GO films exhibit intensity dependence when exposed to femtosecond laser pulses. Importantly, we reported for the first time on the NLO properties of GO–Ni-NP and GO–Fe-NP composite films, and their β values are increased by 2.3 and 2.6 times, respectively. The NLA of the GO–Ag-NP composite membrane that we reported exhibited a notable enhancement, showing an increase of 2.9 times in the β value. Because of the strong synergistic coupling effect between MNPs and GO, the NLO properties of the composite are significantly improved. The synthesized GO–Ni-NP, GO–Fe-NP, and GO–Ag-NP composites are promising NLO materials that can be safely applied as effective optical limiters in a variety of military and medical operations to protect photosensitive devices from damage by strong light pulses. These composites have potential applications in all-optical switches and optical limiters, providing new possibilities for the application of optical limiters, optical modulation, and optical switches.

See the supplementary material for the modified Hummer’s method for graphene oxide.

This study was funded by the Henan Key Laboratory of Intelligent Manufacturing Equipment Integration for Superhard Materials (Grant No. JDKJ2022-01) and the Key Lab of Modern Optical Technologies of Education Ministry of China, Soochow University.

The authors have no conflicts to disclose.

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Qingqing Sun is studying for a master’s degree at the State Key Laboratory of Precision Measuring Technology and Instruments in the School of Precision Instruments and Opto-Electronics Engineering at Tianjin University in China. Her research interests include nonlinear optics, femtosecond laser processing, and semiconductor Raman spectroscopy.

Bing Dong is studying for a doctorate at the State Key Laboratory of Precision Measuring Technology and Instruments in the School of Precision Instruments and Opto-Electronics Engineering at Tianjin University. His research interests are femtosecond laser processing and semiconductor Raman spectroscopy.

Ying Song is studying for a doctorate at the State Key Laboratory of Precision Measuring Technology and Instruments in the School of Precision Instruments and Opto-Electronics Engineering at Tianjin University. Her research interests include the preparation of silicon carbide color centers by ion implantation, three-dimensional Raman and photoluminescence spectral characterization, and models of spectral depth profiling.

Jianshi Wang is studying for a master’s degree at the State Key Laboratory of Precision Measuring Technology and Instruments in the School of Precision Instruments and Opto-Electronics Engineering at Tianjin University. His research interests include ultrafast femtosecond laser processing, Raman spectroscopy, and nanoparticle fabrication.

Mengzhi Yan is studying for a master’s degree at the State Key Laboratory of Precision Measuring Technology and Instruments in the School of Precision Instruments and Opto-Electronics Engineering at Tianjin University. His research interests include heuristic algorithms, molecular-dynamics simulation, and detecting defects in the ultrawide-band-gap semiconductor material gallium oxide by machine learning.

Zongwei Xu is a Professor at Tianjin University and a Doctoral Supervisor. His research interests include ultra-fast energy beam (ion, laser) processing, Raman and photoluminescence spectroscopy characterization, wide-bandgap semiconductor devices, micro-cutting tools, and nano-cutting technology.