Synergistically Enhanced Optical Limiting Property of Graphene

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Synergistically Enhanced Optical Limiting Property of Graphene Oxide Hybrid Materials Functionalized with Pt Complexes Rui Liu,* Jinyang Hu, Senqiang Zhu, Jiapeng Lu, and Hongjun Zhu* Department of Applied Chemistry, College of Chemistry and Molecular Engineering, Nanjing Tech University, Nanjing, 211816, China S Supporting Information *

ABSTRACT: Recently, graphene-based materials have become well-known nonlinear optical materials for the potential application of laser protection. Two new graphene oxide−platinum complex (GO−Pt) hybrid materials (GO−Pt-1, GO−Pt-2) have been fabricated through covalent modification and electrostatic adsorption of different Pt complexes with GO. The structural and photophysical properties of the resultant hybrid materials were studied. The nonlinear optical properties and optical power limiting (OPL) performance of Pt complexes, GO, and GO−Pt hybrid materials were investigated by using Z-scan measurements at 532 nm. At the same transmittance, the results illustrate that functionalization of GO makes GO−Pt hybrid materials possess better nonlinear optical properties and OPL performance than individual Pt complexes and GO due to a combination of nonlinear scattering, nonlinear absorption, and photoinduced electron and energy transfer between GO and Pt complex moieties. Furthermore, the nonlinear optics and OPL performance of GO−Pt-2 are better than those of GO−Pt-1, due to not only the excellent optical limiting of Pt-2 and more molecules per area of GO but also the way of combination of Pt-2 and GO. KEYWORDS: graphene oxide, Pt complex, optical limiting, photophysics, nonlinear optics

1. INTRODUTION Recently, laser technology has been widely utilized in the civilian and military fields with applications in laser surgery, industrial processing, communications, optical imaging, and assault weapons.1 The high intensity of a laser beam can be potentially hazardous to human eyes and optical devices; thus, much attention has been focused on avoiding damage from accidental laser radiation.2−4 Consequently, optical power limiting (OPL) technology has been making great progress under enormous demand.5 For OPL materials, it is known that the high transmission remains under sustainable light intensities, whereas the decreasing transmission occurs gradually with increasing intensities. Nevertheless, the development of powerful optical limiters is a huge challenge because excellent optical limiting materials are still limited. To date, in order to develop highly efficient optical limiters, different mechanisms have been applied to reduce laser intensity, such as absorption, reflection, and diffraction. Among these, it is worth noting that nonlinear optical (NLO) materials possessing strong nonlinear absorption or scattering responses are potential optical limiters for different laser pulses.1 Apparently, to achieve a better OPL effect, materials designed by a single strategy cannot satisfy various applications. Thus, multimechanistic combination and synergistically enhanced effects have been considered as promising methods to design nextgeneration OPL materials. Due to the merits of multiple photophysical properties and varied applications in organic light-emitting diodes, low power © 2017 American Chemical Society

upconversion, photocatalysis, OPL, etc., much attention has been focused on cyclometalated Pt(II) complexes.6 In addition, the optical properties and OPL performance of Pt(II) complexes could be easily tuned by structural modification of their ligands, including phosphine ligands, N-heterocyclic ligands, and acetylide ligands. By choosing the appropriate ligands, broadband and intensive reverse saturable absorption (RSA) or two-photon absorption (TPA) from the visible to the near-infrared region could be obtained, which is superior to that of other materials, such as C60, metalloporphyrins, and metallophthalocyanines.7−10 Typical Pt(II) complexes with a nonlinear absorption property are slightly colored in solution due to their main absorption bands located below ca. 400 nm and an extremely weak absorption band in the region of visible light, which endow good transparency to the OPL materials. It is worth noting that Pt(II) complexes also show superb photochemical and thermal stability, making them a class of well-known nonlinear optical materials for the potential application of laser protection. Graphene has attracted remarkable interest recently, owing to its excellent optical transparency, mechanical flexibility, and large surface area.11,12 However, the prospective applications of graphene are hampered by its poor solubility and processability, because these properties are the first requirements for many Received: July 21, 2017 Accepted: September 1, 2017 Published: September 1, 2017 33029

DOI: 10.1021/acsami.7b10585 ACS Appl. Mater. Interfaces 2017, 9, 33029−33040

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ACS Applied Materials & Interfaces Scheme 1. Schematic Representation of the Structures for GO−Pt-1 and GO−Pt-2

Scheme 2. Synthesis of GO−Pt-1 and GO−Pt-2

or electrostatic adherence, by porphyrin,18−20 phthalocyanine,21−24 oligothiophene,25 nanoparticles,26−29 carbazole,30 polymer,31,32 and ionic complexes33 have been reported. The enhanced OPL and NLO performance of hybrid materials was observed for nanosecond pulses at 532 nm by the synergistic effect of nonlinear mechanisms (RSA, TPA, and NLS) and photoinduced electron or energy transfer (PET/ET). To the best of our knowledge, however, the preparation of GO hybrid materials covalently and noncovalently functionalized with Pt(II) acetylide complexes, as well as their OPL performance, have not yet been reported. The hybrid materials are expected to qualify the improved OPL effect by the synergism of different mechanisms. Meanwhile, a more in-depth study is required on the different OPL behaviors of covalently and noncovalently functionalized GO hybrid materials in one system, which is still a gap in this research field. Encouraged by these considerations, in this study, two new hybrid materials containing graphene oxide functionalized with cyclometalated square-planar Pt(II) complexes via an amidation reaction and electrostatic absorption were developed (GO−Pt1 and GO−Pt-2, Scheme 1). Attachment of Pt(II) complexes to the surface of the GO, which can significantly improve its solubility, was characterized by transmission electron micros-

practical applications. Meanwhile, as a two-dimensional carbon sheet, graphene oxide (GO) not only keeps the features of graphene but also possesses oxygen-containing functional groups, such as carboxyl, epoxide, and hydroxyl groups, in the basal area and edge. The presence of these functional groups allows the GO to readily swell and disperse in common solvents. Furthermore, these groups could permit the derivation of GO with small molecules and polymers, which are covalently linked to its oxygen functionalities or noncovalently absorbed on its surface.13 Feng et al.14 observed the broadband NLO and OPL properties of the graphene system, including GO nanosheets, graphene nanosheets (GNS), GO nanoribbons, and graphene nanoribbons for nanosecond pulses (532 and 1064 nm). Moreover, it has been proved that GO-based materials exhibit multiple NLO mechanisms, such as RSA, TPA, and nonlinear scattering (NLS).15−17 These unique features make GO and its derivatives very promising candidates for designing OPL materials. GO is an excellent platform that can be employed to design and prepare novel materials with improved OPL properties by covalent or noncovalent combinations of other OPL-active substances. Over the past decade, a series of graphene-based hybrid materials functionalized, through covalent combination 33030


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as thin films with KBr (spectroscopic grade). XPS spectra of all samples were measured on a Thermo ESCALAB 250 spectrometer with a monochromatized Al Kα X-ray source (Ephoton = 1486.7 eV). Raman spectra of GO-based materials were performed on a HR800 (JY) spectrometer with Ar+ (532 nm) as the excitation source. The thermal behaviors of all samples were documented by TGA curves on a Netzsch Sta 409PC instrument in N2 atmosphere. The content of Pt in GO−Pt-1 and GO−Pt-2 was determined by the inductively coupled plasma-atomic emission spectroscopy (ICP-AES), which was performed on a JY2000 Ultrace ICP atomic emission spectrometer. The UV−vis absorption and fluorescence spectra were obtained by using a U-3310 UV spectrophotometer (Hitachi) and LS-55 spectrofluorometer (PerkinElmer), respectively. The samples were dissolved in DMF and then degassed with high-purity N2 in a quartz cell for 30 min before measurement. The triplet excited-state lifetimes and transient absorption (TA) spectra of samples in degassed DMF were also obtained on an Edinburgh LP980 laser flash photolysis spectrometer, and the third harmonic output of a Nd:YAG laser (355 nm, 4.1 ns pulse width, 1 Hz repetition rate) was used as excitation source. The triplet excited-state absorption coefficient (εT) at the maximum transient absorption band of each sample was calculated by the singlet depletion method. Relative actinometry was conducted to determine the triplet quantum yields (ΦT) by using SiNc in benzene (ε590 = 70 000 M−1·cm−1, ΦT = 0.20) as the reference. The NLO and OPL performances of the samples were investigated by open-aperture Z-scan using a Nd:YAG laser (EKSPLA, PL2143B) and 532 nm pulses of 4 ns, operating at 10 Hz repetition rate.37 To facilitate comparison, all sample concentrations in dry DMF were adjusted to the same 70% linear transmittance at 532 nm in NLO and OPL experiments. All samples were measured in 1 mm quartz cells. NLO coefficients were calculated according to the literature.23,30 The function of normalized transmittance and the position (z) is offered by TN(z) = log[1+ p0(z)]/p0(z) and p0(z) = p00/[1 + (z/z0)2], where z0 represents the length of the beam and p00 = βeffI0Leff. βeff and I0 represent the effective nonlinear extinction coefficient and the intensity of the laser at focal point, respectively. Leff = [1 − exp(−α0L)]/α0, and Leff represents the effective length of the sample, which was determined by the linear absorption coefficient (α0) and actual optical path length through the sample.

copy (TEM), atomic force microscopy (AFM), Raman spectra, Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), ultraviolet−visible (UV−vis) absorption, and steadystate fluorescence, which confirmed the successful functionalization of the GO by Pt-1 and Pt-2. A remarkable enhancement of OPL performance at 532 nm was observed for these hybrid materials by virtue of the efficient combination of varied NLO mechanisms. Moreover, the OPL performance of GO−Pt-2 prepared by electrostatic absorption was better than that of GO−Pt-1 prepared by covalent bonding, and the influences on NLO properties of the composition and combination forms of GO and Pt(II) complex components are discussed. As a result, GO−Pt hybrid materials exhibited much improved OPL performance than GO and Pt(II) complexes at 532 nm, indicating an extraordinary synergistic enhancement effect via both the covalent and noncovalent links of GO to Pt(II) complexes.

2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. All chemicals were purchased from Adamas-beta. GO was purchased from Suzhou Graphene Science and Technology Co. Ltd. Dichloromethane (DCM), dimethylformamide (DMF), triethylamine (Et3N), tetrahydrofuran (THF), and N,Ndiisopropylethylamine (DIEA) were dried and distilled under N2 before use. 4,6-Diphenyl-2,2′-bipyridine34 and phenylacetylide ligands35,36 were synthesized according to the reported literature methods. Synthetic routes for the hybrid materials are shown in Scheme 2. 2.2. Preparation of GO−Pt-1 Hybrid Material. GO−Pt-1 was synthesized according to literature procedures.22 GO (50 mg) and excess of SOCl2 (15 mL) were refluxed for 24 h in the presence of DMF (catalytic amount). The redundant SOCl2 was removed by distillation. Then Pt-1 (300 mg) and Et3N (15 mL) were added to the GO−COCl suspension in DMF (anhydrous, 50 mL) under N2 protection. The reaction mixture was sonicated for 30 min first and then refluxed for 3 days. After finishing the reaction, the mixture cooled to room temperature. Water (800 mL) was added to the reaction solution to precipitate the product. The residue was washed with ethanol, collected by centrifuge at 10 000 rpm, and dried under vacuum to obtain the hybrid GO−Pt-1. 2.3. Preparation of GO−Pt-2 Hybrid Material. The preparation of GO−Pt-2 is also shown in Scheme 2, and GO−Pt-2 was synthesized according to the reported procedures.33 Under ultrasound, GO (50 mg) was dissolved in deionized water (100 mL), then 2% Na2CO3 solution (10 mL) was added to adjust the solution pH to ≈11. Then the mixture was sonicated for 60 min to completely ionize the carboxy group in GO. After removing the excess Na2CO3 by washing with distilled water, the pH was adjusted between 7 and 8. A 100 mL portion of Pt-2 (5 × 10−3 M) aqueous solution was added into the above residue. The resulting mixture was then sonicated at 25 °C for 3 h to enable GO to absorb Pt-2 sufficiently. Finally, the mixture was centrifuged at 10 000 rpm, washed with ethanol or THF, and dried under vacuum to obtain GO−Pt-2. 2.4. Instruments and Measurements. 1H NMR spectra of Pt complexes were measured on a Bruker AV-400 spectrometer (DMSOd6 as solvent, tetramethylsilane as internal standard). The results of elemental analyses for C, N, and H were obtain on a Vario El III elemental analyzer. High-resolution mass (HRMS) spectra were collected on a matrix-assisted laser desorption/ionization-time of light mass spectrometer (MALDI-TOF-MS). High-quality TEM images were taken on a JEM-2100 electron microscope, and sample was fabricated by dispersing material in DMF under ultrasound and then dropping the mixture on carbon-coated copper grids. AFM images of GO-based materials were collected using the SPM D3100. Fourier transform infrared (FT-IR) spectra in this work were obtained on a Nicolet Nexus 470 spectrometer, and all samples were prepared

3. RESULTS AND DISCUSSION 3.1. Synthesis. On the basis of previous reports, in general, the organic functionalization of GO could involve two methods:38 (i) covalent bonds between organic molecules with special functional groups and the oxygen-containing groups of GO and (ii) the formation of π−π interactions or electrostatic effects between aromatic organic compounds and GO. The two approaches are extraordinarily fascinating because of the fact that they allow the combination of all kinds of organic molecules onto the graphene sheets directly, which may result in effective interaction between the two parts and ultimately affect their photophysical and optoelectronic behaviors. Therefore, versatile strategies including the amidation reaction between the carboxylic groups of GO and the amino groups of Pt complexes, as well as electrostatic interaction between GO with negative charge and Pt complexes with pyridine quaternary ammonium salts, were employed to prepare GO-based materials. Both methods successfully lead to the attachment of complexes to the GO surface without degrading its properties. The as-prepared hybrid materials (GO−Pt-1 and GO−Pt-2) were obtained by washing the product thoroughly with organic solvents. The GO-based materials display better dispersibility than GO in DMF and result in a brown dispersion solution [Figure S5, Suppoorting Information (SI)]. The structural features, photophysical properties, synergistic NLO properties, and OPL performance of GO-based hybrid materials are discussed below. 33031


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Figure 1. TEM image of GO (a), GO−Pt-1 (b), and GO−Pt-2 (c) sheets. AFM images and the thickness of GO (d), GO−Pt-1 (e), and GO−Pt-2 (f) sheets on a mica surface.

3.2. Structure Characterization. 3.2.1. Morphological Analysis. The morphology of GO and hybrid materials was obtained from the TEM and AFM measurements. As shown in Figure 1a, the TEM image reveals that GO edges are smooth, without stacking and wrinkles. Meanwhile, the average thickness of the GO sheet was found to around 0.908 nm (Figure 1d), which are in accordance with previous works.39−41 The average thickness of the GO sheet is larger than the theoretical value (0.34 nm) due to the large amounts of oxygen-containing functional groups present on its surface and edge. However, when GO was functionalized with Pt-1 complex by amidation reaction and Pt-2 complex by electrostatic absorption, a wrinkled texture with scrolled edge appears on the surface of both GO−Pt-1 and GO−Pt-2 hybrid sheets, as shown in Figure 1b,c. Similarly, the average thickness of GO−Pt-1 of ca. 3.7 nm and GO−Pt-2 of ca. 7.8 nm (Figure 1e,f) was higher than the thickness of GO in AFM images. The increased thickness may demonstrate the presence of the Pt-1 and Pt-2 complexes on the GO surface. 3.2.2. Raman Analysis. Raman spectroscopy is widely applied to research the carbon framework of graphene materials, identifying the existence of defects. The functionalization of graphene usually results in the formation of more defects in the graphene sheets, which thus provides evidence that the functionalization has actually occurred. The obvious structural differences among GO, GO−Pt-1, and GO−Pt-2 were observed in their Raman spectra (Figure 2, λex = 532 nm). The Raman spectrum of GO shows two characteristic bands at about 1357 cm−1 (D band) and 1595 cm−1 (G band), corresponding to the disordered modes and the sp2 carbon atoms, respectively.42,43 In contrast to GO, the G and D bands of GO−Pt-1 are broadened and shifted by 12 and 20 cm−1, respectively. Moreover, a similar result of the blue-shift for D and G bands in GO−Pt-2 compared with those of GO was also obtained. The observation indicates that the functionalization of the carbon framework of GO by the Pt complex could impact the position and shape of the D and G bands. In

Figure 2. Raman spectra of GO and GO−Pt-1 and GO−Pt-2 hybrid materials.

addition, functionalization of GO could lead to changes of the ID/IG ratio (intensity ratio of D to G band).20,23 ID/IG ratios increased from 0.86 for GO to 1.02 for GO−Pt-1, whereas it should be noted that the ID/IG ratio of GO−Pt-2 did not change significantly compared with that of GO in this study. This could be explained by the Pt-2 complex containing a crowd of aromatic rings and also undergoing π−π interactions with basal planes of GO, simultaneously, which is different from the interactions between Pt-1 and GO.23 The result of high ID/ IG ratios for GO−Pt-1 is consistent with the more-scrolled edges observed in the TEM images of GO−Pt-1 compared to those of GO−Pt-2. The different kinds of functionalization play an important role in the ID/IG ratio and further influence the structure and properties of GO. 3.2.3. FT-IR Studies. FT-IR spectroscopy is an essential method to verify the functionalization of GO with metal/ complex moieties. The FT-IR spectra of GO, Pt-1, GO−Pt-1, Pt-2, and GO−Pt-2 are shown in Figure 3. In the FT-IR spectrum of GO, the characteristic absorption peaks located at 1735 cm−1 (CO) and 1411 cm−1 (O−H) are attributed to carboxyl groups. The absorption peaks at 3409 cm−1 (O−H) 33032


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Figure 3. FT-IR spectra of (a) GO, Pt-1, and GO−Pt-1 and (b) GO, Pt-2, and GO−Pt-2.

Figure 4. XPS spectra of (a) GO and GO−Pt-1 and the amplified N 1s XPS spectra for (b) Pt-1 and (c) GO−Pt-1.

and 1049 cm−1 (C−O) are characteristic vibrations of hydroxyl groups on the GO surface. The peak at 1635 cm−1 can be ascribed to the stretching vibrations of CC or nonoxidized graphitic domains in GO. These main characteristic absorption bands of GO are consistent with previous reports.44 In Figure 3a, the main characteristic absorption peaks of Pt-1 complex at 3417 and 2075 cm−1 correspond to the N−H characteristic stretching band of the amino group and the CC characteristic stretching band, respectively. In comparison with GO and Pt-1, the characteristic peak observed in the spectrum of GO− Pt-1 at 2058 cm−1 originates from the CC bond, which indicates that the Pt-1 complex was introduced onto GO. Meanwhile, the peak at 1735 cm−1 almost vanishes and a new peak appears at 1651 cm−1, which is assigned to the characteristic CO absorption peak of the amide group. These results clearly confirm the formation of an amido bond between the Pt-1 molecule and GO.19,45 In the spectrum of Pt2 (Figure 3b), the characteristic absorption peaks at 2969 and 2098 cm−1 could be assigned to the stretching vibrations of C− H in the ethyl chain and the CC stretching peak, respectively. For hybrid material GO−Pt-2, the spectrum clearly shows the presence of vibrational bands resulting from both GO and Pt-2. The peaks at 1730 and 2067 cm−1 can be

attributed to the CO group originating from GO and the stretching vibrations of CC coming from Pt-2. It is worth noting that the vibrational frequency is slightly different from that of Pt-2 due to the strong ionic and π−π interactions between Pt-2 and GO.33 The observations also indicate that the interaction of GO and Pt-2 is electrostatic absorption. These results confirmed the successful formation of GO−Pt complex hybrid materials via an amidation reaction and electrostatic absorption, respectively. 3.2.4. XPS Studies. The attachment of the Pt complexes onto GO surface was also supported by X-ray photoelectron spectroscopy. The XPS survey spectra of GO and GO−Pt-1 are shown in Figure 4. It is clear that only two peaks, centered at 531 and 285 eV, could be attributed to O 1s and C 1s in the spectrum of GO.46 After the covalent functionalization of GO, the XPS spectrum of GO−Pt-1 shows two additional peaks at 77 and 399 eV that originated from Pt 4f and N 1s in Pt-1 complex, respectively, which implies a successful incorporation of Pt-1 complex into the hybrid material. In addition, this deduction is also confirmed by the analysis of the N region in XPS spectra, which offers further evidence concerning the presence of amide groups on the functionalized GO surface. As shown in Figure 4b,c, the binding energies of the N 1s region in 33033


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oxygen functional groups, which is consistent with previous reports in the literature.48,49 Pt-2, with an onset decomposition temperature of 324 °C, shows better thermal stability than Pt-1, the onset decomposition temperature of which is 211 °C. This may be related to the existence of amino groups in Pt-1. After coupling Pt-1 and Pt-2 to GO, the thermal stability of resultant GO−Pt-1 and GO−Pt-2 is better than that of GO. The mass of GO−Pt-1 and GO−Pt-2 still remains ca. 69% and 62% at 950 °C, respectively, which further confirms that strong interactions exist between GO and Pt complexes. Furthermore, the thermal stability of GO−Pt-1 was better when the attachment was obtained by covalent bonding than through electrostatic deposition. 3.3. Optical Properties. 3.3.1. UV−Vis Absorption Spectra. Figure 6 displays the UV−vis absorption spectra of GO, Pt-1, GO−Pt-1, Pt-2, and GO−Pt-2 in DMF. As shown in Figure 6a, GO exhibits a broad absorption, extending to 800 nm. The absorption in the ultraviolet region is attributed to the π−π* transitions of the sp2 π-conjugated network from GO. For the Pt-1 complex, it shows structured major absorption bands below 400 nm that arise from intraligand π−π* transitions. Moreover, the broad but less structured band at 400−600 nm originates from metal-to-ligand charge transfer (1MLCT)/intraligand charge transfer (1ILCT)/ligand-to-ligand charge transfer (1LLCT) transitions, based on other reported Pt(II) C^N^N acetylide complexes.9 After the covalent combination of GO and Pt-1, the absorption peak at 295 nm appearing in the spectrum of GO−Pt-1 should correspond to the combination of the intraligand π−π* transitions of the Pt-1 moiety and the π−π* transitions of GO. In comparison with the spectrum of Pt-1, the slight red-shift (5 nm) of the π−π* transitions absorption peak could arise from some alteration of the ground state of the Pt-1 complex due to covalent attachment to GO. In Figure 6b, the Pt-2 complex also

the Pt-1 complex located at 397.78, 398.57, and 399.29 eV are attributed to C−N, CN, and NH2, respectively.47 In contrast to Pt-1, the peaks of GO−Pt-1 at 398.93 eV (CN) and 399.63 eV (NH2) show a red-shift on account of the electronwithdrawing effect of GO. The peak at 400.56 eV (N−CO) indicates the formation of an amide bond between GO and Pt1.22 A similar outcome for GO−Pt-2 hybrid material is presented in Figure S6 (SI). Taken together, the XPS spectra provide additional confirmation of the successful combination of Pt complexes with GO. 3.2.5. Thermal Properties. The thermal performance of GO, Pt-1, Pt-2, GO−Pt-1, and GO−Pt-2 was evaluated by TGA measurement under a N2 atmosphere. As shown in Figure 5,

Figure 5. TGA curves of (a) Pt-2, (b) Pt-1, (c) GO, (d) GO−Pt-2, and (e) GO−Pt-1.

upon heating to 265 °C, GO is unstable and suffers a 35% weight loss, which is attributed to the pyrolysis of erratic oxygen functional groups on GO. An additional 10% weight loss above 275 °C comes from the elimination of more stable

Figure 6. UV−vis absorption spectra of (a) GO, Pt-1, and GO−Pt-1 and (b) GO, Pt-2, and GO−Pt-2 in DMF. Concentration dependence of the UV−vis absorption of (c) GO−Pt-1 and (d) GO−Pt-2 in DMF [concentrations in mg/L are 6 (spectra a), 12 (spectra b), 18 (spectra c), 24 (spectra d), 30 (spectra e), and 36 (spectra f), respectively]. Insets are the plots of absorbance at 295 nm (GO−Pt-1)/286 nm (GO−Pt-2) versus concentration. 33034


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Figure 7. Emission spectra of (a) Pt-1 and GO−Pt-1 (λex = 420 nm) and (b) Pt-2 and GO−Pt-2 (λex = 457 nm) in deoxygenated DMF, with the same value of absorbance at the excitation wavelength (0.2).

Figure 8. Nanosecond time-resolved TA spectra in deoxygenated DMF solution (λex = 355 nm): (a) Pt-1, (b) Pt-2, (c) GO−Pt-1, and (d) GO−Pt2.

shows strong, sharp π−π* absorption bands at 288 and 331 nm, and the weak absorption band at 417 nm could be assigned to 1MLCT/1LLCT/1ILCT transitions, similar to those presented by the Pt-1 complex. The low-energy absorption band of the Pt-2 complex with an electron-withdrawing substituent (pyridine quaternary ammonium salts unit) on the acetylide ligand is blue-shifted in comparison with the Pt-1 complex with an electron-donating substituent (amino group) on the acetylide ligand, which is also consistent with other Pt(II) acetylide complexes bearing C|N|N ligands.9 The GO− Pt-2 hybrid material displays similar absorption peaks compared to Pt-2. Notably, the trend of red-shift from 331 to 338 nm (Δλ = 7 nm) is in accordance with the covalent attachment to GO. Meanwhile, GO−Pt-1 and GO−Pt-2 have a very broad absorption from the ultraviolet to the visible region, which also suggests the interaction between Pt complexes and GO units.33,50 The broad optical window of hybrid materials GO−Pt-1 and GO−Pt-2 is quite beneficial to extend the

spectral range for the broadband OPL application. Moreover, when the absorbance value at 700 nm of GO−Pt-1 and GO− Pt-2 was kept identical, the absorbance value at 286 nm for GO−Pt-2 is twice than that of GO−Pt-1 at 295 nm (as shown in Figure S8, SI), which indicated more Pt-2 complexes per graphene sheet. ICP-AES analyses also show that Pt makes up 3.61% of the total weight in GO−Pt-1 and 4.23% in GO−Pt-2 hybrid materials. Excellent dispersion is particularly important for the applications of graphene and its derivatives. Solution-phase UV−vis spectra have been widely used to evaluate the dispersion of GO according to Beer’s law.19,20 The absorption spectra of GO−Pt-1 and GO−Pt-2 in DMF solutions were measured at different concentrations and are shown in Figure 6c,d. The absorption values at 295 nm for GO−Pt-1 and 286 nm for GO−Pt-2 against different mass concentrations (mg/L) exhibit a nice linear relationship at low concentration. The mass extinction coefficients of GO−Pt-1 and GO−Pt-2 were 33035


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Table 1. Electronic Absorption and Emission (room temperature) Parameters for Complexes Pt-1, Pt-2, GO−Pt-1, and GO−Pt2 λabsa/nm (ε/104 L·mol−1·cm−1) Pt-1 Pt-2 GO−Pt-1 GO−Pt-2

λemb/nm (τem/ns, Φem)

290 (3.5), 370 (0.8), 477 (0.4) 288 (3.8), 331 (2.9), 417 (1.2) 295, 378, 453 286, 338, 416

581 559 578 558

(95, 0.005) (320, 0.031) (3.56, 0.0007) (1.09, 0.0001)

λT1‑Tnc/nm (τTA/ns, εT1‑Tn/L·mol−1·cm−1, ΦT) 540 564 533 539

(110, 3317, 0.06) (420, 9542, 0.14) (12, 1103, 0.01) (20, 2874, 0.03)

a

Absorption band maximum (λabs) and molar extinction coefficient (εmax) in DMF. bEmission wavelength (λem) and emission quantum yield measured in a degassed aqueous solution of Ru(bpy)3Cl2 as the standard for the complexes and hybrid materials. cNanosecond transient absorption band maxima, triplet extinction coefficients, triplet excited-state lifetimes, and quantum yields in deoxygenated DMF. SiNc in C6H6 was used as the reference (ε590 = 70 000 L·mol−1·cm−1, ΦT = 0.20).

Figure 9. (a) Open-aperture Z-scan curves, (b) scattering response, and (c) optical limiting performance of GO, Pt-1, Pt-2, GO−Pt-1, GO−Pt-2, and C60 with the same linear transmittance of 70% to 4 ns, 532 nm optical pulses.

Pt-1, Pt-2 shows an emission band at 558 nm with a hypochromatic shift of ca. 23 nm. Meanwhile, the almost complete quenching of the emission intensity for the GO−Pt-2 in DMF was observed. Additionally, phosphorescent lifetimes of the Pt-1, Pt-2, GO−Pt-1, and GO−Pt-2 were measured. Among them, the decay lifetimes of Pt-1 and Pt-2 were monoexponentially fitted with a lifetime of 95 and 320 ns, respectively. However, the measurable decay lifetimes of GO− Pt-1 and GO−Pt-2 were shortened to 3.56 and 1.09 ns, respectively, thus indicating the triplet excited-state deactivation of Pt-1 and Pt-2 in the hybrid material. The two competitive processes of photoinduced electron transfer (PET) and energy transfer (ET) result in the effective phosphorescence quenching in hybrid materials because the Pt complex can serve as an energy and electron donor, while graphene serves as a good acceptor of energy and electrons.20,33 In the case of GO−Pt hybrids with the same GO moiety, the PET/ET process should be especially affected by the Pt moiety and the interaction between the GO and Pt complexes. More efficient phosphorescent quenching in GO−Pt-2 than in GO−Pt-1 suggests that the PET/ET process between Pt-2 and GO would be further enhanced.52

calculated from the slope of the linear equation according to Beer’s law, and the values are 0.039 and 0.042 L/(mg·cm), with R values of 0.995 and 0.986, respectively. In addition, the absorbance value of GO−Pt-1 and GO−Pt-2 in DMF at other wavelengths also accords with Beer’s law. These outcomes demonstrate the excellent dispersibility of the hybrid materials in DMF, which is helpful for the follow-up application in the field of OPL devices. 3.3.2. Phosphorescence Spectroscopy. Phosphorescence spectroscopy could be used as an effective method to investigate the electron donor−acceptor interactions. In order to evaluate the excited-state interactions of the Pt complex and GO in the hybrid materials, emission spectra of Pt-1, Pt-2, GO−Pt-1, and GO−Pt-2 are shown in Figure 7. For the convenience of comparison, the absorbance value at the excitation wavelength of all samples was adjusted to be 0.2.51 Pt-1 in DMF shows a maximum emission band at 581 nm, which is a typical phosphorescent emission according to related reports and the sensitivity to oxygen,8,9 while GO−Pt-1 shows a weak emission peak at 578 nm. Furthermore, the emission intensity of GO−Pt-1 was quenched by 83% in comparison with that of Pt-1. Similarly, compared with the emission peak of 33036


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ACS Applied Materials & Interfaces 3.3.3. Transient Absorption (TA) Spectra. The nanosecond TA spectra of the Pt complexes and GO-based hybrid materials were studied to realize the triplet excited-state behaviors and to predict the range where RSA arises. The nanosecond timeresolved TA spectra of complexes and hybrid materials in DMF are presented in Figure 8, and the triplet excited-state lifetimes are listed in Table 1. Both Pt-1 and Pt-2 possess bleaching bands below ca. 500 nm and broad positive absorption bands from 500 to 700 nm. The position of bleaching bands corresponds to the lowest-energy ground-state absorption of Pt complexes. The lifetimes of the triplet excited state for Pt-1 and Pt-2 deduced from the TA decay are in accord with those measured from the emission decay. This fact indicates that the TA spectra for Pt-1 and Pt-2 originating from the excited state could be the same as for the emitting state. Considering the short TA lifetimes and the shape of absorption bands, the triplet excited states of two complexes could be attributed to the 3MLCT/3ILCT states.9 Furthermore, The transient difference absorption spectra of GO−Pt-1 and GO−Pt-2, like Pt-1 and Pt-2, also exhibit a broad and positive absorption in the range 400−800 nm. While the continuous absorption in the 700−800 nm region arises from GO, the strong absorption in the 400−700 nm range may be assigned to the combination of GO and Pt complexes.53−55 It is also worth noting that the peaks of GO−Pt-1 and GO−Pt-2 undergo a hypsochromic shift compared with only Pt-1 and Pt-2, respectively. Meanwhile, the triplet excited-state lifetimes of the hybrid materials are shortened. On the basis of these TA and phosphorescence studies, we infer that the PET and ET processes could occur from the Pt complex moiety and GO simultaneously, which is consistent with the previous reports.19,30 These facts indicate that in the four materials, reverse saturable absorption happens at 532 nm. 3.4. Nonlinear Optical and Optical Power Limiting Properties. Lasers at the 532 nm wavelength have been widely used in industrial processes, medical treatment, and scientific research. However, the human eye and optical sensors are so sensitive to this wavelength that laser protection at 532 nm is particularly important in practical applications. The NLO and OPL properties of GO, Pt complexes, and GO-based hybrid materials were evaluated by using the Z-scan technique at 532 nm. During these experiments, as the sample with NLO behavior is moved slowly into focus, the laser intensity increases and the NLO effect enhances. All samples were individually dispersed in DMF by 30 min of ultrasonic processing and kept the same linear transmittance of 70% at 532 nm.19,25,56 As shown in Figure 9a, upon excitation by 4 ns laser pulses of 532 nm, open-aperture Z-scan curves of all samples display decreased normalized transmittance as samples were closed to the focus. The normalized transmittances of GO, Pt-1, Pt-2, GO−Pt-1, GO−Pt-2, and benchmark OPL material C60 at the maximal input influence decrease to 87%, 84%, 75%, 45%, 28%, and 48%, respectively. All six materials exhibit obvious NLO properties, and the transmittance curves of GO−Pt-1 and GO−Pt-2 clearly indicate the greater reduction of transmittance at Z = 0 mm, which is comparable to the results for C60. Table 2 summarizes the information about the nonlinear absorption coefficient (βeff) and the thirdorder nonlinear susceptibility (Im{χ(3)}) of GO, Pt-1, Pt-2, GO−Pt-1, and GO−Pt-2. After functionalization with Pt complexes, the GO−Pt hybrid materials possess much larger βeff and Im{χ(3)} than GO and Pt complexes, which indicates a remarkable synergistic effect of the combination between GO

Table 2. Linear and Nonlinear Optical Coefficients of GO, Pt-1, Pt-2, GO−Pt-1, and GO−Pt-2 in DMFa sample

T (%)

α0 (cm−1)

GO Pt-1 Pt-2 GO−Pt-1 GO−Pt-2

68.9 69.7 70.4 69.9 69.3

3.72 3.61 3.51 3.58 3.67

βeff (cm/GW) 9.1 9.8 17.2 64.3 77.3

± ± ± ± ±

0.17 0.28 0.35 0.51 0.23

Im{χ(3)} (×10−12 esu) 3.13 3.37 5.93 22.1 26.6

± ± ± ± ±

0.07 0.09 0.11 0.14 0.08

α0 = linear absorption coefficient. βeff = nonlinear extinction coefficient. Im{χ(3)} = imaginary third-order susceptibility.

a

and Pt complexes. Among them, GO−Pt-2 exhibits the largest third-order susceptibility and nonlinear extinction coefficient at 532 nm, which is also improved compared to those materials in early reports.22,24,30 Furthermore, NLS is regarded as an important factor for the NLO effect of graphene and its derivatives.57 Figure 9b shows the fluence-dependent scattering measurement at an angle of 40° to the propagation axis of the transmitted lasers. The NLS responses of GO−Pt-1 and GO− Pt-2 were also remarkably enhanced when compared to individual GO, Pt-1, Pt-2, and C60, which exhibited tiny scattering response. Therefore, GO−Pt-1 and GO−Pt-2 are able to act as effective NLO materials. The OPL performance of different materials was studied in DMF with the same linear transmittance at 532 nm. The pure DMF solvent displays no OPL performance under the same conditions, implying that the observed OPL response should originate from the samples. As shown in Figure 9c, the OPL performance of GO and Pt-1 is almost identical at 532 nm. The property of OPL for Pt-2 is better than that of Pt-1, which is consistent with our early studies. The improvement of RSA is because of the electron-withdrawing substituent of the acetylide ligands, which could lead to a decrease of the ground-state absorption and an improvement of the triplet excited-state absorption.9 It can be observed that the OPL performance of GO−Pt-1 and GO−Pt-2 is much better than that of the individual Pt-1, Pt-2, and GO. In addition, the value of entering into the nonlinear optical region was 0.064 J/cm2 for GO−Pt-2, 0.070 J/cm2 for GO−Pt-1, 0.187 J/cm2 for Pt-1, 0.215 J/cm2 for Pt-2, and 0.278 J/cm2 for GO. Therefore, hybrid materials can not only enhance the property of OPL but also realize the protection of weaker laser intensity, which is very meaningful. Furthermore, it can be found that GO−Pt-2 hybrid materials have the largest effective optical limiting responses of the five materials tested. The OPL threshold (F50, defined as the incident density where the transmittance falls to 50%) for different samples was studied. The GO−Pt-2 hybrid material reveals the lowest F50 value (0.85 J/cm2) in comparison with GO−Pt-1, Pt-1, Pt-2, and GO, and the transmission decreases to 0.25 as the incident fluence reaches 1.79 J/cm2. The limiting threshold value of GO−Pt-2 is better than that of the benchmark OPL material C60 (2.2 J/cm2), GO nanosheets (>3.0 J/cm2), multiwalled carbon nanotubes (1.3 J/cm2),14,58 and MoS2 nanotubules with a diameter of 300−400 nm (1.1 J/ cm2).59 Consequently, these results further demonstrate that the preparation of GO−Pt-1 and GO−Pt-2 hybrid materials by an amidation reaction and electrostatic absorption not only improves the solubility of the hybrid material but also greatly enhances their NLO and OPL performance. The strongly enhanced NLO performance and OPL response of the GO−Pt-1 and GO−Pt-2 hybrid materials can be attributed to a synergistic effect between the two 33037



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components. Pt(II) complexes bearing acetylide ligands are well-known reverse saturable absorbers in the visible region. The NLO and OPL properties of a GO suspension for nanosecond pulses at 532 nm commonly result from NLS induced by microbubbles and microplasmas,with TPA originating from the sp3 domains.60 In addition, we deem that two kinds of scattering centers are responsible for the optical scattering behavior of GO-based materials. When the incident is high and focalized on GO-based materials, one scattering center originated from GO−Pt-1 and GO−Pt-2 microplasma that formed through the complexes (Pt-1 and Pt-2) rapidly transferring the excitation energy to the GO sheets, and the other center is the microbubbles derived from the surrounding solvent of microplasmas. Furthermore, taking the strong fluorescence quenching for GO−Pt-1 and GO−Pt-2 into consideration, the photoinduced electron and energy transfer between GO and Pt(II) complexes occurred due to the excellent behaviors of GO as acceptor and of the Pt(II) complex moieties as donors, which has been observed for the hybrid materials of GO for optical limiting. In addition, ICPAES analyses show more Pt-2 molecules per area of GO. Therefore, the reasons for the better OPL performance of GO−Pt-2 are not only the excellent optical limiting of Pt-2 and more molecules per area of GO but also the means of combination between Pt-2 and GO.

AUTHOR INFORMATION

Corresponding Authors

*R.L.: phone, +86-25-83172358; fax, +86-25-83587428; e-mail: [email protected]. *H.Z.: phone, +86-25-83172358; fax, +86-25-83587428; email: [email protected]. ORCID

Hongjun Zhu: 0000-0002-8227-6064 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors greatly acknowledge the financial support in part by the National Natural Science Foundation of China (21602106), the Natural Science Foundation of Jiangsu Province-Outstanding Youth Foundation (BK20170104), the Industry−Academy−Research Prospective Joint Project of Jiangsu Province (BY2016005-06), and the National Key Research and Development Program of China (2016YFB0301703).

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REFERENCES

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4. CONCLUSION The synthesis, structure, and nonlinear optical properties of two GO−Pt hybrid materials were reported. The results of TEM, AFM, Raman, FT-IR, XPS, and TGA confirm the successful fabrication of hybrid materials based on covalent functionalization and electrostatic adsorption between GO and Pt complexes. Additionally, the photophysical properties were measured by UV−vis absorption, steady-state fluorescence, and nanosecond transient difference absorption. The NLO properties and OPL performance of GO, Pt complexes, and GO hybrid materials were investigated using Z-scan measurements at 532 nm with 4 ns laser pulses. The results indicate that functionalization of GO results in the GO−Pt hybrid material having much larger NLO properties and OPL performance than those of individual GO or Pt complexes, which could be attribute to a combination of NLS and TPA arising from GO, RSA originating from the Pt complex moiety, and the photoinduced electron and energy transfer from the donor Pt complex moiety to the acceptor GO. The present GO−Pt hybrid materials are expected to be good candidates for optical limiting. More attention will be paid to in-depth research concerning the solid state of graphene-based hybrid materials in further work, which could be used to fabricate films or glasses for practical applications.

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Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b10585. The synthetic route of Pt complexes; 1H NMR and MS of Pt-1 and Pt-2; photographic image of the samples dispersed in DMF; XPS spectra of GO, GO−Pt-2, and GO−Pt-1; PL excitation spectra of hybrids; and FT-IR of GO are provided (PDF) 33038


Research Article


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Synergistically Enhanced Optical Limiting Property of Graphene

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