1. Introduction
It is well known that the conductive, photoelectric organic or polymeric materials contain π-conjugated units and their photophysical properties are generally modulated by the degree of conjugation of the π electrons, for example, increasing or decreasing the size of the π-conjugation chain or ring to tailor the degree of π-conjugation, or introducing electron-donating/withdrawing groups to regulate the electronic state density for the desired bandgap width and photophysical properties. These modulation methods are very similar to the n- or p-doping in conventional inorganic semiconductors, which control the conductivity type (n- or p-type) and carrier concentration. Non-conjugated-polymer-derived carbon nanomaterials, as a type of hybrid materials, represent another type of approach to develop conductive, photoelectric, and photoluminescent multifunctional composites [1,2,3]. The key to designing materials and controlling their microstructures is to solve the charge delocalization and control the charge behavior of materials with different sizes, dimensions, and interfacial interactions between components, phases, and dimensional contacts to exploit the surface effects of materials. Among them, defects and interfaces play an important role in the carrier behavior. Due to the diversity of non-conjugated polymer materials, the development of carbon nanomaterials or hybrid multifunctional nanocomposites has unlimited scope because non-conjugated polymers are rich in the carbon element.
Sodium alginate (SA) is an abundant, biocompatible, and inexpensive natural polymeric material that has been widely used in tissue engineering [4,5], drug delivery [6,7,8,9], wound healing [10], fuel cells [11], sodium-ion batteries [12], ultrafiltration membranes [13], water and wastewater treatment [14,15,16], removal of organic pollutants [17], detection and removal of radioactive metal ions [18], photocatalytic removal of hexavalent chromium [19], removal of toxic metal ions [20,21], supercapacitor electrodes [22], soft and flexible actuators [23], responsive materials [24], organic/inorganic nanocomposites [25], and so on. Otherwise, sodium alginate contains -COOH groups. The presence of -COOH groups of the polymer easily leads to chemical crosslinking and functionalization, which facilitates the development of a large number of hybrid multifunctional materials and expands the use of non-conjugated polymers in the design of functional nanocomposites.
Among inorganic functional materials, PbS is a typical narrow bandgap semiconductor material. A key feature of PbS is its bandgap scalability, which can cover the visible light region and part of the infrared. PbS-based nanocomposites have good applications in photodetectors [26,27,28,29,30,31,32,33], solar cells [34,35,36,37,38,39,40,41,42,43], thermoelectric applications [44], optical switches [45], photocatalytic hydrogen production [46], NIR photoluminescence [47,48], broadband neuromorphic vision sensing [49], gas sensors [50], biosensors [51], water splitting [52], NIR responsive capacity [53], etc. Reviewing the relative research progress, much research is devoted to doping, heterostructures, surface modification, property tailoring, and device applications, for example, Bi-doped PbS [54,55], silver-doped PbS [56], Eu-doped PbS [57], and organic molecule-doped PbS [58]. Heterostructures include PbS/perovskite composite [59,60,61], organic/inorganic hybrid [62,63,64,65,66,67,68,69], Cu2-xS/PbS heterostructure [70,71], CdS/PbS heterostructures [72,73,74,75], MoS2/PbS heterostructures [76,77,78], PbS/WS2 heterostructure [79], PbS/ZnS core/shell nanocrystals [80], PbS/SnS2 [81], PbS/MnS [82], Ni-doped PbS/WO3/BiVO4 [83], ZnO/PbS heterojunction [84,85], PbS/TiO2 nanohybrids [86,87,88], PbS/graphene [89,90,91,92,93,94], carbon nanotube/PbS nanohybrids [95,96,97,98,99], PbS/carbon black [100], etc. For studies of PbS-based nanomaterials, there are better prospects for reducing toxicity and maintaining good photophysical properties.
Since the properties of nanomaterials are strongly dependent on size and dimension, the controlled preparation of PbS-based nanomaterials is still very important [101,102,103,104,105,106,107,108,109,110,111,112,113]. Due to the high free energy of the surface of PbS nanostructures, the surface is usually treated to avoid aggregation. Since there are many surface defects of PbS, in order to exert their physical and chemical properties, suitable passivating agents are usually selected to treat their surfaces [114,115,116,117,118,119,120,121,122,123,124,125,126]. Passivating agents can be divided into inorganic and organic ligands based on the type of material and their electronic effects, which include charged donors or acceptors (oleates, halides, ammoniums, etc.), neutral donors (amines, phosphines are typical of neutral donors), and neutral acceptors (metal oleates, metal halides, etc.). In combination with the corresponding literature analysis, other defect passivators have also been extensively studied, such as halide ions (I−, Br−, Cl−), PbI2 and PbBr2, oleic acid (OA), 1,2-ethanedithiol (EDT), 1-pyrenebutyric acid (PBA), 1,4-benzenedithiol, 1,2-ethanedithiol, 1,3-mercaptopropionic acid (MPA), -SCF3 and CF3(CF2)14COOH (pFA), thiolates, thiophenolate, and thiocyanate anion (SCN-). Among the organic ligands, the length of organic ligand chain and its electronic effect have a great influence on its photophysical properties. The interaction of electrons between ligands and PbS is the main basis for the selection of suitable ligands.
For photoactive materials, the photophysical process is extremely complex, involving defects, band structure, recombination centers, luminescence, scattering, free carrier generation, separation, and transport. This is also one of the major research topics in materials physics and applied physics [127,128,129,130,131,132,133,134,135,136,137,138,139,140,141]. Some representative examples of studies in this area are as follows. Lei and co-workers [127] enhanced the photoinduced carrier transfer by coupling the InZnP quantum dots with PbS via interfacial interaction. Nordin and co-workers [128] studied charge transfer in hybrid organic–inorganic PbS nanocrystals. Shen and co-workers [129] reported the ultrafast carrier dynamics in PbS quantum dots. Li and co-workers [130] discussed the mechanisms of large Stokes shift in isolated and coupled PbS quantum dots. Voznyy and co-workers [131] investigated the origin of the Stokes shift in PbS nanocrystals. Zherebetskyy and co-workers [132] discussed the tolerance of intrinsic defects in PbS quantum dots. Balazs and co-workers [133] studied the reduction of charge trapping in PbS quantum dot solids. Dantas and co-workers [134] studied the radiative versus non-radiative optical processes in PbS nanocrystals. Kushnir and co-workers [135] studied the dynamics of photoexcited carriers in polycrystalline PbS and PbS/ZnO heterojunctions, focusing on the influence of grain boundaries and interfaces. Moroz and co-workers [136] suppressed carrier scattering in CdS-encapsulated PbS nanocrystal films. Yang and co-workers [137] suppressed the interfacial charge recombination of PbS quantum dots. Nestoklon and co-workers [138] studied the exciton interaction with acoustic phonons in PbS nanocrystals. Aerts and co-workers [139] reported highly efficient carrier multiplication in PbS nanosheets. Kennehan and co-workers [140] studied the dynamic ligand surface chemistry of excited PbS quantum dots. Xia and co-workers [141] performed surface fluorination to control the bandgap of PbS quantum dots. Li and co-workers [142] fabricated the infrared light emitting diodes based on PbS quantum dots. Ruggieri and co-workers [143] reported the PbS quantum dot thin film X-ray monitors. Zhang and co-workers [144] investigated the photoemission of PbS quantum dots. He and co-workers [145] reported the mid-infrared response of PbS quantum dot solids. Bederak and co-workers [146] studied the S-rich PbS quantum dots. Shi and co-workers [147] balanced the NIR II fluorescence and photothermal effect of Au-PbS nanoparticles. Li and co-workers [148] enhanced the visible light photoenergy conversion of PbS quantum dots coupled with Au nanoparticles using their plasmonic effects. Mamiyev and co-workers [149] reviewed the progress of PbS nanostructures, and so on. These studies provide some useful physical mechanisms involving surfaces, interfaces, defects, grain boundaries, recombination, and photodynamic processes.
Reviewing the above studies, it can be seen that the use of carbon materials to modify PbS nanostructures can not only passivate the defects, but also reduce their toxicity of PbS to some extent, since carbon materials have good biocompatibility [89,90,91,92,93,94,95,96,97,98,99,100]. In addition, carbon materials and their derivatives contain a certain amount of C sp2 hybridization, which can improve their charge transport performance at different scales. Carbon materials include graphene, carbon nanotubes, fullerene series, carbon dots, and their derivatives. Among them, carbon dots are one of the most promising materials in recent years due to their good fluorescence properties and biocompatibility. Compared to carbon quantum dots, carbon dots, and graphene quantum dots, non-conjugated-polymer-derived carbon materials are more challenging due to the complexity of the aggregation state structures. The diversity of structural design is their greatest advantage and challenge for multifunctional composite applications. Polymer-derived carbon materials contain high C content and controlled heteroatoms according to the desired properties and requirements of different elemental doping. Multi-element doping can be easily achieved through polycondensation and blending processes. By enhancing the electronic interactions between the interfaces of the nanocomposite, the effect that the doping amount is difficult to increase can be compensated. Therefore, inorganic semiconductors modified via polymer-derived carbon materials would be effective, low toxicity, and simple. It provided a flexible approach to the modification of inorganic semiconductors. However, polymer-derived carbon materials are rich in defects and require passivation to improve their charge transport. The balance between ordered and disordered structures is a major challenge for polymer-derived carbon materials.
Based on the previous studies of polymer-derived carbon materials [1,2,3], increasing the degree of π-electron conjugation and crosslinking of the polymer precursor would increase the photoluminescence of the resulting carbon dots. Sodium alginate is considered to be rich in -COOH groups. The chemical crosslinking of the -COOH groups of the polymer occurred upon the addition of the divalent metal ions and automatically resulted in the formation of a sodium alginate gel. Although polymer gels have a wide range of applications, such as drug release applications, artificial muscles, adsorption fields, self-healing materials, soft and flexible robots, actuators, gel polymer electrolytes for sodium-sulfur batteries, lithium-ion batteries, supercapacitors, and solar cells, the transformation of polymer gels into multifunctional materials is still an attractive prospect in the field of materials science. In our previous studies, carbon nanofibers were synthesized using carboxymethyl cellulose, and the electronic interaction between carbon nanofibers and Cu nanoparticles was investigated [150]. In another work, the interfacial interaction of Mo2O3/carbon dots derived from chitosan was investigated [151]. These studies provided a low-cost and green chemistry method to develop advanced multifunctional composites and have some reference for the design and application of light-responsive multifunctional and smart materials. However, the above preparation of nanocomposites is carried out in two steps, which is relatively time-consuming. In this study, the PbS-based nanocomposites were derived from polymer gel, and their photophysical properties were investigated. By replacing conventional defect passivators with polymer-gel-derived carbon materials, the photocurrent signals were obtained in the broadband light spectral region. The synergies of in situ preparation of each component of the nanocomposites and mutual defect passivation were realized. Not only was the toxicity of PbS reduced to some extent, but also the aggregation of PbS was avoided and the surface defects were effectively passivated. This would be a simple approach to develop a series of multifunctional inorganic multifunctional hybrid materials using abundant non-conjugated polymer resources. The interdisciplinary and application of more polymer systems in frontier areas will be expanded upon. The ZnO/PbS heterostructure system has been reported in our previous publication [152]. Herein, the synthesis and its photophysical properties of PbS/C nanocomposite with one-pot preparation is introduced.
2. Materials and Methods
2.1. Materials
Sodium alginate (AR) (purity 99.9%) was from Tianjin Basf Chemical Co., Ltd. (B407, No.8 South Binguan Road, Hexi District, Tianjin, China) Lead acetate (AR) (purity 99.0%) was from the Tianjin Tianhe Chemical reagent factory. Thiourea (AR) (purity 99.0%) was from the Tianjin Taixing Chemical reagent factory.
2.2. Preparation of Sodium Alginate Solution
A total of 1 g of sodium alginate and 200 mL of H2O were added to a 500 mL glass flask, stirred, and sonicated for 5–10 min. The sodium alginate solution was saved for later use.
2.3. Preparation of PbS/Carbon Nanocomposite Derived from Polymer Gel
A total of 50 mL sodium alginate solution and 0.5 g lead acetate were added together and stirred for 1–3 min. The polymer gel formed rapidly and automatically. Then, 0.5 g thiourea was added as a sulfur source. The hydrothermal reaction conditions and sample treatment were the same as in our previous report [151].
Similarly, 50 mL of sodium alginate solution, 10 mL of graphene oxide nanoribbon (the concentration of graphene oxide nanoribbon was about 4.3 mg/mL; its synthesis is also shown in our previous report [153,154]), and 0.5 g of lead acetate were added together and stirred for 1–3 min. The polymer gel formed quickly. Then, 0.5 g thiourea was added as a sulfur source. The hydrothermal reaction conditions and sample treatment were the same as in our previous report [154] for comparison of increasing C sp2.
2.4. Characterization of SEM, TEM, UV–VIS-NIR, XRD, and Raman Spectra
The characterization of the SEM (scanning electron microscope), TEM (transmission electron microscope), UV–VIS-NIR (UV-VIS-NIR spectrophotometer), and XRD (X-ray powder diffraction) was shown in our previous report [150,151]. The instruments used were with the ZEISS Gemini SEM500 (Oberkohen, Germany), JEM-1011 (Japan Electronics Co., Ltd., Tokyo, Japan), TU-1810 spectrophotometer (Beijing Puxi General Instrument Co., Ltd., Beijing, China), and XRD-7000 from SHIMADZU (Shimadzu, Kyoto, Japan).
The Raman spectra were characterized as follows: The sample suspension was dip-coated on the glass substrate and dried at room temperature. The Raman spectra were determined using a PHS-3C confocal Raman spectrometer (HORIBA, Kyoto, Japan). The operating wavelength and power density of the laser radiation were 785 nm and 5 mW, respectively.
2.5. Photocurrent Measurements of the Nanocomposite to the Visible Light and Part of NIR
The determination of the photocurrent signal of the resulting nanocomposite in the visible light and part of the NIR was shown in our previous report [150]. In this study, untreated A4 printing paper was used as the substrate, and the electrodes were graphite electrodes with 5B pencil drawings (Au gap electrodes on PET film in previous studies). The electrode structure is shown in Scheme 1. The photocurrent signal to some typical light sources such as 650 nm (100, 50, 5 mW) and 808, 980, and 1064 nm NIR (10, 20, 50, 100, 200 mW) was determined using an LK2000A Electrochemical Work Station (LANLIKE Chemistry and Electron High Technology Co., Ltd. (Tianjin, China)) with 1 V DC bias applied, and the current of the thick film was measured by computer recording before and after irradiation of the light sources.
2.6. Tentacle Sensitivity Examination of the PbS/C Composite Nanosheets to the Applied Force
To investigate the tentacle sensitivity of the PbS/C composite nanosheets to the applied force, the structure of Scheme 1 was bonded to the EVA (ethyl vinyl acetate) foam (thickness approximately 2 mm) with a pressure-sensitive adhesive. The force was applied by deforming the EVA foam. The electrical responses were preliminarily measured using the LK2000A Electrochemical Work Station from LANLIKE Chemistry and Electron High Technology Co., Ltd. (China), under different compressive forces (such as 20, 50, 100 g weight of the balance) or finger touch with 1 V DC bias applied [155].
3. Results and Discussion
PbS nanomaterial is one of the infrared-active materials. The photophysical properties of PbS-based nanomaterials are strongly dependent on their size effect and dimension effect. The PbS nanomaterials are easy to accumulate or adsorb foreign molecules due to a large number of surface defects and high surface energy. Therefore, surface treatment is usually required in the controlled synthesis of PbS nanomaterials. The choice of surface treatment is based on the electron donor and acceptor effects to avoid their aggregation and to exert their excellent photophysical properties. In nanocomposites, grain boundary defects and interfacial spacing also strongly influence the carrier transport generated by light triggering. Some common surface defect passivators are summarized below. They are listed in Table 1.
As shown in Table 1, organic ligands are mostly compounds containing -COOH, -NH2, -SH, and phosphate groups. These groups are easily coordinated with Pb in PbS. Surface defect passivation can be either in situ passivation during the preparation process of PbS-based nanomaterials or post-treatment. Since carbon material has excellent charge transport performance, we intended to combine in situ formation of PbS and defect passivation with carbon materials in one step. Since sodium alginate is rich in -COOH groups, crosslinking is easy to achieve. Among the many crosslinking agents, the divalent or multivalent metal ions can lead to chemical crosslinking of the -COOH groups of the polymer and form a sodium alginate gel. In this study, lead salt was used as the crosslinking agent to prepare a polymer gel, which was then converted into the PbS nanocomposite. The representative SEM and TEM images of the resulting PbS/C nanocomposite are shown in Figure 1 and Figure 2, respectively.
As shown in Figure 1 of the SEM images with different magnification (5000×, 10,000×), the resulting nanocomposite showed a nanosheet shape, and the size of the nanosheets was not uniform. The thickness of the nanosheets was very thin. The size of these nanosheets was approximately in the range of 0.5–2 µm. The SEM images with different magnifications (5000×, 10,000×) reflected the consistency of the overall morphology and the local morphology. This morphology was also verified by the transmission electron microscopy. This is shown in Figure 2. As shown in the TEM image, there were some flocculants on the surface and edge of the nanosheets. These flocculants should be polymer-derived carbon materials. The depth of the defect energy level has a significant effect on the photoelectric properties. Deep level defects are easily able to trap photogenerated carriers and require defect passivation. It is expected to have some passivation effect on the surface and edge defects and improve charge transfer while avoiding aggregation of the PbS material. The in situ preparation of PbS/C nanocomposites is expected to realize this idea. The passivation effects of defects with different energy levels were investigated by excitation with different wavelength light sources. The formation process of the PbS/C nanocomposite derived from the polymer gel is shown in Scheme 2.
The XRD results of the polymer-gel-derived PbS/C composite nanosheets are shown in Figure 3.
As shown in Figure 3, the strong diffraction peaks at 25.94°, 30.04°, 43.12°, 50.91°, 53.50°, 62.51°, 68.93°, 70.98°, and 78.88° were the peaks of (111), (200), (220), (311), (222), (400), (331), (420), and (422) planes of PbS (PDF# 05-0592) for the polymer-gel-derived PbS/C composite nanosheets that were observed, respectively. Therefore, the polymer gel-derived composite nanosheets contained a PbS component.
The UV–VIS-NIR absorbance curve of PbS/C composite nanosheets derived from polymer gel is shown in Figure 4.
As shown in Figure 4, it was found that the PbS/C composite nanosheets derived from the polymer gel exhibited a certain degree of absorption in the visible light region and the NIR.
The Raman spectra of the polymer-gel-derived PbS/C composite nanosheets are shown in Figure 5.
In the experiment of studying the Raman spectra of PbS/C composite nanosheets derived by polymer gel, it is found that PbS/C composite nanosheets had some fluorescence when using the 532 nm and 633 nm wavelength laser resource excitation. In the process of photoexcitation kinetics, the generation of fluorescence, Raman, and free carrier production is a competitive process (Scheme 3). The probability of Raman scattering is very low, and it is difficult to characterize the Raman signal of fluorescent materials. When the excitation was performed using a 785 nm light source, a small Raman signal was able to be observed. It is shown in Figure 5.
As shown in Figure 5, the band at about 1580 cm−1 corresponding to C sp2 of G peak was a little weak in the sample of PbS/C nanocomposite derived by polymer gel, but it was able to be discerned. The band at about 1270 cm−1 belonged to the D peak of the disordered structure, and this band was also clear in the sample of PbS/C nanocomposite derived by polymer gel. The 2D band at 2689.5 cm−1 was also observed. A small D, G, 2D band was also able to be observed under the fluorescence interference of PbS/C composite nanosheets. The Raman signal had a large noise, mainly because the fluorescent material was difficult to characterize its Raman signal. It was the result of fluorescence signal interference with Raman. This is only a reference for photodynamic processes. In turn, it also showed how to balance the above three competitive processes by material microstructure, defects passivation, interfacial interactions, etc.
As shown in Scheme 3, the focus of this research is on how to suppress the non-radiative energy consumption by controlling the aggregation state of the nanocomposite derived from the non-conjugated polymer gel, as well as how to improve the generation of free electrons and holes by light triggering. Deep-level defects are not conducive to the extraction of photogenic carriers and need to be passivated. Although defect passivation for PbS nanomaterials is diverse, some inorganic ligands, organic small-molecule ligands, and polymer ligands are widely used. However, the passivation principle is very similar, all based on the interaction between electrons (Table 1). Based on some of our research experience in carbon materials derived from various polymer materials, the structure of carbon nanomaterials derived from polymer gel should be very complex, as it contains the combination of ordered and disordered structure, and the content of the C sp2 ordered structure is not high. The ratio of ordered and disordered of carbon materials is related to the degree of cross-linking, cyclization, and carbonization processes of linear polymers. The material design is expected to form more than five, six-membered ring structures of the C sp2 hybrid. It is expected that the carbon materials derived from non-conjugated polymer gel will still have a low content of graphitic C sp2. On the other hand, carbonization of polymers can produce a variety of ring structures and disordered structures. The disordered structure and defects of carbon materials can cause a scattering effect and increase the non-radiative energy consumption. It also showed the diversity of ordered and disordered structure for non-conjugated polymer-derived carbon materials by hydrothermal carbonization. For a visual illustration, the combination of ordered and disordered structure is shown in Scheme 4.
As shown in Scheme 4, the combination of ordered and disordered structures of the aggregation state of polymer-derived carbon materials is very complex. Since the degree of crosslinking affects the photophysical properties of carbon materials, and five- and six-membered rings are more stable, but the effect of the degree of crosslinking of the polymer on the five- and six-membered rings of C sp2 structure remains to be further investigated. The many kinds of ring structures and disordered structures in the carbonization process of polymer are mainly due to the complexity of the structure of its precursor polymer. The hybrid cluster structure is its main characteristic. A detailed explanation of the photophysical mechanism remains a great challenge.
In our previous publications [150], a PET film substrate and Au gap electrodes were used to investigate the photoelectronic signal in most of our research. Paper is an abundant, inexpensive, and natural polymer fiber. It has potential applications in wearable and printed flexible devices. However, its main disadvantages are high roughness, multi-porosity, and some impurities. These factors are detrimental to charge transport in flexible devices. In general, it is not easy to obtain photoelectric signals using untreated paper as a substrate. However, in some material systems, the concentration of photogenerated carriers is quite high, and photoelectric signals can be obtained using untreated paper as a substrate. Here, the electrodes are the low-cost graphite electrodes drawn in 5B pencil. Several typical light sources such as 650, 808, 980, and 1064 nm were selected to study the photoelectric signals using untreated paper as a substrate. These light sources mainly cover some representative light sources from visible light to the NIR.
The representative results are shown in Figure 6, Figure 7, Figure 8 and Figure 9.
As shown in Figure 6, Figure 7, Figure 8 and Figure 9, it was found that the polymer-gel-derived PbS/C composite nanosheets exhibited good photocurrent signals to 100 mW 650 nm, 200 mW 808 nm, 100 mW 980 nm, and 20 mW 1064 nm light sources. Since different wavelengths of light correspond to different energies, these representative light sources, ranging from the visible light to the near infrared, effectively excite electrons from the ground state to the excited state and then convert them into free photogenerated carriers. The effectiveness of in situ passivation of defects is also demonstrated. This one-step preparation not only avoids the aggregation of nanomaterials and improves the dispersion but also effectively passivates the defects and improves the extraction of photogenerated carriers.
Based on the accumulation of previous research, polymer-derived carbon materials have more defects and poor charge transport performance. Due to the good charge transport and flexibility of graphene nanoribbons, nanocomposites with large grain boundary spacing can be connected. Therefore, a comparative experiment was conducted by adding a small amount of graphene oxide nanoribbons. The comparative results are shown in Figure 10 and Figure 11 under similar experimental conditions.
As shown in Figure 10 and Figure 11, after adding a small amount of graphene oxide nanoribbons, the resulting nanocomposite showed similar photocurrent signals for 100 mW 650 nm, 200 mW 808 nm, 100 mW 980 nm, and 20 mW 1064 nm light sources. This indicates that the addition of graphene oxide nanoribbons had little effect on the extraction of photocurrent signals. On the one hand, C sp2 hybridization was conducive to charge transport, and on the other hand, the oxygen-containing groups of graphene oxide nanoribbons were not conducive to charge transport, and the combined results of these two factors may have a little effect. Since polymer-derived carbon materials have a large number of defects, there were also many defects on the surface and edges of the PbS nanosheets. The two types of defects passivate each other and promote photogenerated charge transport at the interface. It also shows that the PbS/C composite nanosheets had good charge transport performance without the need to add graphene nanoribbons. Similarly, some comparative experiments with other relative polymers did not achieve similar effects for PbS in situ preparation. This suggests that there is a large gap between the ordered and disordered structures of carbon materials prepared using different polymer precursors.
The dependence of the photocurrent responses of PbS/C composite nanosheets derived from polymer gel on the power of typical excitation light sources is shown in Figure 12 and Figure 13.
As shown in Figure 12 and Figure 13, it was found that the polymer-gel-derived PbS/C composite nanosheets still showed good photocurrent in up to 5 mW 980 nm light sources. However, there was almost no response to 5 mW 650 nm light sources. This shows that the PbS/C composite nanosheets were more sensitive to the NIR.
It can be seen from the above results that the resulting PbS/C nanocomposite in this study showed good photocurrent response from the visible to the NIR region.
This method is also used in the synthesis of several other inorganic hybrid functional materials. Some oxides and sulfides/carbon nanocomposites derived from various polymeric materials have been investigated in our studies. Some similar results have been obtained. The generality of the method of nanomaterial preparation is demonstrated. It provides more scope for the development of a range of functional hybrid materials using non-conjugated polymers. It also expands the potential applications in the design of some advanced multifunctional materials and devices using non-conjugated polymers as precursors. The interdisciplinarity and application of more polymer systems in frontier areas will be expanded upon further.
To investigate the tentacle sensitivity of the resulting nanocomposite to force, the prototype device for investigating the photocurrent behavior was subjected to a finger touch force. The electrical response is shown in Figure 14.
As shown in Figure 14, the film current increased when the compressive force was applied with a finger touch. Conversely, the current decreased when the compressive force was released. This suggests that the resulting nanocomposite exhibited some force sensitivity. Since it is not easy to control the magnitude of the force by touch, a 100 g weight was applied to the prototype device, and the electrical response is shown in Figure 15.
As shown in Figure 15, the film current decreased when the compression force was applied with a 100 g weight. Conversely, the current increased when the compression force was released. Since the resulting nanomaterial was coated on the A4 printing paper, the paper is a kind of natural polymer. Applying the correct compression force would cause the distance between the nanosheets to increase and the film current to decrease. If a larger compression force was applied (such as the touch of a finger), the distance between the nanosheets would become smaller and the film current would increase. Otherwise, since the resulting nanocomposite is more sensitive to the infrared light, the increase in film current may be a contribution of the NIR irradiation of the finger, which deserves further investigation.
The effects of different weights on the sensitivity of the film were also examined. The results are shown in Figure 16.
As shown in Figure 16, its sensitivity increased as the compressive force increased. Since the force was applied by deforming the polymer, EVA foam was a viscoelastic material. Hysteresis was inevitable. The effect of cycles of applied force on the sensitivity was investigated. The results are shown in Figure 17.
As shown in Figure 17, the first and second compression forces applied were significantly different. This was mainly due to the presence of permanent deformation of the EVA foam. The later cycle was basically stable. These studies were only preliminary, and the improvement in hysteresis was mainly achieved by improving the composition, microstructure, and dynamic fatigue properties of the polymers used.
In summary, the design of nanocomposites mainly focuses on the modulation of material properties through interactions between components, sizes, dimensions, phases, etc. There are many interfacial theories to guide the design of nanocomposites to improve the physical and chemical properties, such as chemical bonding theory, interfacial transition layer theory, and so on. All of them involve the chemical and physical interactions. Among them, interfacial charge transfer is the direct experimental evidence of interfacial interaction, and regulating the interfacial interaction can improve the physical and chemical properties of nanocomposites, which can meet the interdisciplinary application. This study provides some valuable information on the controlled preparation and photophysical properties of nanocomposites from an interdisciplinary point of view as part of the research for the interdisciplinary field.
4. Conclusions
In conclusion, PbS/C composite nanosheets were obtained by one-pot carbonization using sodium alginate gel. The results showed that the non-conjugated polymer-gel-derived PbS/C composite nanosheets coated on an untreated A4 printing paper substrate exhibited broadband spectral photocurrent signals. It is expected to have potential applications in broadband flexible photodetectors, tentacle sensors, or interdisciplinary fields. The photophysical mechanism was discussed. This study provided a low-cost and simple method to synthesize PbS nanocomposites modified with carbon nanostructures, which exhibited good photocurrent signals in the NIR region. This one-step preparation method not only avoids the aggregation of nanomaterials and improves the dispersion but also effectively passivates the defects and enhances the extraction of photogenerated carriers. It demonstrates the effectiveness of in situ defect passivation. Mutual defect passivation during the in situ preparation of PbS and carbon nanomaterials is a simple approach to improve the photophysical properties. It also helps in the development of NIR-responsive inorganic hybrid functional materials using non-conjugated polymer systems. The interdisciplinary and application of more polymer systems in frontier areas will be expanded.
Conceptualization, methodology, investigation, writing—original draft preparation, writing—review and editing, funding acquisition, resources, X.M.; investigation, X.Z., investigation, M.G. and Y.W.; resources, G.L.; all authors analyzed the data; all authors discussed the results of the paper. All authors have read and agreed to the published version of the manuscript.
This study did not involve in ethical issues.
Not applicable.
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy.
Thanks go to You Wang and Guang Li of Zhejiang University for the fabrication of several electrodes and for checking the English language of the paper. TEM was performed by Chunsheng Wang, SEM was taken by Fang Tian and Jie Su, and Raman spectroscopy was carried out by Jie Su and Weiwei Wang at the Structural Composition Testing Center, School of Chemistry and Chemical Engineering, Shandong University. Some students, such as Rui Fang, Wangzhen Liu, Yansong Liu, Wenxiu Qiu, Zesong Wang, Wenli Zhang, Yongqi Zhao, Bingjie Yang, et al., performed part of the experiments.
We declare that we have no conflicts of interest.
Footnotes
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Enlarge this image.Scheme 1. The structure of the electrodes in this study (the electrodes are the graphite electrodes with 5B pencil drawings).
Enlarge this image.Figure 1. The representative SEM image of PbS/C nanocomposite derived by polymer gel with different magnifications (5000×, 10,000×).
Enlarge this image.Figure 2. The representative TEM image of PbS/C nanocomposite derived by polymer gel in different regions (Left: 60,000×; Right: 30,000×).
Enlarge this image.Scheme 2. The formation process of PbS/C composite nanosheets derived by polymer gel.
Enlarge this image.Figure 3. The XRD results of the PbS/C composite nanosheets derived by polymer gel.
Enlarge this image.Figure 4. The UV–VIS-NIR absorbance curve of the PbS/C composite nanosheets derived by polymer gel.
Enlarge this image.Figure 5. The Raman spectra of the PbS/C composite nanosheets derived by polymer gel.
Enlarge this image.Figure 6. The transient-state photocurrent responses of the PbS/C composite nanosheets derived by polymer gel to 100 mW 650 nm.
Enlarge this image.Figure 7. The transient-state photocurrent responses of the PbS/C composite nanosheets derived by polymer gel to 200 mW 808 nm.
Enlarge this image.Figure 8. The transient-state photocurrent responses of the PbS/C composite nanosheets derived by polymer gel to 100 mW 980 nm.
Enlarge this image.Figure 9. The transient-state photocurrent responses of the PbS/C composite nanosheets derived by polymer gel to 20 mW 1064 nm.
Enlarge this image.Figure 10. The comparative transient-state photocurrent responses of the PbS/C composite nanosheets derived by polymer gel containing graphene oxide nanoribbons to 650 and 980 nm light resources ((A) 100, 50, 5 mW 650 nm; (B) 100, 50, 5 mW 980 nm; (C) 5 mW 650 nm; (D) 5 mW 980 nm).
Enlarge this image.Figure 11. The comparative transient-state photocurrent responses of the PbS/C composite nanosheets derived by polymer gel containing graphene oxide nanoribbons to 650, 808, 980, and 1064 nm ((A) 100 mW 650 nm; (B) 200 mW 808 mW; (C) 100 mW 980 nm; (D) 20 mW 1064 nm).
Enlarge this image.Figure 12. The dependence of photocurrent responses of the PbS/C composite nanosheets derived by polymer gel on the power of excitation of 650 nm (100, 50, and 5 mW).
Enlarge this image.Figure 13. The dependence of photocurrent responses of the PbS/C composite nanosheets derived by polymer gel on the power of excitation of 980 nm (100, 50, and 5 mW).
Enlarge this image.Figure 14. The tentacle sensitivity of the PbS/C composite nanosheets to the compression force of a finger touch.
Enlarge this image.Figure 15. The tentacle sensitivity of the PbS/C composite nanosheets to a compression force of about 100 g.
Enlarge this image.Figure 16. The effects of different external forces applied (20, 50, 100 g, etc.) on the tentacle sensitivity of PbS/C composite nanosheets (the external forces applied are as follows: (A) 20 g; (B) 50 g; (C) 100 g).
Enlarge this image.Figure 17. The tentacle sensitivity of the PbS/C composite nanosheets to compression force of about 100 g ((A) 100 g; (B) 100 g after more than 10 cycles).
Some common surface defect passivating agents for PbS nanomaterials [
| Inorganic Ligands | Organic Ligands |
|---|---|
| Halide ions (I−, Br−, Cl−) | oleates |
| PbI2 and PbBr2, PbI2 | amines |
| Thiocyanate anion (SCN−), etc. | oleic acid (OA) |
| 1,2-ethanedithiol (EDT) | |
| 1-pyrene butyric acid (PBA) | |
| 1,4-benzenedithiol | |
| 1,2-ethanedithiol | |
| -SCF3 | |
| CF3(CF2)14COOH (pFA) | |
| 1,3-mercaptopropionic acid (MPA) | |
| acetonitrile | |
| P3HT | |
| MAPbI3, CsPbI3, CsxMAyFAzPbX3, formamidinium lead iodide | |
| trioctylphosphine | |
| thiolates | |
| thiophenolate | |
| polyvinylpyrrolidone |
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; Zhang, Xintao 1 ; Gao, Mingjun 1 ; Wang, You 2 ; Li, Guang 2 1 School of Environmental and Material Engineering, Center of Advanced Functional Materials, Yantai University, Yantai 264005, China;
2 National Laboratory of Industrial Control Technology, Institute of Cyber-Systems and Control, Zhejiang University, Hangzhou 310027, China;