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Article

The Interface Interaction of C3N4/Bi2S3 Promoted the Separation of Excitons and the Extraction of Free Photogenerated Carriers in the Broadband Light Spectrum Range

by
Xingfa Ma
1,*,
Xintao Zhang
1,
Mingjun Gao
1,
Ruifen Hu
2,
You Wang
2 and
Guang Li
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
*
Author to whom correspondence should be addressed.
Inorganics 2025, 13(4), 122; https://doi.org/10.3390/inorganics13040122
Submission received: 5 February 2025 / Revised: 29 March 2025 / Accepted: 10 April 2025 / Published: 12 April 2025
(This article belongs to the Special Issue Synthesis and Application of Luminescent Materials, 2nd Edition)
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Abstract

:
Exciton generation and separation play an important role in the photoelectric properties and the luminescence performance of materials. In order to tailor the defects and grain boundaries and improve the exciton separation and light harvesting of the graphitic carbon nitride (g-C3N4) nanosheets, a C3N4/bismuth sulfide (Bi2S3) nanocomposite was synthesized. The photoelectric properties of the 405, 532, 650, 780, 808, 980 and 1064 nm light sources were studied using Au electrodes and graphite electrodes with 4B and 5B pencil drawings. The results indicate that the C3N4/Bi2S3 nanocomposite exhibited photocurrent switching behavior in the broadband light spectrum range. It is noted that even with zero bias applied, a good photoelectric signal was still measured. The resulting nanocomposite exhibited good photophysical stability. Physical mechanisms are discussed herein. It is suggested that the interfacial interaction of C3N4 and Bi2S3 in the nanocomposite creates a strong built-in electric field, which accelerates the separation of excitons. Therefore, as a dynamic process of photoexcitation, fluorescence, the photoelectric effect, and scattering are three main competing processes; the separation of excitons and the extraction of free photogenerated charge can be used as a reference for the fluorescent materials or other photoelectric materials studies as photophysical properties. This study also serves as an important reference for the design, defect and grain boundary modulation or interdisciplinary application of functional nanocomposites, especially for the bandgap modulation and suppression of photogenerated carrier recombination.

1. Introduction

Graphitic C3N4 is one of the most important 2D polymeric materials, with a bandgap of about 2.7 eV. The utilization of the solar spectrum is limited. Although the field of application is wide, such as dye-sensitized solar cells [1], photocatalytic carbon dioxide (CO2) reduction [2,3,4,5,6], photocatalytic hydrogen evolution [7,8], and other photocatalytic fields [9,10,11,12,13,14], due to absorption in the visible range, it is mainly used in catalysis and photocatalysis fields due to the abundance of defects and the utilization of visible light. Due to its structural similarity to the graphene, it contains a large conjugated structure and a high content of N atoms, whose lone pairs of electrons can regulate its electronic structure and physicochemical properties. However, the high photogenerated carrier recombination, abundant defects and poor conductivity limit the applications of g-C3N4 in multidisciplinary fields. Defects can localize charge, reflecting catalytic activity, but are detrimental to charge transport, making it difficult to fully exploit the two-dimensional surfaces of the material. To address these key issues, the construction of heterojunctions is an important way to exploit the excellent physicochemical properties of g-C3N4 and broaden its applications in interdisciplinary fields.
Sulfides are also an important class of 2D semiconductor materials. Among them, bismuth sulfide (Bi2S3) is a non-toxic material. It has good chemical stability, a low energy gap (1.3 eV) and good absorption of near-infrared light. It has good development prospects in the field of environmentally friendly materials. It has been extensively studied in H2 production under visible light irradiation [15,16,17], photocatalytic oxygen evolution [18], lithium-ion batteries [19,20,21], Zn-ion batteries [22], potassium-ion batteries [23], supercapacitors [24,25,26], resistive switches [27], chemical sensors and biosensors [28,29,30,31], carbon dioxide photoreduction [32], electrocatalytic CO2 reduction [33], photodetectors [34,35], etc. Bi2S3-based nanocomposite systems include oxide heterojunctions [36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54], such as bismuth trioxide (Bi2O3)/Bi2S3 heterostructures, Bi2S3/Bi2S3-xOx, indium oxide (In2O3)/Bi2S3 heterojunctions, Bi2S3/titanium oxide (TiO2) heterostructures, nickel tungstate (NiWO4)/Bi2S3, Bi2S3/Bi0.5Na0.5TiO3 compounds, ferroferric oxide (Fe3O4)/Bi2S3, bismuth vanadium oxide (BiVO4)/Bi2S3 heterostructures, Bi2S3/zinc oxide (ZnO) heterostructures, Bi2S3/bismuth ferrite (BiFeO3) heterojunctions, Bi2S3/BiFeO3 heterojunctions, tungsten trioxide (WO3)/BiVO4/Bi2S3 heterojunctions, etc. Sulfide heterojunctions [55,56,57,58,59,60,61,62,63,64] include Bi2S3/ZnIn2S4 heterojunctions, n-Bi2S3/lead sulfide (p-PbS) heterojunctions, Bi2S3/molybdenum disulfide (MoS2), AgBi3S5/Bi2S3, ferrous disulfide (FeS2)/Bi2S3 composites, Bi2S3/cadmium indium sulfide (CdIn2S4) heterojunctions, Cu2CdSnS4/n-Bi2S3 heterojunctions, Cu2CdSnS4/n-Bi2S3 heterojunctions and Bi2S3/tin disulfide (SnS2) heterojunctions. There also exists boron nitride heterojunctions [65], g-C3N4 heterojunctions [66,67,68,69,70,71,72], carbon nanomaterial heterojunctions [73,74], metal/Bi2S3 heterojunctions [75,76,77,78,79,80,81,82], including Au, Ag, Ni and Bi/Bi2S3 heterojunctions, doping systems [83], and so on.
The control of the microstructure of materials and the study of their physical properties remains an important prerequisite for interdisciplinary applications. Some representative examples are listed below. Kim and co-workers [84] reported a high performance phototransistor based on 2D Bi2S3. Kilcoyne and co-workers [85] studied gate-tunable transport characteristics of Bi2S3 transistors. Andzane and co-workers [86] examined the photoconductive properties of Bi2S3 nanowires synthesized using an anodized alumina membrane as a hard template with light sources of wavelengths of 500 to 900 nm. Xi and co-workers [87] reported optical switches based on Bi2S3 nanowires. Bao and co-workers [88] presented photoswitchable Bi2S3 nanowires. Kunakova and co-workers [89] studied the space–charge-limited current mechanism in Bi2S3 nanowires. Chen and co-workers [90] reported the nonlinear optical switching of Bi2S3 nanorods.
From the above literature analysis, it is clear that Bi2S3 (1.3 eV) and C3N4 (2.7 eV) can make complementary use of visible and NIR light, considering the energy band. The Bi2S3/C3N4 nanocomposite is expected to cover the visible and the near-infrared range. It not only modulates a large number of defects in the C3N4 material but also significantly improves the material’s utilization of the near-infrared range. This is beneficial for the development of low-cost, broadband-responsive, and environmentally friendly materials. In addition, composites are multi-phase, multi-component, and multi-dimensional integrations. The interfacial interaction is an important prerequisite for the embodiment of synergy and complementarity. The interfacial charge transfer of the Bi2S3/C3N4 nanocomposite provides direct experimental evidence of interfacial interactions and reflects the controllability of its chemical and physical properties. The most desirable goal in the design of composites is to reflect the synergy and complementarity of the components, phases, and dimensions.
For photoactive materials, the photoexcitation kinetics not only reflect the photophysical properties and related physical mechanisms but also pose a great challenge to the microstructure control of the materials, in particular bandgap engineering, defect modulation, and interfacial interactions. As a dynamic process of photoexcitation, fluorescence, the photoelectric effect, and scattering are three main competing processes. The extraction of free photogenerated charge can be used as a reference for studies of fluorescent materials with photophysical properties. Excitons are important quasiparticles in the process of light excitation, and their production, separation, and recombination involve many photophysical processes and multidisciplinary applications. The recombination of excitons is divided into radiative and non-radiative processes. These different photophysical processes reflect different photophysical properties and multidisciplinary applications. The desired properties can be achieved by modulation of the material microstructure.
For photodetection, the more studied material systems are two-dimensional graphene, molybdenum disulphide, black phosphorus, and boron nitride material systems. The C3N4 material system is mainly focused on catalytic and photocatalytic fields due to the higher number of defects and the absorption of visible light. The defects tend to produce electron-localized states, which have good chemical activity but are unfavorable for charge transport. For interdisciplinary applications where improved charge transport properties are desired, the increasing number of electronically delocalized states is required. This can modulate the redistribution of photogenerated charges and exploit the surface and interfacial effects of the material. This requires defect passivation of the material to enhance charge transport. Inspired by research fields such as photodetection and solar cells, we are investigating the control of defects, interfacial interactions, exciton separation, and free photogenerated charge extraction of the C3N4 material system from an interdisciplinary perspective. A previous publication [91] focused on iron (III) oxide (Fe2O3)/C3N4 interface and defect control. In this contribution, the interfacial charge transfer of the Bi2S3/C3N4 nanocomposite aggregation states in a broadband region and the suppression of recombination of photogenerated carriers are studied.

2. Results and Discussion

In the process of synthesizing the Bi2S3/C3N4 nanocomposite in this study, two types of precursors were used as a source of sulfur, namely sodium thiosulfate and thiourea. Due to the ease of obtaining the optoelectronic signals, the main focus here is on the Bi2S3/C3N4 nanocomposite synthesized with sodium thiosulfate. The representative TEM and SEM images of the Bi2S3/C3N4 nanocomposite are shown in Figure 1 and Figure 2.
As shown in Figure 1, both components have a distinct morphology. One component is a lamellar morphology. The other component is nanofibers, nanorods, and nanoparticles. The two components are tightly bound and well dispersed. The interfacial contact between the two components is called a heterojunction. This morphology is conducive to interfacial charge transfer between the two components. The nanosheets belong to the C3N4, and the size is about 50–200 nm or so. The Bi2S3 is attached to the surface of the C3N4 nanosheets as a flocculent layer of varying size.
As shown in Figure 2, one component is a lamellar morphology. The other components are nanofibers, nanorods, and nanoparticles. Some nanofibers are very long and fine. Their length is about 1 μm or more. The length of nanorods is about 150–200 nm, and their width is about 30–40 nm. There are fewer nanoparticles. Local magnification is shown in the right part of Figure 2. These nanofibers and nanorods should be the flocculent layer of varying size in the TEM (shown in Figure 1).
The XRD results of the Bi2S3/C3N4 nanocomposite are shown in Figure 3A. The UV–Vis–NIR results of the Bi2S3/C3N4 nanocomposite are shown in Figure 3B.
As shown in Figure 3A, the resulting nanocomposite exhibits a high degree of crystallinity. The diffraction peaks at 15.68°, 17.54°, 22.44°, 23.79°, 24.98°, 27.52°, 28.70°, 31.91°, 35.64°, 35.80°, 42.73, 45.61, 46.62, 49.16, 51.70, and 52.71° contribute to the peaks of the (020), (120), (220), (101), (130), (021), (211), (221), (240), (420), (421), (440), (501), (251), (620), and (312) planes of Bi2S3 (17-0320), respectively. The diffraction peak at 27.3° belongs to the peak of (002) of C3N4 (PDF#50-1250). Therefore, the resulting nanocomposite contains Bi2S3 and C3N4 components.
As shown in Figure 3B, two absorption peaks at 780 and 960 nm are clearly observed. The absorption band edge of the Bi2S3/C3N4 nanocomposite is larger than that of 1100 nm. This is because the bandgap of Bi2S3 is about 1.3 eV. The absorption peak at about 960 nm should belong to Bi2S3.
With regard to fluorescence and photoelectric materials, there are some common issues, and solving these key problems would be a useful reference for interdisciplinary fields. Suppression of non-radiative energy consumption is expected to enhance the fluorescence performance or photoelectric properties. Non-radiative energy consumption is closely related to the defects in the material, which involves the scattering of electrons and phonons. Scattering can produce a photothermal effect, which has applications in photothermal catalysis and biomedical fields. The photothermal effect is also susceptible to the photothermoelectric effect. Strong Raman scattering can be used in chemical sensors and biosensors. Therefore, the production, separation, and recombination of excitation include many physical processes and interdisciplinary applications. Radiative recombination is mainly responsible for fluorescence. When a spin transition is involved, phosphorescence is emitted. Fluorescence occurs when a material is irradiated with incident light of a certain wavelength, absorbs light energy, and enters an excited state; the excited state is unstable and immediately de-excites and emits light of a wavelength longer than that of the incident light. Scattering can be divided into Rayleigh scattering and Raman scattering. Rayleigh scattering refers to the elastic collision of photon and matter. No energy exchange occurs; only the direction of photon motion changes. This scattered light is called Rayleigh scattered light, and its frequency is the same as the frequency of the incident light. Raman scattering refers to the photon and matter inelastic collision. The direction of photon movement changes at the same time, so photon and matter energy exchange occurs. The photon part of the energy is transferred to the material or from the material to obtain part of the energy. The incident frequency of light is slightly lower or slightly higher than this scattered light, which is called Raman scattering light. The photoelectric effect refers to the conversion of light of a specific wavelength into an electrical signal by materials, and it is divided into photovoltaic effects and photoconductive effects. The photoelectric effect is closely related to the separation of excitons. The separation of excitons produces free electrons and holes which are involved in solar cells, photodetectors, information, photocatalysis, etc. The generation of excitons is closely related to the energy band structure, interfaces, and defects of the material. The main study fields of excitation are shown in Scheme 1.
Based on the analysis in Scheme 1, this study focuses on the generation of free electrons and holes by light excitation in the broadband region. This requires the designed nanocomposites themselves to have a strong built-in electric field that promotes photogenerated electron/hole separation and transport. It also reflects the synergy and complementarity of functional nanocomposites. The photoconductive response to the light was investigated using Au gap electrodes and a PET film substrate, referring to the photodetector structure of the photoconductor type. The representative results are shown in Figure 4 and Figure 5.
As shown in Figure 4 and Figure 5, the Bi2S3/C3N4 nanocomposite in the aggregation state with Au gap electrodes and a PET film substrate shows good photocurrent responses to 405, 532, 650, 808, and 980 nm of 50 mW and 1064 nm of 40 mW light sources. In the visible region, the ratio of on/off at 650 nm is greater than that at 405 or 532 nm. The ratio of on/off at 405 nm is almost equal to that at 532 nm. In the NIR range, the ratio of on/off at 808 nm is greater than that at 980 nm, and the ratio of on/off at 980 nm is higher than that at 1064 nm. This shows that the Bi2S3/C3N4 nanocomposite in the aggregation state displays a fast photoelectric response in the NIR region. By further reducing the power of the excitation light, the representative results are shown in Figure 4C and Figure 5B,C.
As shown in Figure 4C and Figure 5C, it is found that the Bi2S3/C3N4 nanocomposite in the aggregation state with Au gap electrodes and a PET film substrate at 5 mW 650 and 980 nm light sources still showed fast photocurrent responses and still had a high signal-to-noise ratio.
The characteristics of the photocurrent response to some representative light sources with Au gap electrodes and a PET film substrate are listed in Table 1.
As shown in Table 1, it was found that the Bi2S3/C3N4 nanocomposite in the aggregation state responded significantly differently to the light sources of different wavelengths and powers, which may be related to the absorption of light and defects with different energy levels. The defects with different energy levels would affect the trapping and release of photogenerated electrons, which can affect the photocurrent signals at different wavelengths of light. Of course, good optical absorption is a prerequisite for obtaining photocurrent signals.
The detection of the photoelectric signal depends not only on the material itself but also on the electrode material and its interface. For this reason, Ag fibers were also investigated as electrodes integrated on A4 printing paper with conductive adhesive. The representative results are shown in Figure 6.
As shown in Figure 6, the Bi2S3/C3N4 nanocomposite in the aggregation state with Ag fiber electrodes and A4 printing paper as substrates still showed good photocurrent responses to the representative visible light and the NIR light. This indicates that the Bi2S3/C3N4 nanocomposite in the aggregation state can relatively easily detect photoelectric signals because the untreated paper has a large number of defects. This shows that the number of photogenerated electrons is much larger than the number of electrons trapped by the defect, thus manifesting a positive photoelectric effect.
Based on the above experimental results, in order to further reduce electrode cost, the Bi2S3/C3N4 nanocomposite in the aggregation state using the electrodes with 4B pencil drawings and A4 printing paper as substrates were studied at 50 mW 405, 532, 650, 780, 808, and 980 nm and 40 mW 1064 nm. The representative results are shown in Figure 7.
The characteristics of the photocurrent response to some representative light sources using the graphite electrodes with 4B pencil drawings and A4 printing paper as substrates are listed in Table 2.
As shown in Table 2, it was found that the Bi2S3/C3N4 nanocomposite in the aggregation state using the graphite electrodes with 4B pencil drawings and A4 printing paper as substrates still shows good photocurrent responses to the light sources with different wavelengths from the visible region to the NIR region.
Combined with Figure 7, the Bi2S3/C3N4 nanocomposite in the aggregation state using graphite electrodes with 4B pencil drawings and A4 printing paper as substrates shows good photocurrent responses to the 50 mW 405, 532, 650, 780, 808, and 980 nm and 40 mW 1064 nm light sources. Interestingly, the Bi2S3/C3N4 nanocomposite in the aggregation state using the graphite electrodes with 4B pencil drawings shows a negative photoconductive response to the 5 mW 980 nm light source. It has no response to the 5 mW 650 nm light source. This indicates that the Bi2S3/C3N4 nanocomposite in the aggregation state can relatively easily acquire photoelectric signals. The defect effects of the graphite electrodes with 4B pencil drawings are significantly higher than those of the interface of Au gap electrodes. Since the content of the graphite electrodes with 4B pencil drawings is about 79%, they must contain a large number of defects. These defects are detrimental to the extraction of photogenerated electrons. It is also shown that the photogenerated electrons are completely trapped by the defect under 5 mW 980 nm irradiation. Photogenerated holes accumulate to form a depletion layer in the opposite direction to the applied bias voltage, thus reducing the photocurrent. The nanocomposite shows a negative photoconductive response to the 5 mW 980 nm light source. This is the result of an imbalance between the concentration of material defects and the concentration of photogenerated electrons.
Since the Bi2S3/C3N4 nanocomposite in the aggregation state can relatively easily detect photoelectric signals, it is likely that the Bi2S3/C3N4 nanocomposite possesses a strong built-in electric field that inhibits the recombination of photogenerated carriers. Therefore, the effects of bias on photoelectric signals were examined by selecting representative wavelengths. The representative results are shown in Figure 8 (the Bi2S3/C3N4 nanocomposite sample has been stored at room temperature for about 4 years).
As shown in Figure 8, with 0 V bias applied, the Bi2S3/C3N4 nanocomposite in the aggregation state using graphite electrodes with 5B pencil drawings and A4 printing paper as substrates still shows good photocurrent responses to the 50 mW 780 and 980 nm light sources. When a 1 V bias is applied, the photocurrent signal is significantly enhanced. It is shown that the application of a bias voltage further enhances the photogenerated charge separation and transfer. Using light sources of other wavelengths, the aggregation state of the Bi2S3/C3N4 nanocomposite has also been studied with 0 V bias applied using graphite electrodes with 5B pencil drawings and A4 printing paper as substrates for its responses to the 50 mW 405, 650, 780, and 980 nm and 20 mW 1064 nm light sources. The representative results are shown in Figure 9 and Figure 10.
As shown in Figure 9 and Figure 10, at 0 V bias, the Bi2S3/C3N4 nanocomposite in the aggregation state using graphite electrodes with 5B pencil drawings (the graphite content is 82%) and A4 printing paper as substrates with the 50 mW 405, 650, and 100 mW 980 nm and 20 mW 1064 nm light sources still shows good photocurrent responses. These are very interesting results. On the one hand, the graphite electrodes with 5B pencil drawings have a low cost. On the other hand, the Bi2S3/C3N4 nanocomposite sample has been stored at room temperature for about 4 years. This illustrates that the Bi2S3/C3N4 nanocomposite has good stability in terms of its photophysical properties.
Combining the electrode materials and substrate, it is found that these results are very useful because the Au electrodes have a low contact barrier and high cost. Among the carbon materials, graphene and carbon nanotubes have been widely used. Compared with graphene and carbon nanotube electrodes, graphite electrodes with pencil drawings have a large number of defects, and the untreated printing paper also has a large number of defects. The Bi2S3/C3N4 nanocomposite in the aggregation state using graphite electrodes with 4B and 5B pencil drawings and A4 printing paper as substrates still shows good photoelectric signals in a wide spectral range. This indicates that the Bi2S3/C3N4 nanocomposite in the aggregation state should be highly photoactive. The number of photogenerated electrons is much larger than the number of electrons trapped by the defect. The material itself can suppress the recombination of photogenerated carriers.
The photoelectric signal using Au gap electrodes and a PET film as substrates in this study was tested 4 years ago. Therefore, the effect of bias on the photocurrent signals using Au gap electrodes and this nanocomposite sample was re-tested after 4 years of storage at room temperature. The representative results are shown in Figure 11 and Figure 12.
As shown in Figure 11 and Figure 12, the bias has little effect on the extraction of photoelectric signals, suggesting that the charge transfer between the interfaces of the components in this nanocomposite creates a strong built-in electric field, which is sufficient to facilitate the separation of photogenerated charges. The photoelectric signals are much better than those of the graphite electrodes with pencil drawings and A4 printing paper as substrates. In particular, the ratio of on/off to a 200 mW 808 nm light source at zero bias reaches two orders of magnitude. Good results were also obtained with other wavelengths of the representative light sources. However, poor photostability at 50 mW 532 nm was observed for several of the representative light sources studied, indicating an imbalance between the capture and release of photogenerated electrons.
The properties of the photocurrent response to some representative light sources using the Au gap electrodes and PET film as substrates at 0 V bias with the resulting nanocomposite sample after 4 years of storage at room temperature are listed in Table 3.
As shown in Table 3, the Bi2S3/C3N4 nanocomposite in the aggregation state with the Au gap electrodes and PET films as substrate at 0 V bias still shows good photocurrent signal responses to the visible and NIR regions even though the resulting nanocomposite sample was stored at room temperature for 4 years. It is shown that the charge transfer between the resulting nanocomposite interfaces generates a strong built-in electric field, which promotes exciton separation and inhibits photogenerated charge recombination. It is also shown that the resulting nanocomposite has good photophysical stability.
To account for photodoping and baseline variations, comparative experiments were performed with 1 V bias applied by selecting the same baseline and representative light source as used approximately 4 years ago. The results are shown in Figure 4B,C, Figure 5 and Figure 7. The photocurrent responses obtained showed clear selectivity and power dependence for different wavelengths of light sources.
There are many factors affecting baseline noise, such as grain boundaries, thermal effects, quantum noise, scattering noise, etc. In order to compare the results in the same figure with different wavelength light source excitations, the Bi2S3/C3N4 nanocomposite sample stored for 4 years was re-coated on the Au gap electrodes of the PET film. The comparative experiments were carried out with 0 V bias applied using some representative light source excitations under similar experimental conditions. The representative results are shown in Figure 13, Figure 14, Figure 15 and Figure 16 and Table 4. Since the thickness of the cast thick film is not easy to control, the grain boundaries have a significant effect on the charge transport. Grain boundaries as a class of defects have a significant effect on the physicochemical properties of materials. It is therefore not possible to make a direct quantitative comparison of these data with those from four years ago, so only qualitative or semi-quantitative characterization of the photoelectric signal was carried out in this study. These results indicate that the resulting nanocomposite still shows good photoelectric signals for light sources of different wavelengths and powers, despite the sample being stored at room temperature for 4 years. Some adsorbates, such as oxygen, may be present on the surface of the nanocomposite during the 4 years of storage at room temperature. However, the latest results of the study show that these adsorbates did not reduce the extraction of photogenerated carriers. This further supports the good photophysical stability of the resulting nanocomposite.
As shown in Figure 13, the photocurrent sensitivity at 50 mW 650 nm is much better than that at 50 mW 780 and 405 nm. The photocurrent response at 780 nm and 405 nm is comparable. The on/off ratio at 780 nm is higher than that at 808 nm and much higher than that at 980 and 1064 nm. The on/off ratio at 980 nm and 1064 nm is comparable. It shows good selectivity for different wavelengths of light sources.
As shown in Figure 14 and Figure 15, the resulting nanocomposite still shows good photoelectric signals to 5 mW 650, 780, 808, and 980 nm light sources. As the excitation power decreases, the switching ratio decreases significantly. It shows good selectivity and power dependence for different wavelengths of light sources. In general, the on/off ratios for the representative light sources from the visible to the near-infrared region are much better than those shown in Figure 7C,D, Figure 9 and Figure 10. This indicates that the defects of the device using the graphite electrodes with pencil drawings and A4 printing paper as substrates are significantly higher than those of the Au gap electrodes and PET film. These defects greatly reduce the extraction of photogenerated currents. The devices with gold electrodes and the PET film have significantly higher switching ratios than the graphite electrodes with pencil drawings and A4 printing paper.
As shown in Figure 16, the base current fluctuates somewhat but still shows good reproducibility. The base current of the second test is significantly higher than that of the first test. This is due to the fact that it is difficult to recover 100% of the current in the cycle check tests, although the switching ratio is approximately the same. There are many factors that affect baseline noise, such as grain boundaries, thermal effects, quantum noise, scattering noise, and so on. It is therefore extremely important to keep the conditions as similar as possible when comparing experiments using different light source excitations. The latest experimental results, although not directly quantitatively comparable to those from 4 years ago, still show that the resulting nanocomposite samples show good photoelectric signals after 4 years of room-temperature storage. These results further support the existence of built-in electric fields in the resulting nanocomposites. The charge transfer between the nanocomposite interfaces creates a charge depletion layer, resulting in the formation of a built-in electric field. When the resulting nanocomposites are exposed to the light, electrons move from the ground state to the excited state. Excitons are formed by the interaction of Coulomb forces. The exciton separation to form free electrons and holes must be facilitated by the presence of a built-in electric field or applied bias voltage. The Bi2S3/C3N4 nanocomposite produces a broad spectrum of photoelectric signals at zero bias, indirectly indicating the presence of the built-in electric field. It also reflects the interfacial charge transfer of the resulting nanocomposite.
As shown in Table 4, the aggregation states of the Bi2S3/C3N4 nanocomposite with the Au gap electrodes and PET film as substrates after 4 years of storage at room temperature in response to 5 mW 808, 780, and 980 nm light sources still show a higher on/off ratio compared to at 650 nm. It shows a high signal to noise ratio. It is also indicated that the resulting nanocomposite is more sensitive to the NIR region (780, 808, 980, and 1064 nm light sources) (it is shown in Figure 13). On the other hand, the photoelectric properties of the resulting nanocomposites were characterized several times during the 4-year period and still showed good photoelectric signals, indicating a good photophysical stability of the Bi2S3/C3N4 nanocomposite.
In summary, good absorption is an important prerequisite for obtaining a photoelectric signal. In this study, it is found that light absorption is not exactly positively correlated with the photoelectric signal. The linear dependence is not good. This is closely related to the defect and the defect energy level of the material. The qualitative and semi-quantitative characterization of the photoelectric properties has been carried out. The extraction of a good optoelectronic signal is based on a combination of materials, interfaces, grain boundaries, and device structure.
The interfacial interaction between Bi2S3 and C3N4 nanosheets is shown in Scheme 2. The bandgap of C3N4 is about 2.7 eV. It is possible to utilize light with wavelengths less than 459 nm. In this study, thiourea was used as the starting material for the synthesis of C3N4. This resulted in the formation of S-doped C3N4. Sulfur doping results in an absorption red shift of C3N4 due to the lone electron pair of the S element. Referring to the relevant literature, the bandgap of S-doped C3N4 is about 2.28 eV. The CB is located at −0.80 eV, and the VB is at 1.48 eV [92]. The bandgap of Bi2S3 is about 1.3 eV. The CB is located at −0.80 eV, and the VB is at 0.51 eV [93]. Therefore, the absorption of the near-infrared light is mainly by Bi2S3. From the energy band position, the resulting nanocomposite should belong to a type I heterojunction. The photogenerated carrier was easily recombined for a type I heterojunction. However, the conduction bands of the two materials are close to each other. Defects or impurities are inevitably created or introduced during the preparation process of nanocomposites. This increases the chances of photoexcitation and trapping. The interfacial charge transfer between Bi2S3 and C3N4 nanosheets would create a strong built-in field that accelerates the separation of the excitation. This ultimately leads to the efficient extraction of free electrons/holes in the broad spectral range at 0 V bias. This may be due to the introduction of intermediate states in the impurity energy levels or defect energy levels, leading to the formation of type II heterojunctions which are susceptible to charge separation. On the other hand, photo-doping effects have the potential to improve the effective separation of photogenerated carriers. It can be seen that the defects, impurities, and interfaces of the nanocomposite have a significant effect on its photoelectric properties, fluorescence, and photothermal effects. Modulation of these key issues can improve certain physical properties of nanocomposites and satisfy multidisciplinary applications. This is an important part of research on microstructure modulation and property enhancement of materials.
To explore the mechanism of the Bi2S3/C3N4 nanocomposite, Raman and PL (fluorescence) analyses were carried out. The results are shown in Figure 17 and Figure 18.
Since the structure of C3N4 is similar to the structure of graphene, it should have a G band similar to that of graphene. As shown in Figure 17, the Raman bands at 1605.8 cm−1 (C=N stretching vibration), 1510.8 cm−1 (asymmetric C-N stretching vibration mode), and 1225 cm−1 (C sp2 bending vibration) are very weak. Perhaps there is fluorescence interference or low content in the resulting nanocomposite. In contrast, the Bi2S3 has distinct Raman bands at 145 cm−1, 428.2 cm−1, 965.5 cm−1, etc. The Raman band at 965.5 cm−1 is broader, suggesting broadening due to more defects. The results are consistent with those of Bi2S3 nanoflowers and nanorods reported in [94].
As shown in Figure 18, the PL peaks are also weak. Perhaps the fluorescence is reduced or even quenched by the charge transfer. Some weak PL peaks at 434.98, 492.76, 506.73, 657.12, and 684.26 nm can be identified with light excitation at a wavelength of 325 nm. According to [95,96], the PL peaks at 434.98, 492.76 should come from C3N4. The PL peaks at 506.73, 657.12, and 684.26 nm can be attributed to the defective luminescence of Bi2S3. Some weak PL peaks at 642.48 and 743.67 nm are observed with light excitation at a wavelength of 532 nm. These peaks also originate from the defect luminescence of Bi2S3. Overall, the resulting nanocomposites have weak fluorescence, which also reflects the strong charge transfer of the resulting nanocomposite. The weak fluorescence property is further supported by the extraction of photoelectric signals, which was shown to be good in this study. These preliminary results help us to explore the microstructure and photophysical properties of the nanocomposite.

3. Materials and Methods

3.1. Materials

Bismuth(III) nitrate pentahydrate (AR) was purchased from Macklin (Shanghai, China). Sodium thiosulfate (AR) was purchased from Tianjin Guangcheng Chemical Reagent Co., Ltd. (Tianjin, China). Thiourea (CP, chemical pure) was purchased from Tianjin Taixing Reagent Factory (Tianjin, China).

3.2. Preparation of Graphitic Carbon Nitride

Graphitic carbon nitrides (C3N4) were synthesized following a similar method to that in [91], using thiourea as the starting material.

3.3. Preparation of the Bi2S3/C3N4 Nanocomposite

To 20 mL of C3N4 suspensions, 40 mL of H2O was added. Then, 0.5 g of bismuth(III) nitrate pentahydrate and an appropriate amount of sodium thiosulfate were added, and the mixtures were ultrasonicated for about 10 min. The hydrothermal synthesis conditions were 120–130 °C for 8–24 h. The Bi2S3/C3N4 nanocomposite was obtained.

3.4. Characterization of SEM, TEM, UV–Vis–NIR, XRD, Raman, and PL

Characterization using scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–Vis–NIR (UV–Vis–NIR spectrophotometer), X-ray powder diffraction (XRD), and Horiba HR Evolution was shown in our previous report [91]. The instruments used were ZEISS Gemini SEM300 (Jena, Germany), JEM-1011 (Japan Electronics Co., Ltd., Tokyo, Japan), the TU-1810 spectrophotometer (Beijing Puxi General Instrument Co., Ltd., Beijing, China), XRD-7000 from SHIMADZU (Shimadzu, Kyoto, Japan), and Horiba HR ‌Evolution (Horiba, Kyoto, Japan). The Raman and PL analyses were performed on the Bi2S3/C3N4 nanocomposite powder samples and measured with Horiba HR ‌Evolution. The excitation wavelength of the Raman spectrum was 532 nm, and the power was 5 mW. The excitation wavelength of PL was 325 and 532 nm, and the power was 10 and 5 mW, respectively.

3.5. Photocurrent Measurements of the Bi2S3/C3N4 Nanocomposite in the Aggregation State with Light Sources of Different Wavelengths

The Bi2S3/C3N4 nanocomposite suspension was cast onto Au electrodes on a polyethylene terephthalate (PET) film. Referring to the photodetector structure of the photoconductor type, the structure of the electrodes is shown in Scheme 3. The photoconductive response to light of different wavelengths was determined. The details are given in [91]. We applied 0 V and 1V DC bias. Another electrode was used with 4B and 5B pencil drawings and A4 printing paper as substrates in this study. The bias applied was 0 and 1 V. Ag fiber electrodes were integrated on A4 printing paper with conductive adhesive.

4. Conclusions

A Bi2S3/C3N4 nanocomposite was synthesized. The resulting nanocomposite in the aggregation state showed broadband spectral photocurrent switching behavior from the visible to the NIR region using graphite electrodes with 4B and 5B pencil drawings and A4 printing paper as substrates. A reversible photocurrent response phenomenon was observed with a decrease in the excitation light source power. This is the result of an imbalance between the concentration of material defects and the concentration of photogenerated electrons. Even with 0 V bias applied, the Bi2S3/C3N4 nanocomposite in the aggregation state still exhibited good photocurrent signals. This illustrates that the Bi2S3/C3N4 nanocomposite has a strong built-in field which suppresses the recombination of photogenerated carriers, so the free carriers could be extracted very easily. Otherwise, the resulting nanocomposite has good photophysical stability. The separation of excitation by light triggering could be applied to various applications in multidisciplinary fields. This study also provides a useful reference for the design, defect and grain boundary modulation, and interdisciplinary application of nanocomposites, especially for the bandgap modulation and suppression of photogenerated carrier recombination.

Author Contributions

Conceptualization, methodology, investigation, writing—original draft preparation, writing—review and editing, funding acquisition, resources, and investigation, X.M.; investigation, X.Z., M.G., R.H. 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.

Funding

This project was supported by the Natural Science Foundation of Shandong Province (project no. ZR2013EMM008).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available upon request from the corresponding author. The data are not publicly available due to privacy issues.

Acknowledgments

Thanks go to You Wang and Guang Li of Zhejiang University for the fabrication of several electrodes and for checking the English in this paper. SEM, Raman, and PL analyses were carried out by Jie Su, and TEM analysis was performed by Chunsheng Wang at the Structural Composition Testing Center, School of Chemistry and Chemical Engineering, Shandong University. Some students, such as Chenchen Hu, took part in the experiments.

Conflicts of Interest

We declare that we have no conflicts of interest.

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Figure 1. The representative TEM image of the Bi2S3/C3N4 nanocomposite (the scale bar is 200 nm).
Figure 1. The representative TEM image of the Bi2S3/C3N4 nanocomposite (the scale bar is 200 nm).
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Figure 2. The representative SEM image of the Bi2S3/C3N4 nanocomposite (the scale bar is 200 nm).
Figure 2. The representative SEM image of the Bi2S3/C3N4 nanocomposite (the scale bar is 200 nm).
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Figure 3. The XRD results and UV–Vis–NIR results of the Bi2S3/C3N4 nanocomposite ((A) XRD; (B) UV-Vis).
Figure 3. The XRD results and UV–Vis–NIR results of the Bi2S3/C3N4 nanocomposite ((A) XRD; (B) UV-Vis).
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Scheme 1. The generation, separation, and recombination of excitation involved in physical processes and their main applications.
Scheme 1. The generation, separation, and recombination of excitation involved in physical processes and their main applications.
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Figure 4. The comparative photocurrent responses of the Bi2S3/C3N4 nanocomposite in the aggregation state with Au gap electrodes and a PET film substrate to some representative light sources ((A) the visible light (25 W); (B) 50 mW 405, 532, and 650 nm light sources; (C) 100, 50, and 5 mW 650 nm) (1 V bias applied).
Figure 4. The comparative photocurrent responses of the Bi2S3/C3N4 nanocomposite in the aggregation state with Au gap electrodes and a PET film substrate to some representative light sources ((A) the visible light (25 W); (B) 50 mW 405, 532, and 650 nm light sources; (C) 100, 50, and 5 mW 650 nm) (1 V bias applied).
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Figure 5. The comparative photocurrent responses of the Bi2S3/C3N4 nanocomposite in the aggregation state with Au gap electrodes and a PET film substrate to some representative light sources ((A) 50 mW 808, 50 mW 980 nm, and 40 mW 1064 nm; (B) 100, 50, and 10 mW 808 nm; (C) 100, 50, and 5 mW 980 nm) (1 V bias applied).
Figure 5. The comparative photocurrent responses of the Bi2S3/C3N4 nanocomposite in the aggregation state with Au gap electrodes and a PET film substrate to some representative light sources ((A) 50 mW 808, 50 mW 980 nm, and 40 mW 1064 nm; (B) 100, 50, and 10 mW 808 nm; (C) 100, 50, and 5 mW 980 nm) (1 V bias applied).
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Figure 6. The comparative photocurrent responses of the Bi2S3/C3N4 nanocomposite in the aggregation state with Ag fiber electrodes and A4 printing paper as substrates to some representative light sources ((A) 100 mW 650 nm; (B) 200 mW 808 nm; (C) 100 mW 980 nm; (D) 25 W visible light) (1 V bias applied).
Figure 6. The comparative photocurrent responses of the Bi2S3/C3N4 nanocomposite in the aggregation state with Ag fiber electrodes and A4 printing paper as substrates to some representative light sources ((A) 100 mW 650 nm; (B) 200 mW 808 nm; (C) 100 mW 980 nm; (D) 25 W visible light) (1 V bias applied).
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Figure 7. The comparative photocurrent responses of the Bi2S3/C3N4 nanocomposite in the aggregation state using graphite electrodes with 4B pencil drawings and A4 printing paper as substrates to some representative light sources ((A) 50 mW 405, 532, 650, and 808 nm; (B) 50 mW 780, 808, 980, and 40 mW 1064 nm; (C) 50 and 10 mW 808 nm; (D) 100, 50, and 5 mW 980 nm) (1 V bias applied).
Figure 7. The comparative photocurrent responses of the Bi2S3/C3N4 nanocomposite in the aggregation state using graphite electrodes with 4B pencil drawings and A4 printing paper as substrates to some representative light sources ((A) 50 mW 405, 532, 650, and 808 nm; (B) 50 mW 780, 808, 980, and 40 mW 1064 nm; (C) 50 and 10 mW 808 nm; (D) 100, 50, and 5 mW 980 nm) (1 V bias applied).
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Figure 8. The comparative photocurrent responses of the Bi2S3/C3N4 nanocomposite in the aggregation state using graphite electrodes with 5B pencil drawings and A4 printing paper as substrates to some representative light sources ((A) 50 mW 780; (B) 100 mW 980 nm) (0, 1 V bias applied) (the Bi2S3/C3N4 nanocomposite sample has been stored at room temperature for about 4 years).
Figure 8. The comparative photocurrent responses of the Bi2S3/C3N4 nanocomposite in the aggregation state using graphite electrodes with 5B pencil drawings and A4 printing paper as substrates to some representative light sources ((A) 50 mW 780; (B) 100 mW 980 nm) (0, 1 V bias applied) (the Bi2S3/C3N4 nanocomposite sample has been stored at room temperature for about 4 years).
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Figure 9. The comparative photocurrent responses of the Bi2S3/C3N4 nanocomposite in the aggregation state using graphite electrodes with 5B pencil drawings and A4 printing paper as substrates to some representative light sources (0 V bias applied) ((A) 50 mW 405 nm; (B) 50 mW 650 nm; (C) 50 mW 780 nm) (the Bi2S3/C3N4 nanocomposite sample has been stored at room temperature for about 4 years).
Figure 9. The comparative photocurrent responses of the Bi2S3/C3N4 nanocomposite in the aggregation state using graphite electrodes with 5B pencil drawings and A4 printing paper as substrates to some representative light sources (0 V bias applied) ((A) 50 mW 405 nm; (B) 50 mW 650 nm; (C) 50 mW 780 nm) (the Bi2S3/C3N4 nanocomposite sample has been stored at room temperature for about 4 years).
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Figure 10. The comparative photocurrent responses of the Bi2S3/C3N4 nanocomposite in the aggregation state using the graphite electrodes with 5B pencil drawings and A4 printing paper as substrates to some representative light sources (0 V bias applied) ((A) 100 mW 980 nm; (B) 20 mW 1064 nm) (the Bi2S3/C3N4 nanocomposite sample has been stored at room temperature for about 4 years).
Figure 10. The comparative photocurrent responses of the Bi2S3/C3N4 nanocomposite in the aggregation state using the graphite electrodes with 5B pencil drawings and A4 printing paper as substrates to some representative light sources (0 V bias applied) ((A) 100 mW 980 nm; (B) 20 mW 1064 nm) (the Bi2S3/C3N4 nanocomposite sample has been stored at room temperature for about 4 years).
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Figure 11. The comparative photocurrent responses of the Bi2S3/C3N4 nanocomposite in the aggregation state with Au gap electrodes and a PET film as substrates to some representative light sources ((A) 50 mW 780; (B) 200 mW 808 nm; (C) 100 mW 980 nm; (D) 20 mW 1064 nm) (the red line: 0 V bias applied; the black line: 1 V bias applied) (the Bi2S3/C3N4 nanocomposite sample has been stored at room temperature for about 4 years).
Figure 11. The comparative photocurrent responses of the Bi2S3/C3N4 nanocomposite in the aggregation state with Au gap electrodes and a PET film as substrates to some representative light sources ((A) 50 mW 780; (B) 200 mW 808 nm; (C) 100 mW 980 nm; (D) 20 mW 1064 nm) (the red line: 0 V bias applied; the black line: 1 V bias applied) (the Bi2S3/C3N4 nanocomposite sample has been stored at room temperature for about 4 years).
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Figure 12. The comparative photocurrent responses of the Bi2S3/C3N4 nanocomposite in the aggregation state with Au gap electrodes and a PET film as substrates to some representative light sources ((A) 50 mW 405; (B) 50 mW 532 nm; (C) 50 mW 650 nm) (0 V bias applied) (the Bi2S3/C3N4 nanocomposite sample has been stored at room temperature for about 4 years).
Figure 12. The comparative photocurrent responses of the Bi2S3/C3N4 nanocomposite in the aggregation state with Au gap electrodes and a PET film as substrates to some representative light sources ((A) 50 mW 405; (B) 50 mW 532 nm; (C) 50 mW 650 nm) (0 V bias applied) (the Bi2S3/C3N4 nanocomposite sample has been stored at room temperature for about 4 years).
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Figure 13. The comparative photocurrent responses of the Bi2S3/C3N4 nanocomposite in the aggregation state with Au gap electrodes and a PET film as substrates to some representative light sources ((A) 50 mW 405, 650, and 780 nm; (B) 50 mW 780, 808, and 980 nm and 20 mW 1064 nm) (0 V bias applied) (the Bi2S3/C3N4 nanocomposite sample has been stored at room temperature for about 4 years).
Figure 13. The comparative photocurrent responses of the Bi2S3/C3N4 nanocomposite in the aggregation state with Au gap electrodes and a PET film as substrates to some representative light sources ((A) 50 mW 405, 650, and 780 nm; (B) 50 mW 780, 808, and 980 nm and 20 mW 1064 nm) (0 V bias applied) (the Bi2S3/C3N4 nanocomposite sample has been stored at room temperature for about 4 years).
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Figure 14. The comparative photocurrent responses of the Bi2S3/C3N4 nanocomposite in the aggregation state with Au gap electrodes and a PET film as substrates to some representative light sources with different powers ((A) 5, 50 mW 650 nm; (B) 5, 50 mW 780 nm) (0 V bias applied) (the Bi2S3/C3N4 nanocomposite sample has been stored at room temperature for about 4 years).
Figure 14. The comparative photocurrent responses of the Bi2S3/C3N4 nanocomposite in the aggregation state with Au gap electrodes and a PET film as substrates to some representative light sources with different powers ((A) 5, 50 mW 650 nm; (B) 5, 50 mW 780 nm) (0 V bias applied) (the Bi2S3/C3N4 nanocomposite sample has been stored at room temperature for about 4 years).
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Figure 15. The comparative photocurrent responses of the Bi2S3/C3N4 nanocomposite in the aggregation state with Au gap electrodes and a PET film as substrates to some representative light sources with different powers ((A) 5, 50 mW 808 nm; (B) 5, 50 mW 980 nm) (0 V bias applied) (the Bi2S3/C3N4 nanocomposite sample has been stored at room temperature for about 4 years).
Figure 15. The comparative photocurrent responses of the Bi2S3/C3N4 nanocomposite in the aggregation state with Au gap electrodes and a PET film as substrates to some representative light sources with different powers ((A) 5, 50 mW 808 nm; (B) 5, 50 mW 980 nm) (0 V bias applied) (the Bi2S3/C3N4 nanocomposite sample has been stored at room temperature for about 4 years).
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Figure 16. The reproducibility of photocurrent responses of the Bi2S3/C3N4 nanocomposite in the aggregation state with Au gap electrodes and a PET film as substrates in response to some representative light sources (50 mW 808 nm) (0 V bias applied) (the Bi2S3/C3N4 nanocomposite sample has been stored at room temperature for about 4 years).
Figure 16. The reproducibility of photocurrent responses of the Bi2S3/C3N4 nanocomposite in the aggregation state with Au gap electrodes and a PET film as substrates in response to some representative light sources (50 mW 808 nm) (0 V bias applied) (the Bi2S3/C3N4 nanocomposite sample has been stored at room temperature for about 4 years).
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Scheme 2. The interfacial interaction between Bi2S3 and S-doped C3N4 nanosheets.
Scheme 2. The interfacial interaction between Bi2S3 and S-doped C3N4 nanosheets.
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Figure 17. The Raman spectrum of the Bi2S3/C3N4 nanocomposite.
Figure 17. The Raman spectrum of the Bi2S3/C3N4 nanocomposite.
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Figure 18. The PL spectra of the Bi2S3/C3N4 nanocomposite ((A) 325 nm wavelength excitation; (B) 532 nm wavelength excitation).
Figure 18. The PL spectra of the Bi2S3/C3N4 nanocomposite ((A) 325 nm wavelength excitation; (B) 532 nm wavelength excitation).
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Scheme 3. The structure of the electrodes in this study.
Scheme 3. The structure of the electrodes in this study.
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Table 1. The response time, recovery time, and on/off ratio of the Bi2S3/C3N4 nanocomposite in the aggregation state with Au gap electrodes and a PET film substrate with some representative light sources (1 V bias applied).
Table 1. The response time, recovery time, and on/off ratio of the Bi2S3/C3N4 nanocomposite in the aggregation state with Au gap electrodes and a PET film substrate with some representative light sources (1 V bias applied).
Excitation Light Wavelength (nm)Response Time (s)Recovery Time (s) Ratio of On/Off
50 mW, 405 nm12.012.92.367
50 mW, 532 nm3.48.62.154
50 mW, 650 nm7.912.02.618
5 mW, 650 nm123.41.982
50 mW, 808 nm12.07.72.570
10 mW, 808 nm8.64.31.759
50 mW, 980 nm8.54.32.049
5 mW, 980 nm3.48.51.482
40 mW, 1064 nm4.34.31.785
Table 2. The response time, recovery time, and on/off ratio of the Bi2S3/C3N4 nanocomposite in the aggregation state using graphite electrodes with 4B pencil drawings and A4 printing paper as substrates with some representative light sources (1 V bias applied).
Table 2. The response time, recovery time, and on/off ratio of the Bi2S3/C3N4 nanocomposite in the aggregation state using graphite electrodes with 4B pencil drawings and A4 printing paper as substrates with some representative light sources (1 V bias applied).
Excitation Light Wavelength (nm)Response Time (s)Recovery Time (s) Ratio of On/Off
50 mW, 405 nm16.37.71.681
50 mW, 532 nm16.34.39.333
50 mW, 650 nm20.612.02.533
50 mW, 780 nm8.612.01.176
50 mW, 808 nm16.37.74.942
10 mW, 808 nm5.225.11.173
100 mW, 980 nm16.312.01.739
50 mW, 980 nm4.38.61.247
40 mW, 1064 nm29.917.41.063
Table 3. The response time, recovery time, and on/off ratio of the Bi2S3/C3N4 nanocomposite in the aggregation state with Au gap electrodes and a PET film as substrates after 4 years storage of at room temperature with some representative light sources (0 V bias applied).
Table 3. The response time, recovery time, and on/off ratio of the Bi2S3/C3N4 nanocomposite in the aggregation state with Au gap electrodes and a PET film as substrates after 4 years storage of at room temperature with some representative light sources (0 V bias applied).
Excitation Light Wavelength (nm)Response Time (s)Recovery Time (s) Ratio of On/Off
50 mW, 405 nm13.16.68.60
50 mW, 532 nm67.16.55.15
50 mW, 650 nm6.63.413.56
50 mW, 780 nm1.35.09.41
200 mW, 808 nm19.34.8181.31
100 mW, 980 nm3.46.64.53
20 mW, 1064 nm10.016.63.09
Table 4. The response time, recovery time, and on/off ratio of the Bi2S3/C3N4 nanocomposite in the aggregation state with Au gap electrodes and a PET film as substrates after 4 years storage of at room temperature with low-power representative light source excitation (0 V bias applied).
Table 4. The response time, recovery time, and on/off ratio of the Bi2S3/C3N4 nanocomposite in the aggregation state with Au gap electrodes and a PET film as substrates after 4 years storage of at room temperature with low-power representative light source excitation (0 V bias applied).
Excitation Light Wavelength (nm)Response Time (s)Recovery Time (s) Ratio of On/Off
5 mW, 650 nm7.98.41.55
5 mW, 780 nm15.911.73.51
5 mW, 808 nm4.28.05.69
5 mW, 980 nm4.27.92.90
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Ma, X.; Zhang, X.; Gao, M.; Hu, R.; Wang, Y.; Li, G. The Interface Interaction of C3N4/Bi2S3 Promoted the Separation of Excitons and the Extraction of Free Photogenerated Carriers in the Broadband Light Spectrum Range. Inorganics 2025, 13, 122. https://doi.org/10.3390/inorganics13040122

AMA Style

Ma X, Zhang X, Gao M, Hu R, Wang Y, Li G. The Interface Interaction of C3N4/Bi2S3 Promoted the Separation of Excitons and the Extraction of Free Photogenerated Carriers in the Broadband Light Spectrum Range. Inorganics. 2025; 13(4):122. https://doi.org/10.3390/inorganics13040122

Chicago/Turabian Style

Ma, Xingfa, Xintao Zhang, Mingjun Gao, Ruifen Hu, You Wang, and Guang Li. 2025. "The Interface Interaction of C3N4/Bi2S3 Promoted the Separation of Excitons and the Extraction of Free Photogenerated Carriers in the Broadband Light Spectrum Range" Inorganics 13, no. 4: 122. https://doi.org/10.3390/inorganics13040122

APA Style

Ma, X., Zhang, X., Gao, M., Hu, R., Wang, Y., & Li, G. (2025). The Interface Interaction of C3N4/Bi2S3 Promoted the Separation of Excitons and the Extraction of Free Photogenerated Carriers in the Broadband Light Spectrum Range. Inorganics, 13(4), 122. https://doi.org/10.3390/inorganics13040122

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