Visible-light photocatalytic capability and the mechanism investigation of a novel PANI/Sn₃O₄ p–n heterostructure

Abstract

A novel polyaniline (PANI)/Sn₃O₄ heterojunction composed of PANI nanofibers and Sn₃O₄ nanosheets was fabricated by a facile physical milling technique. Modification of Sn₃O₄ with a PANI conductive polymer contributes to facilitating interfacial charge transfer efficiency, and thus, significantly enhances the visible-light Rhodamine B (RhB) photo-degradation. Results indicate that PANI/Sn₃O₄ heterostructures with 10 wt% PANI reached the maximum degradation efficiency (around 97%) for RhB within 5 h, which is 2.27 times higher than that of Sn₃O₄ alone. This improvement is due to the p–n heterostructure formation in PANI/Sn₃O₄. Moreover, the outcome of reactive species capturing experiments demonstrated that in PANI/Sn₃O₄, holes made the largest contribution to RhB degradation under visible light illumination, while hydroxyl radicals showed less significance under the same conditions. In addition, the photocatalytic mechanism was proposed based on evidence from the reactive species test and energy band structure analysis.

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1. Introduction

Rapid industrial development has generated a variety of water pollution problems that are posing serious challenges to environmental remediation. Recently, the removal of organic contaminants and decolorization of dyes in waste water have become highly significant to environmental protection,¹ as the dyes exhibit stable chemical properties, toxicity, high photo-stability and low biodegradability.² More seriously, most of these dyes will be lost during processing and discharged as waste water, which means potential harm to human beings, animals and plants. Many dyes are toxic, carcinogenic and mutagenic, such as Rhodamine B. Photocatalysis is a promising technology for water purification not only because of its low-cost, but also due to the eco-friendliness – no secondary pollution is created during the reaction process.³ Thus, it has become a research hot spot with respect to organic contaminant degradation. Mixed-valence or non-stoichiometric tin oxide (Sn₃O₄),⁴ as an n-type semiconductor, can effectively catalyze the degradation of target contaminant. Theoretical and experimental studies show that the Sn₃O₄ nanosheets have superior visible light response and photoelectric characteristics owing to the existence of oxygen vacancies.⁵

The Sn₃O₄ nanosheets' high photocatalytic activity is due to its nanocrystalline properties, mixed-valence and layered nanosheet morphology.⁶ However, the visible light absorption region of pure Sn₃O₄ nanosheets is limited, and the electron–hole pairs can be inclined to quickly recombine. As such, single component Sn₃O₄ undergoes significant charge recombination at the surface throughout the majority of the photocatalyst, which hinders its stimulation of photocatalytic properties. Thus, researchers have developed and tested several methods for depressing photogenerated carrier recombination on the Sn₃O₄ surface. Previous studies have demonstrated that forming a heterojunction between Sn₃O₄ and other materials tremendously boosts photocatalytic performances. For instance, Huda et al. researched that Sn₃O₄/TiO₂/Ti depicted superior decomposition of acid yellow 17 under visible light irradiation;⁷ while Chunmei Li et al. investigated the mes-Sn₃O₄/g-C₃N₄ heterostructure and determined that it dramatically enhanced TC-HCl photocatalytic degradation in water.⁸

Polyaniline (PANI), a p-type, visible-light-responsed, active semiconductor material, has attracted comprehensive attention from the research community on account of its good conductivity, non-toxicity, easy preparation, high stability, unique electrochemical property and physical behaviour.⁹ In addition, PANI acts as a photo-sensitizer, predominantly enhancing the charge and carrier separation and migration.¹⁰ Ma et al. reported that PANI/SnO₂ heterostructure with enhanced photocatalytic activity toward methyl orange.¹¹ Shirmardi et al. determined that ZnSe/PANI nanocomposite displayed photocatalytic property for the removal of methylene blue and chromate ions.¹² However, the coupling of p-type PANI with n-type Sn₃O₄ for photocatalytic application has rarely been reported until now. Given the good conductivity of PANI nanofibers, we proposed the construction of a novel PANI/Sn₃O₄ p–n heterostructure to accelerate carrier separation rate for improved photocatalytic performance. Integration of PANI and Sn₃O₄ changes the structural, optical and electronic performance of this composite material,¹³ and the PANI/Sn₃O₄ obtained broader light responsive spectra and boosted the interface charge transfer effectively, which significantly enhanced the photocatalytic performance to a degree.

In this work, Sn₃O₄ nanosheets were synthesized using a general hydrothermal method, after which PANI nanofibers were prepared via a in situ polymerization process.¹⁴ Then, using PANI nanofibers as a modified composition for Sn₃O₄ nanosheets, the p–n PANI/Sn₃O₄ heterojunction was constructed via a mechanical milling method. Compared to single Sn₃O₄, the PANI/Sn₃O₄ nano-heterostructure exhibited a dramatic improvement in terms of photocatalytic property and stability. It is ascribed to the mutual interaction between Sn₃O₄ and PANI in RhB degradation under visible light. Thus, this PANI/Sn₃O₄ heterostructure shows promising potential for practical application in organic pollutant degradation. This work provides a feasible and novel approach for further research on the application of polymers in the field of photocatalysis.

2. Experimental

2.1. Materials

Tin(II) dichloride dihydrate (SnCl₂·2H₂O), citric acid (C₆H₈O₇·2H₂O), aniline monomer (C₆H₅NH₂) and ammonium persulfate [(NH₄)₂S₂O₈] (APS) were bought from Sigma Aldrich. Hydrochloric acid (HCl), ethanol and sodium hydroxide (NaOH) were received from China National Medicines Corporation, Ltd. All the chemicals in our experiments were employed without further purification.

2.2. Synthesis

2.2.1. Synthesis of Sn₃O₄ nanosheets. Sn₃O₄ was fabricated using a hydrothermal method.¹⁵ 20 mmol SnCl₂·2H₂O and 50 mmol C₆H₈O₇·2H₂O were together dissolved in 65 mL of DL water, then the solution was continuously stirred for 2 h. Under continuous stirring conditions, NaOH particles were added to the obtained suspension to regulate pH until neutral. Subsequently, the homogeneous solution was slowly transferred to the 100 mL Teflon-lined autoclave. Then, the autoclave was heated at 180 °C for 6 h. After hydrothermal reaction, the obtained precipitate was separated by centrifugation, and then washed several times using DL water and ethanol separately. At last, the product was dried at 35 °C for 6 h in vacuum.

2.2.2. Synthesis of PANI nanofibers. PANI nanofibers were prepared by a in situ chemical polymerization method.^14,16 In accordance with the standard procedure, 73 μL aniline was poured into 10 mL hydrochloric acid (1 M) and with magnetic stirring at 0–5 °C for 1 h. Then 10 mL of 1 M hydrochloric acid containing 0.18 g APS with 1 : 1 molar ratio was dripped to the above mixed solution and maintained under magnetical stirring conditions for 24 h at low temperature (0–5 °C). The product was centrifuged, washed with DL water and ethanol separately, and then dried at 40 °C for 10 h under vacuum.

2.2.3. PANI/Sn₃O₄ composite synthesis. The PANI/Sn₃O₄ composites were prepared via a solvent evaporation and physical milling process.^17,18 First, 10 mg PANI and 190 mg Sn₃O₄ were separately dispersed in beakers containing 30 mL DL water and sonicated for 30 minutes so as to accelerate fully dispersion. Then the suspensions containing PANI and Sn₃O₄ were mixed under stirring conditions and heated continuously in a water bath to accelerate solvent evaporation. When the solvent was completely evaporated, the composite was transferred to an oven for drying. The dried samples were ground in an agate mortar, thereby preparing the PANI/Sn₃O₄ powder with a 5% PANI mass ratio. The additional sample PANI/Sn₃O₄ hybrids with different ratios were identically prepared through varying blending ratios. The composites with different PANI weight ratios were denoted as PANI/Sn₃O₄-5%, PANI/Sn₃O₄-10%, PANI/Sn₃O₄-30% and PANI/Sn₃O₄-50%.

2.3. Characterizations

The following techniques were exploited to determine the as-prepared samples' physical and chemical properties. Fourier transform-infrared (FT-IR) spectrums were tested over the region 4000–400 cm⁻¹ from a Tensor 37 with a KBr pellet. The crystal forms were measured by X-ray diffraction (XRD) named Smartlab with Cu Kα radiation (45 kV, 200 mA, λ = 1.5406 Å). Microstructures and morphologies of materials were employed by field emission scanning electron microscope (FE-SEM, Hitachi SU8010, Japan) and high resolution transmission electron microscope (HR-TEM, TecnaiG2 F20, FEI, USA) at 1 kV and 200 kV, respectively. The nitrogen adsorption–desorption isotherm was determined at 77 K (Quantachrome Instruments, Boynton Beach, FL, USA). Ultraviolet-visible diffuse reflectance spectra (UV-vis DRS) were obtained from 200–800 nm via a spectrometer (Shimadzu UV2600). Photoluminescence (PL) spectra were conducted on the F-7000 with 248 nm as the excitation wavelength. X-ray photoelectron spectroscopy (XPS) spectra were measured by the Thermo ESCALAB 250XI photoelectron spectroscope system using Al Kα radiation. Electron spin resonance (ESR) tests were measured using a Bruker E500 ESR spectrometer (Billerica, MA) with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) in methanol. Total organic carbons (TOC) were obtained on a multi N/C 2100 (Analytik Jena AG, Germany) TOC analyzer. The photocurrent response (I–t) curve was tested at 0 V, electrochemical impedance spectroscopy (EIS) was measured at open-circuit voltage with 0.01 Hz to 1000 kHz using this three-electrode cell by a Multi-Channel Autolab M204. The photocatalyst thin film on fluorine-doped tin oxide (FTO) was usually regarded as the working electrode with irradiation area of 1 cm².¹⁹ The platinum sheet and Ag/AgCl electrode were regarded as the counter electrode and reference electrode, respectively. The electrolyte was Na₂SO₄ (0.5 M) aqueous solution.

2.4. Photocatalytic Rhodamine B degradation

The samples' photocatalytic property was investigated by monitoring the Rhodamine B aqueous solution (RhB, 10 mg L⁻¹) decomposition process. Typically, 50 mg sample was poured into a 50 mL RhB solution. Then the suspension was stirred without light irradiation for 1 h to ensure the adsorption desorption-equilibrium. Subsequently, suspension was shined by a 300 W xenon lamp equipped with a 420 nm cut-off filter under continuous stirring. At pre-specified time intervals, certain aliquots were collected and then centrifuged, after that the concentration of the residual RhB aqueous solution was detected with the UV-2550 by observing the peak intensity at 554 nm. To confirm the materials' cyclic stability, after reaction the residual catalyst was collected for circular photocatalytic tests. The RhB removal efficiency was calculated as η (%) = (1 − C_t/C₀) × 100, where η (%) was the degradation efficiency, and C₀ (mg L⁻¹) and C_t (mg L⁻¹) were the initial and reaction RhB concentrations,¹⁹ respectively.

2.5. Mechanism study

Typical active species in the degradation process, such as hydroxyl radical (·OH), holes (h⁺) and superoxide anion radicals (·O²⁻) could be conducted using scavengers.²⁰ In this test, tert-butyl alcohol (t-BuOH),²¹ ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA-2Na)⁸ and benzoquinone (BQ)¹⁹ were introduced into the reaction solution as ·OH, h⁺ and ·O²⁻ scavenger, respectively. The trapping experiment was similar to the process of RhB degradation, except for a certain amount of scavengers were poured into RhB aqueous solution before the addition of catalysts.

3. Results and discussion

3.1. FT-IR analysis

The FT-IR spectrum of pure PANI, Sn₃O₄ and PANI/Sn₃O₄ heterostructure are examined in Fig. 1a. PANI spectrum depicted vibration peaks at 1576, 1478, 1287, 1351, 1118 and 830 cm⁻¹.²² The peaks at 1576 and 1478 cm⁻¹ are associated with the C=C stretching vibration mode of the quinoid and benzenoid rings,²³ respectively. The peaks observed at 1287 and 1351 cm⁻¹ resulted from C–N stretching, whereas the peak located at 1118 cm⁻¹ corresponds to the C=N stretching of a secondary aromatic amine.²⁴ Finally, the peak at 830 cm⁻¹ is attributed to the degree of electron delocalization and C–H out of plane vibration,²⁵ which corresponds to the polaron-related transition in the PANI. For the most part, these characteristic peaks correspond to PANI peak,²⁶ as described in literature. The characteristic Sn₃O₄ peak was observed at 486.73 cm⁻¹ and represents Sn–O vibrations.²⁷ It was worth noting that the peak intensity of PANI/Sn₃O₄ composite was strengthened due to the introduction of PANI. The increased peak intensity might be due to the interaction between the PANI nanofibers and Sn₃O₄ nanocrystals.²⁸ In addition, these peaks of as-prepared samples were shifted slightly compared to their original locations in pure Sn₃O₄ samples, indicating the successful modification of mesoporous Sn₃O₄ nanosheets and PANI nanofibers.²⁹

Fig. 1 (a) FT-IR spectra and (b) XRD patterns of the pure PANI, Sn₃O₄ and PANI/Sn₃O₄ composite.

3.2. X-ray diffraction analysis

The XRD patterns of pure PANI, Sn₃O₄ and PANI/Sn₃O₄ composite are depicted in Fig. 1b. For Sn₃O₄ nanosheets, there were characteristic peaks at 2θ = 24.10°, 27.08°, 32.32°, 33.01°, 37.07°, 52.68°, 55.88°, 63.54°, 65.49° and 67.25° corresponding to the (101), (111), (1̄21), (210), (130), (311), (24̄1), (312), (03̄3) and (13̄3) Sn₃O₄ crystal facets.³⁰ Thus, the peak locations are consistent with standard Sn₃O₄ (JCPDS card, no. 16-0737).³¹ In addition, the average crystallite sizes were estimated from XRD peaks, and equalled 13.56 nm. The PANI nanofibers showed a peak at 15° which confirmed the polymer chain's two-dimensional arrangement.³² And, the characteristic peak detected at 25° could be due to the periodicity parallel and perpendicular to the PANI,^33,34 implying an amorphous structure of the synthesized polymer PANI.^35,36 In case of composites, no impurity peaks were detected.²⁹ And all the PANI/Sn₃O₄ composites had the same profile as single Sn₃O₄ owing to the fact that the crystal structure of Sn₃O₄ wasn't changed by PANI.³⁷

3.3. Morphology

The prepared samples' morphology and microstructure were shown by SEM. Fig. 2a depicts a low magnification SEM image that displays distinct, hierarchical flower-like structures with a scale size of ∼0.2–1.0 μm in diameter; while Fig. 2b, a high magnification image, shows Sn₃O₄ composed of numerous, thin nanoflakes with thickness ranging from 5 to 10 nm. Fig. 2c and d are FE-SEM images of PANI nanofiber with diameters ranging from 40 to 90 nm, and its lengths ranging from 0.5 to 2 μm. The PANI/Sn₃O₄-10% composites (Fig. 2e and f) exhibit PANI morphology with a short-chained fibrous structure and Sn₃O₄ nanoflakes scattered at the edge and tips of the PANI nanofibers.

Fig. 2 SEM images of the PANI (a and b), Sn₃O₄ (c and d) and PANI/Sn₃O₄-10% heterostructures (e and f).

The SAED pattern and high-resolution TEM of Sn₃O₄ were displayed in Fig. 3a and b. Besides, the HRTEM analysis of PANI/Sn₃O₄-10% heterostructure (Fig. 3c) was also demonstrated to know about detailed information on the morphology and structure. Presented in Fig. 3b and c, the lattice fringes with an interplanar spacing of 0.33 and 0.28 nm were attributed to the (111) and (1̄21) crystal orientation of triclinic Sn₃O₄,¹⁵ respectively. TEM of PANI, Sn₃O₄ and PANI/Sn₃O₄-10% hybrid were performed in Fig. 3d–g, which were consistent with SEM results. The 3D hierarchical flower-like Sn₃O₄ tended to randomly deposit on the PANI nanofibers surfaces,³⁸ and almost amorphous PANI nanofibers were located on the surfaces and edges of Sn₃O₄ nanosheets. Apparently, the results of HR-TEM and TEM were consistent with the XRD analysis results, which was indicative of the successful formation of PANI/Sn₃O₄ heterostructure. The EDS analysis of PANI/Sn₃O₄-10% composite was performed in Fig. 3h–k, which symbolized the existence of C, N, O and Sn elements in heterojunction, and Sn₃O₄ nanosheets were adhered well on the surface of PANI nanofibers.³⁹ With the aggregation of Sn₃O₄ nanosheets self-assembled, the amount of reflected light within the interconnected nanoflakes multiplies, which improves the hierarchical structure's light absorption capacity.⁴⁰ Significantly, PANI nanofibers as light-harvesting materials, it could drastically strengthen the light absorption for PANI/Sn₃O₄ heterostructure, which could dramatically facilitate the separation and transmission of photoinduced carriers,¹⁹ further increasing the photocatalytic performances.

Fig. 3 SAED pattern and HRTEM image of Sn₃O₄ (a and b), HRTEM image of PANI/Sn₃O₄-10% (c); TEM images of pristine Sn₃O₄ (d), PANI nanofibers (e), PANI/Sn₃O₄-10% hybrid (f and g); EDS mapping of PANI/Sn₃O₄-10% heterostructure (h–k).

3.4. XPS analysis

XPS spectra is used to analyze the valence states and superficial chemical ingredient of PANI/Sn₃O₄-10% heterostructure.⁴¹ Fig. 4a showed that the Sn 3d in PANI/Sn₃O₄ composite were decomposed into two bimodal peaks, respectively. And the peaks displayed at 495.02 and 486.56 eV were assigned to Sn(II), while the peaks of 487.03 and 495.51 eV were ascribed to Sn(IV).⁴² The O 1s spectra of PANI/Sn₃O₄-10% exhibited in Fig. 4b were fitted into two peaks at 530.57 and 531.67 eV, which could be assigned to the lattice oxygen bounded to tin atoms (Sn–O) and induced by the oxygen vacancies⁴³ within the composites, respectively. As for the N 1s spectrum (Fig. 4c), the main peak line could be dissolved into four peak line. The peaks corresponding to quinoid di-imine nitrogen and benzenoid diamine nitrogen⁴⁴ were located at 398.54 and 399.52 eV, respectively. The peaks displayed at 401.19 and 402.69 eV could be attributed to oxidized amine and protonated imine,⁴⁵ respectively. In Fig. 4d, the C 1s spectra of PANI/Sn₃O₄-10% composite was deconvoluted into five peaks located at 284.57, 285.15, 285.78, 286.67 and 288.95 eV. The first four peaks were associated with the C–C/C–H, C–N/C=N, C–N⁺/C=N⁺/C–Cl and C=O/C–O,⁴⁵ respectively. In addition, the peak at 288.95 eV could be due to the oxygen-containing functional groups.^46,47 This might be owing to the formation of new chemical bonds in PANI/Sn₃O₄-10% heterostructure, i.e., the PANI nanofibers were chemical bounded on the surface of Sn₃O₄ nanoflakes by C–O–Sn bonds in PANI/Sn₃O₄-10% p–n hybrids.¹⁵ Fig. 4e showed the survey spectra of PANI/Sn₃O₄-10% hybrid using bare PANI and Sn₃O₄ as comparisons. As we can see, the peaks of C 1s and N 1s were observed for PANI, while Sn₃O₄ revealed many peaks attributed to tin, oxygen and adventitious carbon from test.¹⁵ In the survey spectrum of PANI/Sn₃O₄-10%, it was similar to that of bare Sn₃O₄ except that the peak of N 1s and C 1s for PANI/Sn₃O₄-10% composite originates from the introduction of PANI nanofibers. The survey spectrum indicated that PANI/Sn₃O₄-10% hybrid was consist of Sn, O, C and N elements, confirming the successful fabrication of PANI/Sn₃O₄ p–n heterostructure. These results are well in agreement with the HR-TEM and EDS mapping analysis.⁴⁸

Fig. 4 The high-resolution XPS spectra of (a) Sn 3d, (b) O 1s, (c) N 1s, (d) C 1s of PANI/Sn₃O₄-10% composite and (e) survey XPS spectra of pristine PANI, Sn₃O₄ and PANI/Sn₃O₄-10% hybrid.

3.5. Burnauer–Emmett–Teller (BET) analysis

It has been well established that photocatalytic property is dependent on the BET surface area and porosity of nanoscale materials. The BET surface area, pore volume and average pore size of all materials are displayed in ESI Table S1.† PANI showed a stronger adsorption ability, which was in good agreement with the larger specific surface area (Fig. S1(a), ESI†). Moreover, the PANI's better adsorption capability might be related to the adsorption of imines in its molecular chain, which effectively improved the adsorption and degradation of organic contaminant for PANI/Sn₃O₄ composite. Fig. 5 showed that the PANI/Sn₃O₄-10% composite's N₂ adsorption/desorption behaviour parallels that of a typical IV isotherm, and depicted type H₃ shaped hysteresis loops.⁸ And the pore size distribution and N₂ adsorption–desorption curves with different weight ratio of PANI in composite materials were provided (Fig. S1(c–f), ESI†). The enlarged BET surface area and pore volume of the PANI/Sn₃O₄ composites, relative to bare Sn₃O₄, can exhibit better adsorption ability, leading to that heterostructure may supply more surface reaction sites.⁴⁹ For highly mesoporous materials, a hysteresis loop was observed owing to capillary condensation.⁵⁰ Such a self-organized mesoporous structure could be important in photocatalysis, as it provides efficient transmission path for reactant and product molecules.⁵¹ The mesoporous structures of PANI/Sn₃O₄ composite promote absorption of additional dye molecules, which is favourable to the dye degradation process. Therefore, compared to Sn₃O₄ nanosheets, the PANI/Sn₃O₄ composite is expected to exhibit predominant photocatalytic performance.

Fig. 5 N₂ adsorption–desorption isotherms of PANI/Sn₃O₄-10% nanocomposites. Inset is the hierarchical PANI/Sn₃O₄-10% pore-size distribution.

3.6. UV-vis analysis

The PANI, Sn₃O₄ nanosheets and the PANI/Sn₃O₄ heterostructure' light absorption properties are shown in Fig. 6a. The Sn₃O₄ sample showed photoabsorption capability from UV to visible region, with a light absorption edge of ∼506 nm. With the mass ratio increase of polyaniline in composites, the peak shifts toward the higher wavelength which means that the optical absorption range of PANI/Sn₃O₄ composites also increase gradually, i.e., the composite's light absorption ability has been strengthened than that of pure Sn₃O₄ in visible region. Moreover, the PANI/Sn₃O₄ heterostructure absorbed more visible light, probably owing to the light absorption of PANI in composite that could be assigned to the polaron transition,⁵² which was a typical characterization of conductive polymer PANI. The Tauc equation was employed (eqn (1)) to quantitatively estimate the optical band gap.

αhν = A(hν − E_g)² (1)

where α is the absorption coefficient, E_g is the optical band gap, hν is the photon energy, and A is a constant that does not depend on the photon energy.⁸ Until now, the Sn₃O₄ band gap was reported as ∼2.61 eV,⁵³ 2.75 eV,⁵⁴ 2.85 eV ⁵⁵ and 2.56 eV.⁵⁶ The PANI nanofibers' band gap has been reported as ∼2.80 eV⁵⁷ by many researchers. The Tauc plots in Fig. 6b indicated that the optical band gap value of Sn₃O₄ and PANI/Sn₃O₄-10% samples were 2.56 eV and 2.06 eV, respectively. Therefore, the composite was effectively doped with PANI performing the role of the photosensitizer. Such results indicate that the heterojunction better utilizes visible energy, and subsequently increased the photocatalytic efficiency.

Fig. 6 (a) UV-vis DRS of PANI, Sn₃O₄ and PANI/Sn₃O₄ heterostructures and (b) Tauc plots of Sn₃O₄ and PANI/Sn₃O₄-10% heterostructure.

3.7. Photocatalytic activity of hybrid PANI/Sn₃O₄ composite

The photocatalytic property of materials was assessed by decomposing RhB dye under visible light illumination. In Fig. 7a, the residual concentration of RhB solution decreased with the time of visible light illumination increased. When the weight ratio of PANI in PANI/Sn₃O₄ was <10%, photocatalytic activity in the PANI/Sn₃O₄ increased with the continuously increasing PANI weight ratio; but the degradation rate decreased when the weight ratio of PANI in PANI/Sn₃O₄ was >10%. The results showed that the loading amount of PANI relative to Sn₃O₄ had a significant impact on the photocatalytic performance.⁵⁸ In Fig. 7b, the PANI/Sn₃O₄-10% composite exhibited the best photocatalytic activity when the PANI mass ratio reached 10%. Using this composite, 96.84% of the RhB was degraded in 5 h, which is faster than the pure Sn₃O₄ (42.71%). In general, the improved photocatalytic performance of polymer-inorganic composite could mainly be caused from the synergistic effect of PANI and Sn₃O₄. According to literature research, the performance comparison of relevant catalyst for organic dyes degradation as presented in ESI Table S2.† Results showed that the PANI/Sn₃O₄ were still competitive among the listed photocatalysts for organic dyes degradation to some extent. Simultaneously, our research group will continue to study and enhance the photocatalytic property. The UV-visible absorption spectrum of PANI/Sn₃O₄-10% showing RhB dye photocatalytic degradation was shown in Fig. 7c. The RhB dye's absorption peak intensity decreased and blue shift indicated that the chromophore group in RhB molecule structure was destroyed by PANI/Sn₃O₄-10% composite during the reaction.⁵⁹ As the decolorization doesn't signify that the dyes have been completely oxidized into harmless final products.⁶⁰ Therefore, the total organic carbon (TOC) analysis was performed on dispersions at initial and final visible-light irradiation time points. The TOC concentration decreased from 8.42 mg L⁻¹ to 2.24 mg L⁻¹ after the photodegradation process over PANI/Sn₃O₄-10% for 5 h, i.e., TOC removal achieved 73.40%, indicating that RhB degradation was accompanied by partial mineralization.⁶¹ It is noteworthy that mechanical blends dramatically improved the photocatalytic activity to some extent, which could be related to the photoelectron energy in terms of dye degradation and the adsorption behaviour on the conducting polymer's surface.⁶² The conducting polymer PANI has shown a strong photoresponse in visible region, which has the high capacity photogenerated holes. Importantly, PANI nanofibers act as a limited delayed-action part in the PANI/Sn₃O₄ composite by offering a transition platform for photoelectrons. Thus, it has been demonstrated that conductive polymer PANI nanofibers perform an essential role in enhancing photocatalytic property, as PANI decreases recombination of photoinduced carriers and boosts effectively charge transfer in PANI/Sn₃O₄ composite.

Fig. 7 (a) Photocatalytic RhB degradation curves of synthesized samples under visible light irradiation; (b) photocatalytic performance of PANI/Sn₃O₄ composite with different PANI mass ratios on RhB dye degradation; (c) temporal evolution of the RhB absorption spectra; (d) cycling runs for RhB photo-degradation of PANI/Sn₃O₄-10% under visible light irradiation.

3.8. Photocatalytic stability

RhB degradation recycling experiment was conducted on PANI/Sn₃O₄-10% to evaluate the reuse efficiency under visible light irradiation after its initial use. Fig. 7d showed that the photocatalytic performance of PANI/Sn₃O₄-10% did not significantly decrease after three runs. In fact, the degradation rate of RhB could decrease to 92.44% from maximum 96.84%. This decline might be owing to the loss of weight in washing catalyst process. The results showed PANI/Sn₃O₄-10% exhibited relatively stable photocatalytic activity during RhB photodegradation.

3.9. Charge transport and separation

The separation efficiency of photoexcited carriers acts as a decisive part in photocatalytic process. To testify the photocatalytic mechanism rationality, transient photocurrent, EIS and photoluminescence were all employed,⁶³ respectively. The transient photocurrent responses of Sn₃O₄ and PANI/Sn₃O₄-10% were depicted in Fig. 8a. When illuminated by visible light, the photo-current density over PANI/Sn₃O₄-10% composite was increased significantly,⁶⁴ but then dropped rapidly when turned off the light. The enhanced photocurrent of PANI/Sn₃O₄ hybrid may due to the improved transition of photoinduced carriers via the conductive polymer PANI. EIS study is a powerful path to study the carriers transfer and separation efficiency.⁶⁵ And it was also investigated to detect conductivity of the Sn₃O₄ and PANI/Sn₃O₄-10%. As we know, the semi-arc shape reflects the speed of electron transmission.⁶⁶ In Fig. 8b, Nyquist arc radii of PANI/Sn₃O₄-10% composite was smaller than that of bare Sn₃O_4, and it signified that PANI/Sn₃O₄-10% heterostructure could receive less resistance for transport of interfacial electrons and holes compared with pure Sn₃O₄.⁶⁷ In short, the promoted photocatalytic property is chiefly owing to the excellent interfacial charge separation.⁶⁸

Fig. 8 (a) Instant photo-current density of the synthetic samples under visible light illumination; (b) EIS spectra of Sn₃O₄ and PANI/Sn₃O₄-10% measured at open-circuit voltage.

Generally speaking, the photoluminescence spectra can intuitively provide information on the separation and recombination of photoexcited carriers. In Fig. 9a, the PANI/Sn₃O₄-10% heterostructure shows a low fluorescence intensity relative to Sn₃O₄. Thus, the photocatalytic performance of PANI/Sn₃O₄-10% is evidently improved with modification by the conductive polymer PANI, which restrains the recombination and boosts the separation efficiency of photo-excited carriers to some extent.

Fig. 9 (a) PL emission spectra of Sn₃O₄ and PANI/Sn₃O₄-10% composite; (b) photocatalytic performance of PANI/Sn₃O₄-10% to RhB solutions with the addition of different radical trapping agents under visible irradiation.

3.9.1. Photodegradation mechanism. In photocatalytic process, ·O²⁻, ·OH and h⁺ are usually served as active species that eventually oxidize contaminants.⁶⁹ Thus, in order to verify and analyze the radical intermediate products during this photocatalytic reaction, ESR tests were performed. In Fig. 10a and b, no characteristic signals were detected in darkness, signifying radicals were generated in photocatalytic process.³⁹ The characteristic peak signals of DMPO⁻·OH and DMPO⁻·O²⁻ appeared when the PANI/Sn₃O₄-10% composites were exposed to visible illumination.⁷⁰ What's more, the characteristic signals' intensity gradually increased with illumination time. Thus, the existence of hydroxyl and superoxide radicals in the reaction system was confirmed. In this work, trapping experiments investigated active species generation, as well as their impact on the degradation course.^19,71 Fig. 9b showed that the RhB degradation ratio decreased when BQ, t-BuOH and EDTA-2Na, respectively, were added to the PANI/Sn₃O₄-10% photocatalyst reaction system. More importantly, the RhB photocatalytic degradation efficiency dropped significantly when EDTA-2Na was added, which strongly indicated that h⁺ was primary reactive species in the degradation process. Similarly, RhB degradation decreased sharply after the addition of the scavenger BQ. By contrast, when t-BuOH was poured into the reaction system, the photodegradation process decelerated slowly, indicating that ·OH had less influence on the system. Consequently, h⁺ and ·O²⁻ have a strong oxidation ability, which induces the photocatalytic degradation of organic contaminant.

Fig. 10 ESR signals of (a) DMPO⁻·OH and (b) DMPO⁻·O²⁻.

Based on the experimental conclusion of photocatalytic degradation, photoelectrochemical test and photogenerated carrier trapping, the mechanism of RhB photo-degradation was proposed. We suggest that the promotion of photocatalytic performance is mainly due to PANI and Sn₃O₄ mixing, which improves the charge separation efficiency. In the PANI/Sn₃O₄ hybrid systems, PANI nanofibers serve as a dispersing template, hole acceptors and transfer channels; and the PANI/Sn₃O₄ heterojunction synergetic effect depresses recombination of the photo-induced charge carriers.⁷² The electronic and energy band structure of materials play a crucial part in properties.⁷³ Fig. 11 shows the carriers separation mechanism and photocatalytic reaction on the PANI/Sn₃O₄ composite. Previous literature have reported the PANI optical band gap value to be 2.70 eV;⁷⁴ while according to the experimental results, Sn₃O₄ band gap was 2.56 eV.⁵⁶ The literature reports that PANI's lowest unoccupied molecular orbital level (LUMO) (−1.90 eV) is lower than the Sn₃O₄ conduction position (−1.11 eV);^30,57 whereas Sn₃O₄ valence position (+1.46 eV) is higher than that of PANI (+0.8 eV).^30,75 Thus, the PANI's LUMO level is more negative than the Sn₃O₄ nanosheet conduction band position, and Sn₃O₄ can be used as a photoelectron sink in hybrid composite.²¹ The Sn₃O₄ valence band is more positive than PANI's highest occupied molecular orbital, so PANI can be used as the acceptor of photogenerated holes for composite to some extent. When light falls on the surface of a composite catalyst, the conductive polymer PANI generates a π–π* transition,¹¹ delivering the excited electrons to Sn₃O₄ nanosheets' CB, after which the electrons are transferred to the photocatalysts' surface to react with oxygen. Moreover, the PANI is also a better material for hole migration.⁷⁶ The photogenerated holes in the Sn₃O₄ VB transfer to PANI's HOMO orbital,⁷² then emigrate to the surface of photocatalysts.²¹ In this way, the photogenerated carriers could be efficiently separated, indicating the benefit of applying the coupled system.²⁸ RhB is excited to RhB* under visible light illumination,⁴¹ followed by photoinduced electron transfer from RhB* to CB of Sn₃O₄,²⁸ which is scavenged by dissolved O₂ to form superoxide radical anion owing to the fact that the CB position of both Sn₃O₄ and PANI located above the reduction potential of O₂/·O²⁻.⁷⁷ The hydroxyl radicals are produced owing to the reaction of holes with H₂O.²⁸ The reaction yields ·OH and ·O²⁻ that are able to oxidize the adsorbed contaminant and form colorless small degraded products.⁷⁸ The introduce of PANI nanofibers not only improved light absorption range of PANI/Sn₃O₄ heterostructure, but also initiated the synergistic effect between Sn₃O₄ and PANI. Importantly, this might be due to the p–n junction formation at the Sn₃O₄ and PANI interface, which facilitates the transfer of photogenerated carriers. The photogenerated electron–hole pairs were effectively inhibited, which improved the photocatalytic performance.

Fig. 11 Schematic representation of the photocatalytic mechanism.

4. Conclusions

In summary, a novel PANI/Sn₃O₄ catalyst, that effectively improves photocatalytic activity, was successfully synthesized using mechanical blends. Compared with pure Sn₃O₄, the PANI nanofibers introduced in PANI/Sn₃O₄ heterostructure reduced the agglomeration of Sn₃O₄ nanosheets.⁷⁹ Furthermore, the PANI acted as an integral part in facilitating the charge separation efficiency and photocatalytic activities of PANI/Sn₃O₄ composite due to the PANI nanofibers' high conductivity. It has been demonstrated that PANI nanofibers ensured sufficient light absorption, dye molecular adsorption and electron transfer, also provided good conductive channels to enhance photosensitive reactions in PANI/Sn₃O₄ composite. The p–n heterostructure electrical field built at the PANI/Sn₃O₄ interface and the interface housing photoinduced carriers at the energy band level between that of PANI and Sn₃O₄ have accelerated the separation of photoinduced hole–electron pairs and effectively improved photocatalytic activity. Thus, this PANI/Sn₃O₄ p–n heterojunction could be applied as an efficient photocatalyst for organic pollutants decomposition under visible light. This concept shows good application prospects in environmental purification and has the potential to be implemented on an industrial level in the future.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support from National Natural Science Foundation of China (No. 21902081), Science and Technology Key Project of Henan Province (No. 192102210031, No. 172102210429), Doctoral Scientific Research start-up Foundation of Pingdingshan University (No. PXY-BSQD-2018026) and National Scientific Research Project Cultivating Foundation of Pingdingshan University (No. PXY-PYJJ-2019004) are gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra07562c

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