Pengfei
Tan
a,
Lu
Yang
*b,
Hele
Liu
b,
Yi
Zhang
ab,
Binhua
Zhou
a and
Jun
Pan
*a
aState Key Laboratory for Powder Metallurgy, Central South University, Changsha 410083, P. R. China. E-mail: jun.pan@csu.edu.cn
bHunan Key Laboratory of Applied Environmental Photocatalysis, Changsha University, Changsha 410022, P. R. China. E-mail: z20180829@ccsu.edu.cn
First published on 20th November 2023
Developing novel catalysts with excellent photocatalytic hydrogen evolution activity is crucial for expediting current research on solar-chemical energy conversion. In the present study, unique Sn3O4/CdS composites were carefully designed and prepared using a facile hydrothermal method to achieve outstanding photocatalytic H2 production activity. Various techniques were employed to determine the crystalline structure, chemical composition, microstructure, and optical and electrochemical characteristics of the prepared samples. The experimental results indicate that zero-dimensional (0D) CdS quantum dots were tightly attached to the surface of three-dimensional (3D) Sn3O4 nanoflowers. Furthermore, enhanced light-absorbing capacity and accelerated electron–hole separation and transfer were achieved after incorporating CdS quantum dots into Sn3O4 nanoflowers. The generation rate of hydrogen using the optimal sample (Sn3O4/CdS QDs-2) was about 20.74 μmol g−1 h−1, which was 2.86 and 3.16 times those of pure Sn3O4 and CdS, respectively. In addition, the possible photocatalytic H2 production mechanism of Sn3O4/CdS nanocomposites was also revealed. This work is highly desirable to provide valuable inspiration for rational design and preparation of efficient photocatalysts for H2 evolution.
In recent years, Sn3O4, a non-stoichiometric tin-based oxide consisting of Sn2+ and Sn4+, has attracted much attention and been recognized as a potentially excellent photocatalyst due to its natural abundance, nontoxicity, thermodynamic stability, responsiveness to visible light, and favorable band-edge position for hydrogen production.7,8 However, the pristine Sn3O4 material also encounters the disadvantages of fast electron–hole combination, limited visible light absorption range, and poor stability, which exist in most single-component photocatalysts, leading to unsatisfactory performance and seriously affecting its practical applications.9–11 The construction of heterojunctions provides an idea for the solution of the above issues. To this end, researchers have been successful in constructing a wide variety of photocatalytic hydrogen generation catalysts based on Sn3O4. For instance, Zn0.5Cd0.5S/Sn3O4 (ZCS/SO) composites prepared by Jiang and colleagues using a one-step hydrothermal method exhibited an enhanced hydrogen production rate, which is due to the improved light absorption range and electron–hole separation efficiency.10 Additionally, Wang's group prepared Sn3O4/TiO2 NT composites through a hydrothermal method and achieved a higher photocatalytic H2-production rate compared to that of the single component.9 Cui et al. synthesized two dimensional (2D)-2D g-C3N4/Sn3O4 heterojunctions using a calcination strategy. In comparison to g-C3N4 and Sn3O4, the H2 evolution rates of the heterojunctions were 5.4 and 3.2 times higher, respectively.12 From the above successful cases, it is clear that it is possible to develop excellent Sn3O4-based photocatalysts by constructing composite structures with wide light absorption and unique photogenerated charge transfer pathways. It should be noted, however, that the construction of composite structures frequently necessitates the selection of semiconductor materials with suitable energy band structures and that a reasonably close contact between the components in the composites is required for the full effect.3,13–15 Therefore, the task of selecting suitable semiconductor materials that can effectively enhance the photocatalytic H2-production activity when combined with Sn3O4 to create composite materials remains a challenge, and further research in this area is still ongoing.
As an n-type semiconductor, CdS has garnered considerable interest due to its abundant earth resources, visible light response, suitable electronic structure, and exceptional catalytic performance.4 In recent years, it has been reported that CdS can couple with many semiconductors (ZnO,16 Co9S8,17etc.) to produce composites with enhanced H2-production performance.4 Given the extremely matched energy band edge positions of CdS and Sn3O4, it is promising to accelerate charge separation and migration by carefully designing and constructing Sn3O4/CdS composites with a unique energy band arrangement, thus realizing efficient hydrogen production. In addition, a lot of research has also been done on the morphology of heterojunctions because of the remarkable structure–activity relationship in photocatalysis. It is noted that zero-dimensional (0D)/three-dimensional (3D) composites constructed by modifying 3D nanostructures with 0D quantum dots (QDs) not only have good interfacial contacts for accelerated carrier separation, but also optimize multiple light reflection/scattering with the help of the hierarchical structure to improve the light energy utilization.17–19 However, the attempt of constructing 3D/0D Sn3O4/CdS composites for enhanced HER has not been reported so far.
Inspired by the aforementioned analysis, in this study, by confining 0D CdS QDs on 3D Sn3O4 nanoflowers using a simple hydrothermal method, novel 3D/0D Sn3O4/CdS composites were carefully designed and constructed for the first time. The as-prepared catalysts’ activity under visible light was revealed by photocatalytic HER experiments. It is interesting to find that the coupling of Sn3O4 and CdS can dramatically improve the HER activity. By analyzing the optical and electrochemical properties of the composites, the enhanced efficiency of Sn3O4/CdS composites is elucidated, and a potential photocatalytic mechanism is suggested. This work opens up ideas for further research on efficient Sn3O4-based visible-light-driven photocatalysts with enhanced H2-evolution activity.
The preparation process of pure Sn3O4 flower-like structures was the same as that of the Sn3O4/CdS QD composites described above, except that the CdS QD solution was not added.
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Fig. 3 SEM images of Sn3O4/CdS QDs-1 (a), Sn3O4/CdS QDs-2 (b), Sn3O4/CdS QDs-3 (c), and Sn3O4/CdS QDs-4 (d). |
TEM images of pure Sn3O4 are shown in Fig. S2b and c (ESI†). These images reveal that a flower-like structure is uniform throughout the material, and demonstrate that the flower-like structure is formed of stacked thin nanosheets. From the TEM image of CdS (Fig. S2d, ESI†), it is clear that the prepared pure CdS QDs are quantum dots with small sizes. Fig. 4a and b depict the TEM images of the Sn3O4/CdS QDs-2 composite sample, in which the structural features of the thin nanosheets are clearly visible and are not disrupted after coupling with CdS QDs, confirming the SEM results. Moreover, the high-resolution transmission electron microscopy (HRTEM) image reveals that CdS QDs in the composite sample appeared on the nanosheets of Sn3O4 and were uniformly dispersed (Fig. 4c and Fig. S3, ESI†). A favorable contact interface is formed between the two components. The interface promotes the establishment of a pathway for carrier transportation and facilitates the efficient carrier movement. The composite sample exhibits distinct lattice stripes (Fig. 4d). And, a lattice spacing of 0.369 nm corresponds to the (101) crystal plane of Sn3O4, confirming the presence of Sn3O4 in the prepared catalysts. CdS can be identified in the composite sample because the characteristic lattice spacing of 0.205 nm is consistent with its (220) crystal plane. Furthermore, the nanostructure of Sn3O4/CdS QDs can be corroborated by the high-angle annular dark-field (HAADF) image (Fig. 4e) and energy dispersive spectroscopy (EDS) elemental mapping images (Fig. 4f–i and Fig. S4, ESI†), from which it is clear that Sn, O, Cd, and S are all present in the composite structure and are distributed evenly throughout the nanostructure.
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Fig. 4 TEM (a) and (b) and HRTEM (c) and (d) images of Sn3O4/CdS QDs-2. (e)–(i) HAADF and EDS mapping images of Sn, O, Cd, and S in Sn3O4/CdS QDs-2. |
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Fig. 5 High-resolution XPS spectra of (a) Sn 3d, (b) O 1s, (c) Cd 3d, and (d) S 2p in Sn3O4 and Sn3O4/CdS QDs-2. |
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Fig. 6 (a) DRS of Sn3O4, CdS QD, and Sn3O4/CdS QDs-2 samples. (b) Tauc and (c) and (d) Mott–Schottky plots of Sn3O4 and CdS QDs. |
In addition, the Mott–Schottky (M–S) curves provide insight into the catalysts’ semiconductor type and energy band structure. Fig. 6c and d show the M–S curves of Sn3O4 and CdS QDs. It can be seen that both exhibit a linear region with a positive slope, indicating that both of them are n-type semiconductors. The flat band potentials of Sn3O4 and CdS QDs are −0.61 and −0.88 eV (vs. NHE, pH = 7), respectively, obtained from the tangent intercepts of the M–S curves. In general, the conduction band potential (ECB) of n-type semiconductors can be considered to be approximately equal to their flat band potentials.12,13,22 Thus, the ECB values of Sn3O4 and CdS QDs are estimated to be around −0.61 and −0.88 eV (vs. NHE, pH = 7), respectively. Meanwhile, according to the following equation: Eg = EVB − ECB, and in combination with the Eg of Sn3O4 and CdS QDs in Fig. 6b, the valence band edge positions (EVB) of Sn3O4 and CdS can be calculated as 1.99 and 1.52 eV (vs. NHE, pH = 7), respectively.28
Fig. 7a shows the fluctuation curves of hydrogen production over time for different photocatalysts. It can be seen that the photocatalytic activities of both pure Sn3O4 and bare CdS QDs are poor under the above conditions, which is due to the rapid recombination of photogenerated carriers. Surprisingly, the coupling of Sn3O4 with CdS QDs enhances the hydrogen production performance of all Sn3O4/CdS QD composite samples. In order to explain more directly the change in performance after the addition of CdS QDs, the H2-evolution rates of the corresponding products are shown in Fig. 7b. The comparison of the hydrogen production rates of other Sn3O4-based photocatalysts is shown in Table S2 (ESI†). Pure Sn3O4 and bare CdS QDs exhibit average hydrogen production rates of 7.25 and 6.55 μmol g−1 h−1, respectively. Among a series of composites, Sn3O4/CdS QDs-2 shows the highest H2 production rate of 20.74 μmol g−1 h−1, surpassing those of pure Sn3O4 and CdS by 2.86 and 3.16 times. The hydrogen production rates of the composite samples show the following trend: Sn3O4/CdS QDs-2 (20.74 μmol g−1 h−1) > Sn3O4/CdS QDs-3 (17.85 μmol g−1 h−1) > Sn3O4/CdS QDs-1 (15.87 μmol g−1 h−1) > Sn3O4/CdS QDs-4 (14.25 μmol g−1 h−1). It can be seen that more CdS do not always mean higher photocatalytic performance since it might have a negative impact on the catalyst's dispersion, interfacial contact ability, and light absorption capacity at high concentrations. The cyclic stability test (Fig. S5, ESI†) shows that the H2 production rate of the prepared catalyst does not decrease considerably after four reactions, indicating that the material has good stability. The photocatalytic H2 evolution curve of Sn3O4/CdS QDs-2 with an error bar is shown in Fig. S6 (ESI†), which further proves the reliability and stability of Sn3O4/CdS QDs-2. In addition, the XRD and TEM results of Sn3O4/CdS QDs-2 after the photocatalytic reaction are shown in Fig. S7 and S8 (ESI†). The results show that the morphology and crystal structure of the sample did not change significantly after the photocatalytic hydrogen production reaction.
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Fig. 7 (a) Photocatalytic H2-production versus time and (b) the rates of H2 production for different samples. |
To investigate the separation and transfer efficiency of photogenerated carriers in the catalysts, various experiments were performed, including transient photocurrent analysis (I–t curves), electrochemical impedance spectroscopy (EIS), steady-state photoluminescence (PL) spectroscopy, and time-resolved photoluminescence (TR-PL) spectroscopy. In general, the higher the transient photocurrent density, the higher the interfacial carrier separation efficiency of the catalyst, and the better the photocatalytic performance.29,30Fig. 8a shows the transient photocurrent response plots of CdS QD, Sn3O4, and Sn3O4/CdS QDs-2 nanocomposites under visible light irradiation for several on–off cycles. The results show that the transient photocurrent response of the Sn3O4/CdS QDs-2 composite photocatalyst is significantly enhanced when used as working electrodes compared to pristine Sn3O4 or CdS, suggesting that the photogenerated charge separation and transfer of Sn3O4/CdS QD nanocomposites is more capable under light irradiation. This conclusion can also be confirmed by the Nyquist plots of the corresponding samples shown in Fig. 8b. For the optimal sample of Sn3O4/CdS QDs-2, the radius of a semicircle arc is the smallest compared with that of pristine CdS QDs and Sn3O4, respectively. This implies that Sn3O4/CdS QDs-2 has the smallest electron-transfer resistance, signifying that it has the fastest electron transfer, which is favorable for the efficient separation and transfer of carriers.31 The recombination rate of charge carriers in the catalysts was further assessed by PL. Fig. 8c reveals the fluorescence intensity of Sn3O4 and a series of Sn3O4/CdS QD nanocomposites. Generally, the low signal of fluorescence peak indicates a low recombination probability of photo-induced electron–hole pairs, thus showing high photocatalytic activity.32,33 It is not difficult to find that the PL intensity of pure Sn3O4 is the highest, and the PL intensity of Sn3O4/CdS QDs-2 is the lowest. The results indicate the recombination of photogenerated carriers in the composite material is sufficiently suppressed, which also corroborates with the experimental outcomes of I–t and EIS. Additionally, the lifetime of photoinduced carriers in the photocatalysts was analyzed using TR-PL spectra.34 The average lifetime (τav) can be obtained by fitting the decay process with a bi-exponential function: τav = (A1τ12 + A2τ22)/(A1τ1 + A2τ2).27 The specific fitting data are summarized and shown in Table S3 (ESI†). From the TR-PL spectra (Fig. 8d), the τav values of Sn3O4, CdS QD, and Sn3O4/CdS QDs-2 composites are 1.78, 1.37 and 2.25 ns, respectively. Apparently, the fluorescence lifetime of the Sn3O4/CdS QDs-2 nanocomposite is increased compared with those of Sn3O4 and CdS QDs. The increase in the carrier lifetime implies an increase in the utilization of electron–hole pairs during the photocatalytic process and a boost in the activity of photocatalysis.35 This is also consistent with the above findings on photocurrent, EIS, and hydrogen production properties.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3nj03611a |
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2024 |