Synergistic effect of noble metal Pt and Zn0.25Cd0.75S QDs on ZnO: Broad spectral response and its outstanding photocatalytic performance

doi:10.21203/rs.3.rs-4375026/v1

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Synergistic effect of noble metal Pt and Zn_0.25Cd_0.75S QDs on ZnO: Broad spectral response and its outstanding photocatalytic performance

https://doi.org/10.21203/rs.3.rs-4375026/v1

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In this paper, based on the modification of semiconductor ZnO by solid solution heterojunction and noble metal photoreduction, Pt/Zn_0.25Cd_0.75S QDs/ZnO composite with a broad spectral response was synthesized, and crystal structure, morphology, optical properties, specific surface area and electrochemical properties of composites were investigated and discussed. The prepared composite has a skeleton structure, in which the solid solution Zn_0.25Cd_0.75S mainly exists in the form of quantum dots (QDs), and Pt is mostly simple nanoparticles. After ZnO was modified by solid solution Zn_xCd_1−xS QDs and precious metal Pt with surface plasmon resonance effect, the composite has strong light absorption ability in the visible region. Compared with the ZnO monomer, the specific surface area of the nanoparticle framework has a significant enhancement, thus increasing the active sites for the photocatalytic reaction. In addition, the results of the transient photocurrent response tests and the electrochemical impedance tests show that Pt/Zn_0.25Cd_0.75S QDs/ZnO composite has a better carrier separation efficiency with the fastest electron transfer rate and the lowest charge transfer resistance compared with other reference systems. Furthermore, the composite exhibit excellent photocatalytic performance in the multi-mode photocatalytic degradation of dye molecules. The results of photocatalytic water splitting into hydrogen show that the hydrogen production capacity of Pt/Zn_0.25Cd_0.75S QDs/ZnO composite is 33.67 mmol·g^− 1 in 8 h, which is 207 times higher than that of commercially available P25. Combined with the results of the capture experiments, it is finally determined that the possible photocatalytic mechanism of the composite Pt/Zn_0.25Cd_0.75S QDs/ZnO is more inclined to be the effect of "Z-type" heterostructure.

Zn0.25Cd0.75S QDs

ZnO

Broad spectral response

Photocatalysis

Photocatalytic hydrogen production

Along with the progress of society and industry, the utilization of solar energy for the photocatalytic decomposition of pollutants and meanwhile the production of hydrogen through photocatalysis of water has become a successful approach towards tackling environmental pollution and energy shortage^[1–4]. ZnO has been extensively studied among numerous semiconductor materials for photocatalysis due to its advantages of high activity, chemical stability, non-toxic, wide availability, and low cost^{[5, 6]}. However, since ZnO has a wide band gap (3.2 eV)^[7], it can only be excited at wavelengths below 385 nm^[8], which severely limits the large-scale application of ZnO. Therefore, a variety of methods have been used to design new ZnO-based photocatalytic composite materials, such as the use of noble metal loading (like Au, Ag, Pt, etc.)^[9–11], ion doping^[12–14], constructing heterogeneous structures^[15–17] and other methods to improve the solar light energy utilization efficiency of ZnO-based photocatalytic materials.

Metal sulfides have relatively small band gaps, which enable them to be used as photocatalysts in visible light driven reactions, while the design of heterostructures has advantages such as high light capture, delayed electron–hole recombination, and fast charge transport^[18]. Therefore, it is an effective method to improve semiconductor ZnO by constructing heterostructures between metal sulfides (such as CdS, In₂S₃, CuS, etc.) and semiconductor ZnO. Lu et al.^[19] prepared clustered ZnO/CdS nanotube heterojunction photocatalyst on fluorine-doped tin oxide (FTO) conductive glass substrate by a two-step hydrothermal method. The unique clustered tubular structure, wide light absorption range, and high carrier transfer rate made the composite material have excellent photocatalytic hydrogen production activity. The photocatalytic hydrogen production activity of the ZnO/CdS nanotube heterojunction photocatalyst reached 7669 µmol·g^− 1·h^− 1, and the apparent quantum efficiency (AQE) was 55.25%. However, CdS and other narrow band gap photocatalysts have disadvantages such as high photoelectron–hole recombination rate, easy agglomeration of particles, easy photocorrosion, and poor stability, which seriously restrict their practical application^[20]. To this end, researchers have tried to overcome this problem by adding Zn atoms to form a solid solution of CdS and ZnS^{[21, 22]}. The synthesized Zn_xCd_1−xS has the band structure and light absorption range of CdS with strong reduction capability^[23], and also maintains the stability of ZnS^[24]. Most importantly, during the synthesis process, the band structure of the solid solution can be significantly altered by changing the ratio of Zn to Cd, and the tunable band structure brings a wider application prospect for this series of materials^[25–27]. Liu et al.^[28] synthesized a highly efficient SiO₂/ZnCdS/NiS photocatalyst by self-assembly (ZnCdS loaded on SiO₂) and photo-deposition (NiS loaded on ZnCdS). Through the synergistic effect of the three, the production and separation of photoelectrons was effectively promoted, thus improving the photocatalytic performance. Under visible light (λ > 420 nm), the hydrogen production activity of SiO₂/ZnCdS/NiS was as high as 8.4 mmol·g^− 1·h^− 1. Compared with traditional solid solution Zn_xCd_1−xS, Zn_xCd_1−xS QDs is considered to be one of the most promising photocatalysts for hydrogen production by water decomposition due to its excellent light trapping performance, high electron transmitting capacity, suitable conduction band position and large specific surface area^[29]. Qi et al.^[30] prepared ZnCdS QDs by sulfate reducing bacteria (SRB) reduction method. ZnCdS QDs had remarkable photocatalytic activity and stability in visible light, especially when Zn/Cd = 3:1, ZnCdS QDs had the best photocatalytic performance for hydrogen production, up to 3.752 mmol·h^− 1·g^− 1. In addition, in terms of optimizing the photocatalytic performance of ZnO-based composites, the precious metals plasmon resonance effect such as Pt^[31], Au^[32] and Ag^[33] acts as a conduction band between semiconductor, thereby increasing the utilization efficiency of visible light and reducing the Fermi level of capturing photogenerated electrons is also a very effective method. Yang et al.^[34] synthesized ZnO/Zn₃In₂S₆/Pt composites with S-scheme/Schottky heterojunction using step-by-step hydrothermal process and in situ photoreduction deposition, showing efficient photoexcited charge separation/transfer. ZnO/Zn₃In₂S₆/Pt composites had high photocatalytic performance and cyclic stability against photocorrosion, and the generated H₂ reached 3.5 mmol·g^− 1·h^− 1.

Therefore, in this paper, in order to improve the photocatalytic performance of semiconductor ZnO, on the one hand, Zn_0.25Cd_0.75S/ZnO was prepared by modification of Zn_0.25Cd_0.75S QDs on semiconductor ZnO to increase the specific surface area, expand the light absorption range, enhance the photocatalytic active sites and improve light absorption range. On the other hand, Pt was loaded on the surface of Zn_0.25Cd_0.75S/ZnO by photoreduction method to reduce the photocorrosion of Zn_0.25Cd_0.75S QDs and extend the lifetime of photogenerated carriers, thus the synthesized Pt/Zn_0.25Cd_0.75S QDs/ZnO has much higher photocatalytic performance.

2.1 Preparation of Pt/Zn_0.75Cd_0.25S QDs/ZnO

0.2 g of Zn_0.25Cd_0.75S/ZnO was dissolved in 50 mL of deionized water, and then 1 mL of chloroplatinic acid (H₂PtCl₆·6H₂O) solution (10 mmol·L^− 1) was added. The above mixture was sonicated and stirred well, followed by adding 5 mL of methanol (CH₃OH) as a hole trapping agent. After that, the mixture was illuminated using 300 W Xe light for 2 h. Finally, the solution was centrifuged, washed and dried to obtain Pt/Zn_0.25Cd_0.75S/ZnO composite.

For other relevant experimental information, please refer to the supplementary materials.

3.1 XRD analysis

In order to investigate the crystalline structure of the prepared composites, X-ray diffraction (XRD) examination of Zn_0.25Cd_0.75S QDs, Zn_0.25Cd_0.75S QDs/ZnO, and Pt/Zn_0.25Cd_0.75S QDs/ZnO were carried out in this paper, and the results are shown in Fig. 1. As can be seen from Fig. 1(a), Zn_0.25Cd_0.75S QDs do not have any diffraction peaks that exactly correspond to CdS (JCPDS#41-1049)^[35] and ZnS (JCPDS#05-0566)^[36]. However, based on the comparison of peak intensities, the diffraction peaks of ZnS and CdS exist in Zn_0.25Cd_0.75S QDs, and these diffraction peaks shift regularly. The diffraction peaks of Zn_0.25Cd_0.75S QDs shift to higher angles in comparison to CdS, while shift to lower angles compared with ZnS^[37]. This suggests that Zn_0.25Cd_0.75S QDs material is not a mixture of ZnS and CdS, but belongs to Zn_xCd_1−xS solid-solution quantum dots.

Meanwhile, XRD image of Zn_0.25Cd_0.75S QDs/ZnO given in Fig. 1(b) shows that the diffraction peaks of the composite Zn_0.25Cd_0.75S QDs/ZnO at 27.5°, 45.3°, and 54.2° correspond to those of Zn_0.25Cd_0.75S QDs. Moreover, the diffraction peaks at 32.0°, 34.5°, 36.5°, 47.6°, 56.6°, 62.9°, 66.4°, 68.0°, 69.1°, 72.6°, 81.4°, and 89.6° correspond to the standard PDF cards of ZnO (JCPDS#36-1451)^[38], indicating that the composite material consists of Zn_0.25Cd_0.75S QDs and ZnO. In addition, the composite has very high diffraction peaks at 32.0°, 34.5°, and 36.5° consistent with the (100), (002), and (101) crystal planes of ZnO, proving that ZnO has a good crystallinity and exists as a hexagonal fibrillar zincite structure in the composites^[39].

Furthermore, XRD results of Pt/Zn_0.25Cd_0.75S QDs/ZnO show that the diffraction peaks of Pt/Zn_0.25Cd_0.75S QDs/ZnO correspond to those of Zn_0.25Cd_0.75S QDs/ZnO without obvious shift (Fig. 1(c)), indicating that Zn_0.25Cd_0.75S QDs/ZnO is present in the composite Pt/Zn_0.25Cd_0.75S QDs/ZnO. By comparing the intensities of diffraction peaks between Pt/Zn_0.25Cd_0.75S QDs/ZnO and Zn_0.25Cd_0.75S QDs/ZnO in Fig. 1(d), the intensities of ZnO in Pt/Zn_0.25Cd_0.75S QDs/ZnO are weaker, whereas the diffraction peaks of Zn_0.25Cd_0.75S QDs in Pt/Zn_0.25Cd_0.75S QDs/ZnO are sharper, which suggests that the presence of Pt promotes the growth and crystallisation of Zn_0.25Cd_0.75S QDs. However, the diffraction peaks of Pt are not observed in Fig. 1(c), which may be due to the small amount of its deposition.

3.2 UV–vis/DRS analysis

As can be seen from Fig. 2(a), the monomer ZnO synthesized in this work has an absorption edge wavelength of 401 nm with a strong absorption in the UV region^[40]. Meanwhile, the absorption edge wavelength of the monomer Zn_0.25Cd_0.75S QDs is 526 nm, displaying a certain absorption in the visible region. The absorption edge of Zn_0.25Cd_0.75S QDs/ZnO shows an obvious redshift compared with ZnO, with the wavelength of 457 nm. After the deposition of noble metal Pt, the absorption range of Pt/Zn_0.25Cd0.75S QDs/ZnO composite is further broadened and the absorption edge is redshifted to 497 nm.

The band gap energies of ZnO, Zn_0.25Cd_0.75S QDs, Zn_0.25Cd_0.75S QDs/ZnO and Pt/Zn_0.25Cd_0.75S QDs/ZnO were calculated from Fig. 2(b) using the Kubelka–Munk formula (see Formula S1)^[41], and the corresponding results are given in Table S1. The bandgap value of the composite material decreases after combining the wide bandgap semiconductor material ZnO with narrow bandgap Zn_0.25Cd_0.75S QDs. Meanwhile, the loading of Pt further reduces the bandgap value of the composite, preliminarily suggesting that both the heterostructure and the loading of noble metals can change the bandgap value of the material.

Meanwhile, the absorption edge wavelength of the monomer Zn_0.25Cd_0.75S QDs is 526 nm, displaying a certain absorption in the visible region. The absorption edge of Zn_0.25Cd_0.75S QDs/ZnO shows an obvious redshift compared with ZnO, with the wavelength of 457 nm. After the deposition of noble metal Pt, the absorption range of Pt/Zn_0.25Cd0.75S QDs/ZnO composite is further broadened and the absorption edge is redshifted to 497 nm.

3.3 SEM and SEM-EDS analysis

In order to investigate the surface morphology, Pt/Zn_0.25Cd_0.75S QDs/ZnO composite was analyzed by scanning electron microscopy (SEM), and SEM images at different scales are shown in Fig. 3. It can be seen from Fig. 3(a–c) that the prepared composites are made up of interconnected nanoparticles, forming a skeleton-like morphology.

At the same time, the composite material presents pore structure with a relatively loose state. This structure is not only conducive to the transfer of substances, but also easy to adsorb dyes. From the magnified SEM photograph in Fig. 3(b), it can be observed that the nanoparticles have a rough surface and a uniform size of about 10 nm. And EDS analysis results of Pt/Zn_0.25Cd_0.75S QDs/ZnO in Fig. 3(d–h) shows that the elements of Zn, O, Cd, S and Pt are uniformly distributed on the surface of the material, indicating the presence of the above-mentioned elements in the composite material with the content of each element shown in Fig. S2.

3.4 TEM and HR-TEM analysis

For investigating the morphological structure and composition of the Pt/Zn_0.25Cd_0.75S QDs/ZnO composites, TEM analyses were carried out. In Fig. 4(a–c), it can be clearly observed that the prepared composites consist of several nanoparticles agglomerated together with a size of about 10 nm, which is consistent with the SEM results. Fig. 4(d), Fig. S3 and Fig. 4(e–j) show the HR-TEM maps and the Fast Fourier Transform (FFT) of Pt/Zn_0.25Cd_0.75S QDs/ZnO, respectively. From Fig. 4(d) and Fig. S3, it can be seen that the average particle size of Zn_0.25Cd_0.75S QDs is about 2–3 nm, which proves that the solid solution Zn_0.25Cd_0.75S exists in the form of quantum dots on the surface of ZnO^[42]. In addition, the lattice spacing in Fig. 4(e) is n=0.278 nm, which belongs to the (100) crystal faces of ZnO^[43]. The lattice spacing in Fig. 4(f) is n=0.296 nm, corresponding to the (101) crystallographic plane of the solid solution Zn_xCd_1-xS QDs^[44]. The lattice spacing in Fig. 4(g) is n=0.203 nm, which corresponds to the (200) crystal plane of Pt, demonstrating the presence of noble metal Pt monomers in the composite^[45]. The above lattice spacing results indicate the presence of hexagonal fibrillar zincite phase ZnO, metallic Pt, and Zn_0.25Cd_0.75S QDs in the prepared composites.

3.5 XPS analysis

In this paper, the elemental distribution and valence states on the surface of the composite material Pt/Zn_0.25Cd_0.75S QDs/ZnO were obtained by XPS detection and the results are shown in Fig. 5. From Fig. 5(a), it can be seen that the surface of the composite material contains Zn, Cd, Pt, S and O elements. Among them, Zn 2p at 1044.5 eV and 1021.4 eV in Fig. 5(b) corresponds to Zn 2p_3/2 and Zn 2p_1/2, respectively, indicating the presence of Zn²⁺ on the surface of the composite^[46]. Figure 5(c) shows the XPS energy spectrum of Cd, where the binding energies of 411.0 eV and 404.3 eV correspond to Cd 3d3/2 and Cd 3d5/2, respectively, indicating that the Cd element in the composites is present at + 2 valence^[47]. The binding energies of 71.7 eV and 74.7 eV correspond to Pt 4f_5/2 and Pt 4f_7/2 in Fig. 5(f), respectively, indicating that the Pt element exists in the form of Pt⁰ monomers on the surface of the composite^[48]. Meanwhile, the binding energies of S 2p at 162.2 eV and 161.0 eV in Fig. 5(e) are consistent with those of S 2p_3/2 and S 2p_1/2, indicating that S exists in the − 2 valence state^[49]. In addition, there are characteristic peaks of O 1s in Fig. 5(f) at 530.8 eV and 532.3 eV, corresponding to the O atoms in Zn–O formed between O²⁻ and Zn²⁺ ions and in the form of hydroxyl groups on the surface of the composite, respectively^[50].

3.6 N₂ adsorption–desorption analysis

In order to investigate the surface physicochemical properties of the composites, N₂ adsorption–desorption tests were carried out on ZnO, Zn_0.25Cd_0.75S QDs/ZnO, and Pt/Zn_0.25Cd_0.75S QDs/ZnO, and the results are shown in Fig. 6 and Table S2. As can be seen from Fig. 6, according to the IUPAC definition^[51], the N₂ adsorption–desorption isotherms of the above materials belong to type IV adsorption curve with H3-type hysteresis loops, indicating that all materials have mesoporous structure.

According to the data in Table S2, it can be seen that the specific surface area of ZnO is small, and after the loading of Zn_0.25Cd_0.75S QDs, the specific surface area becomes approximately five times larger than the monomer ZnO, which further proves that solid solution Zn_0.25Cd_0.75S exists on the surface of the semiconductor ZnO in the form of quantum dots, resulting in the rough surface and increased specific surface area. However, after loading with Pt, the specific surface area of Pt/Zn_0.25Cd_0.75S QDs/ZnO only has a small increase of 0.31 m²·g^− 1 compared with Zn_0.25Cd_0.75S QDs/ZnO, which may be due to the low loading amount of Pt.

3.7 Photoluminescence analysis

In order to investigate the photogenerated electron–hole recombination rate of the composites, photoluminescence was used to examine the composites in this paper, and the results are shown in Fig. 7. The photoluminescence spectra of ZnO, Zn_0.25Cd_0.75S QDs/ZnO, and Pt/Zn_0.25Cd_0.75S QDs/ZnO samples were obtained under the excitation of wavelength of 300 nm. As can be seen in Fig. 7, ZnO has the highest emission peak, indicating a high electron–hole recombination rate. And after combining with Zn_0.25Cd_0.75S QDs, the intensity of the emission peak is significantly reduced, which indicates that the construction of heterostructures in the composites can improve the efficiency of photogenerated electron–hole separation. In addition, when the noble metal Pt is loaded on the surface of Zn_0.25Cd_0.75S QDs/ZnO composite, Pt/Zn_0.25Cd_0.75S QDs/ZnO composite shows the lowest fluorescence intensity, and thus Pt/Zn_0.25Cd_0.75S QDs/ZnO has a good carrier separation performance, which is because the noble metal Pt has the ability to transfer electrons on the effect of surface plasmon resonance (SPR), thereby increasing the electron transfer path and effectively suppressing the recombination of electrons and holes^[52].

3.8 Electrochemical testing

In order to investigate the photogenerated carrier separation efficiency of ZnO, Zn_0.25Cd_0.75S QDs, Zn_0.25Cd_0.75S QDs/ZnO, and Pt/Zn_0.25Cd_0.75S QDs/ZnO, transient photocurrent response tests and electrochemical impedance tests were carried out, and the results are shown in Fig. 8. The transient photocurrent response can further confirm the electron–hole transfer ability of ZnO, Zn_0.25Cd_0.75S QDs, Zn_0.25Cd_0.75S QDs/ZnO, and Pt/Zn_0.25Cd_0.75S QDs/ZnO. The higher the photocurrent intensity, the faster the carrier migration on the surface of the photocatalyst^[53]. As shown in Fig. 8(a), the transient photocurrent intensity of the composite Zn_0.25Cd_0.75S QDs/ZnO is almost four times higher than that of ZnO, which is because the formation of heterostructure after the combination of ZnO and Zn_0.25Cd_0.75S QDs promotes the photogenerated current intensity, as the result of generating a large number of electrons with directional motion in Zn_0.25Cd_0.75S QDs/ZnO. After the deposition of the noble metal Pt, the transient photocurrent intensity of the composite Pt/Zn_0.25Cd_0.75S QDs/ZnO is greatly enhanced, which is about 1.5 times higher than that of Zn_0.25Cd_0.75S QDs/ZnO and 5 times higher than that of the monomer ZnO, indicating that the SPR effect of the noble metal Pt effectively inhibits the recombination of photogenerated electrons and holes, and thus improving the survival rate of photogenerated carriers.

In addition, the charge transfer efficiencies of all above materials were tested as shown in Fig. 8(b). The electrochemical impedance results are as follows: Pt/Zn_0.25Cd_0.75S QDs/ZnO > Zn_0.25Cd_0.75S QDs/ZnO > Zn_0.25Cd_0.75S QDs > ZnO. In general, the material with smaller radius of electrochemical impedance has lower charge transfer resistance and higher transfer efficiency^[54]. The figure shows that Pt/Zn_0.25Cd_0.75S QDs/ZnO has the smallest radius, indicating that it has the fastest electron transfer rate and the lowest charge transfer resistance.

3.9 Photocatalytic experiment results of Pt/Zn_0.25Cd_0.75S QDs/ZnO composite

In order to investigate the photocatalytic performance of Pt/Zn_0.25Cd_0.75S QDs/ZnO series of materials, multi-mode photocatalytic experiments were carried out using methyl orange (MO) as a model molecule to compare the differences in photocatalytic performance under different light sources, and the relevant results are shown in Fig. 9. According to the results of Fig. 9(a), it can be seen that the degradation efficiency of Pt/Zn_0.25Cd_0.75S QDs/ZnO is obviously improved, which is basically in line with the results of the above-mentioned UV–vis diffuse absorption spectroscopy analysis. The high photocatalytic degradation efficiency of Pt/Zn_0.25Cd_0.75S QDs/ZnO is due, on the one hand, to the synergistic effect between Zn_0.25Cd_0.75S QDs composite and the monomer ZnO with strong absorption ability in the UV region, on the other hand, to the modification of Pt. In order to have a clearer understanding of the effect and influence of different catalysts on the degradation of MO under UV light irradiation, the reaction kinetics graphs are given in this paper as Fig. 9(b), which is calculated as -ln(C_t/C₀) = kt ^[55], and the calculated rate constants of the reaction, k, are shown in Table S3. From Fig. 9(b) it can be seen that -ln(C_t/C₀) is essentially linear with the reaction time t, indicating that the degradation of the dye (MO) by the composites follows quasi-primary reaction kinetics.

In addition, in order to further investigate the photocatalytic performance of the composite Pt/Zn_0.25Cd_0.75S QDs/ZnO, experiments were carried out on the degradation of MO by the composites under different light source conditions, and the results are shown in Fig. 9(c) and Fig. 9(d). The degradation efficiency of Pt/Zn_0.25Cd_0.75S QDs/ZnO on MO is about 4.5 times and 1.5 times that of the monomer ZnO under visible light and simulated sunlight irradiation, respectively, indicating that Pt/Zn_0.25Cd_0.75S QDs/ZnO are able to significantly improve the utilization of sunlight during the photolysis process and consequently has a good degradation activity.

Furthermore, in this paper, the hydrogen production performance of ZnO, Zn_0.25Cd_0.75S QDs, Zn_0.25Cd_0.75S QDs/ZnO, and Pt/Zn_0.25Cd_0.75S QDs/ZnO was investigated by using a mixture solution of 0.1 M sodium sulfide (Na₂S) and 0.1 M sodium sulfite (Na₂SO₃) as a sacrificial agent, and the results are shown in Fig. 10. Under the irradiation of a 300 W xenon lamp for 8 h, Zn_0.25Cd_0.75S QDs show good hydrogen-producing activity compared with ZnO. Meanwhile, the hydrogen production of Zn_0.25Cd_0.75S QDs/ZnO composites is about 14 times higher than that of the monomer ZnO, suggesting that the construction of heterojunction structure has an obvious enhancement on the hydrogen production performance of ZnO.

In addition, further deposition of the noble metal Pt on the composite Zn_0.25Cd_0.75S QDs/ZnO results in a hydrogen production of 33.67 mmol·g^− 1, which is 33 times higher than that of the monomer ZnO. This is because the loading of Pt effectively inhibits recombination of photoinduced electrons and holes, leading to a significant increase in the hydrogen production capacity of Pt/Zn_0.25Cd_0.75S QDs/ZnO composite.

In order to speculate the possible reaction mechanism of the composite Pt/Zn_0.25Cd_0.75S QDs/ZnO in the photocatalytic reaction, reactive group trapping experiments were performed under UV light conditions with MO as a molecular model using different trapping agents: p-benzoquinone (BQ), disodium ethylenediaminetetraacetic acid (EDTA-2Na) and isopropanol (IPA), and the results are shown in Fig. S4. As can be seen from Fig. S4, the photocatalytic degradation of MO by the composite Pt/Zn_0.25Cd_0.75S QDs/ZnO is significantly reduced after the addition of various scavengers, which is attributed to the decline in the number of reactive groups in the reaction system, leading to the decreased photocatalytic activity in the composite Pt/Zn_0.25Cd_0.75S QDs/ZnO. It can be seen that there are three types of reactive groups in the composites: ·O₂⁻, h⁺ and ·OH, and their quantitative relationship is ·O₂⁻> h⁺> ·OH.

3.10 Possible photocatalytic reaction mechanism of Pt/Zn_0.25Cd_0.75S QDs/ZnO

According to Eq. (2) and Eq. (3) (see Formula S2), the values of conduction band and valence band were calculated and the results are given in Table S4. Based on the above results and related experimental data, the possible photocatalytic reaction mechanism of Pt/Zn_0.25Cd_0.75S QDs/ZnO composites was speculated. As shown in Fig. 11, under simulated sunlight irradiation, Pt can be used as an electronic transducer to improve the efficiency of solar energy usage due to the difference in Fermi energy levels with ZnO. According to the conventional energy band structure theory, since the conduction band value of Zn_0.25Cd_0.75S QDs (-0.41 eV) is more negative than that of ZnO (-0.31 eV), the photogenerated electrons transfer from Zn_0.25Cd_0.75S QDs to the conduction band of ZnO, and the electrons in the conduction band of ZnO interact with the dissolved oxygen in water to generate ·O₂⁻, which can degrade the dye molecules; and in the process of photolysis of water to produce hydrogen, the electrons in the conduction band of ZnO reduce water to H₂. In addition, the valence band of ZnO (2.89 eV) is more positive than that of Zn_0.25Cd_0.75S QDs (2.05 eV), so that photogenerated holes transfer from ZnO to the valence band of Zn_0.25Cd_0.75S QDs. Since the valence band value of Zn_0.25Cd_0.75S QDs is 2.05 eV, which is less than the redox potential for the formation of ·OH (2.4 eV vs NHE), Zn_0.25Cd_0.75S QDs are not capable of generating ·OH, which does not match with the captured active species, and therefore the photocatalytic mechanism demonstrated in Fig. 11(a) does not agree with the results of photocatalytic capture experiments. Consequently, this study prefers the "Z-type"energy band structure theory based on Pt as the electronic transducer between ZnO and Zn_0.25Cd_0.75S, as shown in Fig. 11(b).

At first, under simulated sunlight irradiation, due to the relatively low redox capacity of the electrons in the conduction band of ZnO and the holes in the valence band of Zn_0.25Cd_0.75S QDs, Pt serves as an electron mediator to recombine the electrons and holes produced by the two and thus facilitates the separation of photogenerated carriers. Meanwhile, the photocorrosion of Zn_0.25Cd_0.75S QDs is effectively suppressed, resulting in the retention of electrons in the conduction band of Zn_0.25Cd_0.75S QDs and holes in the valence band of ZnO. The e⁻ in the conduction band of Zn_0.25Cd_0.75S QDs has a large reducing capacity to produce ·O₂⁻ from O₂ and mineralize MO to CO₂ and H₂O;In addition, the h⁺ in the valence band of ZnO is also strongly reducing, and one part reacts with H₂O to produce hydroxyl radicals, followed by participating in the mineralization of pollutant molecules to produce CO₂ and H₂O; the other part reduces H₂O to H₂ in the presence of a sacrificial agent. The composite material transfers photogenerated carriers through the "Z-type" energy band structure theory and undergoes photodegradation and hydrogen production from photolytic water by the combined action of three different active species, which corresponds to the results of the capture experiments presented above.

In this paper, on the basis of hydrothermal and photoreduction methods, skeleton-like Pt/Zn_0.25Cd_0.75S QDs/ZnO composite consisting of nanoparticles was successfully prepared to show a broad spectral response. The composite Pt/Zn_0.25Cd_0.75S QDs/ZnO exhibits efficient photocatalytic activity in multi-mode photocatalytic experiments. The combination of solid solution Zn_0.25Cd_0.75S QDs leads to a red shift of the light absorption range compared with ZnO, and at the same time, the synergistic effect between Zn_0.25Cd_0.75S QDs/ZnO and the noble metal Pt with SPR effect enhances the utilization efficiency of solar energy. In addition, Pt as an electron acceptor can effectively reduce the photogenerated electron–hole recombination efficiency, which improves the total quantum efficiency of the photocatalytic system. This paper provides a simple and effective method to improve the photocatalytic performance of ZnO semiconductors.

Ethical Approval

The submitted work is original and not have been published elsewhere in any form or language.

Funding

Funding was supported by National Natural Science Foundation of China (21376126), the Heilongjiang Provincial Natural Science Foundation of China (LH2021B031), the Fundamental Research Funds in Heilongjiang Provincial Universities of China (145109104), Scientific Research Foundation for Scholars funded by Zhejiang University of Technology (2022181010002), Innovation Project of Qiqihar University Graduate Education (YJSCX2020037), College Students' Innovative Entrepreneurial Training Program Funded Projects of Qiqihar University (202310232026X, S202310232132) and Qiqihar University in 2020 College Students Academic Innovation Team Funded Projects.

Availability of date and materials

Data and materials will be made available on request.

Conflicts of interest

The authors declare no competing financial interest.

Author Contribution

Yating Liu: Conceptualization; Methodology; experiments and collect data; data analysis; Writing-Original draft preparation; Research on reaction mechanism.Xiaodan Zhao: experiments and collect data; data analysisTianyu Hu: Draft articles and make critical revisions for important intellectual content; Fund supportYingru Sun: Photocatalytic degradation performance test; Hydrogen production performance test by photolysis of waterSiyu Li: Sample preparation; Software; Investigation.Siqi Ding: Instrument reservation; Visualization; Investigation.Yan Yu: Provide technical support for instrument operation; SupervisionLi Li: Make a significant contribution to the research concept and design, data analysis and interpretation; Draft articles and make critical revisions for important intellectual content; Fund support

Acknowledgements

This study is supported by the National Natural Science Foundation of China (21376126), the Heilongjiang Provincial Natural Science Foundation of China (LH2021B031), the Fundamental Research Funds in Heilongjiang Provincial Universities of China (145109104), Scientific Research Foundation for Scholars funded by Zhejiang University of Technology (2022181010002), Innovation Project of Qiqihar University Graduate Education (YJSCX2020037), College Students' Innovative Entrepreneurial Training Program Funded Projects of Qiqihar University (202310232026X, S202310232132) and Qiqihar University in 2020 College Students Academic Innovation Team Funded Projects.

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Synergistic effect of noble metal Pt and Zn_0.25Cd_0.75S QDs on ZnO: Broad spectral response and its outstanding photocatalytic performance

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental sections

2.1 Preparation of Pt/Zn_0.75Cd_0.25S QDs/ZnO

3. Results and discussion

3.1 XRD analysis

3.2 UV–vis/DRS analysis

3.3 SEM and SEM-EDS analysis

3.4 TEM and HR-TEM analysis

3.5 XPS analysis

3.6 N₂ adsorption–desorption analysis

3.7 Photoluminescence analysis

3.8 Electrochemical testing

3.9 Photocatalytic experiment results of Pt/Zn_0.25Cd_0.75S QDs/ZnO composite

3.10 Possible photocatalytic reaction mechanism of Pt/Zn_0.25Cd_0.75S QDs/ZnO

4. Conclusion

Declarations

Ethical Approval

Funding

Availability of date and materials

Conflicts of interest

Author Contribution

Acknowledgements

References

Additional Declarations

Supplementary Files

Status:

Version 1

Synergistic effect of noble metal Pt and Zn0.25Cd0.75S QDs on ZnO: Broad spectral response and its outstanding photocatalytic performance

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental sections

2.1 Preparation of Pt/Zn0.75Cd0.25S QDs/ZnO

3. Results and discussion

3.1 XRD analysis

3.2 UV–vis/DRS analysis

3.3 SEM and SEM-EDS analysis

3.4 TEM and HR-TEM analysis

3.5 XPS analysis

3.6 N2 adsorption–desorption analysis

3.7 Photoluminescence analysis

3.8 Electrochemical testing

3.9 Photocatalytic experiment results of Pt/Zn0.25Cd0.75S QDs/ZnO composite

3.10 Possible photocatalytic reaction mechanism of Pt/Zn0.25Cd0.75S QDs/ZnO

4. Conclusion

Declarations

Ethical Approval

Funding

Availability of date and materials

Conflicts of interest

Author Contribution

Acknowledgements

References

Additional Declarations

Supplementary Files

Status:

Version 1

Synergistic effect of noble metal Pt and Zn_0.25Cd_0.75S QDs on ZnO: Broad spectral response and its outstanding photocatalytic performance

2.1 Preparation of Pt/Zn_0.75Cd_0.25S QDs/ZnO

3.6 N₂ adsorption–desorption analysis

3.9 Photocatalytic experiment results of Pt/Zn_0.25Cd_0.75S QDs/ZnO composite

3.10 Possible photocatalytic reaction mechanism of Pt/Zn_0.25Cd_0.75S QDs/ZnO