Composite photocatalysts based on Cd_1−xZn_xS and TiO₂ for hydrogen production under visible light: effect of platinum co-catalyst location

Abstract

Ternary composite photocatalysts based on titania and solid solutions of CdS and ZnS were prepared and studied by a set of physicochemical methods including XRD, XPS, HRTEM, UV-vis spectroscopy, and electrochemical tests. Two synthetic techniques of platinization of Cd_1−xZn_xS/TiO₂ were compared. In the first case, platinum was deposited on the surface of synthesized Cd_1−xZn_xS (x = 0.2–0.3)/TiO₂ P25; in the second one, Cd_1−xZn_xS (x = 0.2–0.3) was deposited on the surface of Pt/TiO₂ P25. The photocatalytic properties of the obtained samples were compared in the hydrogen evolution from TEOA aqueous solution under visible light (λ = 425 nm). The Cd_1−xZn_xS (10–50 wt%; x = 0.2–0.3)/Pt (1 wt%)/TiO₂ photocatalysts demonstrated much higher photocatalytic activity than the Pt (1 wt%)/Cd_1−xZn_xS (10–50 wt%; x = 0.2–0.3)/TiO₂ ones. It turned out that the arrangement of platinum nanoparticles precisely on the titanium dioxide surface in a composite photocatalyst makes it possible to achieve efficient charge separation according to the type II heterojunctions and, accordingly, a high rate of hydrogen formation. The highest photocatalytic activity was demonstrated by 20% Cd_0.8Zn_0.2S/1% Pt/TiO₂ in the amount of 26 mmol g⁻¹ h⁻¹ (apparent quantum efficiency was 7.7%) that exceeds recently published values for this class of photocatalysts.

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1. Introduction

Currently, particular attention is focused on novel methods for renewable energy production. The finite nature of fossil resources (such as coal, crude oil, and natural gas) stimulates the search for and development of alternative energy technologies and fuel sources. Hydrogen is considered as the most prospective fuel.^1–3 However, current hydrogen production capacities are obviously insufficient for making significant progress towards hydrogen energy. Photocatalytic hydrogen production attracts particular attention as it, first, uses renewable sources, such as water and solar light, and, second, integrates hydrogen evolution and water purification from organic impurities,^4–6 and thus ensures environment-friendly energy generation.

Triethanolamine (TEOA) is one of the most popular sacrificial reagents.^4–6 It has a wide range of application areas, for example, surfactant production, medicine, cosmetology, etc. Purification of water from TEOA is an important practical task. It is well-known that photocatalytic methods allow one to decompose various organic compounds, including amines,^7–15 with hydrogen evolution. Photocatalytic hydrogen production from aqueous TEOA solutions is often carried out over g-C₃N₄-based^4,5,8–10 or composite g-C₃N₄/TiO₂-based^11–15 photocatalysts. Titanium dioxide is most often used as a semiconductor photocatalyst for various applications, because TiO₂ is a relatively cheap and non-toxic material.^16–18 The main disadvantage of using pure TiO₂ for photocatalytic hydrogen production is its insensitivity to visible light owing to wide band gap of 3.2 eV,¹⁹ and, consequently, low-efficient functioning under the influence of sunlight. An effective method for sensitizing titanium dioxide is the creation of composite systems with interfacial heterojunctions, in which TiO₂ contacts a semiconductor that absorbs visible light, for example, cadmium sulfide or Cd_1−xZn_xS solid solution.^20,21 However, photocatalytic hydrogen evolution from TEOA using photocatalysts based on TiO₂ and CdS–ZnS solid solutions has been studied insufficiently as yet.

One of the most appropriate methods for enhancing the activity of a semiconductor photocatalyst consists in depositing co-catalysts on its surface, mainly noble metals, especially platinum.^9,10 Also, heterostructure creation improves the activity of the semiconductor photocatalyst in the hydrogen production.^22–24 Previously, platinum was deposited on the surface of synthesized composites CdS/TiO₂ or Cd_1−xZn_xS/TiO₂.^25–29 However, in recent works it was clearly shown that the deposition of platinum between two semiconducting phases, sulfide and titania, is more efficient.^30,31 Usually, for CdS/TiO₂ and Cd_1−xZn_xS/TiO₂ photocatalysts, the type II heterojunctions were considered,^25–30 however, in recent works, the Z-scheme³¹ or S-scheme^32–34 charge transfer mechanisms are assumed. It is interesting to define what location of platinum is preferable for hydrogen production from TEOA solutions, and therefore which type of heterojunctions is realized. Earlier, we have shown for the photocatalytic CO₂ reduction that the order in which the components are deposited affects the activity of the ternary Pt/Cd_1−xZn_xS/TiO₂ photocatalysts.²⁰

Thus, in this research we compared two synthetic techniques of platinization of Cd_1−xZn_xS/TiO₂. In the first case, platinum was deposited on the surface of synthesized Cd_1−xZn_xS (x = 0.7–0.8)/TiO₂ P25; in the second one, Cd_1−xZn_xS (x = 0.7–0.8) was deposited on the surface 1%Pt/TiO₂ P25. The photocatalytic properties of these samples were compared in hydrogen evolution from TEOA aqueous solution under visible light (λ = 425 nm). A comprehensive comparison of the photocatalytic properties of the two type photocatalysts, Pt/Cd_1−xZn_xS/TiO₂ and Cd_1−xZn_xS/Pt/TiO₂, based on the detailed characterization of the samples, was carried out for the first time. We have defined the location and electron properties of platinum and built an electron transfer scheme for two types of photocatalysts.

2. Experimental

2.1. Photocatalyst preparation

The composite photocatalysts y% Cd_l−xZn_xS/TiO₂ (x is a molar ratio [Zn]/([Zn]+[Cd]), x = 0.3 or 0.2; y is a weight content of Cd_l−xZn_xS, y = 10, 20, 50) were synthesized by the following method. TiO₂ Evonik P25 was dispersed in water, after that the proper amount of 0.1 M CdCl₂ and 0.1 M Zn(NO₃)₂ was added. At the first stage, 0.1 M NaOH was added, and the reaction mixture was stirred during 15 min. After that the 2.5-fold excess of 0.1 M Na₂S was added. The obtained mixture was stirred for 60 min, and then the samples were washed with water and dried at 50 °C.

The ternary systems were prepared by two different ways.²⁰ In the first case, 1 wt% of Pt was deposited on the surface of y% Cd_l−xZn_xS/TiO₂ samples. In general, platinization of the photocatalyst was carried out by the method described elsewhere.³⁵ The photocatalyst was impregnated by hexachloroplatinic acid solution (0.1 M); after that the 2.5-fold excess of sodium borohydride was added. The mixture was stirred during 20 min, then the precipitate was collected, washed out by distilled water and dried at 50 °C. The obtained samples were marked as Pt-yCd(1 − x)-P25, e.g. Pt-10Cd0.8-P25 (1% Pt/10% Cd_0.8Zn_0.2S/TiO₂ Evonik P25).

In the second case, the corresponding values of Cd_1−xZn_xS were precipitated on 1% Pt/TiO₂ surface. These samples were labeled as yCd(1 − x)-Pt-P25. The deposition of Cd_l−xZn_xS was carried out by the method described above. The abbreviations of some samples are given in Table 1.

Table 1 Textural and optical properties of y% Cd_l−xZn_xS/TiO₂ samples with x = 0.2, 0.3

Sample	Abbreviation	SBET, m^2 g^−1	Vpore, cm^3 g^−1	APD,^a nm	Absorption edge, nm	Band gap, eV
a average pore diameter calculated from adsorption–desorption data.
Cd0.8Zn0.2S	—	125	0.24	8	512	2.42
10% Cd0.8Zn0.2S/TiO2	10Cd0.8-P25	52	0.47	36	471	2.63
20% Cd0.8Zn0.2S/TiO2	20Cd0.8-P25	66	0.48	29	482	2.57
50% Cd0.8Zn0.2S/TiO2	50Cd0.8-P25	68	0.37	22	479	2.59
Cd0.7Zn0.3S	—	112	0.19	7	500	2.48
10% Cd0.7Zn0.3S/TiO2	10Cd0.7-P25	51	0.45	43	463	2.68
20% Cd0.7Zn0.3S/TiO2	20Cd0.7-P25	68	0.39	23	464	2.67
50% Cd0.7Zn0.3S/TiO2	50Cd0.7-P25	58	0.31	22	470	2.64
TiO2 P25 (anatase/rutile)	P25	55	0.49	11	400	3.10

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2.2. Sample characterization

The samples were characterized by various physicochemical techniques. The phase composition of the photocatalysts was examined using a D8 Advance diffractometer (Bruker) with Cu Kα radiation. The optical properties of the samples were studied by diffuse reflectance spectroscopy in the range of 250–800 nm. The spectra were recorded with a Shimadzu UV-2501 spectrophotometer with an ISR-240A diffuse reflectance unit. The specific surface areas (S_BET), pore volumes, and average pore sizes of the photocatalysts were obtained from the low temperature N₂ adsorption–desorption (N₂ adsorption at 77 K) by using an ASAP 2400 apparatus.

The structure and microstructure of the photocatalysts were studied by HRTEM using a ThemisZ electron microscope (Thermo Fisher Scientific, USA) operated at an accelerating voltage of 200 kV. The microscope was equipped with a corrector of spherical aberrations, which provided a maximum lattice resolution of 0.06 nm, and a SuperX spectrometer (TFS, USA). Images were recorded using a Ceta 16 CCD sensor (Thermo Fisher Scientific, USA). For electron microscopy studies, the samples were deposited on perforated carbon substrates attached to aluminum grids using an ultrasonic dispersant.

The XPS measurements were performed with a photoelectron spectrometer SPECS using Al Kα irradiation (hν = 1486.6 eV; 200 W). Binding energy (BE) scale was preliminarily calibrated by the position of the peaks of Au 4f_7/2 (BE = 84.0 eV) and Cu 2p_3/2 (BE = 932.67 eV) core levels. The binding energy of peaks was calibrated by the position of the C 1s peak at 284.8 eV (ref. 36) corresponding to the hydrocarbons presented on the samples surface. The spectra analysis and peak fitting were performed with XPS Peak 4.1 software. For the quantitative analysis, the integral intensities of the spectra were corrected by their respective atomic sensitivity factors.³⁷

2.3. Photocatalytic hydrogen production

The catalytic properties of the prepared photocatalysts were studied in the hydrogen evolution from alkaline solution of triethanolamine. A typical procedure was described earlier.³⁵ The experimental set-up for the photocatalytic activity measurements is described in detail in ESI (Fig. S1a†). A mixture containing 10 mL of triethanolamine (TEOA), 90 mL of 0.11 M NaOH, and 50 mg of the tested photocatalyst was placed to a reactor. Then the reactor was purged with argon for removing oxygen, after that the system was illuminated with 425-LED (56 mW cm⁻², Fig. S1b†). The hydrogen concentration was measured by a gas chromatograph “Khromos GKh-1000” (Khromos, Russia). The apparent quantum efficiency (AQE) was calculated by the following formula:
where W – reaction rate (μmol min⁻¹), N_ph – photon flux (μeinstein min⁻¹). For all experiments N_ph = 285 μeinstein min⁻¹.

2.4. Photoelectrochemical experiments

Photoelectrochemical experiments were conducted using a two-electrode cell (see Fig. S1c and d†).³⁸ The working electrode was FTO with the photocatalyst deposited by the drop-casting method described earlier.³⁸ The counter electrode was Cu₂S/brass, the electrolyte was a solution consisting of 1 M Na₂S_n and 0.1 M NaCl. Photoelectrochemical characteristics were obtained with using a potentiostat–galvanostat P-45X (Russia). The current–time curves were registered at 0 V. The impedance data were recorded over a frequency range of 0.8 to 10⁵ Hz with amplitude of 10 mV at 0.2 V. The light source was 425-LED (56 mW cm⁻², Fig. S1b†).

3. Results and discussion

3.1. Photocatalyst characterization

3.1.1. XRD analysis. Bulk properties of the synthesized samples were studied by XRD technique. Fig. 1 shows the XRD patterns of the y% Cd_l−xZn_xS/TiO₂ photocatalysts (x = 0.2–0.3, y = 10, 20, 50 wt%) and Evonik P25. One can see that commercial Evonik P25 consists of anatase (PDF # 21-1271) and rutile (PDF # 21-1276) phases. These phases are found in all composite samples. Deposition of solid solutions of CdS and ZnS does not lead to any changes in the position and intensity of titania peaks. The average crystalline sizes of rutile and anatase phases are the same for all studied samples and equal 30 and 19 nm, respectively. Peaks of the Cd_l−xZn_xS phases are not identified for samples containing 10 wt% of the corresponding solid solution. However, as seen in Fig. 1a and b, the increase in the amount of the solid solutions leads to the growth of the intensity of peak located at ∼44°. This peak may be attributed to (220) reflex of the Cd_l−xZn_xS (x = 0.2 and 0.3) solid solutions with a disordered structure.³⁹ Other typical peaks of Cd_l−xZn_xS are not observed in the XRD patterns, may be due to small particle size of Cd_l−xZn_xS. The x parameter in Cd_1−xZn_xS was determined using the Vegard rule; the reference points were ZnS (PDF # 05-0566, lattice constant a = 5.406 Å) and CdS (PDF # 42-1411, lattice constant a = 5.818 Å) with a cubic structure. It was shown that the sample 50% Cd_0.8Zn_0.2S/TiO₂ contains Cd_1−xZn_xS solid solution with a lattice parameter equal to 5.75 Å (x ∼0.2); whereas the sample 50% Cd_0.7Zn_0.3S/TiO₂ contains Cd_1−xZn_xS solid solution with a lattice parameter equal to 5.71 Å (x ∼0.3). The average crystalline size of Cd_1−xZn_xS estimated with Scherer's equation was less than 2 nm.

Fig. 1 The XRD patterns of photocatalysts y% Cd_l−xZn_xS/TiO₂ with x = 0.3 (a) and x = 0.2 (b). TiO₂ Evonik P25 is labeled as P25. PDF # 21–1271 and PDF # 21–1276 cards were used for anatase and for rutile identification.

3.1.2. Textural properties. The textural properties of the composites and pristine Cd_l−xZn_xS and TiO₂ are presented in Table 1. Table 1 shows that the surface areas of the composite materials 10% Cd_0.8Zn_0.2S/TiO₂ and 10% Cd_0.7Zn_0.3S/TiO₂ are close to S_BET of TiO₂ P25. The increase in solid solution content to 20 wt% leads to the growth of the surface area due to the increase in the content of Cd_l−xZn_xS phase with a high surface area. However, the further increase of sulfide content in the samples to 50 wt% does not lead to the increase in the surface area. Earlier, it was demonstrated that if the content of Cd_l−xZn_xS is much than 40 wt%, sulfide particles over titania surface form aggregates.⁴⁰ Table 1 also shows that pore volume gradually falls with the increase of solid solution content for y% Cd_l−xZn_xS/TiO₂ samples likely due to the deposition of sulfides inside the pores of titania.

3.1.3. UV-vis spectroscopy. UV-visible diffuse reflectance spectra were recorded to study the optical properties of the y% Cd_l−xZn_xS/TiO₂ photocatalysts (x = 0.2–0.3, y = 10, 20, 50 wt%) and Cd_l−xZn_xS solid solutions. Fig. 2a and b demonstrate that titanium dioxide absorbs predominantly UV-light^41–43 while Cd_l−xZn_xS are sensitive to visible light.^44–46 One can see that the obtained spectra of the prepared composites are the superposition of the individual spectra of TiO₂ and Cd_0.8Zn_0.2S or Cd_0.7Zn_0.3S, respectively. The UV-vis spectroscopy proves the multiphase composition of the obtained photocatalysts. It is worth noting that the addition of Cd_l−xZn_xS to TiO₂ expands its light absorption in the visible region and changes the absorption edges. Fig. 2c and d show the F(R) value of the synthesized photocatalysts; further, the absorption edges of the composite samples and Cd_l−xZn_xS were estimated using the Tauc function for direct semiconductors, F(R)²(hν)² (Fig. 2e and f); the adsorption edge of TiO₂ was determined using this function for indirect semiconductors, F(R)^1/2(hν)^1/2 (not shown). Band gap values of the prepared samples were calculated from absorption edges and summarized in Table 1. Table 1 shows that the absorption edges of Cd_0.8Zn_0.2S and Cd_0.7Zn_0.3S are equal to 512 and 500 nm, respectively; for TiO₂, the absorption edge is equal to ca. 400 nm, likely due to the presence of rutile phase in TiO₂ Evonik P25. The absorption edges of the prepared Cd_l−xZn_xS/TiO₂ photocatalysts are 460–470 nm and 470–480 nm for Cd_0.7Zn_0.3S/TiO₂ and Cd_0.8Zn_0.2S/TiO₂, respectively, as shown in Table 1. This blueshift may be attributed to the changes in the electronic structures of the samples and quantum-size effect.⁴⁰ Note that all composite samples absorb visible light with the wavelength of 425 nm, which is used in the light source in this study.

Fig. 2 UV-vis spectra (a and b), F(R) function (c and d), and Tauc plots for direct semiconductor (e and f) of the y% Cd_1−xZn_xS/TiO₂ samples with x = 0.2 (a, c and e) and x = 0.3 (b, d and f). Emission spectra of 425-LED are shown in (d).

3.1.4. XPS analysis. The photocatalysts were investigated by the XPS method in details (Table 2). Fig. S2† shows that all tested samples are composed of six elements – Cd, O, Pt, S, Ti, and Zn. The XPS spectra of the samples Pt-20Cd0.8-P25, Pt-50Cd0.8-P25, and 50Cd0.8-Pt-P25 are shown in Fig. 3. The binding energies of Cd 3d (see Fig. 3a) and Zn 2p (see Fig. 3b) were equal to 405.4 eV and 1022.3 eV (ref. 47–49) for all samples, which may be attributed to the sulfides of the corresponding metals. Fig. 3c demonstrated thar the S 2p spectrum consists of two main peaks located at 161.8 eV and 169 eV. These peaks correspond to S²⁻ and SO₃²⁻/SO₄²⁻ forms, respectively.^40,50 The Ti 2p spectrum (not shown) displays the characteristic peak at 458.8 eV, which is typical for Ti⁴⁺ in the titanium dioxide.³⁶ Additionally, the atomic ratios were calculated by XPS, and are also given in Table 2. According to the XPS data, x in Cd_l−xZn_xS is equal to 0.25–0.35 that is in a good agreement with a theoretical value (x = 0.2). One can see that the [Cd + Zn]/[Ti] surface ratio is equal to 0.51–0.57 for samples Pt-20Cd0.8-P25 and 20Cd0.8-Pt-P25 and reaches about 0.7 for samples Pt-50Cd0.8-P25 and 50Cd0.8-Pt-P25. Thus, a good coverage of the titania surface is achieved in both cases. Also, it is likely that in samples with a lower sulfide content more dispersed particles are formed. The main difference between the samples prepared by different methods is the state of platinum. Fig. 3d shows the Pt 4f XPS spectra of three samples – 50Cd0.8-Pt-P25, Pt-50Cd0.8-P25, and Pt-20Cd0.8-P25. One can see that in sample Pt-20Cd0.8-P25 platinum exists in two forms, namely, as metal with the binding energy of 71.2 eV and as oxidized Pt²⁺ in PtO with the binding energy of 72.6 eV.^51,52 Only metallic platinum is observed in photocatalyst 50Cd0.8-Pt-P25, whereas only oxidized Pt²⁺ was detected in photocatalyst Pt-50-Cd0.8-P25. Table 2 demonstrates that only metallic platinum is found in samples prepared by the Cd_0.8Zn_0.2S deposition on 1% Pt/TiO₂, 20Cd0.8-Pt-P25 and 50Cd0.8-Pt-P25. Thus, the preparation method, in which first platinum is deposited on titanium dioxide, and then a solid solution of cadmium and zinc sulfides is deposited, leads to the formation of metallic platinum, likely due to the “sealing” of the metal particles. Earlier it was shown that metallic platinum enhances the photocatalytic activity of the samples.⁵³

Table 2 Surface properties of Pt-yCd0.8-P25 and yCd0.8-Pt-P25 (y = 20 and 50 wt%) based on XPS data

Sample	[Pt^0], %	[Pt^2+], %	[S^2−], %	[SOx^2], %	[Cd]/[Cd + Zn]	[Pt]/[Cd + Zn]	[Pt]/[Ti]	[Cd + Zn]/[Ti]
Pt-20Cd0.8-P25	36	64	84	16	0.74	0.019	0.013	0.51
20Cd0.8-Pt-P25	100	0	89	11	0.73	0.033	0.017	0.57
Pt-50Cd0.8-P25	0.0	1.0	85	15	0.65	0.028	0.019	0.70
50Cd0.8-Pt-P25	100	0	80	20	0.66	0.016	0.011	0.68

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Fig. 3 Cd 3d_5/2 (a), Zn 2p_3/2 (b), S 2p (c), Pt 4f (d) core-level XPS spectra of 50Cd0.8-Pt-P25, Pt-50Cd0.8-P25, and Pt-20Cd0.8-P25.

3.1.5. HRTEM and HAADF-STEM investigations. Samples Pt-20Cd0.8-P25 and 20Cd0.8-Pt-P25 were studied by the HRTEM and HAADF-STEM methods with EDX elemental mapping. Fig. 4a shows the HRTEM image of the Pt-20Cd0.8-P25 photocatalyst. One can see that there are three different types of particles in the analyzed sample. The first one are cylinder and spherical particles with a size up to 50 nm (Fig. 4a and b). The second type particles comprizes the solid solutions of cadmium sulfide and zinc sulfide with the average size of about 10 nm; these particles form aggregates with a size of 20–30 nm (Fig. 4a and c). The third found type of the particles are nanoclusters of platinum whose size reaches 5–6 nm, being quite large for noble metal particles. Fig. 4a and b show that platinum clusters are located both on the surface of sulfide and titania. Fig. 4g and h also demonstrate that Pt is deposited both on the surface of TiO₂ and Cd_l−xZn_xS solid solution. However, a careful examination of these figures reveals that there are islands of highly concentrated sulfur and platinum located at the same area (blue circles in Fig. 4g and h). Thus, platinum is predominantly loaded on the solid solutions of CdS and ZnS nanoparticles. In further mechanism considerarions, we assume that platinum is completely located on sulfide particles. EDS data (Fig. S3 and S4†) confirms that the sample consists of TiO₂ and Cd_l−xZn_xS, and Pt, at the same time, the distribution of particles of a solid solution of sulfides on the surface of titanium dioxide is not very uniform.

Fig. 4 HRTEM images (a and b), HAADF-STEM (c) and elemental mapping of Ti/Zn/Cd/Pt (d), Ti (e), O (f), S (g), Pt (h) of photocatalyst Pt-20Cd0.8-P25.

The TEM images and elemental mapping of the 20Cd0.8-Pt-P25 sample are summarized in Fig. 5. Images in Fig. 5a and b show the presence of titania with a particle size of 30–50 nm and clusters of disordered Cd_l−xZn_xS. One can see the platinum particles with a size of 2–4 nm (Fig. 5a and b). Also, element mapping shows that platinum is located both on titania particles and Cd_1−xZn_xS domains. To elucidate the location of platinum nanoparticles, we obtained the dark-field HAADF-STEM images of sample 20Cd0.8-Pt-P25 with a high resolution (Fig. 6a and b). One can see that platinum nanoparticles as to surround the titanium dioxide particles. Thus, according to the electron microscopy data, in sample 20Cd0.8-Pt-P25 the platinum clusters are located over titania and between two semiconductors and have smaller sizes than in sample Pt-20Cd0.8-P25. Thus, the electron microscopy data confirm that the Pt-20Cd0.8-P25 and Cd0.8-Pt-P25 photocatalysts differ only by platinum location and particle size, and have the same morphology of titania and cadmium and zinc sulfide solid solutions, and phase composition.

Fig. 5 HRTEM images (a and b), HAADF-STEM (c) and elemental mapping of Ti/Zn/Cd/Pt (d), Ti (e), O (f), S (g), Pt (h) of photocatalyst 20Cd0.8-Pt-P25.

Fig. 6 Dark field HAADF-STEM images of photocatalyst 20Cd0.8-Pt-P25 with a scale 10 nm (a) and 20 nm (b).

To sum up, the photocatalyst characterization has shown that two synthetic techniques – deposition of platinum over Cd_1−xZn_xS (x = 0.2–0.3)/TiO₂ P25 surface and deposition of Cd_1−xZn_xS (x = 0.2–0.3) over platinized Pt/TiO₂ P25 – lead to the formation of photocatalysts with the same bulk structure; however, in the first case quite large platinum nanoparticles are located predominantly over Cd_1−xZn_xS surface, platinum exists in the oxidized Pt²⁺ state; in the latter case, small platinum nanoparticles are located over titania surface and all platinum is in metallic state.

3.2. Photocatalytic activity

The activity of the synthesized photocatalysts was studied in the hydrogen evolution in alkaline solution of TEOA under visible light with a wavelength of 425 nm. It has been previously shown, that for CdS-based photocatalyst the use of TEOA as a sacrificial agent lead to the highest efficiency as compared to all other substrates. The results showed that amine might be strongly bound to the CdS surface and therefore effectively consume the holes as compared to alcohol and sugars.⁴ To facilitate the proton abstraction during the chain process of TEOA oxidation and therefore to prevent charge recombination the reaction was carried out in an 0.11 M NaOH aqueous solution.⁵⁴ N-bis (2-hydroxyethyl) hydroxylamine and acetaldehyde are recognized as the main TEOA oxidation by-products.⁴ First, the photocatalysts without platinum were tested; the results of these experiments are listed in Fig. 7a. The rate of hydrogen evolution over titanium dioxide under visible light was equal to zero; single-phase Cd_0.7Zn_0.3S and Cd_0.8Zn_0.2S possessed noticeable activity. Fig. 7a shows that the samples containing 10 wt% and 20 wt% of Cd_1−xZn_xS exhibited the same photocatalytic activities, which were similar to those of Cd_0.7Zn_0.3S and Cd_0.8Zn_0.2S. However, as the content of Cd_1−xZn_xS increases to 50 wt%, the hydrogen production accelerates rapidly. Note that the activity of composite 50% Cd_1−xZn_xS/TiO₂ P25 (x = 0.2–0.3) samples was 3.5-times higher that the activity of corresponding single phase solid solutions. Perhaps, this phenomenon may be caused by two reasons. Firstly, the enhancement of Cd_1−xZn_xS content in the photocatalyst increases the light absorption in the range where the LED emits, from 400 to 480 nm, which leads to the growth of the amount of the photoinduced electrons and holes and the photocatalytic activity. Secondly, it is well-known that heterojunctions between the solid solutions of CdS and ZnS and titania are realized^55,56 and formed at the phase separation boundaries. High content of small Cd_1−xZn_xS particles provides a higher quantity of possible heterojunctions that enhances electron lifetime and favors the photocatalytic hydrogen production. One can see that the composite Cd_1−xZn_xS/TiO₂ P25 samples containing Cd_0.7Zn_0.3S are more active than the photocatalysts with Cd_0.8Zn_0.2S solid solution. This trend was observed earlier for single phase solid solutions Cd_1−xZn_xS in hydrogen production from aqueous solutions of Na₂S/Na₂SO₃.^57–59 This phenomenon may be assigned to a higher conduction band position of Cd_0.7Zn_0.3S, which reflects the reduction ability of the semiconductor and favors the target reaction.

Fig. 7 Photocatalytic hydrogen production over Cd_1−xZn_xS/TiO₂ P25 (a), Pt-yCd(1 − x)-P25, and yCd(1 − x)-Pt-P25 (b). Conditions: C₀(TEOA) = 10 vol%, C₀(NaOH) = 0.1 M, V = 100 mL, C_cat = 0.5 g L⁻¹, light source – 425-LED. Point 100% means the activity of 1%Pt/Cd_0.7Zn_0.3S and 1%Pt/Cd_0.8Zn_0.2S.

To improve the photocatalytic properties of the composites, platinum was deposited by two methods. The first method includes Pt deposition on the surface of pre-synthesized y% Cd_l−xZn_xS/TiO₂ (samples Pt-yCd0.8-P25), while in the second method the Cd_l−xZn_xS solid solutions were loaded on the 1% Pt/TiO₂ (samples yCd0.8-Pt-P25). Also, platinum (1 wt%) was deposited on the surface of Cd_0.7Zn_0.3S and Cd_0.8Zn_0.2S. The catalytic activities of the studied photocatalysts are depicted in Fig. 7b. Analysis of the data in Fig. 7 allows conclusion that the depositon of platinum significantly (in some cases, more than an order of magnitude), improves the photocatalytic activity of the samples. The lowest hydrogen productivities, 2.9 and 2.6 mmol h⁻¹ g⁻¹, were possessed by the 1%Pt/Cd_0.7Zn_0.3S and 1%Pt/Cd_0.8Zn_0.2S single-phase samples, respectively. Composite platinized photocatalysts produced from 5 to 26 mmol h⁻¹ g⁻¹ of hydrogen (AQE = 1.4–7.7% at 425 nm) that exceed the latest published data (see Table 3 ^60–74). The highest activity was possessed by photocatalyst 20Cd0.8-Pt-P25 (20% Cd_0.8Zn_0.2S/1% Pt/TiO₂ P25). Fig. 7b also demonstrates that the yCd(1 − x)-Pt-P25 samples are more active than Pt-yCd(1 − x)-P25. The main reason of this observation is the different mechanisms by which electrons and holes migrate into the samples prepared according to two schemes.

Table 3 Comparison of the photocatalytic activity with the recently published data

	Sample	Sacrificial agents	Light source	Activity, mmol (H2) h^−1 g^−1	Ref.
1	Au/TiO2/gC3N4	1 vol% TEOA	150 W ceramic–metal-halide Hg lamp	0.4	11
2	3% Pt/2% Na2Fe2Ti6O16/g-C3N4	10 vol% TEOA	Xe lamp with 420 nm cut-off filter	0.4	60
3	1% Pt/W18O49/g-C3N4	10 vol% TEOA	Xe lamp, λ > 400 nm	0.9	61
4	1% C-dots/50% g-C3N4/TiO2	10 vol% TEOA	350 W Xe arc lamp	0.6	62
5	20% g-C3N4/TiO2	10 vol% TEOA	250 W visible light source	1.1	63
6	C-quantum dots/TiO2/Pt	0.33 M TEOA, pH = 9	Xe lamp with 420 nm cut-off filter	1.5	64
7	Pt–g-C3N4–(Au/TiO2)	25 vol% TEOA	Xe lamp with AM1.5 filter	1.52	65
8	18%CdS/2%Pt/N–NaNbO3	10 vol% TEOA	Xe lamp with 420 nm cut-off filter	1.8	66
9	40% CdS/4% PdAg/g-C3N4	10 vol% TEOA	Xe lamp with 420 nm cut-off filter	3.1	67
10	Pd0.22Pt0.78–TiO2/C	5 vol% TEOA	Xe lamp with 420 nm cut-off filter	2.9	68
11	N–TiO2/g-C3N4@NixP	10 vol% TEOA	Xe lamp with 350 nm cut-off filter	5.4	69
12	0.25% Pt/Eosin Y-doped TiO2	0.25 M TEOA, pH = 7	AM 1.5 G with 420 nm cut-off filter	3.6	70
13	1% Pt/SrTiO3	10 vol% TEOA, pH = 7, 0.1 g L^−1 eosin Y	Xe lamp with 420 nm cut-off filter	6.6	71
14	2.5% PtNix/g-C3N4	10 vol% TEOA	Xe lamp	8.5	72
15	0.33% Pt/TiO2/P-doped g-C3N4	10 vol% TEOA	Xe lamp with 420 nm cut-off filter	11.6	73
16	Cu/TiO2	17 vol% TEOA, pH = 6	Xe lamp with 420 nm cut-off filter	5.9	74
17	1%Pt/20% Cd0.8Zn0.2S/TiO2	10 vol% TEOA, 0.1 M NaOH	425-LED	11.5	This work
18	20% Cd0.8Zn0.2S/1%Pt/TiO2			26.4

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According to the difference in the positions of Cd_l−xZn_xS and TiO₂ bands in the ternary system, the following types of heterojunctions can take place: type II heterojunctions, Z-scheme, and S-scheme.³³ In the case of platinized photocatalyst, the Z-scheme heterojunctions are hardly possible owing to the formation of a Schottky barrier between Pt and Cd_l−xZn_xS or TiO₂.³³ Also, the type II heterojunctions between the semiconductors may be realized.^26,27 In this case, under light irradiation electrons and holes are generated in the Cd_1−xZn_xS conduction band (CB) and valence band (VB), respectively. Since the position of TiO₂ CB is lower than that of Cd_0.8Zn_0.2S, photoinduced electrons can migrate to CB of titania, whereas photogenerated holes remain in VB of sulfide solid solution. Thus, in this case, electrons are collected in CB of TiO₂, and holes – in VB of Cd_l−xZn_xS. On the contrary, when implementing S-scheme, photogenerated electrons remain in CB of Cd_l−xZn_xS, and holes – in VB of titania providing high reducing and oxidizing ability, respectively.^32–34 Platinum is an effective co-catalyst for hydrogen production. Thus, in the case of the S-scheme heterojunctions, it is advantageous for platinum to be located on the sulfide surface, and in the case of the type II heterojunctions – over titanium dioxide. In the present study, samples yCd(1 − x)-Pt-P25 with platinum deposited on titania surface are much more active than samples Pt-yCd(1 − x)-P25 with platinum located over Cd_l−xZn_xS nanoparticles that allows suggestion that exactly the type II heterojunctions occur in the system. Besides, Fig. 2d clearly shows that titanium dioxide does not absorb irradiation from the LED source used in the experiments, whereas Z-scheme and S-scheme assume the light absorption by both semiconductors.

Fig. 8 shows probable scheme of heterojunctions for Pt-yCd(1 − x)-P25 and yCd(1 − x)-Pt-P25 photocatalysts. As described above, the photoinduced electrons can migrate to the CB of titanium dioxide, whereas photogenerated holes remain in the VB of the sulfide solid solution. Also, for Pt-yCd(1 − x)-P25 (Fig. 8a), photogenerated electrons can move to platinum deposited on the surface of Cd_1−xZn_xS; in this case both electrons and holes remain on the surface of Cd_1−xZn_xS. The activity of composite samples Pt-y-Cd(1 − x)-P25 (y = 20–50%; x = 0.2–0.3) reaches the level of ∼10 mmol h⁻¹ g⁻¹, and at least 3.5–4 times exceeds the activity of photocatalysts 1%Pt/Cd_0.7Zn_0.3S and 1%Pt/Cd_0.8Zn_0.2S. Thus, it turns out that the type II heterojunctions provide efficient spatial separation of charges and improve the catalyst activity in the hydrogen evolution. However, the CB position of TiO₂ vs. NHE is not negative enough (ca.−0.1 V), with a low reducing ability of electrons; moreover, judging by the TEM data, platinum is deposited mainly on sulfide nanoparticles, and reduction proceeds over pristine titania. Therefore, the activity in the hydrogen production is not so high in comparison to the photocatalysts, synthesized according to the second method.

Fig. 8 Scheme of the possible type II heterojunctions realized in the case of the Pt-yCd(1 − x)-P25 (a) and yCd(1 − x)- Pt-P25 (b) photocatalyst.

Fig. 7 shows that photocatalysts yCd(1 − x)-Pt-P25 (y = 10–20%; x = 0.2) possess the highest activity, 23–26 mmol h⁻¹ g⁻¹ of hydrogen, more than twice exceeding that of samples with the same bulk structure, Pt-yCd(1 − x)-P25 (y = 10–20%; x = 0.2). In the yCd(1 − x)-Pt-P25 photocatalysts, platinum particles are located over titania nanoparticles. In this case, photogenerated electrons first migrate to CB of TiO₂ and then to platinum particles, deposited over the surface of titania; photogenerated holes remain in the Cd_1−xZn_xS VB (Fig. 8b). High activity of the yCd(1 − x)-Pt-P25 (y = 10–20%; x = 0.2) samples confirms that the transfer of electrons in this direction has a positive effect on the separation of photogenerated charges. Pt particles at the interface between sulfide and titanium dioxide can also be effective co-catalysts for the evolution of hydrogen, although their number is less than the number of Pt particles on the TiO₂ surface. Another reason for the high activity of yCd(1 − x)-Pt-P25 is that it contains Pt only in the metallic state, while in Pt-yCd(1 − x)-P25 platinum occurs both as Pt⁰ and Pt²⁺. The metallic platinum accelerates the photocatalytic reaction to a greater extent than the oxidized one.⁵⁴ Certainly, it is difficult to provide unambiguous arguments in favor of a particular heterojunction mechanism, but our experiments clearly show that the deposition of platinum on titanium dioxide, followed by deposition of Cd_1−xZn_xS, leads to the production of highly active photocatalysts. Indeed, the activity of the 20% Cd_0.8Zn_0.2S/1% Pt/TiO₂ P25 photocatalyst in H₂ evolution by an order of magnitude exceeds that of a platinized solid solution 1%Pt/Cd_0.8Zn_0.2S. At the same time, our preliminary studies of these systems in the reduction of CO₂ showed that the deposition of platinum on the particles of sulfide solid solutions was preferable.²⁰

Stability is one of the most important characteristics of a photocatalyst. Thus, stability tests were carried out with samples Pt-20Cd0.8-P25 and 20Cd0.8-Pt-P25 (Fig. 9). After four 1.5 h runs, the hydrogen production rate was 57 and 72% of the initial rate for photocatalysts Pt-20Cd0.8-P25 and 20Cd0.8-Pt-P25, respectively. That is, the sample of the second type, 20Cd0.8-Pt-P25, possesses the higher activity and stability, most likely due to the improved charge separation. Unfortunately, it is very difficult to obtain highly stable composite sulfide photocatalysts, because photogenerated holes oxidize sulfide solid solutions.⁵³

Fig. 9 Stability tests for photocatalyst Pt-20Cd0.8-P25 and 20Cd0.8-Pt-P25. Conditions: C₀(TEOA) = 10 vol%, C₀(NaOH) = 0.1 M, V = 100 mL, C_cat = 0.5 g L⁻¹, light source – 425-LED. The reactor was purged with argon after every run.

3.3. Photoelectrochemical tests

The photoelectrochemical properties of 1% Pt/TiO₂, 20% Cd_0.8Zn_0.2S/TiO₂, and 20Cd0.8-Pt-P25 were studied. Fig. 10a demonstrates that composite photocatalyst 20Cd0.8-Pt-P25 prepared by the second route provides a higher photocurrent generation than both platinized titania and 20% Cd_0.8Zn_0.2S/TiO₂. The main reason of this fact is high photogenerated charge separation associated with the type II heterojunctions which is realized under visible light. It is seen also that photocurrent density obtained over 20Cd0.8-Pt-P25 decreases during first two on–off cycles, and remains constant afterwards. In opposite, the photocurrent densities recorded over 1% Pt/TiO₂ and 20% Cd_0.8Zn_0.2S/TiO₂ continuously decline due to photocorrosion processes, such as self-oxidation of the CdS and ZnS solid solutions. The ternary composite photocatalyst is photoelectrochemically active and stable. The samples were investigated by the impedance spectroscopy and electron lifetimes were calculated.³⁸ The electron lifetimes of 1% Pt/TiO₂, 20% Cd_0.8Zn_0.2S/TiO₂, 20Cd0.8-Pt-P25 were 2.7 ms, 5.8 ms, and 8.9 ms, respectively. One can conclude that the higher electron lifetime values favor both the photocatalytic hydrogen production and photocurrent generation.

Fig. 10 The I–t curves (a) and Nyquist plots (b) of 1% Pt/TiO₂, 20% Cd_0.8Zn_0.2S/TiO₂, and 20Cd0.8-Pt-P25; electrical equivalent-circuit model used for the impedance data approximation (c).

Fig. 10b shows the Nyquist plots obtained over the tested photoelectrodes. The main trend observed is that low values of impedance semicircle radii led to enhanced photocurrent density. Additionally, the impedance data was approximated by the equivalent-circuit model suggested earlier.⁷⁵ The model consists of the series resistance R_D in parallel with two resistance–capacitance loops (R_W–C₁, R_K–C₂) (see Fig. 10c). The series resistance R_D includes all the contact and material resistances, such as resistance of the FTO surface and electrolyte solution. R_W and C₁ were attributed to the charge-transfer resistance and double-layer capacity for the processes at the photoelectrode/electrolyte interface, whereas R_K and C₂ described the electrochemical processes at the counter electrode/electrolyte interface. It was shown that series resistance R_D values did not exceed 10 Ohm that was typical of the electrolyte resistance. Parameters R_W and R_K decreased in the consequence 1% Pt/TiO₂/FTO > 20% Cd_0.8Zn_0.2S/TiO₂/FTO > 20Cd0.8-Pt-P25. It should be noted that photocurrent density and electron lifetime grew in the same consequence. The ternary composite photoelectrode demonstrated low resistivity and high electron lifetime. It means that the charge separation in the space was more efficient for the 20Cd0.8-Pt-P25/FTO sample as proved by the electrochemical methods.

4. Conclusions

Ternary composite samples consisting of Cd_1−xZn_xS, titania P25 and platinum were synthesized using a different sequence of metal deposition. In the first case, platinum (1 wt%) was deposited on the surface of synthesized Cd_1−xZn_xS (x = 0.2–0.3)/TiO₂ P25; in the second case Cd_1−xZn_xS (x = 0.7–0.8) was deposited on the surface platinized titania. This approach made it possible to obtain photocatalysts with the same bulk structure while the properties of deposited platinum were rather different. According to the data of physicochemical studies, including HRTEM with mapping, in photocatalysts synthesized by the first method (1% Pt/Cd_l−xZn_xS/TiO₂) relatively large platinum particles are located predominantly on the Cd_1−xZn_xS surface, Pt can exist both in metallic Pt⁰ and oxidized Pt²⁺ states; photocatalysts synthesized by the second method (Cd_l−xZn_xS/1% Pt/TiO₂) contain small platinum particles which are located on the TiO₂ surface, the whole platinum is in metallic state.

The photocatalytic properties were tested in hydrogen production from alkaline aqueous solution of TEOA under visible light irradiation with a wavelength of 425 nm. It was found that the y% Cd_l−xZn_xS/1% Pt/TiO₂ photocatalysts demonstrated much higher photocatalytic activity than y% 1% Pt/Cd_l−xZn_xS/TiO₂ samples. Thus, the arrangement of platinum nanoparticles precisely on titanium dioxide surface in a composite photocatalyst makes it possible to achieve efficient charge separation according to type II heterojunctions and, accordingly, a high rate of hydrogen formation. Also, the deposition of a solid solution of sulfides on the surface of platinized titania makes it possible to “seal” the platinum particles and keep them in a metallic state, whereas deposition of platinum over Cd_1−xZn_xS/TiO₂ composite leads to formation of mixed Pt/PtO_x co-catalyst.

The highest photocatalytic activity was observed for 20% Cd_0.8Zn_0.2S/1% Pt/TiO₂ which provided hydrogen production of 26 mmol g⁻¹ h⁻¹ (apparent quantum efficiency was 7.7%) that exceeds the latest published values. This photocatalyst possessed high photoelectrochemical stability. It should be noted that, despite the apparent simplicity, the determination of the preferred location of platinum particles in the TiO₂–Cd_1−xZn_xS systems was carried out for the first time.

Authors contributions

A. V. Zhurenok: conceptualization, validation, visualization, writing—original draft preparation; D. V. Markovskaya: investigation, data curation, visualization, writing—original draft preparation; E. Yu. Gerasimov: investigation, visualization; S. V. Cherepanova: data curation, formal analysis; A. V. Bukhtiyarov: data curation, formal analysis; E. A. Kozlova: writing—original draft preparation, supervision, project administration, funding acquisition. All authors have read and agreed to the published version of the manuscript.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the Ministry of Science and Higher Education of the Russian Federation within the governmental order for Boreskov Institute of Catalysis (project AAAA-A21-121011390009-1) and also was partially supported by the Russian Foundation for Basic Research (project no. 20-33-70086). The authors are grateful to T. V. Larina for DRS analysis. The studies were conducted using the equipment of the Center of Collective Use “National Center of Catalyst Research”.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra06845h

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