Multi-mode photocatalytic degradation and photocatalytic hydrogen evolution of honeycomb-like three-dimensionally ordered macroporous composite Ag/ZrO₂

Abstract

A three-dimensionally ordered macroporous (3DOM) composite, 3DOM Ag/ZrO₂, was prepared by polystyrene (PS) colloidal spheres as a template, combined with the one-step method of vacuum impregnation. The composition, crystalline structure, morphology and surface physicochemical properties were characterized by ICP-AES, XRD, UV-Vis/DRS, XPS, SEM and N₂ adsorption–desorption measurements. The results showed that the 3DOM Ag/ZrO₂ crystal type was mainly the tetragonal ZrO₂ phase, and Ag was present in the form of metallic silver, which resulted in strong absorption in the visible region due to the effect of surface plasmon absorption (SPA). The composite exhibited a honeycomb-like, three-dimensionally ordered macroporous structure that was permeable, open, arranged in order, with holes closely connected with each other, which was a good reproduction of the PS crystal template. The photocatalytic activity of 3DOM Ag/ZrO₂ was investigated using Congo red as the model molecule. Compared to ZrO₂, P25 and Ag/ZrO₂, 3DOM Ag/ZrO₂ exhibited better photocatalytic activity under multi-modes including UV, visible light, simulated solar light and microwave-assisted irradiation. Moreover, the 3DOM Ag/ZrO₂ composite had a positive effect on the degradation of different types of organic pollutants under UV light. In addition, during the study of photocatalytic hydrogen production, the hydrogen yield of the 3DOM Ag/ZrO₂ composite was 18 times that of commercial P25, which also showed excellent photocatalytic activity.

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Introduction

It is well known that the advantages of solar energy are enormous and inexhaustible, and are unmatched by other energy sources to a certain extent. As such, the study of photocatalytic technology using solar energy has captured much attention.^1–3 Currently, as an advanced technology, photocatalytic technology has broad potential applications in many areas, such as anti-bacterial materials, water treatment, air purification, hydrogen production from water, and so on.^4–6 Hydrogen has become an ideal energy source; in the case of igniting hydrogen, coal, and gasoline with the same weight, hydrogen can generate the most energy and the main product is water, which will not pollute the environment; there is no ash or waste gas. As such, photocatalytic hydrogen evolution is becoming a research hotspot.^7–9

The study of photocatalytic technology cannot be separated from photocatalytic materials. At present, the materials used in photocatalysis as the main body of the catalyst are mainly semiconductor materials such as TiO₂, ZnO and ZrO₂.^10–12 Among them, ZrO₂ is the only one that has both acidic and basic properties, and it is also the only metallic oxide that has oxidizability and reducibility. Since ZrO₂ is a kind of p-type semiconductor,¹³ as the carrier of the catalyst, it is easy to produce oxygen holes, which can produce interactions with active components. Therefore, it has more excellent performance than catalysts supported by other materials. However, as a single semiconductor, zirconia has a high carrier recombination rate and low quantum efficiency, and these faults limit its practical application to some degree. In view of this, most of the present research is focused on the modification of ZrO₂ by doping, compounding and other means to improve its photocatalytic activity.¹⁴ In the relevant literature, ZrO₂ is usually complexed with TiO₂. Polisetti et al. synthesized TiO₂/ZrO₂ and carried out degradation experiments on Alizarin Cyanine Green (ACG), but the experiments proved that the composite had no visible light activity.¹⁵ Yu et al. synthesized Ti_1−xZr_xO₂ solid solutions by the citric acid complexing method, and the solid solutions exhibited higher photocatalytic activity than pure anatase TiO₂ for the degradation of acetone in air.¹⁶ Sajid et al. proposed that the deposited or doped metals in semiconductors may improve electron–hole separation and promote the interfacial electron transfer process, attributed to high Schottky barriers at the metal–semiconductor contact region, and noble metals acting as electron traps, which can enhance the efficiency of the photocatalyst.¹⁷ Wang et al. prepared CdS/ZrO₂ by using the microwave-assisted hydrothermal process, and studied the photocatalytic activity of CdS/ZrO₂ with different proportions; the results showed that the photocatalytic effect of 30% CdS/ZrO₂ was the best.¹⁸ Sasikala et al. proposed a catalyst including ZrO₂, TiO₂ and CdS, which showed higher photocatalytic activity for hydrogen production. Moreover, the co-catalyst, adding 11.5% Pd to the TiO₂–CdS–ZrO₂, increased the lifetime of the light carrier, enhancing its photocatalytic activity.¹⁹

In addition, studies were done on optimizing the morphology of composite materials, and the activities of the catalysts were improved by changing the morphologies.^20–22 Three-dimensionally ordered macroporous materials (3DOM) have been one of the main research areas in the field of porous materials in recent years. Since three-dimensionally ordered macroporous materials have the characteristics of general porous materials, in addition to such traits as transparent space, neat and orderly arrangement, uniform pore size distribution, they have good application prospects in many fields.^23–27 3DOM materials have characteristics of both ordered macroporous structures and solid material, which provide potential utilization value in physics and chemistry. Usually, the framework of 3DOM material may be composed of either a single-component or multi-components, and it has good compatibility, which leads to the easy construction of the novel functional materials that possess both specific composition and macroporous structure, with ordered cycle characteristics, whose uniform pore size is adjustable, with tunnels arranged in order. Compared to other samples synthesized by conventional methods, the unique pore structure of the 3DOM material is very beneficial for substances getting into the holes from all directions, which can reduce the diffusion resistance of the material to some extent and provide higher diffusion efficiency. Among the traditional methods of preparing three-dimensionally ordered macroporous materials, the colloidal crystal template method is the most practical. Jiao et al. synthesized a 3DOM TiO₂-supported CeO₂ photocatalyst by this method, and under the action of heterojunctions and the slow light effect of photonic crystals (3DOM structure), the composite material exhibited high catalytic activity for photocatalytic reduction under simulated solar irradiation.²⁸ Arandiyan et al. synthesized Pt/3DOMCe_0.6Zr_0.3Y_0.1O₂ and 3DOM Ce_0.6Zr_0.3Y_0.1O₂ by the colloidal crystal template method, under the action of the unique 3DOM structure, and strong interactions between Pt nanoparticles. The as-synthesized 3DOM Pt/Ce_0.6Zr_0.3Y_0.1O₂ showed good low temperature reduction, and high catalytic performance for methane combustion.²⁹

Based on the above works and our existing research, our group has carried out a series of work on three-dimensionally ordered macroporous materials prepared by different kinds of composites, and some better results were obtained.^30–32 We have found that a variety of multi-layer, open 3DOM composite materials can be synthesized by polystyrene (PS) colloidal spheres templates. The composites were prepared in order to form an excellent 3DOM material, of which the photocatalytic activity can be effectively improved. In this paper, we introduce a three-dimensionally ordered macroporous structure into the synthesis of a Ag/ZrO₂ composite by using the PS colloidal crystal template method, and improve the photocatalytic activity of the composite by utilizing the open and permeable porous structure. Using n-butoxypolyethylene zirconium as the precursor, self-assembly with silver is carried out, taking advantage of the one-step method of vacuum impregnation and the enhanced light absorption by the surface plasmon absorption (SPA) effect of Ag, to broaden the application range of ZrO₂ and improve the activity of the 3DOM Ag/ZrO₂ nanocomposite. This study of the 3DOM Ag/ZrO₂ nanocomposite covers two aspects, photocatalytic degradation and photocatalytic hydrogen evolution.

1. Experimental

1.1 Materials

n-Butoxypolyethylene zirconium (C₁₆H₃₆O₄Zr) was purchased from Shanghai Chunhe Biotechnology Co. Ltd., and the triblock poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) copolymer (P123) (EO₂₀PO₇₀EO₂₀, M_w = 5800) was purchased from Energy Chemical Company. NaOH, K₂S₂O₈ and isopropanol were purchased from Kaitong Chemical Reagent Co. Ltd., Tianjin, China. Degussa P25 (an efficient and industrial photocatalyst), styrene, p-benzoquinone (BQ), methanol (MA), disodium ethylenediamine tetraacetic acid (EDTA-2Na), Congo red (CR), rhodamine B (RhB), crystal violet (CV), malachite green (MG) and phenol (Ph) were purchased from Guangfu Testmart, China. All reagents were analytical grade. Doubly distilled water was used in all experiments.

1.2 Synthesis of Ag/ZrO₂

First, P123 was dissolved in isopropanol, then n-butoxypolyethylene zirconium was added quickly under stirring conditions, followed by the addition of AgNO₃ solution; the Ag : Zr molar ratio was 1 : 50. After treating the product at 200 °C for 2 h, following the formation of Ag/ZrO₂ gel, the as-obtained sample was vacuum dried, ground and calcined in a muffle furnace at 500 °C for 7 h; it was then washed several times with ethanol and distilled water, and marked as Ag/ZrO₂. Under the same experimental conditions, the sample without the addition of AgNO₃ was marked as ZrO₂.

1.3 Synthesis of 3DOM Ag/ZrO₂

PS colloidal spheres synthesized by an emulsifier-free emulsion polymerization method³¹ were added to methanol, stirred for about 30 min at a constant speed, and then filtered. AgNO₃ solution and n-butoxypolyethylene zirconium were added to 10 mL isopropanol solution with Ag and Zr in a molar ratio of 1 : 50. This solution was added to the treated PS colloidal spheres and stirred for 1 hour. The mixture was subjected to vacuum impregnation for 12 h, and then leached. The product was calcined in a muffle furnace at 500 °C for 7 h, and marked as 3DOM Ag/ZrO₂.

1.4 Characterization of photocatalysts

The phase and composition of the as-prepared samples were detected by X-ray diffraction studies (XRD), using a Bruker-AXS (D8) X-ray diffractometer, with Cu Kα X-ray radiation source under 60 kV, and 80 mA, and with 2θ ranging from 20° to 80°. ICP-AES studies were carried out on an Agilent Technologies 7500ce instrument. The optical properties of the samples were analyzed by UV-Vis diffuse reflectance spectroscopy (DRS), using a UV-Vis spectrophotometer (TU-1901) equipped with a diffuse reflectance system, over a wavelength range of 200–800 nm. The chemical composition analysis of the sample surfaces was carried out by X-ray photoelectron spectra (XPS) using an ESCALAB 250 Xi spectrometer with Al Kα radiation at 300 W, and vacuum pressure of 1 × 10⁻⁶ Pa. The morphologies and microstructures of the as-prepared samples were investigated by scanning electron microscopy (SEM) (Hitachi S-4300), with working voltage of 5 kV. The specific surface areas of the samples were measured by a Brunauer–Emmett–Teller specific surface area instrument (Beishide Instrumentation Technologies (Beijing) Ltd., Model 3H-2000PS2) with nitrogen adsorption at 77 K.

1.5 Photocatalytic degradation studies

The photocatalytic activities of the 3DOM Ag/ZrO₂ composite were evaluated by photocatalytic degradation of crystal violet under multi-modes including UV, visible light, microwave-assisted and simulated solar light irradiation. Some of the experimental reaction devices of the multi-mode light catalytic reaction were homemade. The UV light source was a 125 W high-pressure mercury lamp (built-in type, with maximum emission at 313.2 nm); the visible light was obtained from a 400 W Xe lamp (built-in type, main emission lines greater than 410 nm, the inner sleeve was made of No. 11 glass to filter out ultraviolet radiation from the Xe lamp). A microwave discharge electrode less lamp (MDEL, built-in type, UV emission wavelength was mainly located at 280 nm) was used in the microwave-assisted test; the power was 15 W, and it was a U shaped model. The output power of the microwave was about 600 W.³³ The simulated sunlight source was a 1000 W Xe-lamp (external type, Shanghai Bilon Instruments Co., Ltd., the emission spectrum was close to the full spectrum; we carried out different experiments to check that O₃ could not be produced during the photocatalytic reaction process to discard the participation of the oxidizing agent in the decomposition of the dye). Congo red (CR) was used as a model dye.

The process of the photocatalytic reaction is as follows:

(1) UV light mode. Prior to irradiation, 0.15 g photocatalyst were suspended in 90 mL of CR (50 mg L⁻¹) solution by ultrasound (ca. 10 min), and then the suspension was magnetically stirred for 30 min in the dark, to ensure the adsorption/desorption equilibrium between CR and photocatalyst powders. The photocatalytic experiment was carried out in the photocatalytic reaction device. The high-pressure mercury lamp was placed into a jacketed quartz tube. The quartz tube was soaked in the solution (which was continuously magnetically stirred), and the suspensions were kept at constant temperature by circulating water through the jacket during the entire process. At given time intervals, 3 mL of suspension were sampled and centrifuged to remove the photocatalyst particles, then, the absorption spectrum of the centrifuged solution was recorded using a TU-1901 UV-Vis spectrophotometer (China). The change in CR concentration was determined by monitoring the optical intensity of the absorption spectra at 499 nm. The absorbance values of RhB, CV, MG and Ph under similar photocatalytic conditions were measured at their respective λ_max values.

(2) Visible light mode. 0.30 g photocatalyst were suspended in 220 mL of CR (50 mg L⁻¹), and the subsequent steps of the experiment were the same as above, except the lamp source was a 400 W Xe lamp.

(3) Microwave-assisted mode. 500 mL of CR (50 mg L⁻¹) solution and 0.50 g photocatalyst were taken in a microwave reactor and stirred for 30 min in the dark to ensure the adsorption/desorption equilibrium between CR and photocatalyst powders. The microwave reactor employed for the degradation of CR was purchased from Yuhua Instrument limited company of China. It consisted of a cylindrical glass reactor (capacity 600 mL) with a 600 mm long water reflux condenser, connected through a communication pipe. The microwave discharge electrode less lamp (MDEL) was placed into the reaction solution, with about 2/3 of the MDEL being in the reaction solution. Three silicone tubes were connected to the equipment, which could let water in and out, and let air out through the hole on the microwave reactor. Air was bubbled into the solution through a sintered glass filter, fixed at the bottom of the reactor for passing oxygen, as well as for mixing the catalyst and the solution. At given time intervals, 3 mL of suspension were sampled and centrifuged to remove the photocatalyst particles. Then, the absorption spectrum of the centrifuged solution was recorded using a TU-1901 UV-Vis spectrophotometer (China).

(4) Simulated solar light mode. During the process of the simulated solar light irradiation, 90 mL of CR (50 mg L⁻¹) solution and 0.15 g photocatalyst were taken in a quartz photo-reactor and stirred for 30 min in the dark to ensure the adsorption/desorption equilibrium between CR and photocatalyst powders. The reactor was placed 8.5 cm away from the light source. The suspensions were kept at room temperature by circulating thermostated ethanol through the jacket. The subsequent steps of the experiment were the same as for the UV light mode above.

To determine the photocatalytic oxidation pathways of 3DOM Ag/ZrO₂, EDTA-2Na (1 mM), BQ (1 mM) and MA (1 mM) were selected as scavenger agents for holes (h⁺), oxygen radical anions (·O₂⁻) and hydroxyl radicals (·OH), respectively, for the photocatalytic degradation of CR over the 3DOM Ag/ZrO₂ composite. The concentration of CR during the photocatalytic reaction in the presence of EDTA-2Na, BQ or MA was determined by measuring the absorption of the CR solution at 499 nm.

1.6 Photocatalytic hydrogen evolution test

Photocatalytic hydrogen evolution experiments were carried out in a reactor with a closed and connected vacuum circulatory system. 0.1 g photocatalyst was dispersed in 50 mL doubly distilled water, and Na₂S·9H₂O was added as a sacrificial agent. After vacuum degassing, the test for photocatalytic hydrogen evolution began under the conditions of stirring. The light source was a 300 W Xe lamp, high purity nitrogen was the carrier gas, the output pressure was 0.4–0.5 MPa, and working voltage and working current were about 20 mV and 50 mA, respectively. In the reaction process, the temperature of the reactor was kept at 5 °C by circulating cooling water. Gas was collected after a certain irradiation time, by using online gas chromatography to analyze the hydrogen production; the reaction was carried out for 8 h. The column was 5 Å molecular sieve column, and the detector was a thermal conductivity detector (TCD). Based on the peak areas of the different reaction times, the production of hydrogen was calculated, and the catalytic activity of the photocatalyst was measured by the total hydrogen production over 8 h.

2. Results and discussion

2.1 ICP and XRD analysis

The loadings of Ag in the samples were determined using the ICP-AES technique, and the results are shown in Table 1. According to the results of the ICP-AES test, we can get two pieces of information. First, low amounts of Ag are actually present in the 3DOM Ag/ZrO₂ and Ag/ZrO₂. Second, the difference in synthesis method can affect the Ag content in the composites. Compared with 3DOM Ag/ZrO₂, Ag/ZrO₂ had less Ag, attributed to the fact that in some areas, Ag had not been formed and in others, it was washed away by ethanol and distilled water.

Table 1 Crystallite size (D*), band-gap energy (E_g), BET surface areas (S_BET), average pore diameters (D), pore volumes (V_total), cell parameters and ICP-AES data of 3DOM Ag/ZrO₂ and Ag/ZrO₂^a

Sample	SBET (m^2 g^−1)	D (nm)	Vtotal (cm^3 g^−1)	D* (nm)	Eg (eV)	Crystal parameters		Mole ratio of precursor n(Ag) : n(Zr)	Actual mole ratio n(Ag) : n(Zr)
						a/Å	c/Å
a The Scherrer formula:^18 D = Kλ/B cos θ (where K is the Scherrer constant; D is the average thickness of the grains perpendicular to the plane direction (nm); B is the measured half-width of the diffraction peak sample; θ is diffraction angle; and λ is the wavelength of X-rays).
Ag/ZrO2	54.9	15.3	0.210	10.6	3.51	3.6029	5.1450	1 : 50	1 : 57
3DOM Ag/ZrO2	33.3	20.0	0.166	15.1	3.25	3.5983	5.1555	1 : 50	1 : 48

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The crystal structure and crystallinity of the 3DOM Ag/ZrO₂, Ag/ZrO₂, ZrO₂ and Ag were studied by XRD, and the results are shown in Fig. 1. Pure ZrO₂ is in the tetragonal phase, and the diffraction peaks are located at 30.22°, 34.96°, 50.29° and 60.22° (PDF 50-1089).³⁴ Pure Ag is in the cubic phase and the main diffraction peaks are 38.3°, 44.39°, 64.55° and 77.51° (PDF 04-7083). Fig. 1 shows that the peaks of 3DOM Ag/ZrO₂ at 2θ values of 30.2°, 35.2°, 50.4°, 60.0° in the XRD patterns were indexed to the ZrO₂ tetragonal phase (011), (110), (112), (121) crystalline planes, respectively, which showed that doping with Ag did not cause changes in the ZrO₂ crystals. In addition, the characteristic diffraction peaks of Ag were not observed in the XRD pattern of 3DOM Ag/ZrO₂. This may be due to the high dispersion of Ag on the surface of the composite.³⁵ Compared with that of pure ZrO₂ (a = b = 3.6072 Å, c = 5.1482 Å), the lattice parameters of 3DOM Ag/ZrO₂ did not significantly change (a = b = 3.5983 Å, c = 5.1555 Å), showing that Ag did not enter into the ZrO₂ crystal lattice, therefore, Ag is more likely to be dispersed on the surface of ZrO₂ in the form of nanoparticles. Compared with ZrO₂, the lattice parameters of Ag/ZrO₂ decreased slightly, which may be due to the defects made by the temperature-programmed hydrothermal treatment during the synthesis process, resulting in a decrease in the degree of crystallinity.³⁶ From Fig. 1, it can also be seen, that the diffraction peak of 3DOM Ag/ZrO₂ is sharper than that of ZrO₂, indicating that the crystallinity of 3DOM Ag/ZrO₂ is higher. According to the Scherrer formula,¹⁸ the grain sizes of the samples are shown in Table 1, where we see that the grain size of 3DOM Ag/ZrO₂ is significantly larger than that of Ag/ZrO₂, attributed to the different types of synthesis methods, and the effect of 3DOM on the grain.³⁷

Fig. 1 XRD patterns of 3DOM Ag/ZrO₂, Ag/ZrO₂, ZrO₂ and Ag.

2.2 XPS analysis

In order to study the chemical forms of the various elements on the surface of the composite, 3DOM Ag/ZrO₂ was analyzed by XPS and the results are shown in Fig. 2. It can be seen that the surface of 3DOM Ag/ZrO₂ has mainly three elemental species: Ag, Zr and O. According to Fig. 2(a), the binding energy of zirconium element on the 3DOM Ag/ZrO₂ composite is 181.9 eV and 184.1 eV, respectively, corresponding to Zr 3d_5/2 and Zr 3d_3/2, in the Zr⁴⁺ oxidation state.³⁸ Fig. 2(b) shows the binding energy of the silver element in the composite; the peaks at 368.0 eV and 374.0 eV are attributed to Ag 3d_5/2 and Ag 3d_3/2, respectively. The spin energy interval is 6.0 eV, indicating that Ag was present in the form of metallic silver.^39,40 In addition, Fig. 2(c) shows O 1s peaks at 529.9 eV and 531.2 eV, resulting from the lattice oxygen and the adsorbed oxygen in the composite, respectively.⁴¹

Fig. 2 XPS spectra of 3DOM Ag/ZrO₂: (a) Zr 3d, (b) Ag 3d and (c) O 1s.

2.3 SEM analysis

To study the morphology and the size of the composites, SEM analysis was carried out on polystyrene and 3DOM Ag/ZrO₂; the results are shown in Fig. 3. Fig. 3(a) and (c) show the SEM images of PS colloidal spheres under different magnifications. Fig. 3(a) shows the synthesized polystyrene colloidal spheres template has a highly ordered array, the surfaces of the PS colloidal spheres are smooth, the sizes are relatively uniform and the average diameter is ca. 300 nm. The PS colloidal spheres are arranged closely, and each polystyrene colloidal sphere is simultaneously exposed to six other spheres, presenting a “honeycomb-like” hexagonal close-packed structure, attributed to the relatively tight arrangement when the PS colloidal spheres are self-assembled.⁴² Fig. 3(b) and (d) show the SEM images of the composite after calcination to remove the PS colloidal spheres. The product presents a three-dimensionally ordered macroporous interconnection state, and the bottom of each large hole is composed of three small windows, while the holes are connected, open and transparent. This indicates that the product has replicated well the structure of the PS colloidal spheres template. Because the removal of the PS colloidal spheres template was carried out by calcined treatment, if the temperature control was not uniform at the time of calcination, there would be a small amount of the template not being completely removed. This can be seen in the upper right corner of Fig. 3(b), where the PS colloidal spheres were not fully calcined. The three-dimensionally ordered macroporous materials are arranged in order, with uniform sizes of ca. 210 nm. Looking closely at the images and the enlarged view of the partial regions in Fig. 3(b and d), it is not difficult to find that the pore walls of these ordered macroporous pores are closely packed with Ag/ZrO₂ nanocrystals. Comparing Fig. 3(a and c) and (b and d), we see that the pore size of the 3DOM composite is smaller than the diameter of the PS template, due to the high temperature of the calcination process causing the shrinkage of the aperture.⁴³

Fig. 3 SEM images of PS (a and c) and 3DOM Ag/ZrO₂ (b and d).

2.4 UV-Vis/DRS analysis

The UV-Vis diffuse reflectance spectra (UV-Vis/DRS) were obtained to investigate the optical absorption of 3DOM Ag/ZrO₂; the results are shown in Fig. 4 and Table 1. It can be seen from Fig. 4 that compared to ZrO₂, the absorption peaks of 3DOM Ag/ZrO₂ and Ag/ZrO₂ are both red-shifted to the visible region. This result can be attributed to the surface plasmon absorption (SPA) of Ag nanoparticles in the composites.⁴⁴ Additionally, the absorption of the 3DOM Ag/ZrO₂ sample prepared by the templated self-assembly of PS colloidal spheres is in the range of 250–800 nm, and is significantly higher than that of Ag/ZrO₂. This is attributed to the slight differences in doping amounts of Ag in the two composites, due to the different synthesis methods, leading to the different light absorption properties of the Ag/ZrO₂ and 3DOM Ag/ZrO₂ composites. It is also due to the special morphology of the 3DOM material producing multiple scattering of light, so as to increase the contact time between light and the composite material.⁴⁵ At the time of UV-Vis/DRS detection, the open and transparent macroporous structure of 3DOM Ag/ZrO₂ caused the light reflection by the Ag in the deep layer easier, and produced a stronger absorption response.

Fig. 4 UV-Vis/DRS spectra (a) and Kubelka–Munk energy curve plots of ZrO₂, Ag/ZrO₂ and 3DOM Ag/ZrO₂ (b).

The bandgap energy values (E_g) of ZrO₂, Ag/ZrO₂ and 3DOM Ag/ZrO₂ were calculated using the Kubelka–Munk plot (Fig. 4(b)). The band gap (E_g) of the semiconductors were obtained from eqn (1):⁴⁶

αhν = A(hν − E_g)^n/2 (1)

In the aforementioned formula, α, ν and A are the absorption coefficient, light frequency and proportionality constant, respectively. The n can be determined by the type of optical transition of a semiconductor (i.e., n = 1 for direct transition and n = 4 for indirect transition). The results are shown in Table 1. Since the E_g is smaller, the excited required wavelength is greater under illumination, so the electronic transition occurs more easily and the quantity is larger. According to Table 1, the lower E_g value of 3DOM Ag/ZrO₂ can affect its light-absorbing properties; therefore, the 3DOM Ag/ZrO₂ composite will have a higher photocatalytic activity.

2.5 N₂ adsorption–desorption analysis

To evaluate the surface physicochemical properties of the as-synthesized composites, N₂ adsorption–desorption measurements were obtained for 3DOM Ag/ZrO₂, and compared with the prepared composite Ag/ZrO₂; the results are shown in Fig. 5 and Table 1. According to Fig. 5, the N₂ adsorption–desorption isotherms of 3DOM Ag/ZrO₂ and Ag/ZrO₂ are type IV adsorption curves,⁴⁷ which are typical for mesoporous structures. The biggest characteristic of this type of curve is the hysteresis loop of 3DOM Ag/ZrO₂, belonging to the H3 type.³¹ This type of isotherm and its hysteresis loop are caused by the aggregation of the capillary and the aggregation of the particles in the structure. The hysteresis loop of Ag/ZrO₂ belongs to the H1 type,⁴⁸ and the adsorption and desorption of the hysteresis loop are parallel, corresponding to round tubular structure openings at both ends.⁴⁹ The mesoporous structure of 3DOM Ag/ZrO₂ could be attributed to the large-holed wall of the composite material. As shown in Fig. 5, the BET surface area of Ag/ZrO₂ is larger than that of 3DOM Ag/ZrO₂, which could be due to the addition of the templating agent P123 in the synthesis process of Ag/ZrO₂, resulting in the increase in specific surface area.⁵⁰ It can be seen from Fig. 5 that the pore size of 3DOM Ag/ZrO₂ is more uniform than that of Ag/ZrO₂ in the BJH pore size distribution curves of both kinds of composite materials, and this could be attributed to the 3DOM synthesis method used in this paper. As a result of using the methanol vacuum impregnation method during the synthesis, the precursor is uniformly filled in between the polystyrene microspheres; resulting in the uniform and orderly internal structure of 3DOM Ag/ZrO₂.

Fig. 5 N₂ adsorption–desorption isotherms of 3DOM Ag/ZrO₂ (a) and Ag/ZrO₂ (b) (insets are the BJH pore size distribution curves).

2.6 Photocatalytic activity for the degradation of dyes

In order to investigate the photocatalytic performance of the 3DOM Ag/ZrO₂ composite, a series of multi-mode photocatalysis experiments was carried out under UV light, visible light, simulated sunlight and microwave-assisted irradiation; the results are as shown in Fig. 6. To study the photocatalytic properties of 3DOM Ag/ZrO₂, we used commercial P25 for comparison to more objectively evaluate the photocatalytic effect of 3DOM Ag/ZrO₂.^51,52 According to Fig. 6, the photocatalytic activity of 3DOM Ag/ZrO₂ is significantly higher than the other samples under UV light irradiation conditions. In Fig. 6(a), −ln(C_t/C₀) (C₀ refers to the initial concentration of the reaction solution after the completion of the adsorption experiments, and C_t is the concentration of the reaction solution at the time t) has a linear relationship with reaction time t, which shows that the degradation of Congo red dye follows pseudo first-order kinetics. After calculation, the apparent reaction rate constants of the degradation of Congo red by direct photocatalysis, P25, ZrO₂, Ag/ZrO₂ and 3DOM Ag/ZrO₂ are 0.00030, 0.00847, 0.00041, 0.00299 and 0.01489 min⁻¹, respectively; therefore, under UV light irradiation conditions, the order of catalytic activity is 3DOM Ag/ZrO₂ > P25 > Ag/ZrO₂ > ZrO₂ > direct photocatalysis. Furthermore, the photocatalyst 3DOM Ag/ZrO₂ was used for the degradation of different organic contaminants, including Congo red (CR), rhodamine B (RhB), crystal violet (CV), malachite green (MG) and phenol (Ph), under similar conditions of UV irradiation as shown in Fig. 6(b). It can be seen from Fig. 6(b) that the degradation effect of 3DOM Ag/ZrO₂ on the different types of dyes and phenol is better under UV light, showing that 3DOM Ag/ZrO₂ has extensive application for different types of organic matter degradation. Fig. 6(c) presents the results of photocatalytic degradation of CR under other different modes, including visible light, simulated solar light and microwave-assisted irradiation. As shown in Fig. 6(c), for each mode described above, the degradation effect is not as obvious as UV light, but compared with direct photocatalysis, the degradation rate is obviously improved. Furthermore, the degradation rate under microwave-assisted irradiation is faster than the other two modes due to the microwave effect promoting the reaction process.^53,54 Fig. 6(c) also exhibits that the photocatalytic activity of 3DOM Ag/ZrO₂ is significantly higher than that of the other photocatalysts under microwave-assisted irradiation, and 3DOM Ag/ZrO₂ has a relatively high activity in these experimental conditions. This may be due to the incorporation of Ag in 3DOM Ag/ZrO₂, which causes a red shift and enhancement in light absorption, helping to improve the photocatalytic activity. The transparent hole structure of 3D ordered macroporous materials can effectively reduce the diffusion resistance of the material, which make the dyes and other reactant molecules reach the active center more easily. The catalyst has a highly ordered macroporous structure, which can enhance its periodicity, and increase the contact surface between the dye molecules and the catalyst, so more reactive sites can further enhance its activity.

Fig. 6 (a) UV photocatalytic degradation of CR using different photocatalysts. Inset: kinetics curves; (b) UV photocatalytic degradation of different organic pollutants using 3DOM Ag/ZrO₂; (c) photocatalytic degradation of CR using different photocatalysts under multi-modes, including visible light (t = 180 min), microwave-assisted (t = 90 min) and simulated solar light irradiation (t = 240 min); (d) UV photocatalytic degradation of CR capture test using 3DOM Ag/ZrO₂.

The active groups in the system of 3DOM Ag/ZrO₂ during the process of photocatalysis were investigated, and p-benzoquinone (BQ), methanol (MA) and EDTA-2Na were added as scavenger agents to capture superoxide radicals (·O₂⁻), hydroxyls (·OH) and holes (h⁺), respectively, for comparison with the experiment without capture agents (Fig. 6(d)). The results show that the addition of the three trapping agents had an inhibiting effect on the degradation of Congo red.^32,55 There was also a certain degree of capture of the active substances of the photocatalytic reaction, which suggests that the active groups of 3DOM Ag/ZrO₂ in the photocatalytic process are mainly superoxide radicals (·O₂⁻), hydroxyls (·OH) and holes (h⁺).

2.7 Photocatalytic hydrogen evolution

In order to investigate the activity of the 3DOM Ag/ZrO₂ composite in the photocatalytic hydrogen evolution reaction, the experiments were carried out for 8 h. Fig. 7 shows that the hydrogen production efficiencies of ZrO₂ and P25 are lower; mainly due to photoinduced electrons and holes of the semiconductor monomer quickly compound and cause the reverse hydrogen reaction.¹⁸ The activity of the photocatalytic hydrogen evolution of 3DOM Ag/ZrO₂ was much higher than that of Ag/ZrO₂ and the two monomers, and was attributed to a certain amount of Ag nanoparticle doping in the composite materials, which increased the absorption of light and enhanced the photocatalytic activity. In addition, the result that the hydrogen production efficiency of 3DOM Ag/ZrO₂ was higher than that of the Ag/ZrO₂ sample showed that the three-dimensionally ordered macroporous structure was the key to improving the hydrogen production activity. Introducing the three-dimensionally ordered structure, of which the open inner surface favors the reduction reaction, the open pores can help to produce H₂ in a timely manner and reduce the reverse reaction.

Fig. 7 Hydrogen production amount for 3DOM Ag/ZrO₂, Ag/ZrO₂, ZrO₂ and P25.

2.8 Possible photocatalytic reaction mechanism

Based on the above results and related experimental data, we speculated the possible photocatalytic mechanism of 3DOM Ag/ZrO₂, as shown in Fig. 8. It can be seen that, on the one hand, the visible response of 3DOM Ag/ZrO₂ is enhanced due to the Ag surface plasmon resonance effect, which generates more photogenerated carriers, and improves the photocatalytic activity. On the other hand, after the p-type semiconductor ZrO₂ loads Ag nanoparticles, due to the work function of Ag (4.26 eV (ref. 56)) being greater than that of ZrO₂ (3.5 eV (ref. 15)), the Schottky barrier is formed between them.¹⁷ ZrO₂ is excited to produce electrons and holes in the photocatalytic process, and electrons are gathered in the contact surfaces of Ag and ZrO₂, due to the Schottky barrier function. Meanwhile, the noble metal Ag can act as electron traps and promote electron–hole separation, increasing electron transfer between interfaces, thereby enhancing the photocatalyst efficiency. According to the formulae, E_(CB) = χ − E_e − 0.5E_g and E_(VB) = E_(CB) + E_g,⁵⁷ the redox potential of the conduction band and valence band of the composite material are −0.22 and 3.03 eV, respectively. The result not only satisfies the conditions of photocatalytic degradation of organic matter, but the potential of the valence band of the sample is more negative than hydrogen, so the sample also has photocatalytic hydrogen evolution activity. Also, the photogenerated electrons reacting with O₂ adsorbed on the catalyst surface generates superoxide radicals, and at the same time, the holes and the water generate hydroxyl radicals and superoxide radicals. The hydroxyl radicals can degrade organic pollutants completely, and even mineralize CO₂ and H₂O, and so on. In this paper, the prepared 3DOM Ag/ZrO₂ is a three-dimensionally ordered macroporous material with multilayers, where the open pore structure allows the organic contaminants to be in sufficient contact with the photocatalyst, and improves the photocatalytic reaction efficiency further. At the same time, the position of the conduction band of the 3DOM Ag/ZrO₂ is negative to the hydrogen electrode reaction potential (0 eV), which can satisfy the requirements of the hydrogen evolution reaction; i.e., conduction band electrons may reduce water into hydrogen. Since the as-synthesized composite 3DOM Ag/ZrO₂ can simultaneously satisfy the above conditions of degradation and hydrogen production, it may have both photocatalytic degradation and photocatalytic hydrogen evolution activity. In addition, introducing the three-dimensionally ordered macroporous structure can also help produce H₂ in a timely manner, and improve the photocatalytic reaction efficiency further.

Fig. 8 Possible photocatalytic reaction mechanism of 3DOM Ag/ZrO₂.

Conclusion

The 3DOM Ag/ZrO₂ composite, having a uniform, transparent and “honeycomb-like” macroporous structure, was prepared using PS colloidal spheres as a self-assembling template using a methanol vacuum impregnation method and post-processing calcinations. The composite not only has a good crystal structure, but Ag, in the form of metallic silver, can also enhance the absorption of the composite in the visible region, due to the SPA effect. By processing using the colloidal crystal template, the large-holed wall was piled up with nanoparticles. The photocatalytic activity of 3DOM Ag/ZrO₂ was higher than that of direct photolysis, P25, ZrO₂ and Ag/ZrO₂ in the multi-mode photocatalytic degradation of Congo red, which has a certain effect on the degradation of many organic pollutants. 3DOM Ag/ZrO₂ also exhibits good ability for photocatalytic hydrogen evolution, demonstrating that the synergistic effect after Ag and ZrO₂ were complexed and the three-dimensionally ordered macroporous structure introduced into the synthesis of the composite helped to improve its photocatalytic activity.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (21376126), Natural Science Foundation of Heilongjiang Province, China (B201106), Scientific Research of Heilongjiang Province Education Department (12511592, 12541888), Government of Heilongjiang Province Postdoctoral Grants, China (LBH-Z11108), Open Project of Green Chemical Technology Key Laboratory of Heilongjiang Province College, China (2013 year), Postdoctoral Researchers in Heilongjiang Province of China Research Initiation Grant Project (LBH-Q13172), Innovation Project of Qiqihar University Graduate Education (YJSCX2015-ZD03), College Students' Innovative Entrepreneurial Training Program Funded Projects of Qiqihar University (201510221077) and Qiqihar University in 2015 College Students Academic Innovation Team Funded Projects.

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