The three-dimensional ordered macroporous structure of the Pt/TiO2–ZrO2 composite enhanced its photocatalytic performance for the photodegradation and photolysis of water

Using polystyrene (PS) spheres as a template, three-dimensional ordered macroporous Pt/TiO2–ZrO2 (3DOM Pt/TiO2–ZrO2) composites were prepared by vacuum impregnation combined with photoreduction. The crystal structure, composition, morphology, optical absorption, and surface physicochemical properties of the as-synthetized samples were characterized by X-ray diffraction (XRD), UV-visible diffuse reflectance spectroscopy (UV-vis/DRS), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and N2 adsorption–desorption analyses. The results showed that the 3DOM Pt/TiO2–ZrO2 composites were mainly composed of anatase TiO2 and tetragonal ZrO2 crystal phases, in which Pt mainly existed as a single species. In addition, the as-synthesized composites had open, three-dimensionally ordered macroporous structures that could enhance their multi-mode photocatalytic degradation performance under UV, visible light, simulated solar light, and microwave-assisted irradiation. Moreover, the 3DOM Pt/TiO2–ZrO2 composites exhibited the best photocatalytic water splitting performance as compared to other systems.


Introduction
A study on semiconductor photoelectrochemistry was published in Nature by Fujishima and Honda in 1972. 1 The concept of exciting TiO 2 semiconductors with UV light to split water has now become the basis for modern solar fuel research, 2 and research on photocatalytic materials that are benecial to human beings in the management of environmental pollution and improvement of living space has attracted signicant attention. 3 At present, TiO 2 is a more widely used semiconductor photocatalyst with high photocatalytic efficiency and chemical stability and low cost; however, the recombination of photogenerated electron-hole pairs and the low quantum efficiency have limited the practical application of single semiconductor photocatalysts, such as TiO 2 , to some extent. Therefore, research on improving the photocatalytic efficiency of composite materials via the synergistic effect of the combination of a single semiconductor photocatalyst, such as TiO 2 , with other single semiconductor photocatalysts is increasing. The optical absorption range of TiO 2 monomer is less than 400 nm, and the quantum efficiency is low. 4 The combination of appropriate semiconductor materials and TiO 2 can use the synergistic effect to further enhance the photocatalytic activity of TiO 2 . ZrO 2 is an oxide with a wide bandgap (ca. 3-5 eV). Moreover, the surface of ZrO 2 has both acidic and basic properties; therefore, ZrO 2 exhibits oxidation and reduction properties and shows photocatalytic activity. 5,6 ZrO 2 is a typical ptype semiconductor with three crystal forms: it is monoclinic at room temperature and undergoes a tetragonal phase transition with a change in temperature. The tetragonal phase has high catalytic activity and has been extensively studied. 7,8 In recent years, researchers have oen improved the photocatalytic performance of composites by various methods during the synthesis process. 9 For the rst time, Velev et al. used polystyrene colloidal microspheres as templates to prepare ordered macroporous SiO 2 . 10 Because the preparation process is relatively simple and the pore structure is easy to be controlled, this approach has been developed as one of the most popular preparation methods to achieve three-dimensional ordered macroporous composites; Stein et al. have used 3DOM a-Al 2 O 3 as a support for metallic silver in the ethylene epoxidation reaction and found that the catalytic efficiency of the 3DOM a-Al 2 O 3 support is signicantly higher than that of the commercial a-Al 2 O 3 support. 11 Thus, studies on the use of polystyrene microspheres as templates to prepare composites have shown that a three-dimensional ordered macroporous structure is excellent to the reaction process of reactants and product molecules into and out, which can increase the reactive sites and improve the catalytic activity to a certain extent.
In addition, the modication of semiconductors with precious metals has attracted signicant attention. The change in the electron distribution in the system affects the surface properties of the semiconductor and can improve its photocatalytic activity; studies on the deposition of noble metals, including Ag, Pt, Rh, Pd, Ru, Au, Ir, etc., have been reported, and studies in which Pt is used are more common because the modication effect of Pt is best as compared to that of other noble metals. [12][13][14][15] The photocatalytic water splitting production of Pt/TiO 2 could reach 33.0 mmol aer 6 h of light irradiation, as reported by Zou et al. 16 Therefore, high-activity materials for photocatalytic hydrogen production are expected to be prepared by doping Pt into semiconductor TiO 2 and ZrO 2 composites. [17][18][19] Based on the existing experimental studies, polystyrene (PS) spheres were used as templates to prepare three-dimensionally ordered macroporous Pt/TiO 2 -ZrO 2 composites by photoreduction. On the one hand, the noble metal Pt was supported as an electron trap to suppress the recombination of photogenerated electron-hole pairs in the semiconductor TiO 2 and ZrO 2 to effectively capture the photogenerated electrons and promote the separation of photogenerated electron-hole pairs. Moreover, the captured photogenerated electrons can be better involved in the reaction of hydrogen production. On the other hand, the characteristics, such as single pore size, uniform pore size distribution, and orderly arrangement, of three-dimensional ordered macroporous materials endow 3D porous materials with higher porosity, larger specic surface area, and stronger surface adsorption capacity, which can effectively improve the photocatalytic activity of composite materials.
In this study, the photocatalytic degradation of 3DOM Pt/ TiO 2 -ZrO 2 composites was investigated by multi-mode photocatalytic experiments. Moreover, the photocatalytic performance of the composites was further investigated. The experimental result indicated that the three-dimensional ordered macroporous structure of the composite material enhanced the photocatalytic performance.

Materials
Titanium tetraisopropoxide (TTIP, purity 98%) was purchased from New Jersey, USA; zirconium n-butoxide (C 16 H 36 O 4 Zr) was purchased from Shanghai Chunhe Biotechnology Co., Ltd.; chloroplatinic acid was purchased from Shanghai Jingjing Biochemical Technology Co., Ltd.; tri-block copolymer (P123) was purchased from Aldrich Company in the United States; styrene, commercial photocatalyst (Degussa P25), K 2 S 2 O 8 , methyl orange (MO), congo red (CR), methylene blue (MB), and salicylic acid (SA) were purchased from Tianjin Mitsufu Fine Chemicals Research Institute. All chemicals were of analytical grade (AR) and used as received without purication; secondary distilled water was employed for all the experiments.

Preparation of 3DOM Pt/TiO 2 -ZrO 2
(a) At rst, 2.0 g of polystyrene (PS) spheres synthesized by the non-emulsication polymerization technology reported in the literature 20 was impregnated with 8 mL methanol, stirred for 30 min, then ltered, and naturally dried. Then, n-butoxide and titanium tetraisopropoxide were mixed uniformly at a volume ratio of 1 : 4. The treated PS spheres were then slowly added, stirred for 1 h, and lled in under vacuum. The resulting solid mixture was calcined for 8 h to obtain the composites marked as 3DOM TiO 2 -ZrO 2 .
(b) The 3DOM TiO 2 -ZrO 2 composites were placed in 50 mL of deionized water and sonicated for 10 min. Then, 3.0 g of sodium sulde was added as a sacricial agent in the photoreduction reactor, and 0.15 mL of platinum standard solution was added. The mixed solution was then irradiated for 1 h using a 300 W xenon lamp for light reduction of Pt, washed four times with deionized water and ethanol, and vacuum dried to obtain the composites marked as 3DOM Pt/TiO 2 -ZrO 2 .
(c) Typically, 14 mL of ethanol and 2 mL of isopropanol were added into a 50 mL beaker, and titanium tetraisopropoxide and zirconium n-butoxide at a volume ratio of 1 : 4 were added. The mixture was continuously stirred at room temperature, and a small amount of deionized water (ca. 0.5 mL) was added until a gel-like material appeared. The resulting gel was dried for 12 h. Finally, the resulting solid was calcined at 600 C for 8 h to obtain the desired nanocomposites TiO 2 -ZrO 2 .
(d) The Pt/TiO 2 -ZrO 2 composites used for comparative analysis were prepared by hydrothermal synthesis and photoreduction with the same ratio of feedstock. The other monomers were prepared under the same experimental conditions.

Characterization
The phase and composition of the as-prepared samples were determined using a Bruker-AXS (D8) X-ray diffractometer (XRD) equipped with Cu Ka as the X-ray radiation source at 60 kV, 80 mA, and 2q ranging from 20 to 80 ; X-ray photoelectron spectroscopy (XPS) was performed using an ESCALAB 250Xi spectrometer equipped with an Al Ka radiation source at 300 W. SEM analysis of the samples was carried out using a Japanese Hitachi S-4700 scanning electron microscope, and the working voltage was 5 kV. The specic surface area and pore size of the samples were measured using a specic surface area instrument (Beishide Instrumentation Technologies Ltd., Model 3H-2000PS2, Beijing, China) involving nitrogen adsorption at 77 K. The UV-vis absorption spectra were obtained using a UV-vis spectrophotometer (Model TU-1901) in the wavelength range of 200-800 nm, and BaSO 4 was used as a reference. The absorbance of the sample solution was determined using a TU-1901 UV-visible doublebeam spectrophotometer (Beijing Purkinje General Co.). The photolysis of water to hydrogen was measured using a Lab-Solar-IIIAG photocatalytic line analysis system (Perfect Light Ltd., Beijing, China). Photocatalytic hydrogen production was analyzed via a GC7900 gas chromatograph (Tianmei Instrument Co., Ltd., Shanghai) operating at a voltage of 50 mV and a current of 20 mA using the external standard method.

Photocatalytic tests
The photocatalytic experimental reactors under different modes were self-made. The UV light source was a 125 W high-pressure mercury lamp (the main emission wavelength was 313.2 nm), the visible light source was a 400 W xenon lamp (the main emission wavelength was greater than 410 nm), and the simulated daylight light source was a 1000 W xenon lamp (external type, Shanghai Bao Xun Instrument Co., Ltd., the emission spectrum was close to the full spectrum). The microwave photocatalytic experiments used H-type microwave electrodeless lamps as the light source with a power of 15 W and a microwave reactor output power of 600 W; details about the microwave photocatalytic reactor can be found in the literature. 21 In the experiments, 0.15 g, 0.3 g, 0.15 g, and 0.5 g of catalyst were dispersed in the newly prepared solution (concentration of 50 mg L À1 ), and the volume was 90 mL, 220 mL, 100 mL, and 500 mL, respectively. The suspension was sonicated for 10 min, stirred in the dark for 30 min, and then placed in a photocatalytic reactor for photocatalytic experiments. Samples were obtained at regular intervals during the reaction, and their absorbance was measured at l max by a UV-vis spectrophotometer.

XRD analysis
To investigate the crystal structure of 3DOM Pt/TiO 2 -ZrO 2 , X-ray diffraction analysis of 3DOM Pt/TiO 2 -ZrO 2 , 3DOM TiO 2 -ZrO 2 , monomer TiO 2 , and ZrO 2 was carried out, and the results are shown in Fig. 1 (401) crystal planes. The diffraction peaks of 3DOM TiO 2 -ZrO 2 in the three-dimensional ordered macroporous composites are mainly located at 25.36 , 31.06 , 36.10 , 48.11 , 53.90 , and 62.87 . The diffraction peak of the labelled ring in Fig. 1 corresponds to the weaker TiO 2 -ZrO 2 diffraction peak, indicating the presence of Ti-O-Zr bonds in the composites. 24 The diffraction peaks of 3DOM Pt/TiO 2 -ZrO 2 in the three-dimensionally ordered composites are mainly located at 25.36 , 31.06 , 36.10 , 48.11 , 53.90 , and 62.87 . The characteristic peak of Pt does not appear because of the small doping amount of Pt. The abovementioned results show that the 3DOM TiO 2 -ZrO 2 and 3DOM Pt/TiO 2 -ZrO 2 composites mainly consist of a TiO 2 anatase phase and ZrO 2 tetragonal phase. 25 The average crystallite size of each synthesized product was calculated according to the Scherrer formula d ¼ Kl/(B cos q). The results are shown in Table 1, where d is the average grain diameter of the crystal grains, K is a constant (0.89), B is the diffraction peak half-width, l is the X-ray wavelength of 0.154178 nm, and q is the Bragg angle corresponding to the diffraction peak. 26 As seen in Table 1, the crystallite size of the composites is smaller than that of pure ZrO 2 and TiO 2 . Moreover, the incorporation of Pt can affect the crystallite size of the composites to a certain extent. As can be seen from the data presented in Table 1, the crystallite size of the 3DOM Pt/TiO 2 -ZrO 2 composites is signicantly larger than that of the 3DOM TiO 2 -ZrO 2 composites; this is attributed to the lattice distortion of the composite material due to Pt doping. Moreover, the unit cell parameters further prove that the grain size increases with Pt doping.

UV-visible diffuse reectance spectroscopy
To understand the optical absorption properties of 3DOM Pt/ TiO 2 -ZrO 2 , UV-vis diffuse reectance spectroscopy of the monomer TiO 2 and ZrO 2 and Pt/TiO 2 -ZrO 2 and 3DOM Pt/TiO 2 -ZrO 2 composites was carried out, as shown in Fig. 2. The absorption performance of 3DOM Pt/TiO 2 -ZrO 2 in the ultraviolet region is enhanced as compared to that of TiO 2 and ZrO 2 ; this indicates that the incorporation of Pt changes the optical absorption properties of TiO 2 and ZrO 2 . Moreover, compared with that of the Pt/TiO 2 -ZrO 2 composites, the absorption of 3DOM Pt/TiO 2 -ZrO 2 composites is higher, and some redshis occur in the UV region, indicating that the three-dimensional ordered macroporous structure can further enhance the absorption of the material. However, the visible light absorption where a is the absorption coefficient, n is the optical frequency, and A is the proportionality constant. As obtained from the abovementioned equation, the E g values of TiO 2 , ZrO 2 , Pt/TiO 2 -ZrO 2 , and 3DOM Pt/TiO 2 -ZrO 2 are 3.16 eV, 4.03 eV, 3.56 eV, and 3.02 eV, respectively. From the as-obtained results, it can be seen that the E g value of 3DOM Pt/TiO 2 -ZrO 2 decreases obviously as compared to that of TiO 2 , ZrO 2 , and the composite Pt/ TiO 2 -ZrO 2 . To a certain extent, it indicates that the Pt/TiO 2 -ZrO 2 composites with a three-dimensional ordered macroporous structure will show higher photocatalytic activity.

XPS analysis
To study the valence of the surface elements of 3DOM Pt/TiO 2 -ZrO 2 composites, XPS analysis was carried out. It can be seen from Fig. 3a that there are four elements, i.e. Pt, Zr, Ti and O, on the surface of the 3DOM Pt/TiO 2 -ZrO 2 composites. Fig. 3b shows the XPS spectrum of O 1s in the composites with the binding energies of 527.27 eV and 531.25 eV, indicating the presence of lattice oxygen and adsorbed oxygen, respectively. 28 Fig. 3c shows the XPS spectrum of Ti in the Ti 2p 3/2 and Ti 2p 1/2 binding energy regions, and the binding energy is 458.25 eV and 463.75 eV, respectively, indicating that Ti is in the Ti 4+ form. 29 Fig. 3d shows the XPS spectrum of Zr in the Zr 3d 5/2 and Zr 3d 3/2 binding energy regions, with the binding energies of 181.69 and 184.08 eV, respectively, indicating that Zr is in the Zr 4+ form. 30 Fig. 3e shows the XPS spectrum of Pt in Pt 4f 7/2 and Pt 3d 5/2 in the composites, with the binding energies of 71.51 eV and 75.45 eV, respectively, indicating that Pt exists as a single species. 31  In addition, a small number of defects in some areas are related to the arrangement of the PS microspheres, and the size of the microspheres depends mainly on the diameter of the polystyrene (PS) sphere template. It can be observed from Fig. 4 that the pore size of 3DOM Pt/TiO 2 -ZrO 2 is much smaller than that of the polystyrene (PS) colloidal spheres; this is due to the shrinkage of pores during the calcination process for removing the polystyrene (PS) colloidal spheres.

N 2 adsorption-desorption analysis
To study the surface physicochemical properties of 3DOM Pt/ TiO 2 -ZrO 2 composites, N 2 adsorption-desorption tests for Pt/ TiO 2 -ZrO 2 , 3DOM TiO 2 -ZrO 2 , TiO 2 -ZrO 2 , and 3DOM Pt/TiO 2 -ZrO 2 were carried out, as shown in Fig. 5 and Table 2. As seen in Fig. 5, the N 2 adsorption-desorption isotherms of Pt/TiO 2 -ZrO 2 , 3DOM TiO 2 -ZrO 2 , TiO 2 -ZrO 2 , and 3DOM Pt/TiO 2 -ZrO 2 are type IV adsorption curves. 32 According to the IUPAC denition, they have typical mesoporous structures. The hysteresis loop of the Pt/TiO 2 -ZrO 2 composites belongs to H2 type, and the hysteresis loop of this kind of materials is characterized by the straight hole model. Moreover, the hysteresis loop of 3DOM Pt/TiO 2 -ZrO 2 and 3DOM TiO 2 -ZrO 2 is of H3 type and caused by the aggregation of the capillary and the aggregation of the particles in the structure. In addition, the hysteresis loop of TiO 2 -ZrO 2 belongs to the bicyclic structure of H4 type. 33 3DOM Pt/TiO 2 -ZrO 2 shows a mesoporous structure mainly due to the Paper macroporous wall of the composites. It can be seen from Fig. 5a and d that the structure is different mainly because the composites shown in Fig. 5a have been synthesized using polystyrene spheres as templates and form a threedimensionally ordered macroporous structure, whereas the composites shown in Fig. 5d are composed of TiO 2 and ZrO 2 , which do not have a three-dimensionally ordered structure.
As can be seen from Table 2, 3DOM Pt/TiO 2 -ZrO 2 shows a certain decrease in the specic surface area (BET) as compared to 3DOM TiO 2 -ZrO 2 ; this may be because the doping of Pt has blocked some large pores that results in a smaller surface area.
Compared with TiO 2 -ZrO 2 , Pt/TiO 2 -ZrO 2 has a larger surface area; this may be because the presence of some amorphous structures in the material results in a larger surface area. In addition, the specic surface area of Pt/TiO 2 -ZrO 2 composites is larger than that of 3DOM TiO 2 -ZrO 2 ; this can be attributed to their different hysteresis loops and synthesis methods. Due to the use of the methanol vacuum impregnation method in the synthesis process, the precursors are very evenly lled within the pores of polystyrene microspheres, which can improve the photocatalytic activity of the composite materials.

Photocatalytic activity for the degradation of dyes
To investigate the photocatalytic activity of 3DOM Pt/TiO 2 -ZrO 2 composites, multi-mode photocatalytic experiments were carried out under UV light, visible light, simulated sunlight, and microwave-assisted irradiation. The results are shown in Fig. 6. It can be seen from Fig. 6A that the 3DOM Pt/TiO 2 -ZrO 2 composites show the best degradation effect on the organic pollutant malachite green (MG) under ultraviolet light irradiation, and the catalytic activity is remarkably enhanced aer the addition of Pt; this is similar to the result of the previous UVvisible diffuse reectance absorption spectroscopy analysis. Due to the strong absorption capacity of monomer TiO 2 and monomer ZrO 2 in the ultraviolet region, the synergistic effect between Pt and TiO 2 can be further improved by Pt doping. Upon comparing the results of direct degradation by different catalysts under UV light, it is found that 3DOM Pt/TiO 2 -ZrO 2 exhibits increased photocatalytic activity for the degradation of malachite green under ultraviolet irradiation. From Fig. 6B, it is clear that Àln C t /C 0 has a substantially linear relationship with the reaction time t; this indicates that the degradation of malachite green follows quasi-rst order reaction kinetics. The catalytic activity order for dye degradation was 3DOM Pt/TiO 2 - It can be seen from Fig. 6C that 3DOM Pt/TiO 2 -ZrO 2 has the highest photocatalytic degradation rate of malachite green under visible light degradation, simulated sunlight degradation, and microwave-assisted degradation; this indicates that the three-dimensional ordered macroporous structure can enhance the photocatalytic activity to a certain extent. However, the photocatalytic activity of 3DOM Pt/TiO 2 -ZrO 2 composites under visible light is far less than that under UV light because 3DOM Pt/TiO 2 -ZrO 2 has little absorption in the visible region (380-800 nm); therefore, its photocatalytic activity in the visible region is not high. This result is consistent with the UV-vis diffuse reectance spectroscopy analysis results. Moreover, compared with other catalytic materials, 3DOM Pt/TiO 2 -ZrO 2 has better degradation effect on malachite green under the simulated sunlight condition. The abovementioned results show that the degradation effect of 3DOM Pt/TiO 2 -ZrO 2 is further increased, and 3DOM Pt/TiO 2 -ZrO 2 has more obvious practical application effects on the dye model molecule under simulated sunlight irradiation.
Finally, the catalytic activity of the three-dimensional ordered macroporous composites under microwave irradiation is higher than that under ultraviolet light, visible light, and simulated sunlight. This is partly because under the microwaveassisted condition, the used electrodeless lamp also emits ultraviolet light, but the lamp power (15 W) is smaller than that of the ultraviolet lamp (125 W). On the other hand, due to the good crystallinity and narrow bandgap of 3DOM Pt/TiO 2 -ZrO 2 , which can induce better activity, the macroporous structure can enhance the contact area with pollutants. More contact active sites further enhance the activity of the catalyst.
As can be seen in Fig. 6D, the 3DOM Pt/TiO 2 -ZrO 2 composites show a certain degradation effect on malachite green, congo red, methylene blue, methyl orange, and salicylic acid within 120 min; this indicates that the 3DOM Pt/TiO 2 -ZrO 2 composites display a certain degree of universality in the degradation of organic pollutants.

Photocatalytic hydrogen evolution
As seen in Fig. 7, compared with P25, TiO 2 , ZrO 2 and Pt/TiO 2 -ZrO 2 , 3DOM Pt/TiO 2 -ZrO 2 possesses a certain ability in hydrogen production; this is attributed to the unique large-pore structure of 3DOM Pt/TiO 2 -ZrO 2 that allows light to be internally reected multiple times. Moreover, the neat and orderly large pore arrangement reduces light consumption and enhances the light absorption efficiency. According to the nitrogen adsorption-desorption test results, 3DOM Pt/TiO 2 -ZrO 2 composites have a larger specic surface area to varying degrees that can improve the photocatalytic activity of the sample. Upon doping the noble metal Pt, the light absorption of the catalyst is improved. Moreover, the introduction of Pt increases the migration paths of the photogenerated carriers and reduces the recombination of photogenerated electronhole pairs. In addition, Pt acts as a surface active site and increases the hydrogen production activity.

Possible photocatalytic reaction mechanism
As seen in Fig. 8a, the major active factor in the 3DOM Pt/TiO 2 -ZrO 2 composite is ($OH), whereas superoxide radicals ($O 2 À ) and holes (h + ) play an auxiliary role. Based on the abovementioned results of the catalytic activity and related experimental data, a possible photocatalytic reaction mechanism of 3DOM Pt/TiO 2 -ZrO 2 is speculated, as shown in Fig. 8b. On the one hand, the photocatalytic activity of 3DOM Pt/TiO 2 -ZrO 2 can be improved due to the plasmon resonance effect on the surface of the noble metal Pt. On the other hand, when Pt nanoparticles  are loaded on the n-type semiconductor TiO 2 and the p-type semiconductor ZrO 2 , the work function (5.65 eV) of Pt has a higher bandgap energy than that of the semiconductor TiO 2 ; therefore, a Schottky barrier is formed between them. 34 Due to the Schottky barrier, when the photocatalytic reaction of TiO 2 is conducted under light, the electrons from the separated electron-hole pairs aggregate on the contact surface of Pt and TiO 2 . Moreover, Pt can act as an electron trap, promoting the separation of electron-hole pairs and increasing the electron transfer between interfaces. The three-dimensional ordered macroporous structure provides a good channel for the transfer of the active material, thereby improving the photocatalytic activity of the material. 35 The positions of the conduction band (CB) and valence band (VB) are calculated by the following formula: The redox potentials of the conduction band and the valence band of TiO 2 are À0.18 eV and 2.98 eV, respectively. The oxidation-reduction potentials of the conduction band and the valence band of ZrO 2 are À0.62 eV and 3.41 eV, respectively. This satises the conditions for the photocatalytic degradation of organic pollutants, and 3DOM Pt/TiO 2 -ZrO 2 has the ability to photolyse water to produce hydrogen. As shown in Fig. 8b, the photoelectrons in the conduction band can react with oxygen adsorbed on the catalyst surface to generate superoxide radicals  ); moreover, the holes (h + ) remaining in the valence band can react with water to form hydroxyl radicals ($OH), which can degrade organic pollutants; in general semiconductor photocatalysts, the hydroxyl radicals do not easily contact with organic pollutants, whereas the three-dimensional ordered macroporous structure allows the organic pollutants to directly contact the hydroxyl radicals such that these pollutants can be degraded more effectively.
Moreover, the conduction band potential of 3DOM Pt/TiO 2 -ZrO 2 composites is lower than that of the hydrogen electrode (0 eV), thus meeting the condition for hydrogen evolution, and 3DOM Pt/TiO 2 -ZrO 2 can decompose water into hydrogen. Therefore, the 3DOM Pt/TiO 2 -ZrO 2 composites can catalyze both the photocatalytic degradation and photolysis of water to produce hydrogen. On the one hand, the three-dimensional ordered macroporous structure can allow organic pollutants to directly contact the active material, thereby enhancing photocatalytic degradation. On the other hand, it can provide a convenient channel for the generated hydrogen and improve the efficiency of hydrogen evolution. Therefore, the threedimensional ordered macroporous structure can enhance the photocatalytic activity of Pt/TiO 2 -ZrO 2 .

Conclusion
Herein, three-dimensional ordered macroporous Pt/TiO 2 -ZrO 2 composites were prepared by vacuum impregnation combined with photoreduction using polystyrene (PS) spheres as a template synthesized via emulsion-free polymerization. The crystal structure of 3DOM Pt/TiO 2 -ZrO 2 was better than that of other composites. Moreover, the composite material treated by the crystal template arranged in a neat and orderly manner, and the walls of the pores were formed by nanoparticle accumulation. Pt was present in the composite material as a simple substance, which could bring about some strong absorptions and redshis in the ultraviolet region. The hydrogen production activity and photocatalytic degradation activity of the threedimensional ordered macroporous Pt/TiO 2 -ZrO 2 composites were higher than those of other systems; this indicated that the synergistic effect of the three materials and the unique threedimensional ordered macroporous structure were conducive to improving the photolytic hydrogen production and photocatalytic degradation activity.

Conflicts of interest
There are no conicts to declare.