Facile synthesis of TiO₂/Ag₃PO₄ composites with co-exposed high-energy facets for efficient photodegradation of rhodamine B solution under visible light irradiation

Abstract

In this study, TiO₂/Ag₃PO₄ composites based on anatase TiO₂ nanocrystals with co-exposed {101}, {010}/{100}, {001} and [111]-facets and Ag₃PO₄ microcrystals with irregular and cubic-like polyhedron morphologies were successfully synthesized by combining hydrothermal and ion-exchange methods. The anatase TiO₂ nanocrystals with different high-energy facets were controllably prepared via hydrothermal treatment of the exfoliated [Ti₄O₉]²⁻/[Ti₂O₅]²⁻ nanosheet solutions at desired pH values. The Ag₃PO₄ microcrystal with different morphologies was prepared via the ion-exchange method in the presence of AgNO₃ and NH₄H₂PO₄ at room temperature, which was used as a substrate to load the as-prepared anatase TiO₂ nanocrystals on its surface and to form TiO₂/Ag₃PO₄ heterostructures. The apparent rate constant of the pH 3.5-TiO₂/Ag₃PO₄ composite was the highest at 12.0 × 10⁻³ min⁻¹, which was approximately 1.1, 1.2, 1.4, 1.6, 13.3, and 24.0 fold higher than that of pH 0.5-TiO₂/Ag₃PO₄ (10.5 × 10⁻³ min⁻¹), pH 7.5-TiO₂/Ag₃PO₄ (10.2 × 10⁻³ min⁻¹), pH 11.5-TiO₂ (8.8 × 10⁻³ min⁻¹), Ag₃PO₄ (7.7 × 10⁻³ min⁻¹), blank sample (0.9 × 10⁻³ min⁻¹), and the commercial TiO₂ (0.5 × 10⁻³ min⁻¹), respectively. The pH 3.5-TiO₂/Ag₃PO₄ composite exhibited the highest visible-light photocatalytic activity which can be attributed to the synergistic effects of its heterostructure, relatively small crystal size, large specific surface area, good crystallinity, and co-exposed high-energy {001} and [111]-facets. The as-prepared TiO₂/Ag₃PO₄ composites still exhibited good photocatalytic activity after three successive experimental runs, indicating that they had remarkable stability. This study provides a new way for the preparation of TiO₂/Ag₃PO₄ composite semiconductor photocatalysts with high energy crystal surfaces and high photocatalytic activity.

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1. Introduction

With the rapid development of industrialization, energy and environmental crises have become the key factors restricting the sustainable development of human society. Therefore, it is very urgent to search for suitable semiconductor photocatalysts to make full use of solar energy to split water into hydrogen, convert carbon dioxide into fuels, store energy, and degrade the organic wastewater discharged from the textile industry.^1–5 In recent decades, different types of semiconductor photocatalysts, such as carbon-cloth functionalized transition metal based electrocatalysts,⁶ quantum dot-based photocatalysts,^7–9 iron (Fe)-doped ZrO₂,¹⁰ metal–organic framework (MOF)-based heterostructured catalysts,¹¹ and ZnO/Bi₂WO₆ nanohybrids¹² have been reported. Among the well-known oxide semiconductor photocatalysts, titanium dioxide (TiO₂) has been proven to be the best choice due not only to its excellent photo-oxidization ability and low cost but also its long-term photostability and chemical stability and innocuousness.^13,14 However, the photocatalytic efficiency of TiO₂ still needs to be further improved for its practical application. The photocatalytic efficiency of TiO₂ is mainly dependent on the phase structure, crystallization, crystal size, specific surface area, and surface energy.^15,16 However, based on the principle of surface energy minimization (0.44 J m⁻² for {101} facets < 0.53 J m⁻² for {010}/{100} facets < 0.90 J m⁻² for {001} facets < 1.09 J m⁻² for {110} facets < 1.61 J m⁻² for {111} facets), the proportion of high-energy crystal surfaces in the natural and synthetic anatase TiO₂ crystals under equilibrium condition is very small, resulting in the dominant exposed {101} crystal facets (more than 94%) on its surface.^17,18 Since the pioneering work by Wen and co-workers on the synthesis of nanometer-sized anatase TiO₂ crystals with a large percentage of {101} facets by using the delaminated [Ti_1.73O₄]^1.07− nanosheets as the precursor, there has been intensive interest in the flexible and controllable synthesis of anatase crystals with varied high-energy facets, such as {001}, {010}/{100}, {110} and {111} facets.^19–22 Recently, we synthesized high-energy {010}, {001}, and [111]-faceted anatase TiO₂ nanocrystals by using the delaminated [Ti₄O₉]²⁻ and [TiO₃]²⁻ nanosheets as the precursors in the presence and absence of capping agent, which displayed superior photocatalytic and photovoltaic performance.^23–25 Although the exposed high-energy crystal surface of anatase TiO₂ crystals will be conducive to improving the photocatalytic activity and dye-sensitized solar energy performance, however, anatase TiO₂ crystals cannot suitable for applications under visible light irradiation due to its wide band gap (3.2 eV), resulting in the lower energy conversion efficiency in practical application.²⁶

In order to overcome above limitation, it is of great significance to extend the light absorption range of the anatase TiO₂ crystals to the visible light region.^27–29 Silver orthophosphate (Ag₃PO₄) is a semiconductor photocatalyst with a narrow band gap of 2.45 eV, and is often to decompose organic contaminants and oxidize water to produce oxygen under visible light irradiation.³⁰ However, the narrow band gap energy and low valence band (VB) and conduction band (CB) position of Ag₃PO₄ result in high recombination rate and weak redox capacity of photogenerated electrons and holes, which severely weaken the photocatalytic activity of Ag₃PO₄.^26,31 Therefore, it is an effective strategy to form a heterojunction by coupling anatase TiO₂ crystals with Ag₃PO₄ photocatalyst for improving the photocatalytic activity under visible-light irradiation.^32,33 Zhang et al. synthesized one-dimensional heterostructured Ag₃PO₄/TiO₂ photocatalyst with improved photocatalytic activity for degradation of 4-nitrophenol in simulant wastewater under visible light.³⁴ An et al. reported that the floating HGMs-TiO₂/Ag₃PO₄ composites exhibited superior photocatalytic performance than that of pure Ag₃PO₄ and TiO₂/Ag₃PO₄ for degradation of methylene blue solution under visible light irradiation.³ Xu et al. reported that the magnetic Ag₃PO₄/TiO₂/Fe₃O₄ heterostructured nanocomposite showed enhanced photocatalytic performance for the degradation of acid orange 7 under visible light irradiation.³⁵ Hamrouni et al. synthesized Ag doped TiO₂–Ag₃PO₄ (Ag@TiO₂–Ag₃PO₄) composites by coupling sol–gel and precipitation methods, which significantly improved the photocatalytic activity than that of the TiO₂–Ag₃PO₄ and the benchmark TiO₂ Evonik P25 for degradation of 4-nitrophenol solution under solar light irradiation.³⁶

In this study, anatase TiO₂ nanocrystals with different high energy facets were successful synthesized by using the exfoliated two-dimensional [Ti₄O₉]²⁻/[Ti₂O₅]²⁻ nanosheets, which were compounded with Ag₃PO₄ microcrystals to form a series of heterostructured TiO₂/Ag₃PO₄ composites. To our knowledge, this is the first time to study the TiO₂/Ag₃PO₄ photocatalysts formed by the combination of the anatase TiO₂ nanocrystals with high energy crystal surface and Ag₃PO₄ with different morphologies. Various catalyst characterization of the synthesized TiO₂/Ag₃PO₄ composites confirmed that TiO₂ nanocrystals with co-exposed high-energy facets were successfully attached to the surface of Ag₃PO₄ microcrystals. In comparison to the commercial TiO₂ and the pure Ag₃PO₄ samples, the heterostructured TiO₂/Ag₃PO₄ composites exhibited good photocatalytic activity for the degradation of rhodamine B under visible light irradiation, which can be attributed to the separation of the e⁻ (in Ag₃PO₄ crystal) and h⁺ (in TiO₂ nanocrystal) inhibits the charge recombination. For the as-prepared TiO₂/Ag₃PO₄ composites, the pH 3.5-TiO₂/Ag₃PO₄ exhibited the highest photocatalytic activity, which can be attributed to the synergistic effects of its relative small crystal size, large specific surface area, good crystallinity, and co-exposed high-energy {001} and [111]-facets. However, although the as-prepared TiO₂/Ag₃PO₄ composites exhibited good stability, the photocatalytic performance needs to be further improved for their practical application.

2. Materials and methods

2.1 Synthesis of TiO₂ nanocrystals

The K₂Ti₄O₉/K₂Ti₂O₅·xH₂O composite was prepared via solid-state synthesis in the present of 14.5109 g K₂CO₃ (0.105 mol, Tianjin Bodi Chemical Co., Ltd., Tianjin, China) and 15.9800 g TiO₂ (0.200 mol, Tianjin Bodi Chemical Co., Ltd., Tianjin, China) at 900 °C for 24 h. The obtained K₂Ti₄O₉/K₂Ti₂O₅·xH₂O composite (10.0 g) was dissolved in 1 M HCl aqueous (1 L, Sichuan Xilong Chemical Co., Ltd., Chengdu, China) for three days under continuous magnetic stirring conditions to obtain H₂Ti₄O₉·H₂O/H₂Ti₂O₅·H₂O composite. Then, 6.0 g H₂Ti₄O₉·H₂O/H₂Ti₂O₅·H₂O composite, 7.0 g tetramethylammonium hydroxide (TMAOH, Shanghai Dibai Biological Technology Co., Ltd., Shanghai, China) and 50 mL deionized water were mixed uniformly, which were hydrothermally treated at 100 °C for 24 h in a homogenous reaction (KLJX-8A, Yantai Keli Chemical Equipment Co. Ltd., Yantai, China) under stirring conditions to prepare TMA⁺-intercalated H₂Ti₄O₉·H₂O/H₂Ti₂O₅·H₂O compound. The resulting white TMA⁺-intercalated compound was dispersed in 500 mL of deionized water under stirring conditions for three days to obtain the nanosheets solutions containing of H₂Ti₄O₉/H₂Ti₂O₅ compound. The above nanosheets solutions were adjusted to desired pH values (0.5–11.5) at 180 °C for 24 h to prepared TiO₂ nanocrystals.

2.2 Synthesis of Ag₃PO₄ crystals

Silver orthophosphate (Ag₃PO₄) crystals were synthesized by using an ion-exchange method, using AgNO₃ (Beijing Beihua Fine Chemicals Co., Ltd., Beijing, China) and NH₄H₂PO₄ (Beijing Guangfu Technology Development Co., Ltd., Beijing, China) as the starting materials and NH₄F (Tianjin Bodi Chemical Co., Ltd., Tianjin, China) as a capping agent at room temperature. The Ag₃PO₄ sample was prepared according to the stoichiometry of AgNO₃ and NH₄H₂PO₄ to 3 : 1 (3AgNO₃ + NH₄H₂PO₄ = Ag₃PO₄↓ + NH₄NO₃ + 2HNO₃). In a typical synthesis, 3.8228 g (0.0225 mol) AgNO₃ and 0.8626 g (0.0075 mol) NH₄H₂PO₄ were dissolved in 150 mL of deionized water, respectively. Then, 1.0 g NH₄F (0.027 mol) was dissolved in the NH₄H₂PO₄ aqueous solution. After that, the AgNO₃ solution (0.15 mol L⁻¹) was transferred to pear-shaped separatory funnel and added to the NH₄H₂PO₄ aqueous solution (0.05 mol L⁻¹) drop by drop under continuous magnetic stirring. The yellow Ag₃PO₄ precipitate was obtained after 2 h, which was washed with lots of deionized water to remove unwanted ions, kept in the dark and dried at ambient temperatures.

2.3 Synthesis of TiO₂/Ag₃PO₄ composites

The well dispersed TiO₂ colloidal suspensions were obtained by dispersed 1.2 g as-prepared TiO₂ in 200 mL deionized water under stirring for 2 h. Then, 0.3 g Ag₃PO₄ precipitate was added to the above TiO₂ colloidal suspensions and kept under stirring for 2 h to generate TiO₂/Ag₃PO₄ composites (w(TiO₂) = 80%, w(Ag₃PO₄) = 20%). Finally, the composites were collected by filtering, which were washed several times, and dried at room temperature.

2.4 Characterization

The crystal structure of obtained samples were characterized by powder X-ray diffractometer (XRD) on a XRD-6100 (Shimadzu, Kyoto, Japan) with monochromated Cu Kα radiation (λ = 1.5406 Å). The data were collected for scatting angles (2θ) from 5 to 80° with a scanning speed of 8° min⁻¹. The morphology of the samples were investigated by using cold field emission scanning electron microscope (FESEM, JSM-7500F, Japan). The crystalline nanostructures were investigated using transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) (Tecnai G² F20 S-TWIN, FEI, America). The specific surface areas of the as-prepared samples were determined by using the Brunauer–Emmett–Teller (BET) method (Autosorb-IQ3, Quantachrome, America). UV-Vis-NIR spectra of the samples were obtained by using a Cary Series UV-Vis-NIR Spectrophotometer (Agilent Technologies, Cary 5000). The absorbance of rhodamine B solution was recorded within the wavelength range of 350–650 nm by using a TU-1901 UV-vis spectrophotometer (Beijing Purkinje General Instrument Co. Ltd).

2.5 Photocatalytic activity evaluations

The photocatalytic activities of the as-synthesized TiO₂/Ag₃PO₄ composites were evaluated by monitoring the degradation of rhodamine B (RhB). The irradiation source was provided by a 300 W xenon lamp equipped with a 400 nm cutoff light filter and the wavelength ranges from 400 nm to 600 nm. Typically, 75 mg TiO₂/Ag₃PO₄ composite was suspended in 150 mL RhB solution (10 ppm). Prior to illumination, the suspensions were magnetically stirred for 2 h in the dark to achieve adsorption–desorption equilibrium. At intervals of 15 min, 5 mL of suspensions were taken out and centrifuged at 2500 rpm for 10 min to remove the TiO₂/Ag₃PO₄ composites. The changes of RhB concentration during xenon light irradiation were determined by using a TU-1901 ultraviolet-visible spectrophotometer at the maximum absorption wavelength of RhB (554 nm) with deionized water as the reference solution. For comparison, the commercial TiO₂ powder (∼70.9% anatase and ∼29.1% rutile), and as-prepared Ag₃PO₄ powder were also used as the photocatalytic references. The stability and recyclability of the TiO₂/Ag₃PO₄ composites were investigated by the degradation experiments of the 10 ppm RhB solution (150 mL).

3. Results

3.1 Structure and morphological characterization

The XRD patterns of the K₂Ti₄O₉/K₂Ti₂O₅·xH₂O and H₂Ti₄O₉·H₂O/H₂Ti₂O₅·H₂O composites depicted characteristic structures. The diffraction peaks at 10.08°, 14.23°, 22.49°, 28.08°, 30.24°, 31.04°, 41.30°, 43.34° and 48.04° corresponded to the (200), (201), (−203), (310), (311), (004), (512), (205) and (020) crystal facets of K₂Ti₄O₉, which depicted the characteristic monoclinic structure of K₂Ti₄O₉ (JCPDS card no. 32-0861, space group: C2/m, lattice parameter: a = 19.968 Å, b = 3.746 Å, c = 12.025 Å and β = 114.01°) (Fig. 1(a)). The diffraction peaks at 11.10°, 32.96°and 33.78° corresponded to the (200), (10–2) and (201) crystal facets of K₂Ti₂O₅·xH₂O, which depicted the characteristic monoclinic structure of K₂Ti₂O₅·xH₂O (JCPDS card no. 46-0224, space group: C2/m, lattice parameter: a = 6.605 Å, b = 3.069 Å, c = 5.665 Å and β = 99.98°) (Fig. 1(a)). Fig. 1(b) shows XRD patterns of proton exchanged phase, the diffraction peaks located at 9.70°, 17.70°, 22.49°, 30.04°, 37.48° and 43.72° are ascribed to (20–1), (40–1), (203), (403) and (80–5) crystal facets of H₂Ti₄O₉·H₂O, respectively, and the diffraction peaks located at 9.70°, 19.40° and 24.22° are ascribed to (200), (400) and (110) crystal facets of H₂Ti₂O₅·H₂O, respectively. The basal spacing was changed from 8.77 Å for K₂Ti₄O₉ (or 11.10 Å for K₂Ti₂O₅·xH₂O) to 9.11 Å for H₂Ti₄O₉·H₂O (or 9.11 Å for H₂Ti₂O₅·H₂O), indicating the protonation of K₂Ti₄O₉/K₂Ti₂O₅·xH₂O composite occur successfully. Based on the above analysis, it can be seen that the K₂Ti₄O₉/K₂Ti₂O₅·xH₂O composite and the protonated products H₂Ti₄O₉·H₂O/H₂Ti₂O₅·H₂O have been successfully prepared.

Fig. 1 XRD patterns of the layered (a) K₂Ti₄O₉/K₂Ti₂O₅·xH₂O composite and the protonic titanate (b) H₂Ti₄O₉·H₂O/H₂Ti₂O₅·H₂O composite.

Fig. 2(a) presents the XRD pattern of the Ag₃PO₄ obtained by simple ion-exchange method. The diffraction peaks at 20.89°, 29.72°, 33.12°, 36.60°, 52.72°, 55.06°, 57.30°, 61.68° and 71.92° corresponded to the (110), (200), (210), (211), (222), (320), (321), (400) and (421) crystal facets of Ag₃PO₄, which depicted the characteristic cubic structure of Ag₃PO₄ (JCPDS card no. 06-0505, space group: P43̄n, lattice parameter: a = 6.013 Å and β = 90°) (Fig. 1(a)). After hydrothermal treatment of the exfoliated H₂Ti₄O₉/H₂Ti₂O₅·H₂O nanosheets composites, both composites transformed into anatase phase TiO₂ completely. The TiO₂/Ag₃PO₄ composites were prepared by mixed the obtained anatase TiO₂ nanocrystals and Ag₃PO₄ in water. Fig. 2(b–h) shows the XRD patterns of the obtained TiO₂/Ag₃PO₄ composites, except for the characteristic diffraction peaks of the Ag₃PO₄ crystals, other diffraction peaks at around 25.32°, 37.84°, 38.60°, 48.06°, 53.98°, 55.04°, 62.84°, 68.82°, 70.34° and 75.24° corresponded to the (101), (004), (112), (200), (105), (211), (204), (116), (220) and (215) crystal facets of anatase TiO₂ (JCPDS card no. 21-1272, crystal system: tetragonal, space group: I4₁/amd, lattice parameter: a = 3.7852 Å and c = 9.5139 Å). The diffraction peaks of TiO₂/Ag₃PO₄ composites are shifted slightly to the right, which can be attributed to the basic crystal plane spacing (d_basic) of the crystal plane varies slightly. The d_basic values of TiO₂(101) (Ag₃PO₄(110)) are 3.514 (4.244), 3.509 (4.236), 3.507 (4.234), 3.512 (4.247), 3.515 (4.250), 3.512 (4.244), and 3.512 Å (4.244 Å) for the pH 0.5-TiO₂/Ag₃PO₄, pH 1.5-TiO₂/Ag₃PO₄, pH 3.5-TiO₂/Ag₃PO₄, pH 5.5-TiO₂/Ag₃PO₄, pH 7.5-TiO₂/Ag₃PO₄, pH 9.5-TiO₂/Ag₃PO₄, and pH 11.5-TiO₂/Ag₃PO₄, respectively. Moreover, for the as-prepared pH 5.5-TiO₂/Ag₃PO₄ composite, a weak impurity peak was observed at 18.04°. The intensities of anatase TiO₂ and Ag₃PO₄ crystals indicate that the TiO₂/Ag₃PO₄ composites are well crystallized and no diffraction peaks attributed to rutile or brookite are detected. It can be seen that with increasing pH value, the peak intensities of anatase TiO₂ increase and the width of the (101) crystal facets diffraction peak of anatase TiO₂ (2θ = 25.32°) become narrow, indicating the increase of the average crystalline sizes and relative crystallinity of the TiO₂/Ag₃PO₄ composites. The diffraction peaks of TiO₂(101)/Ag₃PO₄ composites synthesized are relatively broad, which may be ascribed to the small size of TiO₂/Ag₃PO₄ composites. Based on the broadening of (101) peaks of the TiO₂/Ag₃PO₄ composites specimens (b–h) in Fig. 2, the average crystalline size of the specimens can be calculated as 23.0, 23.2, 23.5, 26.4, 28.0, 29.7, and 32.0 nm for pH 0.5-TiO₂/Ag₃PO₄, pH 1.5-TiO₂/Ag₃PO₄, pH 3.5-TiO₂/Ag₃PO₄, pH 5.5-TiO₂/Ag₃PO₄, pH 7.5-TiO₂/Ag₃PO₄, pH 9.5-TiO₂/Ag₃PO₄, pH 11.5-TiO₂/Ag₃PO₄, respectively.

Fig. 2 XRD patterns of the (a) Ag₃PO₄, (b) pH 0.5-TiO₂/Ag₃PO₄, (c) pH 1.5-TiO₂/Ag₃PO₄, (d) pH 3.5-TiO₂/Ag₃PO₄, (e) pH 5.5-TiO₂/Ag₃PO₄, (f) pH 7.5-TiO₂/Ag₃PO₄, (g) pH 9.5-TiO₂/Ag₃PO₄, and (h) pH 11.5-TiO₂/Ag₃PO₄ composites specimens obtained from the exfoliated H₂Ti₄O₉/H₂Ti₂O₅·H₂O nanosheets composites.

Morphology of the TiO₂/Ag₃PO₄ composites and the pure Ag₃PO₄ specimens was determined by FESEM. The FESEM images of the TiO₂/Ag₃PO₄ composites that were synthesized under different pH values conditions (pH 0.5–11.5) are shown in Fig. 3(a)–(g). Results show that there were no significant differences in the morphology of the TiO₂/Ag₃PO₄ composites synthesized at pH 0.5–11.5, and all the nanocrystals are severely agglomerated together. When the pH is 0.5, many square rod-shaped anatase nanocrystals with about 70–160 nm in length and 40–50 nm in width, a lot of cuboid-shaped anatase nanocrystals with 25–110 nm in length and 20–60 nm in width, lots of shuttle-like anatase nanocrystals with the size about 60–110 nm in length and 30–50 nm in width and a large number of egg-like anatase nanocrystals with about 30–60 nm in the central axis length and 15–30 nm in the central axis width are observed, as shown in Fig. 3(a). Fig. 3(b) shows the FESEM image of the pH 1.5-TiO₂/Ag₃PO₄ composite, it can seen that some square rod-shaped anatase nanocrystals with about 60–140 nm in length and 30 nm in width, some cuboid-shaped anatase nanocrystals with 35–50 nm in length and 30–40 nm in width, several shuttle-like anatase nanocrystals with about 90–150 nm in length and 45 nm in width, and a large numbers of egg-like anatase nanocrystals with about 20–70 nm in length and 15–40 nm in width are observed. When the pH value rises to 3.5, many cuboid-shaped anatase nanocrystals with 40–80 nm in length and 35–70 nm in width, some shuttle-like anatase nanocrystals with 95–185 nm in length and 40–80 in width, lots of spheroidal anatase nanocrystals with 20–45 nm in diameter, and several diamond-shaped anatase nanocrystals with 50–90 nm in length and 20–60 nm in width are observed, as shown in Fig. 3(c). Fig. 3(d) shows the representation FESEM image of the pH 5.5-TiO₂/Ag₃PO₄ composite prepared by mixed the pH 5.5-TiO₂ and Ag₃PO₄ samples. As shown in Fig. 3(d), egg-like anatase nanocrystals with a size of 30–60 nm and 30–70 nm in length in high yield, some shuttle-like anatase nanocrystals with a size of about 20–50 nm in width and 60–150 nm in length, and cuboid-shaped anatase nanocrystals with a size of 30–70 nm in width and 55–90 nm in length are observed. The FESEM image in Fig. 3(e) shows that the pH 7.5-TiO₂/Ag₃PO₄ composite that has two main morphologies, shuttle-like anatase nanocrystals with 40–200 nm in length and 25–50 nm in width, and cuboid-shaped anatase nanocrystals with 40–95 nm in length and 30–55 nm in width. Fig. 3(f) and (g) show FESEM images of the pH 9.5-TiO₂/Ag₃PO₄ and pH 11.5-TiO₂/Ag₃PO₄ composites, respectively. It can be seen that the prepared composites have similar morphologies, square rod-like (or cuboid-shaped) anatase nanocrystals with a length of about 50–130 nm (or 25–90 nm) and a width of about 30–50 nm (25–70 nm), spheroidal anatase nanocrystals with 20–95 nm (or 20–95 nm) in diameter, and shuttle-like anatase nanocrystals with a length of about 30–210 (or 30–215 nm) nm and a width of about 20–65 (or 20–85 nm) nm for pH 9.5-TiO₂/Ag₃PO₄ (or pH 11.5-TiO₂/Ag₃PO₄) composites. FESEM images of Ag₃PO₄ microcrystals are shown in Fig. 3(h) and (i), it can be seen that well-dispersed irregular Ag₃PO₄ polyhedrons with about 3–12 μm in length and 2.5–9.0 μm in width (or thickness), and cubic-like particles with the size about 1.5–7.0 μm were obtained. And the surface of the Ag₃PO₄ crystals is rough, which is formed by the agglomeration of many nanoparticles with the size about 30–50 nm in diameter (Fig. 3(i)). Based on the above analysis, the Ag₃PO₄ crystals were not observed in the TiO₂/Ag₃PO₄ composites, which can be ascribed to the fact that the sizes of Ag₃PO₄ crystals were micrometer while the anatase TiO₂ crystals were nanometer, and TiO₂ nanocrystals were bound to the surface of Ag₃PO₄ microcrystals.

Fig. 3 FESEM images of (a) pH 0.5-TiO₂/Ag₃PO₄, (b) pH 1.5-TiO₂/Ag₃PO₄, (c) pH 3.5-TiO₂/Ag₃PO₄, (d) pH 5.5-TiO₂/Ag₃PO₄, (e) pH 7.5-TiO₂/Ag₃PO₄, (f) pH 9.5-TiO₂/Ag₃PO₄, and (g) pH 11.5-TiO₂/Ag₃PO₄ composites specimens, and (h and i) Ag₃PO₄ specimens.

The FESEM images and the corresponding elemental distribution maps of TiO₂/Ag₃PO₄ composites were achieved by energy dispersive spectrometer (EDS). As shown in Fig. 4, the appearance of Ag and P elements in EDS further demonstrated successful impregnation of Ag₃PO₄. The analysis of the results shows the atomic ratio of Ag to Ti is about 1 : 27.86, 1 : 71.55, 1 : 58.27, and 1 : 121 for pH 0.5-TiO₂/Ag₃PO₄, pH 3.5-TiO₂/Ag₃PO₄, pH 7.5-TiO₂/Ag₃PO₄, and pH 11.5-TiO₂/Ag₃PO₄ composites, respectively.

Fig. 4 FESEM images and the corresponding EDS with elemental mapping images of (a and b) pH 0.5-TiO₂/Ag₃PO₄, (c and d) pH 3.5-TiO₂/Ag₃PO₄, (e and f) pH 7.5-TiO₂/Ag₃PO₄, and (g and h) pH 11.5-TiO₂/Ag₃PO₄ composites specimens.

The TEM and HRTEM images further reveal the detailed surface morphology of the obtained TiO₂/Ag₃PO₄ composites products, as shown in Fig. 5 and 6. For pH 0.5-TiO₂/Ag₃PO₄, shuttle-like anatase nanocrystals with the length of about 30–85 nm and the width of about 15–25 nm, and square rod-shaped anatase nanocrystals with the length of about 25–140 nm and the width of about 15–50 nm are observed (Fig. 5(a)), which corresponds to the results of FESEM (Fig. 3(a)). The square rod-shaped nanocrystals with a lattice of 0.353 nm (or 0.359 nm) can be indexed to the (101) planes of the anatase, and the egg-like nanoparticle with a lattice fringes of 0.359 nm also can be indexed to the (101) planes of the anatase (Fig. 5(b)–(f)). The lateral planes of square rod-shaped nanocrystals are parallel to (101) planes, indicating that the exposed facets are {101} facets (Fig. 5(b) and (c)). The lattice fringe has d-spacing values of 0.236 and 0.246 nm, corresponding to (004) and (103) planes of anatase TiO₂, respectively (Fig. 5(c)). The long axis of the shuttle-like anatase nanocrystals is perpendicular to (004) planes, indicating that the exposed facets are {001} facets of the top and bottom planes (Fig. 5(c)). In Fig. 5(e), the lattice fringes of the irregular crystals with lattice spacings of 0.235 and 0.353 nm can be assigned to the (004) and (101) planes of the anatase TiO₂, respectively. And the angle between the (004) and (101) facets is 68°, implying that the irregular crystals expose {010} facets on its surface. The coexistence of various morphologies of the pH 3.5-TiO₂/Ag₃PO₄ composites was further investigated by TEM and HRTEM, as shown in Fig. 5(j–l). For the cuboid-shaped anatase nanocrystals, the TEM images depict the nanocrystals with 15–50 nm in length and 15–30 nm in width (Fig. 5(g)), and the lattice fringe has d-spacing values of 0.353 (or 0.360) and 0.353 nm, corresponding to (101) and (011) planes of anatase TiO₂, respectively (Fig. 5(h) and (l)). The interior angle between (101) and (011) planes of 82° is in good agreement with the theoretical value, which indicates that the preferentially exposed crystal facets of the cuboid-shaped anatase is perpendicular to [111] crystal zone axis (expressed as [111]-facets). For the shuttle-like anatase nanocrystals, the TEM images depict the nanocrystals with 15–120 nm in length and 10–45 nm in width (Fig. 5(g)), and the lattice fringe has d-spacing values of 0.360 nm, corresponding to (101) planes of anatase TiO₂ (Fig. 5(k)). For the diamond-shaped anatase nanocrystals, the TEM images depict the nanocrystals with 35–85 nm in length and 15–35 nm in width (Fig. 5(g)), and the lattice fringe has d-spacing values of 0.191 and 0.360 (or 0.364) nm, corresponding to (200) and (101) planes of anatase TiO₂, respectively (Fig. 5(h)–(j)). The lateral planes of the diamond-shaped anatase nanocrystals is parallel to (101) planes, indicating that the exposed facets are {101} facets of the lateral planes. For the square rod-shaped anatase nanocrystals, the TEM images depict the nanocrystals with 40–135 nm in length and 20–30 nm in width (Fig. 5(g)), and the lattice fringe has d-spacing values of 0.360 nm, corresponding to (101) planes of anatase TiO₂ (Fig. 5(l)). The top and bottom planes of square rod-shaped anatase nanocrystals are parallel to (101) planes, indicating that the exposed facets are {101} facets.

Fig. 5 TEM and HRTEM images of (a–f) pH 0.5-TiO₂/Ag₃PO₄ and (g–j) pH 3.5-TiO₂/Ag₃PO₄ composites specimens.

Fig. 6 TEM and HRTEM images of (a–f) pH 7.5-TiO₂/Ag₃PO₄ and (g–o) pH 11.5-TiO₂/Ag₃PO₄ composites specimens.

Fig. 6(a)–(f) shows the TEM and HRTEM analysis results of the pH 7.5-TiO₂/Ag₃PO₄ composite. The size of cuboid-shaped anatase nanocrystals has a size about 30–60 nm in length and 20–35 nm in width, as shown in Fig. 6(a). The size of shuttle-like anatase nanocrystals is about 25–250 nm in length and 20–75 nm in width (Fig. 6(a) and (d)), and the lattice fringe of 0.349, 0.476 and 0.360 nm corresponds to the distance between two adjacent (10-1), (002) and (101) planes of anatase TiO₂, and the intersection angles between (10-1) and (002), (101) and (002), and (101) and (10-1) planes are 68.3°, 68.3°, and 43.4°, respectively, as shown in Fig. 6(b). The high crystallized shuttle-like TiO₂ surfaces with the clear lattice fringes of the anatase phase are also observed from Fig. 6(c) and (f). Two set of non-parallel lattice fringes with the d-spacing values of 0.349 and 0.238 nm, corresponding to (101) and (004) atomic planes of anatase phase (Fig. 6(c)). The lattice spacing of 0.352 and 0.476 nm of the truncated shuttle-like TiO₂ anatase TiO₂, corresponding to the distance between two adjacent (101) or (002) planes, and the intersection angle between (101) and (002) planes is 68.3°, as shown in Fig. 6(e). Based on the above TEM and HRTEM analysis and the Wulff construction model, the shuttle-like anatase TiO₂ nanocrystals preferentially expose the {010} facets, {101} facets, and {001} facets on the four lateral planes, the eight isosceles trapezoid planes, and the two top/bottom surfaces, respectively, and the directional grown direction is along the [001]-direction. The size of shuttle-like (or cuboid-shaped) anatase nanocrystals is about 50–180 nm (or 25–100 nm) in length and 25–50 nm (or 20–80 nm) in width, as shown in Fig. 6(g, j and m). {010} facets exposed TiO₂ exhibits a typical shuttle-like morphology with lattice fringes of 0.353 (or 0.360) and 0.174 (or 0.238, 0.482) nm attributed to (101) and (006) (or (004), (002)) crystallographic planes, respectively, and an interfacial angle of 68.3° between the {101} and {001} planes, as shown in Fig. 6(h, i and k). In addition, {010} facets exposed shuttle-like TiO₂ nanocrystals also has d-spacing values of 0.173 (or 0.234, 0.189) and 0.482 (or 0.238, 0.238) nm, corresponding to (105) (or (112), (200)) and (002) (or (004), (004)) crystallographic planes, respectively, and an interfacial angle of 26.7° (or 60.6°, 90°) between the {105} (or (112), (200)) and {112} (or (004), (004)) planes, as shown in Fig. 6(k, l and o). Fig. 6(n) exhibits a typical TEM image of cuboid-shaped anatase nanocrystals, the fringe spacing of 0.191 nm corresponding to the (200) planes of anatase TiO₂, indicating that the exposed crystal facets of the top/bottom of the nanocrystals are {100} facets.

The morphology and microstructure of the Ag₃PO₄ crystals were further analyzed by TEM and HRTEM images, as shown in Fig. 7. As can be seen in Fig. 7(a), the obtained Ag₃PO₄ crystals contains some irregular polyhedrons with the lengths of 1.0–3.7 μm and a cubic-like crystals with the lengths of about 1.75 μm and the widths of about 1.45 μm, respectively, which is in agreement with the results observed by the SEM images (Fig. 3(h) and (i)). The lattice fringes of 0.269 (or 0.262) and 0.247 (or 0.239) nm match well with the (210) and (2-1-1) (or (211)) planes of irregular polyhedral Ag₃PO₄ crystals, respectively (Fig. 7(b)–(d)). And the angle between the (210) and (2-1) facets of 57° agrees well with the theoretical value 56.8°, according to calculated result from the lattice constants of Ag₃PO₄ (cubic, space group P43̄n, JCPDS 06-0505, and a = 6.013 Å). Based on the above TEM and HRTEM analysis, the Ag₃PO₄ specimens in the TiO₂/Ag₃PO₄ composites were not observed, which can be attributed to the deposition of nanoscale anatase TiO₂ crystals on the microsized Ag₃PO₄ crystals via an in situ precipitation process.

Fig. 7 (a) TEM and (b–d) HRTEM images of Ag₃PO₄ specimens.

3.2 Growth mechanism of the TiO₂/Ag₃PO₄ composites

According to the results of the XRD, SEM and HR-TEM observation, the possible growth mechanism for the formation of TiO₂/Ag₃PO₄ hybrids can be expressed as follows. Firstly, the TMA⁺-intercalated H₂Ti₄O₉/H₂Ti₂O₅ compounds (TMA⁺-H₂Ti₄O₉/H₂Ti₂O₅) were exfoliated into nanosheets solutions under stirring conditions.

TMA⁺-H₂Ti₄O₉ + H₂O → TMA⁺ + [Ti₄O₉]²⁻ + H₃O⁺ (1)

TMA⁺-H₂Ti₂O₅ + H₂O → TMA⁺ + [Ti₂O₅]²⁻ + H₃O⁺ (2)

The positive ions of TMA⁺ and H₃O⁺ located on surface of [Ti₄O₉]²⁻/[Ti₂O₅]²⁻ nanosheets to balance the negative charge of [Ti₄O₉]²⁻/[Ti₂O₅]²⁻ so that the nanosheets remain electrically neutral. Then, the nanosheets solutions containing of [Ti₄O₉]²⁻/[Ti₂O₅]²⁻ compounds (pH = 0.5–11.5) were transformed to anatase TiO₂ nanocrystals under hydrothermal conditions by the following reaction.

[Ti₄O₉]²⁻ + 2H⁺ → 4TiO₂ + H₂O (3)

[Ti₂O₅]²⁻ + 2H⁺ → 2TiO₂ + H₂O (4)

[Ti₄O₉]²⁻ + H₂O → 4TiO₂ + 2OH⁻ (5)

[Ti₂O₅]²⁻ + H₂O → 2TiO₂ + 2OH⁻ (6)

Acidic condition is beneficial for reactions (3) and (4), neutral and basic conditions are favorable for reactions (5) and (6). In this process, the [Ti₄O₉]²⁻/[Ti₂O₅]²⁻ nanosheets were transformed firstly to nanosheet-like anatase TiO₂ crystals by an in situ topotactic dehydration reaction.³⁷ Then the nanosheet-like anatase TiO₂ crystals were split into anatase TiO₂ nanocrystals with various morphologies and different exposed facets by dissolution–recrystallization process along their different planes.

The micro-sized Ag₃PO₄ crystals were synthesized by using an ion-exchange method, using AgNO₃ and NH₄H₂PO₄ (3Ag⁺ + H₂PO₄⁻ = Ag₃PO₄↓ + 2H⁺). The anatase TiO₂ nanocrystals with various morphologies and different exposed facets and Ag₃PO₄ precipitate were well dispersed into deionized water under stirring to form suspension solution. The micro-sized Ag₃PO₄ polyhedrons with larger particle surface, which could absorb more nano-sized anatase TiO₂ nanocrystals onto their surfaces via an in situ precipitation process to form the heterostructured TiO₂/Ag₃PO₄ composites.

3.3 UV-vis adsorption spectra of Ag₃PO₄, TiO₂/Ag₃PO₄ and TiO₂

The UV-visible absorption spectrum was applied to examine the optical properties of pure Ag₃PO₄, TiO₂ and TiO₂/Ag₃PO₄ composites. As observed in Fig. 8, the UV-Vis NIR spectrum of pure TiO₂ sample only exhibits the fundamental absorption band edge (395 nm) in the UV light region, and the absorption band edge almost no more exists in the visible wavelength range. The pure Ag₃PO₄ sample shows strong adsorption with absorption band edge at around 500 nm, which is equivalent to the band gap energy of 2.45 eV, in an good agreement with the results reported previously.³⁸ However, for the prepared TiO₂/Ag₃PO₄ composites at different values of pH, except for adsorption band edge (less than 408 nm) in the UV light region, a feature band edge (510 nm) of pure Ag₃PO₄ appears in the visible light range based on the UV-Vis NIR spectrum. The absorption edges of TiO₂/Ag₃PO₄ composites are shifted slightly toward higher wavelength relative to pure Ag₃PO₄, indicating TiO₂ in the composites is coupled to Ag₃PO₄. The above analysis show that the as-prepared TiO₂/Ag₃PO₄ composites can be used for visible light photocatalytic reactions.

Fig. 8 UV-Vis NIR Spectra of pure TiO₂, pure Ag₃PO₄, pH 0.5-TiO₂/Ag₃PO₄, pH 3.5-TiO₂/Ag₃PO₄, pH 7.5-TiO₂/Ag₃PO₄, and pH 11.5-TiO₂/Ag₃PO₄ composites.

3.4 Photocatalytic activities for the degradation of rhodamine B solutions

Recently, different types of photocatalysts, such as Mn-doped ZrO₂,³⁹ carbon quantum dots,⁴⁰ MOFs,¹¹ BaTiO₃,⁴¹ were used to degrade the organic pollutants. In this study, the photocatalytic activities of the TiO₂/Ag₃PO₄ composites were evaluated by degradation of the carcinogenic textile dye rhodamine B (RhB, adsorption band: 554 nm). The degradation efficiency of all the specimens is expressed as (c₀ − c_t)/c₀ × 100%, where c₀ and c_t represent the initial and residual concentration of the RhB, respectively. Prior to illumination, the suspensions were magnetically stirred in the dark for 2 h to make the RhB dyes reach achieve adsorption–desorption equilibrium on the surface of TiO₂/Ag₃PO₄ composites.⁴² The adsorption values (mol(RhB) g(TiO₂/Ag₃PO₄)⁻¹) of RhB on the surface of TiO₂/Ag₃PO₄ composites were 4.0 × 10⁻⁶, 7.0 × 10⁻⁶, 5.5 × 10⁻⁶, and 4.5 × 10⁻⁶ mol g⁻¹ for pH 0.5-TiO₂/Ag₃PO₄, pH 3.5-TiO₂/Ag₃PO₄, pH 7.5-TiO₂/Ag₃PO₄, and pH 11.5-TiO₂/Ag₃PO₄ samples, respectively. These results indicated that the enhancement order of adsorption binding of the RhB to the TiO₂/Ag₃PO₄ was pH 0.5-TiO₂/Ag₃PO₄ < pH 11.5-TiO₂/Ag₃PO₄ < pH 7.5-TiO₂/Ag₃PO₄ < pH 3.5-TiO₂/Ag₃PO₄, and that the strong anchoring of the RhB onto the surface of pH 3.5-TiO₂/Ag₃PO₄ could improve the photocatalytic activity. The commercial TiO₂ powder (∼70.9% anatase and ∼29.1% rutile) and Ag₃PO₄ powder were used as the photocatalytic references. Fig. 9(a) shows the variation of the absorption of rhodamine B (RhB) in the presence of pH 0.5-TiO₂/Ag₃PO₄ composite under the Xe light irradiation for 120 min. The peak position at 554 nm gradually moved towards the short-wavelength direction (i.e., hypsochromic shift) and the intensity gradually decreased, indicating the partial N-de-ethylation and the destruction of structure of the polycyclic aromatic hydrocarbon by the gradual decolorization of the RhB solution.⁴³

Fig. 9 (a) Temporal evolution of rhodamine B (RhB) over pH 0.5-TiO₂/Ag₃PO₄ composite, (b) photocatalytic degradation of RhB solution over blank, TiO₂, Ag₃PO₄, and TiO₂/Ag₃PO₄ composites under Xe light irradiation with a 400 nm cutoff filter, (c) the kinetic fit of the RhB degradation by blank, TiO₂, Ag₃PO₄, and TiO₂/Ag₃PO₄ composites, and (d) the corresponding apparent rate constants.

After exposure to visible light for 120 min, the degradation of RhB was as follows: pH 3.5-TiO₂/Ag₃PO₄ (75.6%) > pH 0.5-TiO₂/Ag₃PO₄ (72.2%) > pH 7.5-TiO₂/Ag₃PO₄ (65.8%) > pH 11.5-TiO₂/Ag₃PO₄ (61.3%) > Ag₃PO₄ (56.0%) > blank (10.8%) > the commercial TiO₂ (5.8%), as shown in Fig. 9(b). Obviously, the as-prepared TiO₂/Ag₃PO₄ composites exhibit enhanced photocatalytic performance for the degradation of RhB compared to the commercial TiO₂ powder and Ag₃PO₄ powder. The enhanced photocatalytic performance can be attributed to the TiO₂/Ag₃PO₄ heterostructures, which can absorb more visible light and inhibit the recombination of photoelectrons and holes.³⁴ Fig. 10 shows a possible photocatalytic mechanism for the photodegradation of RhB over the TiO₂/Ag₃PO₄ heterostructures under visible light irradiation. The valence band (VB) potential (+2.90 eV vs. NHE) and conduction band (CB) potential (+0.45 eV vs. NHE) of Ag₃PO₄ are more positive than those of TiO₂ (VB potential: +2.70 eV, and CB potential: −0.30 eV), which imply that the photon generated electrons (e⁻) of TiO₂ nanocrystal will be quickly transferred to the CB of Ag₃PO₄ crystal, whereas the photon generated holes (h⁺) of Ag₃PO₄ crystal will be migrated to the VB of TiO₂ nanocrystal under visible light irradiation.^30,44 The separation of the e⁻ (in Ag₃PO₄ crystal) and h⁺ (in TiO₂ nanocrystal) inhibits the charge recombination, which leads to the improvement of the photocatalytic activity of TiO₂/Ag₃PO₄ composites.³² The h⁺ and e⁻ have oxidation and reduction, respectively. Under visible light irradiation, the h⁺ in the VB of TiO₂ nanocrystal can directly oxidize the organic dye RhB and the water molecules adsorbed to the surface of TiO₂ photocatalyst to form RhB oxidation and ˙OH radicals, respectively.³ At the same time, the e⁻ in the CB of Ag₃PO₄ crystal can directly reduce the oxygen molecules adsorbed to the surface of Ag₃PO₄ photocatalyst to form strong oxidizing capacity of hydrogen peroxide (H₂O₂) to oxidize and degradation RhB. Moreover, Ag₃PO₄ is reduced to Ag by e⁻ in the photocatalytic process. The 10 mg L⁻¹ RhB solution (10 ppm) was not completely degraded due to the addition of more RhB solution (150 mL) and fewer catalysts (75 mg), and the liquid level of RhB solution was far away from the light source (25 cm). However, TiO₂ exhibited very low photocatalytic activity for the photodegradation of RhB, only 5.8% degradation efficiency, even lower than 10.8% for the blank without any photocatalysts under the Xe light irradiation for 120 min, implying that the TiO₂ actually had no any photocatalytic activity. Based on the discussion results of TiO₂ and the blank, it is reasonable that the presence of photocatalyst has a shielding effect on the degradation of RhB under the Xe light irradiation.⁴⁵

Fig. 10 Possible photocatalytic mechanism for the photodegradation of RhB over the TiO₂/Ag₃PO₄ composites under visible light irradiation.

Since the process of photodegradation of RhB solution followed the pseudo-first-order reaction kinetics model, the fitted pseudo-first-order reaction plots, the correlation coefficient and the corresponding apparent rate constant (k_app) are shown in Fig. 9(c) and (d), respectively. The correlation coefficients (R²) are 0.968, 0.971, 0.943, 0.955, 0.997, 0.939, and 0.977 for the blank, TiO₂, Ag₃PO₄, pH 0.5-TiO₂/Ag₃PO₄, pH 3.5-TiO₂/Ag₃PO₄, pH 7.5-TiO₂/Ag₃PO₄, and pH 11.5-TiO₂/Ag₃PO₄, respectively. The pH 3.5-TiO₂/Ag₃PO₄ composite exhibited the highest k_app value (12.0 × 10⁻³ min⁻¹), which was approximately 24.0, 13.3, 1.6, 1.4, 1.2, and 1.1 times larger than those of the commercial TiO₂ (0.5 × 10⁻³ min⁻¹), blank (0.9 × 10⁻³ min⁻¹), Ag₃PO₄ (7.7 × 10⁻³ min⁻¹), pH 11.5-TiO₂/Ag₃PO₄ (8.8 × 10⁻³ min⁻¹), pH 7.5-TiO₂/Ag₃PO₄ (10.2 × 10⁻³ min⁻¹), and pH 0.5-TiO₂/Ag₃PO₄ (10.5 × 10⁻³ min⁻¹) samples, respectively. The pH 3.5-TiO₂/Ag₃PO₄ composite had the highest k_app value, indicating that the pH 3.5-TiO₂/Ag₃PO₄ composite had the highest photocatalytic activity.

The stability and recyclability of photocatalyst is one of the important parameters for its practical applications. Herein, the stability and recyclability of the pure Ag₃PO₄ and TiO₂/Ag₃PO₄ composites were evaluated by examining their recyclability in the photodegradation of RhB. Fig. 11 exhibited the repetitive photocatalytic degradation of RhB solution (10 mg L⁻¹, 150 mL) during three sequential runs under identical conditions. After each run, TiO₂/Ag₃PO₄ and Ag₃PO₄ photocatalysts were collected by centrifugation and washed with deionized water for several times and the fresh RhB solutions with the same concentration (10 mg L⁻¹) were used for next run. The photocatalytic efficiency of the TiO₂/Ag₃PO₄ and Ag₃PO₄ photocatalysts remained almost unchanged after three successive experimental runs, indicating that the synthesized photocatalysts had remarkable stability.

Fig. 11 Recycling studies of the pure Ag₃PO₄, pH 0.5-TiO₂/Ag₃PO₄, pH 3.5-TiO₂/Ag₃PO₄, pH 7.5-TiO₂/Ag₃PO₄, and pH 11.5-TiO₂/Ag₃PO₄ photocatalysts for the photocatalytic degradation of RhB solution.

As is well-known, the photocatalytic activity is not only related to the heterostructure of TiO₂ nanocrystals, but also influenced by other factors, such as crystalline phase, crystalline size, crystallinity, specific surface area, exposed facets, and so on.⁴⁶⁻⁴⁸ For the synthesized TiO₂/Ag₃PO₄ composites, the crystalline form (anatase) and proportion (w(TiO₂) : w(Ag₃PO₄) = 4 : 1) of TiO₂ are the same, indicating that the influence of TiO₂ crystal form and proportion on photocatalytic activity in TiO₂/Ag₃PO₄ composites is negligible. In the TiO₂/Ag₃PO₄ composites, the average crystalline sizes of pH 0.5-TiO₂/Ag₃PO₄, pH 3.5-TiO₂/Ag₃PO₄, pH 7.5-TiO₂/Ag₃PO₄, and pH 11.5-TiO₂/Ag₃PO₄ were 58.5, 68.5, 87.5 and 80.0 nm, respectively, by measuring 200 particles in the FESEM images with Particle Size Distribution Calculation Software (Fudan University, China). And the specific surface areas were 32.6, 27.8, 21.8, 24.4 m² g⁻¹ for pH 0.5-TiO₂/Ag₃PO₄, pH 3.5-TiO₂/Ag₃PO₄, pH 7.5-TiO₂/Ag₃PO₄, and pH 11.5-TiO₂/Ag₃PO₄, respectively. It is known that smaller crystal size and larger specific surface (favorable for the RhB adsorption) contribute to the enhancement of photocatalytic activity in the photochemical reaction, which is attributed to its strong oxidation-reduction capability and more active sites.^49,50 However, the pH 3.5-TiO₂/Ag₃PO₄ composite displayed the highest photocatalytic activity, although the crystal size (68.5 nm) is much bigger than that (58.5 nm) of the pH 0.5-TiO₂/Ag₃PO₄ composite, and the specific surface area (27.8 m² g⁻¹) slightly smaller than that (32.6 m² g⁻¹) of the pH 0.5-TiO₂/Ag₃PO₄ composite, indicating that it is also very significant to establish a balance between crystal size and specific surface area to improve the photocatalytic performance. On the other hand, the crystallinity of the pH 0.5-TiO₂/Ag₃PO₄ composite is better than that of pH 3.5-TiO₂/Ag₃PO₄ composite, which inhibits the recombination of photogenerated charge carriers (photogenerated electrons and holes), resulting in a relative good photoactivity.¹⁵

The surface energies of {101}, {010} (or {100}), {001}, {110} and [111]-facets of anatase TiO₂ are 0.44, 0.53, 0.90, 1.09, and 1.61 J m⁻², respectively.^21,51 In generally, in the photocatalytic reaction, the crystal surface with high-energy crystal facets usually exhibits high photocatalytic activity. Based on the discussion above, the anatase TiO₂ nanocrystals preferentially co-exposed the {101}/{001}/{010} facets, {101}/{001}/[111]-facets, {101}/{010} facets, {101}/{010} (or {100}) facets on their surfaces in the pH 0.5-TiO₂/Ag₃PO₄, pH 3.5-TiO₂/Ag₃PO₄, pH 7.5-TiO₂/Ag₃PO₄, pH 11.5-TiO₂/Ag₃PO₄ composites, respectively. Hence, the improvement of photocatalytic activity of the pH 3.5-TiO₂/Ag₃PO₄ can also be attributed to the coexistence of high-energy {001} and [111]-facets. According to the discussion above, the pH 0.5-TiO₂/Ag₃PO₄ composite possesses a relative small crystal size, large specific surface area, good crystallinity, and co-exposed high-energy {001} and [111]-facets, the synergistic effects resulting in the highest photocatalytic activity of the pH 3.5-TiO₂/Ag₃PO₄ composite.

4. Conclusions

In summary, TiO₂/Ag₃PO₄ composites composed of anatase TiO₂ nanocrystals with co-exposed {101}, {010}/{100}, {001} and [111]-facets and Ag₃PO₄ microcrystals with irregular polyhedrons and cubic-like crystals were successfully synthesized by combining hydrothermal and ion-exchange methods. The Ag₃PO₄ microcrystals were used as a substrate to load the anatase TiO₂ nanocrystals on their surface and to form TiO₂/Ag₃PO₄ heterostructures. To investigate the photocatalytic performance, the carcinogenic RhB solution was selected as model pollutant because it widely used in the textile industry. Compared with the commercial TiO₂ and the pure Ag₃PO₄ microcrystals, the heterostructured TiO₂/Ag₃PO₄ composites exhibited excellent photocatalytic activity for the degradation of rhodamine B under visible light irradiation, which can be attributed to the separation of the e⁻ (in Ag₃PO₄ crystal) and h⁺ (in TiO₂ nanocrystal) inhibits the charge recombination. For the as-prepared TiO₂/Ag₃PO₄ composites, the pH 3.5-TiO₂/Ag₃PO₄ composite exhibited the highest photocatalytic activity which mainly attributed to the synergistic effects of its relative small crystal size, large specific surface area, good crystallinity, and co-exposed high-energy {001} and [111]-facets. Moreover, this study provides new way for the preparation of TiO₂/Ag₃PO₄ composite semiconductor photocatalysts with high energy crystal surfaces. However, although the as-prepared TiO₂/Ag₃PO₄ composites exhibited good stability, the photocatalytic performance needs to be further improved for their practical application.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was financially supported by the Applied Basic Research Project of Shanxi (No. 201901D111303), the Doctor Research Funds of Jinzhong University, the Shanxi “1331 Project” Key Innovation Team (No. PY201817), the Jinzhong University “1331 Project” Key Innovation Team (No. jzxycxtd2017004 and No. jzxycxtd2019005), Scientific and Technologial Innovation Programs of Higher Education Institutions in Shanxi (No. 2019L0883), the Educational Department of Liaoning Province (No. L2019036), the Liaoning Provincial Natural Science Foundation (No. 20170540583), the National Science Foundation of China (No. 51272030 and 51572031), and the Grants-in-Aid for Scientific Research (B) (No. 26289240) from the Japan Society for the Promotion of Science and Kagawa University.

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