Fabrication of TiO₂ nanosheets via Ti³⁺ doping and Ag₃PO₄ QD sensitization for highly efficient visible-light photocatalysis

Abstract

Visible-light photocatalysis has attracted much attention in environmental remediation and sustainable energy utilization, however, it is still a great challenge to develop highly efficient and stable visible-light photocatalyst. Herein, we developed Ag₃PO₄ quantum dots (QDs) sensitized and Ti³⁺-doped TiO₂ nanosheets (NS) via a solvothermal/in situ precipitation method. The TiO₂/Ag₃PO₄ ratio in the composite was tuned from 3 : 1 to 1 : 4 to optimize the dispersion and size of Ag₃PO₄ QDs, and the best dispersed Ag₃PO₄ QDs with the smallest size (ca. 2 nm) was obtained for TA1 : 3. The characterizations confirm that abundant Ti³⁺ defects are introduced into TiO₂, and the interaction between Ag₃PO₄ QDs and TiO₂ NS is in the form of Ag–O–Ti bonds, which benefit the visible-light absorption and accelerates the charge separation. Moreover, the well-matched band structures drive the electrons to Ag₃PO₄ and holes to TiO₂ {001} faces, respectively. Therefore, TA1 : 3 shows a 1.7-fold, 1.4-fold and 5-fold higher activity than bulk Ag₃PO₄ in MO, phenol photodegradation, and PEC water splitting, respectively. In addition, the sample shows relatively high photostability. Thus, we believe that the rational design of heterostructures based on the matched band and abundant defects can fabricate the highly reactive photocatalysts.

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1. Introduction

Photocatalysis has attracted considerable attention as it provides a new and benign approach to meet the challenges from the environment, energy and sustainability.^1–3 In order to effectively utilize the visible light, a range of photocatalysts have been designed and fabricated, such as Ag₃PO₄, CdS, WO₃, C₃N₄, doped TiO₂ or ZnO, and so on.^4–12 Among them, Ag₃PO₄ has received much attention because of its excellent visible-light response, due to the suitable bandgap (2.45 eV) and very high quantum yield for photooxidation (such as O₂ revolution).^4,13–16 However, there are several drawbacks that hinder the practical application of Ag₃PO₄ in photocatalysis: first, the total quantum efficiency of Ag₃PO₄ is relatively low due to the inherent fast charge recombination. Second, the particle size of Ag₃PO₄ (0.5 to 2 μm) is too large to achieve high photocatalytic performance.^17–19 Third, AgNO₃ is a common precursor for the fabrication of Ag₃PO₄, but it is very expensive, prohibiting the large-scale production of Ag₃PO₄. To address these issues, many strategies have been developed to rationally design the stable and efficient Ag₃PO₄-based composites using other band-matched materials, such as BiPO₄,²⁰ Fe₃O₄,²¹ AgBr,²² ZnO,²³ SnO₂,²⁴ CdS,¹⁶ g-C₃N₄,⁷ etc. Specially, the quantum dot (QD) of Ag₃PO₄ serving for the sensitization to the composites can realize the unilateral charge separation and spatial isolation.

Making composite with Ag₃PO₄, TiO₂ nanosheets (TiO₂ NS, with {001} facet mainly exposed) is one of the most potential candidates.²⁵ Yang et al. made breakthrough in synthesizing TiO₂ NS with high percentage of {001} facets.²⁶ After that, TiO₂ NS have been studied extensively in photocatalysis, such as photoelectrochemical (PEC) water splitting for hydrogen production, environmental remediation, CO₂ photoreduction and solar cells,^8,27–32 attributing to its high photoactivity, nontoxicity, low cost and chemical inertness.³³ The in situ observation of fluorogenic reaction confirmed that,³⁴ on a TiO₂ NS, the photoinduced electrons preferentially transfer to {101} facets with the holes to {001} facets. Since the exposed surface of {001} faces is more than 80% on TiO₂ NS, it can provide abundant hole-trapping sites for charge-pairs isolation. Importantly, the band structure of Ag₃PO₄ is well matched with TiO₂, and the photoinduced holes prefer to transfer from Ag₃PO₄ to TiO₂, with electrons reversely to Ag₃PO₄.^19,35 Therefore, Ag₃PO₄ QD/TiO₂ NS composite will realize the spatial isolation of photoinduced charges.

But TiO₂, with the wide band gap of 3.2 eV, cannot be exited by visible light. The Ti³⁺ doping is an important approach to extend the visible-light absorption,^36–39 because of the introduced localized Ti³⁺ states with energies 0.15–1.18 eV below the conduction band minimum of TiO₂. And the relevant experiments confirmed the photoactivity improvement of Ti³⁺-doped TiO₂ under visible light.⁴⁰

In this work, we fabricate highly photoactive TiO₂ NS via Ti³⁺ doping and Ag₃PO₄ QDs sensitization, by a solvothermal/in situ precipitation method. The TiO₂/Ag₃PO₄ ratio was tuned to optimize the QDs' size and dispersion, and the interaction between TiO₂ and Ag₃PO₄ was also investigated. The sensitization of Ag₃PO₄ QDs and introduction of Ti³⁺ on TiO₂ NS result in low charge recombination possibility and unilateral charge transfer, giving rise to the photoactivity of degradation and photoelectrochemical (PEC) water splitting.

2. Experimental

2.1. Materials

CH₃COOAg was obtained from Tianjin Kermei Chemical Reagent Co., Ltd. Tetrabutyltitanate (TBT), anhydrous ethanol, NaOH, hydrogen fluoride (HF, 40 wt%), methyl orange (MO), Na₂SO₄ and Na₂HPO₄·12H₂O were purchased from Tianjin Guangfu Fine Chemical Research Institute. Phenol was obtained from J&K Chemical. Deionized water (>16.0 MΩ cm) was prepared in the lab and used in all experiments. All the reagents were used as received.

2.2. Sample preparation

2.2.1 Preparation of TiO₂ nanosheets (NS). TiO₂ NS were prepared by the hydrothermal method.²⁷ In a typical synthetic process, 25 mL TBT was dissolved in 3.5 mL HF aqueous solution (40 wt%. Caution! Hydrofluoric acid is extremely corrosive and a contact poison, and it should be handled with extreme care). The mixed solution was stirred vigorously for 20 min. Then the obtained precursor solution was transferred to a 100 mL Teflon-lined autoclave. The autoclave was heated at 180 °C for 36 h. After that, the obtained white powders were centrifuged, washed with deionized water and ethanol, and dried at 60 °C overnight. 0.5 g of the prepared sample was dispersed in 20 mL 10 M NaOH aqueous solution in order to remove the surface F⁻ of the sample. Then the mixture was ultrasonically treated for 60 min. Finally, the sample was rinsed with deionized water and ethanol for several times, and dried at 60 °C overnight. The resulted white powders are {001} faceted TiO₂ NS.

2.2.2 Decoration of Ag₃PO₄ QDs on TiO₂ NS. Ag₃PO₄ QDs decorated TiO₂ NS were prepared by a precipitation method under room temperature. 50 mg of as-prepared TiO₂ NS were dispersed in 50 mL deionized water with the assistance of ultrasonication for 10 min. Then, 0.2 g CH₃COOAg was added into the solution under stirring. After that, 40 mL 0.01 M Na₂HPO₄ aqueous solution was dropped in and kept stirring for 120 min in dark. The obtained powders were centrifuged, washed with deionized water, and dried at 60 °C overnight. The mass ratio of TiO₂ to Ag₃PO₄ was tuned from 3 : 1 to 1 : 1, 1 : 3 and 1 : 4 by modulating the added amount of CH₃COOAg, and the resulted samples were named as TA3 : 1, TA1 : 1, TA1 : 3, TA1 : 4, respectively. Pure Ag₃PO₄ was also prepared using the above mentioned method but without the addition of TiO₂ NS.

2.3. Characterizations

XRD characterization was conducted using a D/MAX-2500 X-ray diffractometer equipped with Cu Kα radiation. SEM images were observed using a field-emission scanning electron microscope (Hitachi S-4800). High-resolution TEM observations were carried out with a Tecnai G² F-20 transmission electron microscope. Surface composition and chemical states were analyzed with a PHI-5000 X-ray photoelectron spectroscope (XPS) equipped with Al Kα radiation, and the binding energy was calibrated by the C1s peak (284.6 eV) of the contamination carbon. UV-vis diffuse reflectance spectra (UV-vis DRS) were recorded with a Hitachi U-3010 spectrometer equipped with a 60 mm diameter integrating sphere using BaSO₄ as the reflectance sample. Steady-state photoluminescence (PL) spectra were measured by a Horiba JobinYvon Fluorolog3-21.

2.4. Photocatalytic reaction

The photocatalytic activities of samples were evaluated by degradation of MO and phenol in a cylindrical quartz vessel under visible light irradiation. 50 mg photocatalyst was dispersed in 100 mL MO (0.06 mM) or phenol (0.2 mM) solution with magnetic stirring. The reactor was kept in dark for 20 min after the adsorption–desorption equilibrium. The reactor was then vertically irradiated by a 300 W high-pressure xenon lamp (CEL-HXUV300, Beijing Aulight Co., Ltd.) with an optical filter (λ ≥ 400 nm). The irradiation area of the light source was ca. 20 cm². The temperature of the reaction was controlled at 25 ± 1 °C. Samples were withdrawn, centrifuged and analyzed using UV-vis spectrometer (U-3010, Hitachi Ltd.), by monitoring the characteristic absorption wavelength of 463 nm and 270 nm for MO and phenol, respectively.

2.5. Photoelectrochemical (PEC)/electrochemical tests

PEC/electrochemical properties were measured using an Ivium Vertex electrochemical workstation in a three-electrode cell with a Pt wire as the counter electrode and an Ag/AgCl reference electrode. Na₂SO₄ (0.2 M) aqueous solution was used as electrolyte solution. The working electrode was prepared by dip-coating sample slurry on an F-doped tin oxide (FTO) glass electrode (1 cm × 1 cm). Electrochemical impedance spectroscopy (EIS) measurements were carried out with a sinusoidal ac perturbation of 10 mV applied over the frequency range of 0.01–10⁵ Hz.

3. Result and discussion

3.1. Crystal structure

The XRD patterns of prepared samples are shown in Fig. 1. The TiO₂ NS is in the phase of anatase, with the main diffraction peak at 2θ = 25.3° ([101] plane). After the decoration of Ag₃PO₄, its diffraction peaks appear with the main peak at 2θ = 33.3° ([210] plane), and the peak intensity gradually increases with the decrease of TiO₂/Ag₃PO₄ mass ratio. Finally, the pure Ag₃PO₄ is produced when no TiO₂ NS are added into the synthetic solution. For Ag₃PO₄ and Ag₃PO₄/TiO₂ composites, no diffraction peaks of metallic Ag or other impurities have been detected.

Fig. 1 XRD patterns of TiO₂, Ag₃PO₄ and Ag₃PO₄/TiO₂ composites, the standard XRD patterns of Ag₃PO₄ and TiO₂ anatase are based on JCPDS card no. 06-0505 and no. 21-1272, respectively.

3.2. Sample morphology

As shown in Fig. 2a, SEM images show that pure TiO₂ are square nanosheets with the average length of ca. 70 nm and the thickness of ca. 8 nm, which are the same results with the previous work.²⁷ For pure Ag₃PO₄ (Fig. 2c), the diameter of irregular spherical particles is about 30–400 nm, and the serious aggregation is observed. From TEM images (Fig. 2b and d), pure TiO₂ and Ag₃PO₄ both show high degree of crystallinity. The lattice fringe spacing of 0.19 nm (Fig. 2b) is attributed to anatase (200) and (020) atomic planes, indicating TiO₂ NS are mainly exposed with {001} facets.⁴¹ In Fig. 2d, the lattice fringe of 0.24 nm is referred to Ag₃PO₄ (211) planes.¹⁵

Fig. 2 SEM images of TiO₂ NS (a and b) and bulk Ag₃PO₄ (c and d).

The aggregation of Ag₃PO₄ is harmful for photoactivity, while the well dispersed nanoparticles are very promising.⁴² Hereby, TiO₂ NS serve as the support for the well dispersed growth of Ag₃PO₄ nanoparticles. As shown in Fig. 3, the decorated Ag₃PO₄ are nanoparticles and their aggregations have been inhibited effectively. There are no obvious size changes for TiO₂ NS, while the average size of Ag₃PO₄ nanoparticles is dependent on its deposited amount. Under low amount loading (TA3 : 1, Fig. 3a–c), Ag₃PO₄ nanoparticles loosely disperse on TiO₂ NS, and they have the wide size range from 2–14 nm with the average size of 6 nm. With the increase of Ag₃PO₄ loading amount (TA1 : 1, Fig. 3d–f; and TA1 : 3, Fig. 3g–i), the average particle size of Ag₃PO₄ is reduced to 4.5 nm and 2 nm, respectively, and their size distribution becomes narrower, especially for TA1 : 3. However, further increase of Ag₃PO₄ loading amount (TA1 : 4) causes the aggregation of Ag₃PO₄ nanoparticles with the average size of 4.2 nm (Fig. 3j–l). Accordingly, using TiO₂ NS as support, Ag₃PO₄ QDs have been successfully fabricated. For all samples, Ag₃PO₄ QDs show high degree of crystallinity with the lattice fringe spacing of 0.24 nm ((211) planes).

Fig. 3 TEM images and size distribution of Ag₃PO₄ for TA3 : 1 (a–c), TA1 : 1 (d–f), TA1 : 3 (g–i) and TA1 : 4 (j–l).

3.3. Surface composition

As shown in Fig. 4, XPS characterizations of TiO₂, Ag₃PO₄ and TA1 : 3 were conducted to investigate the chemical composition and status of the materials. For pure Ag3d spectra of Ag₃PO₄ (Fig. 4a), the peaks at ca. 373.8 eV and 367.7 eV are assigned to Ag3d_3/2 and Ag3d_5/2 of Ag⁺, respectively.^19,43 After the loading of Ag₃PO₄ QDs on TiO₂ NS (TA1 : 3, Fig. 4e), the locations of Ag3d_3/2 and Ag3d_5/2 attributing to Ag⁺ are almost the same as that of pure Ag₃PO₄, while the new peaks at 365.9 eV and 371.8 eV are owing to the formed Ag–O–Ti bonds.⁴³ The low-energy shift of Ag⁺ binding energy in the form of Ag–O–Ti is resulted from the weaker electrical negative of Ti than that of P. The presence of the Ti–O–Ag bonds indicates a strong covalent interaction exists between Ag₃PO₄ QDs and TiO₂ NS, confirming the formation of the heterojunction structure, which is vital for electron–hole separation.⁴⁴

Fig. 4 XPS spectra of Ag₃PO₄ (a–c), TiO₂ NS (d) and TA1 : 3 (e–h): (a and e) Ag3d spectra; (b and f) O1s spectra; (c and g) P2p spectra; (d and h) Ti2p spectra.

Fig. 4b shows the high-resolution O1s spectra of pure Ag₃PO₄, with a symmetric peak located at ca. 531.3 eV, which is attributed to O–P species.⁴³ However, the decoration of Ag₃PO₄ QDs on TiO₂ NS alters the chemical environments of O species (Fig. 4f). The asymmetric peak is divided into four peaks centered at 532.8 eV, 531.2 eV, 530.2 eV and 529.0 eV, which are related to oxygen species in adsorbed H₂O, P–O, Ti–O–Ag and Ti–O moieties, respectively.^42,43,45 Similar to the results of Ag3d spectra, the appearance of Ti–O–Ag suggests the interaction between Ag₃PO₄ QDs and TiO₂ NS. P2p spectra of pure Ag₃PO₄ and TA1 : 3 are shown in Fig. 4c and g. These two materials both show one peak with the binding energy of ca. 132.5 eV, attributing to P⁵⁺ in PO₄³⁻.⁴³

The oxidation state of Ti element in TiO₂ NS is shown in Fig. 4d, two kinds of Ti species are found. The peaks at the binding energy of 458.8 eV (Ti2p_3/2) and 464.3 eV (Ti 2p_1/2), and their splitting of 5.7 eV are referred to Ti⁴⁺ species of bulk TiO₂.^27,43,45 The other is the defected Ti³⁺ with the binding energy centered at 457.7 eV and 463.7 eV.^27,45 This kind of Ti³⁺ defects are caused during the hydrothermal treatment in the presence of HF. Previous calculation confirmed that Ti³⁺ tends to be stabilized at five-coordinated surface sites,⁴⁶ and actually, all surface Ti atoms on {001} TiO₂ NS are five-coordinated.⁴⁷ As shown in Fig. 4h, Ag₃PO₄ QDs-TiO₂ NS (TA1 : 3) still maintain the doped Ti³⁺. But the characteristic peaks of Ti2p_1/2 and Ti2p_3/2 become broader as compared to pure TiO₂ NS, which indicates the existence of new Ti species. There are two new peaks located at 459.8 eV and 466.9 eV, which are owing to the bond of Ti–O–Ag.⁴³ The higher binding energy of Ti in Ti–O–Ag is resulted from the stronger electrical negative of Ag than Ti. From the above XPS results of TA1 : 3, it can be seen that Ti³⁺–TiO₂ NS and Ag₃PO₄ QDs are not just physically mixed, however, they have chemical bond of Ti–O–Ag, suggesting that Ag₃PO₄ QDs have been effectively loaded on Ti³⁺–TiO₂ NS.

3.4. Optical absorption and charge separation

UV-vis DRS spectra of Ag₃PO₄, TiO₂ and Ag₃PO₄ QDs-TiO₂ NS are exhibited in Fig. 5a. It clearly shows that bulk Ag₃PO₄ can absorb visible light with a wavelength less than 546 nm, corresponding to the band gap of ca. 2.27 eV,¹⁹ while TiO₂ NS have an absorption edge at ca. 420 nm, with the band gap of ca. 2.95 eV. The narrowed band gap is owing to the Ti³⁺ doping.¹ With the gradually increasing amount of Ag₃PO₄ QDs in the composites, the absorption edges are red-shifted and the calculated band gap energy (inset in Fig. 5a) is obviously decreased. The introduction of Ti³⁺ and loading of Ag₃PO₄ QDs on TiO₂ NS make the composite to show optical absorption in the visible-light range, and this will increase the visible-light catalytic activity of TiO₂ NS.

Fig. 5 (a) UV-vis DRS and (b) steady-state PL spectra of Ag₃PO₄ QDs-TiO₂ NS composites. The inset in (a) the corresponding plots of transformed Kubelka–Munk function versus the photo energy.

The efficient separation of photo-induced charges is vital for the high photocatalytic activity.³ The photoluminescence (PL) spectra of Ag₃PO₄ and Ag₃PO₄ QDs-TiO₂ NS composites are tested (Fig. 5b). The emission peaks at 420 nm are corresponding to the shallow donor level of Ti³⁺ state to the valence band (2.95 eV). The Ag₃PO₄ has the strongest emission band between 450 and 750 nm, due to the rapid recombination of excited electrons and holes (2.22 eV). The results are consistent with the UV-vis absorption. Moreover, the decoration of Ag₃PO₄ QDs on TiO₂ NS greatly influences the PL intensity, with obvious decrease of PL intensity for the composites as compared to pure Ag₃PO₄. Initially, the emission peak intensity gradually declines with the decrease of TiO₂/Ag₃PO₄ ratio. Especially when the ratio of TiO₂/Ag₃PO₄ is 1 : 3, the PL emission intensity is the lowest. However, when TiO₂/Ag₃PO₄ ratio is lower than 1 : 3, such as TA1 : 4, the PL emission intensity increases again, but is still lower than pure Ag₃PO₄. The PL spectra indicate that the composition of Ag₃PO₄ QDs and TiO₂ NS inhibits the recombination of photoinduced charge pairs, and TA1 : 3 possesses the highest charge-separation efficiency.

3.5. Photocatalytic performance

The model photocatalytic reactions, degradation of organic compounds (methyl orange-MO and phenol), were conducted to evaluate the photoactivity of as-synthesized samples. During the photodegradation, the formation of hydroxyl radical (˙OH) caused by electrons and holes is crucial.^33,48–50 Another important model, photoelectrochemical (PEC) water splitting, is an important model to evaluate the activity of photocatalyst.^48–51 It is emerging as the promising methods for solar hydrogen and oxygen generation, and has attracted increasing interest. In most case, the performance of PEC cell is largely determined by the properties of the electrode. Hence in this work, the above two typical photocatalytic tests were carried out to evaluate the photoactivity of the materials.

The photocatalytic activities of TiO₂, Ag₃PO₄, and Ag₃PO₄ QDs-TiO₂ NS composites were firstly evaluated by the degradation of MO and phenol under visible-light irradiation (Fig. 6a–d). Before irradiation, the samples were stirred in the dark for 20 min to reach adsorption–desorption equilibrium. Under visible-light irradiation, TiO₂ NS without Ti³⁺, treated by 5 h thermal calcination under 500 °C, show very low visible-light activity with only ca. 1.5% MO degraded and almost no degradation of phenol observed in 100 min. But, TiO₂ NS with abundant Ti³⁺ shows obvious visible-light-catalytic activity, with the reaction rate constant (k) of 0.002 min⁻¹ (MO degradation) and 0.004 min⁻¹ (phenol degradation). Therefore, Ti³⁺ doping plays an important role in visible-light response for TiO₂. Meanwhile, Ag₃PO₄ shows relatively high photoactivity with k of 0.022 min⁻¹ for (MO degradation) and 0.021 min⁻¹ for phenol degradation, respectively.

Fig. 6 Photoreaction and the corresponding pseudo-first-order kinetic fitting of MO degradation (a and b) and phenol degradation (c and d), photoelectrochemical water splitting ((e), I–t curve, E = 1.0 V vs. RHE) under visible light, and (f) EIS curves.

Ag₃PO₄ QDs sensitized TiO₂ NS exhibit much higher photocatalytic activity than TiO₂ NS. With the increase of Ag₃PO₄ QDs amount, from TA3 : 1 to TA1 : 1 and then to TA1 : 3, the photoactivities are enhanced significantly (Fig. 6a–d). Particularly, TA1 : 3 exhibits the highest activity with k values of 0.037 min⁻¹ for MO degradation and 0.030 min⁻¹ for phenol degradation, which are respectively 1.7-fold and 1.4-fold higher than those of bulk Ag₃PO₄. However, further increasing the amount of Ag₃PO₄ QDs causes the decrement of photoactivity, such as TA1 : 4.

The similar trend is also observed in photoelectrochemical (PEC) water splitting (Fig. 6e), TiO₂ NS exhibit the visible-light response but the photocurrent density is ca. 8 μA cm⁻² (at 1.0 V vs. RHE), while that for Ag₃PO₄ is ca. 10 μA cm⁻². Interestingly, the composite, TA1 : 3, shows very high PEC performance with the photocurrent density of ca. 52 μA cm⁻² (at 1.0 V vs. RHE), which is about 5-fold higher than that of pure Ag₃PO₄. The above results indicate that the loading amount of Ag₃PO₄ QDs on TiO₂ NS is very important for the high photoactivity.

With smaller size and better dispersion of Ag₃PO₄ QDs, TA1 : 3 provides more active sites for reactant adsorption, mass transfer and further degradation. Hence, the composite structure with appropriate amount and well dispersion of Ag₃PO₄ QDs is optimal to achieve high photoactivity. Besides of the structure, the charge-separation efficiency also plays the important role in photocatalysis. To improve the effective charge separation, the formation of heterostructure is one of the most potential strategy.³³ As shown in Fig. 7, the VB and CB edge potentials of Ag₃PO₄ are 2.9 eV and 0.45 eV, respectively, while those of TiO₂ are 2.7 eV and −0.25 eV (for Ti³⁺ state), respectively, which are both negative to that of Ag₃PO₄.⁴³ Therefore, Ti³⁺–TiO₂ and Ag₃PO₄ can form well matched heterostructure. When the composite is irradiated by visible light, the electrons of Ag₃PO₄ VB can be excited to CB and creates holes in VB. Then, the holes are rapidly transferred to VB of TiO₂ {001} facets, while the electrons are maintained in Ag₃PO₄ CB. In contrast, the photoinduced electrons of Ti³⁺–TiO₂ will transfer to Ag₃PO₄ QDs, with the holes in VB. In this case, the charge-pairs separation and spatial location realize on the composite under visible light irradiation. Importantly, the key to obtain high charge-separation efficiency is the well dispersion of Ag₃PO₄ QDs on TiO₂ NS. Therefore, TA1 : 3 shows the lowest emission intensity in PL spectra (Fig. 5b). In addition, the electrochemical impedance spectroscopy (EIS) technique was also used to elucidate the kinetics and mechanism of photocatalytic performance, and smaller arc radius implies smaller charge transfer resistance. Since the charge transfer resistance (R_ct) value is inversely proportional to the electron transfer rate, TA1 : 3 has the fastest electron transfer rate, which is consistent with PL spectra and the photocatalytic performance.

Fig. 7 Schematic diagram of the photocatalytic mechanism of Ag₃PO₄ QDs-TiO₂ NS composite under visible light irradiation.

Finally, the photostability of TA1 : 3 was tested in MO photodegradation (Fig. 8a) and PEC performance (Fig. 6e). After 5 cycles for photodegradation and 10 cycles for PEC water splitting, TA1 : 3 still retains more than 90% of the initial photoactivity. For the recycled samples, no other phases besides Ag₃PO₄ and TiO₂ anatase are observed from the XRD patterns (Fig. 8b), and the doped Ti³⁺ are also very stable (Fig. 8c). But the careful characterization of Ag3d XPS shows the appearance of trace Ag⁰ in the recycled sample (Fig. 8d), which is the reason for the indistinctively decreased stability in photocatalysis.

Fig. 8 5-Cycle photodegradation of MO catalyzed by TA1 : 3 under visible light (a), XRD patterns (b), and Ti2p (c) and Ag3d (d) XPS spectra of recycled TA1 : 3.

4. Conclusions

Ag₃PO₄ QDs sensitized and Ti³⁺ doped TiO₂ NS have been successfully synthesized via a solvothermal/in situ precipitation approach. The TiO₂/Ag₃PO₄ ratio was tuned from 3 : 1 to 1 : 4 to optimize the composite heterostructure, and the well dispersed Ag₃PO₄ QDs with small size of ca. 2 nm was obtained for TA1 : 3. XPS data indicate Ag₃PO₄ QDs and Ti³⁺–TiO₂ NS are not simply physical mixture but interact with each other in the form of Ag–O–Ti connections, which can accelerate the charge separation and transfer between the well matched band structures of Ag₃PO₄ QDs and Ti³⁺–TiO₂ NS. Therefore, TA1 : 3 shows 1.7-fold, 1.4-fold and 5-fold higher activity than bulk Ag₃PO₄ in MO, phenol photodegradation, and PEC water splitting, respectively. In addition, Ag₃PO₄ QDs-TiO₂ NS composite shows excellent photostability. Our work reveals that the rational design of heterostructure based on the matched band structure along with abundant defects can fabricate the efficient materials for photocatalysis.

Acknowledgements

The authors appreciate the supports from the National Natural Science Foundation of China (21506156, U1462119), the Tianjin Municipal Natural Science Foundation (15JCZDJC37300, 16JCQNJC05200), Shandong Provincial Natural Science Foundation (ZR2015PB006, ZR2015BL022), and Science and Technology Key Projects of Shandong Province (2014GGH217001).

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