Journal ofMaterials Chemistry A

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View Article OnlineView Journal | View Issue

Designed synthe

aDepartment of Chemistry, Tsinghua Univer

zhuyf@tsinghua.edu.cnbAnhui Provincical Key Laboratory for Degra

Environment, School of Chemistry and

University, Fuyang 236037, P. R. China

† Electronic supplementary informationand S2. See DOI: 10.1039/c9ta00580c

Cite this: J. Mater. Chem. A, 2019, 7,6482

Received 16th January 2019Accepted 18th February 2019

DOI: 10.1039/c9ta00580c

rsc.li/materials-a

6482 | J. Mater. Chem. A, 2019, 7, 648

sis of a p-Ag2S/n-PDI self-assembled supramolecular heterojunction forenhanced full-spectrum photocatalytic activity†

Jun Yang,a Hong Miao,a Wenlu Li,a Huiquan Lib and Yongfa Zhu *ab

Herein, an efficient full-spectrum responsive p-Ag2S/n-PDI (perylenediimide) heterojunction was

successfully constructed. The self-assembled PDI nanostructure was formed via hydrogen bonding and

p–p stacking. Ag2S quantum dots were tightly loaded onto the surface of PDI nanofibers via a two-step

electrostatic process. When the mass ratio of Ag2S to PDI was 1 : 0.6, the p-Ag2S/n-PDI heterojunction

showed optimum photocatalytic properties. The full-spectrum photocatalytic activity of p-Ag2S/n-PDI

was found to be 5.13 and 1.79 times higher than pure PDI for phenol degradation and O2 evolution,

respectively. The results showed that Ag2S quantum dots promoted the well-organized p–p stacking

degree of the self-assembled PDI, which was helpful for the migration of photo-generated electrons

along the quasi-one-dimensional p–p stacking of PDI. Simultaneously, Ag2S quantum dots were found

to enhance the light absorption of Ag2S/PDI. More interestingly, the p-Ag2S/n-PDI heterojunction

exhibited excellent photoelectric properties, indicating more effective separation of carriers, which arose

as a result of the built-in electric field between the Ag2S and PDI. Besides this, the p-Ag2S/n-PDI

heterojunction produces more active species than pure PDI, resulting in a much stronger oxidation

ability. This work details some interesting ideas for designing efficient heterojunction photocatalysts that

have a supramolecular organic nanostructure.

1. Introduction

Invigorated by the present worsening of the environment andserious energy crisis, efficient use of sustainable energy sourcesis becoming more and more important. Among all of therenewable energies, solar energy has been widely used due to itsnon-pollution, cleanliness and abundance.1 In consideration ofmaking the most of solar energy, it is one of the most chal-lenging missions to search for photocatalysts with a broadspectral response. The solar spectrum consists of about 5% UVand 95% visible and near-infrared light. At present, most pho-tocatalysts, represented by titanium dioxide, can only absorbultraviolet light.2 For this reason, researchers have put a lot ofeffort into developing visible light catalysts, such as C3N4,3

Bi2WO6 (ref. 4) and BiVO4.5 Unfortunately, their photocatalyticperformance is very poor under full spectrum light. It is theultimate aim to realize the conversion of light and chemicalenergy by using the full solar spectrum in photocatalysis.

sity, Beijing 100084, P. R. China. E-mail:

dation and Monitoring of Pollution of the

Materials Engineering, Fuyang Normal

(ESI) available: Fig. S1–S11, Tables S1

2–6490

Therefore, it is indisputable that searching for full-spectrum-response photocatalysts is crucial.

At present, organic photocatalytic materials are widelyused.6,7 Unlike inorganic semiconductors, organic semi-conductors have many preponderances, such as various struc-tural modications, tunable optical and electronic properties,low cost, and rich element resources. Organic catalysts mainlycomprise three types: organometallic complexes,8,9 covalentorganic polymers10–12 and non-covalent self-assembly supra-molecular organics.13,14 Recently, a representative n-typeorganic semiconductor, PDI-based materials (PDI ¼ per-ylenediimide), have been used in photocatalytic degradationand light-driven water splitting,15,16 due to them having highelectron affinity, superior photo-thermal stability, and excellentcharge mobility properties. PDI-based semiconductors areresponsive to visible light. However, PDI-based materials havevery weak photocatalytic properties under UV light and in thefull spectrum region. Besides this, they are confronted with theeasy recombination of photo-induced carriers. Many modica-tion methods have been reported in order to improve the pho-tocatalytic properties of PDI-based semiconductors, which havegenerally intended to increase the internal electric eld of PDI.The modication methods include forming a p–p stackingstructure,17 adjusting the electron acceptor–donor units18 andintroducing functional groups.19 These methods can improvethe electron transfer rate to different degrees, but their effects

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are negligible in terms of the utilization of ultraviolet andinfrared light to realize full-spectrum-response photocatalysis.As a result, it is very signicant to explore an effective way toimprove the full spectrum photocatalytic properties of PDI-based semiconductors.

Silver sulde (Ag2S) is a p-type semiconductor, which isresponsive to solar light due to its narrow band gap.20,21 Thereported exciton Bohr radius of Ag2S is 2.2 nm.22 When theradius of Ag2S is larger than the exciton Bohr radius, it hasa continuous band structure. Ag2S has been widely applied inphotovoltaic cells, photoconductors, and superionic conductorsbecause of its good chemical stability, excellent optical prop-erties and electronic transport.23–26 In addition, photocatalystsmodied with Ag2S have been reported, such as Ag2S/Bi2WO6,27

ZnO/CdS/Ag2S,28 and Ag2S/TiO2.29 The conduction band (CB) ofAg2S is at around 0 eV vs. NHE, which means that it can absorbultraviolet, visible and infrared light. As a result, silver suldehas great potential to be applied in full spectrum photo-catalysis. However, there is also the issue of the easy recombi-nation of photoinduced carriers on account of the narrow bandgap energy.

Considering the disadvantages of PDI and Ag2S, we thoughtthat it was worth constructing a novel inorganic/organic hybridheterojunction p-Ag2S/n-PDI to investigate its photocatalyticability. On the one hand, Ag2S can broaden the absorption oflight, which is conducive to forming a full spectrum-responsivecomposite photocatalyst. On the other hand, the p–n hetero-structure forms a build-in internal electric eld, directed fromthe PDI (n-type) to Ag2S (p-type), facilitating the transfer of holesand electrons in opposite directions.30–32 Simultaneously, Ag2Scontributes to hole transfer,33,34 which is conducive to theseparation of photo-generated charge carriers. Furthermore, thephoto-induced charge behavior of a p–n-type heterojunctioncontaining self-assembled PDI has scarcely been reported.Herein, a novel composite p-Ag2S/n-PDI photocatalyst wassuccessfully designed, which exhibits satisfactory photo-catalytic activity under ultraviolet, visible and full spectrumlight. The work presents a new viewpoint for constructing p–nheterojunctions based on self-assembled organic catalysts.

2. Experimental section2.1 Sample preparation

The bulk PDI was synthesized according to a reported method,19

the details of which can be found in the ESI.†2.2.1 Preparation of the self-assembled nano PDI. 0.5432 g

of bulk PDI was dispersed in deionized water (200 mL). Then,triethylamine (800 mL) was added under vigorous stirring. Aerstirring for 1 hour, hydrochloric acid (4.0 M, 35 mL) was addedand then the mixture was stirred again for 3 hours to form theself-assembled PDI nanobers as a dark red solid, which wasfully washed. When the pH value of the ltrate became neutral,the wet red solid was collected through a 0.22 mm ltermembrane and was then placed in a vacuum drier at 70 �C.

2.2.2 Preparation of the p-Ag2S/n-PDI composite. Ag2S/PDIcomposites were obtained via an in situ precipitation method.Firstly, different quality of self-assembled PDI was dispersed in

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a denite volume of AgNO3 solution (0.025 mol L–1), stirred for60 min, and then sonicated for 30 min. Secondly, the mixturewas stirred for 180 min aer adding a certain volume of0.025 mol L�1 (NH4)2S dropwise, and was then subjected toultrasonic treatment for 180 min. The nal solid was fullywashed with deionized water and dried at 70 �C in a vacuumoven. The series of photocatalysts prepared were labelled as AP(1 : x), where AP is the abbreviation for Ag2S/PDI and 1 : xrepresents the mass ratio of Ag2S to PDI for x ¼ 0.2, 0.4, 0.6, 0.8,1, 1.4, 2. The pure Ag2S was prepared in the same way withoutadding nano PDI.

2.2 Characterization of the materials

The X-ray powder diffraction (XRD) spectra were obtained ona Bruker D8 Advance X-ray diffractometer. The morphology ofthe materials was observed by transmission electron micros-copy (TEM, Hitachi HT 7700 electron microscope). High-resolution TEM (HR-TEM) was carried out using a JEM 2010Felectron microscope. The UV-vis diffuse reection spectra (DRS,Hitachi U-3010) were recorded to investigate the light absorp-tion capacity of the materials. A Bruker VERTEX 700 spec-trometer was used to record Fourier-transform infrared (FT-IR)spectra. Raman spectra were recorded on a HORIBA HR800spectrometer. Solid photoluminescence spectra (PL, EdinburghFS5 spectrophotometer) were recorded at an excitation wave-length of 612 nm. Transient uorescence spectra were obtainedusing a QY-2000 uorescence spectrometer. X-ray photoelectronspectroscopy (XPS) was carried out using a PHI-Quanter spec-trometer. The zeta potential (x) was measured to study theelectrical properties of the catalyst surface (Horiba SZ-100 NanoParticle analyser). Electron paramagnetic resonance (EPR, JES-FA200) spectroscopy was used to detect active species.

Photoelectrochemical measurements are detailed in theESI.†

2.3 Photocatalytic performance evaluation

The catalytic performance was evaluated via phenol degrada-tion (5 ppm) under visible light (500 W xenon lamp, 420 nm cut-off lter), ultraviolet light (ultraviolet lamp, 254 nm), and fullspectrum light (500 W xenon lamp, without lter), respectively.Specically, photocatalyst (25 mg) and phenol solution (5 ppm,50 mL) were rstly dispersed in a quartz tube via ultrasonictreatment. The solution was stirred without light for 1 hour toachieve adsorption and desorption equilibrium. Then, thequartz tubes were irradiated by light (ultraviolet light, visiblelight, full spectrum light) and the reaction solution (2.5 mL) wasextracted every 20 minutes. The concentration of phenol wasdetected by high performance liquid chromatography (HPLC) at270 nm. Besides this, a 500 W xenon lamp with differentbandpass lters (350� 15 nm, 380� 15 nm, 420� 15 nm, 450�15 nm, 500 � 15 nm, 550 � 15 nm, 600 � 15 nm, 650 � 15 nm,700� 15 nm) was used to research the relationship between thephotocatalytic properties and the light absorption of thesamples.

The photocatalytic water oxidation experiments are detailedin the ESI.†

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Fig. 1 TEM images of (A) PDI, (B) Ag2S and (C) Ag2S/PDI and (D) theHR-TEM image of Ag2S/PDI (1 : 0.6).

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3. Results and discussion3.1 The structure of the p-Ag2S/n-PDI heterojunctioncomposite

The preparation of the p-Ag2S/n-PDI composite is presented inScheme 1 and the morphologies of the obtained catalysts wereanalysed by TEM (Fig. 1). In detail, the formation of the PDInanowires can be attributed to hydrogen bonding and p–p

stacking (pH ¼ 4.2). The length of nano PDI was 100–300 nmand the width of nano PDI was approximately 20 nm (Fig. 1A).Ag2S/PDI composites with different mass ratios were synthe-sized via a two-step electrostatic process. As can be seen inFig. S1 and Table S1,† the average zeta potential (x) of nano PDIwas �85.0 mV. Obviously, the surface of the self-assembled PDIis negatively charged,17,35 making it easy for positively chargedsilver ions (Ag+) to adsorb to the surface of nano PDI via elec-trostatic interactions. Aer adding sulons (S2�), silver sulde(Ag2S) immediately formed on the surface of the PDI nanowires.The average zeta potential (x) of Ag2S/PDI was around�79.3 mV,with a surface charge more positive than that of PDI. Thechange in the surface charge indicated that Ag2S successfullycombined with PDI to some extent. As shown in Fig. 1B, theaverage particle size of the pure Ag2S nanoparticles is in therange of 7–10 nm and there is no obvious aggregation. Theradius of Ag2S is large compared with the reported exciton Bohrradius of Ag2S,22 indicating that the as-prepared Ag2S hascontinuous energy levels.22 Furthermore, the morphology of theAg2S/PDI composite is shown in Fig. 1C, where it can be seenthat the Ag2S quantum dots are closely combined with nanoPDI. The HRTEM image of p-Ag2S/n-PDI is shown in Fig. 1D. Atightly integrated interface was obviously formed between thePDI and Ag2S and the two phases of Ag2S and PDI can be clearlydistinguished. The fringe interval of 0.244 nm is in accordancewith that of the (121) plane of Ag2S. The effective interfaceintegration of p-Ag2S and n-PDI helps to form an internalelectric eld directed from the PDI (n-type) to the Ag2S (p-type),which is conducive to the migration of photon-generated

Scheme 1 Schematic illustration of the synthesis of the self-assembled

6484 | J. Mater. Chem. A, 2019, 7, 6482–6490

carriers. According to the above analysis, the Ag2S/PDI hetero-junction (p–n type) was successfully constructed.

In addition, the XRD patterns are shown in Fig. 2A, whichwere measured to determine the phase composition. The P1peak of nano PDI was around 3.34–3.55 A, corresponding to thed-spacing of p–p stacking.36 The intensity ratio of P1 to P0 (IP1/IP0) is usually used to evaluate the degree of the self-assembly ofPDI. Obviously, the value of IP1/IP0 here (the line of pure PDI) isgreater than 1 and this indicates that the PDI has highly orderedp–p stacking,19 which is helpful for electron delocalization andconducive to the transfer of photo-induced electrons. All of thepeaks of Ag2S here are in accordance with those of monoclinicAg2S, matching the No. 14-0072 JCPDS card. The characteristicpeaks of Ag2S/PDI with different mass ratios all indicated thecoexistence of PDI and Ag2S. No other diffraction peaks weredetected, showing that Ag2S does not react with PDI. In

PDI and Ag2S/PDI composite.

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Fig. 2 (A) XRD patterns of Ag2S, PDI, and Ag2S/PDI; (B) UV-vis diffuse reflection spectra (DRS) of the self-assembled PDI and Ag2S/PDI (Ag2S/PDIis abbreviated as AP, 1 : x represents the mass ratio of Ag2S to PDI).

Fig. 3 Photocatalytic degradation of the rate constants k of samplesunder (A) visible light irradiation (where B-PDI represents bulk-PDI, N-PDI represents nano-PDI, 1 : x represents the mass ratio of Ag2S toPDI), (B) ultraviolet irradiation, (C) full spectrum irradiation and (D)wavelength-dependent photodegradation results of Ag2S/PDI (1 : 0.6).

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addition, the typical P1 peak of Ag2S/PDI shis toward highangles in comparison to that of pure PDI, indicating that Ag2S/PDI has a smaller d-spacing of the p–p stacking, which meansa higher electron cloud overlap density, which is benecial forthe efficient transfer of carriers.19 The results showed that theAg2S quantum dots enhanced the p–p stacking density of PDI,which was probably caused by the built-in potential between thetwo phases. Fig. 2B shows the UV-vis DRS spectra of the pho-tocatalysts. The light absorption of p-Ag2S/n-PDI was obviouslyenhanced compared with that of pure PDI, which could help tofacilitate the light–chemical energy conversion in the full-spectrum region. Fig. S2† shows that the band gap of p-Ag2S/n-PDI (1 : 0.6) is about 1.68 eV, which is narrower than that ofpure PDI (1.73 eV), and the band gap of Ag2S is 1.81 eV.37

To further analyze the structures of the samples, Raman andinfrared measurements were performed (Fig. S3†). The peak inthe FTIR spectrum of pure PDI at 1648 cm�1 for C]C can beascribed to a benzene ring and that of 1686 cm�1 for C]Ostretching can be attributed to the presence of carboxyl groups(Fig. S3A†). The infrared absorption peaks of p-Ag2S/n-PDI werefound to be shied to shorter wavelengths, which maybe indi-cates the existence of interactions between PDI and Ag2S. TheRaman spectra are shown in Fig. S3B.† The peaks of PDI at1565 cm�1 and 1278 cm�1 can be respectively attributed to C]Cand CH stretching vibrations.38 The p–p stacking degree can beevaluated by the strength ratio of 1565 cm�1/1278 cm�1.19 Theintensity ratio of Ag2S/PDI (1.16) was larger than that of purePDI (1.09), demonstrating that Ag2S promoted the degree of p–p stacking, which contributed to the migration of photo-induced electrons. Besides this, the Raman spectra of Ag2S/PDI showed a shi compared to PDI, indicating an interac-tion between PDI and Ag2S.

XPS was used to study the chemical composition and surfacestate of the samples (Fig. S4†), the results of which can be foundin the ESI.† What is noteworthy is that the Ag 3d peak positionof Ag2S/PDI (1 : 0.6) is shied in the direction of lower bindingenergy in comparison with pure Ag2S, revealing a strong

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interaction between PDI and Ag2S, which indicates the existenceof electron transfer in the two phases.39,40

3.2 Photocatalytic performance of the samples

The photocatalytic properties were investigated via phenoldegradation. As shown in Fig. 3A, the catalytic performance ofthe p-Ag2S/n-PDI heterojunction was obviously enhanced undervisible light (l > 420 nm). The corresponding rst order kineticscurve tting is exhibited in Fig. S6B.† The photocatalyticprocess was found to be in accordance with pseudo rst orderkinetics. When the mass ratio of Ag2S to PDI was 1 : 0.6, thecomposite exhibited the optimum catalytic activity with a rateconstant k (0.0259 min�1) 6.93 times that of pure PDI(0.00374 min�1). The Ag2S/PDI (1 : 0.6) composite degraded94.48% of the phenol aer visible light irradiation for 120 min.

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Fig. 4 Amount of evolved oxygen in photocatalytic water oxidationwith PDI and Ag2S/PDI (1 : 0.6) under full spectrum irradiation (theinset shows the rate of oxygen production).

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Furthermore, the novel p-Ag2S/n-PDI exhibited more excellentphoto-degradation activity in comparison with some existingvisible-light photocatalysts, such as Bi2WO6,4 commercial self-assembled PDI,17 and g-C3N4.41 The related photocatalyticperformance data are listed in Table S2.† HPLC was used todetect the intermediates (Fig. S5A†). The peak intensity (3.106min) of phenol was observed to gradually decrease with irradi-ation. In addition, several speculated intermediate productswere detected (Fig. S5B†). The peaks at 2.363 and 1.112 minmaybe attributed to p-benzoquinone and maleic acid, respectively.17

Besides these, the peaks at 0.392 and 0.778 min may be relatedto the small molecule dissolution of nano PDI.

The ultraviolet-light (UV-light) photocatalytic performance ofpure Ag2S, bare PDI and the Ag2S/PDI (1 : 0.6) composite isshown in Fig. 3B. Dramatically, the p-Ag2S/n-PDI (1 : 0.6)showed optimal photocatalytic properties (k ¼ 0.0142 min�1) ofaround 8.07 times higher than those of nano PDI (k ¼0.00176 min�1). The full spectrum photocatalytic performanceis shown in Fig. 3C. It is obvious that the Ag2S/PDI (1 : 0.6)composite exhibited the best activity, with an apparent rateconstant (k ¼ 0.0298 min�1) 5.13 times higher than that of purePDI (k ¼ 0.00581 min�1).

Simultaneously, as shown in Fig. 3D, the photo-degradationresults alongside the wavelength indicated that the photo-catalytic degradation rate may be connected to the opticalabsorption. The trends in the phenol degradation of Ag2S/PDI(1 : 0.6) were positively related to its optical absorption prop-erties, implying that optical absorption played a key role in thephotocatalytic degradation. Furthermore, the UV-vis DRSspectra manifested that the Ag2S/PDI heterojunction has moreefficient light absorption compared with pure PDI (Fig. 2B). It isworth noting that even under 380 � 15 nm UV-light and 700 �15 nm visible light (band-pass lter), Ag2S/PDI showed consid-erable photocatalytic activity, suggesting that p-Ag2S/n-PDI hasan extended spectral response that nearly covers the whole UVand visible spectrum.

PDIs have been used in water splitting systems.42 Fig. 4shows the amount of oxygen produced by PDI and Ag2S/PDI(1 : 0.6) under full spectrum irradiation. For Ag2S/PDI (1 : 0.6),the rate of oxygen production is 34.6256 mmol g�1 h�1, nearly1.79 times higher than that of PDI (19.3312 mmol g�1 h�1).

As is well known, the stability of photocatalysts is a criticalissue. As shown in Fig. S7,† Ag2S/PDI (1 : 0.6) showed highphotocatalytic stability in an acid environment (pH ¼ 4.3).Besides this, the XRD and IR spectroscopic measurements ofthe p-Ag2S/n-PDI (1 : 0.6) composite showed no differencesbefore and aer reaction (Fig. S8†), manifesting that a stablestructure of PDI was maintained aer the catalytic reaction.

Fig. 5 Photocurrents of PDI and Ag2S/PDI (1 : 0.6) under (A) visiblelight (>420 nm) and (B) UV-light. (C) Electrochemical impedancespectroscopy (EIS) Nyquist plots of pure PDI and p-Ag2S/n-PDI(abbreviation: AP) composite and (D) room temperature PL emissionspectra of the as-prepared samples.

3.3 Mechanism of the enhanced photocatalytic activity

3.3.1 The photoinduced charge behavior analysis. Theefficient separation of photo-induced charge carriers is bene-cial for catalytic reaction. Therefore, photoelectric tests wereperformed to investigate the separation efficiency of the elec-tron–hole (e�–h+) pairs. The photocurrent response signal of p-Ag2S/n-PDI was much higher than that of PDI under visible light

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and UV light (Fig. 5A and B), indicating that p-Ag2S/n-PDIfacilitates the separation of photo-induced charge carriers.

Besides this, Fig. 5C shows the results of electrochemicalimpedance spectroscopic (EIS) measurements. The diameter ofthe arc reects the charge-transfer resistance.43 The pure PDIand p-Ag2S/n-PDI composite showed smaller arc radii undervisible light than in the dark, indicating that p-Ag2S/n-PDI andpure PDI could both be excited under visible light. At the sametime, the arc radius of p-Ag2S/n-PDI was much smaller than thatof nano PDI, suggesting that the p–n heterojunction decreasedthe charge transfer resistance. The EIS results show that there isa tight connection between PDI and Ag2S, which is benecial forcharge migration. This means that the p-Ag2S/n-PDI hetero-junction facilitates the separation of photo-induced chargecarriers. Fundamentally speaking, this is mainly due to the

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built-in electrical potential between PDI and Ag2S, directed fromthe n-PDI to the p-Ag2S, which facilitates the migration of e�

and h+ in opposite directions.As is well known, uorescence emission arises as a result of

the recombination of e�–h+ pairs. Thus, lower PL intensityindicates a lower recombination of e�–h+ pairs.44 As can beclearly seen from Fig. 5D, there is an obvious uorescencesignal at 728 nm for pure PDI. However, the intensity of theemission peak decreased aer combination with Ag2S, implyingthat the p-Ag2S/n-PDI heterojunction facilitates the separationof e�–h+ pairs. Furthermore, the uorescence emission peak ofAg2S/PDI (at around 700 nm) shows a signicant blue shicompared with that of pure PDI, indicating that the addition ofAg2S prolonged the life of the e�–h+ pairs.18,46 The resultdemonstrates once again that there is a strong interactionbetween Ag2S and PDI, that is, the built-in electric eld from then-type PDI to the p-type Ag2S. The interaction redistributes thecharge density at the interface.

Surface photovoltage (SPV) measurements were further usedto research the separation process of the carriers.45 Fig. 6presents the SPV spectra of Ag2S/PDI and PDI. The SPV signal ofPDI from 400 to 700 nm is caused by the transition of electronsfrom the valence band (VB) to the conduction band (CB).Obviously, the SPV signal of Ag2S/PDI presents a much strongerresponse than that of pure PDI. The UV-vis absorption involvesthe absorption of a variety of photons, but the SPV is onlyinuenced by electronic transitions. Therefore, the SPV resultsindicate that the charge carrier separation efficiency of Ag2S/PDIis better than that of pure PDI.47 The SPV measurements wereconducted under the same conditions, so the obvious differencebetween the SPV responses of the PDI and p-Ag2S/n-PDI couldbe attributed to the interfacial interaction between the PDI andAg2S. It is speculated that a built-in electric eld may have beenformed at the interface of Ag2S and PDI, which means thatphoton-generated carriers can be effectively separated. Besidesthis, the uorescence lifetime provides direct evidence forelectron transfer (Fig. S9†). The lifetime of p-Ag2S/n-PDI wasslightly longer than that of PDI, indicating that Ag2S/PDI hasmore effective separation of the carriers.

Fig. 6 Surface photovoltage spectroscopy of PDI and Ag2S/PDI(1 : 0.6).

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3.3.2 Active species involved in photocatalytic reactions.Active species play a critical role in photocatalysis, whichmainlyinvolves O2c

�, h+, $OH and singlet oxygen (1O2). The dominantactive species were detected via free radical trapping experi-ments. Fig. 7A and B respectively show the phenol degradationcurves of Ag2S/PDI (1 : 0.6) with the addition of formic acid (h+

scavenger), p-benzoquinone (O2c� scavenger), t-BuOH ($OH

scavenger) and AgNO3 (electron scavenger) under ultravioletlight and visible light. The catalytic properties were observed toobviously decrease aer adding formic acid, indicating that theholes (h+) are the main active species. The positive SPV signal(Fig. 6) also indicates that the holes participate in photocatalyticreaction. Besides this, the photocatalytic activity reduced aeradding benzoquinone, demonstrating that superoxide radicals(O2c

�) also take part in the photocatalytic degradation. Thehydroxyl radicals showed almost no obvious effect on the pho-tocatalytic degradation. Simultaneously, the presence of O2c

�

and 1O2 were also conrmed through EPR measurements. Nosignal response for O2c

� was observed in the dark (0 min) uponthe Ag2S/PDI composite. However, obvious signals appearedaer illumination. Simultaneously, the peak intensity increasedwith the continuous exposure of light, which further indicatedthat O2c

� was generated in the photocatalytic process (Fig. 7C).Besides this, strong singlet oxygen signals were detected by EPRspectroscopy for the p-Ag2S/n-PDI sample (Fig. 7D). Singletoxygen also has a certain oxidation capacity, which can oxidizeintermediate products. Furthermore, Fig. S10† showsa comparison of the EPR spectra of PDI and Ag2S/PDI in termsof the detection of O2c

�, 1O2 and $OH, respectively.The results indicate that the composite Ag2S/PDI produces

more O2c� and 1O2 than pure PDI. In particular, the content of

singlet oxygen in Ag2S/PDI reached about 7 times higher thanthat in the pure PDI under light. In addition, the EPRmeasurements detected very weak hydroxyl radical signals in

Fig. 7 Effects of different scavengers on phenol degradation in thepresence of Ag2S/PDI (1 : 0.6) under (A) visible light irradiation and (B)UV-light. (C) EPR spectra of the Ag2S/PDI (1 : 0.6) composite fordetection of O2c

� under visible light irradiation (l > 420 nm) and (D)EPR spectra of the Ag2S/PDI (1 : 0.6) composite for detecting singletoxygen.

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both pure PDI and Ag2S/PDI. The results demonstrated thathydroxyl radicals are not the dominant active species, which isconsistent with the results of the free radical trappingexperiment.

3.3.3 The separation process of photo-induced carriers andthe catalytic process. Photocatalytic redox reactions and theformation of active species are thermodynamically related tothe energy band structures of catalysts. Fig. S2† shows the bandgaps of PDI, Ag2S and Ag2S/PDI, which were respectivelyconrmed to be 1.73 eV, 1.81 eV and 1.68 eV. Mott–Schottkytests were used to determine the semiconductor types of PDIand Ag2S. A negative slope of the linear curve (1/C2 vs. potential)represents a p-type semiconductor, and a positive slope repre-sents an n-type one.48

As shown in Fig. S11,† the slopes of PDI (Fig. S11A†) and Ag2S(Fig. S11B†) are positive and negative, respectively, indicatingthat PDI and Ag2S are n-type and p-type semiconductors,respectively.17,49 The at-band potential of pure PDI was foundto be �0.24 eV vs. NHE (Fig. S11A†). According to previousreports, the CB positions of n-type semiconductors are deeperthan the at-band potential (0.1–0.2 eV).50 Therefore, theconduction band bottom of PDI was determined to be �0.34 eVvs. NHE (the difference value was set as 0.1 eV here). Thus, thevalence band edge (1.39 eV) of PDI can be obtained using theequation “EVB ¼ ECB + Eg”. The potentials of the VB and CB ofAg2S were conrmed using the Mulliken electronegativitytheory (eqn (S1) and (S2)†). The obtained CB and VB edgepotentials are as follows: PDI : ECB ¼ �0.34 eV, EVB ¼ 1.39 eV;Ag2S : ECB ¼ �0.44 eV, EVB ¼ 1.37 eV. The interlaced bandpositions between the p-Ag2S and n-PDI contribute to thetransfer of photo-induced e� and h+.

According to previous analysis, the separation process ofcarriers and the photocatalytic process are shown in Fig. 8.Firstly, the p-Ag2S and n-PDI both absorb photons to generatee� and h+ in the CB and VB, respectively. Next, the photo-induced electrons at the conduction band at the bottom of p-Ag2S transfer to the CB of n-PDI. Based on the above study, the

Fig. 8 The separation process of photo-generated carriers and the pho

6488 | J. Mater. Chem. A, 2019, 7, 6482–6490

Ag2S quantum dots can promote the ordered p–p stackingdegree. As a result, this is helpful for electron delocalization andthe migration of photo-generated electrons, leading to morephoto-generated electrons that can transfer along the p–p

stacking of PDI. The CB potential of PDI (�0.34 eV, vs. NHE) ismore negative than the Eq (O2/O2c

�) (�0.33 eV, vs. NHE). Thus,O2c

� can be generated in the CB of PDI.51 This is consistent withthe active species capture experiment and EPR results. Super-oxide radicals (O2c

�), as highly active oxidants, can decomposeor even mineralize organic pollutants. In addition, theconduction band positions of PDI (�0.34 eV) and Ag2S (�0.44eV) are both above the Eq (O2/

1O2) (+0.34 eV vs. NHE), so the p-Ag2S/n-PDI heterojunction has the ability to generate 1O2.Besides this, the holes generated in the VB of PDI can easilytransfer to the VB of Ag2S, revealing the strong oxidation abilityfor the splitting of water and organic pollutants. It is worthnoting that the built-in electrical potential in the direction ofthe PDI to the Ag2S greatly facilitates the separation of photo-induced e�–h+ pairs. In short, the p–n heterojunction Ag2S/PDI realizes high separation efficiencies of e�–h+ pairs andmore efficient full-spectrum photocatalytic performance.

4. Conclusion

In conclusion, a novel effective self-assembled supramolecularheterojunction p-Ag2S/n-PDI was successfully constructed viaan in situ precipitation method. The p-Ag2S/n-PDI hetero-junction shows superior ultraviolet-light, visible-light and full-spectrum photocatalytic performance. The satisfactory photo-catalytic properties can be mainly attributed to three factors:rstly, the p–p stacking degree of PDI is promoted by Ag2S,which is helpful for the migration of photo-generated electronsalong the quasi-one-dimensional p–p stacking. Simulta-neously, Ag2S enhances the absorption of light to facilitate thelight–chemical energy conversion. Secondly, the built-in elec-trical potential between Ag2S and PDI is conducive to moreeffective separation of photon-generated carriers. Finally, the p-

tocatalytic process.

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Ag2S/n-PDI heterojunction can produce more active speciesthan pure PDI, resulting in much stronger oxidation ability. Inshort, the work provides some interesting ideas for designingefficient heterojunction photocatalysts with a supramolecularorganic nanostructure.

Conflicts of interest

There are no conicts to declare.

Acknowledgements

This work was partly supported by the Chinese National ScienceFoundation (21437003, 21673126, 21761142017, 2162100-3) andthe Collaborative Innovation Center for Regional Environ-mental Quality.

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