Ag-Ag2S quantum-dots modified TiO2 nanorod arrays with enhanced photoelectrochemical and photocatalytic properties

Accepted Manuscript Ag-Ag2S quantum-dots modified TiO2 nanorod arrays with enhanced photoelectrochemical and photocatalytic properties Yong Zuo, Jinjun Chen, Haocheng Yang, Miao Zhang, Yanfen Wang, Gang He, Zhaoqi Sun PII:

S0925-8388(18)34406-2

DOI:

https://doi.org/10.1016/j.jallcom.2018.11.274

Reference:

JALCOM 48500

To appear in:

Journal of Alloys and Compounds

Received Date: 26 September 2018 Revised Date:

20 November 2018

Accepted Date: 21 November 2018

Please cite this article as: Y. Zuo, J. Chen, H. Yang, M. Zhang, Y. Wang, G. He, Z. Sun, Ag-Ag2S quantum-dots modified TiO2 nanorod arrays with enhanced photoelectrochemical and photocatalytic properties, Journal of Alloys and Compounds (2018), doi: https://doi.org/10.1016/j.jallcom.2018.11.274. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Ag-Ag2S Quantum-Dots Modified TiO2 Nanorod Arrays with Enhanced Photoelectrochemical and Photocatalytic Properties Yong Zuoa, Jinjun Chenb, Haocheng Yanga, Miao Zhanga, Yanfen Wanga, Gang Hea,

a

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Zhaoqi Suna* School of Physics & Materials Science, Anhui University, Hefei 230601, PR China; b

Electrical Engineering College, Guizhou University, Guiyang 550025, PR China.

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Abstract: In this paper, highly ordered titanium dioxide nanorod (TNR) arrays decorated with Ag-Ag2S quantum-dots (QDs) were synthesized on FTO substrate

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through hydrothermal treatment and in-situ vulcanization. The content of Ag2S QDs on Ag/TNR was controlled by changing the immersion time of Ag/TNR in the N-Ndimethyl formamide (DMF) solution containing dissolved sulfur element. The morphology, structure, composition and optical, photoelectric and photocatalytic

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properties of QDs/TNR arrays were studied in detail. In the visible range, the QDs/TNR array with a soaking time of 12 h exhibits the maximal photocurrent density (0.082 mA/cm2) and photodegradation rate (62.2%), which is 5.5 and 1.85

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times higher than that of pure TNR. The improvement of these properties could be attributed to synergy effect between Ag and Ag2S materials. That is, Ag QDs can

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stably capture and transmit photo-generated electrons from the TNR surface, whereas the Ag2S promoter is regarded as the interface active sites to enhance photocatalytic reactions. Therefore, the synergistic effect of Ag-Ag2S can significantly enhance the photoelectrochemical and photocatalytic propertise of the TNR arrays.

Keywords: Ag-Ag2S quantum-dots; TiO2 nanorod arrays; photoelectrochemical and photocatalytic properties; synergistic effect.

*Corresponding author. E-mail address : [email protected] (Zhaoqi Sun).

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1.Introduction Nowadays, environmental issues and energy crisis are becoming increasingly urgent, which requires people to find promising and low-cost methods for clean energy production. Under such circumstance, the utilization of semiconductor

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nanoarray structure in photoelectrochemical (PEC) and photovoltaic (PV) devices, for instance, luminous diodes, thin film solar cells and solar water decomposition systems, has gained wide attention due to their distinct geometric, electronical and optical

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characteristics [1-4]. For the PEC cell, sunlight drives the photoelectrode to generate electron-hole (e-h+) redox couples [5]. The semiconductor plays a key role in this

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process, as different types or forms the photoelectrode of PEC and PV will result in different performance [6].

As a typical semiconductor material, TiO2 has many advantages, including extremely high chemical stability, non-toxicity, economy, and no secondary pollution,

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[7-8] etc. It is widely used in gas sensors, biological materials, water purification applications, and one-dimensional (1D) nanostructures. Therefore, it has become one of the research hotspots in the field of semiconductor photocatalysis. Recently, many

preparation

and

modification

methods.

For

example,

different

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properties,

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research groups have devoted themselves to the study of TiO2 nanomaterials for its

morphologies of TiO2 include nanoparticles, nanorods, nanowires, nanotubes, and their applications in photocatalytic, photoelectrochemical cells, photovoltaic cells and other fields. Highly ordered TiO2 nanorod arrays have been utilized to increase effectiveness in collecting electronic grain boundaries by reducing unwanted electron losses recombining with redox couples [9-11]. However, as a wide band gap semiconductor material (band gap energy is 3.2 eV), TiO2 has its own defects. It must be activated by ultraviolet light and has low solar energy utilization, but high

ACCEPTED MANUSCRIPT photocarrier recombination rate at the same time. Therefore, in order to optimize the performance of TiO2, many scholars have taken a series of methods to improve these deficiencies. The current methods frequently used include noble metal deposition [1213], ion doping [14-18], dye sensitization [19-20] and surface modification with

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semiconductor [21] etc. However, these methods are difficult to be applied widely due to their own limitations, such as high cost, low cycle rate, and complex processes. Therefore, it is quite necessary and valuable to develop new and simple methods to

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further modify the TiO2 to improve its properties. In this case, the current study considers the synergistic effect between metallic silver and silver sulfide, which may

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be an effective and facile way to improve the defects of TiO2.

Regarded as a kind of superior semiconductor material, for unique characteristic of forbidden band width (0.92 eV) [22-24] and good photoelectrochemical performance, silver sulfide is widely used in the manufacture of photovoltaic

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materials, thermoelectric materials, electronic materials, and has broad application prospects in the fields of sensing, catalysis and light absorption. At the same time, Ag quantum dots have been identified to significantly enhance the light-absorbing

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properties of materials due to the localized surface plasmon resonance (LSPR) effect

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[25-27]. It can be interpreted as the coherent oscillations of the conduction band electrons on the surface of noble metal particles under visible light irradiation, which can significantly enhance the absorption of visible light and accelerate the separation of charge carriers under ultraviolet light irradiation [28-29]. Therefore, Ag-Ag2S QDs co-modified TNR arrays can effectively improve photoelectrochemical and photocatalytic properties. In this paper, we synthesized TNR arrays modified by Ag-Ag2S QDs through simple hydrothermal and wet chemical methods [30-31]. Ag2S QDs are formed on the

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of

Ag

by

in-situ

vulcanization.

The

morphology,

struture,

photoelectrochemical and photocatalytic properties were investigated at great length. The results show that QDs/TNR arrays can greatly improve solar absorption, photoelectrochemical performances and photocatalytic activities. After the above

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experiment, the metal synergy effect between Ag-Ag2S was proposed to explain the enhancement of the properties [32].

2.Experimental

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2.1. Materials

Titanium butoxide (TBOT), silver nitrate (AgNO3), S powder, DMF (N-N-

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dimethylformamide, AR) were purchased from Aladdin Industrial Corporation. Fluorine-doped tin oxide (FTO) conducting glasses were purchased from WuHan Jinge Solar Energy Technology Co., Ltd. Concentrated hydrochloric acid (HCl), absolute ethanol and acetone were purchased from Sinopharm Chemical Reagent Co.,

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Ltd. All chemical reagents were analytical grade and used without further purification. 2.2. Synthesis of Ag-Ag2S/TNR arrays

Firstly, the FTO substrates were ultrasonically cleaned in acetone, absolute

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ethanol and DI water in a proper order, then drying under N2 atmosphere. The TNR

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arrays were manufactured in the same way that mentioned in previous report. We used a two-step wet chemistry method to deposit Ag-Ag2S QDs on a TNR arrays. Then the prepared TNR arrays were immersed in 0.1 M AgNO3 solution and irradiated under a 36 W UV lamp for 40 minutes. After the above reaction was finished, the samples were rapidly rinsed with a large amount of deionized water, and then dried in an oven to obtain Ag/TNR arrays. Subsequently, the above-mentioned sample was soaked in 20 mL of DMF solution containing 0.01 g of S powder to promote the conversion of Ag to Ag2S, the soaking times were 4 h, 8 h, 12 h and 16 h

ACCEPTED MANUSCRIPT separately, and four groups of Ag2S were obtained. Then the above samples were washed several times with ethanol, and finally dried under N2 atmosphere. Meanwhile, we also prepared the original TNR and Ag/TNR samples for comparison. 2.3. Characterization methods

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The crystalline structure and phase composition were tested by X-ray diffractometer (XRD, MAC M18XHF) using Cu Kα (1.54184 Å) radiation (cathode voltage: 40kV, current: 100 mA). The field emission scanning electron microscope

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(FE-SEM, Hitachi, S4800) was used to analyze the morphology of all samples. The microstructure was examined by high resolution transmission electron microscopy

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(HRTEM, Hitachi, JEM-2100). X-ray photoelectron spectroscopy (XPS, Thermo, ESCALAB250) was applied to determine surface composition. The ultraviolet-visible (UV-Vis) absorption spectra were recorded by UV-Vis spectropho tometer (Shimadzu, UV-2550). Fluorescence spectrophotometer (FL, Hitachi, F-4500) was used to

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measure the photoluminescence (PL) spectra, within which the Xe lamp was operated as an excitation source operating at an excitation wavelength of 325nm. 2.4. Photoelectrochemical tests

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Photoelectrochemical tests were gained through an electrochemical workstation

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(CHI660D, Shanghai Chenhua Limited). The photocurrent density curves were executed under irradiation of AM 1.5 G simulated sunlight in a three-electrode system, with QDs/TNR photoanodes, Pt foil and a Ag/AgCl (saturated with KCl) electrode as the working, counter and reference electrode separately. The blending aqueous solution containing 0.25 M Na2S and 0.35 M Na2SO3 was used as electrolyte. 2.5. Photocatalytic activity Photocatalytic activity of the ready samples (1×1 cm2) was studied by photodegradation of methyl orange (MO) solution (15 mg/L) under a 24 W high

ACCEPTED MANUSCRIPT pressure mercury lamp, which gives out light of 404.7, 435.8, 546.1, and 577.0-579.0 nm. The range between the high pressure mercury lamp and the sample was 5.0 cm. The absorbance of the MO solution was measured every 30 mins and the overall irradiation time was 180 mins.

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3. Results and Discussion 3.1. Morphology and structure.

Figure 1 presents the XRD patterns of different samples. According to the

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standard card (JCPDS no.21-1276), the diffraction pattern of TNR can be indexed to rutile TiO2. As shown in curve (a), the three diffraction peaks at 36.1○, 54.43○, and

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62.7○ are in line with the indexing planes of (101), (105), and (002), respectively. The highly intensified rutile TiO2 characteristic peaks manifest that the arrays are catalytically synthesized on the FTO substrate [33]. Similar indexing patterns can be observed in curves (b), (c), (d), (e) and (f). No other forms of crystal diffraction peaks

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are detected, which indicates a pure rutile phase. But owing to small dimension and low content of Ag or Ag2S QDs deposited on the TNR [34], no diffraction peaks of metallic Ag or Ag2S were detected.

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The morphology and size of samples can be observed by SEM. As shown in

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Figure 2, image (a) is the original TNR. Obviously the TNR arrays grow approximately perpendicularly on the FTO surface and have a diameter of about 100150 nm. After the deposition of Ag QDs, it can be seen from Figure 2(b) that the surface of the TNR contains a large number of Ag quantum dots and its morphology does not change significantly. The images of (c)-(f) are samples obtained at different soaking time. With increasing reaction times, the Ag QDs continuously transform into Ag2S, and the number and size of Ag2S QDs are continuously increasing as well.

ACCEPTED MANUSCRIPT The microstructure of the samples is further characterized by TEM and HRTEM. Figure 3 exhibites TEM and HRTEM images of Ag-Ag2S(12h)/TNR arrays. Figure 3(a) clearly reveals a rod-like TiO2 with a diameter about 140 nm, which is consistent with the SEM results. It can be seen from Figure 3(b) that some Ag and Ag2S

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quantum dots are attached to the surface of the TNR, and the diameter of these quantum dots is about 13 nm by calculating the statistical average value. The HRTEM images display four categories of lattice fringes as presented in Figure 3(c) and 3(d).

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The lattice fringe spacing of 0.33 nm is in line with the spacing of the (101) planes of the rutile TiO2, 0.23 nm and 0.20 nm are ture of the (111) and (200) lattice planes of

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the face-centered-cubic Ag, respectively, and 0.16 nm is in line with the spacing of the (-223) planes of the Ag2S, indicating Ag and Ag2S quantum dots are deposited on the surface of TNR. It can be further seen that the diameter of Ag quantum dots is about 10 nm, while the diameter of Ag2S quantum dots on the surface is about 3 nm.

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3.2. Composition analysis

The elemental compositions and their chemical states of Ag-Ag2S/TNR arrays can be obtained by XPS. Figure 4 presents the corresponding results. The survey result in

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Figure 4(a) indicating all peaks are attributed to Ti, Ag, O, S, and C, and their strong

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line spectra are exhibited in Figure 4(b)-(f). As shown in Figure 4(b), two strong peaks at 458.8 and 464.3 eV are severally allocated to the binding energies of Ti 2p3/2 and Ti 2p1/2 with an energy difference of 5.5 eV, which is attributed to the Ti4+ oxidation state. For the Ag 3d spectrum (Figure 4(c)), it shows that the peaks of 368.2 and 374.2 eV can be applied separately to Ag 3d5/2 and 3d3/2 of Ag0 ions [35]. Furthermore, the peaks at 367.8 and 373.8 eV are in accord with Ag 3d5/2 and 3d3/2 of Ag+ ions [36]. The S 2p3/2 and 2p1/2 peaks correspond to 160.9 and 162.1 eV in the Figure 4(c). The difference of energy is 1.2 eV, which corresponds to the 160–164 eV

ACCEPTED MANUSCRIPT scope expected for S during the vulcanization phases. From Figure 4(d), the O 1s profile is unsymmetric and can be fitted to two Gaussian-Lorentzian peaks (α and β locating at 529.9 and 531.7eV separately) through the nonlinear least squares fitting program. This means the presence of two distinct O states in the sample. The peaks of

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α and β are related to the lattice oxygen of TiO2 and the chemisorbed oxygen of the surface hydroxyl separately. The above results are coincident with the reports in the previous literature [37-40]. Therefore, it can be concluded that Ag and Ag2S QDs are

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successfully deposited on the TNR arrays [41]. 3.3. Optical properties

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Figure 5 demonstrates the UV-vis absorption spectrum of different samples. It can be seen from the Figure 5 that the untreated TNR can only absorb ultraviolet light at about 400 nm. When the Ag QDs are sensitized, Ag/TNR exhibits a slight redshift of the absorption edges and higher absorption intensities because of the LSPR effect [42-

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43]. As some Ag QDs transformed into Ag2S QDs at different reaction time, the absorption edge of samples enlarge into the visible light region and the absorption intensity also increases, resulting in a significant red shift. It can be found that the

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sample with reaction time of 12 h has the best absorption coefficient. However, the

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absorption coefficient of the sample decreases when the reaction time continues to increase. This may be caused by the increased size of the Ag2S QDs, which decreases the specific surface area and thus reducing the absorption of light by the sample. Therefore, UV-vis spectroscopy can also provide compelling evidence for metallic Ag deposition and in-situ sulfide of Ag2S were formed on the surface of TiO2. Photoluminescence (PL) spectrum can be applied to test the efficiency of charge carrier migration, trapping, transfer and separation. The PL spectra of different samples can be obtained from Figure 6 (excitation wavelength of 325 nm). It can be

ACCEPTED MANUSCRIPT seen that there are three major peaks at 413 nm, 452 nm, and 469 nm, two small peaks at 482 nm and 493 nm. The 452 nm and 469 nm peaks are photoluminescence because of band edge free excitons and defects. Meanwhile, the small peaks at 482 nm and 493 nm are caused by bound exciton-induced photoluminescence [44].

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Compared with the pure TNR, the peak intensity of the PL spectrum of Ag/TNR and Ag-Ag2S/TNR was reduced. This shows that the photo/electron-hole pair recombination rate of the Ag/TNR and Ag-Ag2S/TNR samples is reduced. Through

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observation, it was found that the reaction time from 4 h to 16 h, PL peak intensity weakened first decreased and then increased, indicating that in a certain reaction time,

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moderate Ag2S QDs doping can effectively inhibit the photo-carrier recombination. However, when the reaction time is too long and too many Ag2S QDs are generated, these excess particles will form new e-h+ recombination sites and hinder the separation of carriers. Moreover, as the reaction time increases, the intrinsic excitation

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tends to decrease compared with the defect excitation peak intensity, which indicates that more deposition of Ag2S QDs leads to more oxygen defects [45-46]. 3.4. Photoelectrochemical Measurements

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In order to obtain further evidence of electron-hole transfer mechanism, transient

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photocurrent is tested by electrochemical workstation. The light and dark current densities of samples under different conditions are shown in Figure 7. The initial anodic photocurrent spike is resulted from the separation of the e-h+ pairs at the semiconductor/electrolyte interface: The holes are transferred to the surface of the semiconductor, trapped and reduced by substances in the electrolyte, whereas the electrons are moved to the back contact. After attaining the spike, the photocurrent sharply decreases in a short time until it reaches a steady state. For the exposed TNR electrode, it produces a relatively low photocurrent density of 0.015 mA/cm2. After a

ACCEPTED MANUSCRIPT primary sensitization of Ag nanoparticles, the photocurrent density of the sample reaches 0.043 mA/cm2. As the reaction time increased, Ag QDs were gradually converted to Ag2S. The photocurrent density of samples increased continuously and reached the best value at 12 h (0.082 mA/cm2), which was about 5.5 times higher than

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bare TiO2. This consequence is also consistent with the results of the PL spectra, indicating that the synergy between Ag-Ag2S can efficaciously accelerate the charge transfer and inhibit the recombination rate of the e-h+ pairs, finally improving the

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photoelectric properties. For the sample of 16 h, the photocurrent density decreased and the PL spectra showed the same trend. This may be attributed to the oversized

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Ag-Ag2S QDs blocking the surface active sites of TNR and increase the recombination rate of the e-h+ pairs, which reduces the photocurrent density. 3.5. Photocatalytic activity

The photocatalytic activity of samples is characterized by measuring the

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photodegradation rate of methyl orange (MO) solution by ultraviolet light irradiation. The results of different samples are presented in Figure 8 and Table 1. The photocatalytic degradation efficiency can be characterized by the Lambert-Beer Law

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and the Langmuir-Hinshelwood model [47] viz.:

η (%) =

ln (

Co − C t A − At = o × 100 Co Ao

C0 ) = Kα t Ct

[1]

[2]

C0 and A0 are the initial concentration and absorbance, while Ct and At are the concentration and absorbance after t mins’ degradation. Kα is the apparent first-order rate constant. It can be seen that all sensitized samples exhibit higher photodegradation properties than pure TiO2 from Figure 8(a). Moreover, the sample

ACCEPTED MANUSCRIPT reacting for 12 h has the best photodegradation activity. After 180 minutes of photodegradation, the degradation ratio of Ag-Ag2S(12h)/TNR to MO is 62.2%, while the value of pure TiO2 is only 33.6%. The kinetic research of photodegradation is shown in Figure 8(b). Meanwhile, Table 1 lists the photodegradation rate constants

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and photodegradation ratios. Obviously the slope of QDs/TNR is greater than pure TNR. Such implies that the QDs/TNR samples show a better photocatalytic activity than pure TNR. Clearly, the slope of Ag-Ag2S(16h)/TNR is the largest (K=4.71×10-3

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min-1), which means that the sample has the highest photocatalytic activity.

Table 1. Kinetic parameters of different samples for degradation of MO.

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K (min−1) 2.18×10-3 3.89×10-3 4.12×10-3 4.36×10-3 4.71×10-3 4.15×10-3

Degradation ratio (%) 33.6 51.3 52.4 54.4 62.2 52.8

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Photocatalyst TNR Ag/TNR Ag-Ag2S(4h)/TNR Ag-Ag2S(8h)/TNR Ag-Ag2S(12h)/TNR Ag-Ag2S(16h)/TNR 3.6. Mechanism discussion.

The proposed photocatalysis mechanism for degrading pollution MO over Ag-

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Ag2S/TNR arrays are demonstrated in Scheme 1. Irradiated by UV light, the electrons in the valence band (VB) of pure TNR excite to the conduction band (CB) to generate

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e-h+ pairs. However, the photo-induced e-h+ pairs will recombine quickly without any cocatalyst loading, leading to a low photocatalytic activity. When the surface is decorated with Ag QDs, the Ag/TNR arrays show a slightly increased photocatalytic activity because the photogenerated electrons were quickly captured and transferred from the CB of TiO2. After Ag2S QDs are deposited on the metallic Ag surface selectively, the Ag-Ag2S/TNR arrays obviously exhibit a notably reinforced photocatalytic property. In accord with the work function of metallic Ag (-4.26 eV) [48], the CB potentials of TiO2 (-4.21 eV) [49] and Ag2S (-4.50 eV) [50], an electron

ACCEPTED MANUSCRIPT transfer path in the Ag-Ag2S/TNR is exhibited in Scheme 1(a). Apparently, the photogenerated electrons from the CB of TiO2 can easily pass to the metallic Ag and the Ag2S in turn. In consequence, for the Ag-Ag2S/TNR arrays (Scheme 1(b)), the synergistic mechanism between metallic Ag and Ag2S results in their enhanced

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photocatalytic performance. In other words, Ag QDs are firstly applied as electron trapping agents to fast capture photo-generated electrons from the surface of TiO2 (first (1)), after this, Ag QDs act as an electron-transfer medium to steady carry

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electrons to the Ag2S active sites (second (2)), whereas the Ag2S QDs are served as the active site of the interface catalyst. The separated electrons on the surface of Ag2S

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react with the dissolved O2 to form O2•- free radicals. Then, the O2•- and the holes with intense oxidizing properties can oxidize H2O and dissolve OH- into OH• free radicals (third (3)). Therefore, the organic dyes like MO are oxidized into CO2 and H2O by the participation of holes, OH• and O2•- radicals.

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In the previous reports, the maximum photocurrents of Ag quantum dots modified TiO2 nanosheets (TNS) and TNR were 0.05 mA/cm2 and 0.032 mA/cm2, respectively. This result is lower than that of Ag-Ag2S quantum dot modified TNR (0.082

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mA/cm2), indicating that the electrochemical performance of Ag-Ag2S co-modified

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TNR has been improved. In addition, the degradation efficiency of MO by sensitized TNR of CdS nanosheets was 42.96% in 150 min [51], while the degradation efficiency of MO by Ag-Ag2S quantum dots sensitized TNR was 62.2% within 180 min. These results indicate that the modification of Ag-Ag2S quantum dots can impart better photoelectric and photocatalytic activity to TNR.

4. Conclusions To sum up, highly ordered TiO2 nanorod arrays decorated with Ag-Ag2S QDs are successfully synthesized on FTO by a hydrothermal reaction and a wet chemical way.

ACCEPTED MANUSCRIPT Results indicate that the TNR arrays whose surface modified by Ag-Ag2S QDs can efficiently

broaden

the

range

of

absorption

spectrum

and

advance

the

photoelectrochemical property and the photocatalytic activity in contrast to that of bare TNR and Ag/TNR under the AM 1.5 G simulating sunlight illumination. Within

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the range of visible light, the QDs/TNR arrays with a reaction time of 12h shows the best photocurrent density (0.082 mA/cm2) and photodegradation rate (62.2%). Based on the above experimental results, the improvement of these properties can be

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attributed to the metal synergy effect between Ag-Ag2S. In other words, the Ag QDs are able to stably capture and quickly transmit the photogenerated electrons from TiO2

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surface, whereas the Ag2S QDs are regarded as the interfacial active sites to motivate the degradation of organic pollutants. Therefore, such QDs/TNR arrays might bring novel chances to develop productive materials for energy and environmental

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applications.

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Acknowledgments

This work was supported by the National Natural Science Foundations of China

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(No. 51472003, 51772003, 51701001 and 61663005). The authors would like to thank Yonglong Zhuang and Zhongqing Lin of the Experimental Technology Center of Anhui University, for electron microscope test and discussion.

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Figure Captions Figure 1. XRD patterns of (a)TNR, (b) Ag/TNR, (c) Ag-Ag2S(4h)/TNR, (d) AgAg2S(8h)/TNR, (e) Ag-Ag2S(12h)/TNR, (f) Ag-Ag2S(16h)/TNR.

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Figure 2. SEM images of (a) TNR, (b) Ag/TNR, and (c) Ag-Ag2S(4h)/TNR, (d) AgAg2S(8h)/TNR, (e) Ag-Ag2S(12h)/TNR, (f) Ag-Ag2S(16h)/TNR.

Figure 3. (a)-(b) TEM images and (c)-(d) HRTEM images of Ag-Ag2S(12h)/TNR.

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Figure 4. (a) XPS survey spectrum and the strong line spectra of (b) Ti 2p, (c) Ag 3d, (d) S 2p, (e) O 1s.

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Figure 5. UV-Vis absorbance spectra of different samples. Figure 6. PL spectra of different samples.

Figure 7. Transient photocurrent responses of different samples

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Figure 8. Photocatalytic activity for degradation of MO, (a) photodegradation percentages of different samples, (b) kinetics of different samples Scheme 1. Proposed mechanism of Charge-transfer interaction and photodegradation

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of MO with Ag-Ag2S/TNR.

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ACCEPTED MANUSCRIPT Highlights 1) Synergistic effect between Ag-Ag2S inhibits recombination of electronhole pairs

increases absorption of visible light

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2) Ag-Ag2S quantum dots (QDs) sensitized TiO2 nanorod arrays (TNR)

3) QDs/TNR shows the highest photocurrent and photodegradation

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efficiency