Enhanced visible-light photoelectrochemical conversion on TiO2

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Enhanced visible-light photoelectrochemical conversion on TiO2 nanotubes with Bi2S3 quantum dots obtained by in situ electrochemical method Denilson Freitas, Johan R. González-Moya, Thiago André Salgueiro Soares, Richardson R. Silva, Dyego M. Oliveira, Herman S. Mansur, Giovanna Machado, and Marcelo Navarro ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00375 • Publication Date (Web): 18 Jul 2018 Downloaded from http://pubs.acs.org on July 19, 2018

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ACS Applied Energy Materials

Enhanced visible-light photoelectrochemical conversion on TiO2 nanotubes with Bi2S3 quantum dots obtained by in situ electrochemical method Denilson V. Freitas1‡, Johan R. González-Moya2‡, Thiago A. S. Soares2, Richardson R. Silva1, Dyego M. Oliveira2, Herman S. Mansur3, Giovanna Machado1,2 and Marcelo Navarro1* 1

Universidade Federal de Pernambuco, Departamento de Química Fundamental, Cidade

Universitária, 50670-901, Recife, PE, Brazil. 2

Centro de Tecnologias Estratégicas do Nordeste (CETENE), Departamento de Nanotecnologia,

50.740-540, Recife, PE, Brazil. 3

Universidade Federal de Minas Gerais, Departamento de Engenharia Metalúrgica e Materiais,

31270-901, Belo Horizonte, MG, Brazil. KEYWORDS: photoelectrochemical conversion; electrochemical synthesis; TiO2 nanotubes; in situ sensitization; Bi2S3; quantum dots

ABSTRACT: A new greener strategy to incorporate Bi2S3-mercaptopropionic acid (MPA) quantum dots (QDs) onto TiO2 nanotube (NT) films was developed, using in situ electrochemical synthesis with a graphite/sulfur powder macroelectrode (cathode) and cavity cell. Simultaneously, the obtained QDs were adsorbed on TiO2 NTs, for the photoelectrochemical cell (PEC) assays. After the electrochemical synthesis, different thermal post-treatments were

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performed for growth and crystallization of obtained nanocrystals. The obtained TiO2-Bi2S3 nanostructured composites were characterized by UV-Vis, diffuse reflectance spectroscopy, xray diffraction, scanning electron microscopy and transmission electron microscopy. Photoelectrochemical tests were carried out, where TiO2 NTs/Bi2S3 QDs with 15 min posttreatment at 90°C presented a greater photocurrent density, when irradiated with visible light (427-655 nm region, 10 mW.cm-2) and compared to pristine TiO2 NTs. Such performance can be attributed to the synergetic effect caused by the good interface between TiO2 NTs/Bi2S3 QDs, promoted by the electrochemical method of synthesis employed. The proof of concept evidenced in this work can be potentially extended for the sensitization of TiO2 NTs with other semiconductors for PEC hydrogen generation and solar cell applications.

1. INTRODUCTION Currently, the development of high efficiency photovoltaic and photoelectrochemical devices, at low cost and moderate toxicity, is extremely important for the use on large scale.1,2 TiO2 is the most studied material for these devices, since 1972, when Fujishima and Honda demonstrated the photoelectrochemical properties of TiO2.3 Due to its excellent properties, like: low cost, good chemical and photocorrosion stability, TiO2 is described as an excellent candidate for photovoltaic applications.4 As the material morphology has a great influence in the photovoltaic activity, the nanostructured forms (high surface area materials) has being increasingly studied, such as TiO2 nanotubes (NTs) and nanoparticles.5 However, the large band gap of TiO2 confines the photoelectrochemical activity of the material to the ultraviolet region, requiring a sensitization or doping of the TiO2 based materials to enhance the absorption of visible and IR light.6


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The sensitization with quantum dots (QDs) is a very efficient and studied way to increase the efficiency of photovoltaic and photoelectrochemical devices, due to the versatility of the nanoparticle band gap, which varies according to the size.7 For the applications in optoelectronic devices, the most used QDs are based on Cd6,8 and Pb9,10 chalcogenides, because the efficient electron transfer can occur under visible light excitation.7 However, these QDs are very toxic and infeasible for large scale application, being necessary the study of QDs based on more abundant and less toxic materials.11,12 Bismuth-based QDs (V-VI group) are promising because of its low-toxicity and useful electronic properties, like a short bulk band gap of 1,3 eV and high absorption coefficient.5 These QDs are a powerful tool in optoelectronic devices due to its absorption in the visible and near infrared region.13–16 Different methodologies are employed to sensitize the TiO2 with Bi2S3 semiconductor. One of the most used is the successive ionic layer adsorption and reaction (SILAR) method.17-18 Li et al.17 attached Bi2S3 nanoparticles to TiO2 NT films enhancing the photocurrent when compared to pristine TiO2 NTs. Using the same technique, Zumeta-Dubé et al.19 described the sensitization of the TiO2 NTs with Bi2S3 nanoparticles, however, in the presence of sodium ions the stability of the system decreases, compromising its photovoltaic properties. Electrochemical methods for QDs synthesis have been proposed for the preparation of chalcogen precursors.20–22 The electrosynthesis follows some principles of green chemistry, using milder reaction conditions, avoiding chemical reducing agents and preventing residue formation. Passos et al.21 developed a versatile electrochemical methodology for the synthesis of CdTe quantum dots. The generation of the chalcogenide precursors was carried out in a two-


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compartment electrochemical cell, separated by Nafion® membrane, producing nanoparticles with high size control and low dispersity. Aiming the use of visible light on TiO2 based photoelectrochemical devices, and the development of greener materials and processes, TiO2 NT films were sensitized with Bi2S3 QDs by using a simple and versatile in situ electrochemical method. A graphite/sulfur powder macroelectrode was used as cathode in electrochemical cavity cell, and mercaptopropionic acid (MPA) was used as bifunctional linker, stabilizing the Bi2S3 nanoparticles and promoting its adsorption on the TiO2 NT surface. After the electrochemical synthesis, different thermal posttreatments were carried out to optimize the interface between the semiconductors. The photoanodes obtained were characterized by several techniques and a photocurrent behavior was addressed and discussed. 2. EXPERIMENTAL SECTION 2.1. Chemicals All chemicals were of reagent grade and used without further purification. Elemental Sulfur (S0) powder (100 mesh, Aldrich), Bismuth (III) Chloride (≥ 98%, Sigma-Aldrich), graphite powder (particle size < 20 µm, Aldrich), 3-mercaptopropionic acid (MPA) (≥ 99%, Aldrich), HCl (37%, Aldrich) and NaOH (97%, Quimex) were used as purchased. The ethylene glycol 99% (ETG) and NH4F were purchased from Sigma-Aldrich, acetone PA and titanium foil (98.6%) were from Synth, Na2S•9H2O was from Alfa-Aesar, Na2SO3 was from Proquimios. The water was of Milli-Q grade. 2.2. Preparation of TiO2 NTs by anodization


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TiO2 NTs were obtained by anodization method. Typically, a potential of 30 V was applied to Ti foil (0.48 cm2 of circumference area and 1 mm of thickness), using a DC Power Source, for 1 h in an ultrasonic bath (135 W, UltraCleaner 1600A, Unique®). An ethylene glycol (ETG) solution containing 0.7 wt.% of NH4F and 10 wt.% of water was used as electrolytic solution.23,24 Before the anodization, the Ti foil was cleaned by sonication in acetone por 10 min rinsed with Milli Q water and dried under N2 flux. The anodization was performed in an undivided cell (Figure 1a), using a titanium foil as anode and a platinum foil as cathode. The anodization curve (current vs. time) was recorded with a Minipa ET-2076A computer controlled multimeter. After the anodization process, the samples were rinsed with distilled water and dried under N2 flux. In order to improve the crystallization of the TiO2 NTs, the samples were annealed in an oven at 400°C, for 3 h under atmospheric air.

Figure 1. (a) TiO2 NTs production by electrochemical anodization and (b) electrochemical cavity cell used for the sensitization of TiO2 NTs with Bi2S3 QDs.

2.3. Sensitization of TiO2 NTs with Bi2S3 QDs by in situ electrochemical method


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The TiO2 NTs obtained by anodization were sensitized with Bi2S3 QDs using an in situ electrochemical method. The TiO2 NT samples were placed inside the intermediate compartment of the electrochemical cavity cell containing BiCl3/MPA aqueous solution (Figure 1b), before starting the electrolysis. The cathodic compartment was prepared by compressing 58.5 mg (4.88 x 10-3 mol) of graphite powder mixed to 1.63 mg (5.09 x 10-5 mol) of elemental sulfur powder, and the mixture was pressed under P = 3.2 kg.cm-2 for 10 min. A sintered glass (previously sonicated in 0.2 mol.L-1 NaOH aqueous solution), with a diameter equal to the cavity of the cell (1.0 cm), was placed over the cavity to avoid the dispersion of the graphite, and at the same time allowing the migration of sulfide ions (S2-) formed during the electroreduction. A stainless steel grid was used as anode, placed in the anodic compartment containing 0.1 mol.L-1 NaCl solution, and separated from the intermediate compartment by a Nafion® membrane. The bismuth precursor solution was prepared by diluting 10.53 mg (3.34 x 10-5 mol) of BiCl3 in 2.0 mL of 1.0 mol.L-1 HCl, with subsequent addition of 1.22 x 10-3 mol (103.2 µL) of MPA (a pale-yellow solution was formed). The BiCl3/MPA solution pH was adjusted to 7.0 (0.1 mol.L-1 NaOH) and then diluted to 50 mL by addition of Milli-Q water. The resulting solution was poured into the intermediate compartment together with TiO2 plates/nanotube films. The electrolysis was kept under argon atmosphere to avoid the atmospheric oxygen interference. During the synthesis of Bi2S3-MPA QDs, the precursor molar ratios was fixed to S2-:Bi3+:MPA (2:3:72). Constant current electrolysis (i = -30 mA) was carried out during t = 327 s (Q = 5.09 x 10-5 mol x 2 e- x 96.500 F. mol-1= 9.82 C, and Q = i x t) plus an extended electrolysis time of 173 s (35% of a total time of 500 s) to ensure the theoretical complete electroreduction of the sulfur (i.e., 100% conversion S → S2-).


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After the electrosynthesis, the TiO2 NTs sensitized with Bi2S3-MPA QDs were rinsed in water and dried with N2 flux for further characterization (Ti_Bi_0 sample). A Ti_Bi_0 sample was transfered to a flask containing 10 mL of the Bi2S3-MPA QDs colloidal solution with pH adjusted to 10, and heated at 90°C for 15 min (Ti_Bi_15 sample). Finally, another Ti_Bi_0 sample was transfered to a flask containing 10 mL of Bi2S3-MPA QDs colloidal solution (pH 10) in a teflon-lined stainless steel autoclave, for hydrothermal treatment at 180°C during 30 min (Ti_Bi_30 sample). These thermal treatments were carried out to improve the interface between Bi2S3-MPA QDs and TiO2 NTs. The TiO2 NTs sensitized with Bi2S3 nanoparticles were referred to as Ti_Bi_time, where time relates to the period of heat treatment performed for that sample. 2.4. Materials and Methods Controlled-current electrolyses were carried out by using an Autolab PGSTAT 30 potentiostat/galvanostat and an electrochemical cavity cell (Figure 1b). X-ray diffraction (XRD) patterns were taken on a Bruker X-ray diffractometer model D8 Advance with a CuKα radiation (λ = 1.5418 Å) and with the 2θ range from 20˚ to 60˚ with a step of 0.02˚. The scanning electron microscopy (SEM) images were obtained using a FEI Quanta 200F field emission scanning electron microscope (FESEM) coupled to an electron probe energy disperse x-ray spectrometer (EDS) for mapping surface. The transmission electron micrographs were recorded by a FEI TECNAI G2 high-resolution transmission electron microscope (HRTEM) at 200 kV. For TEM analysis the samples were prepared by dispersing freestanding NTs in isopropanol at room temperature. One drop was deposited on a 400-mesh holey carbon coated Cu grid. The UV-Vis absorption spectra of colloidal Bi2S3-MPA QDs were registered in a Cary spectrophotometer 50/Varian (Xenon lamp) in the 200 - 1100 nm range with 1 nm of resolution. The diffuse reflectance spectra of TiO2 NTs sensitized films were obtained using a CARY 5000


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UV–Vis spectrophotometer (Agilent Technologies) equipped with an integrating sphere, using BaSO4 as a reference. The spectral range was 200 to 800 nm with 1 nm of resolution. Photocurrent

measurements

were

performed

using

a

flat-faced

quartz

photoelectrochemical cell (PEC) with a three-electrode configuration. The samples were used as photoanode, and Ag/AgCl (saturated KCl) as reference electrode with a Pt foil as counter electrode. The electrolyte was Na2S.9H2O (0.1 mol.L-1) and Na2SO3 (0.1 mol.L-1) (pH = 12.5). The S2-/SO32- mixture works as the hole scavenger to improve the efficiency in the PEC system and it can reduce the Bi2S3 photocorrosion.25–28 The 300 W xenon lamp (Newport) with AM 1.5 G filter (50 mW.cm-2) was used for irradiation source. For incident photon to current efficiency (IPCE) measurements were used a LED driver coupled to Metrohn Autolab Potentiostat with 7 LED in the 470-655 nm range with 10 mW.cm-2 of irradiance. 3. RESULTS AND DISCUSSIONS 3.1. Morphology and structure of TiO2 nanotubes TiO2 NTs arrays obtained by electrochemical anodization of Ti foil were monitored in real time via the current vs. time response. The current-time responses (Figure S1, Supporting Information) exhibit a typical behavior of three stages for the formation of TiO2 NTs in electrolytes containing fluoride ions.29 The curves are very similar for all the experiments indicating the reproducibility of the process. The morphology of the TiO2 NTs obtained was evaluated by SEM. Figures 2a and 2b shows the SEM images of typical TiO2 NTs obtained by anodization process. The average value of internal diameter was 75 ± 7 nm, and the average wall thickness was 10 ± 2 nm, estimated by measuring more than 160 NTs using ImageJ software. The average length of NTs is slightly above 2 µm, as obtained from the side view SEM images.


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The crystallization of the TiO2 NT films was improved by annealing the freshly prepared samples in an oven at 400 °C, during 3 h. The anatase phase has a higher photocatalytic activity if compared to amorphous TiO2 due to a higher mobility of charge carriers.30,31 The crystallization in anatase phase was confirmed by the XRD patterns (Figure S2, Supporting Information). After the annealing, the Bragg reflections in 25.4°, 37.8°, 48.2°, 53.9°, 55.1° and 62.8° are present in the diffractogram. These peaks correspond to the crystalline planes (101), (004), (200), (105), (211) and (204) of TiO2 anatase phase (JCPDS file N° 8412-86). The other peaks correspond to metallic Ti used as substrate for anodization.32,33

Figure 2. (a) SEM images of top view and (b) side view of the anodized TiO2 NTs.

3.2. Electrosynthesis of Bi2S3-MPA QDs An electrochemical method was used for the synthesis of Bi2S3-MPA QDs.21,22 An aqueous solution of BiCl3 (metallic precursor) and MPA stabilizer agent was prepared using NaCl as electrolyte (pH = 7) and poured into the intermediate compartment of the electrochemical cavity cell. The elemental sulfur, in the graphite/sulfur powder macroelectrode (cathodic cavity), was electroreduced to S2- ions (Eq. 1) and expelled out the cathode by


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electrostatic repulsion, migrating to the intermediate compartment to react with the Bi3+-MPA complex present in solution. Simultaneously, the oxidation of water occurs on the inox grid surface placed in the anodic compartment separated by a Nafion® membrane. According to the sulfur pourbaix diagram, at pH 7, the S2- species generated in the cathode can also be protonated, giving the HS- species in the intermediate compartment solution (Eq. 2), due to the HS- pKa = 12.92.34 S(s) + 2 e- → S2-(aq)

(1)

S2-(aq) + H2O(l) ⇌ HS-(aq) + OH-(aq)

(2)

The solvation of the Bi2Cl3 in aqueous medium is difficult due to the formation of Bi-OCl gel (Eq.3).35 Thus, the bismuth chloride salt was firstly dissolved in 1.0 mol.L-1 HCl solution, avoiding the formation of the gel (Bi-O-Cl) due to the common ion effect (Eq. 4). After BiCl3 solubilization, the MPA was added and the pH elevated to 7.0, the solution became yellow as it reached pH 4.3, indicating the formation of the Bi3+-MPA complex (Eq. 5). Bi2Cl3(s) + H2O(l) → Bi-O-Cl(s) + 2 HCl(aq)

(3)

Bi2Cl3(s) + HCl(aq) → 2 Bi3+(aq) + 4 Cl-(aq) + H+(aq)

(4)

Bi3+(aq) + x HS-(CH2)2-COO-(aq) + HCl(aq) + NaOH(aq) →

(5)

[Bi3+(HS-(CH2)2-COO-)x](aq) + NaCl(aq) + H2O(l) Bi3+-MPA complex formation was followed by UV-Vis spectra (Figure S3, Supporting Information). The Bi3+/HCl solution presents an absorption band at λmax = 310 nm. After addition of MPA to the solution at pH 2.0 - 4.0, the Bi3+ absorption band disappears and a new band at λmax = 335 nm can be observed, corresponding to the Bi3+-MPA complex absorption. It


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occurs due to the MPA acidic group deprotonation (pKa = 4.34). Increasing the pH to 10, it is possible to observe a red shift to 348 nm, indicating the deprotonation of the thiol group (pKa = 10.84), giving the complex [Bi3+(-S-(CH2)2-COO-)x]. The Bi2S3 QDs synthesis can be explained in three steps. Initially, due to the high Na+ concentration in solution, NaBiS2 colloidal nanocrystals are formed, the precursor of the Bi2S3. According to the equilibrium described in Eq. 2, S2- ions react with the [Bi3+(HS-(CH2)2-COO-)x] complex in the central compartment of the electrochemical cell, in the presence of Na+ ions (Eq. 6). After the electrolysis, the NaBiS2-MPA nanoparticle solution was transferred to a flask and the pH was adjusted to 10.36 The heat treatment is crucial for the particle growth process, thus, during the heating at 90°C, NaBiS2-MPA nanocrystals equilibrate with S2- dissolved species (Eq. 7), resulting in the formation of Bi2S3-MPA QDs (Eq. 8) . If the process occurs via hydrothermal process at 180°C, the crystallization of NaBiS2 may occur simultaneously to the Bi2S3-MPA QDs formation.37 2 S2-(aq) + [Bi3+(HS-(CH2)2-COO-)x](aq) + Na+(aq) → NaBiS2-(HS-(CH2)2-COO-)x (colloid)

(6)

NaBiS2-(-S-(CH2)2-COO-)x (colloid) ⇄ 2 S2-(aq) + [Bi3+(-S-(CH2)2-COO-)x](aq) + Na+(aq)

(7)

3 S2-(aq) + 2 [Bi3+(-S-(CH2)2-COO-)x](aq) → Bi2S3(-S-(CH2)2-COO-)x (colloid)

(8)

3.3. Characterization of Bi2S3-MPA QDs The samples of NaBiS2-MPA nanoparticles and Bi2S3-MPA QDs with different heat treatments (Figure 3) were characterized by XRD. In the sample obtained after electrolysis without heat treatment (Bi_0), it is possible to observe predominantly an amorphous diffraction pattern, with 2θ values of 26.5, 30.3 and 44.4° associated to the planes 111, 200 and 220 of the cubic structure of NaBiS2 nanoparticles (JCPDS card number 8-406).37 The 2θ values of 15.7°,


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17.7°, 22.4°, 25.0°, 28.7° and 35.6° can be observed in the diffractogram of samples after 15 minutes of heat treatment (Bi_15) and 30 minutes (Bi_30) (Figure 4b and 4c), which are associated to the planes 200, 120, 220, 310, 211 and 240, characteristic of the orthorhombic structure of Bi2S3 (JCPDS card number 17-0320).38 In the diffractogram of the Bi_30 sample, it is also possible to observe the low-intensity NaBiS2 planes, 111 and 200, indicating the existence of a low population of NaBiS2 nanocrystals mixed to Bi2S3-MPA QDs. As the NaCl is present in the reaction medium as supporting electrolyte, it is possible to observe peaks at 31.7°, 45.4° and 56.5°, associated to the planes (020), (022) and (222) of the cubic structure of NaCl (JCPDS card number 05-0628).39 Transmission electron microscopy (TEM) images were taken for the analysis of the nanoparticle sizes of the samples Bi_0 (Figure S4a, Supporting Information), Bi_15 (Figure 3a) and Bi_30 (Figure S5a, Supporting Information). The microscopy showed nanoparticles dispersed on the surface of the grid, with the presence of agglomerates. The size histogram of the samples Bi_0 (Figure S4b, Supporting Information), Bi_15 (Figure 3d) and Bi_30 (Figure S5b, Supporting Information) was analyzed, being possible to observe a normal distribution of nanoparticles, with sizes typically ranging from 2 to 7 nm. The average diameter for the Bi_0 sample (3.4 ± 0.7 nm) was relatively smaller than the Bi_15 (4.3 ± 1.0 nm) and Bi_30 (4.2 ± 0.9 nm) samples, which were subjected to heat treatment. The elemental composition of the nanoparticles was determined by energy dispersive spectroscopy (EDS), with Bi and S peaks attributed to Bi2S3 for the samples: Bi_0 (Figure S4c, Supporting Information), Bi_15 (Figure 4c) and Bi_30 (Figure S5c, Supporting Information). In addition, oxygen, carbon and sulfur identified in the EDS can be related to MPA present in the sample, while the high intensity of carbon and copper can be associated to the grid used for the analysis.40


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(a)

(c)

(b)

(d)

Figure 3: (a) XRD patterns for Bi2S3 QDs with different heat treatments, where NaBiS2 (JCPDS card number 8-406), Bi2S3 (JCPDS card number 17-0320) and NaCl (JCPDS card number 05-0628) are identified. (b) Light-field TEM image, (c) EDS analysis and (d) histogram of sizes for the sample Bi2S3 QDs with heat treatment at 90°C for 15 minutes (Bi_15).

The Bi2S3-MPA QDs were also characterized by UV-Vis (Figure 4a). The absorption spectra of the nanoparticles submitted to different thermal treatments showed an absorption band at 600 nm, where the low definition of the bands occurs due to the polydisperse distribution of size for the nanoparticles, obtained in aqueous medium.41


13


The band gap (Eg) of the Bi2S3-MPA samples were estimated by using Tauc plots of of (αhv)2 vs. photon energy (Figure 4b).40 It can be observed small variations on the Eg values, according to the heat treatment of the nanocrystal solutions, presenting Eg values of 1.94 eV for the sample Bi_0, 1.90 eV for Bi_15, and 1.92 eV for Bi_30. These Eg values are higher than that described for the Bi2S3 bulk (1.33 eV) due to the quantum confinement presented by nanosystems.41,42 1.0

0.05

Bi_0 Bi_15 Bi_30

0.8

0.04

(α ν )2 (eV.m-1)2 α hν

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6

0.4

0.2

0.03

0.02

0.01

(a) 0.0 400

500

600

700

Bi_0 Bi_15 Bi_30

Bi2S3 bulk 1.33 eV

(b) 800

0.00 1.00

1.25

1.50

1.75

2.00

2.25

2.50

Energy (eV)

Wavelength (nm)

Figure 4. (a) UV-Vis spectra and (b) Tauc plot for the Bi2S3-MPA QDs solutions Bi_0, Bi_15 and Bi_30.

The low growth rate observed in the UV-Vis spectra (Figure 4a) occurs due to the high concentration of the MPA stabilizer used in the synthesis, which prevents the growth of the nanoparticles. After heat treatment (Bi_15 and Bi_30), the absorption band intensity increases without a significant wavelength shift, indicating a higher conversion efficiency (NaBiS2 to Bi2S3) for the Bi_15 sample. A further heat treatment at 90°C, during 45 minutes, led to the precipitation of a solid and decreasing of the absorption band intensity, indicating the agglomeration of the Bi2S3 nanoparticles and loss of stability. 3.4 Sensitization of TiO2 NTs with Bi2S3 QDs.


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In the beginning of the in situ electrochemical reaction process, a portion of the Bi3+MPA complex diffuses into the nanotubular structure, adsorbing on the TiO2 NT surface through the MPA carboxylate group.43 Starting the electrolysis, the reduced species (S2-) expelled by electrostatic repulsion, migrating to the central compartment of the cell, thus reacting with the Bi3+-MPA complex present in the bulk solution and also on the TiO2 NTs, giving the TiO2-Bi2S3 sensitized samples. After the sensitization of TiO2 NTs with Bi2S3-MPA QDs, no significant changes were observed on the nanotubular morphology. SEM images of the sensitized TiO2 NTs (Figure S6, Supporting Information) showed that the Bi2S3-MPA QDs were not observed on the nanotubular array, because the size of the nanoparticles is smaller than the SEM resolution. On the other hand, the Bi element was identified by the EDS map of the samples (Figure S6, Supporting Information), showing a uniform presence of the Bi element through the TiO2 nanotubular matrix. Thus, it suggests a homogeneous distribution of Bi2S3 onto the TiO2 NTs. TEM images of the derived colloidal Bi2S3-MPA QDs were obtained to roughly estimate the distribution of nanoparticle sizes attached onto the TiO2 NTs arrays. When compared to the growing rate of the nanoparticles in the bulk solution, the growing rate in a confined mesoporous structures presents a limited diffusion.44 Figure 5a shows the TEM images of the Bi2S3-MPA colloidal nanoparticles (Bi_15) obtained in the bulk solution, simultaneously to the Ti_Bi_15 sensitized sample preparation. It was possible to observe the Bi2S3 nanoparticles with 2 to 5 nm of diameter. In the HRTEM images (Figure 5b), the interplanar distance of 0.357 nm was measured, corresponding to the (130) crystallographic planes for orthorhombic Bi2S3 (JCPDS file No 17-320).38


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Figure 5. (a) TEM image and (b) HRTEM image of colloidal Bi2S3 nanoparticles. (c) TEM image and (d) HRTEM image of Ti_Bi_15 sensitized sample.

Figure 5c shows the TEM images of the Ti_Bi_15 sensitized sample, making possible to observe the nanotubular structure of TiO2 matrix. In the HRTEM image of the same sample (Figure 5d), it was possible to identify the spacing of the visible lattice fringes, which were measured to be 0.24 nm and 0.357 nm, in agreement to the interplanar distances of the (103) crystallographic planes for anatase TiO2 (JCPDS file N° 8412-86),32 and the (130)


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crystallographic planes of orthorhombic Bi2S3 (JCPDS file N° 17-320),38 respectively, showing the interaction between the TiO2 and Bi2S3 semiconductors in the sensitized samples. The UV-Vis absorption spectra of the TiO2 NTs and the respective Bi2S3 sensitized samples were obtained by diffuse reﬂectance spectroscopy (DRS), using a Kubelka-Munk function.45 In the Figure 6, it can be observed a large absorption band at 420 nm corresponding to the TiO2 NT sample. The sensitization of the TiO2 nanotubular matrix with NaBiS2/Bi2S3 nanoparticles (Ti_Bi_0) leads to an increase of the absorption band. The Ti_Bi_15 sample presented the same behavior, showing greater increase of the absorption band, due to the increase in Bi2S3 nanoparticles on TiO2 NT surface, as well as the improving of the TiO2-Bi2S3 interface in the nanotubular matrix. Ti_Bi_30 sample did not lead to higher absorption intensity due to the lower conversion rate of NaBiS2 to Bi2S3 nanoparticles,37 following the same tendency observed for the colloidal Bi2S3 QD sample (Bi_30), confirming that the Ti_Bi_15 sample has the optimal condition.

F(R) (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


25

TiO2

20

Ti_Bi_0 Ti_Bi_15 Ti_Bi_30

15 10 5 0 300

400

500

600

700

800

Wavelength (nm) Figure 6. DRS of the TiO2 NTs and the respective Bi2S3 QDs sensitized samples at different conditions.


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The band gap of the TiO2 NT sample was estimated around 3.3 eV, using a Tauc plot of (αhv)1/2 vs. photon energy, using the formula for indirect band gap semiconductors.46,47 This value is very similar to that reported in the literature for the anatase phase of TiO2 (3.2 eV).48 The band gap of the TiO2 sensitized sample (Ti_Bi_15) was estimated to Eg = 2.7 eV using the same Tauc plot. The difference on the band gap energy shows a good interface interaction for that sensitized sample. The characterization results and optical properties determined allow to conclude that the sensitization of TiO2 NTs with Bi2S3 nanoparticles occurs for all the experimental conditions, however, the optimal sensitization was observed for the Ti_Bi_15 sample. 3.5. Photoelectrochemical characterizations Figure 7a shows the photocurrent density vs. potential curves under solar simulated irradiation AM 1.5 G (50 mW.cm-2) for the TiO2 NTs and TiO2 NTs sensitized with Bi2S3 nanoparticles. All the sensitized samples exhibited higher saturate photocurrent than pristine TiO2 NTs, in addition, all sensitized samples reached the photocurrent saturation at a more negative potential. The increase of the PECs overall efficiency probably occurs due to the reduction of the external applied bias.49,50 The Ti_Bi_15 photoanode obtain a higher saturated photocurrent (0.3 mA.cm-2) than that of other samples. It confirms that the Bi2S3 nanoparticles coating the TiO2 NTs surface by the in situ electrochemical method is a simple but effective way to enhance the PEC performance of TiO2 photoanodes.


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0.4 Ti_Bi_0 Ti_Bi_15 ( Ti_Bi_30

0.3

0.5

(a)

dark)

(b)

η = 0.44 %

-2

dark)

AM 1.5 G (50 mW.cm ) Photoconversion (%)

pristine TiO2 (

-2

Photocurrent density (mA.cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.2

0.1

0.4

pristine TiO2

0.3

Ti_Bi_0 Ti_Bi_15 Ti_Bi_30

η = 0.27 % η = 0.27 %

0.2

η = 0.11 % 0.1

0.0 -0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

0.0 -1.0

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Potential (V vs Ag/AgCl)

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

Potential (V vs Ag/AgCl)

Figure 7. Photocurrent vs. potential plots of the TiO2 NTs sensitized with Bi2S3 QDs under AM 1.5 G (50 mW.cm-2) irradiation (a) and photoconversion efficiency (b).

Figure 7b shows the photoconversion efficiency (η) for the Ti_Bi_0, Ti_Bi_15 and Ti_Bi_30 samples. It can be observed that the η values for all the sensitized samples are higher than the pristine TiO2 NTs (η = 0.11%).50 The Ti_Bi_0 sample presented η = 0.27%, after the heat treatment, the photoconversion efficiency of the Ti_Bi_15 sample (η = 0.44%) increased 4 times. Finally, the Ti_Bi_30 sample showed a photoconversion efficiency η = 0.27%, decreasing to the same value observed for the initial Ti_Bi_0 sample. Both Ti_Bi_0 and Ti_Bi_30 samples showed higher concentration of NaBiS2 nanocrystals mixed to Bi2S3 QDs, causing the recombination of the electrons and reduction on the photocurrent efficiency. The photocurrent response of the photoanodes were carried out at 0.0 V bias (vs. Ag/AgCl), under on/off cycles for the simulated solar light AM 1.5 G (50 mW.cm-2) and also in the visible region with different LEDs (450, 505, 530, 590, 617, 627 and 655 nm, 10 mW.cm-2), Figure 8. As observed in the Figure 8a, the photocurrent responses of all the sensitized samples are higher than the pristine TiO2 NTs, when irradiated with simulated solar light. The optimal Ti_Bi_15 sample generated a 0.29 mA.cm-2 photocurrent, improving the photocurrent of pristine TiO2 NTs about 3.2 times. The photostability analysis of the Ti_Bi_15 sample was carried out


19


with simulated solar light for 3600 s, showing a decay of approximately 10% of the initial photocurrent, indicating a good stability (Figure S7, Supporting Information). For the photocurrent obtained by visible light irradiation (Figures 8b-d), a significant increase can be observed when compared to the pristine TiO2 NTs, which has negligible values in the visible region (Figure S8, Supporting Information). Ti_Bi_15 can be considered an efficient system for the visible light absorption and transferring of the photogenerated electrons

off

∆I = 0.29 mA.cm

0.3

-2

-2

∆I = 0.20 mA.cm

0.2 -2

∆I = 0.17 mA.cm

-2

∆I = 0.09 mA.cm

0.1

0.0

on 0

50

100

150

18 -

Pristine TiO2 Ti_Bi_0 Ti_Bi_15 Ti_Bi_30

(a) AM 1.5 G (50 mW.cm-2)

0.4

Photocurrent density (µ A.cm 2)

-2

Photocurrent density (mA.cm )

on the TiO2 PEC operation.

14 12 10 8 6 4 2 0

200

0

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400

600

800

1000

1200

1400

1600

Time (s)

470nm 505nm 530nm 590nm 617nm 627nm 655nm

80

60

40

20

0

14

(d) Ti_Bi_30, 10 mW.cm-2

470nm 505nm 530nm 590nm 617nm 627nm 655nm

-

(c) Ti_Bi_15, 10 mW.cm-2


-

100

470nm 505nm 530nm 590nm 617nm 627nm 655nm

(b) Ti_Bi_0, 10 mW.cm-2

16

Time (s)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 8. (a) Photocurrent response of pristine TiO2 and sensitized samples under AM 1.5 G (50 mW.cm2

) irradiation. (b) Photocurrent response of Ti_Bi_0, (c) Ti_Bi_15 and (d) Ti_Bi_30 with visible

irradiation (427-655 nm region, 10 mW.cm-2) with on/off cycles at applied potential of 0.0 V (vs. Ag/AgCl).

Figures 8b-d show the activity of the TiO2 NTs sensitized samples for specific wavelengths. It can be observed that the Ti_Bi_15 sample presented the highest average photocurrent. For all Bi2S3 sensitized samples, the photocurrent decreases as the wavelength increases, but at 627 nm the photocurrent has a sudden increase, which is associated to the absorption wavelength of the Bi2S3 nanoparticles (Figure 4a), evidencing the sensitization of the TiO2 NTs. Since MPA acts as a bifunctional binder, it is expected a higher concentration of the BiMPA complex on the TiO2 NT surface, prior the Bi2S3-MPA QDs synthesis, as a consequence of the carboxylate group interaction with the TiO2 NTs.51 During the sensitization procedure, the presence of NaBiS2 is inevitable, due to the occurrence of Na+ ions in solution.19 Thus, the Ti_Bi_0 system is characterized by having a low amount of Bi2S3 nanoparticles deposited on the TiO2 NTs surface. The amorphous structure of Bi2S3-MPA QDs and NaBiS2 nanoparticles on TiO2 NTs (Figure 3a) is considered kinetically unstable because the high disorder degree of the system, increasing the instability and the recombination of the e-/h+ pairs.52 The heat treatment diminishes the energy on the surface of the particles, increasing the crystallinity (Figure 3a), resulting to nanoparticles of spherical structure (Figure 3b). The Ti_Bi_15 sample provides a synergetic effect, reducing the recombination and instability, which is the result of the greater conversion of NaBiS2 nanocrystals to Bi2S3 QDs, obtaining a significant increase of the photocurrent.37 For the Ti_Bi_30 system, a competition between


21


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dissolution and crystallization of NaBiS2 nanoparticles should occur,37 affecting the concentration of Bi2S3-MPA nanoparticles on the TiO2 NT surface (Figure 6), thus causing a lower stability and photocurrent density. The IPCE measurements (Table S1, Supporting Information) show that the Ti_Bi_15 optimal photoanode is active throughout the full visible region from blue to red. The measured IPCE is practically related to the contribution of Bi2S3 QDs in the material due to its absorption in the visible region; since the TiO2 NTs only absorbs efficiently in the UV region. The IPCE follows the same absorption profile obtained by DRS showing that the higher the absorption intensity (lower wavelength), the greater the photocurrent generated, and consequently the incident photon-to-current conversion efficiency. The Ti_Bi_15 sample (Figure 8c), besides the high current values, has an excellent stability, indicating an excellent synergy between Bi2S3 nanoparticles and TiO2 NTs. In turn the samples Ti_Bi_0 and Ti_Bi_30 (Figure 8b and 8d), presented a level of instability and consequently low current values.

Figure 9. Schematic energy level diagram for Ti_Bi_15 sample.


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In order to explain the phenomena observed above, the mechanism shown in Figure 9 is proposed. Under solar light excitation, the Bi2S3/TiO2 system can be defined as a type 2 heterojunction, where the electrons photoexcited to the Bi2S3 conduction band are photoinjected to the TiO2 conduction band, whereas the holes perform the opposite movement, guided by the forces of direction of Fermi.53–56 The proposed mechanism should occur for all TiO2 sensitized samples studied, although the different structures observed in each Bi2S3 nanoparticles should affect the photocurrent values observed in Figure 8. The results described in this work proved the successful deposition of Bi2S3 QDs onto TiO2 NTs, using a facile in situ electrochemical synthesis, without the use of reducing agents. The optimal heat treatment allowed the formation of a good interface between the Bi2S3 QDs and TiO2 NTs in the sensitized samples. The attached Bi2S3 QDs in organized way act synergistically and enhances the photoelectrochemical conversion for TiO2 based material using both ultraviolet and visible light irradiation. Under UV irradiation, the Bi2S3 QDs improve the charge separation efficiency, diminishing the charge recombination of excited TiO2 semiconductor. Moreover, under visible light, the Bi2S3 QDs absorb in the full spectrum and the electrons can be transferred to TiO2 NTs, generating H2 in the Pt cathode through an external circuit, notably increasing the photoelectrochemical efficiency of the sensitized samples. 4. CONCLUSIONS The present work provides a simple and green new strategy to incorporate a Bi2S3 semiconductor onto TiO2 NT films, using an in situ electrochemical method. In summary, we have successfully sensitized the TiO2 NTs by Bi2S3 QDs, used as photoanode for PEC hydrogen generation. The Bi2S3 nanoparticles/TiO2 NT matrix enhanced the photocurrent density of the pristine TiO2 NTs, due to the improved electron excitation in the visible region, charge carrier


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separation and transport efficiency. The heat treatment was necessary to sensitize the Bi2S3 nanoparticles/TiO2 NT samples. The concept developed here can be potentially extended to TiO2 NTs sensitized with other semiconductors for a myriad of applications in light harvesting and conversion.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at

http://pubs.acs.org.

The

following

files

are

available

free

of

charge.

Anodization curve of Ti foil, XRD patterns of TiO2 nanotubes before and after annealing, EDS map of sensitized TiO2 NTs with Bi2S3, transmission electron microscopy, sizes histogram and EDS analysis of Bi2S3 QD samples. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡Denilson V. Freitas and ‡Johan R. González-Moya contributed equally to this work. Notes The authors declare no competing financial interests. Funding Sources


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This work is supported by CNPq (401541/2016-9), CAPES, FAPEMIG and FACEPE (APQ1991-1.06/12). ACKNOWLEDGMENT The authors also thank the staff at the Center of Nanoscience, Nanotechnology and InnovationCeNano2I/CEMUCASI/UFMG for the spectroscopy analyses. JRGM acknowledge the scholarship provided by CNPq (312960/2016-6).

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Enhanced visible-light photoelectrochemical conversion on TiO2

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