Effect of Dye Structure on Optical Properties and Photocatalytic

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Article

Effect of Dye Structure on the Optical Properties and Photocatalytic Behaviors of Squaraine Sensitized TiO Nanocomposites 2

Yongling Fang, Zhongyu Li, Baozhu Yang, Song Xu, Xiaojun Hu, Qiaoli Liu, Dandan Han, and Dayong Lu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp502208y • Publication Date (Web): 01 Jul 2014 Downloaded from http://pubs.acs.org on July 5, 2014

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The Journal of Physical Chemistry

Effect of Dye Structure on Optical Properties and Photocatalytic Behaviors of Squaraine Sensitized TiO2 Nanocomposites Yongling Fang, a Zhongyu Li, a,b,c * Baozhu Yang, a Song Xu, a,* Xiaojun Hu, b,* Qiaoli Liu, c Dandan Han, c Dayong Luc a

Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of

Petrochemical Engineering, Changzhou University, Changzhou 213164, P.R. China b

Key Laboratory of Regional Environment and Ecoremediation (Ministry of Education), Shenyang University, Shenyang 110044, P.R. China

c

Department of Materials Science and Engineering, Jilin Institute of Chemical Technology, Jilin 132022, P.R. China

*

Corresponding author. Tel.: +86-519-86330253; Fax: +86-519-85602670 E-mail address: [email protected]; [email protected]; [email protected]

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ABSTRACT TiO2 nanoparticles obtained via improved polymerization-induced colloid aggregation (Im-PICA) method were sensitized by three squaraine dyes to construct dye/TiO2 nanocomposite photocatalysts (1:3 mass ratios) with visible-light response by facile hydrothermal method. Scanning Electron Microscopy (SEM) and Transmission Electron Miscroscopy (TEM) were employed to visually observe the surface morphology of the sample. We studied the visible-light response via UV-vis diffuse reflectance spectroscopy and optical band gap calculation of TAUC plot equation. The dyes possess excellent thermal stability which is ascribed to the decomposition temperature reaching 300 °C based on TG-DTG analysis. The photocatalytic activities of as-prepared SQ/TiO2 composites were presented by the degradation rate of methylene blue (MB) under visible-light irradiation. The ISQ/TiO2 composites achieved best enhancement of photocatalytic activity under 150 min visible-light irradiation. When ISQ/TiO2 nanocomposites were applied at different photocatalytic degradation cycling times, the degradation rate still reached 68%, which reflects that SQs/TiO2 composites have high photostability in the photocatalytic process. When taking the degradation rate and photoresponse into consideration the combination mode between ISQ dye and TiO2 which obtained optimal degradation rate, tended to bond as a hydrogen bond, which exhibited stronger binding force than m-ISQ/TiO2 and c-BSQ/TiO2 with simple physical absorption based on the FT-IR analysis. Using TOC test and LC results, we verified the degree of mineralization of MB molecular.. Based on the fluorescence spectra the highest fluorescence quantum yield of ISQ dye (0.124), is far more than those of c-BSQ (0.036) and m-ISQ (0.02).Molecular structure of the squaraine dyes and the electron distribution of their HOMO and LUMO states were given by geometrical optimization with Gaussian98 at B3LYP/6-311+G(d,p) level. Compared with the optical band gap calculated via TAUC plot equation, we can suggest that its energy was slightly less than the band gap due to the


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absorbed photon. This may related to the localized central electronic, phonon and local energy level between CB and VB. The calculated value of the optical band gap via UV-DRS becomes less than the theoretical calculated energy band gap. Keywords: Squaraine dye; Titanium dioxide; Fluorescence Spectra; Gaussian Calculation; Visible-light photocatalytic activity



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1. INTRODUCTION

Great advancement and prosperity in the area of renewable energy has drawn attention attributable to the existence of environmental issues and limited utilization of fossil fuels. From the viewpoint of resolving the energy crisis, solar energy conversion is most promising. Solar energy, due to its abundant storage, being clean without any by-product and having convenience of exploitation and utilization, has become first choice in the field of renewable energy sources.1-3 Meanwhile, semiconductor photocatalysts play an important role in the process of photodegradation of organic pollutants with light illumination (visible-light or UV light).3-7 Among various kinds of semiconductors, traditional semiconductor titanium dioxide (TiO2) has been widely studied due to its being non-toxic, highly efficient, stable, environmentally friendly and inexpensive from the aspect of various organic compounds degradation.8-14 The wide bandgap of TiO2 and finite utilization of solar energy in high-energy ultraviolet region (less than 5%) extremely limited its application on a large scale. Modification methods such as heterojunction construction, noble metal and nonmetal doping, selecting supporters with excellent electronic transmission capacity and dye sensitization made great contributions to extend the photo-response range to the visible-light region. Employing dye sensitization to modify TiO2 became a wonderful approach to achieve excellent electronic and optical properties, mainly from the aspect of solving two major drawbacks, including the ultrafast recombination rate of photo-generated electron hole pairs and low quantum yield.15 Metal-free organic dyes have been utilized as sensitizers of TiO2 with visible light response for their advantages of having a high molar absorption coefficient, wide absorption bands,


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facile molecular design and a lack of concern with noble metal resources.16-19 Squaraine dyes (SQs) are a class of promising dyes with resonance stabilized zwitterionic structure because of their strong absorption arising from charge transfer between the electron deficient central four member ring, and oxygen moieties with contribution from the substituent on both sides. Sensitizing abilities have been employed as sensitizers in electrography, organic photovaltaic materials, and optical data storage applications with sharp and intense absorption bands in visible and near-IR regions.20-24 SQs have been employed in different cutting-edge technologies such as nonlinear optics, photodynamic therapy, or bio-labeling applications. However, SQs are recently attracting a lot of attention in the field of photovoltaics, dye sensitized solar cells (DSSCs), or organic solar cells.25-29 For the sake of efficient electron transfer from the excited dye to the TiO2 conduction band, the excited-state redox potential should match the energy of the TiO2 CB edge. Electron flow from the light-harvesting moiety of the squaraine dye towards the TiO2 surface was accompanied by light irradiation and excitation. Excellent electronic coupling has been fabricated between the lowest unoccupied orbital (LUMO) of the dye and CB of TiO2 for better π-π* electron delocalization within the whole molecule.1,23,29 SQs consist of two electron donating endgroups (D) and a central electron withdrawing 1,3-disubstituted C4O2 unit (A) forming a donor–acceptor–donor (D–A–D) alignment which determines its inherent photostability. Moreover, the nature of the side groups can also modify the energy levels of the SQs (HOMO and LUMO orbitals) and affect the electron transfer reactions.22,30 Therefore, dye-molecular structure should be optimally designed to make great contribution to the field of solar-to-electric conversion efficiency improvement, and to the price-performance ratio



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improvement at the same time which remains to be a significant challenge. In this present study, we designed and synthesized a series of simple symmetric SQs (their structure formulas and abbreviations were listed below intensively) with different electron donating endgroups and sensitized on the TiO2 surface successfully by facile hydrothermal method. Enhanced photocatalytic degradation rate the ISQ/TiO2 owns is due to charge transfer capacity of different SQ dyes which illuminated via the fluorescence spectra and possible bonding type between SQ dyes and TiO2. TOC test, FT-IR spectra and Chromatogram of LC analysis were employed to investigate the degradation depth of MB in detail. Molecular structure of SQs and the electron distribution of their HOMO and LUMO states were systematically studied, especially discussing the possible influence factors including steric hindrance, charge flow direction, and conjugated degree of dye molecules on the photocatalytic properties of their sensitizing TiO2.

2. EXPERIMENTAL METHODS

2.1. Preparation of SQ/TiO2 Nanocomposites.

TiO2 nanoparticles were fabricated by an

improved polymerization-induced colloid aggregation (Im-PICA) method as referenced. Simultaneously, the synthesis of the three SQs was slightly modified as referenced.

32

31

For

example, the ISQ dye was synthesized according to the following steps: 2.8565 g (0.018 mol) 2,3,3-trimethylindolenine and 1.0240 g (0.009 mol) squaraine were mixed into a three-necked flask in a solvent mixture of 20.0 mL of methylbenzene and 20.0 mL of butanol equipped with a mechanical stirrer and a thermometer, respectively. After heating and refluxing for 6 h, the byproduct water was removed by water segregator and the solvent was wiped out via


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decompression distillation to obtain the green solid. The dyes were sequentially purified by ethyl acetate and recrystallization in the solvent of ethanol to yield the green crystals, namely the indole squaraine dye. Here is shown the molecule structure of indole squaraine dye which abbreviated to ISQ:

O C H

N H

C H

N H

O

[ISQ]

The other two SQs were fabricated according to the identical method, and the molecule structures and their abbreviations are given as follows:

O C H N CH3

C H O

N H3 C

[m-ISQ]

O S Cl

S C H

N CH3

C H O

N H3C

Cl [c-BSQ]

SQ/TiO2 nanocomposites were prepared by hydrothermal method according to the following steps: first, 60 mg dye was dissolved in 120 mL of ethanol under stirring for 2 h to form a uniform solution. Then, 60 mg TiO2 nanoparticles were dispersed into the dye ethanol solution. After being continuously ultrasonicated for 2 h, the mixture solution was transferred into a 250 mL Teflon-lined stainless autoclave. The autoclave was kept at 100 °C for 6 h. After cooled to ambient temperature naturally, the precipitate was leached, washed with deionized water and ethanol repeatedly, and then dried in vacuum oven at 60 °C for 6 h to



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obtain the dye/TiO2 composites. The three composites are marked as ISQ/TiO2, m-ISQ/TiO2, c-BSQ/TiO2, respectively. 2.2. Characterization.

Morphologies of the prepared samples were observed on a

JSM-6360LA scanning electron microscope (SEM, JEOL, Japan) and a JEM-2100 transmission electron microscope (TEM, JEOL, Japan). Raman spectra were measured at room temperature using a LabRAM XploRA Raman spectrometer (Horiba Jobin Yvon, French) with a 532 nm laser focused on a spot about 3 nm in diameter. UV-vis diffuse reflection spectra (DRS) of the photocatalysts were measured by a UV-vis scanning spectrophotometer (Shimadzu UV-2550) using an integrating sphere and BaSO4 as white standard. The thermal stability of the dyes was carried out using the TG-209-F3 Thermogravimetric Analysis Meter (Nestal Company, Germany). Fourier transform infrared spectra (FT-IR) of samples were collected with a Nicolet (PROTéGé 460) spectrometer in the range from 4000 to 600 cm-1. Fluorescent Spectra of different dyes and reference compound RhB were measured with an F-280 spectrometer. Molecular structure of SQs and the electron distribution of their HOMO and LUMO states were given by geometrical optimization with Gaussian98 at B3LYP/6-311+G(d,p) level. 2.3. Photocatalytic Activity.

The photocatalytic activity of the dye/TiO2 composites were

evaluated by photodegradation of the methylene blue (MB). The photocatalytic experiments were carried out by adding 10 mg composites powder into a cylindrical glass vessel with a circulating water jacket containing 50 mL of the MB aqueous solution (25 mg/L). Before irradiation of the simulated visible light, the suspension containing the target contaminant and photocatalysts was continuously stirred in dark for 40 minutes in order to reach an


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adsorption-desorption equilibrium. After that, the suspension was irradiated while stirring by visible light which was simulated by a 500 W Xe lamp. During irradiation, each sample for analysis was taken out at regular intervals. The nanoparticles were taken from the mixture solution with centrifugation at 5000 rpm for 20 min. The clarified solution was analyzed by UV759 UV-vis spectrometer (Shanghai Precision & Scientific Instrument Co., Ltd., China) to obtain the absorbance of the target contaminant at maximum absorption wavelength.

3. RESULTS AND DISCUSSION

3.1. SEM Images.

We selected ISQ as an example to exhibit its SEM morphology changing

before and after sensitizing TiO2 nanoparticles, which is shown in Figure 1. Figure 1a and b exhibit the bare TiO2 SEM images, and Figure 1c and d present the SEM images of the TiO2 sensitized by ISQ dye. It can be seen clearly that the TiO2 nanoparticles dispersed uniformly with a particle size around 0.1 µm microspheres while the ISQ/TiO2 nanocomposites dispersed with smaller particle size. Visual perception of particle size distribution provided insufficient evidence. Therefore, qualitative measurements were necessary to verify the existence of dye. According to the XRD analysis in Figure 1e, the average crystalline sizes of TiO2, the ISQ sensitized TiO2 nanoparticles and ISQ dye was figured out from the Debye-Scherrer formula to be approximately 13.3 nm, 23.2 nm and 43.9 nm, respectively. The average crystalline size of ISQ/TiO2 is between that of TiO2 and the ISQ, and as particle profile get rough and obvious agglomeration after ISQ dye sensitization, we can conclude that dye/TiO2 compositions were successful fabricated. For further study to demonstrate the successful fabrication of ISQ/TiO2 nanocompositions, FT-IR analysis was employed in Figure



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1f. A wide absorption band at 400-800 cm-1 corresponds to the stretching vibration of the Ti-O band, and the absorption bands at 1628 and 3410 cm-1 represent the vibration of adsorbed water.

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The main characteristic absorption bands at 1456-1624 cm-1 are attributed to the

C=C stretching in the four-membered ring and the aromatic heterocycle of ISQ dye. The absence of the C=O stretching vibration at 1700 cm-1 is an indication of the extensive bond delocalization in the four-membered ring of ISQ dye.

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The characteristic peaks of ISQ dye

and TiO2 can be found in the spectrum of ISQ sensitized TiO2 nanoparticles, indicating that the ISQ molecules have well adsorbed to the TiO2 surface. A conclusion can be inferred that the hydrothermal-sensitization method we carried out has successfully fabricated the dye/TiO2 nanocomposites. 3.2. TEM Images.

The TEM images of bare TiO2 (a, b) and the representative ISQ/TiO2

nanocomposites (c, d) are displayed in Figure 2. It can be observed that the bare TiO2 particles exhibit clear particle profile and slight agglomeration with an irregular shape (especially in Figure 2b) which own a size around 30 nm. After being sensitized by ISQ, the basic morphology of the composite was retained. However, particle profile roughened and obvious agglomeration can be seen in Figures 2c,d ascribed to the successful sensitization from the ISQ dye to the TiO2 surface and more tight connections were constructed, which is almost in agreement with the SEM result. 3.3. UV-vis Diffuse Reflectance Spectra.

The UV–vis diffuse reflectance spectra and

optical band gap calculated via TAUC plot equation of bare TiO2 and the dye/TiO2 nanocomposites are shown in Figure 3. Compared with the limited absorption band in the UV region of bare TiO2, it is obvious that the photo-response of the nanocomposites was


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remarkably extended to visible-light region after three SQs sensitization. What’s more, the absorption in the whole region, including the ultraviolet region, is significantly enhanced. After sensitized by different dyes, tremendous distinction is demonstrated among the visible region absorption intensity which ranked in accordance with the following order: c-BSQ/TiO2 > ISQ/TiO2 > m-ISQ/TiO2 > Bare TiO2. The band gap of different samples ranked with ISQ/TiO2 (2.31 eV) < m-ISQ/TiO2 (2.45 eV) < c-BSQ/TiO2 (2.86 eV) < Bare TiO2 (3.01 eV) orders were shown in Figure 3b. Their considerable capacity of visible-light harvesting means they own potential advantage in the field of photochemical reactions. The band gaps of different TiO2 samples are determined by the optical absorption (UV-vis) properties which are associated with the electronic structure features and the quantum size effect. The quantum confinement effect was present due to the blue-shifted absorption edge of the obtained samples,

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and systematic works have already been reported related to the blue

shift of the band gap energy by the quantum size effect. samples were calculated via TAUC plot equation

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Visual band gap of different TiO2

of the original Kubelka-Munk equation

139 and the modified equation 2. The value of the exponent n denotes the nature of the transition, for example, n = 1/2 for indirect transitions and n = 2 for direct transitions.

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Log(αhν) to log(hν) drawings are employed to obtain the exponent n which corresponds to the slope of the tangent, and the slope of the tangent of ISQ/TiO2 turns out to be 2, which corresponds to direct band gap transitions for squaraine sensitized TiO2. K/S = (1-R∞)2/(2R∞) = F(R∞)

(1)

[F(R∞)hν]n = A2 (hν-Eg)

(2)

Where F(R∞) is the so-called remission or Kubelka-Munk function, hν is the photon energy



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and A is a proportionality constant. Obtaining F(R∞) from Eq. (1) and plotting the [F(R∞)hν]2 against hν, a piece of data with good linearity is found for linear fitting, and the band gap Eg of a powder sample can be extracted easily. The intersection of the extended tangent with the x-axis corresponds to the band gap of different samples, respectively. Their absorption spectra calculated by equation 3 were incorporated in Table 1. Eg = 1240 / λ

(3)

As is shown in Table 1, after sensitized with different squaraine dyes, the absorption spectra of bare TiO2 around UV-light region was extended to visible-light range which contains c-BSQ/TiO2 (433.57 nm), m-ISQ/TiO2 (506.12 nm) and ISQ/TiO2 (536.8 nm). From the molecular structure of the c-BSQ, the edge group such as methyl and chlorine atoms of c-BSQ dye determine its covering power on the TiO2 surface exceed over the rest ISQ and m-ISQ dye. And nonmetal element sulphur and nitrogen atoms therein also could provide more active connection points with the TiO2. Compared with the structures of ISQ and m-ISQ, we can conclude confirmedly that the hydrogen bond formed between the hydrogen atom of ISQ and the oxygen atom of TiO2. This stronger acting force could result in higher photocatalytic activity for ISQ/TiO2 composites by taking the expanded visible light region into consideration. 3.4. TG-DTG Analysis.

Thermo-gravimetric (TG) and differential thermo-gravimetric

(DTG) analysis are shown in Figure 4, which represents a dynamic weight loss profile of a dye/TiO2 from room temperature to 850 °C during the thermal oxidation. Owing to the obtained TiO2 particles having been calcined in the muffle at 500 °C for 4 h, the peaks in the differential thermo-gravimetric curve (DTG) exhibited the decomposition of the dye. The


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decomposition temperature of ISQ approached 300 °C, indicating good thermal stability of the SQs due to its rigid four-member ring structure. It can be seen that the existence of one more methyl in the m-ISQ dye may lead its decomposition temperature up to 328 °C (Figure 4b). To our surprise the decomposition temperature of the c-BSQ dye (Figure 4c) is up to 358 °C, which could be interpreted as the influence of the existence of the chlorine atom in the c-BSQ dye. 3.5. Raman Spectrum.

The Raman spectra of the three dye/TiO2 composites are shown in

Figure 5. It is obvious that the characteristic peaks of the anatase TiO2 are retained, and no apparent dye peak can be seen. According to factor group analysis, anatase has six Raman active modes (A1g+2B1g+3Eg). The Raman spectrum of the six allowed modes of an anatase single crystal appear at 144 cm–1 (Eg), 198 cm–1 (Eg), 398 cm–1 (B1g), 515 cm–1 (A1g), 519 cm–1 (B1g) (superimposed with the 515 cm–1 band) and 640 cm–1 (Eg) were concluded by Ohsaka. 43 Based on the Raman spectrum, the intensity of the dye/TiO2 is arranged according to the following orders: m-ISQ/TiO2 > ISQ/TiO2 > c-BSQ/ TiO2. Compared with the m-ISQ/TiO2 nanocomposites, the band of the c-BSQ/TiO2 and ISQ/TiO2 is blueshifted for the sake of the different particle size of the nanocomposites and parts of the peak width are broadening which was caused by non-homogeneity of oxygen vacancy. The peaks appear at 398 cm–1, 515 cm–1 and 640 cm–1 of ISQ/TiO2 and c-BSQ/TiO2 almost invisible, mainly due to the existence of oxygen vacancy. Based on the Raman spectrum, m-ISQ/TiO2 still leaves apparent characteristic peaks of TiO2 behind and retains strong peak intensity which could result in the sensitization between m-ISQ and TiO2 being not as good as imagined. The peak intensity of c-BSQ/TiO2 is lower than ISQ/TiO2, so we can predict that ISQ/TiO2 nanocomposites own an



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excellent sensitization effect, which could induce optimal photoelectric and photocatalytic properties. 3.6. FT-IR Analysis.

Figure 6 shows the FT-IR spectra of ISQ sensitized TiO2 (a), m-ISQ

sensitized TiO2 (b), c-BSQ sensitized TiO2 (c) and TiO2 (d). Wide absorption band around 600-800 cm-1, which corresponds to the stretching vibration of Ti-O band, can be seen after sensitization with three different dyes. The nanocomposites also retain the characteristic peak of dyes in the range of 1000-1800 cm-1, only with weakened absorption intensity. Compared with the FT-IR spectra of m-ISQ and c-BSQ, the absorption band of ISQ/TiO2 at 1172 cm-1 is red-shifted slightly and the peak width is broadening, suggesting a change of the connection type. Based on the molecular structure of ISQ dye charge delocalization is built among the whole system, especially between nitrogen from indole ring, oxygen and C=C. The strength of the N-H bond is weakened, which induces the oxygen from TiO2 structure access to the opportunity to contest with the nitrogen, then a weak chemical bond (hydrogen bond) is gradually formed in the process of competition which is stronger than simple physical absorption. 3.7. Photocatalytic Activity of MB Degradation in Presence of Different Nanoparticles. Figure 7 shows the temporal changes in absorption spectra of MB for different catalysts, and absence of catalyst at 30 min visible light irradiation intervals. From these degradation curves, it can be found that ISQ/TiO2 demonstrated optimal efficiency. Based on the decrease of the absorption peak of MB at 665 nm and the absorption maximum wavelength shift from 665 to 607 nm during the irradiation in the present of ISQ/TiO2, obvious degradation efficiency can be observed. Hypsochromic effects (i.e., blue shifts of spectral bands) resulting from


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N-demethylation of the dimethylamino group in MB occurs concomitantly with oxidative degradation.

33,44

Such blue-shifted absorption is characteristic of N-demethylated derivatives

of MB in aqueous semiconductor TiO2 dispersions under light irradiation. Aside from this, a parallel decrease in intensities and slight blue shift of the bands located at 292 nm also could be observed; these are caused by the N-demethylated degradation concomitantly with the degradation of the phenothiazine (see inset). 45 No other spectral features are evident in the absorption spectra after 150 min of visible-light irradiation, indicating that the depletion of MB concentration in the solution is due to a photochemical composition rather than the sorption of the MB on the sensitized TiO2. The photocatalytic activities of the three SQ/TiO2 nanocomposites under simulative visible light irradiation are shown in Figure 8. Figure 8a illuminates the degradation rate of the MB (25 mg/l, 50 ml) in the presence of 10 mg of the nanocomposites under visible-light irradiation using a 500 W iodine tungsten lamp (Philips, China) with a 400 nm cutoff filter to eliminate the UV light. We can see clearly that the ISQ/TiO2 exhibited a much higher degradation rate than the other photocatalysts, while the photo-degradation rate of m-ISQ/TiO2 was approximately 20% lower than that of ISQ/TiO2 which owned approximately 99% degradation rate after 150 min visible-light irradiation. Moreover, the 18% self-degradation rate of MB indicates that photocatalysts play a significant role in the process of decomposition. Due to the existence of tremendous steric hindrance and electrophilic chloride atom on the surface of TiO2, the degradation rate of c-BSQ/TiO2 was 20% lower than the bare TiO2. The degradation rate of MB in the presence of different photocatalysts was illustrated in Table 2. The kinetic data for the degradation of MB solution in the presence of



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dye/TiO2 nanocomposites under visible light irradiation is demonstrated in Figure 8b. The tendency of the curve was coincident with the apparent first-order model ln (C0/Ct) = kappt. In addition, the determined apparent rate constant (kapp), linear regression coefficient (R2) and degradation efficiency of MB after irradiating at a certain period of time are summarized in Table 2, referring to the molecule structure. It was found that the degradation reaction of MB for all nanocomposites followed the Langmiur-Hinshewood (L-H) first-order reaction model. The kapp value of the ISQ/TiO2 photocatalyst is 0.03594 min-1, which far exceeds that of m-ISQ/TiO2 (0.00902 min-1),

bare TiO2 (0.00492 min-1), c-BSQ/TiO2 (0.00301 min-1), and

MB self-degradation (0.0011 min-1), respectively. Therefore, we can conclude that the ISQ/TiO2 composites exhibit excellent photocatalytic activity compared to others due to the formation of the hydrogen bond between the hydrogen atom of ISQ and the oxygen atom of TiO2, which own stronger acting forces than the simple physical adsorption. 3.8. Photostability of the Photocatalysts and Degradation of MB.

The photocatalytic

stability of ISQ/TiO2 (set as an example to represent other catalysts) nanocomposites was performed with the concentration of MB (25 mg/L), catalyst dosage (10 mg) and irradiation time (2.5 h) for each cycling run. The regeneration of the photocatalyst was done by filtering the suspension to remove the bulk solution, and drying at 60 oC. The recovered ISQ/TiO2 nanocomposites were reused in the next cycle. The result displayed in Figure 9 shows that after 5 successive cycles under the visible-light irradiation, the degradation rate of MB is still 68%, indicating that ISQ/TiO2 nanocomposites possessed excellent photocatalytic stability, and provides strong evidence to confirm that the system itself could be stable for cycling photoactivity tests. Combined with the TG-DTG results, ISQ dye can be elected as


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representative in the family of metal-free organic dyes in the comparison of the photostability and thermostability of different SQ/TiO2. In order to verify the degradation of MB deeply, TOC test (a), FT-IR spectra (b), and Chromatogram of LC analysis result (c) with corresponding UV-vis absorption curves (d) of MB solution irradiated at different reaction times were employed in Figure 10. MB can fade in color due to the breakage of only one double bond, so the TOC (Total Organic Carbon) test becomes the best method to stand for the degradation activity. This analysis was carried out using a total organic carbon (TOC) analyzer (Shimadzu TOC-VCHV) equipped with a platinum catalyst. The TOC test result is given below in Figure 10a. The TOC value of pure MB solution turns out to be 8.5281 mg/L, however, the TOC value of MB solution improved with the increase of illumination time such as 9.9048 mg/L (20 min illuminated via visible-light marked as Sample 1) and 19.351 mg/L (90 min illuminated via visible-light marked as Sample 2). MB was proposed to degrade and mineralize to some complicated organic molecule

46

with larger molecule weight. Finally, with the end of the decomposition

process, MB molecular was degraded thoroughly with a fairly small TOC value of 0.7076 mg/L (marked as Sample 3). The specific data of TOC values of MB solution irradiated at different reaction times and their corresponding integration area are incorporated in Table 3. The pathway for the MB photocatalytic degradation was elucidated from the evolution of IR intensities of MB. FT-IR spectra of MB decomposed on ISQ/TiO2 nanocomposites, naked ISQ dye and pure MB is exhibited in Figure 10b. Due to the sensitization between ISQ and TiO2, the characteristic absorption peaks of ISQ among the wavenumber of 1600-800 cm-1 become weakened, which does not affect judgment of main functional groups of ISQ dye.



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Prior to the photocatalytic degradation, pure MB exhibited prominent bands for the C=N central ring stretching at 1591 cm-1, the CAr–N (i.e., the bond between the side aromatic ring and the nitrogen atom) stretching at 1353 cm-1, the N-CH3 (i.e., the bond between CH3 and the nitrogen atom) stretching at 1245 cm-1. Exposure of MB/ISQ/TiO2 to visible light illumination caused the intensity of the MB bands to decay and the broad shoulder bands between 1200 and 400 cm-1 to rise gradually, as shown in Figure 10b. Only 1587 cm-1 restrained the C=O stretching of the rigid four-member ring structure of ISQ dye. Therefore, all these results demonstrated that the MB molecule might be degraded after 150 min of irradiation of visible light due to some bands and modes of MB were disappeared. Figure 10c and 10d present the chromatograms of initial MB solution (1) and the treated solutions (2 and 3) irradiated at different reaction times with corresponding UV-vis absorption curves of MB solution. LC analysis was also employed using a Wufeng LC with methanol: H2O (50:50) as a mobile flow. Initially, MB showed a single peak at 1.667 min. After 60 min, the area of the peak vanished and two new peaks were identified at 0.899 and 2.68 min, respectively. These new peaks indicated the intermediate compounds produced from the degradation of MB. At 150 min, only a single weak negative spike occurred at 0.833 min which may correspond to the solvent peak. These results are in accordance with the MB 47

Based on previous work, the possible product in

33,46,48

The degraded products of MB turned out to be

degradation reported by Rajeshwar et al. methylene blue oxidation is phenol.

complicated, MB is decomposed and mineralized finally as illumination time goes by. In a word, the results above suggest that molecular MB is degraded and mineralized deeply, the as-prepared photocatalysts own excellent photostability after several photocatalytic cycle


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experiments. 3.9. Hydrogen Bond Mechanism.

The mechanism of the hydrogen-bond interaction

between ISQ and TiO2 was proposed while the possible formation mechanism process was illustrated in Figure 11. After sensitized by ISQ dye, the hydrogen bond was formed between the framework of TiO2 and indolenine squaraine dye molecule. The valid connection sites, nitrogen hydrogen bond of the ISQ dye and the oxygen atom of the TiO2 framework, provide the primary element to form the hydrogen bond which owns a stronger acting force than the simple physical adsorption among the other two SQ/TiO2 nanocomposites. Appropriate utilization of space effect provides more comfortable spatial distance for both TiO2 and indolenine squaraine dye. According to the study of Miguel et al. 49, the type and length of the functional group of dye that are attached to the TiO2 surface take a toll on the catalytic efficiency which can be observed from the result of the photodegradation rate between ISQ and m-ISQ. ISQ/TiO2 composites exhibit excellent photocatalytic activity could ascribed to the existence of the hydrogen-bond interaction, the flat structure of organic inner salt provide favorable edges of the space. 3.10. Fluorescent Spectrometry.

Explicit quantitative measurement is used to explain the

high efficiency of the charge transfer between dye and TiO2. The calculation of fluorescence quantum yield of different dyes could interpret their charge transfer efficiency well. We choose RhB as reference compound due to its larger molar extinction coefficient and higher fluorescence quantum yield with excellent stability, and prepared them in a 1× 10-6 M concentration which is in accordance with the concentration of the dye solutions. Figure 12 exhibit the UV-vis spectrum and fluorescence emission spectrum of fiducial substance RhB



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Page 20 of 49

and three squaraine including ISQ, m-ISQ, c-BSQ. The fluorescence quantum yield of RhB in ethanol solution is reported as 0.89.

49-52

The

calculation of the samples which need to be measured is according to the following formula: Yu = Ys · (Fu / Fs) · (As / Au) Yu and Ys represent the fluorescence quantum yield, Fu and Fs represent integral fluorescence intensity, As and Au represent the absorbance under the excitation wavelength of the samples and reference compound, respectively. The fluorescence quantum yield of different samples at their excitation wavelengths are incorporated in Table 4. Based on the fluorescence quantum yield of different samples, ISQ dye possess the Y value of 0.124 which is far more than c-BSQ (0.036) and m-ISQ (0.02), ISQ dye own the optimal charge transfer capacity. 3.11. Electron Distribution of the HOMO and LUMO States of the Squaraine Dyes.

In

order to take full advantage of the tremendous solar energy, achieve efficient light-harvesting and provide valid carrier transfer, the excited-state redox potential of the dye should match the energy of the conduction band (CB) edge of TiO2. To gain insight into the optimized electron cloud density distributions of the excited states of the squaraine dye and to understand the origin of electronic and optical properties thoroughly, theoretical calculation was conducted under the Gaussian 03 software suite with the B3LYP/6-31G level of theory. The electron distributions of dyes are shown in Figure 13. Molecular orbital calculations on the three sensitizers indicated that the HOMO spread over the π-framework among indole and benzothiazole moiety from the squaraine core. The HOMO of ISQ exhibits greater delocalization localized at the indole moiety with some contributions from the squaraine core


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while m-ISQ and c-BSQ have no significant delocalization, which due to the existence of the methyl from indole and chlorine from benzothiazole, respectively. Considering the LUMO energy level of ISQ (for its whole intramolecular charge delocalization), the squaraine core of the m-ISQ and c-BSQ share the maximization of the π* orbital charge delocalization with the methylene moiety. The excited electron transferred from the HOMO to LUMO orbital after light illumination, the electron density of π-π* framework are shifted from squaraine core to the indole and benzothiazole on both sides which induced efficient charge transfer from dye to TiO2 surface. The overall molecular conjugated degrees change greatly once combined with different functional groups, and the existence of the electron-donating group methyl from m-ISQ and the electron with-drawing group chlorine from c-BSQ, the electron delocalization within the whole molecular remarkably differ from the ISQ which make it difficult for excited electrons to delocalize to different functional groups and further to the semiconductor TiO2 surface. The orbital energy calculation values of the three dyes also have been demonstrated in Figure 13. The calculated value of the optical band gap via UV-DRS should less than or equal to energy band gap calculated via Gaussian Simulation method. 53,54 This could be because the photonic can be absorbed when its energy is slightly less than the band gap, which may related to the localized central electronic, phonon and local energy level between CB and VB. The energy band gap of the three squarine dyes exhibit the order of c-BSQ (2.748eV) < m-ISQ (2.823eV) < ISQ (2.956eV) with increasing charge delocalization capacity and conjugated degrees of molecules which is connected with the difference between ground and excited state energies, instead of connecting with the HOMO−LUMO energy gap.


55

The


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results of the theoretical calculation demonstrated that the excited electrons of ISQ molecular π frameworks can easily delocalized owing to their planar conjugated structures and more easily form a charge transfer loop line than m-ISQ and c-BSQ according to the HOMO and LUMO orbital energy values, which is likely to give rise to higher electron transfer rate, lower electron recombination, and an improved overall photoelectric efficiency. As a flat structured organic inner salt,

56

its highest mobility urges ISQ dye to be widely employed in field-effect

material, optoelectronic material and devices.

4. CONCLUSIONS

Complete nanocomposites fabricated with TiO2 particles, sensitized with a family of squaraine dyes, SQs via facile hydrothermal method have been studied by using a variety of characterization methods. Great thermal stability and photostability of dyes can be achieved from the TG-DTG analysis and recycling experiment. We have shown that the structure of different squaraine dyes have a notable influence on electron distribution and charge transfer capacity. Thus, after squaraine dyes sensitization, the photoresponse range of TiO2 extended to visible-light region is of great importance for the enhanced photocatalytic activity which holds tremendous light harvesting capacity. Moreover, their optical band gap calculated via TAUC plot equation deduced accordingly and specific wavelength of visible-light (absorption spectra of ISQ/TiO2 turns out to be 536.8 nm) was calculated by using E=1240/λ. Molecular structure of the squaraine dyes and the electron distribution of their HOMO and LUMO states were given by geometrical optimization with Gaussian98 at B3LYP/6-311+G(d,p) level. The


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difference of the band gap between the calculation via TAUC plot equation and theoretical calculated energy band gap reveals that the optical band gap via UV-DRS demonstrates more accuracy than theoretical simulated calculation. There is an enhanced photocatalytic degradation rate in the presence of SQs/TiO2, among which ISQ/TiO2 has its own optimal mineralization effect. TOC test and LC analysis are put forward to demonstrate in depth how molecular MB was degraded and mineralized. In the connection type between dyes and TiO2, we suggested that ISQ/TiO2 bonding as hydrogen bond exhibited a stronger binding force and optimal charge transfer capacity than m-ISQ/TiO2 and c-BSQ/TiO2 with simple physical absorption. Given the molecular structure and the electron distribution of the HOMO and LUMO states on the squaraine dyes, ISQ/TiO2 possesses the strongest charge delocalization capacity which could make an influence on its optical and photocatalytic properties. ISQ dye has great potential to become a promising candidate to be widely employed in the field of field-effect material, optoelectronic material and devices.

Acknowledgments This work was financially supported by Open Research Fund Program of the Key Laboratory of Regional Environment and Ecoremediation of Ministry of Education, P. R. China (SYU-KF-E-12), Open Research Fund Program of Key Laboratory of Environmental pollution and regional ecological security, Shenyang University (SYU-KF-L-12), Program for Advantage Discipline of Changzhou University, Research Foundation for Talented Scholars of Changzhou University (ZMF11020007), Project for Six Major Talent Peaks of Jiangsu



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Province (2011-XCL-004), and Research and Innovation Project for College Graduates of Jiangsu Province (CXLX-0711).


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Table 1. Optical Band Gap and Adsorption Spectra of Different TiO2 Samples Bare TiO2

m-ISQ/TiO2

ISQ/TiO2

c-BSQ/TiO2

Band gap (eV)

3.01

2.86

2.45

2.31

absorption spectra (λ/nm)

411.96

433.57

506.12

536.8

Table 2. Apparent Rate Constant of MB Photodegradation and Linear Regression Coefficients of Three SQ/TiO2 Nanocomposites Abbreviation

kapp/(min)-1

R2

Degradation of MB after Irradiating 3 h (%)

ISQ/TiO2

0.03594

0.979

99.51

m-ISQ/TiO2

0.00902

0.992

78.84

Bare TiO2

0.00492

0.949

60.01

c-BSQ/TiO2

0.00301

0.976

39.63

MB Self-Degradation

0.0011

0.991

18.15

Table 3 TOC Values of MB Solution Irradiated at Different Reaction Times and Their Corresponding Integration Area Pure MB aq

Sample 1

Sample 2

Sample 3

TOC concentration / (mg/L)

8.5281

9.9048

19.351

0.7076

Integration area

10869.5

12639

24693

903


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Table 4 Fluorescence Quantum Yield of Different Samples at Their Excitation Wavelength RhB

c-BSQ

m-ISQ

ISQ

A/λmax

0.1012/544

0.3229/665

0.3327/627

0.1437/648

fluorescence intensity/ excitation wavelength(nm)

667/565

215/670.5

106/636.6

252/662.8

A/ excitation wavelength(nm)

0.0279/565

0.2239/670.5

0.194/636.6

0.0756/662.8

Yu

0.89

0.036

0.02

0.124



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

Figure captions Figure 1. SEM images of bare TiO2 (a, b), ISQ sensitized TiO2 nanoparticles (c, d), their XRD patterns (e) and FT-IR (f). Figure 2. TEM images of bare TiO2 (a, b) and ISQ sensitized TiO2 nanoparticles (c, d). Figure 3. UV-vis diffuse reflectance spectra and calculated optical band gap of bare TiO2 and three different SQs sensitized TiO2 composites. Figure 4. Thermo-gravimetric (TG) and differential thermo-gravimetric (DTG) analysis of ISQ/TiO2 (a), m-ISQ/TiO2 (b) and c-BSQ/TiO2 (c) nanocomposites. Figure 5. Raman spectrum of bare TiO2 and three different SQs sensitized TiO2 nanocomposites. Figure 6. FT-IR spectra of ISQ/TiO2 (a), m-ISQ/TiO2 (b), c-BSQ/TiO2 (c) and bare TiO2 (d). Figure 7. Temporal changes in absorption spectra of MB for different catalysts and absence of catalyst (a-e) at 30 min visible light irradiation intervals, and the inset in e shows the wavelength shifts of the MB spectra at 665 and 293 nm with reaction time. Figure 8. Photocatalytic degradation (a) and kinetic data for the degradation (b) of MB solution in the presence of ISQ/TiO2, m-ISQ/TiO2, c-BSQ/TiO2, bare TiO2 nanoparticles and MB self-degradation under ultraviolet light irradiation. Figure 9. Photocatalytic degradation rate of MB for ISQ/TiO2 nanocomposites at different cycling time. Figure 10. TOC test (a), FT-IR spectra (b), and Chromatogram of LC analysis result (c) with corresponding UV-vis absorption curves (d) of MB solution irradiated at different reaction times.


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Figure 11. Hydrogen bond mechanism between ISQ and TiO2. Figure 12. UV-vis spectrum and fluorescence emission spectrum of fiducial substance RhB and three squaraine dyes including ISQ, m-ISQ, c-BSQ. Figure 13. Electronic cloud density distribution of three SQs and their HOMO/LUMO energy level.



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

Figure 1


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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


Figure 2



1.6

(a) [c-BSQ/TiO2]

Abs.

1.2

0.8

[ISQ/TiO2]

0.4 [m-ISQ/TiO2] [Bare

0.0 300

60

400

500 600 Wavelength (nm)

TiO2]

700

800

(b)

50

2

40 [F(R8 )hν ]

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

Page 38 of 49

30 20

c-BSQ/TiO2

2.86eV

10

2.31eV

ISQ/TiO2

2.45eV

m-ISQ/TiO2 3.01eV

0 1.5

2.0

2.5

3.0 3.5 hν (eV)

Bare TiO2

4.0

Figure 3


4.5

5.0

Page 39 of 49

1.00 80

ISQ/TiO2 0.95 DTG (%/min)

(a) 0.90

40

0.85

20

0.80

0 200 287

400 o Temp. ( C)

600

800 500

1.00

m-ISQ/TiO2

400

0.95

TG%

(b)

300

0.90 200

DTG (%/min)

TG%

60

0.85 100 0.80 0 0

200

328 400 o Temp. ( C)

600

800

25

1.00

c-BSQ/TiO2

20

0.96

DTG (%/min)

15

(c) TG%

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


10

0.92 5 0

0.88

-5 0.84

-10 200

358 400 o Temp. ( C)

600

Figure 4


800


144

Bare TiO2 m-ISQ / TiO2

Raman Intensity (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

Page 40 of 49

ISQ / TiO2 c-BSQ / TiO2

640 198

200

398

400

515

600 800 -1 Raman Shift (cm )

Figure 5


1000

1200

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(a) ISQ / TiO2 (b) m-ISQ / TiO2 Transmission

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


(c) c-BSQ / TiO2

(d) Bare TiO2

4000

3500

3000 2500 2000 1500 -1 Wavenumber (cm )

Figure 6


1000

500


2.5

30 min interval MB (25mg/L)

2.5 MB self-degradation

2.0

Cl

Absorbance

Absorbance

S C H

C H

N CH3

N H3C

O

Cl

c-BSQ/TiO

1.5 1.0

2

1.5

1.0

0.5

0.5 0.0 200

300

400 500 600 Wavelength (nm)

700

800

0.0 200

300


2.5


2.5


O S

2.0

Bare TiO2

N CH3

2.0

C H O

Absorbance

m-ISQ/TiO

1.5 1.0

700

800


O C H

2.0

N H 3C

2

1.5

1.0

0.5

0.5 0.0 200

300


2.5

700

2.0

0.0 200

300


665nm

? shift (nm)

30

20

10

293nm

656nm

0

1.0

0

20

40

60 Time (min)

80

100

120

247nm

638nm

ISQ/TiO2 0.0 300

700

30 min interval MB (25 mg/L)

293nm 665nm

40

1.5

0.5

800

60

50

Absorbance

Absorbance

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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618nm 607nm


Figure 7


700

800

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1.0

(a)

0.8

Ct/C0

0.6

0.4 MB self-degradation c-BSQ / TiO2

0.2

Bare TiO2

m-ISQ / TiO2 ISQ / TiO2

0.0 -30

0

30

60 90 Time (min)

120

150

180

(b)

ISQ / TiO2

5

m-ISQ / TiO2 Bare TiO2

4

c-BSQ / TiO2 MB self-degradation

ln(C0/Ct)

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


3 2 1 0 0

30

60

90 120 Time (min)

Figure 8


150

180


1.0 ISQ/TiO2 nanocomposite

0.8 Degradation rate (%)

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

0.0 1

2

3 Cycle time (time)

Figure 9


4

5

Page 45 of 49

30

(a)

TOC Integration area

(c)

25000

1 2 3

0.899 2.68 1.667

20000

TOC (mg/L)

20

0.833

1

15000

Integration area

10000

10

5000

0

0

q MB a pure

le 1 samp

le 2 samp

0.0

le 3 samp

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Retention Time (min)

(b)

2.5

(d)

665

ISQ/TiO2

0 min

MB/ISQ/TiO2

1 2 3

Absorbance

1587

2.0 1.5 1.0

1353 1245

pure MB

1591

Transmission (%)

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


655

0.5 647

4000 3600 3200 2800 2400 2000 1600 1200 -1 Wavenumber (cm )

800

400

0.0 400

Figure 10


500

600 Wavelength (nm)

150 min

700

800


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

Figure 11


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Page 47 of 49

0.35

0.143 0.1412

(a)

RhB c-BSQ m-ISQ ISQ

0.30

Absorbance

0.25 0.20 0.1437

0.15 0.1012

0.10 0.05 0.00 450

500

550

600

650

700

750

Wavelength (nm)

700 Fluorescence Intensity (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


(b)

667

RhB c-BSQ m-ISQ ISQ

600 500 400 300

252 215

200 106

100 0 550

600

650 700 Wavelength (nm)

Figure 12


750

800

Figure 13

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Graphic for manuscript

Three novel squaraine dyes (ISQ/m-ISQ/c-BSQ) were applied to sensitize TiO2 in order to extend the absorption range from UV to visible light region. The possible connection type between ISQ dye and TiO2 was proposed as hydrogen bond. However, simple physical adsorption acting force was existed between m-ISQ (or c-BSQ) and TiO2 with littery physical accumulation to constructing the steric hindrance which may inhibit the exposure of valid connection sites on TiO2 surface. Based on the photocatalytic degradation rate of MB with the same reaction conditions, ISQ/TiO2 nanocomposites exhibited optimal degradation result after 150 min visible-light illumination compared with the system of m-ISQ/TiO2 or c-BSQ/TiO2. The high fluorescence quantum yield of ISQ illuminated the optimal charge transfer capacity the ISQ hold. In the process of MB degradation, TOC test, LC analysis and TG results showed that MB molecular was degraded and mineralized deeply and photocatalysts possess excellent photostability. The results of the theoretical calculation demonstrated that the excited electrons of ISQ molecular π frameworks can easily delocalized owing to their planar conjugated structures and more easier to form a charge transfer loop line than m-ISQ and c-BSQ according to the HOMO and LUMO orbital energy values, which is likely to give rise to higher electron transfer rate, lower electron recombination, and an improved overall photoelectric efficiency.


Effect of Dye Structure on Optical Properties and Photocatalytic

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