- Email: [email protected]
Surface induced charge transfer in CuxIn2-xS3 nanostructures and their enhanced photoelectronic and photocatalytic performance
Solar Energy Materials and Solar Cells 191 (2019) 100–107
Contents lists available at ScienceDirect
Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat
Surface induced charge transfer in CuxIn2-xS3 nanostructures and their enhanced photoelectronic and photocatalytic performance
T
P. Ilanchezhiyana, G. Mohan Kumara, , Fu Xiaoa, C. Sivab, Shavkat U. Yuldasheva, D.J. Leec, H.C. Jeonc, T.W. Kanga ⁎
a
Nano-Information Technology Academy (NITA), Dongguk University, Seoul, Republic of Korea Department of Physics and Nanotechnology, SRM University, Kattankulathur, India c Quantum-Functional Semiconductor Research Center, Dongguk University, Seoul, Republic of Korea b
ARTICLE INFO
ABSTRACT
Keywords: Nanostructures, Semiconductors Photodiodes Photocatalysis
Multi-functional semiconducting nanostructures are gaining popularity for application in photoelectronics, energy storage devices and also in industrial and environmental remediation functions. In this regard, CuxIn2-xS3 nanostructures were investigated in detail for their photoelectrical and photocatalytic performance. Their physico-chemical characteristics were at first studied using X-ray diffraction, Raman, UV–vis absorbance, X-ray photoelectron spectroscopy and high resolution electron microscopic tools. CuxIn2-xS3 based flip chip Schottky diodes were demonstrated to attest their improved conductivity and enhanced photoelectrical performance. The photo switching capabilities of a type II p-n CdTe/CuxIn2-xS3 heterojunction was also investigated. In both the device configurations, the current-voltage (I-V) characteristics revealed the forward current and rectification ratio to improve under lower threshold voltages. The time-dependent photoresponse characteristics affirmed the stability of diodes, augmenting the improved/effective separation of photo generated electron hole pairs under illumination. Additionally, the photocatalytic performances of CuxIn2-xS3 nanostructures were inferred under visible light conditions through effective remediation of methylene blue (MB) dye molecules. The obtained results infer the Cu interaction in tetragonal lattice of CuxIn2-xS3 to promote the surface induced charge transfer mechanism in respective nanostructures, thereby enhancing their photoelectronic and photocatalytic functionalities.
1. Introduction Chalcogenide materials have started to receive tremendous interest in energy and environmental functions for their intrinsic material characteristics. The rich diverse composition availability in this class of materials and their tunable crystal structure with unique morphology endows them their remarkable physico-chemical properties, which in turn identifies them as promising candidates for a wide range of application in electronics, optoelectronics, energy storage and environment remediation functions [1–5]. And over the last few years non-layered metal chalcogenides have also started to receive similar interests for the same reason on par with that of layered materials. So, with these motivation we have worked on towards partial replacement of copper (Cu) ions for indium (In) ions in indium sulphide (In2S3) nanostructures via a complete solution derived approach for photoelectronic and photocatalytic functions. In2S3 is one of the prominent n-type semiconductors in the chalcogenide family with a direct band gap 2–2.4 eV [6–8]. Generally, In2S3
⁎
is said to exhibit three different polymorphic structures such as α, β and γ. Among them the β-phase is said to be the most stable form at roomtemperature, exhibiting a tetragonal structure and finding a number of technological application in optoelectronic devices [8]. And replacing transition metal ions such as Cu in this class of materials could open up new portfolio of complementary physical properties that in turn could render tunable electronic states and realize efficient photodiodes with excellent stability and performance. So, the primary objective of our investigation was to develop a new class of CuxIn2-xS3 nanostructure with improved photosensitivity. This we believe to take place through the coupling of metal d-electrons and defect states in tetragonal matrix [9]. The current investigation was also extended to identify a simple/scaledup approach to produce effective photocatalysts for treatment of dye effluents in wastewater. From these aforementioned viewpoints, a traditional hydrothermal strategy was adopted for the present investigation to accomplish high degree of compositional control among the processed nanostructures [10].
Corresponding author. E-mail address: [email protected] (G. Mohan Kumar).
https://doi.org/10.1016/j.solmat.2018.10.024 Received 22 June 2018; Received in revised form 3 August 2018; Accepted 25 October 2018 0927-0248/ © 2018 Elsevier B.V. All rights reserved.
Solar Energy Materials and Solar Cells 191 (2019) 100–107
P. Ilanchezhiyan et al.
Also, for the first time we have performed the integration of p-type cadmium telluride (CdTe) with n-type CuxIn2-xS3 nanostructures to arrive at the desired photodiode configuration. According to the best of our knowledge there are no reports on p-n CdTe/CuxIn2-xS3 heterojunction diodes for photodiode applications. CdTe, is a pioneering II-VI semiconducting material with an electron affinity of 4.28 eV and direct band gap of 1.5 eV at room-temperature [11,12]. The unique electrical properties of CdTe makes it a promising material in the field of light emitting diodes (LEDs), field effect transistors (FETs), and lasers [13–15]. CdTe has been processed through numerous methodologies such as thermal evaporation, vapor-phase epitaxy, molecular beam epitaxy and electrochemical deposition till date. And for the present study, we have decided to go on with a facile and unique approach by involving sonochemical treatment [16,17]. The chosen p-n CdTe/ CuxIn2-xS3 heterostructure could be regarded as a viable architecture for optoelectronic applications, owing to their bandgap values and intrinsic physical characteristics of CdTe and CuxIn2-xS3. This could also favor a compelling transport of charge carriers across the desired electrodes. IV characteristics established across the p-n heterostructures demonstrated an improved photocurrent with significant reverse leakages. The photocatalytic activity of these nanomaterials was also investigated through performing dye degradation reactions using MB under visible light. Finally, the present study also highlights the potential of CuxIn2xS3 nanostructures as a cost-effective alternate for large-scale manufacturing of high performance multifunctional devices.
the light was irradiated through the transparent (FTO) side of the flip chip geometry.
2. Experiment
2.3.4. Characterization The structural and phase characteristics of In2S3 and CuxIn2-xS3 nanomaterials were examined using Cu Kα radiation (λ = 0.154 nm) in an X-ray RINT 2500 diffractometer. X-ray photoelectron spectroscopic measurements were performed in a PHI 660 XPS spectrometer (using Al Kα radiation (1486.6 eV) as the excitation source). The morphology and dimension of CuxIn2-xS3 nanostructures were examined using a Hitachi S4800 (SEM). Their structural characteristics were inferred using a Micro Raman spectrophotometer (excitation wavelength of 532 nm). The absorbance spectrum was recorded using a Cary UV/VIS spectrophotometer. Electrical characteristics were registered using a Keithley 617 semiconductor parameter analyzer.
2.3.2. CdTe/CuxIn2-xS3 diodes CdTe suspension prepared elsewhere (refer Supplementary information 15, 16) was involved in the fabrication of CdTe/In2S3 diodes. Initially, 0.2 ml of CdTe suspension was spin cast on the In2S3, Cu0.01In2–0.01S3 and Cu0.02In2–0.02S3 films grown on ITO substrates at 1000 rpm and subsequently heat treated on a hot plate at 90 °C for 10 min. Ten-twenty cycles of such procedures were carried out to attain a homogeneous CdTe thin film. The device functionalities of CdTe/ CuxIn2-xS3 diodes were then studied using a semiconductor parameter analyzer after establishing appropriate contacts on CdTe and CuxIn2-xS3 diodes using Ag paste. 2.3.3. Photocatalysis The photocatalytic performance of CuxIn2-xS3 materials were evaluated by measuring the degradation rate of MB dye molecules under visible light illumination. Initially, 10 ml of MB solutions (5 ppm) were taken in three different vials with 5 mg of processed CuxIn2-xS3 samples (where x = 0, 1 and 2 wt%). The above suspensions were irradiated with a compact fluorescent lamp (CFL, model: 20 W (TU), Havel's India Ltd, India). The absorbance of MB solutions were then registered at regular intervals by using a UV–visible absorption spectrophotometer (JASCO model-V650).
2.1. Materials All the chemicals used in the experiments were of analytical grade and used thereafter. Indium nitrate, copper acetate, thioacetamide, acetone and ethanol were procured from Sigma Aldrich. Cadmium telluride (99%) was procured from Alfa Aesar. The transparent conducting indium tin oxide (ITO, 15 Ω/cm2) and fluorine doped tin oxide (FTO, 10 Ω/cm2) coated glass substrates used in the device fabrication were pre-cleaned using acetone, ethanol and deionized (DI) water. 2.2. Synthesis of CuxIn2-xS3 nanostructures
3. Results and discussion
Initially, 0.3 g of indium nitrate and 0.22 g of thioacetamide was dissolved in 100 ml of deionized (DI) water and subjected to continuous stirring for 1 h. The mixture was then transferred to a Teflon lined stainless steel autoclave and subjected to hydrothermal treatment in a hot air oven at 120–160 °C for 3–6 h. A pre-cleaned ITO substrate was additionally placed inside the autoclave. The orange colored precipitates and films deposited on ITO were harvested after washing in ethanol and DI water. The precipitates and films were dried overnight at 60 °C in an oven and used for further studies. A similar procedure was followed for the preparation of CuxIn2-xS3 nanostructures. The x value of Cu i.e. copper acetate in the aforementioned reactions was held at 0, 1 and 2 wt%. The resulting reddish-orange precipitates were collected by washing the yield in ethanol and DI water. The processed nanostructures are here after referred as In2S3, Cu0.01In2–0.01S3 and Cu0.02In2–0.02S3, respectively. The processed thin films were used for diode studies while the powders were involved as photocatalysts.
The crystal structure and phase characteristics of In2S3 and CuxIn2nanostructures were initially studied through their respective powder X-ray diffraction (XRD) patterns shown in Fig. 1a. The diffraction patterns recorded over a range of 20 < 2θ < 100° were noted to be well indexed to the tetragonal β-In2S3 and signify the samples to be well crystallized and agree with the standard card (JCPDS 03-0650459) [18]. Interestingly, the (440) peak position at 48.05° was observed to be slightly shifted towards the lower angles for Cu0.02In2–0.02S3 nanostructures (Fig. 1b). This shift could be attributed to the greater extent of Cu ion replacement in the tetragonal host matrix, as their radius (0.057 nm) is considerably smaller than that of the In ions (0.080 nm) [19,20]. We would also like to add that the absence of indium hydroxide or Cu related secondary phases attests the phase purity in processed nanomaterials. Room-temperature Raman spectroscopic measurements were additionally carried out on the In2S3 and CuxIn2-xS3 nanostructures to investigate their crystal phase through corresponding vibration modes. Generally, these modes are influenced by a number of factors such as stress, defects and structural disorders in a local atomic arrangement. Fig. 1c shows the room-temperature Raman spectra of In2S3 and CuxIn2xS3 nanostructures. Here, the broad vibration modes centered at 300 cm-1 (irrespective of Cu composition) signify the existence of tetragonal β-phase of In2S3 in A1 symmetry [21,22]. The peak position and half width maximum of these intense vibration also signify the mild xS3
2.3. Device fabrication 2.3.1. Flip chip device To arrive at the Schottky device configuration via flip chip method the CuxIn2-xS3 films grown by aforementioned treatments were sandwiched on to a FTO substrate. Here, the thin films coated on ITO acted as the working electrode, after establishing appropriate contacts on FTO and ITO substrates using Ag paste. For photoresponse measurements, 101
Solar Energy Materials and Solar Cells 191 (2019) 100–107
P. Ilanchezhiyan et al.
Fig. 1. (a) XRD patterns illustrating the tetragonal phase of (i) In2S3, (ii) Cu0.01In2–0.01S3 and. (iii) Cu0.02In2–0.02S3 nanostructures. (b) The shift in (440) peak position towards the lower angles as a function of Cu composition in CuxIn2-xS3 nanostructures. (c) Room-temperature Raman spectrum of (i) In2S3, (ii) Cu0.01In2–0.01S3 and (iii) Cu0.02In2–0.02S3 nanostructures attesting their tetragonal β-phase with A1 symmetry. (d) Raman spectrum of sonochemically treated CdTe nanostructures.
effect of Cu ions on the degree of crystallinity in CuxIn2-xS3 nanostructures. The broad peaks at 300 cm-1 were further subjected to careful evaluation to identify the exact position of three characteristic vibrations at 245, 300 and 358 cm-1, respectively. These vibrations (including the shoulder edge at 358 cm-1) characterize the A1g Raman phonon modes in tetragonal β-In2S3 [23,24]. Additionally, the invariance in Raman peak position and band width also suggests the unconspicuous change of local chemical environment in CuxIn2-xS3 nanostructures. Raman spectrum of sonochemically treated CdTe nanostructures is shown in Fig. 1d. Here, three vibration peaks could be observed at 117, 132 and 260 cm-1, which all match well with that of the previous reports on CdTe [25,26]. The first peak along the lower wavenumber designates the acoustic A1 mode of Te, which usually originates from the vibration of elemental Te in CdTe nanostructures [27]. The second peak at 132 cm-1 could be a contribution owed together by Te and transverse optical (TO) phonon mode of CdTe [28]. Likewise, the third peak at 260 cm-1 could be attributed to the elemental Te related vibrations, attesting the existence of CdTe and elemental Te in the processed CdTe nanostructures (refer Supplementary information for further discussion) [16,29]. Next, the optical band gap (Eg) of In2S3 and CuxIn2-xS3 nanostructures were estimated using the aid of UV–Vis absorption spectra. The influence of Cu ions on the Eg of In2S3 and CuxIn2-xS3 nanostructures is illustrated in the Tauc's plot shown in Fig. 2. According to Tauc's explanation, the relation between Eg and absorption coefficient (α) of a direct bandgap semiconductor is expressed as follows,
( h )2 = A(h
Eg )
Fig. 2. Tauc's plot extracted from UV–vis absorption spectrum of (a) In2S3, (b) Cu0.01In2–0.01S3 and (c) Cu0.02In2–0.02S3 made suspensions. The optical band gap values were estimated to be around 2.08–2.35 eV. The inset shows the optical representation of In2S3, Cu0.01In2–0.01S3 and. Cu0.02In2–0.02S3 films grown on ITO substrates.
interaction in the tetragonal lattice. We believe these factors to promote the photon absorption rate, thereby directing the enhanced photoelectronic and photocatalytic activity in Cu0.02In2–0.02S3 nanostructures (discussed later). Eg of CdTe was assumed to be around 1.50 eV from the previous reports in all of our experiments [11,12]. Cu0.02In2–0.02S3 and CdTe nanostructures were additionally subjected to X-ray photoelectron spectroscopic (XPS) measurements to determine the chemical state of various ions and other information related to the nature of chemical bonding. Fig. 3a shows the 3d core level spectrum corresponding to that of In 3d doublet that splits into 3d5/2 and 3d3/2 at 444.08 and 451.55 eV, respectively. The observed peak positions are in consistent to the reported values for In3+ [32,33]. The core level XPS spectrum of S 2p is shown in Fig. 3b. The origin of resolved S 2p1/2 peak at 161.63 eV and shoulder edge at 162.77 eV characterizes the S2− oxidation state of S in Cu0.02In2–0.02S3 [34,35]. The chemical state of Cu and their composition in Cu0.02In2–0.02S3 samples was next investigated using the Cu 2p core level spectrum shown in Fig. 3c to be around 1.33%. This spectrum appears to be
(1)
where A is a constant and hν is the incident photon and Eg is the bandgap energy. And in the present study, Eg of In2S3, Cu0.01In2–0.01S3 and Cu0.02In2–0.02S3 nanostructures was estimated be around 2.35, 2.23 and 2.08 eV by extrapolating the linear portion of the plot at (αhυ)2 = 0. These values are in consistent to the earlier reports and suggests the domination of direct allowed transitions in CuxIn2-xS3 [30,31]. Furthermore, the lowering of Eg values as a function of Cu concentration provides a significant insight on the shift in absorption band edge of β-In2S3. This trend could be related to the surface modifications in CuxIn2-xS3 nanostructures, which actually takes place due to Cu 102
Solar Energy Materials and Solar Cells 191 (2019) 100–107
P. Ilanchezhiyan et al.
Fig. 3. Core-level XPS spectra corresponding to (a) In 3d, (b) S 2p and (c) Cu 2p in. Cu0.02In2–0.02S3 nanostructures. (d) Cd 3d and (e) Te 3d core-level XPS spectra of CdTe nanostructures signify the existence of secondary phases (refer Supplementary information).
characterized by a doublet feature that could be owed to the spin orbit splitting i.e. the Cu 2p3/2 and Cu 2p1/2 peaks at 931.7 and 951.8 eV, respectively. The peak positions and a spin orbit separation of 20.1 eV authenticates the existence of cationic Cu species in Cu0.02In2–0.02S3 systems processed via hydrothermal synthesis [36]. Additionally, this aspect also elucidates the successful distribution of Cu2+ ions on the surface of tetragonal Cu0.02In2–0.02S3. Fig. 3d shows the core level XPS spectrum of Cd 3d, illustrating the spin-orbit split lines of Cd 3d5/2 and Cd 3d3/2 to be centered on 405.17 and 411.95 eV, respectively. And this could be designated to the Cd-Te bonds that exist within CdTe systems. The core level XPS spectrum of Te 3d is shown in Fig. 3e. The occurrence of Te 3d5/2 and Te 3d3/2 transition peaks at binding energy 573.3 and 583.6 eV authenticates the Cd-Te bonding, while the remaining two peaks signature the co-existence of tellurium in oxide form [37,38]. The oxidation of Te could have been resulted due to the sonochemical treatment of CdTe containing Te-rich phases in the aqueous media (refer Supplementary information for further discussion) [16,17]. The morphological features of CuxIn2-xS3 nanostructures were next verified by the aid of microscopic images recorded using scanning electron microscopy (SEM) technique. The micrographs shown in Fig. 4 a-c affirm the random aggregates of CuxIn2-xS3 and their clustered particle-like characteristics. The average size distribution of
nanostructures were registered to be around 50–100 nm, irrespective of Cu composition in CuxIn2-xS3. Fig. 4d shows the low-magnification SEM image of the CdTe nanostructures. Here, CdTe appears to be almost agglomerated in form of clustered nanostructures. Their particle size distribution was further estimated to be around 100–200 nm through their particulate nature. The influence of Cu ions on the charge conductivity of CuxIn2-xS3 nanostructured thin films was ascertained using their I-V curves registered in Fig. 5a. Here, all the devices appear to be of good conductive nature, while the charge selectivity across Cu0.01In2–0.01S3 and Cu0.02In2–0.02S3 thin films appear to be improved on par with that of In2S3 thin films. And this improvement could be regarded as a substantial evidence suggesting the drastic decrease in resistance values (increased charge carriers) upon Cu incorporation in In2S3 lattice. The electrical and optoelectronic properties of CuxIn2-xS3 films deposited on ITO substrates was then determined through the flip chip device shown in Fig. 5b. This junction was actually established between the CuxIn2xS3 films grown on ITO and bare FTO counter electrodes via flip chip method i.e. top press contact. Here, FTO serves as the front contact and CuxIn2-xS3 film deposited on ITO glass acts as the back contact in device geometry. I-V curves of Cu0.02In2–0.02S3 device under dark and illumination condition is alone shown in Fig. 5c due to their improved 103
Solar Energy Materials and Solar Cells 191 (2019) 100–107
P. Ilanchezhiyan et al.
Fig. 4. (a-c) SEM images revealing the morphological evolution of CuxIn2-xS3 nanostructures (value of x being 0, 1 and 2 from a-c). (b) High-magnification SEM representation of sonochemically processed CdTe nanostructures.
conductivity. Here, the curves shows a nonlinear trend that could be visualized through the variation in their symmetric nature, affirming the realization of Schottky like junction at the FTO and Cu0.02In2–0.02S3 interfaces. In contrast to the dark condition, the current increases significantly under light illumination in Fig. 5c, indicating the
contribution from photogenerated carriers. Fig. 5d substantiates the band alignment realized across the Cu0.02In2–0.02S3 based flip chip devices under illuminated conditions. In these devices, the band offset between FTO and Cu0.02In2–0.02S3 and that between ITO and Cu0.02In2–0.02S3 actually directs the generation of asymmetric Schottky
Fig. 5. (a) Linear scale I-V characteristics registered across CuxIn2-xS3 thin films suggesting the improved conductivity as a function of Cu composition (value of x = 0, 1 and 2 from i-iii). (b) Shows the schematic of Flip chip device constructed using CuxIn2-xS3 films obtained via hydrothermal treatment. (c) Semi-log scale I-V characteristics of Cu0.02In2–0.02S3 made flip chip device under dark and illuminated conditions. (d) Energy level diagram suggesting the band offset between FTO and Cu0.02In2–0.02S3 and that between ITO and Cu0.02In2–0.02S3 in the flip chip device. 104
Solar Energy Materials and Solar Cells 191 (2019) 100–107
P. Ilanchezhiyan et al.
Fig. 6. (a) Reveals the typical architecture of p-CdTe/n-CuxIn2-xS3 diodes fabricated through sol-gel spin coating of CdTe nanostructures on CuxIn2-xS3 films. (b) Linear scale I-V characteristics corresponding to that of p-CdTe/n-Cu0.02In2–0.02S3 diodes. (c) Time-dependent photocurrent response of p-CdTe/n-Cu0.02In2–0.02S3 heterojunction for 20 s interval. The transient photocurrent response was registered under 900 W/m2. (d) The band alignment across the CdTe-CuxIn2-xS3 heterojunction approves to promote the effective separation of electron-hole pairs along the respective electrodes.
barriers across the corresponding interfaces due to electron transfer. The improved current values illustrated in Fig. 5c further manifests the potential of Cu0.02In2–0.02S3 nanostructures for efficient photodiode applications. Additionally, we have also investigated the photo switching functionalities of CdTe/CuxIn2-xS3 made p-n diodes, whose typical architecture is shown in Fig. 6a. I-V characteristics of this p-CdTe/nCu0.02In2–0.02S3 diode under dark and illuminated condition is shown in Fig. 6b. Out of the three devices studied, Cu0.02In2–0.02S3 made diodes tend to exhibit an improved photoresponse and signify the increase in Cu composition to improve their device performance considerably. This is the reason why p-CdTe/n-Cu0.02In2–0.02S3 devices are alone discussed in Fig. 6b. Here, the photocurrent increases gradually when the intensity of light increases from 900 to 1100 W/m2. Additionally, the forward and reverse-biased currents also increased in all the three devices under light illumination (not shown), implying the photo excited electron-hole pair's to increase the concentration of majority carriers. In the next stage, time-dependent photoresponse stability of p-CdTe/nCu0.02In2–0.02S3 diode was studied by turning the light on and off periodically. The photo response curve registered over a time interval of 20 s is shown in Fig. 6c. Here, the energy of incident photon density was held at 900 W/m2, while the applied bias voltage was held as function of time at 3 V. The photocurrent of the diode appears to rise under illumination and drop down quickly on turning off without any fluctuation. Such features actually characterize the reproducible response from the diodes and their stability in switching for a number of cycles. The diode also exhibits a maximum photocurrent of 3.4 × 10−8 A and a photo responsivity of about 0.1 AW−1, while subjected to a photon illumination of 900 W/m2. We also found from our investigation results that the device response to light is still significant without any considerable influence on the intensity of incident photon flux. The average
rise time and fall time of the device was estimated from Fig. 6c to be around 2 and 3 s, respectively. Fig. 6d substantiates the band alignment realized across the p-CdTe/ n-CuxIn2-xS3 based heterojunction under illuminated conditions. Here, the type-II energy level alignment could initially promote the efficient transfer of charge carriers across the heterostructure. The incident light directs the generation of electron-hole pairs along the depletion layer of the investigated p-n heterostructure. The width of depletion layer narrows under forward bias condition, thereby resulting with the easy drift of charge carriers along the respective electrodes. Similarly, the depletion width increases under reverse bias condition to restrict the flow of charge carriers across the barrier. The investigated I-V results also approve the p-CdTe/n-Cu0.02In2–0.02S3 heterostructure to promote the effective separation of electron-hole pairs. Next, the processed CuxIn2-xS3 nanostructures were subjected to visible light photocatalytic activity studies via degrading MB dye in aqueous media. The photocatalytic results shown in Fig. 7a augment the fact that the processed Cu0.02In2–0.02S3 nanostructures are much effective in degrading MB under visible light irradiation rather than In2S3 and Cu0.01In2–0.01S3 nanostructures. The enhanced activity in these systems could be resulting from the successful impregnation of Cu ions in the host lattice, which actually promotes the effective separation of electron-hole pairs required for degradation reactions. From the results it is also clear that pristine In2S3 has a considerable amount of photocatalytic activity, probably due to its band gap. The kinetics of degradation reactions were next investigated using Fig. 7b. Here, the linear relationship between ln(C/C0) and reaction time clearly indicates the photodegradation reactions to follow first order kinetics. The apparent rate constants were determined as 0.007, 0.013 and 0.017 min−1 for In2S3, Cu0.01In2–0.01S3 and Cu0.02In2–0.02S3, respectively. The order of photodegradation efficiency could be visualized through Fig. 8a to be as follows Cu0.02In2–0.02S3 > Cu0.01In2–0.01S3 > In2S3.
105
Solar Energy Materials and Solar Cells 191 (2019) 100–107
P. Ilanchezhiyan et al.
transfer from the respective conduction band to neighboring oxygen molecules. Such process are highly essential in degrading MB, as they ensure the prevention of recombination of electrons and holes. The availability of excess charge carriers then direct the redox reaction to generate additional reactive oxygen species required for degrading MB molecules. Furthermore, the effective injection of electrons from MB molecules to Cu0.02In2–0.02S3 nanostructures upon photoexcitation should have also resulted with the reduction of molecular oxygen and oxidative decomposition of electron-deficient MB more effectively. So such factors altogether could be reasoned for the enhanced photocatalytic performance observed in CuxIn2-xS3 [39]. Finally, we would like to summarize that the present study on CuxIn2-xS3 systems via solution based strategies to provide insight on the surface related modifications taking place in CuxIn2-xS3 tetragonal lattice due to Cu interaction and contribute towards their obvious photo-response and enhanced photoconducting behavior for cost-effective photoelectronic functions. And, the investigated results also suggest the lowered Eg values of CuxIn2-xS3 systems to assist in increasing the net photon absorption rate for effective photocatalytic detoxification under visible light radiation. 4. Conclusion CuxIn2-xS3 nanostructures were processed through a facile sol-gel route for energy and environmental remediation applications. Their physico-chemical characteristics were verified by using XRD, Raman, UV, XPS and electron microscopic tools. The improved conductivity in Cu0.02In2–0.02S3 nanostructures was verified via constructing Schottky diodes through traditional flip chip method. Along by, photoelectronic functionalities of CdTe/Cu0.02In2–0.02S3 made p-n heterojunctions were also extensively studied. In all cases, the I-V characteristics revealed the forward current and rectification ratio to improve under lower threshold voltages. The time-dependent photoresponse characteristics further affirmed the stability of diodes, augmenting the improved/effective separation of photo generated electron hole pairs along the interface. Finally, the photocatalytic studies performed under visible light conditions indicate the Cu0.02In2–0.02S3 nanostructures to function as a superior catalyst among the processed three systems.
Fig. 7. (a) Photocatalytic activities of CuxIn2-xS3 nanostructures with MB under visible light (value of x being 0, 1 and 2). (b) Natural logarithm of C/C0 vs. irradiation time used to evaluate the rate constants.
Fig. 8b represents the photocatalytic mechanism involved in the degradation of MB under visible light conditions in presence of CuxIn2xS3 samples. Here, electron-hole pairs are produced in all the samples (irrespective of Cu content) upon photoexcitation and it is these charge carriers that actually assists in degradation of MB molecules. However, in case of Cu0.01In2–0.01S3 and Cu0.02In2–0.02S3 systems the Cu ions existing within the photocatalyst at first directs the surface induced charge separation mechanism to take place via accelerating the electron
Fig. 8. (a) Photocatalytic efficiency of CuxIn2-xS3 samples vs. Time in degrading MB molecules (value of x being 0, 1 and 2). (b) Photocatalytic mechanism involved in the degradation of MB dye in presence of CuxIn2-xS3 samples under visible light conditions.
106
Solar Energy Materials and Solar Cells 191 (2019) 100–107
P. Ilanchezhiyan et al.
Acknowledgement [18]
This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded by the Ministry of Education (No. 2018R1D1A1B07051461), (No. 2018R1D1A1B07051474), (No. 2018R1D1A1B07051406), (No. 2018R1D1A1B07050237), (No. 2018R1D1A1B07051095), (No. 2016R1A6A1A03012877) and (No. 2016R1D1A1B03935948).
[19]
[20] [21]
Appendix A. Supporting information
[22]
Supplementary data associated with this article can be found in the online version at doi:10.1016/j.solmat.2018.10.024.
[23]
References
[24]
[1] C. Tan, H. Zhang, Wet-chemical synthesis and applications of non-layer structured two-dimensional nanomaterials, Nat. Commun. 6 (2015) 7873. [2] Y. Dou, L. Zhang, X. Xu, Z. Sun, T. Liao, S.X. Dou, Atomically thin non-layered nanomaterials for energy storage and conversion, Chem. Soc. Rev. 46 (2017) 7338–7373. [3] D.L. Duong, S.J. Yun, Y.H. Lee, van der Waals layered materials: opportunities and challenges, ACS Nano 11 (2017) 11803–11830. [4] S. Ithurria, B. Dubertret, Quasi 2D colloidal CdSe platelets with thicknesses controlled at the atomic level, J. Am. Chem. Soc. 130 (2008) 16504–16505. [5] G. Mohan Kumar, Fu Xiao, P. Ilanchezhiyan, Sh.U. Yuldashev, A. Madhan Kumar, H.D. Cho, D.J. Lee, T.W. Kang, High performance photodiodes based on chemically processed Cu doped SnS2 nanoflakes, Appl. Surf. Sci. 455 (2018) 446–454. [6] M. Afzaal, M.A. Malik, P. O’Brien, Indium sulfide nanorods from single-source precursor, Chem. Commun. 0 (2004) 334–335. [7] N. Chaudhari, L. Mandal, O. Game, S. Warule, D. Phase, S. Jadkar, S. Ogale, Dramatic enhancement in photoresponse of β-In2S3 through suppression of dark conductivity by synthetic control of defect-induced carrier compensation, ACS Appl. Mater. Interfaces 7 (2015) 17671–17681. [8] M. Mathew, R. Jayakrishnan, P.M. Ratheesh Kumar, C. Sudha Kartha, K.P. Vijayakumar, Anomalous behavior of silver doped indium sulfide thin films, J. Appl. Phys. 100 (2006) 033504. [9] S. Ghosh, M. Saha, V.D. Ashok, A. Chatterjee, S.K. De, Excitation dependent multicolor emission and photoconductivity of Mn, Cu doped In2S3 monodisperse quantum dots, Nanotechnology 27 (2016) 155708. [10] A. Datta, S.K. Panda, D. Ganguli, P. Mishra, S. Chaudhuri, In2S3 micropompons and their conversion to In2O3 nanobipyramids: simple synthesis approaches and characterization, Cryst. Growth Des. 7 (2007) 163–169. [11] R.W. Crisp, G.F. Pach, J.M. Kurley, R.M. France, M.O. Reese, S.U. Nanayakkara, B.A. MacLeod, D.V. Talapin, M.C. Beard, J.M. Luther, Tandem solar cells from solution-processed CdTe and PbS quantum dots using a ZnTe–ZnO tunnel junction, Nano Lett. 17 (2017) 1020–1027. [12] M.A. Islam, K.S. Rahman, K. Sobayel, T. Enam, A.M. Ali, M. Zaman, M. Akhtaruzzaman, N. Amin, Fabrication of high efficiency sputtered CdS: O/CdTe thin film solar cells from window/absorber layer growth optimization in magnetron sputtering, Sol. Energy Mater. Sol. Cells 172 (2017) 384–393. [13] Q. Lin, B. Song, H. Wang, F. Zhang, F. Chen, L. Wang, L. Song Li, F. Guo, H. Shen, High-efficiency deep-red quantum-dot light-emitting diodes with type-II CdSe/CdTe core/shell quantum dots as emissive layers, J. Mater. Chem. C 4 (2016) 7223–7229. [14] M. Shaygan, K. Davami, N. Kheirabi, C. Ki Baek, G. Cuniberti, M. Meyyappan, J.S. Lee, Single-crystalline CdTe nanowire field effect transistors as nanowire-based photodetector, Phys. Chem. Chem. Phys. 16 (2014) 22687–22693. [15] S.G. Kumar, K.S.R. Koteswara Rao, Physics and chemistry of CdTe/CdS thin film heterojunction photovoltaic devices: fundamental and critical aspects, Energy Environ. Sci. 7 (2014) 45–102. [16] P. Ilanchezhiyan, G. Mohan Kumar, Fu Xiao, S. Poongothai, A. Madhan Kumar, C. Siva, Sh.U. Yuldashev, D.J. Lee, Y.H. Kwon, T.W. Kang, Ultrasonic-assisted synthesis of ZnTe nanostructures and their structural, electrochemical and photoelectrical properties, Ultrason. Sonochem. 39 (2017) 414–419. [17] P. Ilanchezhiyan, G. Mohan Kumar, Fu Xiao, A. Madhankumar, C. Siva, Shavkat U. Yuldashev, H.D. Cho, T.W. Kang, Interfacial charge transfer in ZnTe/ZnO nano
[25] [26] [27] [28] [29]
[30] [31] [32] [33] [34] [35]
[36] [37] [38] [39]
107
arrayed heterostructures and their improved photoelectronic properties, Sol. Energy Mater. Sol. Cells 183 (2018) 73–81. H.X. Bai, L.X. Zhang, Y.C. Zhang, Simple synthesis of urchin-like In2S3 and In2O3 nanostructures, Mater. Lett. 63 (2009) 823–825. L. Chow, O. Lupan, G. Chai, H. Khallaf, L.K. Ono, B. Roldan Cuenya, I.M. Tiginyanu, V.V. Ursaki, V. Sontea, A. Schulte, Synthesis and characterization of Cu-doped ZnO one-dimensional structures for miniaturized sensor applications with faster response, Sens. Actuators A 189 (2013) 399–408. G.C. Xie, L. Fang, L.P. Peng, G.B. Liu, H.B. Ruan, F. Wu, C.Y. Kong, Effect of Indoping on the optical constants of ZnO thin films, Phys. Procedia 32 (2012) 651–657. Y. Ji, Y. Ou, Z. Yu, Y. Yan, D. Wang, C. Yan, L. Liu, Y. Zhang, Y. Zhao, Effect of film thickness on physical properties of RF sputtered In2S3 layers, Surf. Coat. Technol. 276 (2015) 587–594. Y. Xiong, Y. Xie, G. Du, X. Tian, Y. Qian, A novel in situ oxidization-sulfidation growth route via self-purification process to β-In2S3 dendrites, J. Solid State Chem. 166 (2002) 336–340. N.M. Gasanly, H. Ozkan, A. Aydinli, I. Yilmaz, Temperature dependence of the Raman-active phonon frequencies in indium sulfide, Solid State Commun. 110 (1999) 231–236. H. Tao, H. Zang, G. Dong, J. Zeng, X. Zhao, Raman and infrared spectroscopic study of the defect spinel In21.333S32, Optoelectron. Adv. Mater. Rapid Commun. 2 (2008) 356–359. J.M. Song, Y.Z. Lin, Y.J. Zhan, Y.C. Tian, G. Liu, S.H. Yu, Superlong high-quality tellurium nanotubes: synthesis, characterization, and optical property, Cryst. Growth Des. 8 (2008) 1902–1908. L. Zhang, C. Wang, D. Wen, Preparation by hydrothermal techniques in a tungstosilicate acid solution system and optical properties of tellurium nanotubes, Eur. J. Inorg. Chem. 22 (2009) 3291–3297. X. Wang, Y. Xu, R. Tong, X. Zhou, Q. Li, H. Wang, Compositional effects and optical properties of CdSeXTe1−X alloyed nanotube arrays, CrystEngComm 17 (2015) 960–966. P.M. Amirtharaj, F.H. Pollak, Raman scattering study of the properties and removal of excess Te on CdTe surfaces, Appl. Phys. Lett. 45 (1984) 789. M. Dharmadasa, O.K. Echendu, F. Fauzi1, N.A. Abdul-Manaf, O.I. Olusola, H.I. Salim, M.L. Madugu, A.A. Ojo, Improvement of composition of CdTe thin films during heat treatment in the presence of CdCl2, J. Mater. Sci: Mater. Electron. 28 (2017) 2343–2352. Y.J. Hsiao, C.H. Lu, L.W. Ji, T.H. Meen, Y.L. Chen, H.P. Chi, Characterization of photovoltaics with In2S3 nanoflakes/p-Si heterojunction, Nanoscale Res. Lett. 9 (2014) 32. Z. Zheng, J. Yu, S. Cheng, Y. Lai, Q. Zheng, D. Pan, Investigation of structural, optical and electrical properties of Cu doped β-In2S3 thin films, J. Mater. Sci: Mater. Electron. 27 (2016) 5810–5817. J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, J. Chastain, R.C. KingJr. (Eds.), Handbook of X-ray Photoelectron Spectroscopy, ULVAC-PHI, Inc., Chigasaki, 1995, pp. 72–73. G. Mohan Kumar, P. Ilanchezhiyan, A. Madhan Kumar, T.S. Shabi, S. Tamil Selvan, S. Suresh, Sh.U. Yuldashev, T.W. Kang, Chemically-derived CuO/In2O3-based nanocomposite for diode applications, CrystEngComm 17 (2015) 5932–5939. S. Peng, P. Zhu, V. Thavasi, S.G. Mhaisalkar, S. Ramakrishna, Facile solution deposition of ZnIn2S4 nanosheet films on FTO substrates for photoelectric application, Nanoscale 3 (2011) 2602–2608. A. Mumtaz, N.M. Mohamed, M. Mazhar, M.A. Ehsan, M.S.M. Saheed, Core–shell vanadium modified titania@β-In2S3 hybrid nanorod arrays for superior interface stability and photochemical activity, ACS Appl. Mater. Interfaces 8 (2016) 9037–9049. G.J. Lee, S. Anandan, S.J. Masten, J.J. Wu, Photocatalytic hydrogen evolution from water splitting using Cu doped ZnS microspheres under visible light irradiation, Renew. Energy 89 (2016) 18–26. T. Potloga, D. Ducaa, M. Dobromir, Temperature-dependent growth and XPS of Agdoped ZnTe thin films deposited by close space sublimation method, Appl. Surf. Sci. 352 (2015) 33–37. Z.T. Kang, H. Menkara, B.K. Wagner, C.J. Summers, R. Durst, Y. Diawara, G. Mednikova, T. Thorson, Effects of annealing atmosphere on the luminescent efficiency of ZnTe:O phosphors, J. Lumin. 117 (2006) 156–162. S. Rengaraj, S. Venkataraj, C. Tai, Y. Kim, E. Repo, M. Sillanpaa, Self-assembled mesoporous hierarchical-like In2S3 hollow microspheres composed of nanofibers and nanosheets and their photocatalytic activity, Langmuir 27 (2011) 5534–5541.
Contents lists available at ScienceDirect
Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat
Surface induced charge transfer in CuxIn2-xS3 nanostructures and their enhanced photoelectronic and photocatalytic performance
T
P. Ilanchezhiyana, G. Mohan Kumara, , Fu Xiaoa, C. Sivab, Shavkat U. Yuldasheva, D.J. Leec, H.C. Jeonc, T.W. Kanga ⁎
a
Nano-Information Technology Academy (NITA), Dongguk University, Seoul, Republic of Korea Department of Physics and Nanotechnology, SRM University, Kattankulathur, India c Quantum-Functional Semiconductor Research Center, Dongguk University, Seoul, Republic of Korea b
ARTICLE INFO
ABSTRACT
Keywords: Nanostructures, Semiconductors Photodiodes Photocatalysis
Multi-functional semiconducting nanostructures are gaining popularity for application in photoelectronics, energy storage devices and also in industrial and environmental remediation functions. In this regard, CuxIn2-xS3 nanostructures were investigated in detail for their photoelectrical and photocatalytic performance. Their physico-chemical characteristics were at first studied using X-ray diffraction, Raman, UV–vis absorbance, X-ray photoelectron spectroscopy and high resolution electron microscopic tools. CuxIn2-xS3 based flip chip Schottky diodes were demonstrated to attest their improved conductivity and enhanced photoelectrical performance. The photo switching capabilities of a type II p-n CdTe/CuxIn2-xS3 heterojunction was also investigated. In both the device configurations, the current-voltage (I-V) characteristics revealed the forward current and rectification ratio to improve under lower threshold voltages. The time-dependent photoresponse characteristics affirmed the stability of diodes, augmenting the improved/effective separation of photo generated electron hole pairs under illumination. Additionally, the photocatalytic performances of CuxIn2-xS3 nanostructures were inferred under visible light conditions through effective remediation of methylene blue (MB) dye molecules. The obtained results infer the Cu interaction in tetragonal lattice of CuxIn2-xS3 to promote the surface induced charge transfer mechanism in respective nanostructures, thereby enhancing their photoelectronic and photocatalytic functionalities.
1. Introduction Chalcogenide materials have started to receive tremendous interest in energy and environmental functions for their intrinsic material characteristics. The rich diverse composition availability in this class of materials and their tunable crystal structure with unique morphology endows them their remarkable physico-chemical properties, which in turn identifies them as promising candidates for a wide range of application in electronics, optoelectronics, energy storage and environment remediation functions [1–5]. And over the last few years non-layered metal chalcogenides have also started to receive similar interests for the same reason on par with that of layered materials. So, with these motivation we have worked on towards partial replacement of copper (Cu) ions for indium (In) ions in indium sulphide (In2S3) nanostructures via a complete solution derived approach for photoelectronic and photocatalytic functions. In2S3 is one of the prominent n-type semiconductors in the chalcogenide family with a direct band gap 2–2.4 eV [6–8]. Generally, In2S3
⁎
is said to exhibit three different polymorphic structures such as α, β and γ. Among them the β-phase is said to be the most stable form at roomtemperature, exhibiting a tetragonal structure and finding a number of technological application in optoelectronic devices [8]. And replacing transition metal ions such as Cu in this class of materials could open up new portfolio of complementary physical properties that in turn could render tunable electronic states and realize efficient photodiodes with excellent stability and performance. So, the primary objective of our investigation was to develop a new class of CuxIn2-xS3 nanostructure with improved photosensitivity. This we believe to take place through the coupling of metal d-electrons and defect states in tetragonal matrix [9]. The current investigation was also extended to identify a simple/scaledup approach to produce effective photocatalysts for treatment of dye effluents in wastewater. From these aforementioned viewpoints, a traditional hydrothermal strategy was adopted for the present investigation to accomplish high degree of compositional control among the processed nanostructures [10].
Corresponding author. E-mail address: [email protected] (G. Mohan Kumar).
https://doi.org/10.1016/j.solmat.2018.10.024 Received 22 June 2018; Received in revised form 3 August 2018; Accepted 25 October 2018 0927-0248/ © 2018 Elsevier B.V. All rights reserved.
Solar Energy Materials and Solar Cells 191 (2019) 100–107
P. Ilanchezhiyan et al.
Also, for the first time we have performed the integration of p-type cadmium telluride (CdTe) with n-type CuxIn2-xS3 nanostructures to arrive at the desired photodiode configuration. According to the best of our knowledge there are no reports on p-n CdTe/CuxIn2-xS3 heterojunction diodes for photodiode applications. CdTe, is a pioneering II-VI semiconducting material with an electron affinity of 4.28 eV and direct band gap of 1.5 eV at room-temperature [11,12]. The unique electrical properties of CdTe makes it a promising material in the field of light emitting diodes (LEDs), field effect transistors (FETs), and lasers [13–15]. CdTe has been processed through numerous methodologies such as thermal evaporation, vapor-phase epitaxy, molecular beam epitaxy and electrochemical deposition till date. And for the present study, we have decided to go on with a facile and unique approach by involving sonochemical treatment [16,17]. The chosen p-n CdTe/ CuxIn2-xS3 heterostructure could be regarded as a viable architecture for optoelectronic applications, owing to their bandgap values and intrinsic physical characteristics of CdTe and CuxIn2-xS3. This could also favor a compelling transport of charge carriers across the desired electrodes. IV characteristics established across the p-n heterostructures demonstrated an improved photocurrent with significant reverse leakages. The photocatalytic activity of these nanomaterials was also investigated through performing dye degradation reactions using MB under visible light. Finally, the present study also highlights the potential of CuxIn2xS3 nanostructures as a cost-effective alternate for large-scale manufacturing of high performance multifunctional devices.
the light was irradiated through the transparent (FTO) side of the flip chip geometry.
2. Experiment
2.3.4. Characterization The structural and phase characteristics of In2S3 and CuxIn2-xS3 nanomaterials were examined using Cu Kα radiation (λ = 0.154 nm) in an X-ray RINT 2500 diffractometer. X-ray photoelectron spectroscopic measurements were performed in a PHI 660 XPS spectrometer (using Al Kα radiation (1486.6 eV) as the excitation source). The morphology and dimension of CuxIn2-xS3 nanostructures were examined using a Hitachi S4800 (SEM). Their structural characteristics were inferred using a Micro Raman spectrophotometer (excitation wavelength of 532 nm). The absorbance spectrum was recorded using a Cary UV/VIS spectrophotometer. Electrical characteristics were registered using a Keithley 617 semiconductor parameter analyzer.
2.3.2. CdTe/CuxIn2-xS3 diodes CdTe suspension prepared elsewhere (refer Supplementary information 15, 16) was involved in the fabrication of CdTe/In2S3 diodes. Initially, 0.2 ml of CdTe suspension was spin cast on the In2S3, Cu0.01In2–0.01S3 and Cu0.02In2–0.02S3 films grown on ITO substrates at 1000 rpm and subsequently heat treated on a hot plate at 90 °C for 10 min. Ten-twenty cycles of such procedures were carried out to attain a homogeneous CdTe thin film. The device functionalities of CdTe/ CuxIn2-xS3 diodes were then studied using a semiconductor parameter analyzer after establishing appropriate contacts on CdTe and CuxIn2-xS3 diodes using Ag paste. 2.3.3. Photocatalysis The photocatalytic performance of CuxIn2-xS3 materials were evaluated by measuring the degradation rate of MB dye molecules under visible light illumination. Initially, 10 ml of MB solutions (5 ppm) were taken in three different vials with 5 mg of processed CuxIn2-xS3 samples (where x = 0, 1 and 2 wt%). The above suspensions were irradiated with a compact fluorescent lamp (CFL, model: 20 W (TU), Havel's India Ltd, India). The absorbance of MB solutions were then registered at regular intervals by using a UV–visible absorption spectrophotometer (JASCO model-V650).
2.1. Materials All the chemicals used in the experiments were of analytical grade and used thereafter. Indium nitrate, copper acetate, thioacetamide, acetone and ethanol were procured from Sigma Aldrich. Cadmium telluride (99%) was procured from Alfa Aesar. The transparent conducting indium tin oxide (ITO, 15 Ω/cm2) and fluorine doped tin oxide (FTO, 10 Ω/cm2) coated glass substrates used in the device fabrication were pre-cleaned using acetone, ethanol and deionized (DI) water. 2.2. Synthesis of CuxIn2-xS3 nanostructures
3. Results and discussion
Initially, 0.3 g of indium nitrate and 0.22 g of thioacetamide was dissolved in 100 ml of deionized (DI) water and subjected to continuous stirring for 1 h. The mixture was then transferred to a Teflon lined stainless steel autoclave and subjected to hydrothermal treatment in a hot air oven at 120–160 °C for 3–6 h. A pre-cleaned ITO substrate was additionally placed inside the autoclave. The orange colored precipitates and films deposited on ITO were harvested after washing in ethanol and DI water. The precipitates and films were dried overnight at 60 °C in an oven and used for further studies. A similar procedure was followed for the preparation of CuxIn2-xS3 nanostructures. The x value of Cu i.e. copper acetate in the aforementioned reactions was held at 0, 1 and 2 wt%. The resulting reddish-orange precipitates were collected by washing the yield in ethanol and DI water. The processed nanostructures are here after referred as In2S3, Cu0.01In2–0.01S3 and Cu0.02In2–0.02S3, respectively. The processed thin films were used for diode studies while the powders were involved as photocatalysts.
The crystal structure and phase characteristics of In2S3 and CuxIn2nanostructures were initially studied through their respective powder X-ray diffraction (XRD) patterns shown in Fig. 1a. The diffraction patterns recorded over a range of 20 < 2θ < 100° were noted to be well indexed to the tetragonal β-In2S3 and signify the samples to be well crystallized and agree with the standard card (JCPDS 03-0650459) [18]. Interestingly, the (440) peak position at 48.05° was observed to be slightly shifted towards the lower angles for Cu0.02In2–0.02S3 nanostructures (Fig. 1b). This shift could be attributed to the greater extent of Cu ion replacement in the tetragonal host matrix, as their radius (0.057 nm) is considerably smaller than that of the In ions (0.080 nm) [19,20]. We would also like to add that the absence of indium hydroxide or Cu related secondary phases attests the phase purity in processed nanomaterials. Room-temperature Raman spectroscopic measurements were additionally carried out on the In2S3 and CuxIn2-xS3 nanostructures to investigate their crystal phase through corresponding vibration modes. Generally, these modes are influenced by a number of factors such as stress, defects and structural disorders in a local atomic arrangement. Fig. 1c shows the room-temperature Raman spectra of In2S3 and CuxIn2xS3 nanostructures. Here, the broad vibration modes centered at 300 cm-1 (irrespective of Cu composition) signify the existence of tetragonal β-phase of In2S3 in A1 symmetry [21,22]. The peak position and half width maximum of these intense vibration also signify the mild xS3
2.3. Device fabrication 2.3.1. Flip chip device To arrive at the Schottky device configuration via flip chip method the CuxIn2-xS3 films grown by aforementioned treatments were sandwiched on to a FTO substrate. Here, the thin films coated on ITO acted as the working electrode, after establishing appropriate contacts on FTO and ITO substrates using Ag paste. For photoresponse measurements, 101
Solar Energy Materials and Solar Cells 191 (2019) 100–107
P. Ilanchezhiyan et al.
Fig. 1. (a) XRD patterns illustrating the tetragonal phase of (i) In2S3, (ii) Cu0.01In2–0.01S3 and. (iii) Cu0.02In2–0.02S3 nanostructures. (b) The shift in (440) peak position towards the lower angles as a function of Cu composition in CuxIn2-xS3 nanostructures. (c) Room-temperature Raman spectrum of (i) In2S3, (ii) Cu0.01In2–0.01S3 and (iii) Cu0.02In2–0.02S3 nanostructures attesting their tetragonal β-phase with A1 symmetry. (d) Raman spectrum of sonochemically treated CdTe nanostructures.
effect of Cu ions on the degree of crystallinity in CuxIn2-xS3 nanostructures. The broad peaks at 300 cm-1 were further subjected to careful evaluation to identify the exact position of three characteristic vibrations at 245, 300 and 358 cm-1, respectively. These vibrations (including the shoulder edge at 358 cm-1) characterize the A1g Raman phonon modes in tetragonal β-In2S3 [23,24]. Additionally, the invariance in Raman peak position and band width also suggests the unconspicuous change of local chemical environment in CuxIn2-xS3 nanostructures. Raman spectrum of sonochemically treated CdTe nanostructures is shown in Fig. 1d. Here, three vibration peaks could be observed at 117, 132 and 260 cm-1, which all match well with that of the previous reports on CdTe [25,26]. The first peak along the lower wavenumber designates the acoustic A1 mode of Te, which usually originates from the vibration of elemental Te in CdTe nanostructures [27]. The second peak at 132 cm-1 could be a contribution owed together by Te and transverse optical (TO) phonon mode of CdTe [28]. Likewise, the third peak at 260 cm-1 could be attributed to the elemental Te related vibrations, attesting the existence of CdTe and elemental Te in the processed CdTe nanostructures (refer Supplementary information for further discussion) [16,29]. Next, the optical band gap (Eg) of In2S3 and CuxIn2-xS3 nanostructures were estimated using the aid of UV–Vis absorption spectra. The influence of Cu ions on the Eg of In2S3 and CuxIn2-xS3 nanostructures is illustrated in the Tauc's plot shown in Fig. 2. According to Tauc's explanation, the relation between Eg and absorption coefficient (α) of a direct bandgap semiconductor is expressed as follows,
( h )2 = A(h
Eg )
Fig. 2. Tauc's plot extracted from UV–vis absorption spectrum of (a) In2S3, (b) Cu0.01In2–0.01S3 and (c) Cu0.02In2–0.02S3 made suspensions. The optical band gap values were estimated to be around 2.08–2.35 eV. The inset shows the optical representation of In2S3, Cu0.01In2–0.01S3 and. Cu0.02In2–0.02S3 films grown on ITO substrates.
interaction in the tetragonal lattice. We believe these factors to promote the photon absorption rate, thereby directing the enhanced photoelectronic and photocatalytic activity in Cu0.02In2–0.02S3 nanostructures (discussed later). Eg of CdTe was assumed to be around 1.50 eV from the previous reports in all of our experiments [11,12]. Cu0.02In2–0.02S3 and CdTe nanostructures were additionally subjected to X-ray photoelectron spectroscopic (XPS) measurements to determine the chemical state of various ions and other information related to the nature of chemical bonding. Fig. 3a shows the 3d core level spectrum corresponding to that of In 3d doublet that splits into 3d5/2 and 3d3/2 at 444.08 and 451.55 eV, respectively. The observed peak positions are in consistent to the reported values for In3+ [32,33]. The core level XPS spectrum of S 2p is shown in Fig. 3b. The origin of resolved S 2p1/2 peak at 161.63 eV and shoulder edge at 162.77 eV characterizes the S2− oxidation state of S in Cu0.02In2–0.02S3 [34,35]. The chemical state of Cu and their composition in Cu0.02In2–0.02S3 samples was next investigated using the Cu 2p core level spectrum shown in Fig. 3c to be around 1.33%. This spectrum appears to be
(1)
where A is a constant and hν is the incident photon and Eg is the bandgap energy. And in the present study, Eg of In2S3, Cu0.01In2–0.01S3 and Cu0.02In2–0.02S3 nanostructures was estimated be around 2.35, 2.23 and 2.08 eV by extrapolating the linear portion of the plot at (αhυ)2 = 0. These values are in consistent to the earlier reports and suggests the domination of direct allowed transitions in CuxIn2-xS3 [30,31]. Furthermore, the lowering of Eg values as a function of Cu concentration provides a significant insight on the shift in absorption band edge of β-In2S3. This trend could be related to the surface modifications in CuxIn2-xS3 nanostructures, which actually takes place due to Cu 102
Solar Energy Materials and Solar Cells 191 (2019) 100–107
P. Ilanchezhiyan et al.
Fig. 3. Core-level XPS spectra corresponding to (a) In 3d, (b) S 2p and (c) Cu 2p in. Cu0.02In2–0.02S3 nanostructures. (d) Cd 3d and (e) Te 3d core-level XPS spectra of CdTe nanostructures signify the existence of secondary phases (refer Supplementary information).
characterized by a doublet feature that could be owed to the spin orbit splitting i.e. the Cu 2p3/2 and Cu 2p1/2 peaks at 931.7 and 951.8 eV, respectively. The peak positions and a spin orbit separation of 20.1 eV authenticates the existence of cationic Cu species in Cu0.02In2–0.02S3 systems processed via hydrothermal synthesis [36]. Additionally, this aspect also elucidates the successful distribution of Cu2+ ions on the surface of tetragonal Cu0.02In2–0.02S3. Fig. 3d shows the core level XPS spectrum of Cd 3d, illustrating the spin-orbit split lines of Cd 3d5/2 and Cd 3d3/2 to be centered on 405.17 and 411.95 eV, respectively. And this could be designated to the Cd-Te bonds that exist within CdTe systems. The core level XPS spectrum of Te 3d is shown in Fig. 3e. The occurrence of Te 3d5/2 and Te 3d3/2 transition peaks at binding energy 573.3 and 583.6 eV authenticates the Cd-Te bonding, while the remaining two peaks signature the co-existence of tellurium in oxide form [37,38]. The oxidation of Te could have been resulted due to the sonochemical treatment of CdTe containing Te-rich phases in the aqueous media (refer Supplementary information for further discussion) [16,17]. The morphological features of CuxIn2-xS3 nanostructures were next verified by the aid of microscopic images recorded using scanning electron microscopy (SEM) technique. The micrographs shown in Fig. 4 a-c affirm the random aggregates of CuxIn2-xS3 and their clustered particle-like characteristics. The average size distribution of
nanostructures were registered to be around 50–100 nm, irrespective of Cu composition in CuxIn2-xS3. Fig. 4d shows the low-magnification SEM image of the CdTe nanostructures. Here, CdTe appears to be almost agglomerated in form of clustered nanostructures. Their particle size distribution was further estimated to be around 100–200 nm through their particulate nature. The influence of Cu ions on the charge conductivity of CuxIn2-xS3 nanostructured thin films was ascertained using their I-V curves registered in Fig. 5a. Here, all the devices appear to be of good conductive nature, while the charge selectivity across Cu0.01In2–0.01S3 and Cu0.02In2–0.02S3 thin films appear to be improved on par with that of In2S3 thin films. And this improvement could be regarded as a substantial evidence suggesting the drastic decrease in resistance values (increased charge carriers) upon Cu incorporation in In2S3 lattice. The electrical and optoelectronic properties of CuxIn2-xS3 films deposited on ITO substrates was then determined through the flip chip device shown in Fig. 5b. This junction was actually established between the CuxIn2xS3 films grown on ITO and bare FTO counter electrodes via flip chip method i.e. top press contact. Here, FTO serves as the front contact and CuxIn2-xS3 film deposited on ITO glass acts as the back contact in device geometry. I-V curves of Cu0.02In2–0.02S3 device under dark and illumination condition is alone shown in Fig. 5c due to their improved 103
Solar Energy Materials and Solar Cells 191 (2019) 100–107
P. Ilanchezhiyan et al.
Fig. 4. (a-c) SEM images revealing the morphological evolution of CuxIn2-xS3 nanostructures (value of x being 0, 1 and 2 from a-c). (b) High-magnification SEM representation of sonochemically processed CdTe nanostructures.
conductivity. Here, the curves shows a nonlinear trend that could be visualized through the variation in their symmetric nature, affirming the realization of Schottky like junction at the FTO and Cu0.02In2–0.02S3 interfaces. In contrast to the dark condition, the current increases significantly under light illumination in Fig. 5c, indicating the
contribution from photogenerated carriers. Fig. 5d substantiates the band alignment realized across the Cu0.02In2–0.02S3 based flip chip devices under illuminated conditions. In these devices, the band offset between FTO and Cu0.02In2–0.02S3 and that between ITO and Cu0.02In2–0.02S3 actually directs the generation of asymmetric Schottky
Fig. 5. (a) Linear scale I-V characteristics registered across CuxIn2-xS3 thin films suggesting the improved conductivity as a function of Cu composition (value of x = 0, 1 and 2 from i-iii). (b) Shows the schematic of Flip chip device constructed using CuxIn2-xS3 films obtained via hydrothermal treatment. (c) Semi-log scale I-V characteristics of Cu0.02In2–0.02S3 made flip chip device under dark and illuminated conditions. (d) Energy level diagram suggesting the band offset between FTO and Cu0.02In2–0.02S3 and that between ITO and Cu0.02In2–0.02S3 in the flip chip device. 104
Solar Energy Materials and Solar Cells 191 (2019) 100–107
P. Ilanchezhiyan et al.
Fig. 6. (a) Reveals the typical architecture of p-CdTe/n-CuxIn2-xS3 diodes fabricated through sol-gel spin coating of CdTe nanostructures on CuxIn2-xS3 films. (b) Linear scale I-V characteristics corresponding to that of p-CdTe/n-Cu0.02In2–0.02S3 diodes. (c) Time-dependent photocurrent response of p-CdTe/n-Cu0.02In2–0.02S3 heterojunction for 20 s interval. The transient photocurrent response was registered under 900 W/m2. (d) The band alignment across the CdTe-CuxIn2-xS3 heterojunction approves to promote the effective separation of electron-hole pairs along the respective electrodes.
barriers across the corresponding interfaces due to electron transfer. The improved current values illustrated in Fig. 5c further manifests the potential of Cu0.02In2–0.02S3 nanostructures for efficient photodiode applications. Additionally, we have also investigated the photo switching functionalities of CdTe/CuxIn2-xS3 made p-n diodes, whose typical architecture is shown in Fig. 6a. I-V characteristics of this p-CdTe/nCu0.02In2–0.02S3 diode under dark and illuminated condition is shown in Fig. 6b. Out of the three devices studied, Cu0.02In2–0.02S3 made diodes tend to exhibit an improved photoresponse and signify the increase in Cu composition to improve their device performance considerably. This is the reason why p-CdTe/n-Cu0.02In2–0.02S3 devices are alone discussed in Fig. 6b. Here, the photocurrent increases gradually when the intensity of light increases from 900 to 1100 W/m2. Additionally, the forward and reverse-biased currents also increased in all the three devices under light illumination (not shown), implying the photo excited electron-hole pair's to increase the concentration of majority carriers. In the next stage, time-dependent photoresponse stability of p-CdTe/nCu0.02In2–0.02S3 diode was studied by turning the light on and off periodically. The photo response curve registered over a time interval of 20 s is shown in Fig. 6c. Here, the energy of incident photon density was held at 900 W/m2, while the applied bias voltage was held as function of time at 3 V. The photocurrent of the diode appears to rise under illumination and drop down quickly on turning off without any fluctuation. Such features actually characterize the reproducible response from the diodes and their stability in switching for a number of cycles. The diode also exhibits a maximum photocurrent of 3.4 × 10−8 A and a photo responsivity of about 0.1 AW−1, while subjected to a photon illumination of 900 W/m2. We also found from our investigation results that the device response to light is still significant without any considerable influence on the intensity of incident photon flux. The average
rise time and fall time of the device was estimated from Fig. 6c to be around 2 and 3 s, respectively. Fig. 6d substantiates the band alignment realized across the p-CdTe/ n-CuxIn2-xS3 based heterojunction under illuminated conditions. Here, the type-II energy level alignment could initially promote the efficient transfer of charge carriers across the heterostructure. The incident light directs the generation of electron-hole pairs along the depletion layer of the investigated p-n heterostructure. The width of depletion layer narrows under forward bias condition, thereby resulting with the easy drift of charge carriers along the respective electrodes. Similarly, the depletion width increases under reverse bias condition to restrict the flow of charge carriers across the barrier. The investigated I-V results also approve the p-CdTe/n-Cu0.02In2–0.02S3 heterostructure to promote the effective separation of electron-hole pairs. Next, the processed CuxIn2-xS3 nanostructures were subjected to visible light photocatalytic activity studies via degrading MB dye in aqueous media. The photocatalytic results shown in Fig. 7a augment the fact that the processed Cu0.02In2–0.02S3 nanostructures are much effective in degrading MB under visible light irradiation rather than In2S3 and Cu0.01In2–0.01S3 nanostructures. The enhanced activity in these systems could be resulting from the successful impregnation of Cu ions in the host lattice, which actually promotes the effective separation of electron-hole pairs required for degradation reactions. From the results it is also clear that pristine In2S3 has a considerable amount of photocatalytic activity, probably due to its band gap. The kinetics of degradation reactions were next investigated using Fig. 7b. Here, the linear relationship between ln(C/C0) and reaction time clearly indicates the photodegradation reactions to follow first order kinetics. The apparent rate constants were determined as 0.007, 0.013 and 0.017 min−1 for In2S3, Cu0.01In2–0.01S3 and Cu0.02In2–0.02S3, respectively. The order of photodegradation efficiency could be visualized through Fig. 8a to be as follows Cu0.02In2–0.02S3 > Cu0.01In2–0.01S3 > In2S3.
105
Solar Energy Materials and Solar Cells 191 (2019) 100–107
P. Ilanchezhiyan et al.
transfer from the respective conduction band to neighboring oxygen molecules. Such process are highly essential in degrading MB, as they ensure the prevention of recombination of electrons and holes. The availability of excess charge carriers then direct the redox reaction to generate additional reactive oxygen species required for degrading MB molecules. Furthermore, the effective injection of electrons from MB molecules to Cu0.02In2–0.02S3 nanostructures upon photoexcitation should have also resulted with the reduction of molecular oxygen and oxidative decomposition of electron-deficient MB more effectively. So such factors altogether could be reasoned for the enhanced photocatalytic performance observed in CuxIn2-xS3 [39]. Finally, we would like to summarize that the present study on CuxIn2-xS3 systems via solution based strategies to provide insight on the surface related modifications taking place in CuxIn2-xS3 tetragonal lattice due to Cu interaction and contribute towards their obvious photo-response and enhanced photoconducting behavior for cost-effective photoelectronic functions. And, the investigated results also suggest the lowered Eg values of CuxIn2-xS3 systems to assist in increasing the net photon absorption rate for effective photocatalytic detoxification under visible light radiation. 4. Conclusion CuxIn2-xS3 nanostructures were processed through a facile sol-gel route for energy and environmental remediation applications. Their physico-chemical characteristics were verified by using XRD, Raman, UV, XPS and electron microscopic tools. The improved conductivity in Cu0.02In2–0.02S3 nanostructures was verified via constructing Schottky diodes through traditional flip chip method. Along by, photoelectronic functionalities of CdTe/Cu0.02In2–0.02S3 made p-n heterojunctions were also extensively studied. In all cases, the I-V characteristics revealed the forward current and rectification ratio to improve under lower threshold voltages. The time-dependent photoresponse characteristics further affirmed the stability of diodes, augmenting the improved/effective separation of photo generated electron hole pairs along the interface. Finally, the photocatalytic studies performed under visible light conditions indicate the Cu0.02In2–0.02S3 nanostructures to function as a superior catalyst among the processed three systems.
Fig. 7. (a) Photocatalytic activities of CuxIn2-xS3 nanostructures with MB under visible light (value of x being 0, 1 and 2). (b) Natural logarithm of C/C0 vs. irradiation time used to evaluate the rate constants.
Fig. 8b represents the photocatalytic mechanism involved in the degradation of MB under visible light conditions in presence of CuxIn2xS3 samples. Here, electron-hole pairs are produced in all the samples (irrespective of Cu content) upon photoexcitation and it is these charge carriers that actually assists in degradation of MB molecules. However, in case of Cu0.01In2–0.01S3 and Cu0.02In2–0.02S3 systems the Cu ions existing within the photocatalyst at first directs the surface induced charge separation mechanism to take place via accelerating the electron
Fig. 8. (a) Photocatalytic efficiency of CuxIn2-xS3 samples vs. Time in degrading MB molecules (value of x being 0, 1 and 2). (b) Photocatalytic mechanism involved in the degradation of MB dye in presence of CuxIn2-xS3 samples under visible light conditions.
106
Solar Energy Materials and Solar Cells 191 (2019) 100–107
P. Ilanchezhiyan et al.
Acknowledgement [18]
This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded by the Ministry of Education (No. 2018R1D1A1B07051461), (No. 2018R1D1A1B07051474), (No. 2018R1D1A1B07051406), (No. 2018R1D1A1B07050237), (No. 2018R1D1A1B07051095), (No. 2016R1A6A1A03012877) and (No. 2016R1D1A1B03935948).
[19]
[20] [21]
Appendix A. Supporting information
[22]
Supplementary data associated with this article can be found in the online version at doi:10.1016/j.solmat.2018.10.024.
[23]
References
[24]
[1] C. Tan, H. Zhang, Wet-chemical synthesis and applications of non-layer structured two-dimensional nanomaterials, Nat. Commun. 6 (2015) 7873. [2] Y. Dou, L. Zhang, X. Xu, Z. Sun, T. Liao, S.X. Dou, Atomically thin non-layered nanomaterials for energy storage and conversion, Chem. Soc. Rev. 46 (2017) 7338–7373. [3] D.L. Duong, S.J. Yun, Y.H. Lee, van der Waals layered materials: opportunities and challenges, ACS Nano 11 (2017) 11803–11830. [4] S. Ithurria, B. Dubertret, Quasi 2D colloidal CdSe platelets with thicknesses controlled at the atomic level, J. Am. Chem. Soc. 130 (2008) 16504–16505. [5] G. Mohan Kumar, Fu Xiao, P. Ilanchezhiyan, Sh.U. Yuldashev, A. Madhan Kumar, H.D. Cho, D.J. Lee, T.W. Kang, High performance photodiodes based on chemically processed Cu doped SnS2 nanoflakes, Appl. Surf. Sci. 455 (2018) 446–454. [6] M. Afzaal, M.A. Malik, P. O’Brien, Indium sulfide nanorods from single-source precursor, Chem. Commun. 0 (2004) 334–335. [7] N. Chaudhari, L. Mandal, O. Game, S. Warule, D. Phase, S. Jadkar, S. Ogale, Dramatic enhancement in photoresponse of β-In2S3 through suppression of dark conductivity by synthetic control of defect-induced carrier compensation, ACS Appl. Mater. Interfaces 7 (2015) 17671–17681. [8] M. Mathew, R. Jayakrishnan, P.M. Ratheesh Kumar, C. Sudha Kartha, K.P. Vijayakumar, Anomalous behavior of silver doped indium sulfide thin films, J. Appl. Phys. 100 (2006) 033504. [9] S. Ghosh, M. Saha, V.D. Ashok, A. Chatterjee, S.K. De, Excitation dependent multicolor emission and photoconductivity of Mn, Cu doped In2S3 monodisperse quantum dots, Nanotechnology 27 (2016) 155708. [10] A. Datta, S.K. Panda, D. Ganguli, P. Mishra, S. Chaudhuri, In2S3 micropompons and their conversion to In2O3 nanobipyramids: simple synthesis approaches and characterization, Cryst. Growth Des. 7 (2007) 163–169. [11] R.W. Crisp, G.F. Pach, J.M. Kurley, R.M. France, M.O. Reese, S.U. Nanayakkara, B.A. MacLeod, D.V. Talapin, M.C. Beard, J.M. Luther, Tandem solar cells from solution-processed CdTe and PbS quantum dots using a ZnTe–ZnO tunnel junction, Nano Lett. 17 (2017) 1020–1027. [12] M.A. Islam, K.S. Rahman, K. Sobayel, T. Enam, A.M. Ali, M. Zaman, M. Akhtaruzzaman, N. Amin, Fabrication of high efficiency sputtered CdS: O/CdTe thin film solar cells from window/absorber layer growth optimization in magnetron sputtering, Sol. Energy Mater. Sol. Cells 172 (2017) 384–393. [13] Q. Lin, B. Song, H. Wang, F. Zhang, F. Chen, L. Wang, L. Song Li, F. Guo, H. Shen, High-efficiency deep-red quantum-dot light-emitting diodes with type-II CdSe/CdTe core/shell quantum dots as emissive layers, J. Mater. Chem. C 4 (2016) 7223–7229. [14] M. Shaygan, K. Davami, N. Kheirabi, C. Ki Baek, G. Cuniberti, M. Meyyappan, J.S. Lee, Single-crystalline CdTe nanowire field effect transistors as nanowire-based photodetector, Phys. Chem. Chem. Phys. 16 (2014) 22687–22693. [15] S.G. Kumar, K.S.R. Koteswara Rao, Physics and chemistry of CdTe/CdS thin film heterojunction photovoltaic devices: fundamental and critical aspects, Energy Environ. Sci. 7 (2014) 45–102. [16] P. Ilanchezhiyan, G. Mohan Kumar, Fu Xiao, S. Poongothai, A. Madhan Kumar, C. Siva, Sh.U. Yuldashev, D.J. Lee, Y.H. Kwon, T.W. Kang, Ultrasonic-assisted synthesis of ZnTe nanostructures and their structural, electrochemical and photoelectrical properties, Ultrason. Sonochem. 39 (2017) 414–419. [17] P. Ilanchezhiyan, G. Mohan Kumar, Fu Xiao, A. Madhankumar, C. Siva, Shavkat U. Yuldashev, H.D. Cho, T.W. Kang, Interfacial charge transfer in ZnTe/ZnO nano
[25] [26] [27] [28] [29]
[30] [31] [32] [33] [34] [35]
[36] [37] [38] [39]
107
arrayed heterostructures and their improved photoelectronic properties, Sol. Energy Mater. Sol. Cells 183 (2018) 73–81. H.X. Bai, L.X. Zhang, Y.C. Zhang, Simple synthesis of urchin-like In2S3 and In2O3 nanostructures, Mater. Lett. 63 (2009) 823–825. L. Chow, O. Lupan, G. Chai, H. Khallaf, L.K. Ono, B. Roldan Cuenya, I.M. Tiginyanu, V.V. Ursaki, V. Sontea, A. Schulte, Synthesis and characterization of Cu-doped ZnO one-dimensional structures for miniaturized sensor applications with faster response, Sens. Actuators A 189 (2013) 399–408. G.C. Xie, L. Fang, L.P. Peng, G.B. Liu, H.B. Ruan, F. Wu, C.Y. Kong, Effect of Indoping on the optical constants of ZnO thin films, Phys. Procedia 32 (2012) 651–657. Y. Ji, Y. Ou, Z. Yu, Y. Yan, D. Wang, C. Yan, L. Liu, Y. Zhang, Y. Zhao, Effect of film thickness on physical properties of RF sputtered In2S3 layers, Surf. Coat. Technol. 276 (2015) 587–594. Y. Xiong, Y. Xie, G. Du, X. Tian, Y. Qian, A novel in situ oxidization-sulfidation growth route via self-purification process to β-In2S3 dendrites, J. Solid State Chem. 166 (2002) 336–340. N.M. Gasanly, H. Ozkan, A. Aydinli, I. Yilmaz, Temperature dependence of the Raman-active phonon frequencies in indium sulfide, Solid State Commun. 110 (1999) 231–236. H. Tao, H. Zang, G. Dong, J. Zeng, X. Zhao, Raman and infrared spectroscopic study of the defect spinel In21.333S32, Optoelectron. Adv. Mater. Rapid Commun. 2 (2008) 356–359. J.M. Song, Y.Z. Lin, Y.J. Zhan, Y.C. Tian, G. Liu, S.H. Yu, Superlong high-quality tellurium nanotubes: synthesis, characterization, and optical property, Cryst. Growth Des. 8 (2008) 1902–1908. L. Zhang, C. Wang, D. Wen, Preparation by hydrothermal techniques in a tungstosilicate acid solution system and optical properties of tellurium nanotubes, Eur. J. Inorg. Chem. 22 (2009) 3291–3297. X. Wang, Y. Xu, R. Tong, X. Zhou, Q. Li, H. Wang, Compositional effects and optical properties of CdSeXTe1−X alloyed nanotube arrays, CrystEngComm 17 (2015) 960–966. P.M. Amirtharaj, F.H. Pollak, Raman scattering study of the properties and removal of excess Te on CdTe surfaces, Appl. Phys. Lett. 45 (1984) 789. M. Dharmadasa, O.K. Echendu, F. Fauzi1, N.A. Abdul-Manaf, O.I. Olusola, H.I. Salim, M.L. Madugu, A.A. Ojo, Improvement of composition of CdTe thin films during heat treatment in the presence of CdCl2, J. Mater. Sci: Mater. Electron. 28 (2017) 2343–2352. Y.J. Hsiao, C.H. Lu, L.W. Ji, T.H. Meen, Y.L. Chen, H.P. Chi, Characterization of photovoltaics with In2S3 nanoflakes/p-Si heterojunction, Nanoscale Res. Lett. 9 (2014) 32. Z. Zheng, J. Yu, S. Cheng, Y. Lai, Q. Zheng, D. Pan, Investigation of structural, optical and electrical properties of Cu doped β-In2S3 thin films, J. Mater. Sci: Mater. Electron. 27 (2016) 5810–5817. J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, J. Chastain, R.C. KingJr. (Eds.), Handbook of X-ray Photoelectron Spectroscopy, ULVAC-PHI, Inc., Chigasaki, 1995, pp. 72–73. G. Mohan Kumar, P. Ilanchezhiyan, A. Madhan Kumar, T.S. Shabi, S. Tamil Selvan, S. Suresh, Sh.U. Yuldashev, T.W. Kang, Chemically-derived CuO/In2O3-based nanocomposite for diode applications, CrystEngComm 17 (2015) 5932–5939. S. Peng, P. Zhu, V. Thavasi, S.G. Mhaisalkar, S. Ramakrishna, Facile solution deposition of ZnIn2S4 nanosheet films on FTO substrates for photoelectric application, Nanoscale 3 (2011) 2602–2608. A. Mumtaz, N.M. Mohamed, M. Mazhar, M.A. Ehsan, M.S.M. Saheed, Core–shell vanadium modified titania@β-In2S3 hybrid nanorod arrays for superior interface stability and photochemical activity, ACS Appl. Mater. Interfaces 8 (2016) 9037–9049. G.J. Lee, S. Anandan, S.J. Masten, J.J. Wu, Photocatalytic hydrogen evolution from water splitting using Cu doped ZnS microspheres under visible light irradiation, Renew. Energy 89 (2016) 18–26. T. Potloga, D. Ducaa, M. Dobromir, Temperature-dependent growth and XPS of Agdoped ZnTe thin films deposited by close space sublimation method, Appl. Surf. Sci. 352 (2015) 33–37. Z.T. Kang, H. Menkara, B.K. Wagner, C.J. Summers, R. Durst, Y. Diawara, G. Mednikova, T. Thorson, Effects of annealing atmosphere on the luminescent efficiency of ZnTe:O phosphors, J. Lumin. 117 (2006) 156–162. S. Rengaraj, S. Venkataraj, C. Tai, Y. Kim, E. Repo, M. Sillanpaa, Self-assembled mesoporous hierarchical-like In2S3 hollow microspheres composed of nanofibers and nanosheets and their photocatalytic activity, Langmuir 27 (2011) 5534–5541.