CuO Quantum Dots Decorated TiO2 Nanocomposite Photocatalyst for

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CuO quantum dots decorated TiO2 nanocomposite photocatalyst for stable hydrogen generation Nagappagari Lakshmana Reddy, Saim Emin, Valluri Durga Kumari, and Shankar Muthukonda Venkatakrishnan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03785 • Publication Date (Web): 21 Dec 2017 Downloaded from http://pubs.acs.org on December 28, 2017

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Industrial & Engineering Chemistry Research

CuO quantum dots decorated TiO2 nanocomposite photocatalyst for stable hydrogen generation Nagappagari Lakshmana Reddya, Saim Eminb, Valluri Durga Kumaric, Shankar Muthukonda Venkatakrishnana* a

Nanocatalysis and Solar Fuels Research Laboratory, Department of Materials Science &

Nanotechnology, Yogi Vemana University, Kadapa - 516003, Andhra Pradesh, INDIA. b

c

Materials Research Laboratory, University of Nova Gorica, SI-500 Nova Gorica, SLOVENIA.

Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology (IICT),

Hyderabad 500007, Telangana, INDIA.

______________________________________________________________________________ * Corresponding author. Tel.:+91-9966845899; Fax: +91-8562225419 E-mail address: [email protected] (M.V.Shankar).

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ABSTRACT An efficient and stable CuO-TiO2 nanocomposite photocatalyst was synthesized by using simple molten-salt method. Characterization by HR-TEM confirmed existence of both TiO2 and CuO in the nanocomposite, revealed hexagonal TiO2 nanoparticles (NPs) with average particles size of 23.8 nm. CuO QDs decorated on TiO2 surface was in the range of 2.2 to 4.6 nm. Photocatalytic experiments for hydrogen (H2) production were carried out under LED (λ=365 nm) lamp and natural solar light. The effect of Cu-loading in CuO-TiO2 NCs and synthesis time were studied. The optimized CuO-TiO2 NCs abbreviated as CuT-4 and CuT-3 showed 27.7 and 9.0 folds superior rate of H2 production compared to pristine TiO2 NPs under LED and solar irradiation respectively. At optimal conditions CuO-TiO2 NCs demonstrated good photoactivity for H2 evolution during 75 h illumination under LED light. The experimental results confirmed the cocatalytic role of CuO for improved H2 generation by minimized recombination of excitons. Keywords: Water splitting; quantum dots; nanocomposite; molten-salt; hydrogen generation.

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1. INTRODUCTION Energy is the top most problem of the present world due to its increasing demand and deflation of fossil fuels. Therefore development of an alternative renewable energy sources is an urgent need. It is well known that hydrogen (H2) is the best alternative energy source for the fossil fuels, due to its high energy content and clean fuel with zero emission. The H2 generation from photocatalytic water splitting is an emerging technology1-3. Hence many semiconducting photocatalytic materials have been developed for sustainable development. Among them titania (TiO2) based composite photocatalysts is an effective approach for solar light absorption and prolong the excitons life-time for superior H2 production4-11. In this direction, narrow band gap metal oxides immobilized on wide band gap TiO2 semiconductor are in demand, for example, Bi2O312, NiO13,26,27, B (boron)14, CoO15,16, RuO217, CuxO18 were reported. Among them the CuO is an important candidate to improve the efficiency of the photocatalytic activity. The surface contact between CuO-TiO2 behaves as p-n type semiconductor hetero-junction that effectively separates photo induced electron-hole pairs due to inner electric field19-21. The H2 production efficiency of CuO-TiO2 photocatalysts were studied in two ways (i) optimization of various experimental parameters such as solution pH, concentration/nature of sacrificial agent, amount of catalyst in suspension, type of light source and intensity22-24, (ii) Tuning of targeted catalyst properties viz., suitable red-ox potential, effective surface-surface interaction, improved visible light absorption, high crystallinity, excitons with longer life-time can have dramatic effects for multi-fold increase in catalytic activity24. Hence, literature survey on synthesis of CuO-TiO2 and CuxO-TiO2 photocatalysts employing most frequently used methods and light sources together with amount of H2 production is listed in Table 1. Xu et al25, explained the beneficial effects of sol-gel method compared with impregnation, chemical reduction and photo-deposition of Cu on TiO2 for photocatalytic H2 generation. The adopted method determines the oxidation state of Cu, its surface concentration and interaction with TiO2. Alternatively, nanocomposites (NCs) synthesized by using impregnation method displayed comparable rate of H2 production than sol-gel and other reported methods36. For example Yu et al54. reported CuO modified TiO2 particles by impregnation method for H2 generation of 2061µmol. h-1. g-1cat under UV-LED light. Hence, solid-state synthesis method is 3



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ideal choice to produce photocatalysts with good crystallinity. For enhanced rate of photocatalytic H2 production, the goal is the preparation of NCs photocatalyst with high crystallinity. However, direct heating of precursors in solid-state synthesis has deleterious effects such as bigger particles besides alterations in crystal structure and composition30. Recently molten-salt method was adopted to synthesize metal oxide or its composites with high crystallinity such as core-shell, ferro-electric and dielectric materials without damaging the crystallinity28,29. Molten-salt method is a solid-state technique which utilizes melted NaCl or KCl as solvents for synthesis of metal oxides or its composites from metal nitrate precursor31-34. The solvent present in the synthesized material can be easily removed with water washing followed by drying in electric oven35. The synthesis parameters like type of salt, precursor composition, synthesis time and temperature and the degree of solubility of the constituents in the salt highly affect the quality and characteristics of the synthesized material. For the first-time, molten-salt method is used to prepare highly photocatalytically active CuOTiO2 NCs. Photocatalytic H2 generation using the optimized CuO-TiO2 NCs in the presence of aqueous-glycerol solution showed 27.7 and 9.0 folds superior rate of H2 production compared to pristine TiO2 NPs under Light Emitting Diode (LED) and solar irradiation respectively. The CuO/TiO2 composite material prepared by the molten salt method has showed excellent photocatalytic activity and stability. Based on experimental studies and characterization results a plausible reaction mechanism was drawn which explain the co-catalytic role of CuO for superior H2 generation through effective utilization of photo generated charge carriers especially electrons (e-) for reduction of H+ ions into H2.

2. EXPERIMENTAL 2.1 Materials and synthesis method Photocatalyst grade titanium dioxide nanoparticles (TiO2 P-25) composed of anatase 80%, rutile 20%, surface area = 51 m2.g-1 and average particle size of 27 nm was procured from Degussa Corporation, Germany. Sodium Chloride (NaCl), Copper Nitrate tri-hydrate (Cu(NO3)2.3H2O) and Glycerol of analytical grade were procured from Merck, India. Double distilled water was used for washing during material synthesis and photocatalytic experiments. 4


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The molten-salt method was adopted for synthesis of CuO-TiO2 nanocomposite. The schematic illustration of CuO-TiO2 was shown in Scheme 1. First, the TiO2 physical mixture was prepared as follows, 0.5 g of NaCl crystals was manually ground (10 min) into fine powder using pestle and mortar followed by addition of TiO2 P-25 (0.5 g) nanoparticles and again ground well (10 min), the resulting mixture was baked at 150 °C for 12 h. Similarly Cu(NO3)2 physical mixture was prepared by mixing 0.5 g of NaCl fine powder with different amount of x% Cu(NO3)2 fine powder (x = 0, 0.5, 1.0, 1.3, 1.5, 1.7 and 2.0 %). Finally, TiO2 and Cu(NO3)2 physical mixtures were transferred into alumina crucible and pressed to form bottom and top layers respectively and subjected to calcination at 500 °C for 4 h. Thus obtained CuO-TiO2 nanocomposite materials washed with distilled water to remove NaCl and the product was dried at 80 °C for 12 h. The nomenclature of the photocatalysts used in the present study was shown in Table 2. 2.2. Characterization techniques The synthesized nanocomposite photocatalysts were thoroughly characterized by using a wide range of techniques. XRD patterns were recorded using a Rigaku Miniflex 600 diffractometer. Transmission electron microscopy (TEM) and high resolution (HR) TEM measurements were carried out by using a JEOL JEM 2100 electron microscope at 200 keV. Xray photoelectron spectra (XPS) was recorded on a Kratos AXIC165 equipped with Mg Kα radiation and hemispherical analyzer Phoibos 150 with 3D-DLD detector (SPECS). The optical properties were carried out by DRS UV-Visible spectroscopy (PerkinElmer, UV Lambda-600, USA). The photoluminescence property of the materials was analyzed using Spectro Fluoremeter (Fluora MAX 4P) at 320 nm excitons wavelength.

2.3 Photocatalytic Experiments The photocatalyst (5 mg) powder was suspended in 50 mL of 5 vol.% glycerol aqueous solution in a Quartz reactor (capacity: 185 mL) sealed with gas leak-proof special rubber septum. Now, the heterogeneous reaction mixture magnetically stirred well (@320 RPM) for adsorptiondesorption equilibrium, in order to maintain inert atmosphere, the reactor was degassed and filled with nitrogen gas (carrier gas in gas chromatograph). Figure 1 displays LED photocatalytic experimental set-up for H2 production. The reactor was kept on single point magnetic stirrer at 5 cm away from LED lamp (λ = 365 nm, Power = 20 W, 5



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Intensity = 1,500±100 lx) and stirred magnetically to maintain heterogeneous mixture. The gas generated was analyzed at periodic intervals. Figure 2 shows photocatalytic experimental set-up under natural solar light irradiation. Here multi-spin magnetic stirrer was used to conduct four parallel experiments during 10 am and 3 pm (average solar light intensity 1.15 ±0.10 x 105 lx). Here four quartz reactors individually clamped with burette stand were placed on multi-spin magnetic stirrer and tilted at an angle of 45 ° for maximum utilize of light photons. An off-line gas chromatograph equipped with TCD detector (Shimadzu GC-2014 with Molecular Sieve-5A column) was used for quantification of H2 gas.

3. RESULTS AND DISCUSSION 3.1 Characterization results Molten salt method was adopted for synthesis of CuO-TNP photocatalysts. The detailed synthesis procedure was provided in the experimental section. Figure 3 displays XRD pattern of TNP, CuT-0 and CuT-4 materials. Two major peaks at 2θ = 25.4 and 27.5° corresponds to (101), (110) crystal planes of anatase - rutile TiO2 well coincidence with standard JCPDS values22. Diffraction peaks of Cu were absent in CuT materials. This may be due to relatively low concentration of Cu, small crystallite sizes or amorphous nature of CuO41. The presence of Cu was confirmed by using XPS analysis and HR-TEM data (vide infra). With increasing Cu content no change in TiO2 phase was observed which indicates that support material TiO2 scarcely undergone phase transformation during synthesis. The TiO2 crystallite sizes (40±3 nm) in CuO-TiO2 nanocomposites were calculated from XRD data by using Scherrer formula44. The crystallite size increased from 36 to 43 nm for the samples abbreviated as CuT-0 and CuT-4. The lack of XRD peak shift as observed in earlier studies for Cu doped TiO2 material44, suggest that Cu occur as CuO phase perhaps in the form of CuO-TiO245. Figure 4(a) displays TEM images of CuT-4 nanoparticles having spheres and hexagonal shapes with varying diameters from 10 to 40 nm, with an average particle size of 23.8 nm (see Figure 4(b)). Figure 4(c) shows HR-TEM image, which confirms crystalline lattice fringes of about 0.357 nm and 0.327 nm corresponds to the distance between two (101) and (110) planes of anatase and rutile TiO2 respectively. Whereas Figure 4(d) shows HR-TEM image of CuT-4, here 6


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deposition of fine CuO nanoparticles are observed. Figure 4(e) clearly confirms dark spots of CuO deposition with particle size ranging from 2.2 to 4.6 nm that is quantum dots range. Figure 4(f) depicts SAED pattern of CuT-4 where the rings corresponding to d-spacing values of 0.373 nm, 0.351 nm, 0.270 nm matches with anatase, rutile -TiO2 and CuO respectively. Hence HRTEM, SAED patters confirms the existence of CuO in TiO2 composite photocatalyst. Similar observations were reported in literature46. Surface elemental analysis and chemical oxidation state of Cu in prepared photocatalysts (CuT-0, CuT-4) were carried out by X-ray photoelectron spectrophotometer (XPS) which is displayed in Fig. 5. The survey spectra of CuT-0 and CuT-4 materials were shown in Figure 5(a), it exhibited prominent peaks corresponding to Ti 2p, O 1s, Cu 2p, and C 1s species, indicating that prepared sample contains all elements of CuO-TiO2 composite. The O 1s spectra in Figure 5(b) exhibited a prominent peak with BE value of 529.9 eV. Ti 2p spectra in Figure 5(c) shows binding energies at 458.8 and 464.5 eV, respectively indicates Ti 2p3/2 and Ti 2p1/2. The splitting between Ti 2p3/2 and Ti 2p1/2 is 5.7 eV which indicates normal state of Ti+4 in the sample47. The weak peaks at 932.6 and 952 eV in Figure 5(d) are identified as Cu 2p3/2 and Cu 2p1/2 respectively, indicating the presence of Cu+2 in CuT-450,51,53. This is more reasonable for CuO to be found at lower binding energy values (932-933 eV) for lower wt% of Cu loading. Since Chary et al55. also examined the CuO modified TiO2-ZrO2 catalyst and found that the variation of B.E value for CuO peaks appears at lower values around 932.2 to 932.9 for low wt% of Cu from 1 to 2.6 wt%, similarly for higher wt% of Cu from 5 to 21 wt%, the CuO peak appeared at higher BE values from 933.5 to 934.2 eV. Hence it is very clear that the B.E. values decreases at lower amount of CuO on catalyst surface. Hence the optimized catalyst consists of Cu in +2 state. Absence of copper is confirmed in CuT-0 material. The binding energy (BE) values are in good agreement with those of CuO24,43,46,47. Based on the XPS data and HR-TEM (vide infra) it is attributed that CuO clusters are distributed on the surface of the TiO2 nanoparticles instead of doping into the TiO2 lattice. Moreover as the optimized material was calcined at higher temperature (500 ⁰C), there was more chance for CuO instead of Cu2O or metallic Cu⁰.

As

shown in Figure 5(c), the Ti 2p spectrum of CuT-0 and CuT-4 shows no significant change in BE values. It indicates that Cu impregnation does not affect the chemical state of TiO2 crystal

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structure. In contrast HR-TEM, SAED, and the measured BE values of Cu in CuT-4 reveals the presence of CuO on TiO2 surface. Figure 6 shows UV-Visible absorption spectra and corresponding band gap energies (inset) of TNP, CuT-0 and CuT-4 photocatalysts. All the photocatalysts exhibited strong absorption at >390 nm, associated with excitation of electrons in the 2p orbital to the Ti 3d level. Compared to TNP the absorption onset of CuT materials showed red shift towards higher light absorption, giving rise to slight decrease in the band gap to 3.03 eV (TNP: 3.13 eV), as calculated by Kubelka-Munk function (see Figure 6 inset) which could be attributed to the presence of CuO. Similar results were shown in literature as well. For example, Tsai et al48 prepared Cu doped TiO2 nanoparticles with different amount of Cu (0 - 5.0 wt%) and observed red shift in the absorption onset towards higher wavelength region compare to un-doped TiO2. Similarly Kumar et al42,49 reported that Cu2O/TiO2 nanoparticles shows red shift in the onset for all Cu deposited samples when compared with pristine TiO2 nanostructures. The deposition of CuO on TiO2 in fact should not change the band-gap of TiO2 but just enhance the absorption caused by CuO in the visible region. PL analysis gives information about opto-electrical characteristics of semiconductor materials, such as defects, surface oxygen vacancies, resulting electron/hole pair separation efficiency and charge carrier lifetimes of semiconductors. Figure 7 displays photoluminescence (PL) spectra of TNP, CuT-0 and CuT-4 photocatalysts. PL peak intensities of the prepared materials are in the following order CuT-4 < CuT-0< TNP. The PL peak intensity of CuT-4 decreased compared to TNP and CuT-0 which reveals that the excitons recombination is minimized in CuT-4 due to prolonged life time of charge carriers. The PL peaks at 440, 451, 468 are attributed to band edge free excitations in anatase TiO2 and the weak signal at 410 confirms the presence of low quantity of rutile in TiO2 based materials as detected in XRD. On the other side, the peaks at 483 and 493 are assigned to the excitonic PL emission of TiO2 which is mainly resulted from surface oxygen vacancies50-52. This broad emission is drastically decreased after deposition of CuO on TiO2 (CuT-4). This may be due to co-catalytic role of CuO, which takes excited charge carriers (e–) from TiO2 and reduces H+ into H2. The quenching in PL spectra is a

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good measure of the extent of excitons separation. The electron-hole separation leads to effective utilization of charge carriers for photocatalytic reactions. 3.2 Photocatalytic activity results The photocatalytic activity of the prepared catalysts was examined under LED and natural solar light irradiation. The detailed photocatalytic experimental procedure was explained in experimental section. The important synthesis parameters viz., amount of copper loading and calcination temperature of CuT catalysts has been studied in detail and correlated with H2 generation under light irradiation18,42. Figure 8(a) depicts the volume of H2 production for 5 h of irradiation time using different amount (wt.%) of copper loading from CuT-0 to CuT-6 catalysts and compared with pristine CuO and TiO2 (TNP). It is apparent that the amount of H2 production increases with time for all 7 catalysts but with respect to copper loading gradually to reach the maximum and then decreases. Among the catalyst tested CuT-4 displayed highest activity which is 27.7 folds higher when compare to TNP. This result clearly indicates that the amount of CuO quantum dots on TiO2 nanoparticles dramatically influences the H2 generation. This enhanced activity was due to effective utilization of photo generated charge carriers from TiO2 by CuO for reduction of H+ into H2. It is observed that pristine CuO failed to display activity for H2 generation under similar experimental conditions. In Figure 8(a), we observed that CuT-0 (0% Cu) displayed enhanced H2 production due to improved crystallinity than TNP. Since crystallinity of the materials shows significant role to improve the H2 generation rate due to efficient separation of charge carriers57. Similarly molten salt method facilitates to improved crystallinity of the synthesized material29, 34. The XRD pattern of TNP, CuT-0 was shown in supplementary information in Figure S1, where the intensities of anatase, rutile phases of TiO2 in TNP and CuT-0 showed differences, as intensities of both phases in CuT-0 are higher than TNP. Further effect of calcination temperature on photocatalytic activity was studied by calcination of CuT-4 at three different temperatures viz., 400, 500 and 600 °C. The CuT-4 material calcined at 500 °C showed the highest photocatalytic activity of 20.36 mmol. h-1. g-1cat as shown in Figure 8(b). The higher activity acquired for CuT-4 is ascribed due to the optimized temperature that created the minimum defect sites which leads to fast transfer of charge carriers to participate in the photocatalytic reactions for efficient rate of H2 production. In addition to that XRD patterns 9



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of CuT-(a), CuT-4 and CuT-4(b) reveals that the rutile phase increases for the sample clacined at 600 ⁰C in supplementary information in Figure S2. Also for the other samples the anatase to rutile ratios has been changing. It reveals that the anatase and rutile percentages of the corresponding TiO2 material can be influenced by calcinations temperatures and thus intern influence the activity of the catalysts. Here for CuT-4(a) has very less intense rutile peaks and CuT-4(b) has very high intense rutile peaks when compare to Cut-4. Hence from this study we have concluded that the intensity of rutile peak can also alter the photocatalytic activity of the catalyst. Here the CuO so obtained can act a co-catalyst which reduced the charge carriers recombination and improved the H2 generation efficiency. Further the effect of NaCl on photocatalytic activity in the molten salt method was compared as well. Here the CuT-4 showed 5.4 folds higher H2 production than the catalyst without NaCl (CuT-4c) in molten salt method see Figure 8(c). This experiment clearly shows that NaCl also strongly affects the photocatalytic activity. This could be due to the chemical reactions taken place by molten salt, the NaCl during synthesis. The higher crystallinity acquired by the catalyst in molten salt process favors the fast transfer of generated charge carriers for catalytic reactions with minimal defect sites. For example Kim et al56 studied the effect of NaCl in Cu2ZnSnS4 (CZTS) films. The authors claimed that NaCl improved the crystallinity of the CZTS films and showed the better performance. The time-on-stream experiments were conducted under LED light irradiation continuously for 75 h to estimate the stability of optimized photocatalyst (Figure 8(d)), quantifying H2 gas at every 5 h intervals. It is clearly observed that the CuT-4 photocatalyst prepared by molten salt method shows increasing rate of H2 generation with irradiation time depicting that optimized photocatalyst acquired excellent photo stability for longer hours, at least 25 h of irradiation time. Further irradiation up to 50 h, the rate was 13.35 mmol. h-1. g-1cat. For 75 h of irradiation time the rate of H2 generation activity was still 10.79 mmol. h-1. g-1cat. The decrease in rate after 50 h is explained as follows: the intermediates formed by decomposition of glycerol have lower number of hydrogen in its structure and poor adsorption-desorption property hence volume of H2 evolution decreases for continuous light irradiation. Alternatively, the generated H2 gets saturation due to pressure in closed vessel; hence it affects the volume of H2 production. Further to conform any structural changes undergone by the catalyst after stability, we taken XRD for the same catalyst after time-on-stream experiments and compared the XRD data of the catalyst 10


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before stability. Interestingly the CuT-4 does not undergone any structural changes, the crystallinity remained same. By this we conclude that the CuT-4 is a stable catalyst for continuous H2 generation for long hours. The XRD data of CuT-4 catalyst before and after stability was shown in Figure 8(e). Similarly the effect of Cu loading was also studied under solar light illumination. For this purpose different wt % of Cu loaded TNP (0 to 2.0 wt% Cu) were tested and quantified the H2 evolution rates using GC as shown in Figure 9(a). Interestingly the optimum wt% of Cu under solar light was varied compare to the optimum wt% of Cu under LED light irradiation. Under UV LED light CuT-4 photocatalyst was optimized whereas under solar light the best performance was obtained from CuT-3 photocatalyst which showed about 21.7 mmol. h-1. g-1cat. The change in performance of the optimized catalysts CuT-4 and CuT-3 under LED and solar light is ascribed to monochromatic and UV-visible spectrum radiation respectively. Since LED emits photons of single wave length (365 nm) with intensity of 1500±100 lx, the TiO2 is more active and generates sufficient charge carriers, hence more CuO species are required to trap the excited electrons for reduction reactions, hence CuT-4 showed optimum activity. Whereas in case of solar light, it emits wide spectrum of light photons with high intensity which is about 1.15 ±0.10 x 105 lx, so that less number of CuO species are sufficient to trap the generated electrons for H2 generation, hence CuT-3 performed higher activity. Recyclability experiments were also conducted under solar light in order to estimate the stability of the prepared photocatalyst. For this purpose, the optimized photocatalyst (CuT-3) was tested for 3 recycles as shown in Figure 9(b). In each cycle the generated gas sample was quantified using GC. The H2 generation rate was almost same for 3 recycles. There is little bit decrease in activity after the 3rd recycle. The good stability acquired by CuT-3 photocatalyst could be due to fast transfer of charge carriers from TiO2 to CuO for H+ reduction reaction and hole scavenging by the glycerol molecules. Plausible reaction mechanism that takes place during photocatalytic reaction was proposed based on the experimental results (Figure 10), more specifically we carried out the experiments under UV-vis light irradiation (Xe lamp). For this purpose we did photocatalytic experiments under 300 W Xe lamp ORIAL instruments (New Port Co., Ltd, USA) without filters and also with two 11



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filters. One is using UV cut-off filter (FSQ-GG400), λ >400 nm and the other is using with band pass filter (FSQ-KG5) λ >400 nm and λ 400 nm)

band

pass

(400

CuO Quantum Dots Decorated TiO2 Nanocomposite Photocatalyst for

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