Synergistic Plasmonic and Upconversion Effect of (Yb,Er) NYF-TiO2

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C: Energy Conversion and Storage; Energy and Charge Transport

Synergistic Plasmonic and Upconversion Effect of (Yb,Er) NYFTiO2/Au Composite for Photocatalytic Hydrogen Generation Anushree A. Chilkalwar, and Sadhana Suresh Rayalu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05480 • Publication Date (Web): 01 Nov 2018 Downloaded from http://pubs.acs.org on November 9, 2018

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The Journal of Physical Chemistry

Synergistic Plasmonic and Upconversion Effect of (Yb,Er)NYF-TiO2/Au Composite for Photocatalytic Hydrogen Generation. Anushree A. Chilkalwar ab and Sadhana S. Rayalu ab*

a.

Environmental Materials Division, CSIR-National Environmental Engineering Research Institute, Nehru Marg, Nagpur-440020

b. CSIR- Network of Institutes for Solar Energy (CSIR-NISE), India

*Corresponding Author: Dr. Sadhana S. Rayalu Chief Scientist and Head, Environmental Materials Division, CSIR-National Environmental Engineering Research Institute (CSIR-NEERI) Nehru Marg, Nagpur (M.S) 440020, India. Email Id: [email protected] Telephone: +91-712-247828; Fax: +91-712-247828

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Abstract: This article deals with the synthesis of broad band absorption material which extends absorption response upto infrared region by using plasmonic metal nanoparticle and upconverting host. The synergistic effect of surface plasmon resonance of Au nanoparticle and upconversion property of (Yb,Er)NYF significantly enhances broad band absorption and photogenerated charge generation and transfer. Au as a plasmonic nanoparticle on (Yb, Er) NYF-TiO2 plays multiple role of harvesting solar energy and exhibiting excellent catalytic activity. The synthesised hetero-structure (Yb, Er) NYF-TiO2/Au tested for donor assisted photocatalytic hydrogen generation shows steady and remarkably high hydrogen generation rate of 350 mol h-1 under solar AM 1.5 which is approximately 3.1 times higher than Au/TiO2 (Degussa P-25). Moreover, a simple synthesis procedure has been rationally designed (Yb, Er) NYF-TiO2/Au which shall provide pathway for futuristic design of plasmon enhanced broad band absorption material.


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Introduction: Conversion of solar energy to chemical energy is a captivating challenge for many researchers working in this field across the globe.1, 2 One of the critical challenge is to develop photocatalyst with efficient light harvesting and utilization capability of solar energy. Uptil now various photocatalyst have been developed and the focus is on absorption in UV and visible light through reduced band gap of semiconductor 3-5, semiconductor with narrow band gap6, Z- scheme pattern photocatalyst7-12, and decoration of dyes etc.13-15 However, utilization of IR energy which accounts almost half (44%) of the total solar spectrum is in its infancy. In recent years, the enthusiasm for lanthanide doped material has emerged as versatile and ideally suited option for application in bioimaging 16, 17, lighting18, solar energy conversion 19, and biosensing 20. It is still remains an issue in the field of conversion of solar energy specific in photocatalytic water splitting research field.21-23 Therefore, it is important to design an efficient photocatalyst that harnesses wide range of sunlight. Upconversion luminescence (Ln3+-doped) has the unique potential to convert the low photonic energy of two or more photon energy into one photon with higher energy.24 The utilization of IR light has been achieved by developing various new kind of material for enhancing upconversion (UC) emission of material by surface plasmon enhancement due to metal nanoparticles. The surface plasmon resonance arising from resonance energy generated when photoinduced frequency of the incident light parallel to the collecting coherent oscillation which is confined to conducting material surface.25,26 Coupling of localized surface resonance plasmon occurs due to free electron oscillation around metal nanoparticle with rare earth doped UC ion with consequent emission of light excitation for nanoparticle. Thus LSPR of metal nanoparticle has been proposed as a suitable approach for enhancing the efficiency of UC emission.27, 28 In recent years, investigation of LSPR of nanoparticle on upconversion emission has



been documented. LSPR enhanced the UC red and green emission intensity by a factor of 2.3 and 3.7 and was first reported by the group of Yan.29 Recently, group of Kagan demonstrated both the possibilities of LSPR to achieve an enhancement in UC as well as matching plasmonic resonance with absorption wavelength or the emission wavelength.30,31 The SPR effect shows remarkable improvement for conversion of solar energy to chemical energy because of excellent light harvesting and electromagnetic field concentrating benefits. Particularly Forster resonant energy transfer (FRET) and Plasmon resonance energy transfer (PRET) are two kind of non-radiative dipole-dipole energy transfer may occur through the increased local electromagnetic field on the surface in specific for two dipoles with overlapping spectra.32-37 In this present study, we developed Au NPs loaded on NaYF4: Yb3+, Er3+-anatase TiO2 represented as (Yb, Er) NYF-TiO2/Au, which can harness sunlight by tapping UV, Visible and NIR photon. This excellent combination of plasmonic and upconversion allows efficient energy and charge transfer between nanostructure. Moreover, this combination largely decreases the e- and h+ pair recombination. The photocatalytic hydrogen evolution rate of (Yb, Er) NYF-TiO2/Au was examined, and significant improvement was observed on TiO2 (Degussa P-25) as well as with (Yb, Er) NYF-TiO2, and (Yb, Er) NYF samples. The samples were evaluated for hydrogen generation at uniform operating conditions with appropriate cut-off filters The (Yb, Er) NYF-TiO2/Au composite was tested in various cut-off filters such as, UV cut off (400-1000 nm), Visible cut-off ( 400 nm,  800 nm) and IR cut-off (200-700 nm) to have better understanding of mechanism of the photocatalyst. In addition, to hydrogen evolution rate and catalyst properties, the present work also provides physical insights that can furnish the plasmon enhance upconversion luminescence of catalyst.


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Experimental section: Synthesis of (Yb, Er) NYF-TiO2: NaYF4, Er3+, Yb3+ i.e. (NYF) co-doped with anatase TiO2 were synthesized by hydrothermal method. Briefly, 0.05 g of NYF procured from Merck was added to isopropanol and sonicated for 20 min at room temperature (Slurry A). Then Diethylene amine was added to slurry A and stirred for 15 min, followed by addition of 2 mL of titanium isopropoxide Ti (iso). The reaction mixture was transferred into stainless steel autoclave with Teflon lining reactor and kept at 200oC for 24h. After 24h, autoclave was cooled to room temperature, and then the precipitate was separated by centrifugation, which was further washed with ethanol thrice. The sample was air-dried at 80oC for 12h. Finally, the dried material was calcined at 350oC at a heating rate of 1oC min-1 for 2h and designated as (Yb, Er) NYF-TiO2. Synthesis of (Yb, Er) NYF-TiO2/1%Au: In 750 mL Inner radiation vessel, 500 mg of (Yb, Er) NYF-TiO2 was taken, and 380 mL of DI water was added with 5% ethanol (Merck). In the same vessel, 1% of Tetra chloroauric (III) acid HAuCl₄*3H₂O was added. Tetra chloroauric (III) acid was procured from Merck. The reaction mixture was exposed to 450W medium pressure mercury light for 1h. After 1h, the reaction mixture was filtered and centrifuged at 5000 rpm and then washed with DI water and ethanol three times to remove impurities. The sample was designated as (Yb, Er) NYF-TiO2/Au and stored in the oven for drying under vacuum at 60oC for 12h. The recovered sample was evaluated for hydrogen generation reaction and tested for characterization.

Characterization:



Powder X-ray diffraction of NYF-TiO2/Au was performed on desktop Rigaku Miniflex-II equipped with 15mA monochromator and Cu k at 30kV radiation source. A transmission microscope (TEM, Techai 3010 at 3000 kV) samples were prepared by colloidal suspension in Isopropanol and drop coated on a copper grid. XPS (X-ray Photoelectron Spectroscopy) measurements to determine chemical states of element and surface chemical composition (XPS, Prevac, Holland) was carried out by system equipped with monochromatic source Al K anode (1486.6 eV). UV-Vis-NIR absorption was conducted on UV-Visible diffuse reflectance spectroscopy (UV-DRS, Agilent).

Photoluminescence (PL) spectra were measured using

Edinburgh, FLS 980 Fluorescence spectrometer at room temperature. The FLS 980 is a computer controlled modular spectrofluorometer which measures emission spectra. Photocatalytic Hydrogen Generation: Photocatalytic hydrogen evolutions were carried out in quartz enclosed gas circulation system. The prepared material 500 mg of photocatalyst were dispersed in 400 mL aqueous solution containing 5% of Ethanol. The solution was maintained at room temperature with continuous stirring. During evolution, high purity Nitrogen (N2) gas was purged. This reaction mixture was further exposed to Solar AM 1.5 with filter UV-Cut off, visible cut off, IR- cut off and under full spectrum. The evolved hydrogen gas was determined continuously using a Shimadzu- 2014 gas chromatograph.

Result and Discussion: X-ray diffraction analysis:


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The powder X-ray diffraction (pXRD) patterns (Figure 1) reveals the phase formation of (Yb, Er) NYF-TiO2 and (Yb, Er) NYF-TiO2/1%Au. All the diffraction peak of (Yb, Er) NYF-TiO2 were well indexed with anatase TiO2 and addition of (Yb, Er) NYF during hydrothermal synthesis did not reveal any impurity peak. Photochemical deposition of Au on (Yb, Er) NYF-TiO2 indicated that the synthesized nanosphere was in a single phase. No Au diffraction peak was observed as concentration was low and was beyond detection limit. According to JCPDS 12-1272, the entire diffraction peak index of anatase TiO2 phase with lattice constant a =3.785 Å and c = 9.513 Å respectively were identified. The diffraction peaks at 2= 25.23o, 36.91o, 37.77o, 48.03o, 53.74o, 55.16o, 62.16o, 68.80o, 70.72o, and 75.13o can be assigned to (101), (103), (004), (200), (105), (211), (104), (116), (220), (215) and (306) faces of TiO2. The diffraction peaks at 17.20o, 30.03o, 30.76o, 39.61o, 43.42o, 53.19o, 53.68o and 86.43o can be assigned to (100), (110), (101), (111), (201), (300), (211) and (321) faces of (Yb, Er) NYF and matches well with commercial NYF, according to JCPDS 16-0334.



Figure 1: Typical pXRD pattern of anatase TiO2, commercial (Yb,Er) NYF, (Yb,Er) NYF- TiO2 and (Yb,Er) NYF-TiO2/1%Au. (* refer to Anatase TiO2 and # refer to (Yb,Er) NYF).

Figure 2 shows pXRD patterns of (Yb, Er) NYF-TiO2 and (Yb, Er) NYF-TiO2/Au enlarged in the range between 24o and 27o. Careful comparison reveals that the peak of (101) of anatase TiO2 is slightly shifted towards lower angles, which confirms the deposition of Au nanoparticle on lattice structure of (Yb, Er) NYF-TiO2. The shift to low diffraction angle after deposition of Au nanoparticle reveals strong interaction between Au nanoparticle and (Yb, Er) NYF-TiO2.38 The full width half maxima of anatase TiO2 diffraction peak was calculated by using Scherrer equation. The average crystal size of anatase TiO2 (101) peak of pXRD of (Yb, Er) NYFTiO2/1%Au is about 7.8 nm, which was found to be in good agreement with TEM discussed subsequently.


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Figure 2: pXRD diffraction peak of (101) anatase TiO2, (Yb, Er) NYF- TiO2 and (Yb, Er) NYFTiO2/1%Au.

SEM: SEM image (Figure 3) of (Yb, Er) NYF-TiO2/1%Au reveals mono-dispersed spherical particles with a diameter of about 1-3 m. After chemical deposition of Au on (Yb, Er) NYF-TiO2 catalyst, it can be clearly seen that the morphology of the catalyst is a homogeneous grey elliptic spheres with thin layer of particles on it. This proves uniform deposition of Au on the surface as compared to bare (Yb, Er) NYF-TiO2. Results also reveal that Au nanoparticles exist at the interface of (Yb, Er) NYF-TiO2 structure.



Figure 3: SEM images of (a, b) (Yb, Er) NYF- TiO2 and(c, d) (Yb, Er) NYF-TiO2/1%Au.

TEM and EDA: TEM images reveal larger particle of (Yb, Er) NYF-TiO2 covered with thin layer of Au nanoparticle to form a nanocomposite structure. (Figure 4) Also, TEM image demonstrates intimate metal-semiconductor interface ((Yb, Er) NYF-TiO2/Au) in the heterostructure. The average particle size of uniformly deposited Au on the surface is about 5-9 nm. The interplaner distance (d) of small particle is about 0.347 nm, which is consistent with (101) lattice spacing of anatase TiO2. The interlayer spacing of 0.267 nm was observed, which was attributed to the lattice plane of (200) of cubic NaYF4. The elemental mapping of (Yb, Er) NYF-TiO2/1%Au reveals the


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presence of Au, Na, Y, F, Ti, O, Er, Yb elements in the synthesized sample as illustrated in Figure S1

Figure 4: TEM Image of (a, b, c) (Yb,Er) NYF-TiO2/1%Au .

XPS: Furthermore, to investigate successful incorporation of (Yb, Er) NYF and Au in TiO2, the microstructure was analyzed by X-ray photon spectroscopy (XPS). Figure 5 (a) shows the full spectrum of (Yb, Er) NYF-TiO2/1% Au which reveals the presence of Ti, O, Au, Na, Y, and F elements. The Au 4f spectra is shown in Figure 5 (b) which exhibits Au 4f5/2, and Au 4f7/2 signals at 83.4 eV and 87.0 eV. This is assigned to the B.E value of the metallic Au0 state.39-41 Figure 5 (c) reveals peak at 458.7 eV and 464.3 eV which is assigned to Ti2p3/2 and Ti2p1/2 respectively. This is in excellent agreement with the binding energy (B.E) of Ti4+ in anatase TiO2.5, 42 Figure 5 (d) shows the asymmetric peak of O1s that indicates that oxygen fitted into two peaks with one located at 530.1 eV for characteristic peak of crystal lattice oxygen Ti-O-Ti.5 While other located at 530.6 eV is attributed to surface hydroxyl radical Ti-OH.42, 43



Figure 5: XPS spectra of photocatalyst (Yb,Er) NYF-TiO2/1%Au. UV-DRS: Figure 6 (a and b) illustrates the UV-DRS spectra, a visual emitting map of (Yb, Er) NYF-TiO2, (Yb, Er) NYF-TiO2/Au and P-25. P-25 shows absorption near 400 nm and (Yb, Er) NYF-TiO2 shows absorption band starting from 500 nm towards shorter wavelength, (Yb, Er) NYF exhibits only one absorption spectrum from 910-1000 nm corresponding to Yb3+ ions transition of 2F7/22F

5/2 in

(Yb, Er) NYF host. After chemical deposition of Au NPs on (Yb, Er) NYF-TiO2 surface,

an obvious light absorption band in the visible range between 500-600 nm was observed which is


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the characteristic peak of SPR.44 The association of different wavelength absorption feature peaks in single hybrid architecture allows broadband photoresponse from UV-Vis-NIR in microspheres.

Figure 6: UV-DRS spectra of (a) TiO2 (P-25) and (b) (Yb, Er) NYF-TiO2, (Yb, Er) NYFTiO2/1%Au and commercial (Yb, Er) NYF.

Photoluminescence: The photoluminescence spectroscopy (Figure 7) further illustrates the photon trapping effect through the upconversion process of (Yb, Er) NYF which transferrs photon energy to highly UVabsorbing TiO2 and visible light absorbing Au NPs. When (Yb, Er) NYF-TiO2 was excited at 980 nm (NIR) light as demonstrated in PL spectra (Figure 7) upconversion emission spectrum yields four emission band i) UV region at 408 nm shows blue emission 2H9/2-4I15/2. ii) Green emission in the visible region two peak at 520 nm and 540 nm corresponding to the 2H11/2-4I15/2, 4S3/2-4I15/2 transition and iii) Red emission ascribing to the 4F9/2-4I15/2 transition of Er3+ ions at 654 nm. After deposition of Au NPs, it was observed that SPR peak of Au NPs overlaps with green emission band peak at 520 nm-540 nm revealing efficient energy transfer to SPR of Au NPs from the upconverting host (Yb, Er) NYF.



Figure 7: Upconversion Emission spectra of (Yb, Er) NYF-TiO2 and (Yb, Er) NYF-TiO2/1%Au.

Photocatalytic hydrogen generation performance: The photocatalytic activity of catalyst (Yb, Er) NYF-TiO2/Au, (Yb, Er) NYF-TiO2, Au/TiO2 (P25) and TiO2 (P-25) was investigated by accessing the photocatalytic activity under separate irradiation sources of UV-Vis-NIR, UV light cut-off filter (400-1000 nm), Visible light cut-off filter ( 400 nm,  800 nm) and IR light cut-off filter (200-800 nm) using solar simulator AM 1.5G (200-1000nm). Also in order to understand the mechanism of (Yb, Er) NYF-TiO2/Au different composite were prepared by varying Au concentration and evaluated for hydrogen generation and comparison was done with (Yb, Er) NYF-TiO2, Au/TiO2 (P-25) and commercial TiO2 (P-25). It is worth mentioning that without illumination in dark condition no hydrogen peak was observed. The requisite concentration of Au on the catalyst was achieved by photodeposition process as


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mentioned earlier. The weight of (Yb, Er) NYF-TiO2 was kept constant and percent loading of Au was varied from 0 to 1.5%. The synthesized photocatalyst was tested for photocatalytic hydrogen evolution. It was observed that hydrogen evolution rate was very low for 0.25%, 0.5%, 0.75%, 1.25% and 1.5% Au on (Yb, Er) NYF-TiO2 composite (Figure S2). Also, without Au deposition on (Yb, Er) NYF-TiO2 shows no hydrogen evolution in photocatalytic reaction. Intermediate deposition of 1wt% Au shows highest hydrogen evolution rate. The decrease in hydrogen evolution rate above 1% Au was attributed to reduction in catalytic active sites of titania or increased particle size of Au with consequent decrease in activity. The photocatalyst (Yb,Er) NYF-TiO2/1%Au generates hydrogen to the tune of 191.5 µmol h-1 in solar AM 1.5 which is significantly higher than the other light cut-off filter used. Figure 8 shows photocatalytic activity by hydrogen generation of (Yb, Er) NYF-TiO2/Au in term of hydrogen evolution rate as investigated under different illumination sources and ethanol as a sacrificial donor. Under UV cut-off (400-1000 nm) illumination photocatalyst was irradiated with visible and IR light. Catalyst show low hydrogen evolution rate as it is well known that TiO2 is poor in absorbing visible light. In this situation integration of Au NPs possibly injects electron into TiO2 due to SPR induced exciton formation. This observation is consistent with some earlier reported finding.45-47



Figure 8: Phtocatalytic hydrogen generation of (Yb,Er) NYF-TiO2/1%Au different variation sources

The hydrogen generation rate of the photocatalyst was under visible light cut-off (400 nm, 800 nm) irradiation implying combined illumination of UV and IR light the plasmonic effect of Au NPs was ruled out as SPR effect of Au NPs cannot generate an exciton. In this case TiO2 absorbs light, generates electrons and hydrogen evolution is facilitated at Au NPs site. In addition, IR light is absorbed by NYF and UV emission takes place which further enhance hydrogen evolution rate. 45, 48 In IR cut-off (200-800 nm) irradiation, wherein both UV and visible light induced photocatalytic performance is prevalent in (Yb, Er) NYF-TiO2/1%Au. In this case the mechanism is the same as mentioned above except for i) No UV emission by NYF and ii) absorption of visible light by Au. This illustrates the importance and significant contribution of NYF in photocatalytic reaction.


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Hydrogen evolution rate with respect to time under solar AM 1.5 (200-1000 nm) illumination for the two catalyst (Yb, Er) NYF-TiO2/1%Au and (Yb, Er) NYF-TiO2 is presented in Figure 9 and Table 1. The hydrogen evolution rate of (Yb, Er) NYF-TiO2/Au is nearly 61 times higher than (Yb, Er) NYF-TiO2. Similarly, on comparision of hydrogen evolution rate of 1% Au deposited on TiO2 (P-25) with (Yb, Er) NYF-TiO2/1%Au, (Figure 9 (b)) enhancement was observed by a factor of 3.2 thus illustrating the enhanced efficiency of (Yb, Er) NYF-TiO2. The enhancement is more pronounced (by a factor of 400) on comparing (Yb, Er) NYF-TiO2/1%Au with P-25 which showed negligible hydrogen evolution rate. Thus (Yb, Er) NYF-TiO2/1%Au, is providing to be efficient upconverting material. 46

Figure 9: Comparison study of photocatalytic hydrogen evolution rate between (a) (Yb,Er) NYFTiO2/1%Au and(Yb,Er) NYF-TiO2 (b) (Yb,Er) NYF-TiO2/1%Au and 1% Au/TiO2(P-25) under solar simulator.

In order to check the stability of hybrid microsphere Figure 9 (a), catalyst (Yb, Er) NYF-TiO2/1% Au was subjected to solar AM 1.5 for 12h. Almost steady hydrogen evolution rate was observed proving its immense potential as efficient, practically feasible and stable photocatalyst.



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Upconversion phosphors (Yb, Er) NYF-TiO2 when combined with TiO2 and Au thus illustrated broad band absorption with consequent increase in hydrogen evolution rate. The results are summarized in Table 1 Table 1: Comparison study of hydrogen evolution rate between TiO2 (P-25), (Yb, Er) NYF-TiO2, 1% Au/TiO2 (P-25) and (Yb, Er) NYF-TiO2/1% Au under solar AM 1.5. Hydrogen evolution rate in µmol h-1

Time (h)

TiO2(P-25)

(Yb, Er) NYFTiO2

1% Au/ TiO2(P-25)

(Yb, Er) NYFTiO2/1%Au

1

ND

1.623

43.7

191.5

2

ND

4.653

100.2

435.5

3

ND

6.026

116.7

440.1

4

ND

6.594

122.9

445.8

5

ND

6.637

126.9

419.8

6

ND

6.577

124.6

400.1

Plausible mechanism for hybrid microsphere (Yb, Er) NYF-TiO2/1% Au: Based on findings of photoluminescence, UV, Vis, IR light cut-off filters induced photocatalytic hydrogen generation reaction (Figure 10) The detailed mechanistic aspect of (Yb, Er) NYFTiO2/1%Au can be summarized as: i. Hybrid microsphere synthesized by hydrothermal and photodeposition of metal nanoparticle shows broad band absorption based on dual benefit of plasmonic and upconverting host. ii. (Yb, Er) NYF-TiO2/1%Au composite is composed of UV, Visible and NIR component wherein TiO2 is highly active semiconductor triggered by UV , Au NPs is triggered by visible light and shows plasmonic effect and upconverting material (Yb, Er) NYF-TiO2 is triggered by IR light .


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iii. As reported by Dai et al and Kang et al, upconversion phosphors are able to convert longer wavelength to shorter wavelength (UV or Visible illumination) through a two-photon mechanism or multiple photons accomplished by anti-stroke shift.49, 50 iv. As reported in previous work of our group Au NPs enhances photocatalytic hydrogen production through a SPR induced charged electron transfer, enhancement in localized magnetic field and increased photon path length for better activity.51 v. In short, the heterostructure is fabricated to facilitate energy transfer between nano-component as well as suppresses charge carriers i.e. electron-holes pair recombination rate. vi. According to experimental results of hydrogen evolution and structural feature of catalyst (Yb, Er)NYF-TiO2/1%Au the mechanism for hydrogen generation may vary depending upon exposure to different illumination light which is discussed below. vii. Upon exposure to visible light cut-off, the photocatalyst is illuminated with both UV and IR light and showed hydrogen evolution rate of 130.19 µmol h-1. On irradiation to UV and IR light TiO2 is triggered by UV light which generates photoexcited electrons and holes, and this photoexcited electron diffuses towards Au NPs. Here Au NP acts as electron sink and active sites for photon reduction to hydrogen. Similarly, photoexcited holes are scavenged by ethanol and creates site for oxidation on VB of TiO2. Thus on exposure to UV and IR, UV emission is absorbed by TiO2 and generates electron and hole pair on TiO2. This electron-hole pair participates in the photocatalytic reaction. viii. Similarly, when the photocatalyst was exposed to UV light cut-off filter (400-1000 nm), it is exposed to visible and IR light only then observed hydrogen evolution rate was 6.71µmol h-1 this accentuates role of UV light in activating TiO2 photocatalyst. In this case charge separation takes place on Au NPs and the excited SPR induced electron is injected into the conduction band of



TiO2 with consequent charge separation and hydrogen generation. This is exactly opposite to what happens in visible cut off filter. Also, Au NPs absorbs green emission (520 nm and 540 nm) from (Yb, Er) NYF to excite SPR effect. The dual effect enhances the hydrogen evolution rate which is higher than Au-TiO2 (P-25). ix. In order to confirm the IR light contribution in photocatalytic reaction, (Yb, Er)NYF-TiO2/Au further was exposed to IR cutoff light, which shows a distinct decrease in the hydrogen generation rate due to the absence of IR light. When light IR cut-off filter (200-700 nm) is used for exposure, the catalyst is irradiated with UV and visible light and showed hydrogen evolution rate of 81.5 µmol h-1. In this situation the photoexcited electron transfers from the TiO2 conduction band to Au NPs same as under visible light cut-off filter. In the absence of upconverting phosphor photocatalytic hydrogen evolution rate is lowered substantially.

Figure 10: Schematic illustration of upconversion luminescence and photocatalytic hydrogen generation mechanism of (Yb, Er) NYF-TiO2/1% Au.


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x. To check the upconversion emission of (Yb, Er) NYF-TiO2/1%Au, the sample was irradiated under 980 nm laser to excite, the Yb3+ ion sensitizer which absorbs the light and is excited due to large absorption cross section than that of Er3+ ions. The electron from Yb3+ ions are excited from 2F

2F

7/2-

5/2

level. Subsequent to excitation, the excited electron comes back to its original ground

state and releases energy and is transferred to active Er3+ ions, which leads to a long-lived population of Er3+ ions from 4I15/2-4I11/2 level. In addition, a similar 980 nm excited photon from Yb3+ ion could populate to the higher energy level of Er3+ ions to 4F9/2, 4F7/2 and 4H9/2 and then some of the excited electron relax to the 2H11/2, 4S3/2 and 4F9/2, etc. Finally, in the radioactive process, these excited electrons would transfer to ground state energy level of Er3+ ions and emit four emissions. The four emission includes a) Blue emission–UV centered peak at 408 nm ascribed to the transition level 2H9/2-4I15/2. b) Two green emission peaks at 520 nm and 540 nm, overlapping with Au NPs SPR band, corresponds to the transition 2H11/2-4I15/2, 4S3/2-4I15/2 of Er3+ ions and c) Red emission peak centered at 654 nm originating from transition of 2F9/2-4I15/2 of Er3+ respectively. xi. The energy from emissions from phosphor in excited state possibly transfers directly to Au nanoparticles through FRET process which in turn excites the Au nanoparticle with prevalent SPR effect. This transfer is possible based on the energy level as reported by Liu et al.52 The strong electromagnetic field induced near the surface of Au nanoparticles due to SPR mediates PRET process between Au nanoparticle and TiO2 surface. The PRET process directly activates more charge carrier pairs in TiO2 structure. The free electrons from the conduction band of TiO2 surface are transferred to Au nanoparticles, while photogenerated holes transfer to the valence band of TiO2. This could significantly reduce recombination rate in TiO2, moreover, Au plays role as co-catalyst and provides active sites for oxidation of ethanol and hydrogen evolution.



Eventually, photogenerated charge carrier reacts with absorbed ethanol (sacrificial donor) on the surface of TiO2 and Au, with consequent generation of hydrogen. xii. On illumination of the photocatalyst composite with solar AM 1.5, all above mechanism is prevalent resulting in unprecedented enhancement in hydrogen evolution rate of 191µmol h-1. The upconversion of solar light by the host (Yb, Er) NYF helps to improve the photocatalytic hydrogen evolution rate by transferring energy to TiO2 and Au through the FRET and PRET. It also needs to be highlighted that the contribution of (Yb, Er) NYF in transferring energy to TiO2 transfer is significantly higher than (Yb, Er) NYF to Au energy transfer. The upconversion photocatalyst composite thus designed is showing promising results in harvesting radiation including UV, visible and NIR. Conclusion: In summary, we have combined effect of upconversion, plasmonic metal, and semiconductor photocatalyst to provide new strategies for creating broadband absorption material for hydrogen generation. The newly designed semiconductor based (Yb, Er) NYF-TiO2 composite plays key role of light concentrator and energy relay for broadband absorption. The structure was designed in such a way that Au nanoparticle which acts as electron sink and as a co-catalyst favors energy or electron transfer, lowering recombination rate. The synthesized (Yb, Er) NYF-TiO2/1%Au composite trap photons over a broadband range from UV-Visible-NIR and also shows enhanced photocatalytic activity.

AUTHOR INFORMATION


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Corresponding Author To whom correspondence should be addressed. Email: [email protected] Author Contributions Both authors contributed in this manuscript and approved the final version. Supporting Information Elemental mapping images of Ti, O, Au, Y, Er, Yb, F, and Na for photocatalyst (Yb, Er) NYFTiO2/1% Au concentration variation study of photocatalyst (Yb, Er) NYF-TiO2/Au and hydrogen generation study with error bar. Conflicts of interest: None

Acknowledgment A.C acknowledges CSIR-RA for fellowship. Characterization portion of this manuscript was carried out under NWP-56 (TAPSUN project). Author would also like to acknowledge Dr. Bhushan Kore for helping in characterization of Photoluminescence sample. KRC No.: CSIRNEERI/KRC/2017/SEP/EMD/2.

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Synergistic Plasmonic and Upconversion Effect of (Yb,Er) NYF-TiO2

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