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<front>
<journal-meta>
<journal-id journal-id-type="publisher-id">Front. Mater.</journal-id>
<journal-title>Frontiers in Materials</journal-title>
<abbrev-journal-title abbrev-type="pubmed">Front. Mater.</abbrev-journal-title>
<issn pub-type="epub">2296-8016</issn>
<publisher>
<publisher-name>Frontiers Media S.A.</publisher-name>
</publisher>
</journal-meta>
<article-meta>
<article-id pub-id-type="publisher-id">1137771</article-id>
<article-id pub-id-type="doi">10.3389/fmats.2023.1137771</article-id>
<article-categories>
<subj-group subj-group-type="heading">
<subject>Materials</subject>
<subj-group>
<subject>Original Research</subject>
</subj-group>
</subj-group>
</article-categories>
<title-group>
<article-title>Surface plasmon resonance effects of silver nanoparticles in graphene-based dye-sensitized solar cells</article-title>
<alt-title alt-title-type="left-running-head">Li et al.</alt-title>
<alt-title alt-title-type="right-running-head">
<ext-link ext-link-type="uri" xlink:href="https://doi.org/10.3389/fmats.2023.1137771">10.3389/fmats.2023.1137771</ext-link>
</alt-title>
</title-group>
<contrib-group>
<contrib contrib-type="author" corresp="yes">
<name>
<surname>Li</surname>
<given-names>Xinxi</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
<xref ref-type="corresp" rid="c001">&#x2a;</xref>
<uri xlink:href="https://loop.frontiersin.org/people/875796/overview"/>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Yang</surname>
<given-names>Wensheng</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Deng</surname>
<given-names>Jian</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
</contrib>
<contrib contrib-type="author" corresp="yes">
<name>
<surname>Lin</surname>
<given-names>Yuanwei</given-names>
</name>
<xref ref-type="aff" rid="aff2">
<sup>2</sup>
</xref>
<xref ref-type="aff" rid="aff3">
<sup>3</sup>
</xref>
<xref ref-type="corresp" rid="c001">&#x2a;</xref>
<uri xlink:href="https://loop.frontiersin.org/people/2161241/overview"/>
</contrib>
</contrib-group>
<aff id="aff1">
<sup>1</sup>
<institution>School of Materials and Energy</institution>, <institution>Guangdong University of Technology</institution>, <addr-line>Guangzhou</addr-line>, <country>China</country>
</aff>
<aff id="aff2">
<sup>2</sup>
<institution>Beijing National Laboratory for Molecular Sciences</institution>, <institution>State Key Laboratory for Structural Chemistry of Unstable and Stable Species</institution>, <institution>College of Chemistry and Molecular Engineering</institution>, <institution>Peking University</institution>, <addr-line>Beijing</addr-line>, <country>China</country>
</aff>
<aff id="aff3">
<sup>3</sup>
<institution>Center for Nanoscience and Nanotechnology</institution>, <institution>Academy for Advanced Interdisciplinary Studies</institution>, <institution>Peking University</institution>, <addr-line>Beijing</addr-line>, <country>China</country>
</aff>
<author-notes>
<fn fn-type="edited-by">
<p>
<bold>Edited by:</bold> <ext-link ext-link-type="uri" xlink:href="https://loop.frontiersin.org/people/760380/overview">Li Tao</ext-link>, Beijing Institute of Technology, China</p>
</fn>
<fn fn-type="edited-by">
<p>
<bold>Reviewed by:</bold> <ext-link ext-link-type="uri" xlink:href="https://loop.frontiersin.org/people/2097313/overview">Khosro Zangeneh Kamali</ext-link>, Australian National University, Australia</p>
<p>
<ext-link ext-link-type="uri" xlink:href="https://loop.frontiersin.org/people/1860581/overview">Min Xi</ext-link>, Chinese Academy of Sciences (CAS), China</p>
</fn>
<corresp id="c001">&#x2a;Correspondence: Xinxi Li, <email>pkdlxx@gdut.edu.cn</email>; Yuanwei Lin, <email>yuanweilin@pku.edu.cn</email>
</corresp>
</author-notes>
<pub-date pub-type="epub">
<day>18</day>
<month>05</month>
<year>2023</year>
</pub-date>
<pub-date pub-type="collection">
<year>2023</year>
</pub-date>
<volume>10</volume>
<elocation-id>1137771</elocation-id>
<history>
<date date-type="received">
<day>04</day>
<month>01</month>
<year>2023</year>
</date>
<date date-type="accepted">
<day>20</day>
<month>04</month>
<year>2023</year>
</date>
</history>
<permissions>
<copyright-statement>Copyright &#xa9; 2023 Li, Yang, Deng and Lin.</copyright-statement>
<copyright-year>2023</copyright-year>
<copyright-holder>Li, Yang, Deng and Lin</copyright-holder>
<license xlink:href="http://creativecommons.org/licenses/by/4.0/">
<p>This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.</p>
</license>
</permissions>
<abstract>
<p>Localized surface plasmon resonance (LSPR) has been applied in photovoltaic devices to improve their photoelectric conversion efficiency. In this study, the mechanism of LSPR in dye-sensitized solar cells (DSSCs) was determined. Specifically, silver nanoparticles were formed by evaporating silver in electro-beam equipment and then annealing it in a tube furnace. The sizes of the nanoparticles varied with the changes in their thicknesses during evaporation and annealing. A DSSC-based graphene was designed to consist of different sizes of metal nanoparticles assembled on a cathode electrode. The photon electric performance of the DSSCs, which depended on Ag nanoparticles, was analyzed in detail, with a particular focus on nanoparticle size. Compared with the DSSC without Ag nanoparticles, the DSSC with LSPR exhibited excellent electric current density and incident photon-to-current efficiency (IPCE) performance due to the LSPR effect. The DSSC assembled with 10&#xa0;nm-thick Ag film and annealed to form nanoparticles exhibited a high IPCE of 70.03%. The IPCE value of this DSSC was 45.15% higher than that of the pure graphene-based DSSC (31.62%). However, Ag nanoparticles increased to a certain degree and became aggregated and concatenated, thereby decreasing the LSPR effect on DSSCs. Therefore, LSPR plays an important role in the photon-electrical performance of DSSCs.</p>
</abstract>
<kwd-group>
<kwd>localized surface plasmon resonance</kwd>
<kwd>dye-sensitized solar cell</kwd>
<kwd>graphene</kwd>
<kwd>silver nanoparticles</kwd>
<kwd>energy materials</kwd>
</kwd-group>
<custom-meta-wrap>
<custom-meta>
<meta-name>section-at-acceptance</meta-name>
<meta-value>Energy Materials</meta-value>
</custom-meta>
</custom-meta-wrap>
</article-meta>
</front>
<body>
<sec id="s1">
<title>1 Introduction</title>
<p>The localized surface plasmon resonance (LSPR) of nanoparticles has attracted significant attention in different applications, such as optical antennas (<xref ref-type="bibr" rid="B31">You et al., 2022</xref>), surface enhanced Raman spectrophotometry (<xref ref-type="bibr" rid="B10">Gu et al., 2022</xref>), and titanium oxide photocatalysts (<xref ref-type="bibr" rid="B1">Andronic et al., 2022</xref>). When metal nanoparticles absorb incident light, the radiated light scatters around the particle surface, and a strong electromagnetic field is generated by an evanescent wave (<xref ref-type="bibr" rid="B11">Hasan et al., 2022</xref>). The nanoparticle surface is localized by the electromagnetic field, which maintains a distance of less than the diameter of the nanoparticle.</p>
<p>LSPR enhancement is affected by many factors, such as particle size, particle shape, incident light wavelength, dielectric constant of materials, and surface state of particles (<xref ref-type="bibr" rid="B12">Hu et al., 2022</xref>; <xref ref-type="bibr" rid="B15">Kim and Ha, 2022</xref>; <xref ref-type="bibr" rid="B35">Zhu et al., 2022</xref>; <xref ref-type="bibr" rid="B34">Zhu and Zhao, 2022</xref>). The LSPR of metal nanoparticles is recognized as an effective approach to improve the performance of photoelectric devices due to its important advantages (<xref ref-type="bibr" rid="B21">Ninawe and Dhawan, 2022</xref>; <xref ref-type="bibr" rid="B23">Rasheed and Sayidmarie, 2022</xref>).</p>
<p>In recent years, the enhancement of dye-sensitized solar cells (DSSCs) by surface plasmon has been reported (<xref ref-type="bibr" rid="B7">Dahlan et al., 2022</xref>; <xref ref-type="bibr" rid="B33">Zhang et al., 2022</xref>). DSSCs can display some advantages such as low-cost fabrication, semi-transparency, and environment friendliness. Thus, DSSCs have been regarded as a suitable source of electrical energy in future solar cells. Gratzel proposed mesoporous titanium dioxide (TiO<sub>2</sub>) films and ruthenium sensitizers to improve the efficiency of DSSCs (<xref ref-type="bibr" rid="B22">O&#x2019;Regan and Gr&#xe4;tzel, 1991</xref>). The energy conversion efficiencies of DSSCs should be efficiently improved to make them feasible and economical photovoltaic devices. In fact, the energy conversion efficiency of DSSCs is affected by many factors, such as the energetic suitability of HOMO-LUMO levels (<xref ref-type="bibr" rid="B2">Badawy et al., 2022</xref>; <xref ref-type="bibr" rid="B19">Majid et al., 2022</xref>), available large surface areas for the dye (<xref ref-type="bibr" rid="B26">Siddika et al., 2022</xref>), transport kinetics of electrons (<xref ref-type="bibr" rid="B17">Koshika et al., 2022</xref>), and losses due to electron hole recombination and back reactions (<xref ref-type="bibr" rid="B6">Bouwens et al., 2022</xref>).</p>
<p>Many researchers have designed different DSSCs, such as various photosensitive substances (<xref ref-type="bibr" rid="B27">Stavitskaya et al., 2022</xref>), electrolytes (<xref ref-type="bibr" rid="B3">Bashir et al., 2022</xref>), and nanostructures (<xref ref-type="bibr" rid="B24">Salimian et al., 2022</xref>), to improve their photon conversion efficiency.</p>
<p>Several approaches have been used to improve the efficiency of DSSCs. One approach is the absorption of ultra-visible and far infrared light to excite the dye and generate many photo-simulated carriers. The electrons are injected into the semiconductor layer. Thus, enhancing the light-harvesting efficiency of the dye is the most effective method to improve the efficiency of DSSCs. LSPR has been applied in various photovoltaic devices, such as organic solar cells (<xref ref-type="bibr" rid="B13">Ike et al., 2022</xref>), thin film silicon solar cells (<xref ref-type="bibr" rid="B32">Zarerasouli et al., 2022</xref>), quantum dot photovoltaics (<xref ref-type="bibr" rid="B16">Kim et al., 2022</xref>), and DSSCs (<xref ref-type="bibr" rid="B14">Kaur et al., 2022</xref>). LSPR effectively improves the solar conversion efficiency of DSSCs by producing a strong electromagnetic field near the surface of metal nanoparticles and enhancing the light absorption force of the dye in the visible range of wavelengths.</p>
<p>The interface mechanisms between the metallic nanostructure and dielectric layer of the designed photovoltaic device have been studied in our previous research. The substrate induced plasmon concentrated light at the interface, enhanced the photo-excitation of dyes, and improved the photoelectric conversion (<xref ref-type="bibr" rid="B18">Li et al., 2015</xref>). In the current research, a DSSC-based graphene consisting of different sizes of evaporated metal nanoparticles is designed. The basic structure of this DSSC is shown in <xref ref-type="fig" rid="F1">Figure 1</xref>. Given the metal nanoparticles with LSPR, the dye is stimulated by the electromagnetic field to improve the efficiency of DSSCs. To investigate the effects of Ag NP-modified DSSCs with various sizes, we adopt different physical, optical, and electrical measurement techniques.</p>
<fig id="F1" position="float">
<label>FIGURE 1</label>
<caption>
<p>Schematic of the device structure.</p>
</caption>
<graphic xlink:href="fmats-10-1137771-g001.tif"/>
</fig>
</sec>
<sec id="s2">
<title>2 Experimental and schematic</title>
<sec id="s2-1">
<title>2.1 Graphene growth</title>
<p>A low-pressure chemical vapor deposition method with methane was utilized to produce a high-quality, single-layer graphene with a carbon-containing precursor under optimal conditions (<xref ref-type="bibr" rid="B28">Sun et al., 2021</xref>). The substrate of the graphene was copper foil, which was initially pretreated with acetic acid and then posited in a split tube furnace to produce the graphene. During graphene growth, the foil was annealed at a temperature of 1,045&#xb0;C under 10 sccm H<sub>2</sub> flow for 60&#xa0;min and then for 15&#xa0;min. The temperature rapidly dropped to room temperature under hydrogen protection.</p>
</sec>
<sec id="s2-2">
<title>2.2 Preparation of metal nanoparticles</title>
<p>Ag alloy thin films with proportional silver volume ratios and thicknesses were evaporated on the surface of the graphene on the copper foil substrate under the condition of 0.1&#xa0;&#xc5;/s and a pressure of 2 &#xd7; 10<sup>&#x2212;4</sup>&#xa0;Pa. Then, the films were annealed at 300&#xb0;C for 30&#xa0;min under H<sub>2</sub> (600 sccm) and Ar (600 sccm) as protective gas atmospheres. As a result, silver nanoparticles formed on the graphene surface. The graphene film deposited silver films, namely, Gr &#x2b; Ag5, Gr &#x2b; Ag10, Gr &#x2b; Ag15, and Gr &#x2b; Ag20 with sizes of 5, 10, 15, and 20&#xa0;nm, respectively. The thicknesses of the Ag films deposited on the graphene increased with the increase of annealing time, and the sizes of the Ag nanoparticles after annealing increased correspondingly. These metal nanoparticles play an important role in the incident photon-to-current efficiency (IPCE) and charge transfer properties of DSSCs.</p>
</sec>
<sec id="s2-3">
<title>2.3 Back electrode/TiO<sub>2</sub>/Ado/electrolyte layer preparation</title>
<p>Rutile TiO<sub>2</sub> (001) single crystals were purchased from MTI Corporation. Each crystal exhibited a mechanically polished side and an atomically flat surface. A 100&#xa0;nm In and a 100&#xa0;nm Ag were evaporated on the rough back side of the TiO<sub>2</sub> crystals. Before the assembly of the Ado molecule dye to the semiconductor, TiO<sub>2</sub> (001) was crossed for chemical polishing by an aqueous HF solution (30% HF, 1&#xa0;min) and was etched with oxygen plasma (15&#xa0;Pa, 30&#xa0;W; 1&#xa0;min) for surface hydroxyl activation. Then, the Ado molecule was assembled on TiO<sub>2</sub> by immersing the samples in a 10&#xa0;mg/mL solution of Ado in water for 48&#xa0;h. An iodide-triiodide (I/I<sub>3</sub>
<sup>&#x2212;</sup>)-based redox electrolyte that contained 0.5&#xa0;M of LiI, 0.05&#xa0;M of I<sub>2</sub>, 0.5&#xa0;M of 4-tert-butylpyridine, and 0.5&#xa0;M of 1-butyl-3-methylimidazolium iodide in acetonitrile was injected in the vacant space. The In/Ag side of the samples was fixed on a copper foil using a special conductive tap, and the back electrode was sealed with epoxy (Epotek 377) and then dried for at least 40&#xa0;min to solidify.</p>
</sec>
<sec id="s2-4">
<title>2.4 SLG/NP layer formation</title>
<p>For the assembly of the SLG/NP layer, the poly(methyl methacrylate) (PMMA) (MicroChem 950 PMMA A6) layer was spined cast on the surface of the SLG/NP samples on copper under the condition of 4,000 revolutions per minute (rpm) for 30&#xa0;s and was then heated to 180&#xb0;C for 3&#xa0;min. Considering that the two sides of the copper foils showed graphene growth, we etched the back side of SLG/NP/PMMA samples by oxygen plasma to remove residual graphene.</p>
<p>Moreover, the copper layer in the back samples was wet eroded in a 1&#xa0;M (NH<sub>4</sub>)<sub>2</sub>S<sub>2</sub>O<sub>8</sub> aqueous solution for 8&#xa0;h to produce an SLG/NP/PMMA film that should float on the surface of the solution. Then, the floating film was fetched by tweezers and was repeatedly washed using copious amounts of ultrapure water and isopropanol. Finally, the SLG/NP/PMMA sample was transferred on the dye layer.</p>
</sec>
<sec id="s2-5">
<title>2.5 Schematic of DSSC</title>
<p>The schematic of the structure of a DSSC is displayed in <xref ref-type="fig" rid="F1">Figure 1</xref>.</p>
<p>
<xref ref-type="fig" rid="F2">Figure 2</xref> illustrates the schematic of the mechanism of the designed graphene-based DSSC device. The metal nanoparticles absorbed incident light and generated LSPR. The energy generated by the nanoparticles through the graphene was transported to the semiconductor surface, and the absorbed Ado molecule was excited on the TiO<sub>2</sub> layer. At the same time, the I<sub>3</sub>
<sup>&#x2212;</sup>/I<sup>&#x2212;</sup> electrolyte prompted the separation of electrons and holes, and electron and hole pairs were generated. The photoactivated electrons through the semiconductor were transported to the back electrode with Ohmic contacts for collection. Meanwhile, the photoactivated holes were transferred to the graphene layer for collection. In sum, the back-metal electrode as a photoanode and graphene as a photocathode were externally connected to form a circuit and achieve photoelectric conversion.</p>
<fig id="F2" position="float">
<label>FIGURE 2</label>
<caption>
<p>Schematic of the mechanism effect of the designed DSSC.</p>
</caption>
<graphic xlink:href="fmats-10-1137771-g002.tif"/>
</fig>
</sec>
<sec id="s2-6">
<title>2.6 Morphology characterizations</title>
<p>The morphology characterizations of metal nanoparticles were investigated by using a field-emission scanning electron microscope (FE-SEM; Hitachi Limited, S4800). The elemental composition and crystal structure of the nanoparticles were evaluated by using a high-resolution transmission electron microscope (HRTEM; FEI, Tecnai F30). The energy dispersive X-ray (EDX) analysis of the nanoparticles revealed the presence of elements in the nanoparticles.</p>
</sec>
<sec id="s2-7">
<title>2.7 Photovoltaic performance evaluation</title>
<p>The power conversion parameters of the single graphene and Ag NP DSSC were measured by using a solar simulator with a light intensity of 100&#xa0;mW/cm<sup>2</sup> calibrated at AM 1.5 with a digital source meter (Science Tech). The current&#x2013;voltage characteristics of the photovoltaic devices were measured by using a semiconductor characterization system (Agilent 4155C). Serial parameters, such as short circuit current density (Jsc) and incident optical power (Pin), were used to analyze the power conversion efficiency of the single graphene and Ag NP-based DSSC.</p>
</sec>
<sec id="s2-8">
<title>2.8 IPCE calculation</title>
<p>For the IPCE spectroscopic measurements, a computer-controlled grating monochromator (Zolix Omni-&#x3bb;150) with a 150&#xa0;W Xe lamp was used to provide monochromatic light illumination to the samples with an optical filter. The corresponding wavelength of monochromatic light illumination was measured by an OPT-2000 spectrophotometer. The corresponding photocurrent was measured with an Agilent 4155C semiconductor parameter analyzer at current time mode. The IPCE was calculated from the measured photocurrents and corresponding monochromatic light wavelength (<italic>&#x3bb;</italic>). The following formula was used to expressed IPCE.<disp-formula id="equ1">
<mml:math id="m1">
<mml:mrow>
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<mml:mi mathvariant="normal">P</mml:mi>
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<mml:mi mathvariant="normal">E</mml:mi>
<mml:mo>&#x3d;</mml:mo>
<mml:mfrac>
<mml:mrow>
<mml:mi mathvariant="normal">h</mml:mi>
<mml:mi mathvariant="normal">c</mml:mi>
</mml:mrow>
<mml:mi mathvariant="normal">&#x3bb;</mml:mi>
</mml:mfrac>
<mml:mtext>&#x2009;</mml:mtext>
<mml:mrow>
<mml:mfenced open="[" close="]" separators="|">
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<mml:mi>A</mml:mi>
</mml:mrow>
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<mml:mi>c</mml:mi>
<mml:mi>m</mml:mi>
</mml:mrow>
<mml:mn>2</mml:mn>
</mml:msup>
</mml:mfrac>
</mml:mrow>
</mml:mfenced>
</mml:mrow>
</mml:mrow>
<mml:mrow>
<mml:mi>l</mml:mi>
<mml:mi>i</mml:mi>
<mml:mi>g</mml:mi>
<mml:mi>h</mml:mi>
<mml:mi>t</mml:mi>
<mml:mtext>&#x2009;</mml:mtext>
<mml:mi>p</mml:mi>
<mml:mi>o</mml:mi>
<mml:mi>w</mml:mi>
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</mml:mrow>
<mml:msup>
<mml:mrow>
<mml:mi>c</mml:mi>
<mml:mi>m</mml:mi>
</mml:mrow>
<mml:mn>2</mml:mn>
</mml:msup>
</mml:mfrac>
</mml:mrow>
</mml:mfenced>
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</mml:mrow>
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</mml:mrow>
</mml:mrow>
</mml:math>
</disp-formula>where <italic>h is</italic> the Planck&#x2019;s constant and <italic>c</italic> is the speed of light. In this research, all the photoelectrical results were measured by using three samples.</p>
</sec>
</sec>
<sec sec-type="results|discussion" id="s3">
<title>3 Results and discussion</title>
<sec id="s3-1">
<title>3.1 Physical characterization and analysis</title>
<sec id="s3-1-1">
<title>3.1.1 UV-visible spectroscopy</title>
<p>The pure Gr and Ag-Gr photocathode samples were tested in the range of 300&#x2013;800&#xa0;nm. As displayed in <xref ref-type="fig" rid="F3">Figure 3</xref>, the peak curve of the Gr &#x2b; Ag10 extinction spectrum is apparent at 428&#xa0;nm, and the Ado molecule absorption peak position is about 415&#xa0;nm. This value nearly matched the absorption peak of Ado on the TiO<sub>2</sub> surface. This condition embodied the effect of surface plasmon resonance that enhanced the incident light utilization.</p>
<fig id="F3" position="float">
<label>FIGURE 3</label>
<caption>
<p>Extinction spectra of PMMA-supported graphene and Ag-NP film on Ado molecular layer assembled on TiO<sub>2</sub> substrate.</p>
</caption>
<graphic xlink:href="fmats-10-1137771-g003.tif"/>
</fig>
<p>The light absorption enhancement was attributed to the LSPR effect, which created a strong electromagnetic field around the Ag NP and produced an effective resonant energy transfer to the adopted dye on the semiconductor (<xref ref-type="bibr" rid="B20">Mathew and Shyju, 2022</xref>). Therefore, Ag NPs are effective for visible light response.</p>
</sec>
<sec id="s3-1-2">
<title>3.1.2 Physical characteristics of silver nanoparticles</title>
<p>As shown in <xref ref-type="fig" rid="F4">Figure 4A</xref>, silver nanoparticles were annealed on ten nm-thick silver film and were thermally evaporated on a single layer graphene surface, which were formed on graphene uniformly. The SEM images indicates that the silver nanoparticles exhibited even distribution, with their average diameter are about 42.1&#xa0;nm, as shown in <xref ref-type="fig" rid="F4">Figure 4B</xref>. The results of HRTEM and EDX characterization of the silver nanoparticles are displayed in <xref ref-type="fig" rid="F4">Figures 4C, D</xref>, respectively. The lattice distance and facet of the pure silver nanoparticles were approximately 2.35&#xa0;&#xc5; and (111). The EDX dispersive peaks obviously displayed the silver element, so it can be induced that Ag NPs were well-assembled on graphene.</p>
<fig id="F4" position="float">
<label>FIGURE 4</label>
<caption>
<p>Characteristic of Ag nanoparticles: <bold>(A)</bold> FE-SEM image of Ag nanoparticles annealed by 10&#xa0;nm Ag film, <bold>(B)</bold> the size distribution histogram of Ag nanoparticles, <bold>(C)</bold> HRTEM image of single Ag nanoparticles, and <bold>(D)</bold> EDX analysis of Ag nanoparticles.</p>
</caption>
<graphic xlink:href="fmats-10-1137771-g004.tif"/>
</fig>
</sec>
<sec id="s3-1-3">
<title>3.1.3 Raman spectroscopy of silver nanoparticles</title>
<p>The Raman spectroscopy of graphene assembled with Ado and silver nanoparticles is shown in <xref ref-type="fig" rid="F5">Figure 5</xref>. For the Raman spectrum of pure graphene, the small G/2D ratio, narrow single symmetric 2D peak (&#x2212;2,694&#xa0;cm<sup>&#x2212;1</sup>), and negligible D peak indicated that graphene comprised a single layer and exhibited relatively high quality (<xref ref-type="bibr" rid="B30">Thapliyal et al., 2022</xref>). The D peak indicated that graphene comprised a single layer and showed high quality. However, for the graphene assembled with silver nanoparticles, it can display only the D peak of 1,353&#xa0;cm<sup>&#x2212;1</sup> is noted, it reveals the small damage on graphene during the formation of silver nanoparticles, and the formation of silver nanoparticles on the graphene surface causes negligible damage on the graphene layer (<xref ref-type="bibr" rid="B4">Bayle et al., 2015</xref>). The peaks of Ado at 1,200&#x2013;1,650&#xa0;cm<sup>&#x2212;1</sup> in the Raman spectra of the Ado/SLG/NP samples were significantly enhanced compared with those in the spectra of Ado on pure graphene. This finding indicated that silver NPs converged light to the Ado layer below the graphene (<xref ref-type="bibr" rid="B29">Suo et al., 2021</xref>).</p>
<fig id="F5" position="float">
<label>FIGURE 5</label>
<caption>
<p>Raman spectra of samples: Raman spectra of Ado on Gr and Ado on Gr/Ag nanoparticles.</p>
</caption>
<graphic xlink:href="fmats-10-1137771-g005.tif"/>
</fig>
</sec>
</sec>
<sec id="s3-2">
<title>3.2 Physical analysis of silver nanoparticles with different diameter sizes</title>
<sec id="s3-2-1">
<title>3.2.1 Extinction of silver nanoparticles with different diameter sizes</title>
<p>UV-vis spectroscopy has been adopted to evaluate the quantity of nanoparticles among different samples. As displayed in <xref ref-type="fig" rid="F6">Figure 6</xref>, the absorbance of the silver nanoparticles with different thicknesses is compared. The main reason is that the free electrons of the metal nanoparticles are in resonant with incidence of photons. As shown in <xref ref-type="fig" rid="F6">Figure 6</xref>, the absorption peak amplitudes increased correspondingly with the increase of the size of the Ag nanoparticles. The absorbance intensity gradually increased when the thickness of the silver film increased from 5&#xa0;nm to 20&#xa0;nm. Meanwhile, the absorption peak presented a blue shift. The increased absorption peak can significantly enhance the peaks of the samples accompany with the increasing size of the metal nanoparticle. Therefore, the surface plasmon resonance correspondingly changed with the changes in the size of the Ag nanoparticles.</p>
<fig id="F6" position="float">
<label>FIGURE 6</label>
<caption>
<p>Extinction spectra of silver nanoparticles with different diameters and sizes after annealing.</p>
</caption>
<graphic xlink:href="fmats-10-1137771-g006.tif"/>
</fig>
</sec>
<sec id="s3-2-2">
<title>3.2.2 SEM characterization of silver nanoparticles with different thicknesses</title>
<p>The suitable size, proper morphology, and relatively high specific surface area of metal nanoparticles benefit the generation of sufficient energy by LSPR. <xref ref-type="fig" rid="F7">Figure 7</xref> shows the FE-SEM images of different Ag nanoparticles. The obtained Ag nanoparticles showed a uniform distribution when the thickness of the silver film increased from 5&#xa0;nm to 10&#xa0;nm. This condition should be contributed to the LSPR effect. However, the sizes of the silver nanoparticles after annealing became aggregated when the thickness of the evaporated silver film surpassed to 15&#xa0;nm. This condition rapidly decreased the LSPR effect.</p>
<fig id="F7" position="float">
<label>FIGURE 7</label>
<caption>
<p>FE-SEM of different silver nanoparticles. <bold>(A)</bold> Gr &#x2b; Ag5, <bold>(B)</bold> Gr &#x2b; Ag10, <bold>(C)</bold> Gr &#x2b; Ag15, and <bold>(D)</bold> Gr &#x2b; Ag20.</p>
</caption>
<graphic xlink:href="fmats-10-1137771-g007.tif"/>
</fig>
</sec>
</sec>
<sec id="s3-3">
<title>3.3 Electrical characterization of DSSCs</title>
<sec id="s3-3-1">
<title>3.3.1 J-V characteristic</title>
<p>The photovoltaic performance and efficiency of different DSSCs, such as pure Gr, Gr &#x2b; Ag5, Gr &#x2b; Ag10, Gr &#x2b; Ag15, and Gr &#x2b; Ag20, were tested by a semiconductor characterization system under standard conditions (1 Sun; input power of 100&#xa0;mW&#xa0;cm<sup>2</sup>; AM 1.5). The current&#x2013;voltage curves are displayed in <xref ref-type="fig" rid="F8">Figure 8</xref>. The Jsc, open-circuit voltage (Voc), fill factor (FF), IPCE, and photon-to-current efficiency (PCE) (<italic>&#x3b7;</italic>) among different solar cells were calculated using a solar simulator when sunlight was simulated on an active area of 0.15&#xa0;cm<sup>2</sup>. The relative data are tabulated in <xref ref-type="table" rid="T1">Table 1</xref>.</p>
<fig id="F8" position="float">
<label>FIGURE 8</label>
<caption>
<p>Current&#x2013;voltage characteristic of different DSSCs containing pure Gr, Gr &#x2b; Ag5, Gr &#x2b; Ag10, Gr &#x2b; Ag15, and Gr &#x2b; Ag20.</p>
</caption>
<graphic xlink:href="fmats-10-1137771-g008.tif"/>
</fig>
<table-wrap id="T1" position="float">
<label>TABLE 1</label>
<caption>
<p>Performance of pure Gr, Gr &#x2b; Ag5, Gr &#x2b; Ag10, Gr &#x2b; Ag15, and Gr &#x2b; Ag20 DSSCs.</p>
</caption>
<table>
<thead valign="top">
<tr>
<th align="left">Samples</th>
<th align="left">Jsc (mA&#x387;cm<sup>&#x2212;2</sup>)</th>
<th align="left">Voc (V)</th>
<th align="left">FF (%)</th>
<th align="left">&#x3b7; (%)</th>
</tr>
</thead>
<tbody valign="top">
<tr>
<td align="left">Pure Gr</td>
<td align="left">12.2</td>
<td align="left">0.73</td>
<td align="left">53.6</td>
<td align="left">5.13</td>
</tr>
<tr>
<td align="left">Gr &#x2b; Ag5</td>
<td align="left">14.3</td>
<td align="left">0.73</td>
<td align="left">53.7</td>
<td align="left">6.26</td>
</tr>
<tr>
<td align="left">Gr &#x2b; Ag10</td>
<td align="left">19.2</td>
<td align="left">0.77</td>
<td align="left">51.6</td>
<td align="left">7.52</td>
</tr>
<tr>
<td align="left">Gr &#x2b; Ag15</td>
<td align="left">16.1</td>
<td align="left">0.76</td>
<td align="left">52.3</td>
<td align="left">6.64</td>
</tr>
<tr>
<td align="left">Gr &#x2b; Ag20</td>
<td align="left">12.6</td>
<td align="left">0.74</td>
<td align="left">53.6</td>
<td align="left">5.27</td>
</tr>
</tbody>
</table>
</table-wrap>
<p>In the present work, Gr &#x2b; Ag10 DSSC exhibited a relatively high PCE of 7.52%, which was obviously higher than that of the pure Gr sample. The 10&#xa0;nm Gr &#x2b; Ag10 displayed a suitable particle size and proper particle distance, which increased the absorbance of incident light and enhanced the SPR effect. However, the nanoparticles began to aggregate and decreased the LSPR effect when the size of the silver particles surpassed a certain degree. For example, in this study, silver nanoparticles formed after annealing of a 15&#xa0;nm thick Ag film are prone to aggregation due to their larger size, which minimizes energy transfer to the dye. Therefore, the Jsc of DSSC decreased when the metal nanoparticles increased to a certain degree.</p>
</sec>
<sec id="s3-3-2">
<title>3.3.2 IPCE performance of different DSSCs</title>
<p>The IPCE spectra of pure Gr, Gr &#x2b; Ag5, Gr &#x2b; Ag10, Gr &#x2b; Ag15, and Gr &#x2b; Ag20 are shown in <xref ref-type="fig" rid="F9">Figure 9</xref>. The IPCE values of Gr &#x2b; Ag5 and Gr &#x2b; Ag10 improved, whereas those of Gr &#x2b; Ag15 and Gr &#x2b; Ag20 did not. This result was due to the serious aggregation of the nanoparticles and the increased recombination between electrons. The electron injection and charge transfer efficiency increased correspondingly with the increase of Ag film thickness (<xref ref-type="bibr" rid="B9">Di and Qin, 2022</xref>).</p>
<fig id="F9" position="float">
<label>FIGURE 9</label>
<caption>
<p>IPCE values of different DSSCs containing different Ag nanoparticles, such as pure Gr, Gr &#x2b; Ag5, Gr &#x2b; Ag10, Gr &#x2b; Ag15, and Gr &#x2b; Ag20: <bold>(A)</bold> IPCE spectra of different DSSCs and <bold>(B)</bold> IPCE values of different DSSCs.</p>
</caption>
<graphic xlink:href="fmats-10-1137771-g009.tif"/>
</fig>
<p>As shown in the analysis in <xref ref-type="fig" rid="F9">Figure 9</xref>, the IPCE of pure G-based DSSC was 31.62%, and the IPCE of Gr &#x2b; Ag10 DSSC was approximately 70.03%, which indicated significant improvement due to the silver nanoparticles. The improvement of electron injection and charge transfer efficiency was mainly due to the deposition of the Ag nanoparticles on graphene that increased the LSPR effect. Thus, the IPCE of DSSC increased correspondingly. However, the Ag nanoparticles began to aggregate and concatenate and decreased the LSPR effect when they were increased to a certain degree, such as the case of Gr &#x2b; Ag15. Therefore, the IPCE values of different DSSCs containing different Ag nanoparticles decreased after the modification of Ag nanoparticles to a certain degree.</p>
<p>Considering the above analysis, the size of the metal nanoparticles is one of the important parameters to the magnitude of the plasmon enhancement effect. Concentrated energies with a strong electromagnetic field were produced by the metal nanoparticles in the DSSCs, and the SPR effect enhanced the incident light scattering and increased the optical path length and Jsc. Meanwhile, the improvement of photo electron generation increased the IPCE. However, the metal nanoparticles began to aggregate and concatenate when their size was increased to a certain degree. The condition decreased the LSPR effect.</p>
</sec>
<sec id="s3-3-3">
<title>3.3.3 Electrochemical impedance spectroscopy of different silver nanoparticles with modified graphene samples</title>
<p>The IPCE values for different DSSCs, such as pure Gr, Gr &#x2b; Ag5, Gr &#x2b; Ag10, Gr &#x2b; Ag15, and Gr &#x2b; Ag20, were investigated. The IPCE values of the DSSCs decreased with the increase in silver nanoparticles to a certain degree. This result was contrary to the incident light absorption. To identify this phenomenon, we measured the internal impedance of the DSSCs.</p>
<p>The EIS parameters are summarized in <xref ref-type="table" rid="T2">Table 2</xref>, and the corresponding diagrams for the DSSCs with and without metal nanoparticles are shown in <xref ref-type="fig" rid="F10">Figure 10</xref>.</p>
<table-wrap id="T2" position="float">
<label>TABLE 2</label>
<caption>
<p>Obtained EIS parameters from fitting Nyquist plots of DSSCs containing Ag nanoparticles of different sizes.</p>
</caption>
<table>
<thead valign="top">
<tr>
<th align="left">Samples</th>
<th align="left">Rt (&#x3a9;)</th>
<th align="left">Rr (&#x3a9;)</th>
<th align="left">Rd (&#x3a9;)</th>
<th align="left">Jsc (mA)</th>
</tr>
</thead>
<tbody valign="top">
<tr>
<td align="left">Pure Gr</td>
<td align="left">2.65</td>
<td align="left">30.13</td>
<td align="left">5.02</td>
<td align="left">12.1</td>
</tr>
<tr>
<td align="left">Gr &#x2b; Ag5</td>
<td align="left">2.39</td>
<td align="left">32.89</td>
<td align="left">6.12</td>
<td align="left">14.3</td>
</tr>
<tr>
<td align="left">Gr &#x2b; Ag10</td>
<td align="left">2.13</td>
<td align="left">41.32</td>
<td align="left">6.37</td>
<td align="left">19.2</td>
</tr>
<tr>
<td align="left">Gr &#x2b; Ag15</td>
<td align="left">2.43</td>
<td align="left">39.58</td>
<td align="left">5.75</td>
<td align="left">16.1</td>
</tr>
<tr>
<td align="left">Gr &#x2b; Ag20</td>
<td align="left">2.61</td>
<td align="left">31.06</td>
<td align="left">5.31</td>
<td align="left">12.6</td>
</tr>
</tbody>
</table>
</table-wrap>
<fig id="F10" position="float">
<label>FIGURE 10</label>
<caption>
<p>Nyquist plots of EIS for different DSSCs, namely, pure Gr, Gr &#x2b; Ag5, Gr &#x2b; Ag10, Gr &#x2b; Ag15, and Gr &#x2b; Ag20.</p>
</caption>
<graphic xlink:href="fmats-10-1137771-g010.tif"/>
</fig>
<p>The photo-electrode/dye/electrolyte interface focused on the LSPR effect during the charge transfer process. In <xref ref-type="fig" rid="F10">Figure 10</xref>, three semicircles, namely, Rt, Rr, and Rd, denote transport, recombination, and diffusion resistances, respectively (<xref ref-type="bibr" rid="B8">Dhonde et al., 2017</xref>). Charge resistance (Rt) and recombination resistance (Rr) were affected by the LSPR of the silver nanoparticles. The first semicircle Rt that corresponded to a high frequency range presented charge transfer resistance. The increase of the middle semicircle Rr indicated a high value of charge recombination resistance accompanied by an increased charge transfer mechanism. Thus, the PCE increased correspondingly (<xref ref-type="bibr" rid="B5">Bhojanaa et al., 2020</xref>; <xref ref-type="bibr" rid="B25">Selvapriya et al., 2022</xref>). For the Gr &#x2b; Ag10 sample, the Rt and Rr values were observed to be 2.13 and 41.32&#xa0;&#x3a9;, respectively. Thus, these findings are very in agreement with the J-V results of DSSCs.</p>
</sec>
</sec>
</sec>
<sec sec-type="conclusion" id="s4">
<title>4 Conclusion</title>
<p>In this study, a DSSC with TiO<sub>2</sub>/Dye/SLG/NP structure was designed to investigate the effects of LSPR enhancement on photovoltaic conversion. The DSSC with LSPR exhibited excellent electric current density and IPCE performance relative to the DSSC without Ag nanoparticles due to the LSPR effect. For the DSSC assembled with 10&#xa0;nm-thick Ag film and annealed to form nanoparticles, Gr &#x2b; Ag10 obtained the highest IPCE of 70.03%, which was 45.15% higher than that of pure Gr (31.62%). Therefore, the suitable quantity of Ag nanoparticles played an important role in enhancing the photon electric conversion efficiency due to the LSPR effect. The proper proportion of metal nanoparticles represents a valuable and efficient method for improving the performance of DSSCs.</p>
</sec>
</body>
<back>
<sec sec-type="data-availability" id="s5">
<title>Data availability statement</title>
<p>The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.</p>
</sec>
<sec id="s6">
<title>Author contributions</title>
<p>XL and YL designed and conducted the whole project. WY and JD performed the experiments and wrote the first draft of the manuscript. XL and YL revised the manuscript, and all the authors made contribution to the data analysis.</p>
</sec>
<sec id="s7">
<title>Funding</title>
<p>This work is financially supported by the National Natural Science Foundation of China (21875046 and 51803036), and the Guangzhou emerging industry development fund project of Guangzhou development and reform commission (2018841).</p>
</sec>
<sec sec-type="COI-statement" id="s8">
<title>Conflict of interest</title>
<p>The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.</p>
</sec>
<sec sec-type="disclaimer" id="s9">
<title>Publisher&#x2019;s note</title>
<p>All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.</p>
</sec>
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