ORIGINAL RESEARCH published: 02 February 2021 doi: 10.3389/fchem.2020.604092 ZnO Nanowires/N719 Dye With Different Aspect Ratio as a Possible Photoelectrode for Dye-Sensitized Solar Cells Karina Portillo-Cortez 1*, Ana Martínez 1 , Monserrat Bizarro 1 , Mario F. García-Sánchez 2 , Frank Güell 3 , Ateet Dutt 1* and Guillermo Santana 1* 1 Departamento de Materiales de Baja Dimensionalidad, Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, Ciudad de México, Mexico, 2 Unidad Profesional Interdisciplinaria en Ingeniería y Tecnologías Avanzadas, Instituto Politécnico Nacional, Ciudad de México, Mexico, 3 ENFOCAT-IN2UB, Universitat de Barcelona, Barcelona, Spain Edited by: The vapor-liquid-solid (VLS) process was applied to fabricate zinc oxide nanowires Anatoly P. Pushkarev, ITMO University, Russia (ZnO NWs) with a different aspect ratio (AR), morphological, and optical properties. The Reviewed by: ZnO NWs were grown on a system that contains a quartz substrate with transparent Bing-Xin Lei, conductive oxide (TCO) thin film followed by an Al-doped ZnO (AZO) seed layer; both films Hainan Normal University, China were grown by magnetron sputtering at room temperature. It was found that the ZnO Lihong Qi, Harbin Engineering University, China NWs presented high crystalline quality and vertical orientation from different structural *Correspondence: and morphological characterizations. Also, NWs showed a good density distribution Karina Portillo-Cortez of 69 NWs/µm2 with a different AR that offers their capability to be used as possible portillo@ciencias.unam.mx Ateet Dutt photoelectrode (anode) in potential future device applications. The samples optical adutt@iim.unam.mx properties were studied using various techniques such as photoluminescence (PL), Guillermo Santana absorption, and transmittance before and after sensitization with N719 dye. The results gsantana@iim.unam.mx demonstrated that NW with 30 nm diameter had the best characteristics as feasible Specialty section: photoelectrode (anode) (high absorption, minimum recombination, high crystallinity). This article was submitted to Also, the present samples optical properties were found to be improved due to the Nanoscience, a section of the journal existence of N719 dye and Au nanoparticles on the tip of NWs. NWs grown in this work Frontiers in Chemistry can be used in different photonic and optoelectronic applications. Received: 08 September 2020 Keywords: ZnO nanowires, high aspect ratio, dye-sensitized solar cell, N719 dye, Au nanoparticles, absorption Accepted: 02 November 2020 spectra Published: 02 February 2021 Citation: Portillo-Cortez K, Martínez A, INTRODUCTION Bizarro M, García-Sánchez MF, Güell F, Dutt A and Santana G (2021) Energy production is satisfied by conventional energetic technologies based on natural gas, coal, ZnO Nanowires/N719 Dye With Different Aspect Ratio as a Possible oil. Several efforts have been performed to mollify the growing demand for electrical energy Photoelectrode for Dye-Sensitized in unification with ecological upkeep through renewable energy development (Abdin et al., Solar Cells. Front. Chem. 8:604092. 2013; Skandalos and Karamanis, 2015; Greenaway et al., 2017; Pathak, 2020). O’Regan and doi: 10.3389/fchem.2020.604092 Grätzel in 1991 proposed a modern kind of renewable solar technology termed as Dye-sensitized Frontiers in Chemistry | www.frontiersin.org 1 February 2021 | Volume 8 | Article 604092 Portillo-Cortez et al. ZnO Nanowires as Photoelectrode in DSSC solar cell (DSSC) (O’Regan and Grätzel, 1991) that convert solar nanostructure density, even high crystalline quality. These useful irradiation to electricity via a similar process to photosynthesis. structural and morphological features can provide excellent In a DSSC device, a dye sensitizer such as N719 dye is one charge transport properties for photovoltaic and nanoelectronics of the most critical parts where the majority of the absorption (Yang et al., 2008; Simon et al., 2013; Ye et al., 2015). The VLS of incident solar radiation occurs to generate electrons that are process involves using liquid metal nanoparticles as a catalyst injected and transported by an Electron Transport Layer (ETL), (Au), and it constructs a eutectic alloy with the seed layer and finally, collected by a TCO substrate (Portillo-Cortez et al., and gas precursor. Then, the gas precursor dissolves into the 2019). The electrons flow through the external circuit toward the liquid catalyst to form a supersaturated solution followed by counter electrode. The regeneration of the oxidized dye is due the nucleation process and NWs growth (Güell and Martínez- to a redox reaction produced inside the electrolyte (Boschloo, Alanis, 2019). For ZnO, the dimensions and quality of NWs are 2019). Outstanding their simplicity in the fabrication technique dependent on the different VLS factors such as chamber pressure, and economical way of processing, these methods have drawn a oxygen ratio, and thickness of the metal catalyst layer (Zhang significant consideration as in contrast to traditional solar cells et al., 2012). (Bisquert et al., 2004; Grätzel, 2005; Wang et al., 2006; Gong In the previous investigations, it has been shown that 1-D et al., 2012, 2017; Jung and Lee, 2013; Scalia et al., 2018; Juang ZnO nanostructures fabricated by different chemical methods et al., 2019) and also present satisfactory conversion proficiency (Gao et al., 2007) can be used as photoelectrodes in DSSC (13 –14%) (Hagfeldt et al., 2010; Parisi et al., 2014; Freitag solar cells. Still, there were remaining some open questions, for et al., 2017). As mentioned earlier, optimizing the structure with example, the effect of the following factors, such as (1) Quality of maximum absorption and least electron-hole recombination is crystallinity and Optimum morphology (diameter and length) of one of the significant challenges in designing and manufacturing NWs on the performance of a DSSC device. (2) Repeatability and of photoelectrodes for their use in the DSSC device. It is also control over the fabrication of NWs. (3) Better surface area for important to mention that the short circuit current and open- the overall enhancement in the efficiency of the device (Saleem circuit voltage of the device is strongly dependent on the overall et al., 2019). One of the disadvantages of ZnO NWs for potential absorption and recombination processes, which is one of the application as photoelectrode could be low chemical stability need of the hour for the fabrication of these devices. One of the when immersed in the acid environment generated by the dye most common semiconductors is TiO2, which is widely used as a dissolution. Fortunately, many proposals have been performed photoelectrode (anode) in a DSSC device due to the great surface to overcome the chemical stability problem. These proposals area of the network nanoparticles and physical and chemical have improved the DSSC device performance (Sakai et al., 2013; stability; however, ZnO is an exciting alternative for the electrode Warnan et al., 2013). due to the similar, even better electrical properties compared to In early works, our research group has demonstrated broad TiO2 (Canto-Aguilar et al., 2017; Boschloo, 2019). control over the growth of ZnO NWs through the VLS ZnO is a semiconductor of the II-VI group. It presents a method, where the effect of seed layer, vertical and random broad direct bandgap of 3.3–3.37 eV at 300 K and a large exciton orientation of the NWs on the structural, morphological and binding energy of 60 meV (Galdamez et al., 2019); also, it has optical properties have been analyzed. Moreover, we have higher electron mobility (130–200 cm2 V−1 s−1 ) and diffusion reported the application of these ZnO NWs in bio-sensing and coefficient (1.1∗ 10−4 cm2 s−1 ). ZnO shows three crystallographic hydrogen production (Serrano et al., 2017; Galdamez et al., phases: hexagonal wurtzite, zinc blend, and rock salt, and can 2019; Galdámez-Martínez et al., 2020a). Additionally, in previous be synthesized with a varied choice of morphologies such work, we have reported the theoretical study of the absorption as nanoparticles, nanorods, nanowires, nanobelts, nanoflowers, spectra of N719 dye together with the ZnO NWs, and it was hierarchical and core-shell structures (Zhang et al., 2012; Yang found that a particular N719 group showed the best alignment of et al., 2015). Among all ZnO nanostructures, ZnO nanowires the bands for the better absorption, charge-transport mechanism, (ZnO NWs) are of the main interest due to their high aspect and rate of regeneration (Portillo-Cortez et al., 2019). From that ratio (AR), the proportion of length/diameter and is related experience, we wanted to start implementing the fabrication of to the effective surface area. Also, ZnO NWs present effective the first photoelectrode prototype based on ZnO NWs grown confinement of photons and carriers (Song et al., 2010; Cui, by the VLS process and their relationship with N719 dye for 2012), and they provide a direct and fast pathway to the improved absorption. transportation of free charge carriers (Karst et al., 2011; Lee et al., In the present work, we show the fabrication of ZnO NWs 2011; Meng et al., 2014). These previous properties suggest ZnO with a different aspect ratio (AR) and good crystallinity obtained NWs as a good candidate for UV laser, photocatalysis, solar cells, by the VLS process. Four different thicknesses of the Au layer are LEDs, and other photonic applications (Hong et al., 2008; Kang utilized to control the AR, shape, and density of NWs. The optical et al., 2008; Wijeratne and Bandara, 2014; Polyakov et al., 2016). properties were studied before and after the sensitization with the The manufacturing techniques of ZnO NWs can primarily N719 dye of the ZnO NWs. To the best of our knowledge, this is be categorized as vapor and solution phase synthesis (physical the first time that AR of ZnO NWs grown by VLS is optimized and chemical) (Cui, 2012; Udom et al., 2013). Vapor-liquid- for potential application as photoelectrodes to be used in a DSSC solid (VLS) is a kind of vapor-phase synthesis (physical) for device. The present research could lead to a suitable merge in ZnO NW. VLS process is widely used to obtain nanostructures modern organic solar cell technology such as Perovskites and with controlled features: dimensions, shapes, distribution, other optoelectronic applications. Frontiers in Chemistry | www.frontiersin.org 2 February 2021 | Volume 8 | Article 604092 Portillo-Cortez et al. ZnO Nanowires as Photoelectrode in DSSC MATERIALS AND METHODS Characterization The structural characterization of the samples was obtained TCO and Seed Layer (SL) Deposition by XRD (Bruker D8) with Cu-Kα radiation of 1.54 Å, 2θ Both TCO and SL films are composed of AZO and were deposited angle from 25◦ to 65◦ . Morphological properties were analyzed on quartz substrates (1.5 × 2.5 cm) by a magnetron sputtering via Field emission scanning electron microscopy (FESEM) in system (mod. H2 INTERCOVAMEX R ). A solid target of AZO a JEOL JSM-7600F. JEOL ARM 200F was used to carry out (99.999 %, ZnO 98 wt %: Al2 O3 wt %, Kurt Lesker R ) was used the Transmission electron microscopy (TEM) measurements. as source material. The chamber pressure was maintained at Optical properties were studied using a Cary-5000 UV-Vis-NIR 9.3 × 10−5 mbar, and a mass flow controller regulated the Ar spectrophotometer with an integrating sphere in the range from gas (99.999%) flow rate. The sputtering process was performed 250 to 950 nm. Kimmon Koha He-Cd laser was used for the similarly, as reported previously (Serrano et al., 2017). Deposition photoluminescence measurements. conditions for TCO were 45 W sputtering power, 5 sccm Ar flow for 12 min, and the substrate-target distance was 4 cm. Subsequently, the SL film was deposited at 45 W, 35 sccm Ar RESULTS AND DISCUSSION flow for 6 min, and the substrate-target distance was kept at The four ZnO NWs samples presented different AR, as listed 10 cm. All the sputtering growth processes were carried out at in Table 1. The variation in AR is due to the four thicknesses room temperature. After both deposition processes, the sample of Au layers deposited as a metal catalyst for the VLS process, is labeled as TCO/SL. that resulted in a change of the diameter of the Au liquid drops and, as a consequence, different diameters of the nanowires are ZnO NW Photoelectrodes Structure by VLS obtained (Güell et al., 2016). Table 1 shows that the diameter of ZnO NW samples were prepared by the VLS method on TCO/SL NWs augmented with increasing the Au layer thickness, whereas substrates previously deposited for their use as a photoelectrode the length was 3 µm approximately for all samples (Please (TCO/SL/NWs). At this point, the TCO/SL samples were coated refer to SEM Micrographs for graphical details and histogram with a thin Au layer via the DC sputtering process with four descriptions, Figure 4). The AR increased with a decrease in different thicknesses (Table 1). The VLS process was performed the diameter of the NWs, and the maximum AR was obtained in a tubular oven (HAT-1200, EVELSA R ). For the carbothermal with a diameter of 30 nm, which states the reverse relationship reduction, ZnO powder (99.999%) and graphite (99.99%) were between the AR and the diameter of NWs. It is essential to placed in a quartz container, whereas the sample TCO/SL mention that VLS offers this advantage of obtaining thin NWs coated with the Au layer was placed in a second substrate compared to conventional chemical synthesis methods. This holder. The quartz containers were placed inside the oven, result is significant because a high value of AR is related to a followed by an annealing process at 950◦ C for 1 h under high surface area in order to perform a superficial process that (3.3 L/min) Ar atmosphere. subsequently could be used for different applications such as gas sensors or catalysis. Sensitization Process of ZnO NW Figure 2 presents the XRD patterns of the NWs samples The four ZnO NW photoelectrodes prototypes were sensitized with different AR. The patterns show the peaks corresponding with 0.5 mM of N719 dye (Ruthenizer 535-bisTBA from to (100), (002), and (101) planes of ZnO, and (111) plane of Solaronix R ) in an ethanolic solution (Sigma Aldrich) at 60◦ C for Au phases according to database sheets ICDD 01-070-2551 and 2 h. Following sensitization, the samples were extracted from the ICDD 03-065-2870 for ZnO and Au, respectively. No secondary dye solution and meticulously cleaned with ethanol to remove the phases were identified for aluminum compounds from AZO dye residues. The elaboration process of ZnO NW photoelectrode layers. Also, the XRD peaks intensity shows a decrease when AR architecture is shown in Figure 1, which displays (A) AZO as a decreased from 100 to 35 (d = 30 nm to d = 85 nm, Table 1). seed layer and (B) Au film deposition followed by (C) ZnO NWs The sample NW (d = 30 nm) showed the highest peak intensity, growth by VLS and (D) obtention of ZnO NWs with different AR. resulting in better crystal quality. (E,F) show the sensitization process of ZnO NWs with N719 dye. All patterns show a preferential crystallographic orientation along (002) peak, which corresponds to a c-orientation perpendicular to the substrate. The texture coefficient τ(hkl) was TABLE 1 | Morphological properties of the ZnO NWs in relation to different Au determined by the followed expression (Hsu et al., 2005; Karst thicknesses. et al., 2011): Sample Au NW NW NW thickness diameter length aspect I(hkl) (nm) (nm) (nm) ratio Io(hkl) (AR) τ(hkl) = P  I(hkl)  (1) 1 N N Io(hkl) NW (d = 30 nm) 1 30 3,000 100 NW (d = 42 nm) 2 42 2,800 67 Where I(hkl) corresponds to the noticed intensity of the (hkl) NW (d = 60 nm) 4 60 2,700 45 plane, Io(hkl) corresponds to the standard intensity of (hkl) plane NW (d = 85 nm) 6 85 3,000 35 from the database sheet, and N is the number of the considered Frontiers in Chemistry | www.frontiersin.org 3 February 2021 | Volume 8 | Article 604092 Portillo-Cortez et al. ZnO Nanowires as Photoelectrode in DSSC FIGURE 1 | Elaboration process of ZnO NWs based DSSC photoelectrode prototype. (A) AZO and (B) Au film deposition, (C) VLS process, (D) ZnO NWs samples, (E) sensitization with N719 dye, and (F) final photoelectrodes. diffraction peaks, N =3 for this work. Table 2 shows the texture of the nanostructures, as shown in the XRD patterns (Figure 2). coefficient for (100), (200), and (101) planes; all the samples have On the other hand, it is well known that radiative recombination, a preferred orientation along the c-axis due to the seed layer. such as photoluminescence, is a kind of loss mechanism, and for However, the crystallite size showed no trend. solar cell photoelectrode applications, it is crucial not to have The photoluminescence spectroscopy (Figure 3) was used as them. It can also be seen that the minimum PL intensity in the a cross-check technique to examine the crystalline quality and case of NWs with 30 nm diameter is the suitable sample as a defect states of ZnO NWs in the present work. The PL relative photoelectrode (anode) in a possible DSSC device. On the other intensities were found to be inversely related to the ZnO NWs hand, broad NWs with a diameter such as 85 nm and intense PL diameter (Minimum for 30 nm, maximum for 85 nm). It is well can be used for different photonic or optoelectronic applications. known that the origin of the intense band in the visible region Figure 4 shows the morphological analysis of ZnO NWs with of the ZnO NWs is due to different defect states on the surface different AR. Figures 4A,E,I,M display the SEM micrographs and inside of the structures. More discussions about the various of the Au drops distribution formed after thermal treatment at luminescence mechanism centers from ZnO NWs can be found 950◦ C on the AZO substrates coated with different thicknesses in our recent review article (Galdámez-Martinez et al., 2020b). As of Au layers as listed in Table 1. As can be seen, the Au drop shown in Figure 3, the luminescence intensity decreases with the size was found to be enhanced (Figure 4) with the growing diminution of the NW diameter or increase in the AR. This is also thickness of the Au layer (Table 1). Figures 4B,F,J,N show a related to a minimal point defect density and better crystallinity planar view of the ZnO NWs with homogeneous distribution, Frontiers in Chemistry | www.frontiersin.org 4 February 2021 | Volume 8 | Article 604092 Portillo-Cortez et al. ZnO Nanowires as Photoelectrode in DSSC FIGURE 2 | XRD patterns of the four ZnO NW samples with different AR. (A) NW (d = 30 nm), (B) NW (d = 42 nm), (C) NW (d = 60 nm) and (D) NW (d = 85 nm). TABLE 2 | Crystallite size and textured coefficient of ZnO NW photoelectrodes. Sample Crystallite size (nm) τ(100) τ(002) τ(101) NW (d = 30 nm) 35.8 0.09 2.97 0.01 NW (d = 42 nm) 33.6 0.03 2.97 0.01 NW (d = 60 nm) 34.7 0.03 2.97 0.01 NW (d = 85 nm) 36.2 0.04 2.90 0.06 which showed a decrement in the density related to the increase in NW diameter. The cross-sectional SEM images of the samples (TCO/SL/NWs) are displayed in Figures 4C,G,K,O. The ZnO NWs present perpendicular growth with respect to the substrate; this result is in good agreement with c-axes orientation obtained in the XRD study, Figure 2. The average length of the NWs was close to 3 µm (Table 1) for all the samples. This result could be FIGURE 3 | PL spectra of the four ZnO NW photoelectrodes with different AR. due to the fact, in the VLS process, the length depends on the source powder quantity (0.2 g) and the VLS process time (1 h), which remained constant for all of the samples in this work. Finally, Figures 4D,H,L,P present the diameter distribution, diameter size of the NWs increased from 30 to 85 nm. In contrast, and surface density (histograms) of NWs obtained for each its uniformity decreased with the diameter increase, as can be photoelectrode prototype calculated from Figures 4B,F,J,N. The seen from the histograms. A high surface density of NWs of Frontiers in Chemistry | www.frontiersin.org 5 February 2021 | Volume 8 | Article 604092 Portillo-Cortez et al. ZnO Nanowires as Photoelectrode in DSSC FIGURE 4 | SEM images of (A,E,I,M) gold drop distribution and, (B,F,J,N) planar view and (C,G,K,O) cross-section view images, and (D,H,L,P) diameter histogram of ZnO NW photoelectrodes with different AR. 66 NW/µm2 was achieved by NW (d = 30 nm) sample, which AR, uniformity, better crystallinity, and higher surface density of decreased with an increase in diameter until 29 NW/µm2 (d the NWs. = 85 nm). These results again confirm NW (d = 30 nm) as After obtaining a majority of the morphological information a suitable sample for prospective photoelectrode applications from the SEM study, additional analysis [TEM and STEM based on ZnO NWs due to desirable properties such as high (Scanning Transmission Electron Microscopy)] was performed Frontiers in Chemistry | www.frontiersin.org 6 February 2021 | Volume 8 | Article 604092 Portillo-Cortez et al. ZnO Nanowires as Photoelectrode in DSSC to cross-check the crystalline quality of the ZnO NWs grown obtained during the VLS process. Figure 5B displays the high- by the VLS process and also the formation of Au on the resolution TEM and Fourier-transformed pattern for the ZnO tip of NWs. Figure 5A shows the micrograph of one of the NW structure (blue) and the Au drop (pink). The ZnO NW deposited ZnO NW (d = 60 nm) to confirm the relationship shows high crystalline quality, and the determined interplanar between the Au diameter and the final diameter of the grown spacing of 0.263 nm concurs with that of the (002) plane in c- ZnO NW. Micrograph presents a clean and smooth longitudinal axes orientation for wurtzite hexagonal ZnO. The lattice spacing feature with Au drop placed on the NW tip that confirms for Au was estimated at 0.238 nm. Figure 5C presents STEM VLS as the nanostructures growth mechanism. After statistical elemental mapping of the ZnO NW where Zn (red) and O (green) studies, the ZnO NWs average diameter was 59 nm, which can be identified homogeneously along the NW, whereas Au is in good concordance with the diameter of the Au catalyst (cyan) is localized on the tip of the NW. (59 nm). The similar diameter of both nanostructures supports Figures 6A,B present the transmittance spectrum and the fact that the Au catalyst determines the diameter of the NWs calculated effective absorption coefficient of the four ZnO NWs FIGURE 5 | (A) TEM image, (B) HRTEM and FFT images and (C) EDS spectrum of single ZnO NW. FIGURE 6 | (A) Transmittance and (B) absorption coefficient of ZnO NW photoelectrodes. Frontiers in Chemistry | www.frontiersin.org 7 February 2021 | Volume 8 | Article 604092 Portillo-Cortez et al. ZnO Nanowires as Photoelectrode in DSSC samples (photoelectrode prototype) with different AR. From Figure 6A, no transmittance signal is observed in the UV region of the electromagnetic spectrum due to the intrinsic absorbance properties of ZnO. Nevertheless, the transmittance increases at a longer wavelength. In the visible region, the transmittance signal increases with increasing the AR (decrease of the diameter) of the ZnO NWs. The peaks located in the visible region (450–600 nm) are associated with surface plasmon resonance (SPR) due to Au nanoparticles localized in the tip of ZnO NWs. Figure 6B displays the absorption coefficient of the NWs (α), which can be determined by the formula shown below (Bedia et al., 2015): 1 100   α= ln (2) d T FIGURE 7 | Absorbance spectra of ZnO NW photoelectrodes with different AR and N719 dye. The insert displays a magnification in the 450–700 nm region. where d corresponds to the thickness of the NWs, and T is the transmittance. As can be seen, α decrease with increasing both the wavelength and the AR of the ZnO NWs. Figure 7 shows the experimental absorbance spectra N719 dye (Bora et al., 2013; Thankappan et al., 2015; Lu et al., measured by the Cary-5000 UV-Vis-NIR spectrophotometer 2016). with an integrating sphere of N719 dye and ZnO NW The sensitized photoelectrode prototype is composed of three photoelectrodes with different AR without sensitization. layers: transparent conductive oxide (TCO), a seed layer (SL), and For N719 dye, two absorption bands were observed in the UV ZnO NWs. Figures 8A,C show the change in transmittance and region (311 and 383 nm), and one absorption band was identified absorption coefficient of the NW (d = 30 nm) photoelectrode in the visible region (525 nm). The absorption signal of the prototype after the sensitization process with N719 dye. As can N719 dye is extended to 700 nm approximately. These optical be seen from Figure 8A, a high transmittance is obtained when properties are suitable for performing a high light-harvesting TCO is coated over the quartz substrate, and further, it decreases ability in a DSSC device. The absorbance signal for ZnO NWs when SL is added. Subsequently, the transmittance signal drops is mainly located in the UV region, which is due to the high when ZnO NWs are grown by VLS, followed by sensitization with bandgap of ZnO. However, the signal intensity was found to N719 dye. The resulted decrease in transmittance is due to the be decreased by decreasing the AR (increasing the diameter) in optical properties of the ZnO NWs, the plasmonic properties of this region. This result supports d = 30 nm as the best possible Au nanoparticles, and the absorption properties of N719 dye. photoelectrode for harvesting the incident radiation due to Figure 8C shows the absorption coefficient (α) of the NW (d the high surface area and density distribution. The inset of = 30 nm) photoelectrode prototype, which presents a variation Figure 7 displays a magnification between 450 and 700 nm for corresponding to deposited layers. For TCO and TCO/SL layers, ZnO NWs and N719 dye. The bands at 513, 559, 566, 577, α oscillate along wavelength, whereas for TCO/SL/NW (d = and 610 nm are associated with the surface plasmon resonance 30 nm), α decreases with increasing the wavelength. However, (SPR) effect due to the optical properties of Au nanoparticles when N719 dye is incorporated as a sensitizer, α increases in located on the tip of ZnO NWs. The band positions presented the visible region for both d = 30 and 60 nm, respectively. A a blue shift and an increase in the FWHM with an increase similar trend can be observed for NW (d = 60 nm), as shown in the size of Au diameter (Figure 4). The surface plasmon in Figures 8B,D. This study is carried out to see the influence resonance (SPR) effect is the combined oscillation of unbound of the diameter on the absorption properties of two different electrons of metallic nanoparticles, energized by the incident NWs. Figures 8E,F display α comparison for photoelectrode lights electromagnetic field. When the electrons of nanoparticles with NW (d = 30 nm) and NW (d = 60 nm) before and after are restricted in three dimensions, the electron oscillations the sensitization process. α for both samples decreased with stimulate an electric field around the nanoparticle, which can be wavelength and increased when the AR decreased. However, after stronger than that generated by incident radiation (Lou et al., the sensitization process with N719 dye (Figure 8F) α increased 2013). For DSSC technology, SPR can be utilized to boost the in the visible region for both of the electrodes irrespective of the absorption ability of the incident radiation by the photoelectrode diameter of NWs. A slight difference can be observed in the UV because Au nanoparticles produce a light scattering process region due to variation in the AR of the samples; however, in the of the incident radiation when both the incident radiation visible region, absorption is due to the combination of both N719 and SPR wavelength are close to each other. In this work, dye and ZnO NWs. the absorption peaks located between 513 and 577 nm are From the previous results and discussions, it can be inferred adjacent to the absorption band of N719 dye in 525 nm, then that the NWs with d = 30 nm is one of the best candidates (best the dispersed light due to the SPR effect can be absorbed by the crystalline, least defect sites, better orientation, the appropriate Frontiers in Chemistry | www.frontiersin.org 8 February 2021 | Volume 8 | Article 604092 Portillo-Cortez et al. ZnO Nanowires as Photoelectrode in DSSC FIGURE 8 | (A,B) Transmittance and (C,D) absorption coefficient of NW (d = 30 nm) and NW (d = 60 nm) photoelectrodes, respectively. (E,F) Comparison of the absorption coefficient of the photoelectrodes before and after sensitization with N719 dye. density of NWs, and minimal recombination losses) for their use Finally, this work can guide the optimal design of as a possible photoelectrode (anode) in DSSC or other organic photoelectrodes (anode) for the DSSC applications and solar cells. The high density of NWs and the high aspect ratio other organic solar cells. can improve the NW surface sites for appropriate functionalizing with the dye. Consequently, an increase in absorption in the dye, generates a greater number of electrons, which will be CONCLUSIONS subsequently transferred to the conduction band (CB) of the ZnO NWs. Due to the high crystalline quality and low-recombination ZnO NW photoelectrodes prototypes with a different aspect process (d = 30 nm), most of the electrons will be transferred to ratio (AR) were grown by the VLS process. The results the ETL layer (without loss), which could improve the electrical show ZnO NWs grow with polycrystalline features and (002) properties of the DSSC device, such as short circuit current and as a preferential plane. The NW dimensions can be well- open-circuit voltage. controlled by the thickness of the Au catalyst, resulting in Frontiers in Chemistry | www.frontiersin.org 9 February 2021 | Volume 8 | Article 604092 Portillo-Cortez et al. ZnO Nanowires as Photoelectrode in DSSC a high AR of 100; moreover, the NWs presented high- and project administration. AM, MB, and MG-S: supervision ordered orientation and vertical alignment. The absorbance and review and editing. FG: revision and partial funding. of the samples introduced a dependence from the AR of All authors contributed to the discussion and revision of NWs and SPR of Au nanoparticles. The NW (d = 30 nm) the manuscript. showed better structural and morphological properties, and its optical properties were further improved by N719 dye addition. FUNDING The obtained results suggest our ZnO NW photoelectrodes prototype (anode) as a potential candidate for their use in KP-C acknowledges CONACyT grant CVU 593132. GS optoelectronic applications. thankfully acknowledge the grants PN 4797 and PAPIIT IG100320. AD recognizes the project PAPIIT IA100219. EU’s DATA AVAILABILITY STATEMENT H2020 research and innovation program under Grant no. 654148 Laserlab-Europe, UIU-2019. The original contributions generated for the study are included in the article/supplementary materials, further inquiries can be ACKNOWLEDGMENTS directed to the corresponding author/s. The author acknowledges the LUCE laboratory of ICAT, AUTHOR CONTRIBUTIONS UNAM and S. R. Islas (ICAT) for UV-vis characterization. The authors gratefully thank Adriana Tejeda, Josué All authors contributed a direct and intellectual way to the work. Romero, Carlos Ramos and Omar Novelo (all at the KP-C: investigation and writing-original manuscript. AD and IIM) for their technical support in XRD, SEM, TEM, and GS: supervision, conceptualization, writing-review and editing, Lab measurements. REFERENCES Gao, Y. F., Nagai, M., Chang, T. C., Shyue, J. J. (2007) Solution derived ZnO nanowire array film as photoelectrode in dye sensitized solar cells. Cryst. Abdin, Z., Alim, M. A., Saidur, R., Islam, M. R., Rashmi, W., Mekhilef, Growth Des. 7, 2467–2471. doi: 10.1021/cg060934k S., et al. (2013). Solar energy harvesting with the application of Gong, J., Liang, J., and Sumathy, K. (2012). Review on dye-sensitized solar cells nanotechnology. Renew. Sustain. Energy Rev. 26, 837–852. doi: 10.1016/j.rser. (DSSCs): fundamental concepts and novel materials. Renew. Sustain. 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Portable high Conflict of Interest: The authors declare that the research was conducted in the voltage integrated harvesting-storage device employing dye-sensitized solar absence of any commercial or financial relationships that could be construed as a module and all-solid-state electrochemical double layer capacitor. Front. Chem. potential conflict of interest. 6:443. doi: 10.3389/fchem.2018.00443 Serrano, A., Arana, A., Galdámez, A., Dutt, A., Monroy, B. M., Güell, F., et al. Copyright © 2021 Portillo-Cortez, Martínez, Bizarro, García-Sánchez, Güell, Dutt (2017). Effect of the seed layer on the growth and orientation of the ZnO and Santana. This is an open-access article distributed under the terms of nanowires: consequence on structural and optical properties. Vacuum 146, the Creative Commons Attribution License (CC BY). The use, distribution or 509–516. doi: 10.1016/j.vacuum.2017.03.010 reproduction in other forums is permitted, provided the original author(s) and the Simon, H., Krekeler, T., Schaan, G., and Mader, W. (2013). Metal-seeded copyright owner(s) are credited and that the original publication in this journal growth mechanism of ZnO nanowires. Cryst. Growth Des. 13, 572–580. is cited, in accordance with accepted academic practice. No use, distribution or doi: 10.1021/cg301640v reproduction is permitted which does not comply with these terms. Frontiers in Chemistry | www.frontiersin.org 11 February 2021 | Volume 8 | Article 604092