1 Introduction

Dye-sensitized solar cells (DSSCs) have attracted great attention in the area of using clean solar energy [1–4]. DSSC is based on a mesoporous structure of a wide band semiconductor material [e.g., titanium oxide (TiO2), zinc oxide (ZnO)] sensitized by a light harvesting material (conventionally metal–organic Ru-based dyes), permeated with a redox electrolyte, and sandwiched by a counter electrode. ZnO is a unique material that has prompted a vast amount of research. Various morphologies and sizes of ZnO materials have led to a wide range of promising applications. ZnO also is one of the most important semiconductor materials as the photoelectrodes of DSSCs and quantum dot sensitized solar cells (QDSSCs) due to its suitable energy-band structure and excellent physical properties [5]. Compared to TiO2, ZnO is a rather unique functional semiconductor material with a wide band gap, higher electron mobility at room temperature (115–155 cm2 V−1 s−1) is higher than that of anatase TiO2 (<10−5 cm2 V−1 s−1) [6], has been recognized as a kind of ideal material to replace TiO2 [6–10]. In addition, the surface structures of ZnO can be easily controlled by growth methods, and have simple tailorability of nanostructure as compared to TiO2 [7, 10, 11]. However, overall solar-to-electric energy conversion efficiency (η) of ZnO-based cells is still relatively low. In recent years, more researchers have been focused on improving the efficiency of ZnO-based cells using nanostructured ZnO materials, especially ZnO one-dimensional (1D) materials (nanowires, nanorods, nanowire/nanorod arrays and nanotubes et al.) which are believed to gain the enhancement of efficiency because of their rapid electron transfer, reduction of charge recombination degree, and collection of carriers through electrical transport pathway [7, 12–19]. However, the use of 1D ZnO nanomaterials limits the surface area and interface of electrode films, resulting in a low η.

Alternatively, another effective method to the enhance efficiency in DSSCs is to build a light scattering layer with enhanced photocapture efficiency and optical absorption in photoelectrode films [20–23]. Until now, a variety of ZnO nanostructures have been fabricated and applied in DSSCs and QDSSCs [19, 24, 25]. Although research related to the applications of ZnO is progressing rapidly, it has been an enormous change to produce uniform ZnO materials. There are many synthesis methods available to produce ZnO nanostructures, including thermal evaporation of ZnO powders [26], vapor-phase transport processes [27, 28], and solution synthesis [29–31]. Low-temperature process, high growth rate, ease of production on large-area substrate, and convenient operation are some of the advantages offered by electrodeposition, which are significant for further industrial applications. This work presents a simple electrochemical method for tuning the degree of extended growth of ZnO nanostructures along the c-axis, giving regular hexagonal sheet-like nanocrystals at one extreme and hexagonal rod at the other. The two-dimensional (2D) hexagonal sheet-like nanocrystals assembled to three-dimensional (3D) ZnO network nanostructures, was directly grown on ITO substrate at low temperature by electrochemical method, and its application on the photoelectrode of DSSCs was investigated by sensitizing the 3D ZnO nanosheets and 1D ZnO nanorods with N719. The QDSSCs based on the photoelectrode of 3D ZnO nanosheets network structures showed superior photoelectrochemical performance to that of 1D ZnO nanorods.

2 Experimental

In a typical electrodeposition process, a piece of Pt foil and a standard silver/silver chloride electrode (Ag/AgCl) were used as the counter and reference electrodes, respectively. Indium-tin oxide (ITO)-coated glass was used as a substrate and working electrode. ITO-coated glass was cleaned using acetone and rinsed with deionized water before use. All of the reagents are of analytical grade and used without further purification. Aqueous 0.1 M KCl solutions with various concentrations of ZnCl2 were used as electrolytes. H2O2 was used as a source of oxygen. Electrodeposition was carried out using an electrochemical analytical instrument (Ivium Stat). The applied potential was operated at −1 V versus Ag/AgCl for 30 min and the solution temperature was maintained at 70 °C. Finally, all of the samples were annealed at 350 °C for 30 min in air.

For use in solar cells, the samples were dye-sensitized with Ru-based N719 dye (cis–bis (isothiocyanato) bis (2,2-bipyridyl 4,4-dicarboxylato) ruthenium(II) bis-tetrabutylammonium) by immersion in dye solution (mixture of acetonitrile and tert-butyl alcohol, volume ratio: 1:1) at 25 °C for 24 h. After dye-sensitization, the samples were rinsed with acetonitrile to remove non-chemisorbed dye and dried in a nitrogen stream. To measure dye-loading on the samples, the dye-sensitized ZnO layers were immersed in 4 mL of 10 mM KOH for 1 h, and afterwards the absortion spectra of the solutions were measured photometrically. To fabricate DSSCs, the ZnO samples were sensitized with Ru-based N719 dye and then sandwiched together with a Pt coated fluorine doped glass counter electrode using a polymer spacer (Surlyn). The electrolyte was a mixture of 0.1 M LiI, 0.03 M I2, 0.6 M 1, 2-dimethyl-3-propylimidazolium iodide, and 0.5 M tert-butylpyridine (TBP) in acetonitrile.

The field-emission gun scanning electron microscope (FE-SEM, Hitachi S-4800) was used to evaluate the morphology of the samples. The crystalline orientations of TiO2 were confirmed by X-ray diffraction (XRD-6000, Bruker). Photovoltaic measurements were made using an AM 1.5 illumination provided by a xenon lamp (150 W) with an optical filter (AM 1.5G) under a power of 100 mW cm−2 (calibrated by a standard silicon solar cell) and the Ivium Stat electrochemical analyzer was employed to measure the photocurrent and voltage (J–V) obtained from an illuminated area of 0.5 cm × 0.5 cm.

3 Results and discussion

The morphology of ZnO is a key factor in its properties. Many applications of ZnO nanostructures have been identified due to their morphological dependence. ZnO nanostructures can be synthesized by a variety of methods, such as chemical-vapor-deposition [32–34], thermal evaporation [35–37], hydrothermal synthesis [38–40], and electrodeposition process [41, 42]. Electrodeposition of ZnO nanostructures is generally based on the generation of OH− ions at the surface of working electrode by electrochemical reduction of precursors such as O2 [43], NO3 − [44], and H2O2 [45] in Zn2+ aqueous solution. During the deposition process, OH− ions are produced in terms of Eqs. (1), (2), or (3) after certain potential is applied. Then Zn2+ ions in the vicinity of working electrode react with OH− ions, resulting in formation of zinc hydroxide (Zn(OH)2) (Eq. 4). Finally, ZnO is produced by dehydration of Zn(OH)2 (Eq. 5). The morphology of the as-synthesized ZnO nanodeposits strongly depends on the experimental conditions, particularly the Zn2+ concentration, which might change the reaction rate of the hydroxylation (Eq. 4) and dehydration (Eq. 5), thereby enabling modifying the growth behavior of ZnO nanodeposits [43, 46].

$${\text{O}}_{2} + 2{\text{H}}_{2} {\text{O}} + 4{\text{e}}^{ - } \to 4{\text{OH}}^{ - }$$
(1)
$${\text{NO}}_{3}^{ - } + {\text{H}}_{2} {\text{O}} + 2{\text{e}}^{ - } \to {\text{NO}}_{2}^{ - } + 2{\text{OH}}^{ - }$$
(2)
$${\text{H}}_{2} {\text{O}}_{2} + 2{\text{e}}^{ - } \to 2{\text{OH}}^{ - }$$
(3)
$${\text{Zn}}^{2 + } + 2{\text{OH}}^{ - } \to {\text{Zn}}\left( {\text{OH}} \right)_{2}$$
(4)
$${\text{Zn}}\left( {\text{OH}} \right)_{2} \to {\text{ZnO}} + {\text{H}}_{2} {\text{O}}$$
(5)

The dehydration reaction plays a leading role in the electrolyte containing role in the electrolyte containing a low concentration of Zn2+ [43, 46]. Thus, Zn(OH)2 can be converted into ZnO as soon as it is produced, leading to formation of 1D ZnO nanostructures (e.g., nanorods, nanowires, or nanopillars) because of the anisotropic growth along the [0001] direction of the hexagonal wurtzite structure [43, 46–51]. However, the formation of ZnO produced by dehydration of Zn(OH)2 could be delayed because of the relatively faster hydroxylation reaction in the case of high Zn2+ concentration. As a result, the growth along [0001] direction might be replaced by other preferred growth direction, such as [\(01\bar{1}0\)] or [\(10\bar{1}0\)] [39, 52], which can give rise to formation of 2D nanostructures (e.g., nanosheets or nanowalls) [42, 43, 46].

Figure 1a, b present the morphology of the ZnO nanostructures obtained at Zn2+ concentrations of 5 and 20 mM, respectively. It can be noticed that the morphology of ZnO nanostructures change from nanorods to nanosheets by increasing the Zn2+ concentration. The effect of Zn2+ concentration on the morphology can be attributed to the difference between the solubility of ZnO and other zinc-containing compounds, such as Zn5(OH)8Cl2H2O [46]. At low Zn2+ concentration, ZnO has a lower solubility than Zn5(OH)8Cl2H2O and is preferentially precipitated from the electrolyte at the electrode surface. Naturally, the c-axis of ZnO is associated with a higher growth rate than other directions and a rod-like morphology is therefore easily obtained. As the Zn2+ concentration is increased, the solubility of ZnO increases more rapidly than Zn5(OH)8Cl2H2O. In this case, Cl− can partially substitute for OH− and sheets-like Zn5(OH)8Cl2H2O rather than ZnO nanorods is formed [53]. The formation of Zn5(OH)8Cl2 can be attributed to the high concentration of Zn2+ in electrolyte, which gives rise to the large ionic products ([Zn2+][OH−]2 or [Zn2+][OH−] x [Cl−] y ) in solution and hence the precipitation of Zn(OH)2 (Eq. 4) or zinc hydroxylchloride compounds (Zn(OH) x Cl y ) (Eq. 6) in the as-synthesized nanodeposits, as demonstrated by Peulon and co-workers [43]. After annealing in the air, the pyrolysis of Zn5(OH)8Cl2H2O will happen (Eq. 7).

$${\text{Zn}}^{2 + } + x\left( {\text{OH}} \right)^{ - } + y{\text{Cl}}^{ - } \to {\text{Zn}}\left( {\text{OH}} \right)_{x} {\text{Cl}}_{y}$$
(6)
$${\text{Zn}}_{5} \left( {\text{OH}} \right)_{8} {\text{Cl}}_{2} {\text{H}}_{2} {\text{O}} \to 5{\text{ZnO}} + 4{\text{H}}_{2} {\text{O}} \uparrow + 2{\text{HCl}} \uparrow$$
(7)
Fig. 1
figure 1

a FE-SEM images of nanostructures electrodeposited on ITO substrates after thermal treatment with different ZnCl2 concentration: a 5 mM; b 20 mM

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Figure 1a is typical FE-SEM top-images of the as prepared films at low and high magnifications, respectively, showing a highly uniform and densely packed array of nanorods. The nanorods have flat hexagonal crystallographic planes, and their diameters are of 50–150 nm. While the sheet-like ZnO is formed with the increasing of Zn2+ concentration as shown in Fig. 1b. It is found that no nanorod structures are formed at the Zn2+ concentration of 20 mM, and 2D ZnO nanosheets are thus obtained. The 2D hexagonal sheet-like nanocrystals assembled to 3D ZnO network nanostructures.

Figure 2 shows the XRD patterns of ZnO nanostructures on ITO substrate. The positions of all diffraction peaks agreed closely with those of the wurtzite ZnO structure. Notably, the changing relative intensities of the (100) and (002) peaks in the XRD patterns of ZnO nanostructures represent a change in shape [54]. A low (100)/(002) ratio reveals the formation of nanorods are oriented along the c-axis, and a large fraction of nonpolar facets. Conversely, a very large (100)/(002) ratio indicates shortening along the c-axis, as for a nanosheet structure, whose surface is dominated by polar facets. Accordingly, the XRD data herein clearly reveals that the significant difference between nanosheets and nanorods in the proportions of polar facets and nonpolar facets. The (001) and (\(00\bar{1}\)) polar planes of the wurtzite ZnO crystal are known to have a high surface energy, and the nonpolar planes parallel to the c-axis are the most stable and have a lower surface energy [53]. The polar facet of ZnO had a highest surface energy than any of the other faces. They have the inherent capacity to absorb the dye. At the same time, the polarity of wurtzite ZnO is a result of its noncentrosymmetric structure with its polar direction along the c-axis, this material exhibits strong spontaneous polarization (P = −0.057 cm−2) [55]. The oppositely charged ions and the strong polarization field in the sheet structure may result in the efficient charge separation of photo-induced carrier.

Fig. 2
figure 2

XRD patterns of ZnO nanostructures with various morphologies

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The photovoltaic properties of the DSSCs constructed with the ZnO nanorod and nanosheet structures are presented in Fig. 3 and Table 1. The variation might be ascribed to the difference in the morphology of the ZnO nanostructures, which can therefore influence the internal surface area of the photoelectrodes. Thus the dye loading of the ZnO nanorod photoelectrode is restricted to the low level (2.86 × 10−8 mol cm−2), leading to the low short-circuit current densities (Fig. 3 and Table 1) and hence the low power conversion efficiencies (1.15 %). Because of the relatively large internal surface area, the power conversion efficiency of the DSSCs increases to 1.59 %, which results in the high dye loading (3.54 × 10−8 mol cm−2) and hence the high short-circuit current density (5.03 mA cm−2). The high J sc and η might be ascribed to the excellent properties of 3D ZnO nanosheet network structures. First, the large specific surface area of 3D ZnO nanosheet network structures increases the dye of N719 and light absorption intensity, and the high porosity facilitates the penetration of electrolyte in the photoelectrode. Second, the polar facet of ZnO nanosheet has a highest surface energy. Therefore, they have the inherent capacity to absorb the dye to increase the short-circuit current density. Finally, the efficient charge separation of photo-induced carriers is improved by the polarity of wurtzite ZnO which is a result of its noncentrosymmetric structure with its polar direction along the c-axis.

Fig. 3
figure 3

J–V characteristics of DSSCs based on ZnO nanorod and nanosheet structures

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Table 1 Photovoltaic performance parameters of DSSCs based on ZnO nanorod and nanosheet structures
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The photocatalytic activities of the ZnO nanorods and ZnO nanosheets were evaluated by studying the photocatalytic degradation of methylene blue (MB) in solution were tested under UV light. Methylene blue (MB), a typical dye contaminant in wastewater, is a blue cationic thiazine dye with maxima of absorbance at 660 (the most prominent), 614, and 292 nm. It has often been used as a model compound to test the photocatalytic degradation of organic materials [56]. All the samples with the size of 1 cm × 2.5 cm were soaked into two identical bottles containing MB (50 mg L−1, 10 mL) and stored in a dark environment for 20 min to reach adsorption equilibrium of MB before exposure to UV light (254 nm, 8 W, 150 μW cm−2) at a room temperature. Figure 4 compares the MB decomposition rate of all the samples over a period of time. After 140 min of photocatalytic reaction, 68 % of MB concentration was removed by sample ZnO nanorods, and 77 % of MB concentration was removed by sample ZnO nanosheets, respectively. It is obviously that the ZnO nanosheets exhibited a much higher photocatalytic activity than the ZnO nanorods.

Fig. 4
figure 4

Comparison of photocatalytic decomposition rates of MB of ZnO nanorods and ZnO nanosheets

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4 Conclusions

In summary, a simple chemical solution route for tailoring morphologies without template or capping agent was used successfully to shape ZnO into 1D nanorods and 2D nanosheets. The 2D hexagonal sheet-like nanocrystals assembled to 3D ZnO network nanostructures, which was directly grown on ITO substrate. The ZnO nanorods and nanosheets were used in the photoelectrode of DSSCs. The DSSCs based on 3D ZnO network nanostructures showed more superior photoelectrochemical performance than that based on 1D ZnO nanorods. The conversion efficiency of 1.59 % achieved by the DSSCs based on 3D ZnO nanosheet network structures. Based on this work, understanding the effect of morphology on photoelectrochemical activity can provide a blueprint for the design of materials in solar cell applications. The large specific surface area of 3D ZnO nanosheet network structures increases the dye loading. Moreover, the polarity effect can further play a significant role on the photo-conversion efficiency.