Journal of Solid State Chemistry 197 (2013) 69–74

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Journal of Solid State Chemistry

0022-45

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Controllable synthesis of ZnO nanograss with different morphologiesand enhanced performance in dye-sensitized solar cells

Shibu Zhu a, Xiangnan Chen a, Feibiao Zuo a, Man Jiang a, Zuowan Zhou a,n, David Hui b

a Key Laboratory of Advanced Technologies of Materials (Ministry of Education), School of Materials Science and Engineering, Southwest Jiaotong University,

Chengdu 610031, PR Chinab Department of Mechanical Engineering, University of New Orleans, New Orleans, LA 70148, USA

a r t i c l e i n f o

Article history:

Received 29 June 2012

Received in revised form

1 September 2012

Accepted 2 September 2012Available online 11 September 2012

Keywords:

ZnO nanograss

Polyethyleneimine

Hydrothermal method

Dye-sensitized solar cells

96/$ - see front matter & 2012 Elsevier Inc. A

x.doi.org/10.1016/j.jssc.2012.09.001

esponding author. Fax: þ86 28 87600454.ail address: zwzhou@at-c.net (Z. Zhou).

a b s t r a c t

A series of ZnO nanograss films grown on fluorine-doped tin oxide coated glass substrates were

synthesized via hydrothermal method by using polyethyleneimine (PEI) as adjusting agent. The films

were characterized by field emission scanning electron microscope (FE-SEM) and X-ray diffraction

(XRD). It was found that the PEI not only affected the aspect ratios of ZnO nanograss but also changed

the geometrical shape of ZnO nanograss. A possible mechanism based on PEI adsorbed on the non-polar

facets of ZnO that governed the growth rate of different directions were proposed to elucidate the

effect of PEI on morphology of ZnO. The ZnO nanograss films were applied to dye-sensitized solar

cells (DSSCs). The results showed that the photocurrent density significantly enhanced, and the

power conversion efficiency increased by 55% based on ZnO nanograss synthesized in a growth

solution containing 7 mmol/L PEI, resulting from the dye loading properties related to the different

morphologies.

& 2012 Elsevier Inc. All rights reserved.

1. Introduction

Dye-sensitized solar cells (DSSCs) based on oxide semiconductor(typically TiO2) film and organic dye or metallorganic-complex dyehave attracted much attention as a potential, cost-effective alter-native to silicon solar cells since they were first proposed by Grätzelet al. in 1991 [1]. Zinc oxide (ZnO) is a wide band gap (3.37 eV), II–VIsemiconductor of great interest for wide area applications [2–5].In the field of DSSCs, ZnO has been considered as a promisingalternative photoelectrode material owing to its similar band gapand comparable electron injection process as that of TiO2 [6,7].Among the various shapes of ZnO nanostructures, well-aligned ZnOnanorods are important for DSSCs because the electronic mobility ofZnO nanorod is about 2–3 orders of magnitude faster than that inTiO2 nanoparticles film [6].

Many methods have been developed to synthesize well-alignedZnO nanorods, such as metal-organic chemical vapor deposition[8], magnetron sputtering deposition [9], pulsed laser deposition[10], electrochemical deposition [11] and hydrothermal methods[12]. Recently, the hydrothermal method has become the subjectof interest in the growth of ZnO nanorods because of its simpleprocedure, moderate-temperature and low cost. Suresh Kumaret al. reported that ZnO nanorods were successfully grown on

ll rights reserved.

glass substrate via a hydrothermal method through controlling theZnO nanostructure seed layer [13]. For fabrication of ZnO nanorodsvia hydrothermal method, however, it is still a challenge to obtainthe nanorod arrays with smaller diameters, less than 100 nm,controllable density, and higher c-axis orientation. Thus, manyattempts have been made to synthesize ZnO nanowires with highaspect ratios. Gao et al. [14] reported a simple method of preparingZnO nanowires with a high aspect ratio of 100–200 by changingthe molar ratio of Zn(II)/NH3, which enlarged the inter-surface areafor dye loading in DSSCs application. Law et al. [15] reported thatthe introduction of polyethylenimine (PEI) into the growth solutionas an additive could efficiently increase the length of ZnO nanowirearrays, receiving ZnO nanowires with a aspect ratio above 125. Fromthen on, although many researchers [16–18] studied the effect ofPEI on the aspect ratio of ZnO nanorods in detail, their resultsindicated that the variation of precursor concentration resultedin a significant change in the density, aspect ratio and alignmentof ZnO nanorods. To our best knowledge, however, there is nosystematically study focusing on the morphological changes afterintroduction of polyethylenimine (PEI).

Thus, in the present article, ZnO nanograss films on fluorinatedtin oxide (FTO) were synthesized by capping agent-assistedhydrothermal method with polyethyleneimine (PEI) as additive.In this process, the different contents of PEI were added into thegrowth solutions to synthesize ZnO nanograss. The effect of PEIconcentration varying from 0 to 7 mmol/L on the structure andmicro-morphology of ZnO nanograss films growth on FTO substrates

www.elsevier.com/locate/jsscwww.elsevier.com/locate/jsscdx.doi.org/10.1016/j.jssc.2012.09.001dx.doi.org/10.1016/j.jssc.2012.09.001dx.doi.org/10.1016/j.jssc.2012.09.001mailto:zwzhou@at-c.netdx.doi.org/10.1016/j.jssc.2012.09.001

S. Zhu et al. / Journal of Solid State Chemistry 197 (2013) 69–7470

was examined. In this case, the liner molecule of PEI not onlyaffected the aspect ratios of ZnO nanograss, but also changed thegeometrical shape of ZnO nanograss, similar to the effect of bothPEI and ammonium on ZnO nanowires [19]. The photovoltaicconversion properties in dye-sensitized solar cells (DSSCs) were alsodiscussed.

2. Experiment

2.1. Preparation and characterization of ZnO nanograss

All chemicals in this experiment were of analytical grade andused as received without further purification. Preparation of zincoxide (ZnO) nanograss was similar to our previous report [20] with asmall difference. First, fluorinated tin oxide (FTO, 15 O/cm2, NipponSheet Glass, Japan) were rinsed ultrasonically in acetone, ethanoland distilled water for 15 min, successively. A ZnO seed layer wasprepared on FTO glass by dip-coating (3 cm/min) in 5 mmol/Lethanolic solutions of zinc acetate, followed by thermal decomposi-tion at 300 1C for 15 min. The obtained seed layer covered FTOglasses were then transferred into a growth solutions of 25 mmol/LZnNO3, 25 mmol/L hexamethylenetetramine (HMTA) and differentconcentrations (0, 1, 3, 5, 7 mmol/L) of polyethyleneimine (branched,low molecular weight, Sigma Aldrich), which was maintained at92 1C for 32 h in a sealed vessel. The growth solutions were refreshedevery 4 h during the reaction period. After reaction, all samples weretaken out to rinse with deionized water and ethanol, successively, forseveral times, and annealed in air at 450 1C for 30 min to remove anyresidual organics.

The surface morphology and microstructure of the ZnO nano-grass were characterized by field emission scanning electron micro-scopy (FE-SEM, JSM-7001F, JEOL) operated at 15 kV. Furtherstructural analysis of individual ZnO nanorod was carried out usingtransmission electron microscopy (TEM) and high-resolution TEM(HRTEM, JEM-2100, JEOL). X-ray diffraction analysis of the as-grownZnO nanograss film was carried out on a Philips X’Pert PRO X-raydiffractometer with a CuKa radiation from 101 to 1001.

2.2. Fabrication and photovoltaic measurement of DSSCs

The prepared ZnO nanograss photoelectrode was immersed intoa 0.5 mmol/L solution of (Bu4N)2Ru(dcbpyH)2(NCS)2 (N719 dye,Dyesol, Australia) in dry ethanol for 3 h in order to obtain the dye-sensitized ZnO nanograss photoelectrode. The sensitized electrodewas sandwiched with platinum coated FTO counter electrodeseparated by a 25 mm thickness hot-melt spacers (Dupont, Surlyn1702). The electrolyte (0.5 mol/L tetrabutylammonium iodide,0.05 mol/L I2 and 0.5 mol/L 4-tertbutylpyridine in acetonitrile) wasthen introduced by injection and capillary force. A mask of 0.25 cm2

as the ZnO nanograss photoanode area is employed during thecharacterization for the calibration of cell area.

To estimate the dye absorbed amount of sensitized ZnOphotoanodes, the photoanodes were separately immersed into0.1 M NaOH solution [21]. The absorbance of the resultingdesorbed N719 solution was measured by a UV-2550 UV–visspectrophotometer (Shimadzu, Japan). The photovoltaic perfor-mance of fabricated DSSCs were conducted on an electrochemicalwork station (CIMPS-2, Zahner, Germany), simulated illumination(100 mW cm�2) was provided by 150 W Xenon Arc Lamp (XBO150W/CR OFR, OSRAM). The electrochemical impedance spectra(EIS) measurements were recorded by applying a 10 mV ac signalover the frequency ranging from 100 mHz to 100 kHz undersimulated illumination at applied bias of open-circuit voltageto analyze the electron transport properties in prepared ZnOnanograss-based DSSCs.

3. Results and discussion

3.1. Morphological and structural characterization

Fig. 1(a)–(e) displays the representative FE-SEM images of thezinc oxide nanograss fabricated on FTO substrates under differentgrowth conditions. Averages were taken of measurements fromthe ZnO nanograss on a single FTO substrate, where the lengthswere measured using the edge tilt-view images and the diameterswere measured using the tilt-view images [22]. The mean valuesof the ZnO nanograss dimension, including the length and diameterestimated from a statistical evaluation of FE-SEM images, are givenin Fig. 1(f). According to FE-SEM observations, the lengths of allsamples were about 11 mm, and ZnO nanograss were almost verticalaligned on the FTO substrates with a high aspect ratio. In compar-ison, the diameters of ZnO nanograss (a)–(e) (named PEI-0, PEI-1,PEI-3, PEI-5 and PEI-7 film, respectively) are 290, 270, 184, 145 and123 nm (Fig. 1(f)), respectively. It can be seen that the nanorodsnumber densities of the as-prepared PEI-0 to PEI-7 filmsare reckoned as ca. 5.3�108, 7.3�108, 1.1�109, 1.3�109 and1.6�109/cm2, respectively. As the concentration of PEI increasedfrom 0 to 7 mmol/L, the diameter of ZnO nanowire decreased from290 nm to 123 nm. This could be attributed to the presence of PEIhindering only the lateral growth of the ZnO nanowire [15,23,24].As a result, the higher ratio of c-axial growth rate to lateral one ledto an increase in aspect ratio from 34 to 93 when PEI concentrationin the growth solution increased from 0 to 7 mmol/L. Furthermore, itwas found that the shape of ZnO nanorod was hexagonal rod like atlower PEI concentration, while trended to cylindrical needle like athigher PEI concentration (7 mmol/L). The possible reason will bediscussed in the following section.

To give further insight into the morphology and structure of theZnO nanostructure, TEM and HRTEM images were performed. Toprepare TEM sample, ZnO nanorods corresponding to Fig. 1(e) werescraped from FTO substrate. Fig. 2(a) shows the typical TEM imageof several individual ZnO nanorods scraped from PEI-7 film.It clearly demonstrated that the diameter of the nanorods wasaround 120 nm, which was in well agreement with the FE-SEMresult (Fig. 1(e)). The structural characteristic of the nanorods ismore distinctly observed in the corresponding HRTEM pattern(shown in Fig. 2(b)). The measured fringe spacing between adjacentlattice planes of the observed nanorod matches well with the latticespacing of the (0001) planes (d¼0.262 nm) of hexagonal wurtziteZnO, suggesting the nanorods are grown along the [0001] direction.This is also verified by the following XRD results.

The crystallinity of the obtained ZnO nanograss on FTOsubstrate was investigated using X-ray diffraction analysis.Fig. 3 gives the XRD patterns of ZnO nanograss grown for 32 hfrom the aqueous solutions with 0, 1, 3, 5 and 7 mmol/L PEI. It canbe found that all diffraction peaks in XRD patterns can be indexedas wurtzite hexagonal structures (JCPDS card No. 36-1451). Thedominant (0002) peaks which appeared in XRD patterns stronglysupported that the films show a strong orientation along [0001]direction, indicating a high degree of orientation along with c-axisthat was perpendicular to the FTO substrates. The resultingobservations can be inferred from FE-SEM observations (insertimages of Fig. 1(a)–(e)). In addition, compared the samples grownunder different conditions, it was obvious that the intensity of(0002) peak decreased with increase of PEI concentration in thereaction solution. This was ascribed to lower degree of verticalalignment of ZnO nanograss when PEI concentration increased inthe growth solution, which could be seen in Fig. 1. We alsoenlarged the (0002) diffraction peak of all the samples, as shownin Fig. 3(b). The positions of these peaks in the as-synthesizedZnO nanograss (PEI-3, PEI-5 and PEI-7) shifted to lower angelsas compared with that of the ZnO nanograss grown without PEI.

Fig. 1. FE-SEM tilt-view (451) images of ZnO nanograss film growth under different conditions (a) 0 mmol/L PEI, (b) 1 mmol/L PEI, (c) 3 mmol/L PEI, (d) 5 mmol/L PEI and(e) 7 mmol/L PEI. (f) Mean diameter and length from (a) to (e). The insets associate with the edge tilt view SEM images of the samples. The scale bars of tilt-view and insert

images are 1 mm and 10 mm, respectively.

Fig. 2. TEM (a) and HRTEM (b) images of an individual ZnO nanorod detached from PEI-7 film.

S. Zhu et al. / Journal of Solid State Chemistry 197 (2013) 69–74 71

Fig. 3. (a) X-ray diffraction patterns of ZnO nanograss under different growth conditions on FTO glass substrates and (b) enlarged (0002) diffraction peaks.

Fig. 4. Schematic diagram showing the growth of ZnO nanograss on ZnO seed FTO substrates with and without PEI.

S. Zhu et al. / Journal of Solid State Chemistry 197 (2013) 69–7472

The results demonstrated that the plane distance along c-axis ofthe ZnO nanograss, grown using hydrothermal method with PEIas additives, is notably greater than that without PEI.

3.2. Effect of polyethyleneimine on morphology of ZnO

nanostructures

In this study, zinc nitrate was used as the Zn2þ precursor, andHMTA acted as a weak base and pH buffer. ZnO was synthesizedbased on the following chemical reactions [25,26]:

(CH2)6N4þ6H2O26HCHOþ4NH3 (1)

NH3þH2O2NH4þþOH� (2)

Zn2þþOH�2Zn(OH)22ZnOþH2O (3)

It is generally accepted that HMTA decomposes slowly inheated aqueous solutions to yield ammonia and formaldehydeas an initial reactant resulting in forming hydroxide ions. Thehydroxide ions further react with Zn2þ to form zinc complexes,and finally forms ZnO nanograss. It is well known that ZnO hasa polar hexagonal wurtzite structure consisting of two polarplanes {0001} and six crystallographic non-polar planes {011̄0}.The positively charged ions produce a positively charged (0001)-Zn terminated and (0001̄)-O terminated polar surface, possessinghigher surface energy and resulting in faster growth rate along[0001] direction. During the whole growth procedure, the reac-tion conditions including the ZnO seed layer were the sameexcept for PEI concentration, so we speculated that PEI concen-tration was the primary factor affecting the ZnO morphologies.

From previous reports [23,27], we know that the PEI is a non-polar polymer with a large amount of amino side groups (–NH2),which can be protonated over a wide range of pH values (3–11)and therefore become positively charged [16]. The initial pH valueof the precursor solution containing Zn(NO3)2 and HMTA is about6.4 at room temperature, falling within the range of PEI protona-tion. Thus, the pH increases proportionally with the PEI concen-tration in the growth solutions. When the pH is higher than theisoelectric point (IEP) of ZnO (pH¼9.5), ZnO nanorods should benegatively charged in the facets [27]. Therefore, positivelycharged PEI molecules should be adsorbed on the facets of ZnOnanorods to reduce the activity of Zn(II) resulting from electro-static affinity, as indicated in Fig. 4, restraining the nanorodsgrowth in the radial direction. Furthermore, when PEI concentra-tion increased, the negative charge on the facets of ZnO nanorodswould increase. Thus, the ratios of c-axial growth rate to lateralone increased with the PEI concentration increasing, resulting inincreasing of the aspect ratio of ZnO nanorods. Furthermore, ithave been demonstrated that when the c-axial growth rate farexceeded the lateral one would lead to induce formation of sharpneedle-like tips of ZnO [28], as shown in Fig. 1(e). However, whenthe concentration of PEI was higher than 10 mmol/L, nothing wasfound both FTO substrates and the growth solution. This phe-nomenon could ascribe to dissolution of ZnO seed in the higherPEI concentration solution.

3.3. Dye-sensitized solar cells performance

The current–voltage characteristic of ZnO nanograss basedDSSCs were studied under simulated sunlight with intensity of

S. Zhu et al. / Journal of Solid State Chemistry 197 (2013) 69–74 73

100 mW cm�2. Fig. 5 shows the measured J–V curves of thenanograss based DSSCs fabricating with the above samplesnumbered PEI-0 to PEI-7 film, probing the effect of PEI concen-tration on the photoelectrochemical performances. The short-circuit current density (Jsc), the open-circuit voltage (Voc), the fillfactor (FF) and the overall power conversion efficiency (Z)deduced from the J–V curves for the DSSCs are summarized inTable 1. For all of the samples, the cells showed open-circuitvoltage (Voc) of ca. 0.63 V, indicating that there was no significanteffect of ZnO nanorod diameter on Voc of DSSCs in this study. Thiswas attributed that the Voc was proportional to the differencebetween the Fermi level of the ZnO electrode and the electro-chemical potential of the redox couple [29]. However, it wasobvious that the short-circuit current density and overall powerconversion efficiency dramatically enhanced with increase of PEIconcentration as compared to those of DSSCs fabricating with PEI-0 film.

Dye loading ability is very important factor for photoelec-trodes of DSSCs. In general, a larger amount of dye absorbance canensure that the incident lights are fully absorbed, leading to alarger short-circuit current density. Herein, the dye absorbedamount on ZnO nanograss photoelectrodes were obtained bymeasuring the UV–vis absorption spectra of solutions containingdyes desorbed from the ZnO film. Fig. 6 illustrates the opticalabsorbance spectra of solutions containing dyes desorbed fromphotoelectrodes fabricated with different ZnO nanograss. Thefigure shows that the absorbance increases with increasing thePEI concentration, and the amounts of dye loading are listed inTable 1. It was found that the amount of dye loading in PEI-7 filmwas 2.01�10�8 mol/cm2, which was as much as 200% comparedto that of PEI-0 film (0.98�10�8 mol/cm2), just as we hadexpected. The results showed that reducing the diameter of ZnOnanograss would enlarge the surface areas of ZnO nanograss

Fig. 5. Current densities against voltage (J–V) characteristics of the DSSCsfabricated with different photoelectrodes.

Table 1Performances and electron transport properties of ZnO nanograss grown using diff

characteristics and electrochemical impedance spectroscopy (EIS) analyses.

ZnO DSSC Dye loading (mol/cm2) Jsc (mA/cm2) Voc (mV) FF

ZnO/PEI-0 0.98�10�8 1.931 630 0.385ZnO/PEI-1 1.06�10�8 2.108 635 0.377ZnO/PEI-3 1.53�10�8 2.419 605 0.413ZnO/PEI-5 1.68�10�8 2.602 608 0.422ZnO/PEI-7 2.01�10�8 2.901 641 0.393

photoelectrodes, resulting in a large dye loading. Thus, the shortcircuit current densities of DSSCs enhanced significantly, whichagreed with the results for dye loading. We ascribe the majority ofthe improvement in cell performance to the enlargement of theinternal surface area within the ZnO nanostructure photoelec-trodes. In addition, the open space between neighboring needle-like nanorods was easy for diffusion of electrolyte into innerregion of photoanode, resulting in fast redox reaction of theelectrolyte at the working electrode interface [30]. The resultclearly shows an effective increase in overall power conversionefficiency of cells by regulating their aspect ratios.

To further examine the electron transport in the photoanode ofthe ZnO nanograss based DSSCs, charge-transport properties wereinvestigated using electrochemical impedance spectroscopy (EIS)under the illumination (100 mW cm�2) by applying a 10 mV acsignal over the frequency range of 10�2–105 Hz at the bias of Voc.An equivalent circuit representing the DSSCs, as illustrated inFig. 7(a), based on the diffusion-recombination model proposedby Bisquert [31] is employed for analyzing the electron transportproperties in the DSSCs. The measured Nyquist plots of impe-dance data of the DSSCs are shown in Fig. 7(b). The fitting resultsare also listed in Table 1. Estimation of the electron transportingparameters in the nanograss portions of the DSSCs was conductedfrom the Nyquist plots according to the procedure demonstratedby Adachi et al. [32]. The charge transfer resistance (Rk) whichrelated to recombination of electrons at the ZnO/electrolyteinterface decreased with increase of PEI concentration, indicatingthat more recombination loss in the solar cells with ZnO nano-grass grown in higher PEI concentration. This phenomenon wasattributed that the surface area enhanced with increase of PEIconcentration. In combination, the values of Rk/Rw were alsoincreased with increase of PEI concentration, suggesting that alarger number of electrons were injected in the conduction band

erent concentrations of PEI determined by photocurrent density–voltage (J–V)

Z (%) Ro (O) Rk (O) keff (/s) ns (/cm3) Deff (cm2/s)

0.47 43.8 204.3 17 1.68�1017 9.59�10�5

0.50 30.0 186.7 17 1.84�1017 1.28�10�4

0.60 22.4 162.2 17.3 2.08�1017 1.52�10�4

0.66 18.7 136.7 19.6 2.18�1017 1.73�10�4

0.73 12.1 106.1 24.7 2.23�1017 2.62�10�4

Fig. 6. Optical absorbance of solutions containing N719 dyes desorbed from thesensitized photoelectrodes fabricated with different ZnO nanograss films.

Fig. 7. (a) General transmission line model of ZnO nanograss-based solar cells.(b) Nyquist plots of DSSCs performed under illumination at the applied bias of Voc.

The solid lines are the fitting results from the equivalent circuit model of ZnO DSSCs.

S. Zhu et al. / Journal of Solid State Chemistry 197 (2013) 69–7474

of ZnO nanograss [33]. These results were consistent with theestimation of the electron densities at the steady state in theconduction band (ns). Moreover, it had been demonstrated thateffective diffusion coefficient of electrons (Deff ) increased as moreelectron were present since the deep trap were filled [34]. Thesewere in accordance with the Jsc results. The above observationsfurther confirmed an effective increase in the overall powerconversion efficiency of cells by adjusting their aspect ratios bymeans of regulating PEI concentration in the growth solutions.

4. Conclusion

In summary, ZnO nanograss films were successfully synthe-sized by means of hydrothermal process without or with addingPEI as a capping surfactant. Detailed morphological and structuralanalyses show that the ZnO nanograss have the hexagonalwurtzite structure, grown vertically on the substrates along the[0001] direction. It has been found that adjusting PEI concentra-tion in the growth solution not only affects the aspect ratio of ZnOnanograss but also changes the geometrical shape of ZnO nanos-tructures. Dye-sensitized solar cell studies show that the overall

energy conversion efficiencies increased by 55% when the PEIconcentration in the growth solution is increased from 0 mmol/Lto 7 mmol/L, resulting from enlarging surface area and henceimproving the loading of N719 dye.

Acknowledgment

This work was financially supported by the National NaturalScience Foundation of China (Nos. 51173148, 90305003) andthe Fundamental Research Funds for the Central Universities(SWJTU11ZT10). Shibu Zhu is also grateful for financial supportfrom the Innovation Fund for Ph.D. Students of SouthwestJiaotong University.

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Controllable synthesis of ZnO nanograss with different morphologies and enhanced performance in dye-sensitized solar cellsIntroductionExperimentPreparation and characterization of ZnO nanograssFabrication and photovoltaic measurement of DSSCs

Results and discussionMorphological and structural characterizationEffect of polyethyleneimine on morphology of ZnO nanostructuresDye-sensitized solar cells performance

ConclusionAcknowledgmentReferences