one - dimensional ( 1-d ) nanostructures become the focus of current research because of the unique properties related with their special structures , such as high surface - to - volume ratios , special compositions , etc . they play important roles in fabricating nanoscale functional electronic , optoelectronic , electrochemical , and mechanical devices . among the numerous 1-d nanostructures , 1-d metal oxide nanostructures have been widely investigated and now have been widely used in many areas , such as catalysts , sensors , ceramics , transparent oxide conductive films ( toc ) and electronic devices [ 16 - 19 ] . with a wide direct band gap of 3.37 ev and a large exciton binding energy of 60 mev , zinc oxide ( zno )   has been attracting attention in both fundamental research and practical applications and has been considered as a promising material for gas sensors , varistors , and optical devices . many kinds of 1-d zno nanostructures have been synthesized till now , such as nanowires , nanotubes , nanobelts , nanorings , nanonails and so on , by many groups including ours [ 21 - 40 ] . it was found that the properties of 1-d zno nanostructures are affected by many factors , such as morphologies , compositions , and alignments [ 30 - 40 ] . in this paper , we present a simple hydrothermal method for the synthesis of aligned zno micro / nanotube arrays on zinc foil . these micro / nanotubes have uniform growth directions along the   orientations with diameters in the range of 100700 nm . room - temperature photoluminescence ( pl ) properties of these micro / nanotubes were investigated and they showed a strong emission peak at 387 nm and 2 weak emission peaks at 422 and 485 nm , respectively . as - synthesized zno micro / nanotube arrays also show hydrophobic properties with a contact angle of 121. finally , single micro / nanotube - based field - effect transistor was also fabricated to investigate their electronic transport properties . all chemical reagents used in the experiment are of analytical grade and used without further purification . in a typical procedure , 5 mmol zinc acetate and equal amount of hexamethylenetetramine ( hmt ) were dissolved in 30 ml deionized water under stirring . and then 10 ml ammonia was put into the above mixed solution . keeping stirring for 15 min , the transparent solution was transferred into a ptfe - lined autoclave with volume of 50 ml . a zinc foil after ultrasonic treatment after sealed , the autoclave was put in an oven and heated at 130c for 7 h. after reaction , the foil was taken out of the autoclave and washed with water for several times and then dried in air . x - ray diffraction ( xrd ) pattern of the products was carried out on a d / max - rb of kaisha x - ray diffractometer with 2 in the range of 3080. scanning electron microscopy ( sem ) and energy dispersive spectrometry ( eds ) were taken on a hitachi s-4800 field - emission scanning electron microscope equipped with an energy - dispersion x - ray detector . the microstructures of the product were investigated using a high - resolution transmission electron microscope ( hrtem , jem-3000f ) . room - temperature photoluminescence ( pl ) was curried out with a spex fl-2t2 fluorophotometer with an excited wavelength of 325 nm . water contact angle ( ca ) of the zno micro / nanotube arrays was measured by using a dataphysics oca20 contact angle system at ambient temperature . all the electrical measurements were carried out using a semiconductor parameter analyzer ( agilent 4156b apparatus ) . figure 1a is a typical sem image of as - synthesized product on zinc foil , which shows the formation of 1-d micro / nanostructures with good alignments . typical micro / nanotubes have hexagonal shapes with diameters in the range of 100700 nm . 1c shows the peaks of only zinc and oxygen , indicating the formation of high purity zno products . a tem image of a single zno nanotube with diameter of around 200 nm is shown in fig . the brightness contrast between the center and the edge indicates the hollow tubular structure , in agreement with the sem result . the clearly resolved lattice fringe is calculated to be around 0.52 nm , in accordance with the ( 0001 ) plane of hexagonal zno crystal . several tens of nanotubes were investigated and they all give similar results , indicating that these nanotubes have preferred growth directions along the   orientations . besides zno micro / nanotubes , some zno nanowires with smaller diameters were also observed in fig . figure 1f is a sem image of the product obtained with a reaction time of 2 h. high - density zno nanowires with diameters smaller than 100 nm are formed on the zinc foil . the result indicates that the final zno micro / nanotubes may grow on the base of small zno nanowires . in fact , the formation of micro / nanotubes from small nanowires has already been observed for several materials , such as zno and zns . a , b sem images , c eds spectrum , d tem image and e hrtem image of as - synthesized zno micro / nanotubes . f sem image of aligned zno nanowire arrays synthesized for short time the crystal structure of the products was also investigated using xrd and the pattern is shown in fig . , all the sharp diffraction peak can be indexed to hexagonal wurtzite zno phase ( jcpds card no.36 - 1451 ) except those small one labeled with asteroidal notation , which come from zn foil used in the experiment . xrd pattern of as - synthesized zno micro / nanotube arrays during hydrothermal synthesis of 1-d nanostructures , it was always found that the salt ions existed in the solution and the ph values of the solution had great influences on the final products . figure 3a , 3b is the sem images of the product obtained when zinc chloride was used instead of zinc acetate . flower - like zno nanostructures were found on a large scale on the zinc foil . each flower has diameter of around 1 m and is composed of numerous small zno nanowires . tem analysis reveals that these small nanowires are single crystals with the growth directions along the   orientations . figure 3c is the sem image of a product obtained under different ph value by using naoh instead of hmt . though zno nanoflowers were also obtained under this condition as those shown in fig . the nanoflowers obtained by using naoh are composed of numerous zno nanoplates instead of nanowires in fig . it was thought that the strong ph values changed the growth rates of different crystal planes and thus resulted in the formation of different structures . c , d zno nanoflowers composed of zno nanoplates to investigate the optical properties of these micro / nanotubes , room - temperature photoluminescence was conducted and a typical spectrum was shown in fig . , it can be seen that the zno micro / nanotube arrays show a sharp and strong emission peak centered at approximately 387 nm , which corresponds to the near - band - edge peak that is responsible for the recombination of free excitons through an exciton - exciton collision process . besides the near - band - edge emission , another two emissions centered at around 422 and 485 nm were also observed in the spectrum , which are the deep - level emissions according to the literature . they are related to the singly ionized oxygen vacancies , and are resulted from the recombination of a photogenerated hole with a singly ionized charge state of specific defects . room - temperature photoluminescence spectrum of the synthesized zno micro / nanotube arrays wettability of nanostructures has attracted great interests in recent years and it is one of the most important properties of nanostructures , which is usually governed by both the chemical composition and the geometrical structure of the solid surfaces . wettability of 1-d zno nanostructures with different morphologies has been studied and the results revealed that different zno nanostructures have quite different wettabilities , such as hydrophobic , superhydrophobic , or hydrophilic properties [ 48 - 51 ] . we also studied the wettability of our aligned zno micro / nanotube arrays by checking the water contract angle on the surface . a contact angle of 121 is observed for the aligned zno micro / nanotube arrays , indicating that the product has a hydrophobic property , which may find applications in self - cleaning and photocatalytic fields . the high water contact angle is believed to be caused by the low surface energy of the ( 0001 ) plane at the micro / nanotube surface combined with the feature size of the sample . typical shape of a water droplet on as - grown zno micro / nanotube arrays single micro / nanotube - based field - effect transistors were then fabricated according to our previous reported technique [ 52 - 54 ] . briefly , the synthesized zno micro / nanotubes were first sonicated into a suspension in isopropanol ( ipa ) and then deposited onto a degenerately doped silicon wafer covered with 500 nm sio2 . photolithography was then performed , followed by ti / au ( 5 nm/100 nm ) deposition to pattern the source and drain electrodes on both ends of the zno micro / nanotubes . figure 6a inset is a top view sem image of the fabricated single - nanotube - device . the channel length between the source and the drain electrodes of the device is 2 m . figure 6a shows the typical gate - dependent current  voltage ( iv ) curves obtained from the device in air . linear current versus voltage was observed for the device , indicating the good ohmic contacts to structure . the applied gate voltages to the device range from 20 to 20 v. the transport data clearly show decrease in conductance for vg < 0 , whereas the conductance increases for vg > 0 , indicating that the present zno micro / nanotubes are of typical n - type semiconducting behavior . the gate effect is relative weak , which is believed to be caused by the existence of high - density defect sites within the micro / nanotubes . vdsids decreases with increasing negative vg , also implies that the zno micro / nanotube are an n - type semiconducting material . a typical ids vds curves obtained at different gate voltages and bids vg curve of a single zno nanotube fet . in summary , zno micro / nanotube arrays have been successfully synthesized via a simple hydrothermal method on a zinc foil substrate . as - synthesized micro / nanotubes are single crystals with growth directions along the   orientations . these micro / nanotubes show strong near - band - edge emission at 387 nm and weak defects - related emissions at 422 and 485 nm , respectively . field - effect transistors were fabricated based on the zno micro / nanotubes , which show n - type semiconducting characteristics . our results show that the hydrothermally synthesized zno micro / nanotubes may be used as self - cleaning photocatalysts as well as building blocks for nanoscale electronic and optoelectronic devices . this work was supported by the high - level talent recruitment foundation of huazhong university of science and technology , the basic scientific research funds for central colleges ( q2009043 ) , the natural science foundation of hubei province ( 2009cdb326 ) , the doctor start - up fund of harbin normal university ( kgb200802 ) , the national natural science foundation of china ( 20871037 ) , the natural science foundation of heilongjiang province ( b2007 - 2 ) and the science technology and research project of education bureau , heilongjiang province ( 11531229 , 12531236 ) . this article is distributed under the terms of the creative commons attribution noncommercial license which permits any noncommercial use , distribution , and reproduction in any medium , provided the original author(s ) and source are credited . this article is distributed under the terms of the creative commons attribution noncommercial license which permits any noncommercial use , distribution , and reproduction in any medium , provided the original author(s ) and source are credited .
we reported the optical and wettability properties of aligned zinc oxide micro / nanotube arrays , which were synthesized on zinc foil via a simple hydrothermal method . 
 as - synthesized zno micro / nanotubes have uniform growth directions along the [ 0001 ] orientations with diameters in the range of 100700 nm . these micro / nanotubes showed a strong emission peak at 387 nm and two weak emission peaks at 422 and 485 nm , respectively , and have the hydrophobic properties with a contact angle of 121. single zno micro / nanotube - based field - effect transistor was also fabricated , which shows typical n - type semiconducting behavior .